These materials are to be used only for the purpose of individual, private study and may not be reproduced in any form or medium, copied, stored in a retrieval system, lent, hired, rented, transmitted, or adapted in whole or in part without the prior written consent of Jeppesen. Copyright in all materials bound within these covers or attached hereto, excluding that material which is used with the permission of third parties and acknowledged as such, belongs exclusively to Jeppesen. Certain copyright material is reproduced with the permission of the International Civil Aviation Organisation, the United Kingdom Civil Aviation Authority, and the Joint Aviation Authorities (JAA). This book has been written and published to assist students enrolled in an approved JAA Air Transport Pilot Licence (ATPL) course in preparation for the JAA ATPL theoretical knowledge examinations. Nothing in the content of this book is to be interpreted as constituting instruction or advice relating to practical flying. Whilst every effort has been made to ensure the accuracy of the information contained within this book, neither Jeppesen nor Atlantic Flight Training gives any warranty as to its accuracy or otherwise. Students preparing for the JAA ATPL theoretical knowledge examinations should not regard this book as a substitute for the JAA ATPL theoretical knowledge training syllabus published in the current edition of “JAR-FCL 1 Flight Crew Licensing (Aeroplanes)” (the Syllabus). The Syllabus constitutes the sole authoritative definition of the subject matter to be studied in a JAA ATPL theoretical knowledge training programme. No student should prepare for, or is entitled to enter himself/herself for, the JAA ATPL theoretical knowledge examinations without first being enrolled in a training school which has been granted approval by a JAA-authorised national aviation authority to deliver JAA ATPL training.
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JA310106-000
ii
© Jeppesen Sanderson Inc., 2004 All Rights Reserved ISBN 0-88487-356-0
Printed in Germany
PREFACE_______________________
As the world moves toward a single standard for international pilot licensing, many nations have adopted the syllabi and regulations of the “Joint Aviation Requirements-Flight Crew Licensing" (JAR-FCL), the licensing agency of the Joint Aviation Authorities (JAA). Though training and licensing requirements of individual national aviation authorities are similar in content and scope to the JAA curriculum, individuals who wish to train for JAA licences need access to study materials which have been specifically designed to meet the requirements of the JAA licensing system. The volumes in this series aim to cover the subject matter tested in the JAA ATPL ground examinations as set forth in the ATPL training syllabus, contained in the JAA publication, “JAR-FCL 1 (Aeroplanes)”. The JAA regulations specify that all those who wish to obtain a JAA ATPL must study with a flying training organisation (FTO) which has been granted approval by a JAA-authorised national aviation authority to deliver JAA ATPL training. While the formal responsibility to prepare you for both the skill tests and the ground examinations lies with the FTO, these Jeppesen manuals will provide a comprehensive and necessary background for your formal training. Jeppesen is acknowledged as the world's leading supplier of flight information services, and provides a full range of print and electronic flight information services, including navigation data, computerised flight planning, aviation software products, aviation weather services, maintenance information, and pilot training systems and supplies. Jeppesen counts among its customer base all US airlines and the majority of international airlines worldwide. It also serves the large general and business aviation markets. These manuals enable you to draw on Jeppesen’s vast experience as an acknowledged expert in the development and publication of pilot training materials. We at Jeppesen wish you success in your flying and training, and we are confident that your study of these manuals will be of great value in preparing for the JAA ATPL ground examinations. The next three pages contain a list and content description of all the volumes in the ATPL series.
iii
ATPL Series Meteorology (JAR Ref 050) • The Atmosphere • Wind • Thermodynamics • Clouds and Fog • Precipitation
• Air Masses and Fronts • Pressure System • Climatology • Flight Hazards • Meteorological Information
General Navigation (JAR Ref 061) • Basics of Navigation • Magnetism • Compasses • Charts
• Dead Reckoning Navigation • In-Flight Navigation • Inertial Navigation Systems
Radio Navigation (JAR Ref 062) • Radio Aids • Self-contained and External-Referenced Navigation Systems
• Basic Radar Principles • Area Navigation Systems • Basic Radio Propagation Theory
Airframes and Systems (JAR Ref 021 01) • Fuselage • Windows • Wings • Stabilising Surfaces • Landing Gear • Flight Controls
• Hydraulics • Pneumatic Systems • Air Conditioning System • Pressurisation • De-Ice / Anti-Ice Systems • Fuel Systems
Powerplant (JAR Ref 021 03) • Piston Engine • Turbine Engine • Engine Construction
• Engine Systems • Auxiliary Power Unit (APU)
Electrics (JAR Ref 021 02) • Direct Current • Alternating Current • Batteries • Magnetism
iv
• Generator / Alternator • Semiconductors • Circuits
Instrumentation (JAR Ref 022) • Flight Instruments • Automatic Flight Control Systems • Warning and Recording Equipment • Powerplant and System Monitoring Instruments
Principles of Flight (JAR Ref 080) • Laws and Definitions • Aerofoil Airflow • Aeroplane Airflow • Lift Coefficient • Total Drag • Ground Effect • Stall • CLMAX Augmentation • Lift Coefficient and Speed
• Boundary Layer • High Speed Flight • Stability • Flying Controls • Adverse Weather Conditions • Propellers • Operating Limitations • Flight Mechanics
Performance (JAR Ref 032) • Single-Engine Aeroplanes – Not certified under JAR/FAR 25 (Performance Class B) • Multi-Engine Aeroplanes – Not certified under JAR/FAR 25 (Performance Class B) • Aeroplanes certified under JAR/FAR 25 (Performance Class A)
Mass and Balance (JAR Ref 031) • Definition and Terminology • Limits • Loading • Centre of Gravity
Flight Planning (JAR Ref 033) • Flight Plan for Cross-Country Flights • ICAO ATC Flight Planning • IFR (Airways) Flight Planning • Jeppesen Airway Manual
• Meteorological Messages • Point of Equal Time • Point of Safe Return • Medium Range Jet Transport Planning
Air Law (JAR Ref 010) • International Agreements and Organisations • Annex 8 – Airworthiness of Aircraft • Annex 7 – Aircraft Nationality and Registration Marks • Annex 1 – Licensing • Rules of the Air • Procedures for Air Navigation
• Air Traffic Services • Aerodromes • Facilitation • Search and Rescue • Security • Aircraft Accident Investigation • JAR-FCL • National Law
v
Human Performance and Limitations (JAR Ref 040) • Human Factors • Aviation Physiology and Health Maintenance • Aviation Psychology
Operational Procedures (JAR Ref 070) • Operator • Air Operations Certificate • Flight Operations • Aerodrome Operating Minima
• Low Visibility Operations • Special Operational Procedures and Hazards • Transoceanic and Polar Flight
Communications (JAR Ref 090) • Definitions • General Operation Procedures • Relevant Weather Information • Communication Failure • VHF Propagation • Allocation of Frequencies
vi
• Distress and Urgency Procedures • Aerodrome Control • Approach Control • Area Control
Table of Contents
CHAPTER 1 Basic DC Terminology Introduction ...................................................................................................................................................1-1 The Electric Circuit .......................................................................................................................................1-1 Current (I)......................................................................................................................................................1-2 Electromotive Force (EMF) ...........................................................................................................................1-3 Potential Difference (PD) ..............................................................................................................................1-3 Voltage (V) ....................................................................................................................................................1-3 Resistance (R) ..............................................................................................................................................1-4 Connecting Resistances in Series or Parallel in a DC Circuit ......................................................................1-5 Ohm’s Law ...................................................................................................................................................1-7 Loads ...........................................................................................................................................................1-7 Kirchhoff’s Laws ...........................................................................................................................................1-7 Electric Power (P) .........................................................................................................................................1-8 Electrical Work .............................................................................................................................................1-9 Electric Unit Prefixes ....................................................................................................................................1-9 Typical Circuit Symbols ................................................................................................................................1-9
CHAPTER 2 Electrical Components Introduction ...................................................................................................................................................2-1 Electric Systems ..........................................................................................................................................2-1 Electric Circuit Faults ...................................................................................................................................2-2 Busbars ........................................................................................................................................................2-3 Protection Devices .......................................................................................................................................2-4 Reverse Current Circuit Breaker (RCCB)......................................................................................................2-6 Switches .......................................................................................................................................................2-6 Electric Generator ......................................................................................................................................2-10 Alternator ...................................................................................................................................................2-10 Electric Motor .............................................................................................................................................2-10
CHAPTER 3 Aircraft Batteries Introduction ...................................................................................................................................................3-1 Lead Acid Battery .........................................................................................................................................3-2 Alkaline Battery (Nickel-Cadmium) ..............................................................................................................3-3 Battery Venting ............................................................................................................................................3-4 Electrolyte Spillage ......................................................................................................................................3-5 Battery Capacity ...........................................................................................................................................3-5 Battery Charging ..........................................................................................................................................3-6 Thermal Runaway ........................................................................................................................................3-6 Battery State of Charge ...............................................................................................................................3-6 Battery Condition Check ..............................................................................................................................3-7 Emergency Use ...........................................................................................................................................3-7 Connection of Batteries ................................................................................................................................3-7 Spare Batteries ............................................................................................................................................3-8 Battery Compartment Inspection ..................................................................................................................3-8
Electrics
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Table of Contents
CHAPTER 4 Magnetism Introduction .................................................................................................................................................. 4-1 Magnetism ................................................................................................................................................... 4-2 Fundamental Laws of Magnetism ............................................................................................................... 4-2 Characteristics of Lines of Magnetic Flux..................................................................................................... 4-3 Classification of Magnetic Materials ............................................................................................................ 4-5 Magnetic Flux .............................................................................................................................................. 4-5 Flux Density ................................................................................................................................................ 4-5 Reluctance .................................................................................................................................................. 4-5 Permeability ................................................................................................................................................ 4-5 Hysteresis ................................................................................................................................................... 4-6 Saturation .................................................................................................................................................... 4-7 Magnetism Produced by Current Flow ........................................................................................................ 4-7 The Electromagnet .................................................................................................................................... 4-10 The Relay .................................................................................................................................................. 4-12 Electromagnetic Induction ......................................................................................................................... 4-12
CHAPTER 5 DC Generator Systems Introduction .................................................................................................................................................. 5-1 Generator Systems ...................................................................................................................................... 5-1 Basic Generator Theory .............................................................................................................................. 5-1 A Simple Generator ..................................................................................................................................... 5-2 Conversion of AC to DC .............................................................................................................................. 5-3 DC Generator System Architecture ............................................................................................................. 5-5 DC Generator Construction ......................................................................................................................... 5-5 Principle of Operation of a DC Generator ................................................................................................... 5-6 Types of DC Generator ............................................................................................................................... 5-6 Voltage Regulator ....................................................................................................................................... 5-8 Cut-Out ....................................................................................................................................................... 5-9 Reverse Current Circuit Breaker ............................................................................................................... 5-10 Busbars ..................................................................................................................................................... 5-10 Power Failure Warning .............................................................................................................................. 5-11 Ground Power ........................................................................................................................................... 5-11 DC Generator System Fault Protection ..................................................................................................... 5-12 Twin-Engine DC Electrical System ............................................................................................................ 5-13 Operation of DC Generators in Parallel ..................................................................................................... 5-14 DC Load Sharing ....................................................................................................................................... 5-14 Operation of an Equalising Circuit ............................................................................................................. 5-15 Single-Engine Aircraft DC Electrical System ............................................................................................. 5-15 Operation of the Alternator ........................................................................................................................ 5-16
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Table of Contents
CHAPTER 6 DC Motors Introduction ...................................................................................................................................................6-1 Motors ...........................................................................................................................................................6-1 The Motor Principle ......................................................................................................................................6-1 DC Motors ....................................................................................................................................................6-2 Back EMF ....................................................................................................................................................6-3 Direction of Rotation ....................................................................................................................................6-3 Types of DC Motor .......................................................................................................................................6-4 Motor Speed Control ....................................................................................................................................6-6 Actuators ......................................................................................................................................................6-8 Split-Field Series Motor ................................................................................................................................6-9 Electromagnetic Brakes .............................................................................................................................6-10 Clutches .....................................................................................................................................................6-10 Instrument Motors ......................................................................................................................................6-10 Architecture of a Starter/Generator System ...............................................................................................6-11 Operation of a Starter/Generator System ...................................................................................................6-11 Inverters .....................................................................................................................................................6-14 Multiple Inverter Installations ......................................................................................................................6-15
CHAPTER 7 Inductance and Capacitance Introduction ...................................................................................................................................................7-1 Inductance ...................................................................................................................................................7-1 Self Induction ...............................................................................................................................................7-2 Inductors ......................................................................................................................................................7-2 Time Constant of an Inductor .......................................................................................................................7-3 Inductors in Series and Parallel ...................................................................................................................7-4 Capacitance .................................................................................................................................................7-5 Factors Affecting Capacitance .....................................................................................................................7-5 Types of Capacitor .......................................................................................................................................7-6 The Charging of a Capacitor ........................................................................................................................7-7 Discharging of a Capacitor ...........................................................................................................................7-8 The Time Constant of a Capacitor ...............................................................................................................7-8 Capacitors in Series and Parallel in a DC Circuit .........................................................................................7-9
CHAPTER 8 Basic AC Theory Introduction ...................................................................................................................................................8-1 Alternating Current........................................................................................................................................8-1 Advantages of AC Over DC .........................................................................................................................8-1 Generating AC .............................................................................................................................................8-1 Simple AC Generator ...................................................................................................................................8-2 AC Terminology ...........................................................................................................................................8-3 Phasor Representation ................................................................................................................................8-6
Electrics
ix
Table of Contents
CHAPTER 9 Single Phase AC Circuits Introduction .................................................................................................................................................. 9-1 Single Phase AC Circuits ............................................................................................................................. 9-1 The Effect of AC on a Purely Resistive Circuit ............................................................................................ 9-1 Power in an AC Resistive Circuit ................................................................................................................. 9-1 The Effect of AC on a Purely Inductive Circuit ............................................................................................ 9-2 Power in an AC Inductive Circuit ................................................................................................................. 9-3 Inductive Reactance (XL) ............................................................................................................................. 9-3 The Effect of AC on a Purely Capacitive Circuit .......................................................................................... 9-4 Power in an AC Capacitive Circuit .............................................................................................................. 9-4 Capacitive Reactance ................................................................................................................................. 9-5 Relationship Between Voltage and Current in Capacitive and Inductive AC Circuits .................................. 9-5 Resistive and Inductive (RL) Series AC Circuit ........................................................................................... 9-5 Resistive and Capacitive (RC) Series AC Circuit ........................................................................................ 9-6 Phase Shift .................................................................................................................................................. 9-6 Resistive, Inductive, and Capacitive (RLC) Series AC Circuits ................................................................... 9-6 Impedance (Z) in a Resistive, Inductive, and Capacitive (RLC) Series AC Circuit ...................................... 9-7 Resistive, Inductive, and Capacitive (RLC) Parallel AC Circuit ................................................................... 9-7 Impedance (Z) in a Resistive, Inductive, and Capacitive (RLC) Parallel AC Circuit .................................... 9-7 Power in a Resistive, Inductive, and Capacitive (RLC) AC Circuit .............................................................. 9-8 Power Factor ............................................................................................................................................... 9-9 AC Series Circuit Example .......................................................................................................................... 9-9 AC Parallel Circuit Example ...................................................................................................................... 9-11
CHAPTER 10 Resonant AC Circuits Introduction ................................................................................................................................................ 10-1 Resonant Circuit......................................................................................................................................... 10-1 Series Resonant Circuit ............................................................................................................................ 10-1 Q Factor in a Series Resonant Circuit ....................................................................................................... 10-3 Parallel Resonant Circuit (Tank Circuit) .................................................................................................... 10-3 Q Factor in a Parallel Resonant Circuit ..................................................................................................... 10-5 Self Resonance of Coils ............................................................................................................................ 10-5 Use of Resonant Circuits .......................................................................................................................... 10-5 Tuning Circuits .......................................................................................................................................... 10-7
CHAPTER 11 Transformers Introduction ................................................................................................................................................ 11-1 Transformers.............................................................................................................................................. 11-1 Construction and Operation ...................................................................................................................... 11-1 Types of Transformers .............................................................................................................................. 11-3 Transformer Rectifier Units ....................................................................................................................... 11-5
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Electrics
Table of Contents
CHAPTER 12 AC Power Generation Introduction .................................................................................................................................................12-1 Power Generation .......................................................................................................................................12-1 Simple Three Phase Generator .................................................................................................................12-1 Star Connection .........................................................................................................................................12-2 Delta Connection .......................................................................................................................................12-3 Advantages of Three Phase Over Single Phase AC Generators ...............................................................12-4 Voltage and Frequency of AC Generators .................................................................................................12-4 Phase Rotation ..........................................................................................................................................12-4 Faults on Three-Phase AC Generators ......................................................................................................12-4 Generator Real and Reactive Load Sharing ..............................................................................................12-5 Types of AC Generator ..............................................................................................................................12-6 Brushless Three Phase AC Generator .......................................................................................................12-7 Constant Speed Drive Unit .........................................................................................................................12-9 Operation of the Hydro-Mechanical CSDU ..............................................................................................12-11 Protection of the Hydro-Mechanical CSDU ..............................................................................................12-11 Integrated Drive Generator ......................................................................................................................12-13 Variable Speed Constant Frequency Power Systems .............................................................................12-13 Auxiliary Power Unit .................................................................................................................................12-14 Emergency Ram Air Turbine ....................................................................................................................12-15
CHAPTER 13 AC Power Generation Systems Introduction .................................................................................................................................................13-1 A Typical Frequency-Wild AC System Architecture ...................................................................................13-1 Operation of a Typical Frequency-Wild AC System ...................................................................................13-2 Fault Protection in a Typical Frequency-Wild AC System ..........................................................................13-2 The Constant Frequency Split Busbar AC System ....................................................................................13-4 Operation of a Constant Frequency Split Busbar AC System ....................................................................13-4 Regulation and Protection of Constant Frequency Units ...........................................................................13-5 Faults on a Constant Frequency Split Busbar AC Generator System ........................................................13-5 Emergency Supplies ..................................................................................................................................13-7 Battery Charger ..........................................................................................................................................13-7 Battery Power ............................................................................................................................................13-7 Ground Handling Bus .................................................................................................................................13-7 Constant Frequency Parallel AC System ...................................................................................................13-8 Operation of a Constant Frequency Parallel AC System ...........................................................................13-8 Reactive Load Sharing .............................................................................................................................13-11 Real Load Sharing ...................................................................................................................................13-11 Paralleling ................................................................................................................................................13-12 Fault Protections in a Constant Frequency AC Parallel System ..............................................................13-12 DC Power Supplies ..................................................................................................................................13-14
CHAPTER 14 AC Motors Introduction .................................................................................................................................................14-1 Alternating Current Motors ..........................................................................................................................14-1 Stator-Produced Rotating Magnetic Field ..................................................................................................14-1 Induction (Squirrel Cage) Motor .................................................................................................................14-2 Two-Phase Induction Motor .......................................................................................................................14-4 Split-Phase Motor ......................................................................................................................................14-4 The Synchronous Motor .............................................................................................................................14-5
Electrics
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Table of Contents
CHAPTER 15 Semiconductor Devices Introduction ................................................................................................................................................ 15-1 Semiconductor Devices ............................................................................................................................. 15-1 Advantages and Disadvantages ................................................................................................................ 15-1 Construction of a Semiconductor .............................................................................................................. 15-1 Doping ....................................................................................................................................................... 15-2 P-Type Material ......................................................................................................................................... 15-2 N-Type Material ......................................................................................................................................... 15-3 P-N Junction Diode ................................................................................................................................... 15-4 Use of Diodes ............................................................................................................................................ 15-6 Zener Diode ............................................................................................................................................... 15-6 Variable Capacitance (VARICAP) Diode.................................................................................................... 15-7 Bi-Polar Transistors ................................................................................................................................... 15-7 Operation of a PNP Bi-Polar Transistor ..................................................................................................... 15-8 Operation of a NPN Bi-Polar Transistor .................................................................................................... 15-9 Disadvantages of Diodes and Transistors ................................................................................................. 15-9 Transistor Applications ............................................................................................................................ 15-10 Integrated Circuits ................................................................................................................................... 15-11 The Advantages and Disadvantages of Integrated Circuits .................................................................... 15-11 Types of Integrated Circuits .................................................................................................................... 15-11
CHAPTER 16 Logic Circuits Introduction ................................................................................................................................................ 16-1 Logic Circuits.............................................................................................................................................. 16-1 Number Systems ....................................................................................................................................... 16-1 Binary Representation ............................................................................................................................... 16-2 Basic Logic Gates ..................................................................................................................................... 16-2 Adder and Subtracter Circuits ................................................................................................................... 16-4 Digital Latch and Flip-Flop Circuits ............................................................................................................ 16-6
CHAPTER 17 Computer Technology Introduction ................................................................................................................................................ 17-1 Computers ................................................................................................................................................. 17-1 Analogue Computers ................................................................................................................................ 17-1 Digital Computers ...................................................................................................................................... 17-1 Computer Architecture .............................................................................................................................. 17-3 Input Devices ............................................................................................................................................ 17-3 Central Processing Unit ............................................................................................................................ 17-4 Output Devices .......................................................................................................................................... 17-5 Storage Devices ........................................................................................................................................ 17-5 Operating Systems .................................................................................................................................... 17-5 Programming ............................................................................................................................................. 17-5
CHAPTER 18 HF and Satellite Airborne Communications Introduction ................................................................................................................................................ 18-1 Airborne Communications .......................................................................................................................... 18-2 Long Range Communications (Up to 4000 Km) ........................................................................................ 18-2 Short Range Communications (Up to 450 Km) ......................................................................................... 18-3 Selective Calling (SELCAL) System .......................................................................................................... 18-4 Satellite Communications (SATCOM) ....................................................................................................... 18-5 Satellite Aircom (SITA)............................................................................................................................... 18-7
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Electrics
INTRODUCTION It is essential to know the basic terminology applied to electricity and electrical components before studying further into specific functions and systems. This chapter explains the most common terminology to provide a basis for further study.
THE ELECTRIC CIRCUIT An electrical circuit usually consists of a power source, a load, a switch, and a conductor, which connects the components together.
The power source, which can be a battery or a generator, provides the pressure that causes electrons to flow in a circuit. When electrons flow, it is referred to as an electric current. In the picture above with the switch open, electrical pressure can be measured on the positive side of the switch but no current flows because there is not a complete circuit and so the filament will not illuminate. As the switch is closed, the circuit is completed and current can flow through the closed contacts of the switch and through the filament, causing it to illuminate. Notice that current has to flow from the supply, through the load, and back to the supply to form a circuit. The filament is considered a load as it uses power and creates heat in the process. Notice that it does not matter whether the switch is between the positive and the load, or between the load and the negative. Also notice that with the switch closed the voltage can be measured on the positive side of the load (indicated in red), but not on the negative side. This is because all the voltage is dissipated across the load.
Electrics
1-1
Chapter 1
Basic DC Terminology
Wires made from copper or steel normally provide the path for the current to flow, and in most cases, the airframe structure is used to complete the circuit back to the supply. A distinction must be made here between electron flow and conventional current flow. In the earliest days of electrical experimentation, electricity was considered to share the same properties as fluid in motion. Fluid flows from high pressure to low, and as voltmeters measure the positive side of the supply as high, the assumption was made that electricity flows from positive to negative, and came to be accepted as conventional flow. With further scientific study came the realisation that electrons, which carry a negative charge, are attracted to the positive end of a supply, and therefore flow from negative to positive. By this time, however, conventional flow theory had become the rule, as it is today. In all diagrams, unless specified otherwise, conventional flow is assumed.
CURRENT (I) Electric current is the flow of electrons in a conductor, but there must be a means to measure this flow. The Coulomb is a charge of 6.25 x 1018 electrons, so it is convenient to use this charge as a yardstick. Therefore, 1 Coulomb passing a given point in 1 second equals 1 ampere, often abbreviated to as amp. Amperes =
Coulombs Seconds
Current in a circuit is measured by connecting an ammeter in line, or in series, with the load, as shown below.
1-2
Electrics
Basic DC Terminology
Chapter 1
ELECTROMOTIVE FORCE (EMF) EMF is the force or pressure that sets electrons in motion, and is a natural result of Coulomb's Law, which states that like charges repel and unlike charges attract. EMF is measured in terms of voltage.
POTENTIAL DIFFERENCE (PD)
Even though a circuit is open, and no current is flowing, a power source still has the potential for current flow. Therefore, whether a battery is connected in a circuit or not, a potential difference still exists between its terminals. The same is true within a circuit or between circuits. For instance, if one part of a circuit is at higher voltage than another part a potential difference exists, and current would flow if a connection was made between them. As with EMF, potential difference is expressed as a voltage.
VOLTAGE (V) The volt is the basic unit of electrical pressure. In order to understand how to measure one volt, it is important to know about resistance. Using fluid flow as an analogy, if water flowing in a pipe meets any resistance, the water flow decreases. Electricity behaves in the same way. Therefore, if current flows through an electrical resistance, the flow rate decreases. One volt of electrical pressure forces 1 ampere through 1 unit of resistance. Voltage is measured using a voltmeter, which must be connected in parallel with (or across) the load or supply.
Electrics
1-3
Chapter 1
Basic DC Terminology
RESISTANCE (R) The unit of resistance is the Ohm. One Ohm exists when it restricts the current flow to 1 amp when a pressure of 1 volt is applied. Resistance opposes current flow and in doing so dissipates the voltage across it, which is why it is said that voltage is dropped across a load. A low resistance would allow a relatively large current to pass through it which in turn creates heat, and heat is energy. Energy over time gives power so it can be said that if heat is being produced then power is being developed. On the other hand, if the value of resistance is very high then little or no current will flow. Metals such as silver and copper have virtually no resistance and are used to conduct electricity. As they have virtually no resistance they must be in series with a resistive load and should be thick enough to withstand the expected current flow, otherwise heating of the conductor will occur. Rubber has a very high resistance and is a non-conductor used for insulation between conductive materials. Cables and wires are comprised of both materials: the metal conductor permits the flow of current along a given path, and the insulation covering it stops the voltage from forcing current out into other paths causing short circuits. A material that is half way between being a conductor and an insulator is known as a semiconductor. On their own, semi-conductors are not particularly useful, but when doped with other elements and fused together, they form the basis of the electronic age. The resistance of a material at a constant temperature is affected by its: ¾ ¾ ¾
Specific Resistance (ρ), the resistance offered by a cube of material at 0°C Length (L) Cross Sectional area (A) R=
ρ xL A
Resistors can have either fixed or variable values. An example of a variable resistor is a rheostat, which is used to control the intensity of a lighting circuit. A material’s temperature can affect its resistance. The resistance of most materials increases with increasing temperature, and these materials have a Positive Temperature Coefficient (PTC). A few materials however, exhibit a decreasing resistance with increasing temperature, and these have a Negative Temperature Coefficient (NTC). In general, most resistive components have a PTC characteristic, and semi-conductors and insulators have a NTC characteristic. NTC is used to advantage with semi-conductors called thermistors, which have a greater change in resistance with temperature than normal resistors, and are used to sense temperature changes, such as in fuel low-level warning systems.
1-4
Electrics
Basic DC Terminology
Chapter 1
CONNECTING RESISTANCES IN SERIES OR PARALLEL IN A DC CIRCUIT Resistances can either be connected in series, in parallel, or in series-parallel combinations. When resistors are connected in series as shown below, the same current flows through each of them, and the total opposition to current flow is equal to the sum of the individual resistances. The supply voltage dissipates across all of the resistors in the series network, and therefore each individual resistor will have a different amount of voltage dropped across it. The sum of all the voltages dropped across each resistor will equal the supply voltage.
Total Resistance (RT)
= R1 + R2 + R3 = 15 + 22 + 31 = 68 Ω
If the resistances connect in parallel with each other, the current flows along two or more paths, as shown below.
As the number of resistances in parallel is increased, the total resistance will decrease, which will draw more current from the supply. The total resistance of a parallel network will always be lower than the smallest resistor in the network. The supply voltage is the same across each resistor in the network. To calculate total resistance of a parallel network, use the following formula:
1 = 1 + 1 + 1 R R3 RT R2 1
1 = 1 + 1 + 1 = 6 4 6 12 12 T
R
12
RT = 6 = 2 ohms
Electrics
1-5
Chapter 1
Basic DC Terminology
In many circuits, a parallel circuit is connected in series with one or more resistors.
To find the total resistance, first calculate the equivalent resistance in the parallel part of the circuit, and then add this value to the series resistance. In the circuit shown above, the total resistance is calculated as follows: PARALLEL PART OF CIRCUIT R
= R1 + R1 = 1 + 1 = 5 9 6 18 TP 2 1
1
Total Parallel Resistance (RTP) = 18 = 3.6 Ω
5
TOTAL CIRCUIT RESISTANCE
RT = RTP + R3 = 3.6 + 2.4 = 6 ohms An alternative and easier method to calculate two resistors in parallel is: RT = R1 X R2 ÷ R1 + R2. For instance, in the parallel circuit above, 9 x 6 ÷ 9 + 6 = 54 ÷ 15 = 3.6. Remember, this only works for two resistors in parallel.
1-6
Electrics
Basic DC Terminology
Chapter 1
OHM’S LAW Ohm’s law states that the current flowing in a circuit is directly proportional to the applied voltage, and inversely proportional to the resistance through which the current flows. Simply stated this means that as voltage is increased current will increase, and as resistance is increased current will decrease. Ohm’s law may be stated by the following formulae: V = IR, R = V/I, or I = V/R It is handy to remember the Ohm’s law triangle. To find an unknown value from the triangle, cover it with your finger and the required formula remains:
Here, V = IR
LOADS The term load refers to any electrical component which consumes power. Loads are connected across the supply voltage, and as more loads are switched on across the supply, the total current increases. Remember that with resistors in parallel, the total resistance is always less than the lowest resistor in the network.
KIRCHHOFF’S LAWS The first law states that the sum of the currents entering a junction must equal the sum of the currents leaving the junction.
The second law states that in a closed circuit, the sum of the voltage drop always equals the supply voltage.
Electrics
1-7
Chapter 1
Basic DC Terminology
In the circuit shown above, a 10-volt battery is connected across a lamp, and as current flows through the circuit, a voltage drop develops across the lamp. The lamp therefore consumes all the energy provided by the battery, and the voltage drop across the lamp equals the supply voltage.
If two identical lamps are connected in series, each consumes half the power in the circuit and there is an equal voltage drop across each. The sum of the voltage dropped across each lamp equals the supply voltage.
ELECTRIC POWER (P) The terms work and power are often confused with each other. To clarify the difference, imagine that two people dig holes of equal dimensions. Both have done an equal amount of work. However, if person A dug the hole in 1 hour, and person B dug his in 2 hours, person A used double the power in order to complete the job sooner. Therefore, power can be seen as the work done divided by the time. As 1 ampere represents work done in 1 second with 1 volt applied, increasing the voltage increases the amount of work done, which equals power. Power is measured in Watts and is calculated using the following formula:
P = VI Using Ohm’s law, the above formula is also expressed as: P = I2 R, if IR is substituted for V, and P = V2 ÷ R, if V/R is substituted for I
1-8
Electrics
Basic DC Terminology
Chapter 1
ELECTRICAL WORK If a potential difference of IV is applied to the ends of a conductor and one coulomb of electricity passes along it, one Joule of work has been done. Electrical work done creates heat and can also result in electromagnetic radiation, as well as motion.
ELECTRIC UNIT PREFIXES For ease of usage and display, electrical units are normally divided into multiples and submultiples. Some of the most commonly used prefixes are as follows: Kilo Mega Giga Tera
Multiples 1 x 103 1 x 106 1 x 109 1 x 1012
Milli Micro Nano Pico
Sub-multiples 1 x 10-3 1 x 10-6 1 x 10-9 1 x 10-12
TYPICAL CIRCUIT SYMBOLS The following symbols normally represent typical circuit components:
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Chapter 1
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Basic DC Terminology
Electrics
INTRODUCTION Electrical circuits form an integral part of an aircraft and must be adequately protected. The flight crew must also be able to select and operate any electrical system safely.
ELECTRIC SYSTEMS Current can return to the source by two methods: the single pole system, better known as earth return, and the dipole system. Single Pole or Earth Return System is used on aircraft constructed from metal, where the airframe acts as a return path between the load and the power source.
This gives an overall reduction in the amount of wiring required and reduces aircraft weight. Dipole or Two-wire System is used on aircraft constructed from non-conductive or non-metallic materials.
In this system, one wire connects the electrical supply to the load, whilst a second wire provides the return path from the load to the power source. This increases the aircraft’s overall mass.
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Chapter 2
Electrical Components
Ground (Earth) is simply a zero or reference point within an electrical circuit and is the metal frame or chassis to which all the various electrical circuits are connected. On an aircraft, the metal airframe is called ground or earth and is at zero volts.
All voltages are measured with respect to the metal structure. In electrics, ground is important because it allows us to have both negative and positive voltages with respect to the metal structure. If a 12-volt battery has a PD between its terminals of 12 volts, then it is not referred to as +12 or -12 volts but simply as 12 volts. The ground reference allows us to express voltages as positive and negative with respect to ground. Remember, ground is a reference point that is considered to be zero or neutral. For example, if the positive terminal of a 12 volt battery is ground, the negative terminal is 12 volts more negative. It follows that the voltage at this terminal with respect to ground is -12 volts. Conversely, if the negative terminal of the battery is connected to ground, the other terminal of the battery will be +12 volts.
ELECTRIC CIRCUIT FAULTS The following faults can occur in an electrical circuit: Short Circuit If the insulation around a wire breaks down or is damaged, it exposes an area of bare conductor. If the wire is at a voltage higher than earth, and the damaged area contacts the airframe in an earth single pole system or the return line in a dipole system, a path of very low resistance will exist. In such circumstances, a very high current flows from the supply through the short circuit bypassing the load and back to the supply.
It is not always guaranteed that a fuse will rupture, or circuit breaker trip straight away. It may happen that the short to earth will initially have a value of resistance which causes a higher than normal current to flow, but just within the rating of the protection device. This could lead to burned wiring and possible fire.
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Electrical Components
Chapter 2
Open Circuit
In any situation where a wire becomes disconnected or breaks, an open circuit fault exists. Its effect is exactly like turning off a switch. Current does not have a complete circuit to flow around, so the system does not work. As there is no current flow, the circuit protection devices do not operate. When a positive wire disconnects, an open circuit exists, but if that wire subsequently touches the airframe, a short circuit occurs from the initial open circuit fault. Static interference During flight, significant levels of static voltage build up on the airframe. If two adjacent areas are electrically isolated from each other, the potential difference between the two could build up to the point that the voltages equalise by discharging across the gap between them, creating a spark in the process. The spark is heard as interference on the radio equipment. The cure for this is bonding. All parts of the airframe and equipment are kept at the same electrical potential using metal braided straps. It is important not to confuse the need for bonding with the static discharge wicks found at the extremities of the airframe. These are fitted to help reduce the build up of static voltage on the airframe by continually discharging it to atmosphere as far away as possible from sensitive equipment. Induced interference All forms of electricity generate magnetic fields. In AC circuits especially, pulsating magnetic fields are created and this can give rise to voltages being induced into adjacent wires, a condition known as cross talk. Obviously, if the circuits affected are sensitive signal circuits such as radio navigation systems, it is important to protect them against such interference. The most common methods of protection are to use twisted pairs or bundles of wires, and enclose them in a metal braided sleeve or screen, which is connected to earth at one end. This accepts the induced voltages and feeds them away to earth.
BUSBARS Busbars form the distribution points from which various systems derive their power and are formed from a solid copper bar. On a simple light aircraft DC system, there may be only one DC busbar, fed from the battery, or the generator if online. On aircraft that are more complex there are many busbars distributed around the aircraft. Busbars are categorised as either AC or DC. Within these categories, there is further sub-division depending on the relative importance of the systems being supplied. For instance, the battery is likely to be the last remaining power source in an emergency, so there will be a vital or emergency busbar, which is hard-wired to the battery. This supplies the most vital services in an emergency, such as the fire extinguishers, shut off valves, etc. Other busbars will be categorised as essential or non-essential. The difference is that the non-essential services, such as galley ovens, can be switched off as a group by disconnecting the non-essential busbar when it is required to reduce the overall load on the generator(s).
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Electrical Components
PROTECTION DEVICES Aircraft electrical circuits use the following protection devices: Fuse This protection device opens or breaks the electrical circuit when excessive current flows. Too much current may ultimately damage either the circuit itself or the system to which it is connected. A fuse is designed to form a weak link in an electrical circuit to protect the majority of the cable between the supply and the load against overheating and burnout. In its simplest form, it consists of a strip or filament of low melting point metal, which is encased in a glass or ceramic envelope.
Fuses are rated in amperes, according to the maximum current they can carry without overheating and rupturing. They are located as near to the supply (busbar) as possible, so that if an excessive current flows due to a short circuit, the fuse can protect all of the cable to the load. Aircraft are required by law to carry spare fuses; minimum stocks of each type being 3 fuses or 10%, whichever is the greater. If a fuse ruptures in flight: ¾ ¾ ¾
Switch off the circuit. Replace the fuse with one of the same value. Switch on the circuit.
A ruptured fuse may be replaced only once. If the fuse ruptures a second time, the flight must continue without the affected system. Current Limiters These devices protect high power circuits where transient high-current conditions may exist, such as certain electric motors that draw a heavy current on initial switch on. They consist of a filament of tinned copper that has a relatively slow temperature rise, allowing an initial over current condition to exist but will rupture if the high current condition persists. The filament is contained in a ceramic housing.
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Electrical Components
Chapter 2
Circuit breaker This device has the same function as a fuse but can be used to restore a circuit when it is reset. Like fuses, circuit breakers are also rated in amperes and are fitted as close to the supply as possible. A circuit breaker is basically a switch that can be opened (tripped) via a bi-metallic strip, as shown below. If an overload current exists, the bi-metallic strip will heat up and distort, causing the latch mechanism to release and open the main contacts of the circuit breaker. A push-pull button pops out, revealing a white band that indicates the circuit breaker has tripped. To reset the circuit breaker, push in the button. Early circuit breaker designs could be manually held in against a fault condition. Although tempting for the pilot on the last leg home, this practice carried inherent risks of system damage or fire. Modern designs are known as trip free and cannot be held in against a fault condition. It is important to learn the difference between trip free and nontrip free.
If a circuit breaker trips: ¾ ¾ ¾ ¾
Electrics
Switch the circuit off. Allow a period of approximately 20-30 seconds to allow the bi-metallic element to cool. Reset the circuit breaker. Switch the circuit on.
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If the circuit breaker trips again, switch off the circuit and do not attempt a further reset. In either case, report the fault on landing. Unlike fuses, circuit breakers can be reset after tripping, so there is no requirement to carry spares. The circuit breaker button functions just like a switch; however, this facility should only be used by ground crew carrying out maintenance in order to isolate a system from the supply.
REVERSE CURRENT CIRCUIT BREAKER (RCCB) Reverse current circuit breakers are used in DC power supplies to protect against short circuits within the generator, and between it and the busbar, which would cause dangerously high currents to flow from the busbar. They operate at high speed and once operated, they mechanically lock and can only be reset manually on the ground with the engine off. Some of the more sophisticated types of RCCB have a separate thermal overload as an additional precaution against a forward current in excess of the power sources safe working capacity.
SWITCHES In most electrical systems, switches are the means of control. Selecting a system on may be made using a simple on/off switch, however some systems or sub-systems should not be selected on together, in which case more complex switches may ensure that one system is isolated before another system can be enabled. A simple switch consists of two contacting surfaces, which can be isolated from each other or brought together by a moveable-connecting link, called a pole. A switch may have an effect on more than one circuit and the number of contacts that can be switched by moving the pole is called the throw. Some examples of these are shown below.
If a switch has only one set of contacts, it is a single pole switch. A switch that operates two or three sets of contacts in one switching action is a double or triple pole switch. Switches operating emergency systems or for non-normal operations are often guarded. A guard must be moved to gain access to operate the switch thereby minimising the risk of inadvertent operation.
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Electrical Components
Chapter 2
The following types of switches may be found on aircraft: Toggle switches (tumbler switches) are general-purpose switches and are used extensively. They range from simple on/off selectors to ganged double or triple throw switches often incorporating a spring-loaded position for intermittent selections, for instance selecting test.
Push switches are used for momentary actions when a circuit is to be completed or interrupted for a finite time. An example of this type of circuit is the start circuit of many turbine aircraft. The start push is held in electromagnetically when operated and released when the engine has reached self-sustaining speed. Many push switches incorporate illuminated lens caps to indicate that the specific circuit has been selected. As with toggle switches, the contacts can be arranged to make or break when operated.
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Rocker switches are single throw or double throw and may have one or both throws spring loaded to the centre-off position. The spring return allows one shot operations such as reset circuits.
Rotary switches are manually operated and are often used as selector switches, such as when selecting a single voltmeter to measure voltage across different busbars or generators.
Micro switches are extensively used throughout aircraft systems in both remote control circuits and remote position sensing and indication. The switch has a snap action and is operated by a spring leaf or roller and cam impinging on the switch-actuating plunger. The actual movement of the spring is very small, typically in the range of a few millimetres.
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Electrical Components
Chapter 2
Some typical circuits using micro switches include: ¾ ¾ ¾ ¾
Landing gear systems Door warning systems Power lever sequencing of system operation (arming of power augmentation systems) Weight on wheels sensing, which isolates circuits that should not operate on the ground
Rheostats are used to alter the amount of current in a circuit by varying the total resistance (e.g. to vary the intensity of panel or flight deck lighting). They normally also have an OFF position to completely remove the current. Time switches are used to perform timing functions. They can be operated by a clockwork mechanism, electric motors or electronically. Obviously, clockwork mechanisms are old technology and are only found in older generations of aircraft. Examples of aircraft-specific timed operations are power switching between heater mats and propeller de-icing, where an electric motor is often the drive for the timing switch and turbine-engine start systems, where electronic timing and switching is employed in modern aircraft. The timing cycles of both electric motordriven time switches and electronic timers can be varied to suit different operating conditions. Mercury switches rely on the fluid properties and electrical conductivity of mercury. Contained in a slightly curved tube of insulating material such as glass or ceramic, mercury can electrically connect between two or three electrodes fixed into the container, forming a switch which is dependent on the tilt of the switch. They are found in instruments such as the Artificial Horizon, where gravity is used as the controlling force, and in any other circuit where gravity is a controlling force.
Pressure switches are used in control and protection circuits and indication circuits where pressure is an important parameter. There are many different types of pressure switches dependent on their application and on the systems in which they are fitted. For instance, a pressure switch installed in a hydraulic circuit is subjected to very high pressure, so the switch itself has to be very robust. They often take the form of solid metal cylinders containing the switch mechanism. An altogether different pressure switch is employed in cabin pressurisation circuits where the weight of a solid metal container can be saved by using a much lighter construction. Thermal switches are sensitive to temperature. Such switches are employed where temperature must be measured or sensed. Most switches in common use are either electronic or are based upon the bending properties of a bi-metallic strip which in turn operates a micro switch (see bimetallic switches below).
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Electrical Components
Proximity switches are similar to micro switches in application. They are either magnetically or electronically operated when a steel or ferrous metal is brought into close proximity to the sensing element. Their reliability is greater than micro switches because they contain no moving parts. Bi-metallic switches are also thermal switches, but specifically use the principle of a bi-metallic strip. Two different metals with different rates of expansion with temperature are fastened together so that the strip will bend when subjected to varying temperatures. By careful design, the strip can be made to operate a snap spring to open or close a micro switch at a specific temperature. They are most often found in cooling circuits for either control or indication.
ELECTRIC GENERATOR An electrical generator is a mechanical device that changes mechanical energy into electrical energy by using permanent magnets or electromagnets with rotating conductors. Engine driven generators produce a voltage that causes current to flow when electrical circuits are switched on. Depending on design, generators may produce DC or AC.
ALTERNATOR As with a generator, an alternator produces electricity. Unlike the generator, alternators are DC machines only. However, the method of producing DC differs from the DC generator as discussed later.
ELECTRIC MOTOR These are electro mechanical devices that convert electrical energy into mechanical energy and are employed extensively throughout aircraft systems.
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INTRODUCTION All aircraft electrical systems include a battery used to: ¾
Supply power to essential services in the event of generator failure
¾
Stabilise the power supplies during switching of transitory loads
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Supply power for engine starting
Batteries are made up of a number of units called cells. Each cell consists of a series of negative and positive plates, immersed in a liquid known as electrolyte.
All cells and batteries store energy in a chemical form, which can be released as electrical energy. The following basic types of cells exist:
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A Primary Cell is not rechargeable and has a limited use in aircraft, where it is mainly used for emergency lighting.
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Secondary Cell batteries are rechargeable, and are the type mainly used in aircraft. They are either of the lead-acid or Nickel-Cadmium (Ni-Cd)/alkaline variety.
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Aircraft Batteries
LEAD ACID BATTERY Each cell of a lead acid battery consists of positive plates of lead peroxide and negative plates of spongy lead, as shown below.
The plates are interleaved, and insulated from each other by plastic separators. An odd number of negative plates is used with one positioned either side of the positive plates to prevent buckling by evening out the thermal distribution. The complete structure is supported in an acid-resistant casing that contains an electrolyte of distilled water and concentrated sulphuric acid to a level just above the plates. Each cell is 2.2 V fully charged and 1.8 V fully discharged. Aircraft batteries of this type consist of either six cells (12 V) or twelve cells (24 V). When a battery is connected to an external circuit, electrons in each cell are transferred through the electrolyte from the spongy lead to the lead peroxide. The net result of the chemical reaction is that a voltage is created across the cells of the battery. Consequently, lead sulphate forms on both plates of each cell. At the same time, the formulation of water dilutes the electrolyte, which takes place during the chemical reaction. For practical purposes, each cell is fully discharged when the specific gravity (SG) or relative density of the electrolyte falls from 1.27 SG (fully charged) to 1.1 SG (fully discharged), which equates to 2.2 and 1.8 V respectively. Any change in the temperature of the electrolyte also varies its specific gravity, so a correction must be made if the temperature is non-standard. The specific gravity of the electrolyte also determines its freezing point, therefore, a discharged battery is more prone to freezing. Batteries constructed from this type of cell must not be left in a discharged condition for extended periods, since the lead sulphate hardens on the plates and cuts down their active area. This process is known as sulphation, and can drastically shorten the life expectancy of a battery.
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Aircraft Batteries
Chapter 3
Lead acid batteries may be recharged by connecting the positive and negative terminals respectively, to the positive and negative terminals of a DC source of a slightly higher voltage than the battery. All of the fore-going reactions are reversed; the lead sulphate is removed from both plates, the positive plate is restored to lead peroxide, the negative plate is restored to spongy lead, and the electrolyte is restored to its original Specific Gravity (SG).
ALKALINE BATTERY (NICKEL-CADMIUM) Each cell of a nickel-cadmium battery in a fully charged condition consists of positive plates of Nickel Oxide and negative plates of pure Cadmium, as shown below.
The plates are interleaved and fully immersed in an electrolyte of dilute potassium hydroxide. The plates and electrolyte are housed in a stainless steel or plastic container. Each cell is 1.2 V (fully charged) and 1.1 V (fully discharged). Batteries of this type for use on an aircraft consist of either twenty cells (24 V) or twenty-two cells (26 V). During discharge, the negative plates turn into cadmium hydroxide, and the positive plates turn into nickel hydroxide. The electrolyte in an alkaline cell has a specific gravity of 1.26, which remains constant whether it is in a charged or discharged condition. Like lead-acid batteries, alkaline batteries are recharged by connecting the positive and negative terminals respectively to the positive and negative terminals of a DC source of slightly higher voltage than the battery. The chemical reaction is reversed, and the plates return to their former states; the negative plates to cadmium, and the positive plates to nickel oxide.
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Aircraft Batteries
BATTERY VENTING As batteries are charged, their temperature increases and volatile hydrogen gas is given off, which is safely vented to atmosphere by way of various systems. In each case, however, a certain amount of water is lost by evaporation, and it is, therefore, necessary to top the battery up to a specific level from time to time with distilled water. ¾
Lead-Acid Battery Venting Lead-acid batteries are vented using one of the following methods: •
The Non-Spill Vent is most commonly used on small aircraft and allows the hydrogen gas to escape, whilst retaining the electrolyte.
• •
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The Cross-Flow Cell System is used on larger aircraft where cabin pressurisation air flows over the tops of the cells and vents the battery to atmosphere.
Alkaline Battery Venting Alkaline batteries give off a mixture of hydrogen and oxygen gases toward the end of charging. As with lead-acid batteries, there are different types of alkaline batteries: •
Semi-Open Battery cells are allowed to gas freely in order to avoid overheating, which can result from overcharging. The gases given off during the chemical reaction are vented safely to atmosphere using a cross-flow venting system. These batteries must also be topped up at regular servicing intervals with distilled water.
•
Sealed Battery cells are completely sealed and require no maintenance.
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Aircraft Batteries
Chapter 3
ELECTROLYTE SPILLAGE Any electrolyte spilled from a battery, normally due to heavy landings and severe turbulence, must be neutralised before it damages the aircraft structure. The neutralising agents for this purpose are as follows:¾
Lead-Acid Battery -
Use a solution of Bicarbonate of Soda.
¾
Alkaline Battery
Use a solution of Boric Acid.
-
It is important that once the area is neutralised, copious quantities of fresh water are used to cleanse the area and prevent corrosion from setting in.
BATTERY CAPACITY The capacity of a battery is measured in Ampere-Hours (AH), and is a measure of the total amount of energy that it contains. It is based on the maximum rated current in amperes delivered by a battery for a known period until it has discharged to a permissible minimum voltage level, which varies according to the size and number of plates in each cell. The following definitions apply: ¾
Rated Capacity is the manufacturer’s stated capacity that is usually stamped on the side of the battery (e.g. 40 AH). This signifies that the battery is designed to last 10 hours when discharged at a 4-Ampere rate, or 1 hour when discharged at a 40Ampere rate.
¾
Actual Capacity is the capacity of the battery as determined by a Capacity Test.
Batteries used in aircraft are normally removed and their capacity checked at specified intervals in a specific battery-charging bay, where the following process takes place: ¾
Fully discharge the battery.
¾
Fully charge the battery.
¾
Discharge the battery at known current level to a minimum permissible voltage level, and note the time taken.
¾
Multiply the current by the time taken to obtain the Actual Battery Capacity.
¾
Compare this value against the battery’s Rated Capacity. Actual Capacity x 100 = 38 x 100 = 95% 40 Rated Capacity
Note: For continued use in aircraft, this value must be 80%, or more.
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Aircraft Batteries
BATTERY CHARGING The following methods are used to charge the batteries whilst installed in the aircraft: ¾
Constant Voltage is used mainly on aircraft fitted with lead-acid batteries. The battery-charging rate is proportional to the difference between the battery and the generator voltage, which in aircraft using 24 V batteries is 4 V (i.e. the generator voltage is normally regulated at 28 V).
¾
Pulse Charging is used mainly on alkaline batteries. Aircraft using this method are fitted with a battery charger supplied by Alternating Current (AC). This source is rectified to provide a constant Direct Current (DC) of approximately 50 amps that continues flowing until the battery is nearly fully charged. The charger goes into a pulse DC current mode to keep the battery topped up. A temperature sensor within the battery is normally designed to reduce or even stop the charging if the battery starts to overheat.
THERMAL RUNAWAY Batteries are capable of performing to their rated capacity when the temperature conditions and charging rates are maintained within the values specified by the manufacturer. If these values are exceeded, Thermal Runaway can occur, which causes violent gassing and boiling of the electrolyte. If this condition continues, the temperature of the battery rises to such a level that it may melt or even explode and cause damage to the aircraft structure. When a battery exceeds a certain temperature, its internal resistance reduces, allowing a higher charging current to flow and the battery temperature to rise. This effect is self-perpetuating, and in some aircraft, particularly those employing alkaline batteries, temperature-sensing devices are located within the batteries to provide a battery-overheat warning on the flight deck. This indicates that the battery should be electrically isolated.
BATTERY STATE OF CHARGE The state of charge of lead-acid batteries can be found by: ¾
Measuring the terminal voltage
¾
Measuring the specific gravity of the electrolyte
Alkaline batteries have a fairly constant voltage output until they are discharged, and the specific gravity of the electrolyte does not alter significantly from fully charged to fully discharged. This makes it difficult to ascertain the state of a battery using the above means, so an alkaline battery must be assumed serviceable unless the voltage is obviously low.
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Aircraft Batteries
Chapter 3
BATTERY CONDITION CHECK An aircraft battery is a vital piece of equipment. Check the following for serviceability prior to flight: ¾
Examine the battery OFF load and note its voltage reading.
¾
Select a specified load and note the new voltage reading.
¾
Compare both ON and OFF load readings, and ensure that the difference between the readings is within a set tolerance.
EMERGENCY USE In an emergency, the aircraft batteries must be capable of maintaining a supply for a minimum period of time, according to JAR: ¾
Main batteries must last at least 30 minutes after total failure of the electrical generating system. (Refer JAR 25.1303).
¾
Emergency Lighting Batteries must last for at least 10 minutes.
CONNECTION OF BATTERIES Batteries that are connected together must be of the same type (i.e. acid and alkaline batteries must never be mixed). Batteries may be connected together as follows:
SERIES CONNECTION If three identical batteries are connected in series, their voltages are added together, but their capacity remains the same as that of an individual battery, as shown below.
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PARALLEL CONNECTION If identical batteries are connected in parallel, their capacities are added together, but the voltage remains the same as that of an individual battery.
SPARE BATTERIES Spare batteries are sometimes carried for operations away from ground servicing facilities, and no attempt should be made to change the batteries in flight.
BATTERY COMPARTMENT INSPECTION Prior to flight, check the battery compartment as follows:
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¾
Check the batteries for security.
¾
Check the electrical connections.
¾
Check for any electrolyte spillage.
¾
Check the vent pipe for security and routing.
Electrics
INTRODUCTION Any study of electricity cannot be conducted without considering the close relationship between it and magnetism. When a current flows in a wire, a magnetic field develops around it. If a magnetic field has movement near a wire, an electric voltage develops in the wire. A magnet has a magnetic field around it that attracts metal objects toward it and can be visualised by sprinkling iron filings on a piece of paper over a magnet. This also shows that the invisible lines of magnetic force flow in circuits or loops, and that they do not cross each other. Magnetic flux is the flow of magnetism around a circuit.
The above illustration also shows that magnetism is concentrated at the extremities of a magnet, called the poles. If it is freely suspended, a magnet always aligns itself in a North-South orientation.
The North-seeking or red pole always points North, and the South-seeking or blue pole always points South. The earliest known form of magnetism is Lodestone, which is a natural mineral found in Asia. It was found that if a piece of this ore was suspended horizontally by a thread, or floated on a piece of wood in water, it would align itself in a North-South direction. Electrics 4-1
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This characteristic led to its use as a compass. Lodestone means leading stone. The north-south alignment occurs because the Earth itself is a huge magnet with its own magnetic field. The fields interact with each other and the Lodestone aligns itself according to the fundamental laws of magnetism. Other than the Earth itself, Lodestone is the only natural magnet. All other magnets are produced artificially. For example, an iron bar becomes magnetised if it is repeatedly rubbed against a piece of Lodestone, and a magnetic field is created if an electric current is passed through a coil of wire. Magnets are additionally classified by their shape and can exist as horseshoe, bar or even ring magnets. Conversely a magnet can be demagnetised by: ¾ ¾ ¾
Heating it to a temperature known as its Curie Point Hitting it with a hammer Placing it in an alternating field created by feeding an alternating current through a coil, known as Degaussing
MAGNETISM FUNDAMENTAL LAWS OF MAGNETISM The fundamental laws of magnetism are as follows: ¾ ¾ ¾
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The line through the poles is called the magnetic axis. Red or blue poles cannot exist separately. Like poles repel each other, and unlike poles attract, as shown below.
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Magnetism
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CHARACTERISTICS OF LINES OF MAGNETIC FLUX Lines of magnetic flux have the following characteristics:
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They have direction or polarity, and the lines of magnetic flux travel externally from the North Pole to the South Pole, as indicated in the following diagram.
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They always form complete loops, where each line of magnetic flux travels back through the body of the magnet to form a complete loop.
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They never cross each other; which is why like poles repel, since lines of magnetic flux having the same polarity can neither connect nor cross. When one field intrudes into another, as shown below, the lines repel, and the magnets tend to move apart.
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They tend to form the smallest possible loops, which is why unlike poles attract. Lines of magnetic flux having the same polarity link up, as shown below, and the resulting loops attempt to shorten by pulling the two magnets together.
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They can be distorted by interacting with other flux lines, as shown below. This is because the lines of magnetic flux pass through soft iron more readily than air, and at the same time, the lines tend to contract to make the smallest possible loops. The iron bar is attracted toward the magnet, and strengthens its overall magnetic field.
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CLASSIFICATION OF MAGNETIC MATERIALS Theoretically, all materials are affected to some extent by a magnetic field, and can be placed in one of the following categories: ¾
Ferromagnetism is the property of a material that enables it to become a permanent magnet (i.e. when placed in a magnetic field, ferromagnetic materials will develop a very strong internal field and retain some of it when the external field is removed). The most common ferromagnetic substances are iron, cobalt, nickel, and alloys of these metals. Above the Curie temperature, thermal agitation destroys the domain structure and the substance becomes paramagnetic. In practice, it is convenient to sub-divide ferromagnetic materials into two classes: •
Hard Iron is a material that is difficult to magnetise. However, once magnetised, it will retain its magnetism unless subjected to a strong demagnetising force. This is a Permanent magnet.
•
Soft Iron is a material, which is easily magnetised, but also easily loses it magnetism when not subjected to a strong magnetising force. This is a Temporary magnet.
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Paramagnetic is the property of a material that has an internal field stronger than that outside and slightly attracts lines of magnetic force when placed in a magnetic field. However, once the magnetic field is removed, random thermal motion destroys the magnetism. Typical materials are platinum, manganese, and aluminium.
¾
Diamagnetic is the property of a material that has an internal field proportional to, but less than that outside and slightly repels lines of magnetic force when placed in a magnetic field. Typical materials are copper and bismuth.
MAGNETIC FLUX Magnetic flux can be considered the equivalent of electric current and is the flow of magnetism. It moves under the influence of Magneto-motive force which can be considered the magnetic equivalent of voltage. The ease with which it flows through a medium is dependent on the material’s reluctance, the equivalent of electrical resistance. Magnetic flux is measured in Webers (Wb).
FLUX DENSITY Flux density is the number of Webers per square metre (Wb/m2) and is known as the Tesla (β).
RELUCTANCE Reluctance is the opposition to magnetic flux, and is similar to resistance in an electrical circuit. It is the ratio of the Magneto-Motive Force (MMF) acting on a magnetic circuit to the magnetic flux (Φ) produced. Reluctance = MMF
φ
PERMEABILITY Permeability (µ) is the ease by which a material accepts lines of magnetic flux and may be compared to conductance in an electrical circuit, which is the ease with which a material or circuit allows current to flow. It is the ratio of B/H, where B is the induced magnetic flux, and H is the magnetising force. The table on the next page shows how the permeability of a material determines its characteristic.
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Magnetism
Material Bismuth Water Copper Air Aluminium Cobalt Nickel Iron
Permeability 0.999833 0.999991 0.999995 1.000000 1.000021 170 1000 7000
Characteristic
Action
Diamagnetic Diamagnetic Diamagnetic Paramagnetic Paramagnetic Ferromagnetic Ferromagnetic Ferromagnetic
Slightly Repelled Slightly Repelled Slightly Repelled Non-Magnetic Slightly Attracted Strongly Attracted Strongly Attracted Strongly Attracted
HYSTERESIS Any ferrous material becomes magnetised to some degree when subjected to a magnetising force. In the diagram below, it is possible to see the effect of a magnetising force on a nonmagnetised iron bar. The H-H axis is the magnetising force and is assumed to be an electromagnet whose magnetic strength can be increased by increasing the electric current through the coil, and vice-versa. As the magnetising force increases from O to +H, the magnetism induced in the bar increases along the curved line O-C. Notice at C that although current could be increased further, the curve has flattened out, indicating that the bar cannot be further magnetised. This is known as saturation.
If the magnetising force is now reduced to zero, there is a residual magnetism left in the bar at D, which is known as remnant flux. If it is intended to completely remove the remnant flux, the magnetising force would have to be applied in the opposite polarity, known as the coercive force. At E, the magnetism in the bar has been removed, but any further increase in the magnetising force toward -H will magnetise the bar in the opposite polarity to that originally achieved, and as before, saturation will eventually be reached. It is worth pointing out that the shape of this hysterisis loop is dependent on the magnetic properties of the bar. Consider a bar which is easy to magnetise, but loses its magnetism on removal of the magnetising force; a paramagnet. The loop would appear thinner as the line O-C would be more upright and D would appear much lower down on the B-B axis. The word Hysterisis means to lag, and this is what happens to the flux density as it lags behind the changing values of the magnetising force.
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Chapter 4
SATURATION Saturation plays an important role in ferromagnetic circuits, where the magnitude of magnetism induced in a piece of iron is proportional to the current creating it. However, if the current is increased beyond a certain point, no further appreciable increase in magnetism occurs, as the iron becomes fully saturated. This is a very important property, and is the principle on which a magnetic amplifier operates.
MAGNETISM PRODUCED BY CURRENT FLOW When current flows through a conductor, a magnetic field is produced around the conductor, and its magnitude is proportional to the current flow.
Electrics
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Chapter 4
Magnetism
The direction of the field depends on the direction of current flow. The Right Hand Grasp Rule is used to determine the direction of the field when a conventional current is flowing.
The thumb points in the direction of the current flow, whilst the fingers wrapping around point in the direction of the magnetic field. In explaining some aspects of electromagnetism, it is also useful to picture current flow looking at the end of a wire, by visualising a feathered arrow. If a cross sectional view of a wire is shown, a cross would indicate flow into the wire, like looking at the back of a feathered arrow. Current flowing out of the wire would be shown as a dot, like looking at the pointed end of an arrow. This principle is illustrated below.
If two wires are placed side by side, the resulting magnetic fields either attract or repel each other dependent on the direction of the current flow, as shown below.
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Electrics
Magnetism
Chapter 4
REPULSION
ATTRACTION
Although a circulating magnetic field around wires has no polarity, where they flow in the same direction they can be considered to share the same polarity and therefore repel each other. Where the currents flow in opposite directions they can be considered to have different polarities and attract each other. The magnetic field produced in a straight piece of wire is of little practical use; it has direction, but no North or South Pole. Unless the current is extremely high, the magnetic field has little strength. The magnetic characteristics are greatly improved by shaping the wire into a loop. The coil of wire, as shown below, now possesses the following characteristics:
Electrics
¾
The lines of flux are closer together in the centre of the loop.
¾
There is an increased flux in the centre of the loop.
¾
North and South Poles are created at the ends of it, and it assumes the same magnetic characteristics as a permanent magnet where lines of magnetic flux emerge from the North Pole, and return via the South Pole, as illustrated below.
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Magnetism
THE ELECTROMAGNET The principle of an electromagnet is that passing current through a loop of wire, establishes a magnetic field. By increasing the number of loops in the wire a coil is formed. The flux density within the core of the coil greatly increases, creating a stronger magnetic field.
This is a Solenoid, and the greater the current that flows through the coil, the greater the flux density. The strength of the magnetic field around a coil (electromagnet), therefore, grows with either an increase in current or an increase in the number of turns. Another method of increasing the strength of the magnetic flux around a coil is to insert a bar of ferromagnetic material into it. This increases permeability within the coil and allows an increased flux density. The magnetic polarity of a solenoid can be established using the right hand grasp rule.
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Magnetism
Chapter 4
Right Hand
N
S
If the fingers of the right hand are wrapped around the coil in the direction of current flow, the thumb will point in the direction of the North Pole.
Electrics
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Magnetism
THE RELAY A relay uses the principle of the electromagnet (solenoid) to attract together or pull apart electrical contacts, and is often used for remote or automatic switching. In the following diagram, a low current through the relay coil can switch a high current through the contacts with the advantage that a smaller, more compact switch can be placed in the cockpit. There is a difference between the voltage at which a relay pulls in (pull in voltage) and the voltage at which it releases (drop out voltage). Due to the greater physical distance between the armature and contacts, the voltage to pull in is greater than that required to release.
ELECTROMAGNETIC INDUCTION If relative motion exists between a conductor and a magnetic field, an electromotive force (EMF) is induced in the conductor, whose magnitude is determined by the following factors: ¾
The strength of the magnetic field
¾
The speed of the conductor with respect to the field
¾
The angle at which the conductor cuts the field
¾
The length of the conductor in the field
These factors are all a natural consequence of Faraday's law, which states that the voltage (EMF) induced in a conductor is directly proportional to the rate at which the conductor cuts the magnetic lines of flux. This principal is the foundation upon which all generators work.
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Electrics
INTRODUCTION Modern aircraft electrical systems are extremely complex and varied. DC systems have now been mostly superseded by AC systems in large commercial aircraft; however, many smaller general aviation aircraft still use DC as their primary electrical system.
GENERATOR SYSTEMS BASIC GENERATOR THEORY All generators work on the principle of magnetic induction, however, the output voltage can vary in both magnitude and form. Where the output is at a constant level, it is called direct current (DC) and is the subject of this chapter. The output may also be in the form of a constantly varying voltage through maximum and minimum levels, known as alternating current (AC) which is dealt with in a later chapter. The magnitude of the voltage produced is dependent on a number of factors: ¾ ¾ ¾ ¾
The strength of the magnetic field The speed at which the conductor cuts the magnetic field The length of the conductor within the magnetic field The angle at which the conductor cuts the magnetic field
The polarity of the induced voltage can be found using Fleming’s Right-Hand Rule for generators. This involves the thumb and the first two fingers of the right hand placed at 90° to each other. The thumb points in the direction in which the conductor is moving. The first finger points in the direction of the magnetic field (N to S). The second finger indicates the polarity of the induced voltage pointing from positive to negative. The second finger also indicates the direction in which conventional current flows in the conductor when it is connected into a circuit. Electrics
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Chapter 5
DC Generator Systems
There is a similar rule for motors known as Fleming’s Left-Hand Rule. To remember which hand to use, memorise the right-hand rule as the gener-righter hand.
A SIMPLE GENERATOR In its simplest form, a generator consists of a single loop of wire, mounted so that it can be rotated within a magnetic field. When rotated, a voltage is induced, which can be taken off the loop by carbon brushes bearing on copper or brass slip rings. The carbon brushes feed the supply to the output terminals. In the diagram below, a voltmeter measures the voltage at the output and it can be seen that the voltage is not at a constant level.
When the armature moves through one revolution at a constant speed, the output voltage rises to a maximum as it cuts the magnetic flux at 90° and falls to zero as it moves in the same direction as the magnetic flux. There are two maximums and two minimums for one revolution of the loop. This varying voltage can be plotted as a sine wave, and is called an AC voltage. The loop is formed on a shaft called the armature which rotates in the centre of the magnetic field. As the loop rotates, the voltages vary according to the angular rotation of the armature, as illustrated in the following diagram.
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CONVERSION OF AC TO DC To convert the AC waveform into a DC waveform, the output must be switched so that the carbon brushes maintain a constant polarity. A device which accomplishes this is called a commutator. The ends of each loop or series of loops terminate in a copper segment. Each is insulated from the other and earth. If a single loop is considered, it should be evident that as each side of the loop passes through the minimum voltage angle, the induced voltage in the loop reverses polarity. At the same time, the segments in contact with the brushes change over, thereby reversing the polarity of each segment and maintaining a constant polarity at the brushes.
When the loop is at 90° to the magnetic field, no voltage is induced and no current flows. At this point, the brushes are in the process of changing segments and in contact with both segments. This effectively short circuits the loop, ensuring no current flow during the change of segments and reducing arcing at the brushes
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DC Generator Systems
a
c
b
e
d
Current
S
N
Current
S
N
S
N
S
N
S
N
Current
Brush Voltage and Lead Current
Degrees of Rotation
0 A
90°
180°
270°
360°
B
C
D
E
The operation of the commutator can be established by referring to the diagram above. At B, apply Fleming's right-hand rule to the left half of the loop as it moves up through the field. This confirms that the current through this part of the loop is from the commutator to the far end. Now, consider the situation at D where the same part of the loop considered above is now on the right and going down. Applying Fleming's right-hand rule again shows that the current has reversed and is now flowing from the far end of the loop toward the commutator. Notice that although the segment has changed polarity, it has also changed brushes, so the polarity of the brushes has not changed. Although an effective DC voltage has been achieved, there are still points of zero voltage shown at A, C, and E giving rise to a pulsed DC, shown in the graph above. To produce a smoother and more constant output voltage, more loops and consequently more segments are added to the commutator. In the diagram below, the output from five loops is shown, and the DC voltage is smoother and more constant.
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Electrics
DC Generator Systems
Chapter 5
DC GENERATOR SYSTEM ARCHITECTURE All aircraft generator systems must be capable of supplying a constant voltage in spite of varying engine speed and electrical load conditions. This is achieved by varying the field strength (excitation) of the generator. The components of a basic single generator system are shown below:
DC GENERATOR CONSTRUCTION The construction of a typical DC generator is shown in the diagram on the next page and consists of the following components:
Electrics
¾
The Yoke is a cylinder of cast iron, which supports the pole pieces of the electromagnetic field.
¾
The Armature is driven by the aircraft engine, and holds the windings (in which the output voltage of the machine is induced) and the commutator.
¾
The Commutator changes the AC voltage induced in the armature into DC voltage.
¾
The Quill Drive is a weak point, which is designed to shear and protect the engine if the generator seizes.
¾
The Suppressor reduces radio interference, which may result from sparking between the brushes and commutator.
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DC Generator Systems
PRINCIPLE OF OPERATION OF A DC GENERATOR When the armature rotates in the magnetic field, the DC voltage induced in the windings is collected at the brushes and fed to the output terminals. Any loads connected across the output terminals will cause a current to flow. This flow of current through the load also flows through the windings. Therefore, the armature develops a magnetic field around it in proportion to the load. The magnetic fields around the armature and the main field winding exist separately and oppose each other, although the strength of the armature field is never greater than the main field. The effect of these two fields causes a motor force that opposes the drive from the engine. This accounts for why a car engine at idle will perceptibly slow down when a heavy electrical load is switched on.
TYPES OF DC GENERATOR Three basic types of DC generator exist, with each differing in how the armature and field windings are electrically connected: Shunt Wound In this arrangement, the field windings are connected in parallel with the armature windings, as shown below.
It is used in all aircraft DC generators, and at a constant speed has a slightly falling voltage output with increasing load.
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Chapter 5
Series Wound In this arrangement, the field windings are connected in series with the armature windings, as shown below.
This type of generator is not used on aircraft, because at a constant speed it has a rising voltage output characteristic with increasing load making it difficult to regulate. Compound Wound This arrangement of the field windings combines the relative advantages of both the shunt and series wound generators. They are more expensive to manufacture than shunt wound generators so their use tends to be restricted to the more expensive end of the commercial aircraft market. Their precise output characteristics can be matched to the aircraft’s specific load versus engine speed range by altering the ratio of the shunt and series windings.
Electrics
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Chapter 5
DC Generator Systems
VOLTAGE REGULATOR It is essential that the voltage output of a generator be maintained within defined limits for all conditions of load and engine speed. As load and speed are necessarily variable, the output can only realistically be controlled by varying the strength of the magnetic field. The voltage regulator achieves this by varying the current through the field windings. Two methods of regulator control are briefly described below. Carbon Pile Voltage Regulator A diagram of a typical carbon pile voltage regulator is shown below. Carbon is granular in structure, so its resistance depends on how compressed it is. If it is compressed, its resistance decreases. Therefore, the resistance of the carbon pile is inversely proportional to compression. The carbon pile forms a variable resistor in series with the field windings. To allow a more rapid build up of voltage on engine start, it is pre-compressed when the generator is offline.
At engine start, the generator needs to build up the voltage rapidly to the regulated value, typically 28V. As the carbon pile is pre-compressed under spring tension, the resistance in series with the field windings is at a minimum. This allows a high current through the field winding. Control over the pile compression is achieved by the voltage coil, which is connected across the output of the generator and attracts an armature, which itself is attached to the spring. An increased current through the voltage coil attracts the armature and the spring away from the carbon pile, therefore, increasing its resistance. With the engine started, the generator voltage builds up until it reaches 28V, at which point the current through the voltage coil stabilises and holds the compression at the required value to maintain 28V. This equilibrium can be upset by either a change in engine speed or electrical load. Consider the operation of the regulator at the start of the take-off roll. As the engine speed increases, the generator speed also increases, and, therefore, voltage rises. As the voltage rises, Ohms law tells us that there must be an increase in current through the voltage coil. This immediately increases the pull on the armature and, therefore, increases the pile resistance. This in turn reduces field current and the excitation. Although the voltage would reduce to the regulated level, system lag will allow a small voltage over-swing.
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DC Generator Systems
Chapter 5
An alternative to varying engine speed is the electrical load. If the pilot turns on a large electrical load, current demand increases and the voltage tends to fall. The falling voltage reduces the current through the voltage coil, consequently reducing the attraction on the armature and increasing the spring compression on the pile. The current through the field windings increases and restores the generator voltage to 28V. Transistorised Voltage Regulator Technological advances have consigned the carbon pile regulator to history as far as new generation aircraft are concerned. The electronic regulator achieves the same aims as older regulators but with the following advantages: ¾ ¾ ¾ ¾
Less maintenance Less weight More reliable Little or no radio interference
A typical electronic regulator senses the generator output voltage using a network of transistors and diodes. It achieves voltage regulation in the same way as the carbon pile regulator in that it varies the current in the field windings.
CUT-OUT The DC generator in an aircraft electrical supply system must be isolated from the battery voltage whenever its output fails or when the engine shuts down. This is normally achieved by a cutout, which is fitted between the generator and the busbar. Many different types of cutout exist, of which the most common is the differential current cut-out, as shown below. The main components in this device are a series (current) coil (DCO) that is wound physically on top of a differential (voltage) coil, which in turn controls the generator line contactor (GLC). The contacts in the cutout are initially closed via the differential coil when the generator output voltage exceeds battery voltage by approximately 0.5V. This in turn causes the GLC to close and connects the generator to the busbar through the series coil. The resulting magnetic field produced by the series coil adds to that already produced by the differential coil and helps to hold the GLC in its closed position. To operate at very low voltages, the differential coil necessarily has thousands of turns of fine wire. However, this means that should the potential difference equalise or reverse across the coil, self-inductance would delay its operation to isolate the generator. To overcome this delay, if the battery voltage exceeds generator voltage, a reverse current through the series coil of around 20 to 30 amps instantly de-energises the cutout contactor. When generator voltage increases again to 0.5V above battery voltage, the cutout energises to re-connect the generator to the busbar.
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DC Generator Systems
REVERSE CURRENT CIRCUIT BREAKER The differential cut-out does not provide complete protection to the generator system, since any short circuit between the differential cut-out and the busbar does not open or Trip the GLC. A Reverse Current Circuit Breaker (RCCB) is fitted as close as possible to the aircraft busbars, and between them and the GLC in order to provide complete protection. The RCCB is designed to operate at a very high speed if the reverse current reaches a value of approximately 300 amps. It remains mechanically locked out until reset. Some RCCB's are fitted with auxiliary contacts, which open the generator field circuit and stop the generator output from feeding the fault condition.
BUSBARS Busbars are current distribution points and are usually standard rectangular sections of high conductivity copper or aluminium, which are categorised as follows:
5-10
¾
Vital Busbars are powered directly from the aircraft battery and used for emergency services such as undercarriage selection, emergency lighting, fire detection, and extinguishing circuits, etc.
¾
Essential Busbars supply equipment essential to ensure the safe flight of an aircraft.
¾
Non-Essential Busbars can be isolated (LOAD SHED) in emergency flight conditions, since the equipment they feed is of very low priority, such as galley supplies and cabin entertainment services.
Electrics
DC Generator Systems
Chapter 5
POWER FAILURE WARNING All generator systems are fitted with a red warning lamp, which illuminates whenever the Generator Line Contactor is open, and the generator is no longer feeding the busbar. Test all warning lamps prior to flight. In older types of aircraft, this is done by pressing a warning light test switch. On modern aircraft, this is automatically initiated whenever the electrical power is first switched on.
GROUND POWER The battery on a modern aircraft has a limited capacity, and is used only in emergencies and for engine starting. On the ground, the battery is only able to supply a minimum of services. Another source of power is necessary during servicing or if power is needed during extended parking. A typical Ground Power system is shown below.
It is important that the aircraft supplies (battery and generator) be disconnected whilst the ground supplies are connected to the aircraft. This is achieved by a short auxiliary pin in the ground power socket, which operates a hold-off relay in the aircraft electrical system. This is necessary because the ground power unit’s (GPU) regulated voltage may not be identical to that of the aircraft generators. For example, if the ground power is too high, there is a risk of overcharging the aircraft battery and damaging electrical equipment. On the other hand, if the ground power voltage output is too low, the first generator to come on line would feed into the GPU and cause instability.
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DC Generator Systems
DC GENERATOR SYSTEM FAULT PROTECTION A typical DC generator system is protected against the following faults:
5-12
¾
Overheat An overheat thermostat is fitted in most aircraft generators, which will cause an overheat warning light to illuminate on the flight deck if the generators cooling air exhaust exceeds approximately 160°C. If this occurs, the generator should be manually switched off.
¾
Seizure If the generator seizes due to a mechanical fault, the aircraft's engine may be damaged. A Quill Drive is fitted between the engine and the generator, which is designed to shear if the generator seizes and automatically disconnects the generator from the engine.
¾
Over-Voltage This condition is usually caused by a malfunction of the voltage regulator and may cause damage to the loads and battery if allowed to continue. An over-voltage sensor is fitted in the system, which will trip the generator off the busbar and de-excite its field. One reset attempt is normally allowed by Recycling the system (i.e. switching the generator OFF and then ON again).
¾
Under-voltage This is explained in the operation of the series coil in the Differential Cut-out.
¾
High Reverse Currents This is explained in the operation of the Reverse Current Circuit Breaker.
Electrics
DC Generator Systems
Chapter 5
TWIN-ENGINE DC ELECTRICAL SYSTEM On a multi-engine aircraft, a generator is normally fitted to each engine gearbox. A typical twinengine turbo-propeller DC electrical system layout is shown below:
The generators are usually connected in parallel and supply the loads together so that: ¾
There will be no break in the supplies if a generator fails.
¾
The system can handle the switching of high transient loads.
¾
The generators can share the loads equally to improve their life expectancy.
The main disadvantage of paralleling generators is that additional circuitry is required to ensure that both machines equally share the loads. Each generator is therefore fitted with an ammeter so that the flight crew can regularly check that the load sharing is correct.
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DC Generator Systems
OPERATION OF DC GENERATORS IN PARALLEL Refer to the diagram above. Generators G1 and G2 are fitted to the No.1 and No.2 engines respectively. When the No.1 engine starts, the generator rotates and produces an output voltage, causing current to flow to the generator field coil via its own voltage regulator. After a short time, the generator’s output reaches its regulated value, which the flight crew can check using the aircraft's voltmeter, and sufficient current flows in the differential coil to close the differential relay. If the generator control switch is in the ON position, the generator line contactor (GLC) subsequently closes, allowing the generator to charge the battery and feed the loads. The flight crew can confirm that the generator is feeding the busbar by the following methods: ¾
The dedicated generator warning light extinguishes.
¾
The generator ammeter reads the current being taken by the battery and loads.
After starting the No.2 engine, the second generator can be brought onto the busbar in the same way as the No.1 generator.
DC LOAD SHARING Whenever the generators are operating in parallel, they must share the aircraft electrical load equally This is achieved by ensuring that their individual output currents are equal under all operating conditions by incorporating an Equalising Circuit (Load Sharing Loop), as shown below, where the equalising coils are wound on the same core as the voltage coil in the voltage regulator. This circuit monitors the generator outputs and automatically adjusts the voltage regulators to ensure equal load sharing.
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DC Generator Systems
Chapter 5
The flight crew can also check that any load sharing is equal by referring to the individual generator ammeters.
OPERATION OF AN EQUALISING CIRCUIT If both generators are equally sharing the load, points X and Y will be at the same potential, because the voltage dropped across R1 and R2 will be the same. If the No.1 generator, for example, takes more than its share of load, the voltage drop across R1 increases, and the voltage drop across R2 decreases. This causes point X to become more negative with respect to point Y and current flows from Y to X through the equalising circuit. The resulting current through each equalising coil in the regulators acts to reduce the voltage of Gen 1 and increase the voltage of Gen 2. The current flowing in the equalising circuit continues until the generator load is equally shared again and the potential difference between X and Y is again zero. Note: The Equalising Circuit only operates when the generators are operating in parallel.
SINGLE-ENGINE AIRCRAFT DC ELECTRICAL SYSTEM Most modern single piston engine aircraft have a 14-volt DC electrical system, which consists of an Alternator and battery combination, as shown below.
The alternator is the primary electrical source when the engine is running and charges the battery. The battery provides the secondary power supply, which is used for initial engine start, and as an emergency power source. There are many differences between the modern alternator and the DC generator discussed earlier. Instead of providing a static excitation field within which the armature provides the output, the low current supply is fed via slip rings and brushes to the rotor. This has the advantage of taking the high current output from the stator windings, so the losses across the commutator of a DC generator are replaced by very small losses across the slip rings. The Alternator is made up of field windings, which are wrapped around a number of pole pieces on a rotating shaft (rotor), and rotate within fixed output windings (stator). The output is fed directly to the rectifier diodes and then to the output terminals.
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Chapter 5
DC Generator Systems
OPERATION OF THE ALTERNATOR In a typical light aircraft system the alternator is selected online by a rocker switch positioned alongside the battery on switch.
Unlike the DC generator, where residual magnetism in the yoke is sufficient to start the generator without any external supply to the field, the alternator, due to its lighter construction, does not have any residual magnetism from which it can initially self-excite. Therefore, the alternator first needs to be separately excited until its output is high enough to be fed back to its own field windings, at which point it becomes self-exciting. Once the alternator is self-exciting, the supply to the field windings is taken from the busbar through the regulator, which controls field current electronically. The field windings are on the rotor, so it is known as a rotating field machine, and the output voltage is induced into the stator windings. The stator windings consist of three coils equally spaced around the rotor, so the voltage produced internally can be considered as three separate AC supplies which must be converted into DC. It is the job of the diode pack to do the conversion. A diode is a semi-conductor device that allows current to flow through it in one direction only. As the internally generated supply is effectively 3 separate AC supplies, an arrangement of 6 diodes is required to convert the supply to DC. A diode that only allows unidirectional current flow is known as a rectifier. The diode pack is alternatively known as the rectifier pack.
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Chapter 5
The output from the alternator is monitored using one of the following ammeter arrangements: ¾
Zero Left Ammeter or Loadmeter In this installation, the ammeter is connected between the generator and the busbar, as shown below. It measures the total load being supplied by the alternator. If the reading drops to zero in flight, it indicates that either the alternator has failed or that no systems are selected.
¾
Centre Reading Ammeter This installation places the ammeter in circuit between the busbar and the battery, as shown below. Under normal conditions, the alternator would supply all the loads and a charging current to the battery as indicated by a positive current reading. If the alternator fails, the battery supplies the busbar and the current draw is indicated as a negative value.
Following an engine start, it is normal for a large charging current to the battery to be indicated until the battery recovers. If the pointer is left of centre, it shows that the battery is discharging, which is a good indication that the alternator has failed. In this situation, the loads should be reduced as much as possible to conserve battery life.
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5-18
DC Generator Systems
Electrics
INTRODUCTION A wide variety of components and systems depend upon mechanical energy, which is often supplied by electric motors. The range and scale of electric motors found in aircraft applications is vast. The table below shows a sample of typical systems that employ electric motors. Equipment
Function
Actuators
Fuel trimming, cargo door operation, heat-exchanger control-flap operation, and landing-flap operation
Control valves
Hot and cold air mixing for air conditioning and thermal de-icing
Pumps
Fuel delivery, propeller feathering, and de-icing fluid delivery
Flight Instruments and Control Systems
Gyroscope operation and Servo control
Many of the above systems combine electro-mechanical functions into an integral component. When a motor is used for only short periods of time, the motor can be reduced in size and power to save weight. However, this does mean that they are operating at the top end of their rated loads, so a cooling off period is often required before selecting repeated operations. The propeller feathering pump motor is an example of this. The construction of a DC motor is virtually the same as that of a DC generator and many machines may be operated as either, such as a starter-generator. Continuously rated motors may be fan cooled, ram air cooled or fuel cooled as in the case of fuel booster pumps, which are immersed in the fuel. On some occasions, an intermediate gearbox is required to match the output speed of the motor to the application.
MOTORS THE MOTOR PRINCIPLE DC motors work in the opposite sense to DC generators. Instead of mechanically rotating the armature in a magnetic field to produce an electrical output, the armature is connected to an electric supply, creating mechanical energy from electrical energy. If a conductor carrying an electric current is placed in a magnetic field, the field around the conductor interacts with the main magnetic field and causes the conductor to move.
Electrics
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Chapter 6
DC Motors
Although the circulating field around a conductor does not possess a North or a South Pole where it flows in the same direction as the main field, they share the same polarity and repel each other. Conversely, if the field flows in opposite directions, they are of opposite polarity and attract. The direction of the current in the conductor therefore determines the direction in which the conductor moves. DIRECTION OF MOTION
N
S N
S
DIRECTION OF MOTION
The direction of this motion can be found using Fleming’s Left-Hand Rule. As with the RightHand Rule discussed earlier, the thumb and first two fingers are placed at 90° to each other. The thumb points in the direction in which the conductor moves. The first finger is field and points in the direction of magnetic flux, north to south. The second finger is current and points in the direction in which current does or would flow. Remember again that the Right-Hand Rule is for generators and can be remembered as the gener-righter hand.
DC MOTORS There is little difference between DC generators and motors, since they both consist of the same essential parts: an armature, field windings, a commutator, and brush gear. The armature and field windings are usually supplied from a common supply. In its simplest form, a motor consists of a single loop of wire (PQ in the diagram below) which is arranged so it can rotate between the pole pieces of a permanent magnet.
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DC Motors
Chapter 6
a P
Q S
N
c
Q
b
Q S
N
P
P S
N
S
N
Current Reversed by Commutator Q Q
270° P
180° Q
P
S
N
Current Reversed by Commutator
90° P
Q
Q
P
0°
e
P
d
P
360° P
Q
Q
The ends of the wire are connected to a DC power supply through the commutator segments and the brushes. Consider the current flow around the loop from P to Q in the above diagram on the far left. Using Fleming’s Left-Hand Rule, see that the loop will rotate anti-clockwise. As the loop moves to 90° to the main field, the motive force on the loop is lost. However, inertia carries the loop through this position. As the current through the loop is reversed by the commutator from Q to P, the rotation of the loop continues anti-clockwise. Just as the generator output improves by adding loops to the armature, the action and power of a DC motor improve in the same way.
BACK EMF The movement of the conductor through a magnetic field induces a voltage in it, which opposes the supply voltage. This can be verified by using both Fleming’s Left- and Right-Hand Rules. In the far left diagram above use the left-hand rule with the thumb pointing down with the first finger in the direction of field N to S. Note the direction of the seCond finger (Current). Now apply the right-hand rule and notice that any voltage induced will opposes the current through the loop. This is back EMF and serves to reduce the armature current. Consider that the back EMF is proportional to speed and the magnetic field The greater the speed of the motor or the strength of the field, the greater the back EMF. (EB α N x Φ), where Φ = field strength and N = armature speed. EB = Back EMF The back EMF can never exceed the supply voltage. At a steady motor speed and load, equilibrium exists between the magnetic field and the current flowing in the armature.
DIRECTION OF ROTATION A motor will turn in a specific direction dependent on the current flow in the armature relative to the field windings, which can be ascertained using Fleming’s Left-Hand rule. If the direction of current flow in either component changes independently of the other, the motor rotates in the opposite direction. It is important to note that changing current flow simultaneously through both components has no effect on direction of rotation.
Electrics
6-3
Chapter 6
DC Motors
TYPES OF DC MOTORS In the same way that DC generators can have the field windings in series with or across the armature, so can DC motors, which include series, shunt, and compound motors. Series Motors With the field windings and armature connected in series, the total current drawn by the motor flows through both the field windings and the armature. This high current requires that the field windings be made from a thicker wire with less turns. Now consider a series motor starting up with no mechanical load attached. Initially, there is a high current demand that creates a strong armature current but a relatively weak field due to the small number of turns in the field winding. Nevertheless, a high torque is produced, because the weak field means that very little back EMF is produced and, therefore, a strong armature current compared to the magnetic field. Remember that at a steady speed, there is a balance between field strength and armature current and that armature current and field strength reduce as back EMF increases. Back EMF is proportional to armature speed As speed builds up, armature current decreases and the field strength weakens further. From a high torque condition at start up, torque reduces as speed increases. Although the machine may run fast off load, its torque or power is low, so any increase in load would rapidly reduce motor speed. Now look at the series motor starting on load. As before, initial current demand will be high, but now the armature cannot accelerate so quickly. This reduces the back EMF allowing a net increase in armature current and therefore field strength, giving high torque to drive the load. As back EMF is proportional to armature speed and inversely proportional to mechanical load, torque must be inversely proportional to back EMF. These factors together give the typical characteristics of series motors: ¾ ¾ ¾
They will race away if started off load Increasing a mechanical load at speed will rapidly slow the motor down When equilibrium is established for a given load, the series motor exhibits high torque but at reduced speed
For all of these reasons, a series motor must be started on load. This usually means that the series motor is permanently connected to the mechanical load, such as actuators connected to flap drives and undercarriage retraction systems. Starter motors are also series machines. The start system will always couple the starter motor with the engine on start up and as soon as the engine starts, the starter motor shuts down and is de-coupled to avoid motor over speed. The diagram below illustrates the speed-to-load characteristic.
6-4
Electrics
DC Motors
Chapter 6
The speed-load characteristic of a series wound motor is such that variations in mechanical load are accompanied by substantial speed variations. A light load will cause it to run at a dangerously high speed, and a high load will cause it to run at low speed. For a given motor: ¾ ¾
Torque is proportional to field strength and armature current, both of which vary in direct proportion to speed. If the load is increased on a motor, the speed reduction reduces back EMF, and torque will increase to drive the load harder but at a reduced speed.
In all the above conditions of speed and torque, always remember that at a steady speed and load, the effect of field and armature current is in balance. Shunt Motors In this type of motor, the field windings are connected in parallel with the armature. The current flowing in the field windings will be fairly constant as the full supply voltage will always be felt across it, and independent of armature current.
As the current through the field windings is independent of the armature current, they are not subject to the high current demand of the armature. This means that the wire can be thinner, allowing more turns, which increases magnetic field for a given electrical current. Now consider the shunt motor on start-up off load. As soon as the supply to the motor is switched on, a strong magnetic field is produced by the field windings, and this immediately induces a large back EMF in the armature, severely limiting armature current. The result of this is to reduce the available torque on start up. However, as there is no mechanical load attached, the torque required is a minimum, and a steady state speed is achieved. Once at a steady state speed, suppose that a mechanical load is attached. The immediate effect is that the armature slows down and back EMF reduces, thereby increasing armature current. The increased armature current in the constant magnetic field increases motor torque and gives the machine a constant speed against load characteristic, as shown by the graph above. If a mechanical load is attached before start up, the rapid build up of back EMF in the strong magnetic field severely limits torque and, consequently, the ability of the motor to run up on load. For this reason, nearly all heavy-duty shunt-wound motors must be run up to speed before the load is connected. The speed to load diagram above is drawn for the normal conditions with the motor already running when the load is connected.
Electrics
6-5
Chapter 6
DC Motors
For a given motor: ¾ ¾ ¾
The field strength is constant, so torque is directly proportional to armature current. The load directly affects the induced back EMF in a proportional manner, giving an increasing torque to counter an increasing load. A shunt-wound motor must be run up off load before the load is attached, unless the load to be driven is light and within the starting torque of the motor.
Compound Motors These motors combine the principal beneficial characteristics of both series and shunt-wound motors, giving higher starting torque with good steady speed to load characteristics.
For example, a motor may be required to develop the high starting torque of a series-wound motor but without the tendency to over-speed when removing the load. Another application may require a motor, which is capable of reducing its speed with increasing load, whilst still retaining the smooth speed control and reliability of a shunt-wound motor when operating off load. These and other requirements can be met by Compounding, or in other words, by combining both series and shunt field windings in the one machine, using one of the following arrangements: ¾
Normal Compounding In this arrangement, a motor is biased toward the shunt-wound type, where the shunt winding produces approximately 60 to 70 per cent of the total flux, whilst the series winding produces the remainder. This retains the desired characteristics of both series and shunt-wound motors.
¾
Stabilised Shunt In this arrangement, the motor is also biased toward the shunt-wound type, and only has a relatively minor series winding. The purpose of this winding is to overcome the tendency of a shunt motor to become unstable when running at or near its maximum operating speed, when subjected to an increased load.
¾
Shunt Limited In this arrangement, the motor is biased toward the series-wound motor and only has a minor shunt field winding incorporated in the field system. The purpose of this winding is to limit the maximum speed when running under off load conditions whilst leaving the torque and general speed characteristics unaltered. Shunt limiting is applied only to the larger type of compound motors (e.g. engine starter motors).
MOTOR SPEED CONTROL Because series motors have a varying speed to load characteristic and are usually permanently attached to the load, they are matched by the system designer, and external speed control is not normally required. Speed control is normally associated with shunt motors. Since there is equilibrium of armature current and field strength at a steady speed, altering either of these increases or decreases motor speed until achieving equilibrium once again. Two methods of control are armature control and field control. 6-6
Electrics
DC Motors
Chapter 6
Armature Control Current through the armature can be directly controlled by connecting a variable resistor in series with it.
IF VARI
M
Supply
Assume the motor in the diagram above is rotating at a steady state. By decreasing the resistance of the variable resistor, the armature current increases. The higher current increases torque until equilibrium is established again but at a higher speed. If the variable resistance is increased, the armature current decreases. The decreased armature current reduces torque and the motor slows down until equilibrium is established again but at a slower speed. This form of speed control is rarely used, as the high armature current requires a larger variable resistor to handle it. Field Control The preferred method of speed control is to control the field strength, which requires a smaller variable resistor due to the smaller field current.
IA
Supply
Variable Resistor
M
If the resistance is reduced, the current through the field windings would increase and the strength of the magnetic field would also increase. Variable resistance would not directly affect the current through the armature. However, as the magnetic field strength has increased, so would the back EMF. The current in the armature would reduce and motor speed would also reduce.
Electrics
6-7
Chapter 6
DC Motors
By increasing the resistance in series with the field winding, the field strength would reduce and, consequently, back EMF in the armature would also reduce. The increase in armature current causes the speed to increase. There is an apparent contradiction in the field control system as an increase in field strength decreases motor speed. It may help to compare the magnetic field to a viscous fluid: The thicker it gets, the harder it is for the armature to turn. The thinner it gets, the easier it is for the armature to turn.
ACTUATORS These are high-speed reversible series-wound motors whose output is normally converted into a driving torque via a step-down gearbox. Motor actuators are self-contained units, which combine electrical and mechanical devices capable of exerting reversible linear thrust over a short distance or alternatively a reversible low-speed turning effort. The following types of actuators exist: Rotary Actuators have a rotary movement and are mainly used to rotate valves in air conditioning and fuel systems.
Linear Actuators are driven directly from a reduction gearbox via a lead screw that extends or retracts a ram or plunger when rotated.
This type of actuator is capable of working against heavy loads and is used to operate trailing edge flaps, trim tabs, and to move variable incidence tailplanes.
6-8
Electrics
DC Motors
Chapter 6
SPLIT-FIELD SERIES MOTOR In this type of motor, the field winding is split into two separate electrical sections establishing two independently controlled magnetic fields. Each winding is selected independently of the other to control the direction in which the motor runs. In the diagram below, a single-pole, double-throw switch controls the motor. However, automated system control may determine motor control via remote switching. Both linear and rotary type actuators are equipped with limit switches to stop their respective motors when the operating ram or output shaft has reached its limit of travel. The switches are of the micro-switch type and are usually operated by a cam driven by a shaft from the actuator gearbox. In some cases, limit-switch contacts are also utilised to complete circuits to indicator lights or magnetic indicators. For example, consider the operation of an air conditioning duct valve, as shown in the following diagram. If the switch is placed in the Open position, current flows in the Open field winding, and then through the armature winding. The two fields will interact, and the armature will rotate, which will cause the cams to rotate at the same time. These cams determine the position of two limit switches, which control the current through the field windings and the position of magnetic indicators or lights that show the position of the valve. 28V d.c.
"Shut"
Limit Switch "A"
Close Winding
Cam Close
Magnetic Indicators or Lights
M
Off Open Cam
Limit Switch "B"
Open Winding
"Open"
As soon as the motor starts to rotate, Limit Switch A breaks the circuit to the SHUT indicator and causes the light to go out. When the valve is in its fully open position, the limit switches are arranged so that Limit Switch A completes the circuit to the CLOSE field winding, whilst Limit Switch B breaks the circuit to the OPEN field winding and operates the OPEN indicator. If the switch is then placed in the CLOSE position, current flows through the CLOSE field winding and then through the armature This causes the motor to rotate in the opposite direction to close the valve. The two field windings are independent of each other. The change of direction occurs because the polarity of the field windings is reversed, but the direction of the current through the armature remains the same. The resultant interaction of the fields causes the armature to run in the reverse direction. When the valve is fully closed, the position of the limit switches reverses, thus completing the circuit to the open field winding and operating the CLOSED indicator.
Electrics
6-9
Chapter 6
DC Motors
ELECTROMAGNETIC BRAKES Most actuators are fitted with electromagnetic brakes that are designed to prevent over-travel when the motor is switched off, such as when a heavy load is being driven. The design of the brake system varies with the type and size of the actuator, but in all cases, the brakes are springloaded to the ON condition whenever the motor is de-energised, thus preventing the actuator from over-running. This means that to motor the actuator, the brakes must be energised OFF to run. The operation of the electromagnetic brake is shown in the following diagram.
Notice that as soon as current flows in the armature, it must also flow in the electromagnetic brake solenoid, consequently releasing the brake.
CLUTCHES Friction clutches are also incorporated in the transmission systems to protect against the effects of mechanical over-loading. They are usually of the single-plate or multi-plate type depending on the size of the actuator.
INSTRUMENT MOTORS Not many instruments utilise DC motors, but of those that do, gyroscopic instruments are the most common. One case in particular is important as a specifically DC instrument and that is the artificial horizon. Consider that even with all the technology incorporated into the latest EFISequipped aircraft, there is still a need to carry a back-up DC attitude indicator or artificial horizon in case of a complete AC electrical failure. Of necessity, the motors found in instruments are specialised and tailored to the instrument in which they are fitted. To save weight and space, they are small and, whenever possible, form part of the instrument itself. To illustrate this point, consider the artificial horizon. This instrument relies on a gyroscope that must be rotated at high rpm for its operation. A gyroscope is a spinning mass. It gains the property of rigidity as it rotates, much like a bicycle wheel gains rigidity when rolling, stopping the bike from falling over. To increase rigidity, the mass of the wheel is concentrated as far from the centre of rotation as possible. A typical artificial horizon DC motor has the commutator and armature fixed at the centre of the gyroscope. The yoke itself forms the rotating mass of the gyro and rotates about the armature, fulfilling the requirements of gyroscopic principles.
6-10
Electrics
DC Motors
Chapter 6
ARCHITECTURE OF A STARTER/GENERATOR SYSTEM A DC generator can run as a motor and vice-versa. This property is used in many turbo-prop aircraft to combine the starter and generator into one unit, thus saving weight. Such a unit is called a starter / generator, and is illustrated below.
It is a compound wound machine, which is coupled to the engine by way of a drive shaft and gear train.
OPERATION OF A STARTER/GENERATOR SYSTEM When the engine start switch is operated, the following sequence of events takes place:
Electrics
¾
Initially, the battery master switch energises both relays connected to it. This places the batteries in parallel and supplies 24 V to both busbars
¾
When the starter switch is pushed, the start relay energises, connecting the starter busbar to the starter motor. The supply is at 24 V at first to reduce the initial starting current and torque, extending the life of the starter motor.
6-11
Chapter 6
¾
6-12
DC Motors
When the engine reaches 10% rpm, a speed sensor energises the paralleling relay (A). This reconnects the batteries in series, supplying 48 V to the motor and dramatically increasing torque and acceleration. In the partial diagram below, trace the lines between the two batteries to see that they are now in parallel.
Electrics
DC Motors
Chapter 6
¾
At 60% engine rpm and with the engine self sustaining, the starter and paralleling relay are de-energised, which removes the power from the starter motor and reconnects the batteries in parallel. If the engine start push was electromechanically held in for the start cycle, it would now be released, and the starter engaged light, if fitted, would extinguish.
¾
When the engine has accelerated to idle speed, the generator control switch is operated. If DC generator output if within limits, the control and protection unit (CPU) is connected to the busbar.
Notice that when the start relay has de-energised, the supply from the armature is removed in preparation for the generator output from the armature to be connected by the CPU to the essential busbar.
Electrics
6-13
Chapter 6
DC Motors
INVERTERS An aircraft may have a DC electrical system or what is called a frequency wild system, which is fully described later. In either case, there may be a requirement for an AC supply of a specific voltage and frequency. Such supplies are derived from machines called inverters, two types of which are described below. The Rotary Inverter is a DC motor that drives an AC generator on a common shaft.
The motor drives the AC generator at a constant speed to give a constant frequency output. This is achieved by adjusting the field excitation of the DC motor. The output voltage of the AC generator is maintained by similarly adjusting its field excitation. This type of inverter has a DC input of 28 volts and produces a 3-Phase AC output of 115 V at 400 Hz. There are large electromechanical losses in most rotary inverters and they are typically only 50% efficient. For instance, a DC input power of 100 W will output just 50 VA at the AC generator. Notice that power is defined differently in DC and AC systems and will be discussed later. Static Inverters differ from the rotary type in that they use solid-state transistorised circuitry and have no moving parts. They are more robust, more reliable and require less servicing. Static inverters cannot match the power output of rotary inverters although most have an improved efficiency of approximately 70%.
6-14
Electrics
DC Motors
Chapter 6
MULTIPLE INVERTER INSTALLATIONS A typical multiple inverter system is shown below and is of the type commonly fitted on twinengine turbo-prop aircraft.
This system consists of three inverters that supply normal constant frequency AC power. The No. 1 and No. 2 inverters are the same type. The No. 3 inverter is an emergency back up to supply the essential busbar only. It can be a smaller lighter type to save weight. Inverters cannot be operated in parallel. To even out the running time between No. 1 and No. 2 inverters, many operators require alternate selection of the inverters for each flight. More advanced systems may monitor the inverter output and provide automatic switching between inverters if a failure occurs and provide appropriate indications in the cockpit.
Electrics
6-15
Chapter 6
6-16
DC Motors
Electrics
INTRODUCTION Inductors and capacitors are components often found in electronic circuits. They react differently to voltage and frequency and are used in detection and control equipment. A coil has inductance and a capacitor has capacitance. However, inductance and capacitance are not restricted to small electronic components. As previously discussed, windings in a motor create a back EMF, which is a direct result of self-inductance.
INDUCTANCE Inductance is the property of a coil that opposes a change in current flow by generating an internal voltage in opposition to the supply voltage. The voltage generated within the coil is due to self-inductance and is at a maximum when the current from the supply is changing rapidly. For this reason, if a constant DC voltage is applied to a coil, there is no variation in supply voltage and, therefore, no varying current to induce an internal voltage in the coil. In other words, no inductance will exist with a DC supply. The variation in voltage when an AC or un-stabilised DC supply are input to a coil will create inductance. Other factors which influence the amount of inductance in a coil are the number of turns in it, and the cross-sectional area of the wire used, both of which are fixed values dependant on circuit or component requirements. The unit of inductance is the Henry (H) and exists when a rate of change in current of 1 amp per second induces 1 volt in the coil. This usually results in a very small figure, so most often the value is expressed in mH (milli-Henrys) or µH (micro-Henrys). The symbol for inductance in formulae is L. For instance, a value of 15 milli-Henrys can be written as: L = 15 mH The following rules are instrumental to understanding inductance: ¾
Electrics
When current flows through a conductor, a magnetic field builds up around it.
7-1
Chapter 7
¾
Inductance and Capacitance
When a conductor is moved through a magnetic field, an EMF is induced in it.
SELF INDUCTION Imagine a situation where an AC current is flowing in a ‘live’ wire and next to this is another wire that has no current flow in it, which can be considered as the ‘dormant’ wire. The magnetic field will build around the live wire to a maximum in one direction before collapsing as the current falls to zero. It then builds to a maximum in the other direction as current flow reverses and flows in the opposite direction. The dormant wire is lying in the influence of the magnetic field produced by the live wire and a voltage is therefore induced in it of an opposite polarity to that in the live wire. Picture a situation where two loops are side by side in a coil. Although they are both part of the same length of wire, the interaction between them is just like the live wire/dormant wire situation described above. Each loop of wire in the coil will be inducing voltage in the adjacent loop which acts to oppose the voltage at the supply. Now take the above process one step further. It has been established that the induced voltage opposes the supply voltage, but such opposition is not always against a rising supply voltage. Opposition can also exist against a falling voltage. A voltage induced in an adjacent loop opposes a rising voltage as the magnetic field strength is increasing. However, when the supply voltage reduces to zero the magnetic field collapses, and this induces a voltage in the adjacent loop of an opposite polarity. This has the effect of maintaining the coil voltage when the supply voltage is reducing. It is easiest to consider the supply voltage and coil voltage as opposite forces, but that the coil voltage is always less than the supply. The interaction between the supply voltage and coil voltage means that current flow does not vary in direct proportion with the supply. The opposition of the coil to rising voltage delays the equivalent rise in current flow. As the supply voltage falls, the current flow is maintained by the collapsing magnetic field about the coil. The net effect of these opposing voltages is that current flow will lag behind the supply voltage. A side effect of the coil field collapsing is that as a switch in the coil circuit opens, a spark occurs at the switch contacts.
INDUCTORS Where using inductors in a circuit, they are chosen with a specific value of inductance. The inductance of a coil can be increased by either raising the number of turns on the coil, and/or by inserting a piece of permeable material, such as soft iron, into the coil.
7-2
Electrics
Inductance and Capacitance
Chapter 7
TIME CONSTANT OF AN INDUCTOR The time it takes for the current in an inductor to reach a steady value depends on the value of the inductance and the value of any resistance in series with it.
Switch Closed
Battery
A
Steady Flux Through Coil
Steady Current
As the voltage induced in an inductor opposes the supply voltage, if a DC supply is supplied to the coil, it takes a finite time for the current to reach a steady value. The time taken is the time constant and can be changed by either altering the value of inductance of the coil or by altering the resistance in series. Bear in mind that once the full supply voltage is reached, it remains steady. No further voltage is induced until the next change in voltage, which occurs when the circuit is switched off. The time constant is not linear because the induced voltage is dependent on a rate of change, which itself is not at a constant rate. As a supply to an inductor is initially switched on, the supply voltage rises rapidly from zero to full voltage in a sine wave curve. The initial rapid rise in supply voltage immediately induces a high voltage in the inductor in opposition, but as the sine curve of the supply voltage flattens, the opposition reduces. The reverse occurs when switching off the supply The time constant is not measured from zero voltage to full voltage but to 63.2% on voltage rise and 36.8% on voltage fall. The time constant can be found by dividing the inductance (H) by the resistance (Ώ). The time for full current and voltage to be developed across the inductor is 5 times the time constant: 5 X L/R The time constant is inversely proportional to resistance.
Electrics
7-3
Chapter 7
Inductance and Capacitance
These graphs show that the resistive voltage (VR) increases and decreases in line with the current (I), whereas the voltage drop across the inductor (VL) falls as the current rises, and vice versa.
INDUCTORS IN SERIES AND PARALLEL Inductors can be connected in a DC electrical circuit in either series or parallel.
When connected in series, the values of inductance for each inductor are added: Total Inductance (LT) = L1 + L2 + L3 When connected in parallel the reciprocals of the individual values of inductance are added together, and the reciprocal of the total gives the total inductance: 1 1 1 1 LT = L1 + L 2 + L3
Inductance calculations are thus similar to resistance calculations in a DC circuit. 7-4
Electrics
Inductance and Capacitance
Chapter 7
CAPACITANCE If two opposing surfaces that contain charges of opposite polarity are close to, but insulated from each other, an electrostatic field would exist between them, maintaining the charges on each surface due to the attraction between the opposite polarities. Remember that opposites attract. The electrostatic field developed maintains the charges on each surface, and the greater the charge maintained, the greater the capacitance. A static charge of 6.28 X 1018 electrons is one Coulomb, and when one coulomb of charge develops between two surfaces with 1 volt applied, 1 unit of capacitance exists, called the Farad. In practice, one unit of Farad is an extremely large value and, therefore, practical values of capacitance are typically very small. The component specifically designed to have a capacitance value is called a capacitor. Because practical values of capacitance are very small, they are rated in terms of microFarads (µF), nanoFarads (nF), and picoFarads (pF). A capacitor comprises two plates of a conductive material separated by an insulator, called a dielectric. The dielectric has a direct influence on the capacitance, because the better the insulator, the greater the charge that can be held on the plates without the voltage causing the charge on the plates to flash over. Because a charge is maintained on the plates by the electrostatic field, the charge remains on the plates even if the component is removed from a circuit. A capacitor is often considered an energy storage device. It will take a finite amount of time for a supply to fully charge a capacitor and, as with the inductor, is known as the time constant. The total number of electrons held on a plate creates an electrical pressure. The charge held by a capacitor is measured in volts. A Capacitor will be charged to a specific voltage level, up to the maximum voltage, which will equal the supply voltage. To help appreciate the difference between the voltage on a capacitor and its capacitance it is useful to consider the formula for capacitance: Capacitance (C) = Q V Where V = voltage across plates and Q = charge in Coulombs It can be inferred from this formula that the largest charge that can be maintained on a capacitor by the smallest possible applied voltage will give the greatest capacitance.
FACTORS AFFECTING CAPACITANCE Capacitors in their simplest form consist of two metal plates separated by a non-conducting material called a dielectric.
Metal foil is often used for the plates, whilst the dielectric may be paper, glass, mica, or another good insulator. A capacitor has a specific amount of capacitance, so if the applied voltage is increased, the charge will similarly increase, so that the ratio of the charge to the voltage remains the same.
Electrics
7-5
Chapter 7
Inductance and Capacitance
The actual amount of capacitance is dependent on the physical shape and size of the capacitor and varies according to the following formula: C= kA d where:
k = type of dielectric A = area of the plates d = distance between the plates
The capacitance of a capacitor is directly proportional to the dielectric constant or the area of the plates and inversely proportional to the distance between the plates.
TYPES OF CAPACITOR Capacitors can be either fixed or variable. The most common types include: ¾
Paper Capacitors are constructed of alternate layers of metal foil separated with similar strips of waxed paper, which act as the dielectric.
¾
Electrolytic Capacitors have a large capacitance for a small physical size. Since electrolytic capacitors are manufactured using an electro-chemical process, they are sensitive to polarity. This limits their use to DC circuits only. Be careful when connecting them into a circuit, since connecting incorrectly will damage the capacitor.
¾
Variable Capacitors consist of multiple plates, which are moved via a rotating shaft.
Note: Capacitors are rated by voltage as well as by their capacitance value. A voltage rating must not be exceeded, or the dielectric may break down and arcing may occur. To withstand higher voltages, the insulating properties of the dielectric must be increased or its thickness increased, both of which will reduce capacitance. To maintain capacitance, the overlapping area of the plates must be increased.
7-6
Electrics
Inductance and Capacitance
Chapter 7
THE CHARGING OF A CAPACITOR If a capacitor is uncharged, the same number of free electrons will exist on both plates, and a voltmeter connected across the plates will read zero volts, as shown in the diagram below.
If a DC voltage is subsequently applied to the plates of the capacitor, it charges up until the potential across the plates is equal and opposite of the supply voltage.
When the switch is closed, the positive terminal of the battery is connected to the upper plate of the capacitor, and the battery attracts the free electrons from the upper plate. This leaves the upper positive plate with a deficiency of electrons and the negative lower plate with an excess of electrons. The positive plate attracts the electrons on the negative plate, but due to the insulator (dielectric) between the plates, no electrons flow between them. The attraction of the positive charge on the upper plate instead tends to pull electrons from the negative terminal of the battery to the lower negative plate. The difference in potential between the plates causes an electric field to build up across the dielectric between them. The capacitor continues to charge until the potential difference between the plates equals the supply voltage.
When this occurs, no further current flows. Current only flows in the circuit whilst the capacitor is charging, and does not pass through the capacitor in a DC circuit. Electrics
7-7
Chapter 7
Inductance and Capacitance
A capacitor retains a charge for a long period of time, and can be charged in either polarity simply by reversing the supply (unless the capacitor is electrolytic). If the supply is removed from the capacitor, the electrical charge on the plates remains for a long time. This can pose a hazard to the unsuspecting, especially with regard to High Energy Ignition Units (HEIU) used for engine starting, where the high capacity can lead to severe injury. With these devices, a safety resistor of high resistance is connected from the capacitor output to earth allowing the capacitor to discharge after approximately 1 minute. Do not touch the HEIU within this time.
DISCHARGING OF A CAPACITOR Theoretically, all of the energy stored in a capacitor can be recovered. A perfect capacitor would use no power, but store it to be released later. This could be used for timing or signalling purposes.
Removing it from the supply and connecting it across a resistor can discharge a capacitor. This causes the current to flow until the capacitor is fully discharged, and its charge has been reduced to zero.
THE TIME CONSTANT OF A CAPACITOR The length of time required for a capacitor to charge or discharge can be calculated if certain circuit values are known. Exponential curves of voltage against time show how the voltage across the plates varies. The two factors affecting the charge and discharge time are the resistance (R) and the capacitance (C). R multiplied by C gives the time required for the capacitor to charge to 63.2% of fully charged state or discharge to 36.8% of its fully charged state. This is known as its time constant (T) and is expressed as: T=RxC where:
T = time in seconds R = resistance in Ohms C = capacitance in Farads
In practice, the time taken for the capacitor to become fully charged or discharged is equal to 5CR.
7-8
Electrics
Inductance and Capacitance
Chapter 7
CAPACITORS IN SERIES AND PARALLEL IN A DC CIRCUIT Like resistors and inductors, capacitors can also be connected in various combinations, as shown below.
¾
Capacitors in Parallel increase the effective area of the plates, and thus increase the overall total capacitance. The formula for calculating the total value of capacitors connected in parallel is:
CT = C1 + C2+ C3... ¾
Capacitors in Series increase the overall thickness of the dielectric, decreasing the total capacitance. The formula for calculating the total value of capacitors connected in series is: 1 CT
= C1 + C1 + C1 1 2 3
Notably, if two capacitors of different values are connected in series in a circuit, the smaller capacitor will have a higher value across it rather than the larger one. To understand why this occurs, consider capacitance in terms of voltage: V=
Q C
Voltage is inversely proportional to the capacitance, so the smaller the capacitor, the higher its voltage.
Electrics
7-9
Chapter 7
7-10
Inductance and Capacitance
Electrics
INTRODUCTION Alternating Current (AC) continually changes its polarity and can vary in both magnitude and how often it changes direction. This differs from Direct Current (DC), which is usually of a constant value flowing in one direction only. The voltage and current in an AC circuit increase from zero to maximum and back to zero again in one direction before reversing and reaching a maximum in the other direction. The effect of an AC supply on resistors, inductors, and capacitors also differs from that of a DC supply.
ALTERNATING CURRENT ADVANTAGES OF AC OVER DC AC is extremely versatile and has the following advantages over DC: ¾
AC can be simply and efficiently changed from one voltage to another using a transformer.
¾
AC generators are simpler and lighter in construction than DC generators for the equivalent power output.
¾
AC can be easily and efficiently changed into DC using rectifiers.
¾
The magnitude or frequency of AC voltages can be easily modified to carry or transmit information as AC signals.
¾
The frequency at which electro-magnetic radio waves can be made to propagate outward from a suitable aerial begins at 3000 Hz or 3 kHz, known as a radio frequency (RF). It is relatively easy to increase an AC supply frequency to the RF level.
GENERATING AC An AC generator converts mechanical energy into electrical energy by using the electromagnetic induction properties of a coil rotating in a magnetic field. The magnitude of the voltage produced is dependent on the following factors:
Electrics
¾
The strength of the magnetic field
¾
The speed at which the conductor cuts the magnetic field
¾
The length of the conductor within the magnetic field
¾
The angle at which the conductor cuts the magnetic field
8-1
Chapter 8
Basic AC Theory
As with a DC generator, the polarity of the induced voltage can be found using Fleming’s RightHand Rule (see page 5-1).
SIMPLE AC GENERATOR In its simplest form, an AC generator consists of a single loop of wire or armature, which is mounted on a shaft, such that it can be rotated within a magnetic field. When it is rotated, an AC voltage is induced in it, which can be easily transferred to an external circuit by means of carbon brushes that bear on slip rings connected to the loop.
When the armature moves through 360°, or through one revolution, at a constant speed the output voltage and current rise to a maximum value in one direction and back to zero, before reversing in polarity. The voltage and current rise to a maximum value in the opposite direction before again returning to zero. The paths plotted by the voltage and current are in the shape of a sine wave whose magnitude and polarity are determined by the actual position of the armature as shown in the following diagram.
8-2
Electrics
Basic AC Theory
Chapter 8
AC TERMINOLOGY The diagram below shows how the voltage output varies when the armature is rotated through 360°.
By convention, the following terminology applies to the resulting sine wave: Cycle This is a complete variation of voltage from zero through a maximum value in each direction and returning to zero. It occurs when the armature of a basic AC generator rotates through one complete revolution (360°). Instantaneous Value This is the value at a specific instant in time. Peak Value The voltage curve rises to a maximum value in one half cycle, and the level at which this occurs is the peak voltage.
Electrics
8-3
Chapter 8
Basic AC Theory
Peak-to-Peak Value This is the total voltage variation between the peak positive voltage and the peak negative voltage. Peak to Peak = 2 x Peak Value Average Value The average value of voltage or current can be calculated by taking a large number of instantaneous values, either positive or negative, and dividing by the number of values taken. Using advanced calculus, it can be shown that the average over a sine curve is 0.637 of the maximum peak value. Root Mean Squared Value RMS (Effective Value) This amount of power or heat will be dissipated by an AC peak current of 1 ampere, compared to the amount of power or heat dissipated by a DC current of 1 ampere when flowing through an identical resistor.
As the AC current only reaches 1A when it reaches the peak voltage, the overall heating effect is reduced compared to the constant 1A current flow in the DC supply. To define the equivalent power of an AC supply when compared to a DC supply, the term Root Mean Squared (RMS) is applied to an AC supply when the above comparison is taken into consideration. For instance, the standard domestic electricity supply is 230 V, and would give an equivalent heating effect as DC at 230 V and is, therefore, the RMS value. The peak voltage value of the domestic supply is approximately 325 V. The RMS value is 0.707 of the peak value: 325 V domestic peak voltage x 0.707 = 230 V Unless specifically stated, all values of AC voltage and current are given as RMS values. In fact, AC voltmeters and ammeters indicate RMS values. Frequency The number of complete cycles in one second is called the frequency of the supply and is measured in hertz. The frequency of an AC waveform is directly related to the speed at which the generator is driven. The standard AC electrical supply in modern aircraft is 200 volts 400 Hz.
8-4
Electrics
Basic AC Theory
Chapter 8
Periodic Time This is the time taken to complete one complete cycle and is the reciprocal of frequency: Periodic Time =
1 Frequency
PHASE AND PHASE ANGLE Consider two AC voltages having the same frequency but with different magnitudes.
Both waveforms cross the zero axis at the same time and are in phase with each other. They also reach their maximum and minimum peak values at the same time. If the waveforms are displaced from each other and cross the zero axis at different points, they are out of phase, as shown below.
The maximum and minimum peak values also occur at different phase angles. By convention, the angular difference between the two waveforms where they cross the horizontal axis and go positive is the phase displacement or phase angle. In the above example, V1 leads V2 by 90° or, alternatively, V2 lags V1 by 90°. If the frequency of V1 and V2 is identical, the phase shift remains constant.
Electrics
8-5
Chapter 8
Basic AC Theory
If the waveforms are 180° out of phase, they are said to be in anti-phase.
PHASOR REPRESENTATION Any AC quantity that produces a sine wave output can be represented as a phasor, which is simply a vector representation that rotates at a constant velocity.
The length of each phasor represents the amplitude of the waveform and its angle with respect to a given reference axis. In this example, the phasors V1 and V2 are both rotating at the same rate, and V1 is leading V2 by 30°. This can be more simply represented by using a phasor diagram, where V1 is taken as the reference phasor.
By convention, the reference phasor is placed in the 3 o’clock position and all phasors rotate in an anti-clockwise direction. In this case, V1 is taken as the reference vector, since it has a phase angle of zero. V2 is 30° behind it and, therefore, lags V1. Any number of voltages and/or currents can be drawn on the same phasor diagram provided that they are all at the same frequency.
8-6
Electrics
INTRODUCTION As voltage is a pressure that forces current to flow in a circuit, then in any circuit, there will be a relationship between voltage and current that depends on whether the total load is capacitive, inductive, or resistive.
SINGLE PHASE AC CIRCUITS THE EFFECT OF AC ON A PURELY RESISTIVE CIRCUIT When an AC supply is applied to a purely resistive component, the current flows through the resistor in one direction and then the other, as illustrated below.
The current varies in both amplitude and direction in sympathy with the AC voltage. Where the load is purely resistive in nature, both reach their maximum and minimum values at the same time. When this condition exists, the two sine waves are in phase and are represented by the phasor diagram as below:
POWER IN AN AC RESISTIVE CIRCUIT As the voltage and current are in phase in a purely resistive circuit, power can be calculated in exactly the same way as it is for a DC circuit, I x V, and is measured in watts. All the power used in a resistive component is dissipated in the form of heat.
Electrics
9-1
Chapter 9
Single Phase AC Circuits
Notice in the power curve above that all the power consumed is positive, even though for the second half of the cycle voltage and current are negative. It is a mathematical fact that the product of two negative values results in a positive value. Therefore all the power in a purely AC resistive circuit does useful work and is called true or real power.
THE EFFECT OF AC ON A PURELY INDUCTIVE CIRCUIT The induced voltage in an inductor caused by the fluctuating magnetic field is always in opposition to the voltage at the supply. Effectively, this opposition voltage delays the current flow and causes a shift between the voltage and current curves in such a manner that current lags behind the voltage.
In theory, if the total load is purely inductive, the phase shift is 90°. V leads I when the circuit is inductive L, or VIL. This can be represented on a phasor diagram as below.
9-2
Electrics
Single Phase AC Circuits
Chapter 9
POWER IN AN AC INDUCTIVE CIRCUIT The instantaneous power is obtained by multiplying the instantaneous values of voltage and current together.
In the first quarter cycle, the values of voltage and current are both positive quantities producing positive power. In the second quarter cycle, the value of the current is still positive, but the value of voltage is now negative, thus producing negative power. This pattern continues and repeats every half cycle of the waveform. The average power is zero, and a perfect inductor dissipates zero real or effective power. The power produced is alternatively known as Reactive Power, and is measured in volts amperes reactive (VAR).
INDUCTIVE REACTANCE (XL) An inductive component (inductor) tends to oppose the change in current flow and, like a capacitor, offers opposition to the flow of alternating current. The counter-EMF induced in an inductor by the varying current opposes the supply voltage. This opposition to current flow is called Inductive Reactance (XL), which is directly proportional to the inductance of the inductor and the frequency of the supply voltage, as shown below. Inductive Reactance (XL ) = 2ΠfL ohms where:
f L XL
= frequency in hertz = inductance in henrys = inductive reactance in ohms
In an AC circuit, an inductor has the same effect on current flow as a resistor. In a purely inductive circuit, the current in the circuit is directly proportional to the applied voltage and inversely proportional to the inductive reactance. IL = V = V XL 2 ∏ fL
If the supply frequency is increased, the inductive current decreases and vice versa. An inductive component may thus be damaged if the frequency is reduced.
Electrics
9-3
Chapter 9
Single Phase AC Circuits
THE EFFECT OF AC ON A PURELY CAPACITIVE CIRCUIT When an AC voltage is applied to a purely capacitive circuit, the capacitor will charge up first in one direction and then in the other.
If the voltage and current are monitored, the current does not rise in phase with the applied voltage as it does in a resistive circuit. In a purely capacitive circuit, the current reaches its maximum value π/2 radians (90°) before the voltage across the capacitor reaches its maximum value. The voltage and current are out of phase, and the voltage lags the current by π/2 radians (90°), or the current leads the voltage by the same angle. This can also be represented on a phasor diagram as follows:
POWER IN AN AC CAPACITIVE CIRCUIT The instantaneous power is given by multiplying the instantaneous values of voltage and current together.
In the first quarter cycle, the values of voltage and current are both positive quantities, producing positive power. In the second quarter cycle, the value of the voltage is still positive, but the value of current is now negative, producing negative power. This pattern continues and repeats every half cycle of the waveform. Although capacitive components do not dissipate any real power, they do develop a voltage that opposes the supply. This opposition has a similar effect as the voltage that drops across a power-consuming device. This effect is called reactive, or wattless power and is measured in volt amp reactive (VAR). 9-4
Electrics
Single Phase AC Circuits
Chapter 9
CAPACITIVE REACTANCE (CAPACITORS AC RESISTANCE) In an AC circuit, the capacitor constantly charges and discharges. This is due to the time lag that exists and the voltage across the capacitor being in constant opposition to the supply voltage. This creates an opposition to current flow (i.e. electrical resistance, which is known as Capacitive Reactance (XC)). Capacitive reactance is inversely proportional to the capacitance of the component and the frequency of the applied voltage, as shown below. Capacitive Reactance (XC ) = where:
f C XC
1 ohms 2Π fC
= frequency in hertz = capacitance in farads = capacitive reactance in ohms
In an AC circuit, a capacitor has the same effect on current flow as a resistor. In a purely capacitive circuit, the current in the circuit is directly proportional to the applied voltage and the capacitive reactance. IC = V = V x 2ΠfC XC If the supply frequency is increased, the capacitive current will increase and vice versa. A capacitive component may thus be damaged if the frequency is increased.
RELATIONSHIP BETWEEN VOLTAGE AND INDUCTIVE AC CIRCUITS IN CAPACITIVE Depending on whether the circuit is inductive or capacitive, the acronym CIVIL is a memory aid as to whether the current leads or lags the voltage.
C I V I L In a Capacitive (C) Circuit, I before V (I leads V). In an Inductive (L) Circuit V before I (V leads I). This is particularly useful when dealing with series or parallel AC circuits. In series AC circuits, current is used as the reference phasor, and in parallel AC circuits, voltage is used.
RESISTIVE AND INDUCTIVE (RL) SERIES AC CIRCUIT When an AC voltage is applied across an RL circuit, a voltage drop takes place across each component, and the same current passes through both. Current is thus taken as the reference phasor in the phasor diagram.
The voltage drop across the resistor (VR) is in phase with the current, and the voltage drop across the inductor (VL) leads the current by π/2 radians (90°). The supply voltage (VS) can then be calculated using the vector sum of these voltages. Electrics
9-5
Chapter 9
Single Phase AC Circuits
RESISTIVE AND CAPACITIVE (RC) SERIES AC CIRCUIT When an AC voltage is applied across an RC circuit, a voltage drop takes place across each component, and the same current passes through both. Current is thus taken as the reference phasor in the phasor diagram.
The voltage drop across the resistor (VR) is in phase with the current, and the voltage drop across the capacitor (VC) lags the current by π/2 radians (90°). The supply voltage (VS) can then be calculated using the vector sum of these voltages, as in the case of the RL series circuit.
PHASE SHIFT The phase shift of a circuit is the angle between the voltage and current vectors. It is a function of the reactive and resistive components. In the case of a series RC circuit, it can be expressed mathematically as tan φ =
XC X and for a series RL circuit as tan φ = L . R R
RESISTIVE, INDUCTIVE, AND CAPACITIVE (RLC) SERIES AC CIRCUITS In an RLC series circuit, current is a common vector, and the voltage drops across the resistor, the inductor, and the capacitor are as shown below.
The voltage drop across the resistor (VR) is in phase with the current. The voltage drop across the capacitor (VC) lags the current by π/2 radians (90°) and the voltage drop across the inductor (VL) leads the current by π/2 radians (90°). The vertical components, VL and VC, are in direct opposition to each other, so the resulting vertical component is (VL – VC). The supply voltage (VS) is found using Pythagoras, as follows. VS =
9-6
VR 2 + (VL − VC ) 2
Electrics
Single Phase AC Circuits
Chapter 9
IMPEDANCE (Z) IN A RESISTIVE, INDUCTIVE, AND CAPACITIVE (RLC) SERIES AC CIRCUIT Impedance is the total opposition to current flow in an AC circuit containing resistance and reactance. In a series AC circuit, it is the vector sum of the inductive reactance (XL), capacitive reactance (XC), and resistance (R) as shown below.
RESISTIVE, INDUCTIVE, AND CAPACITIVE (RLC) PARALLEL AC CIRCUIT In a parallel RLC circuit, voltage is the common vector. The currents through the resistor (IR), the inductor (IL), and the capacitor (IC) are as shown below.
The current through the resistor (IR) is in phase with the voltage. The current through the capacitor (IC) leads the voltage by π/2 radians (90°), and the current through the inductor (IL) lags the voltage by π/2 radians (90°). The vertical components, IL and IC, are in direct opposition to each other, so the resulting vertical component is thus (IL – IC). The supply current (IS) is found using Pythagoras, as follows. IS =
IR 2 + (IL − I C ) 2
IMPEDANCE (Z) IN A RESISTIVE, INDUCTIVE, AND CAPACITIVE (RLC) PARALLEL AC CIRCUIT In a parallel AC circuit, the reciprocal of impedance is the vector sum of the reciprocals of the inductive reactance (XL), the capacitive reactance (XC), and the resistance (R) as shown below. 1 = 1 Z XR
Electrics
2 +
1 1 − X X C L
2
9-7
Chapter 9
Single Phase AC Circuits
POWER IN A RESISTIVE, INDUCTIVE, AND CAPACITIVE (RLC) AC CIRCUIT The types of power that exist in an RLC circuit are shown below:
True or Effective Power (watts) is the amount of power being consumed by the resistive component in an AC circuit. The unit of true power is the watt. Reactive Power (VAR) (wattless power) is the power consumed by the reactive components. The unit of reactive power is volts-amperes reactive (VAR) Apparent Power (VA) is found by measuring the voltage and current being applied to a circuit and multiplying them together. The unit of apparent power is volt-amperes (VA), and most AC equipment is rated in VA. Apparent power (VA) consists of the vector sum of true power (watts) and reactive power (VAR). Power in an AC circuit can alternatively be represented as a triangle, as shown below. Apparent Power (VA)
Reactive Power (VAR)
Real or True Power (Watts)
9-8
Electrics
Single Phase AC Circuits
Chapter 9
POWER FACTOR This is a means of indicating the amount of true power consumed in an AC circuit when the apparent power (VA) is given. The formula is: Power Factor (Cos φ) =
True Power Apparent Power
For example, if the apparent power of a circuit is 1000 volt-amperes and the generator has a power factor of 0.6, the true power is 600 watts (i.e. the generator is only 60% effective). If the power factor is alternatively 1.0 or unity, the true power would be 1000 watts. It is therefore important that the power factor is as close to unity as possible, although this is normally a fixed quantity and cannot be altered.
AC SERIES CIRCUIT EXAMPLE An AC series circuit with a supply voltage of 100 volts has a resistance (R) of 30Ω, a capacitive reactance (XC) of 60 Ω, and an inductance reactance (XL) of 100 Ω, as shown below.
R
C
L
I
Calculate the: a) b) c) d) e)
Electrics
Total impedance (Z) Supply current (I) P.D across each component (VR, VC and VL) True, reactive, and apparent power Power factor
9-9
Chapter 9
Single Phase AC Circuits
Solution:
VL XL - X C
V L - VC
I VR
R
S
I VR
S
S
VC a) Z = b) IS =
R
2
+ (X L − X C ) 2 =
30 2 + (100 − 60 ) 2 = 50 ohms (Ω)
VS = 100 = 2 amps Z 50
c) VR = I x R = 2 x 30 = 60 volts VC = I x XC = 2 x 60 = 120 volts VL = I x XL = 2 x 100 = 200 volts d) True Power = VR x I
= 60 x 2 = 120 watts
Reactive Power = (VL – VC) x I = 80 x 2 = 160 volt-amperes reactive (VAR) Apparent Power = VS x I
e) Power Factor =
= 100 x 2 = 200 volt-amperes (VA)
True Power = 120 = 0.6 lagging Apparent Power 200
This is because the supply voltage is ahead of the supply current in the phasor diagram.
9-10
Electrics
Single Phase AC Circuits
Chapter 9
AC PARALLEL CIRCUIT EXAMPLE An AC parallel circuit with a supply voltage of 100 volts has a resistance (R) of 30.3 Ω, a capacitive reactance (XC) of 10 Ω, and an inductance reactance (XL) of 16.7 Ω, as shown below:
Calculate the: a) b) c) d)
Electrics
Current through each component (IR, IL and IC) Total current (IT) True, reactive and apparent power Power factor
9-11
Chapter 9
Single Phase AC Circuits
Solution:
a) IR = IC = IL = b) IT =
VS R VS XC VS XL
=
100 33.3
= 3 amps
=
100 10
= 10 amps
=
100 16.7
= 6 amps
IR 2 + (IC − IL ) 2 =
c) True Power
3 2 + (10 − 6 ) 2 = 5 amps
= VS x IR
= 100 x 3 = 300 watts
Reactive Power = VS x (IC – IL)
= 100 x 4 = 400 volt-amperes reactive (VAR)
Apparent Power = VS x IT
= 100 x 5 = 500 volt-amperes (VA)
d) Power Factor =
True Power = 300 = 0.6 leading Apparent Power 500
This is because the supply voltage is behind the supply current in the phasor diagram.
9-12
Electrics
INTRODUCTION When a DC voltage is applied to a parallel circuit containing both inductance and capacitance, the capacitor acts like an open circuit and the inductor like a short circuit. This means that XC is infinite while XL is zero. If a very low frequency AC is applied instead of DC and the frequency gradually increased, XL increases and XC decreases. A point is eventually reached where the value of XL is the same as XC. It follows that for any combination of L and C, there is a frequency at which XL equals XC. This is true whether the two components are connected in series or parallel. The condition where XL equals XC is known as resonance, and the frequency at which this occurs is known as the resonant frequency (fo). The resonant frequency can be calculated using the following formula: XL = XC 1 2πfL = 2Π fC 1 fo = 2Π LC Where: f = frequency in hertz, L = inductance in Henrys and C = capacitance in Farads
RESONANT CIRCUIT SERIES RESONANT CIRCUIT When current flows in a series circuit containing a resistor, a capacitor, and an inductor, a voltage develops across each component.
VS =
Electrics
VR 2 + (VC − VL ) 2
10 -1
Chapter 10
Resonant AC Circuits
At resonance, the voltage drop across the capacitor is equal and opposite of the voltage drop across the inductor, and they cancel each other out. Zero Volts VL = V C
VR R
Supply
At resonance, the voltage across the resistance (VR) equals the supply voltage (VS). The capacitor and inductor therefore do not affect the supply, since they provide no opposition to current flow at resonance. The voltage across the individual reactive components can also be many times higher than the supply voltage. Similarly in terms of impedance (Z): Z=
R 2 + (X C − X L ) 2
Both the capacitive and inductive reactances are dependent on the frequency and both alter with changes in frequency, as shown below. ohms XL XC Z
R
0
fo
f(Hz)
With increasing frequency, XC reduces whilst XL increases and vice versa. The value of Z similarly alters, and at resonance, XC = XL, thus Z = R. Minimum impedance thus allows maximum current to flow in the circuit when the resonant frequency is achieved. A series resonant circuit is also known as an acceptor circuit, and is particularly useful in communication equipment, because it increases the sensitivity of the receiver (RX). This is done by enabling signals of a given frequency to be magnified and separated from other signals. The range of frequencies over which it is selective is called the bandwidth of the resonant circuit, as shown in the following diagram.
10-2
Electrics
Resonant AC Circuits
Chapter 10
Bandwidth
By convention, the bandwidth of a series circuit is the separation between two frequencies either side of the resonant frequency, at which the output power falls to half its maximum value.
Q FACTOR IN A SERIES RESONANT CIRCUIT The Q or magnification factor is very important in a series resonant circuit and is defined as the ratio of the reactance to resistance. Q=
XL R
or
XC R
This is the reason the voltage across the reactive components can be very much larger than the supply voltage, because it magnifies the voltage by the factor of Q.
PARALLEL RESONANT CIRCUIT (TANK CIRCUIT) In an ideal parallel resonant circuit containing only pure capacitance and pure inductance, XL will be equal to XC.
SUPPLY
IL=
Under these conditions, an equal amount of energy would first be stored in the capacitor in an electrostatic field and then passed to the inductor to be stored as an electro-magnetic field. This is known as the flywheel effect, and because there is no resistance in the circuit, the oscillation of energy between the capacitor and inductor would continue indefinitely. It follows that since no energy needs to be replaced in the circuit, none is drawn from the AC supply other than the initial amount of energy required to start the oscillation. The circuit appears to the supply to be an open circuit. Practical parallel inductive-capacitive circuits, however, have resistance. Unlike the hypothetical circuit shown, which only stores energy, resistance dissipates it in the form of heat. In a practical tank circuit, the oscillation quickly dies away unless the lost energy is replaced by the supply. If the resistance in the circuit is high, the oscillations quickly damp out, because the energy is rapidly dissipated.
Electrics
10-3
Chapter 10
Resonant AC Circuits
In a normal parallel RLC circuit, the supply current (IS) can be established using a phasor diagram and Pythagoras’s Theorem, as shown below.
IS =
IR 2 + (IL − I C ) 2
At resonance, the current through the capacitor is equal and opposite to the current through the inductor, thus they cancel each other out.
At resonance, the current through the resistance (IR) equals the supply current (IS). The capacitor and inductor affect the supply, since they provide maximum opposition to current flow at resonance. The circulating current in the inductor and capacitor can also be many times greater than the supply current at resonance. The impedance (Z) is maximum and the resultant current a minimum at resonance. Bandwidth
C U R R E N T
fO FREQUENCY
10-4
Electrics
Resonant AC Circuits
Chapter 10
A parallel resonant circuit is also known as a rejector circuit and is particularly useful in communication equipment, because it increases the selectivity of the receiver (RX). This is done by enabling signals of a given frequency to be easily separated from other signals, by magnifying the supply current. The range of frequencies over which it is selective is called the bandwidth of the resonant circuit, as shown in the previous diagram. Often a parallel resonant circuit is too selective and responds to only a very narrow band of frequencies. In these cases, connecting a relatively small value resistor across the tank circuit can increase the bandwidth.
Q FACTOR IN A PARALLEL RESONANT CIRCUIT In a parallel resonant circuit, the supply is applied directly across both C and L, so unlike a series resonant circuit, the current, rather than the voltage, is magnified by a factor of Q. This is determined by dividing the tank current by the source current, as shown below. Q=
I Tank I Source
This is the reason why the current circulating around the reactive components can be very much larger than the supply current.
SELF RESONANCE OF COILS Every coil has a certain value of capacitance and therefore at some value of frequency, the inductor (coil) begins to self resonate.
USE OF RESONANT CIRCUITS The characteristics of resonant circuits make them useful for filtering specific frequencies in electronic circuits, and in this capacity, they are known as filters. The basic filters are: Low-Pass Filter In this circuit, an inductance coil is placed in series, and a capacitor is placed in parallel with the supply.
Low frequencies pass easily through the inductance coil but are blocked by the capacitor, whereas the reverse occurs at higher frequencies. This is because the reactance of the components varies with frequency and thus determines which component passes current more readily. At low frequencies, the inductive reactance (XL = 2ΠfL) is low, whereas at higher frequencies the capacitive reactance (XC =
1 ) is low. A low-pass filter thus passes 2Π fC
frequencies in the lower ranges, but attenuates or reduces the current at frequencies in the higher ranges.
Electrics
10-5
Chapter 10
Resonant AC Circuits
High-Pass Filter In this circuit, a capacitor is placed in series, and an inductance coil is placed in parallel with the supply.
Low frequencies will pass easily through the inductance coil, but will be blocked by the capacitor, whereas at higher frequencies, the reverse occurs. A high-pass filter passes frequencies in the higher ranges, but attenuates or reduces the current at frequencies in the lower ranges. Band-Pass Filter This filter consists of a series LC and a parallel LC circuit, arranged as shown below.
In this arrangement, the impedance of the series LC circuit remains high, except at or near the resonant frequency, whereas the impedance of the parallel LC circuit remains low until this frequency band is reached. The number of circuit components and their resistance also determines the bandwidth of this filter (i.e. the greater the resistance, the greater the bandwidth). Band-Reject Filter This filter consists of a series LC and a parallel LC circuit, arranged as shown below.
In this arrangement, the impedance of the parallel LC circuit remains low, except at or near the resonant frequency, whereas the impedance of the series LC circuit remains high until this frequency band is reached. The resonant frequencies are thus bypassed and blocked from reaching the output. The number of circuit components and their resistance also determines the bandwidth of this filter (i.e. the greater the resistance, the greater the bandwidth).
10-6
Electrics
Resonant AC Circuits
Chapter 10
TUNING CIRCUITS A filter may be used as a tuning circuit if either a variable capacitor or inductor is used. A typical circuit is shown below.
In this circuit, a variable capacitor is used with a fixed resistor. In other circuits, a fixed capacitor is used with a variable inductor, which is altered using a moveable core. Tuning circuits usually have a high selectivity and only allow a narrow band of frequencies to pass, whilst rejecting all others. During its operation, radio signals cut across the antenna and induce signals (currents) of various frequencies to pass through the primary (P) winding of the antenna coil to earth. The resulting electromagnetic waves induce an EMF in the secondary (S) winding of the antenna coil and the variable capacitor (C). When the resonant frequency of the coil is reached, a maximum voltage is developed across the capacitor, and a maximum voltage is applied to the emitter-base of the transistor. This voltage is the input signal to the transistor, which in turn amplifies the relatively weak signal being passed to the tuner. In other cases, a series-resonant circuit is used in the primary circuit, which only allows maximum current to flow in this section at the resonant frequency. This prevents unwanted frequencies from being induced in the secondary winding and increases the system’s selectivity.
Electrics
10-7
Chapter 10
10-8
Resonant AC Circuits
Electrics
INTRODUCTION Transformers are extremely versatile devices that can be used to either step up and step down AC voltages or to step up and step down AC current. They can also allow AC to pass and block DC.
TRANSFORMERS CONSTRUCTION AND OPERATION The most common type of transformer is the voltage transformer, which consists of two windings, the primary winding and the secondary winding. The windings are not electrically connected together, which is a safety feature in AC electrical circuits, but are wound on the same laminated soft iron core.
If an AC voltage is applied to the primary winding, the resultant changing flux links with the secondary winding. The changing flux is concentrated by the iron core and causes an EMF to be induced in the secondary winding. The magnitude of the EMF is proportional to the ratio of the number of turns between the primary and secondary windings. Turns Ratio = Where:-
Electrics
NP V = P NS VS
Vp = Primary voltage Vs = Secondary voltage Np = Primary turns Ns = Secondary turns
11-1
Chapter 11
Transformers
Voltage transformers are categorised depending on the ratio of the turns and are represented by the following symbols.
If there are more turns on the secondary than the primary, it is a step-up transformer. If there are more turns on the primary than the secondary, it is a step-down transformer. Transformers are also extremely efficient (i.e. the amount of power in is approximately equal to the amount of power out), and they are rated in volt-amperes (VA). The following relationship exists between the turns ratio, voltage, and current. I N VP = P = S VS NS IP
where IS = Secondary Current IP = Primary Current If the voltage is stepped up, the current is stepped down. For example, if a transformer has a turns ratio of 1:2, and inputs of 240 V and 5 amps, the outputs will be, respectively: NS VS V = N P P
VS = 2 x 240 = 480 volts 1 IS N = P IP NS
IS = 1 x 5 = 2.5 amps 2 Transformers also consist of inductive components, so it is important that they are operated at their correct frequency and voltage. Any under-frequency condition results in the primary current increasing and the transformer overheating.
11-2
Electrics
Transformers
Chapter 11
TYPES OF TRANSFORMERS In addition to voltage transformers, the following types of transformers also exist: Three-phase transformers (isolation transformers).are widely used on aircraft and consist of three individual isolation transformers, where the primary windings are connected together across a three-phase AC supply, as shown below.
STAR
DELTA
The secondary windings are also connected together. They produce a three-phase output voltage of a value dependent on the supply and the turn’s ratio between the three corresponding winding pairs, which are normally the same. The primary and secondary windings can be alternatively connected in a delta-star configuration, as shown in the following diagram, or connected in starstar or delta-delta. This is dependent on the transformer's particular application.
DELTA
STAR
Auto transformers. are a special type, since they have no electrical isolation between the primary and secondary windings. A single continuous winding is wound on a laminated iron core, where part of the winding is used as the primary, whilst the other part is used as the secondary, as shown below.
These transformers can be used to either step-up or step-down the applied voltage, depending on the winding configuration. In a step-down device, the whole of the winding serves as the primary winding, whilst the lower half of the winding serves as the secondary winding. In this case, there are fewer turns in the secondary than in the primary; so the voltage is stepped-down, but the current is stepped-up. This configuration is typically used to power aircraft instruments where the voltage is stepped-down from 115 V 400 Hz to 26 VAC. The disadvantage of this format is that the full voltage is placed across the load if the coil goes open circuit, since there is no voltage isolation between the two windings.
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Conversely, in a step-up auto transformer, the lower half of the coil is used as the primary, and the entire coil is used as the secondary. In this case, the secondary has more turns than the primary, so the transformer steps-up the voltage and steps-down the current. On aircraft, this arrangement is typically used in windshield anti-icing systems. If the output from the auto transformer can be varied via a moveable tapping, as shown below, it is also known as a variac and is typically used on the flight deck to control the intensity of ultraviolet lighting.
Current transformers differ from the voltage transformer, because the primary circuit consists of a supply feeder cable rather than a winding connected across a supply, as shown below.
CURRENT TRANSFORMER (SECONDARY)
CONDUCTOR (PRIMARY)
In this arrangement, the alternating magnetic field associated with the load current is linked to the current transformer secondary winding via a laminated soft iron core, through which the feeder (primary) passes. The secondary current is used to feed a meter and typically registers the current flowing from an AC generator to the busbar or load. The secondary current can additionally be used to supply power meters and to monitor the load-sharing in an electrical circuit. In AC power generation systems, this type of transformer can also be used as a sensor in a differential protection circuit, as shown on the next page.
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G
This system protects against line-to-line and line-to-earth short circuits on the feeder lines between the generator and the generator circuit breaker (GCB). Doughnut current transformers are placed around the feeder lines and secondary windings of each pair in series opposition to ensure that the full output from the generator passes to the load. Under no fault conditions, the currents at each end of the feeder lines are equal, so the induced EMF is in balance and no current flows to the differential protection relay. If a difference in current of 30-40 amps exists, a signal flows to the protection relay, which instantaneously trips the generator control relay (GCR) and the GCB, thus automatically disconnecting the generator from the system.
TRANSFORMER RECTIFIER UNITS A transformer rectifier unit (TRU) is used to convert AC into relatively smooth DC An example of a simple TRU circuit is that which is used in a car battery charger, as shown below.
This device takes the mains 240 VAC and converts it to approximately 14 VDC to charge the battery. This is achieved by a transformer, which first steps down the AC voltage to a reasonable level and then converts it via a bridge rectifier assembly into DC.
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Transformers
Most large aircraft AC generator systems have dedicated TRUs, which operate on the same principle, although they are slightly more sophisticated. A typical unit is illustrated below.
The TRU that is fitted to an aircraft is typically supplied with 200 V 400 Hz three-phase AC, which is stepped-down through a three-phase star-star wound transformer and changed to 28 VDC by a six-rectifier bridge assembly. The output from the TRU is then fed to the aircraft’s DC busbars. Each TRU has the following basic protections:
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¾
Overheat When operating, most TRUs are cooled by air from a thermostatically controlled cooling fan. If the TRU overheats (150°-200°) due to fan or other failure, a warning light illuminates on the flight deck. The TRU should then be switched off, either manually or automatically.
¾
Reverse Current. When the TRUs are operating in parallel with some other power source, the failure of a rectifier in a TRU can cause a reverse current to flow into it and may even cause a fire. Reverse current protection in the failed TRU is designed to sense the fault current when it reaches approximately 1 amp, and disconnect the TRU automatically from the DC bus bars.
Electrics
INTRODUCTION The majority of large modern aircraft now employ three-phase AC generators, because they are more efficient than their DC equivalents. The most powerful of these are called three-phase machines. The following explanation of three-phase circuits is based on a simple three-phase generator.
POWER GENERATION SIMPLE THREE PHASE GENERATOR A three-phase generator consists of two main parts, as shown below: (RED)
(YELLOW) (BLUE)
The rotor carries the electromagnetic field that is driven by the aircraft engine, whilst the stator carries three sets (pairs) of coils (phase windings). These windings are fixed to one another at angles of 120°, and the phases are AA1, BB1, and CC1 or coloured Red (RR1), Yellow (YY1), and Blue (BB1) respectively, where the A or Red phase is classified as the reference phase. As the rotor rotates, it induces an EMF in each set of windings in turn and produces a sine wave output from each, as shown in the diagram on the next page.
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At any instant, the sum of the EMFs or the currents in a balanced system will add up to zero. These windings supply the output of the generator and are connected in either a Star or Delta configuration, as shown below. STAR
OUTPUT
Most aircraft similarly use three-phase AC motors with delta or star-wound stators.
STAR CONNECTION In the star configuration, one end of each phase winding is connected to a common point called the Neutral (N) or Star Point, whilst the other end of each phase winding is connected to output terminals distributing AC power of different phases.
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In this configuration, the output voltages and currents are respectively: Phase Voltage (VP)
=
3 x Line Voltage (VL)
Line Current (IL)
= Phase Current (IP)
On most aircraft generators, the output voltages are: ¾ ¾
Phase Voltage Line Voltage
= 115 V = 200 V
The vast majority of aircraft AC generators are connected in the star configuration with the neutral point (N) connected directly to earth, which allows: ¾ ¾
The generator to feed unbalanced loads Easy access to the phase voltages
When a three-phase star-connected generator is feeding a balanced load (ABC phases feeding the same current), the net current of all three phases is zero. In this case, no current flows in the neutral line. When unbalanced currents feed the load, the resultant of these currents will flow in the neutral line. If the currents being used are always balanced, there is no need for a neutral point to be fitted. On aircraft, although desirable, it is not practical for the generator to feed balanced loads all of the time, so it is necessary on most generators to connect the neutral point to earth.
DELTA CONNECTION In the delta configuration, the phases are connected in a triangular (delta) format with no common or neutral point.
Unlike the star connection, the phase and line voltages in the delta connection are the same: Line Voltage (VL) = Phase Voltage (VP) However, the line and phase currents differ: Line Current (IL) =
Electrics
3 x Phase Current (IP)
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ADVANTAGES OF THREE-PHASE OVER SINGLE-PHASE AC GENERATORS Three-phase AC generators are preferable to single-phase machines for the following reasons: ¾
Less conductor weight is required for the transmission of a given power.
¾
They can produce a rotating magnetic field, which can be used to operate efficient three-phase AC motors.
¾
Three-phase AC gives smoother rectification than single phase AC
VOLTAGE AND FREQUENCY OF AC GENERATORS Adjusting the field excitation of an AC generator using a voltage regulator controls its voltage output. The output frequency of an AC generator is alternatively dependent on the rotational speed of the rotor and the number of magnetic field poles, as shown in the following formula. Frequency (f) = NP / 60 where:
N = Rotational Speed (rpm) P = Number of pole pairs on the rotor
PHASE ROTATION Three-phase power supplies in an aircraft power system must have a positive phase sequence (i.e. A.B.C, B.C.A, or C.A.B). If any of the phases are crossed over (i.e. A.C.B, C.B.A, or B.A.C), a negative phase sequence would exist and result in the three-phase motor running in the wrong direction.
FAULTS ON THREE-PHASE AC GENERATORS The two main faults that can occur in the output phases and lines of an AC generator are earth and open circuits. The diagram below shows how these faults would affect a star-connected generator.
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The diagram below shows how these faults would affect a delta-connected generator.
GENERATOR REAL AND REACTIVE LOAD SHARING AC loads consume apparent power, which is measured in volt-amperes (VA), and so most AC machines are also rated in VA. On many large aircraft with 3 or 4 engines, the generators are normally run in parallel, and must share the apparent power in terms of true power (watts) and reactive power (volt-amperes reactive, VAR). WATT/VAR meters, as shown below, are fitted to each generator system and allow the flight crew to check that the load sharing between the generators is equal.
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TYPES OF AC GENERATOR The basic types of aircraft AC generators are: Salient-Pole Three-Phase AC Generators are Frequency Wild or Brushed generators, which are mainly used on aircraft with turbo-propeller engines (e.g. F-27) and generate frequency wild AC power. They consist of a rotor with electromagnets fitted to each salient pole, which alternate in polarity around the circumference of the rotor and rotate inside a fixed three-phase stator, as shown in the following diagram.
The outer shell of the machine holds the stator that consists of three fixed star-connected windings, and the generator is cooled by ram air.
QUILL DRIVE
COOLING AIR INLET
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A typical three-phase brushed AC generator, as shown above, would be rated at 22 KVA with an output of 208 V and would supply a full load at this voltage through a frequency range of 280 - 400 Hz. The generator frequency and output voltage vary with rotational speed, so this type cannot be used to operate circuits containing inductive and capacitive components. This type of generator can thus only be used to operate purely resistive circuits, such as the propeller de-icing system on turbo propeller aircraft (e.g. the F27). During its operation, some of the AC output is fed back to the voltage regulator via a three-phase full rectifier pack, which provides a medium to low DC voltage and self excitation of the generator, as shown in the following diagram, although the majority is passed directly to the main AC busbars.
The voltage regulator senses the output from the generator and automatically adjusts the excitation field for varying engine speed and load conditions. The battery is thus no longer required and is manually disconnected from the circuit via the control switch. A temperature sensor and a quill drive protect this type of machine. If the generator overheats, it should be off-loaded, or even switched off and allowed to cool. The quill drive connects the generator to the engine and is designed to shear if the generator seizes, protecting the engine. It is also designed to absorb any mechanical vibrations and produce a smoother output.
BRUSHLESS THREE-PHASE AC GENERATOR This is a highly sophisticated machine and is used on large jet aircraft for generating constant frequency supplies. The brushless generator has the advantage over the brushed type, since it requires less maintenance and is more reliable. It is driven by the aircraft engine via a Constant Speed Drive Unit (CSDU), which maintains a constant generator speed for varying engine speeds and produces a constant frequency output of 400 Hz.
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This type of generator comprises three individual parts as shown in the following diagram. A permanent magnet generator (PMG) initially induces a single-phase AC voltage into the pilot exciter when the rotor is driven via the CSDU. The AC voltage is then full-wave rectified and fed to the main exciter by way of the voltage regulator. As the three-phase windings, which are mounted on a common drive shaft, are rotated within the field, a three-phase AC voltage is induced in the windings. The output is then rectified via a three-phase bridge rectifier circuit, which consists of six diodes that are mounted inside the drive shaft and produces the main DC field. The temperature of the diodes is carefully controlled by ram air-cooling, which is directed down the centre of the shaft. The field coil is also fixed to the common drive shaft .As it rotates, it induces a voltage in the AC output windings. Some of the output is fed back to the voltage regulator and increases the output from the pilot exciter, which in turn increases the output from the main exciter. This sequence of events continues until the generator reaches its regulated AC output line voltage of 200 V and phase voltage of 115 V at 400 Hz.
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CONSTANT SPEED DRIVE UNIT The Constant Speed Drive Unit (CSDU) is a mainly mechanical device, which is positioned between the aircraft engine and the brushless AC generator. On older aircraft, the CSDU and generator are normally separate items, as shown below, and the generator is air-cooled.
The CSDU is designed to keep the generator running at a constant speed, which is usually 8000 rpm for varying engine speeds, and gives a constant output frequency of 400 Hz. One particular type of device is mechanically/hydraulically driven and consists of a self-contained oil system, as shown in the following diagram. A pump assembly provides high-pressure oil, which controls the pumping action of a pump/motor assembly via a centrifugal governor.
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The governor is a mechanical device and is not sensitive enough to give the fine speed trimming required to control the frequency within close limits, 395 - 425 Hz. To achieve the required trim, an electromagnetic coil receives signals from the electrical system load controller and modifies the position of the flyweights in the governor.
SERVO PISTON
The CSDU cylinder block is mechanically linked to the engine drive and as it rotates, the end of the pump pistons stroke against a stationary inclined pump wobbler (swash) plate, as shown below, thus producing a pumping action.
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The angle or inclination of this plate is controlled by a mechanical governor, which varies the hydraulic pressure to the two sides of a piston inside a control cylinder (servo mechanism). As the block rotates, the end of the motor pistons also stroke against an inclined fixed angle motor wobbler (swash) plate assembly, where an eccentric centre plate is sandwiched between two stationary plates. The centre plate is coupled to an output shaft, which drives the generator, and is free to rotate against ball bearings. The pressure exerted on the motor pistons by the pump determines the rotational speed of the centre plate. The higher the pressure, the faster it rotates. A typical analogy of this is a piece of soap on the side of the bath, where the harder it is squeezed, the faster it tends to move away.
OPERATION OF THE HYDRO-MECHANICAL CSDU If the throttle setting is decreased, the engine speed similarly decreases, thus rotating the casing of the CSDU slower and decreasing the pumping action of the hydraulic pump. The engine output speed is now slower than the required generator speed and an over-drive condition exists. The governor senses this, and the angle of the swash plate is increased. Oil is directed to the lefthand side of the piston via the over-drive inlet port, thus increasing the stroke of the pistons. This increases the output pressure from the pump and forces the motor pistons to exert more force on the downhill side of the motor-wobbler assembly. This causes the centre plate to rotate faster than the cylinder block, thus maintaining a constant generator speed. Conversely, if the throttle setting is increased the engine speed similarly increases, thus rotating the casing of the CSDU faster and increasing the pumping action of the hydraulic pump. The engine output is now faster than the required generator speed, and an under-drive condition exists. The governor senses this, and the angle of the swash plate is decreased. Oil is directed to the right-hand side of the piston via the under-drive inlet port, thus decreasing the stroke of the pistons. This decreases the output pressure from the pump and forces the motor pistons to exert less force on the downhill side of the motor wobbler assembly. This causes the centre plate to rotate slower than the cylinder block, thus maintaining a constant generator speed. When the engine output speed equals the required generator speed, the oil pressure and oil flow within the hydraulic system are such that the motor is hydraulically locked (i.e. the cylinder block is locked to the motor and both rotate together as a fixed coupling).
PROTECTION OF THE HYDRO-MECHANICAL CSDU To guard against mechanical failure, the oil pressure and temperature of the CSDU are monitored on the flight deck, as shown in the diagram on the next page.
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If the CSDU fails mechanically, it may cause an over-speed or under-speed (over frequency/under frequency) condition, and the reactive components in the aircraft could be severely damaged. Sensors are fitted to detect any speed change and automatically disconnect the generator from the busbar via the Generator Circuit Breaker (GCB). Conversely, if there is an indication of imminent failure, the CSDU disconnect switch can be manually selected by the flight crew. This operates a solenoid switch, as shown below, and allows the threaded pawl to engage with the coarse thread on the input shaft, separating the dog tooth clutch mechanism.
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This separates the drive between the engine and CSDU and allows the generator to run down. Once the CSDU has been disconnected, it cannot be reset until the aircraft is on the ground with its engine shut down, although the disconnect mechanism can be activated at any time. In order to prevent inadvertent CSDU disconnect, the switches are normally guarded and locked with thin copper wire.
INTEGRATED DRIVE GENERATOR On modern aircraft, the CSDU and generator are normally combined as one unit, which is known as an Integrated Drive Generator (IDG), as shown below, and the generator is alternatively oilcooled.
This is a much lighter and more compact unit.
VARIABLE SPEED CONSTANT FREQUENCY POWER SYSTEMS Variable Speed Constant Frequency (VSCF) systems are fitted to some commercial jet aircraft. In this system, no mechanical CSDU is fitted and the generator’s variable frequency output is converted into a constant frequency AC output of 400 Hz, via solid-state circuitry, as shown below.
VSCF systems are more reliable and offer greater flexibility than a typical CSDU and generator configuration. The generator is still driven directly from the aircraft engine, but the control units of the VSCF system can be mounted virtually anywhere in the aircraft, thus allowing for a more compact engine nacelle.
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AUXILIARY POWER UNIT The auxiliary power unit (APU) is a compact unit, as shown below, which is usually fitted in the tail section of an aircraft and provides electrical power (200 V, 3 Phase, 400 Hz) on the ground.
The APU can also be used to supply compressed air on the ground for engine starting and electrical power in flight during an emergency. Most APUs have their own dedicated 24-volt battery for starting or can alternatively be started from ground power. The main aircraft battery switch must be on to operate the APU control circuits. The APU can drive one or two generators, depending on the type of aircraft, and these are the same type as those fitted to the main engines. The APU does not require a CSDU to maintain a constant frequency output, since the drive from the APU runs at a constant speed via a governor, and can be used up to 25 000 ft.
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EMERGENCY RAM AIR TURBINE In the case of total main electrical AC failure, a Ram Air Turbine (RAT), as shown in the following diagram, can be extended automatically or manually into the airstream.
A variable pitch propeller drives a hydraulic pump, which in turn drives an AC generator at a constant speed and supplies 200 V, 3 Phase, 400 Hz for emergency loads. During the approach to landing, the RAT may become inefficient, so the aircraft batteries take over and supply the necessary loads during the final approach. The RAT can additionally only be restored on the ground and is also inhibited from deployment on the ground.
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Electrics
INTRODUCTION
AC supply systems vary in complexity depending on aircraft type and electrical requirements. There are two categories of AC systems commonly used dependent on whether the output frequency of the generator is controlled or not. They are known as frequency wild and constant frequency systems and are fully described below.
A TYPICAL FREQUENCY-WILD AC SYSTEM ARCHITECTURE In this system, the AC generators are fitted directly to each engine, and unless the engines run at a constant speed, the output frequency varies (frequency-wild).
The output from each generator is normally 200 V three-phase and varies in frequency between 280 and 540 Hz, which corresponds respectively to low and high engine rpm. The generators in this system should not be run in parallel under any circumstance, so their AC output is normally used to feed heating elements only. This is because the elements are purely resistive and are unaffected by changes in frequency. In some systems, part of the frequency-wild output is rectified in a transformer rectifier unit (TRU) and provides an alternative DC supply. The DC supplies may also be paralleled provided that the voltages are matched.
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OPERATION OF A TYPICAL FREQUENCY-WILD AC SYSTEM With the engine started and running, the generator is initially excited by a separate power source (i.e. the battery or ground power), as shown below.
Switching the generator control switch to RESET and thus closing the field relay achieves this. When the generator is producing an output, part of it is fed back through the voltage regulator and Bridge Rectifier Pack to provide the generator field, providing self-excitation. Once the generator is operating at its regulated output voltage of 200 V, the line-contactor closes, and the generator warning light goes out. Moving the control switch to the ON position subsequently deexcites the field relay and removes the source of the initial excitation current. The generator will now be fully self-excited, and the voltage regulator will continue to adjust the field excitation for varying speed load conditions.
FAULT PROTECTION IN A TYPICAL FREQUENCY-WILD AC SYSTEM The following fault protections exist in a twin-engine turbo-propeller frequency-wild AC system:
¾ Overheat
If the generator overheats due to inadequate cooling or overload, a warning light illuminates on the flight deck, and the generator should be manually switched off.
¾ Earth-Leakage
If there is low insulation in the alternator system or loads, a warning light illuminates. If this occurs, switch off the generator.
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¾ Under-Voltage
This fault normally uses the same warning light as that used to indicate an earth leakage fault. The system voltmeter is used to discriminate between an earth leakage fault and an under-voltage fault.
¾ Over-Voltage
If an over voltage occurs, a sensing circuit automatically de-excites the generator and removes it from the busbar. One attempt is usually allowed to reset the system by cycling the control switch between RESET and RUN.
¾ Differential Protection. This system is used to:
• • •
Monitor line-to-line faults Monitor line-to-earth faults Ensure that the output current flowing from the generator is the same as that flowing to the loads and returning to the generator
If one of the above faults exists, the generator is automatically de-excited and is removed from the busbar. One reset may be attempted, but even if the system resets satisfactorily for the rest of the flight, the fault must still be reported on landing.
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THE CONSTANT FREQUENCY SPLIT BUSBAR AC SYSTEM
The following electrical system is typically used on a twin-jet engine aircraft whose AC power supply is 200 V 400 Hz three-phase. APU
No. 1 GENERATOR
No. 1 AC BUSBAR
No. 2 GENERATOR
GROUND POWER
No. 2 AC BUSBAR BUS-TIE BREAKER NON-ESSENTIAL AC CONSUMERS
NON-ESSENTIAL AC CONSUMERS No. 1 TRU
No. 2 TRU
ESSENTIAL BUSBAR ESSENTIAL AC CONSUMERS
DC No. 1 DC BUSBAR
BATTERY BUSBAR VITAL DC CONSUMERS
AC STATIC IVERTER
BATTERY RELAY
ESSENTIAL DC CONSUMERS
No. 2 DC BUSBAR ISOLATION RELAY
NON-ESSENTIAL DC CONSUMERS
The power supply can be derived from four sources: two engine-driven integrated drive generators (IDGs), an Auxiliary Power Unit (APU), and an external power receptacle. These sources should never be paralleled at any time. Under normal operation, the generators independently feed the left and right section loads of the electrical system. The loads fed by these generators are normally indicated on ammeters fitted to each generator output. The APU is used to drive a third generator, which can supply the electrical power necessary for ground operations or act as a substitute for a failed engine-driven generator. External power can also be used instead of APU power on the ground, but not simultaneously.
OPERATION OF A CONSTANT FREQUENCY SPLIT BUSBAR AC SYSTEM The circuit above is shown in the power off condition. On most aircraft, the APU is started by an electrical starter, which is supplied from its own dedicated battery or from the aircraft battery. When the APU is up and running, the generator is selected by the APU generator circuit breaker (GCB) to feed No. 1 and No. 2 main AC busbars. The APU generator then supplies all of the aircraft AC requirements, and the Transformer Rectifier Units (TRUs) supply any DC requirements. If the No. 1 engine is initially started and run up, its dedicated IDG produces the correct output (200 V 400 Hz three-phase) and feeds the No. 1 main AC busbar. However, before it can supply this busbar, the APU power must be removed from the No. 1 main AC busbar by opening the appropriate GCB, followed by the closing of the No. 1 IDG GCB. The No. 1 IDG now feeds the No. 1 main AC busbar and the APU generator continues to feed the No. 2 main AC busbar. When the No. 2 engine is up and running, its IDG alternatively feeds the No. 2 main AC busbar. The APU generator supply must, however, first be removed from the No. 2 busbar before the IDG is allowed to feed it. At this point, the APU is no longer needed to feed the electrical system and is shut down. Both engine-driven IDG AC supplies now operate independently of each other and are kept separated by the Bus-Tie Breaker (BTB).
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If one IDG fails, the BTB between the two systems automatically closes and the serviceable generator feeds both of the main AC busbars. If the APU is started again, it substitutes for the failed generator and the BTB opens. The main aircraft DC supply is maintained by two TRUs (one for each IDG), as follows. ¾
The No. 1 TRU feeds the DC essential busbar
¾
The No. 2 TRU feeds the DC non- essential busbar
The TRUs are kept independent from each other by an isolation relay. If either TRU fails, the isolation relay between the two sides automatically closes, and the serviceable TRU feeds both busbars.
REGULATION AND PROTECTION OF CONSTANT FREQUENCY UNITS Most of these systems have separate or combined solid-state regulation and protection units dedicated to each generator. The regulator is divided into the following parts: ¾
A speed regulator, which senses the output speed or frequency of the IDG and adjusts the IDG to give a frequency output of between 380 and 420 Hz
¾
A voltage regulator, which regulates the output voltage to 200 V ± 5 V by adjusting the IDG's field excitation
A dedicated protection unit houses the circuitry, which detects any faults occurring up to, and including the busbars. Faults within this zone usually have time delays so that any faults occurring after the busbars have time to trip the circuit breakers or blow the fuses.
FAULTS ON A CONSTANT FREQUENCY SPLIT-BUSBAR AC GENERATOR SYSTEM Some faults in a split-busbar generator system cause the IDG to de-excite and its related GCB to open, removing the IDG from its own busbar. These faults are as follows: ¾
Over-Voltage If this type of fault is allowed to persist, it could cause serious damage to cable insulation and components.
¾
Differential Protection This type of protection monitors the following faults: • •
Electrics
A line-to-line or line-to-earth fault, which normally occurs inside the IDG If the current flowing to the busbar is different from the current flowing from the IDG
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Differential faults are detected by current transformers, which sense an imbalance in current between the generator and the busbar. If one of the above faults exists, the generator field is automatically de-excited and the generator removed from the busbar ¾
Over-Frequency If this fault is allowed to continue, it may damage any capacitive circuits due to high currents.
¾
Under-Frequency This fault causes high currents and the overheating of any inductive circuits.
¾
Resetting Many of the faults mentioned have a facility by which the system can be reset. One reset only is usually allowed (i.e. the system is cycled).
Other faults that might occur are:
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¾
Generator Overheat If the generator overheats due to frictional heating or inefficient cooling, an overheat warning is annunciated to the flight crew. If this occurs, switch off the system manually.
¾
IDG Disconnect (CSDU Disconnect) The oil pressure and oil temperature of the IDG is monitored. If the oil pressure drops during a fault, accompanied with an oil temperature rise, the flight crew may elect to operate the IDG disconnect. Once this has been initiated, the system can only be manually reset on the ground with the engine stopped.
¾
Generator Bearing Failure If an excessive clearance exists in the bearings of the engine or APU generators, a bearing failure warning light illuminates on the flight deck.
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EMERGENCY SUPPLIES In the unlikely event that both IDGs and the APU generator fail, AC can still be obtained from: ¾ ¾
The aircraft battery, which automatically feeds the AC essential busbar via a static inverter A Ram Air Turbine (RAT) can be automatically or manually dropped into the airstream to drive an AC generator, which produces a constant frequency output for the AC essential busbar.
If the emergency power supplies are selected, it is normal to shed any non-essential loads (e.g. galleys) in order to prevent overloading the remaining generators, which is known as Load Shedding.
BATTERY CHARGER Modern aircraft are fitted with battery chargers that are supplied from AC power supplies. These provide a DC supply to charge a battery in the shortest possible time, within certain voltage constraints, and without causing excessive gassing. The charger provides a DC current of 45-50 Amps until the charge reaches completion. It then reverts to the pulse mode to prevent the battery voltage from becoming excessive. Comprehensive protection circuitry is provided in the battery charger to give protection against: ¾ ¾ ¾
Over voltage Overheating Battery disconnection
If the battery over-volts, the battery charger is automatically switched off and can only be reset by a push-switch situated on the front of the battery charger. If the charger overheats, it is automatically shut down but resets itself when cooled. If the battery is disconnected, the charger cannot be switched on.
BATTERY POWER The batteries supply secondary DC power. On most aircraft, they also feed essential DC and, through a static inverter, essential AC for a period of 30 minutes or more. Some batteries are additionally fitted in non-pressurised areas in the fuselage and are provided with electrically heated blankets to prevent freezing.
GROUND HANDLING BUS The ground handling busbar is powered from either an APU generator or an external power unit. The busbar is powered automatically whenever external or APU power is available. This busbar is used mainly on the ground to power lights and the refuelling system.
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AC Power Generation Systems
CONSTANT FREQUENCY PARALLEL AC SYSTEM The constant frequency system is almost exclusive to three and four-engine jet aircraft. In older systems, the AC generator and the CSDU are separate items, but on modern aircraft, the two components are combined to form an IDG. In addition to the engine-driven generators, an APU drives a generator, which is capable of supplying the aircraft with power on the ground and at altitudes up to approximately 35 000 ft. The APU may, however, experience difficulties in starting at altitudes above 25 000 ft. Some aircraft also have emergency ram air turbines, which can be deployed in an emergency. The generators are fitted on each engine and are normally run in parallel. However, the system does have the following advantages and disadvantages over Split Busbar AC System: ¾
¾
Advantages When operating in parallel this system: ¾
Provides a continuity of electrical supply
¾
Prolongs the generator life expectancy, since each generator is normally run on part load
¾
Readily absorbs large transient loads
Disadvantages The disadvantages of the system are: ¾
Expensive protection circuitry is required since any single fault may propagate through the complete system.
¾
Parallel operation does not meet the requirements for totally independent supplies.
On most aircraft, only the engine-driven generators can normally be paralleled. The APU or the ground power unit cannot be paralleled with the engine-driven generators or each other. Circuit interlocks prevent this from occurring in the case of incorrect system management.
OPERATION OF A CONSTANT FREQUENCY PARALLEL AC SYSTEM Once all of the above conditions have been satisfied, a ground power available light comes on. When GROUND POWER is selected, the ground power breaker (GPB) closes and allows the ground power to feed the generator busbars. With the No. 1 engine running, its generator is excited when the generator control relay (GCR) is closed, which enables the generator to give an output (200 V three-phase 400 Hz). On closing the generator switch, the external services breaker (ESB) opens, removing ground power. The No. 1 generator circuit breaker closes. This allows the No. 1 generator to supply the necessary aircraft power.
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AC Power Generation Systems
Chapter 13
With the No. 2 engine running and its generator producing the necessary output, it can be paralleled with the No. 1 generator via the synchronising busbars by closing the No. 2 generator's GCB. The following conditions, however, must exist before paralleling can take place between two generators: ¾
Voltages must be within tolerance.
¾
Frequencies must be within tolerance.
¾
Phase displacement must be within tolerance.
¾
Phase rotation must be correct.
Once all of the above conditions have been satisfied, selecting the No. 2 generator switch to ON causes the GCB to close and the No. 1 and No. 2 generators to run in parallel. Both generators must share the real (watts) and reactive (VAR) loads equally. These are monitored on individual generator Watts/VAR meters on the flight deck.
Electrics
13-9
Chapter 13
13-10
AC Power Generation Systems
Electrics
AC Power Generation Systems
Chapter 13
The No. 3 and No. 4 generators are paralleled using the same method as the No. 1 and No. 2. generators. When all of the generators are running, the No. 1 and No. 3 generators are kept separate from the No. 2 and No. 4 generators by a split system breaker (SSB). If any enginedriven generator fails, the SSB automatically closes.
REACTIVE LOAD SHARING Reactive load sharing is achieved by a load-sharing loop, which automatically adjusts the excitation of the paralleled generator fields simultaneously via their individual voltage regulators. A PHASE
A PHASE
A PHASE
A PHASE
GCB
GCB
GCB
GCB
BTB
BTB
BTB
BTB
VOLTAGE REGULATOR
VOLTAGE REGULATOR
VOLTAGE REGULATOR
VOLTAGE REGULATOR
GEN 1 EXCITER FIELD
GEN 2 EXCITER FIELD
GEN 3 EXCITER FIELD
GEN 4 EXCITER FIELD
REAL LOAD SHARING Real load sharing is achieved by a load-sharing loop, which adjusts the magnetic trim in the mechanical governor of the CSDUs simultaneously via their load controllers. GEN 1 A PHASE
Electrics
GEN 2 A PHASE
GEN 3 A PHASE
GEN 4 A PHASE
GCB
GCB
GCB
GCB
BTB
BTB
BTB
BTB
REAL LOAD CONTROLLER
REAL LOAD CONTROLLER
REAL LOAD CONTROLLER
REAL LOAD CONTROLLER
CSDU 1 MAGNETIC TRIM
CSDU 2 MAGNETIC TRIM
CSDU 3 MAGNETIC TRIM
CSDU 4 MAGNETIC TRIM
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Chapter 13
AC Power Generation Systems
PARALLELING The following methods are used to parallel AC generators: Manual Paralleling is an old method of paralleling generators. To facilitate this method, a lamp is fitted across the main contacts of the GCB. When both generators’ outputs are the same, the lamp will darken and go out. When this occurs, the engineer closes the oncoming generator’s control switch. This is known as the lamps dark method of paralleling. Automatic Paralleling When using the automatic paralleling method, the generator switch is selected to on at any time, and once the auto-paralleling circuits sense that both generators are ready for paralleling, the GCB automatically closes.
FAULT PROTECTIONS IN A CONSTANT FREQUENCY AC PARALLEL SYSTEM The following fault protections exist in a parallel generator system: Over-Excitation (Parallel Fault) protection devices operate whenever the excitation to the field of one of the generator increases. This is sensed when the over-excited generator takes more than its share of reactive load. The fault signal has an inverse time function that trips the BTB of the over-excited generator. The voltage regulator or reactive load-sharing circuit could cause this fault. Over-Voltage protection devices operate whenever the system voltage exceeds 225 V. They protect the components in the system from damage due to excessive voltages. This protection device operates on an inverse time function, which means that the magnitude of voltage determines the time in which the offending generator is de-energised by tripping the GCR and GCB. The GCR de-energises the field, and the GCB trips the generator off the busbar. Under-Excitation (Parallel Fault) protection devices operate whenever the excitation of one of the generator fields is reduced. This is sensed when the under-excited generator takes less than its share of reactive load, and a fault signal causes the BTB to trip in a fixed time (3-5 sec). This type of fault could be caused by a fault in the: ¾ ¾ ¾
Reactive load sharing circuit Generator Voltage regulator
Under-Voltage protection devices operate to prevent damage to equipment from high currents and losses in motor loads, which may cause over-heating and burn out. When this device operates, it trips the GCR and GCB in a fixed time (3-5 sec), resulting in the shut-down of that generator. Differential Protection devices operate in the same way as stated in the split-busbar generator system. They operate if any of the following faults exist: ¾ ¾
13-12
A line-to-line or line-to-earth fault If the current flowing to the busbar is different from the current flowing from the generator
Electrics
AC Power Generation Systems
Chapter 13
Instability Protection (Parallel Fault) devices are incorporated in the system to guard against oscillating outputs from the generators, which may cause sensitive equipment to malfunction or trip off. This especially applies to autopilot and radio installations. If the system is operating in parallel, and the No. 1 generator becomes unstable, the instability protection circuits in all generators sense this and trip all of the BTBs. This isolates the unstable generator from the other generators, and the instability protection device continues to operate, tripping its GCR and GCB. The generator, voltage regulator, or CSDU may cause instability. Negative Sequence Voltage Protection devices detect any line-to-line or line–to-earth faults after the differentially protected zone and cause all the BTBs to trip. Overheat warning lights illuminate if a temperature sensor fitted in the generator senses an overheat condition. This fault may be caused by overloading the generator on the ground (no ram air-cooling) or by a blockage in the ram air-cooling duct in flight. If this warning occurs, the pilot should operate the GCR switch, which will cause the GCR and GCB to trip. Over-speed (Over Frequency) devices operate if a fault occurs in the CSDU, which may cause the generator to exceed its specified frequency limits. If left unchecked, this fault damages the aircraft capacitive loads. In older systems, a pressure switch in the CSDU detects this type of fault, but in modern systems frequency sensitive circuits detect it. If an over-speed condition occurs, it causes the GCB to trip and puts the CSDU into underdrive. Under-speed (Under-Frequency) of the CSDU is sensed by an oil pressure switch in the CSDU. This causes the GCB to trip, removing the generator from the busbar, and protecting the loads from an under-frequency. Time delays are fitted in the generator protection system to give the normal circuit protection devices (i.e. circuit breakers and fuses) time to operate, rather than removing a generator from the system.
Electrics
13-13
Chapter 13
AC Power Generation Systems
DC POWER SUPPLIES Primary aircraft DC power supplies are derived from transformer rectifier units, which are supplied from the 200 V AC busbars. The TRUs are normally run in parallel, although some systems have isolation relays installed, which are designed to separate the DC busbars during fault conditions.
13-14
Electrics
INTRODUCTION Since AC motors rely on a constant frequency supply, they are mainly used on larger aircraft. Motors are generally classified as follows: Large motors have an output of 3 KW or more and are normally three-phase machines. Medium to small motors range from 3 KW down to 50 W and are mostly single-phase machines. Motors rated at less than 750 W are referred to as fractional horsepower (FHP) machines. Miniature motors are rated at less than 50 W and are used in instruments and servomechanisms. On aircraft, these motors are either induction or synchronous machines.
ALTERNATING CURRENT MOTORS STATOR-PRODUCED ROTATING MAGNETIC FIELD When a magnet is rotated within a three-phase stator, a three-phase voltage is produced. If this process is reversed (i.e. by connecting the three-phase supply to a three-phase stator), a rotating field is produced, as shown in the following diagram.
Electrics
14-1
Chapter 14
AC Motors
If the stator windings are symmetrically arranged, as shown on the previous page, the magnetic field produced is of constant strength and rotates at a uniform speed, which is dependent on the supply frequency. The magnetic field rotates through one complete revolution during each complete cycle of the AC supply. For example, if the supply has a frequency of 50 Hz, it produces a rotating field of 50 revolutions per second or 3000 (50 x 60) rpm. Every 60° one set of poles is inactive, and does not generate a magnetic field due to the distribution of the input currents, as shown above. However, the other two do produce magnetic fields of equal strength, and the resultant field acts in the direction of the arrow. If a rotor is then placed in the centre of the rotating magnetic field, a magnetic field is induced in it, which locks onto the rotating outer field and turns with it.
INDUCTION (SQUIRREL CAGE) MOTOR The induction motor is one of the most widely used types of AC motor, which is used on aircraft to drive fuel pumps, actuators, and air conditioning. The following diagram shows a typical machine.
The stator is almost identical to that of a three-phase AC generator. When a three-phase AC supply is connected to the stator, it produces a rotating magnetic field, whose speed (synchronous speed) is proportional to the frequency of the supply. The rotor consists of a cylindrical laminated-iron core having a number of copper or aluminium longitudinal bars, which are evenly spaced around its circumference. These bars are joined by end plates, and together form a squirrel cage rotor. 14-2
Electrics
AC Motors
Chapter 14
The rotating outer magnetic field cuts the stationary rotor and induces an EMF or voltage proportional to the rate of change of flux in the squirrel cage. The shorted bars offer little resistance and a large current flows in the bars, as shown below. The passage of current through the bars results in the production of a magnetic field, which in turn interacts with the outer rotating magnetic field.
A torque now exists between the rotor and the stator magnetic fields. This causes the rotor to turn and accelerate in the direction of the stator field, as shown below.
When the applied torque equals the load torque, the motor runs at a speed slightly less than the stator field. The induction motor is an asynchronous machine and possesses similar characteristics to that of a DC shunt-wound motor, as listed below: ¾
Slip speed is the difference between the rotor speed and the synchronous (stator) speed. Slip Speed = Synchronous Speed - Rotor Speed Synchronous Speed = 60 f P Where f = frequency of supply (Hz), and P = number of pole pairs in stator.
¾
Electrics
Reversal of rotation occurs if any two of the motor phases are crossed over.
14-3
Chapter 14
¾
AC Motors
Loss of a phase occurs when the machine is: ¾
Running The motor continues to run at a reduced torque.
¾
Not running The machine does not start, and fuses or circuit breakers blow in the other two phases, causing possible damage to the motor.
TWO-PHASE INDUCTION MOTOR A rotating magnetic field is produced in a two-phase induction motor stator by placing the windings 90° apart, as shown below.
One phase is the reference phase, and the other is the control phase. By varying the phasing and the amplitude of the control phase currents, the direction and speed of rotation can be controlled. This type of motor is, however, not as smooth nor as powerful as a three-phase machine and is used mainly for autopilot servomotors or fuel trim motors.
SPLIT-PHASE MOTOR This is a split-phase induction motor. Two windings; one capacitive and the other resistive, are both connected in parallel across a single-phase AC supply, as shown below.
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Electrics
AC Motors
Chapter 14
The current in the capacitive winding leads the current in the resistive winding by approximately 90° and is known as phase splitting. This type of motor operates like a two-phase AC motor and is used to drive actuators.
THE SYNCHRONOUS MOTOR The stator in this type of motor is identical to that used in an induction motor, except the rotor in this machine alternatively carries its own magnetic field windings, which are supplied from a DC source. When the rotor is energised with DC, it acts like a bar magnet, as shown below, and tries to line itself up with the magnetic field being produced by the stator. The stator is fed with threephase AC and produces a rotating magnetic field, which the rotor follows. This type of motor is a single speed machine, where the actual speed is determined by the speed of the rotating field (i.e. the frequency of the three-phase input). Due to the high inertia between the rotor and stator field, this type of motor does not normally start on its own accord. It has to be started and run up to speed by a pony motor, which is usually an induction motor. When the speed of the driven motor nearly reaches that of the rotating field, it locks on to it and continues to rotate in synchronism with the rotating field. Synchronous motors are used in situations where a constant speed is essential (e.g. gyroscopes).
Electrics
14-5
Chapter 14
14-6
AC Motors
Electrics
INTRODUCTION
Semiconductors are used extensively in most aircraft electronic equipment. The three most common devices are diodes, transistors, and integrated circuits.
SEMICONDUCTOR DEVICES ADVANTAGES AND DISADVANTAGES The advantages and disadvantages of semiconductor devices are: Advantages Components made from semiconductor materials are often referred to as solid-state components, because they are made from solid materials. Semiconductors have largely replaced vacuum tubes, which were made of glass and therefore very fragile, and which consumed large amounts of power, since they required heaters to operate them. Semiconductors are additionally much smaller, lighter, and are much cheaper than vacuum tubes. Disadvantages Semiconductors are highly susceptible to temperature changes, and are easily damaged by excessive heat. For optimal operation they require highly sophisticated temperature control. Solid-state devices are also damaged if supply voltage polarity is not correct.
CONSTRUCTION OF A SEMICONDUCTOR
Neutrons
A semiconductor is a material that, under certain conditions, can act as either a conductor or an insulator. Silicon (Si) and germanium (Ge) are both semiconductive elements, of which silicon is the most popular. Each atom of silicon has four electrons in the outer (valence) shell, as shown in the diagram. Semiconductors are electronically stable, however, doping creates a surplus or deficit of electrons which gives the specific characteristics of semiconductor devices.
Electrics
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Chapter 15
Semiconductor Devices
Single atoms of silicon are of little use, so they are grown into large crystals, which are then cut into wafers for the manufacture of electronic components. The silicon atoms link up with neighbouring atoms to share electrons. A cluster of silicon atoms sharing outer electrons forms a matrix called a Crystal, as shown below.
COVALENT BONDS
The four electrons in the outer shell of each atom are shared with the electrons from the adjoining atoms via Covalent Bonding, and result in the valence shell of each atom in the crystal effectively holding eight electrons. These bonds are so strong that at absolute zero temperature (-273°C), there are no free electrons, and the silicon crystal assumes the properties of an electrical insulator. If the crystal of silicon is subsequently heated or a voltage applied across it, the covalent bonds break down and its characteristics change. The electrons break away from the atom and leave behind a hole in the atom’s outer shell. The free electrons then travel through the silicon as negative electrical charges. As the electrons move from one atom to another, the holes appear as if they are moving from one atom to another in the opposite direction. The movement of holes and electrons forms the basis of a semiconductor.
DOPING Silicon in its pure state is not particularly useful in electronics, so doping is carried out, where the silicon atoms are contaminated with other materials such as phosphorous (P) or boron (B), to give them useful electronic properties. This contamination leaves the silicon atoms with incomplete outer valence shells and a hole is formed in the shell. The holes, which replace the missing electrons, act as positive charges and attract any free electrons within the crystal.
P-TYPE MATERIAL If silicon is doped with indium, it produces a P-type material. Indium atoms only have 3 electrons in their outer shells (trivalent) and are acceptor atoms. This results in vacant electron openings or holes, which are positively charged, being left in the silicon crystal, as shown below.
If a voltage is applied across P-type material, as shown below, the electrons within the crystal tend to move toward the positive terminal of the battery and jump into the available holes of the indium atom near the terminal.
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Semiconductor Devices
Chapter 15
An electron from an adjacent silicon atom then falls into the hole, and the hole appears to move to another location. The electrons move through the material from left to right, whilst the holes move in the opposite direction.
N-TYPE MATERIAL If silicon is doped with phosphorous, it produces an N-type material. Phosphorous atoms have 5 electrons in their outer shell (pentavalent) and are known as donor atoms. Extra electrons, which are negatively charged, are left floating around in the crystal, as shown below.
An N-type semiconductor contains many donor atoms that contribute free electrons, and these are free to drift through the material. The loss of an electron leaves the donor atoms with an overall positive charge and forms positive ions. Electrical current flows in the normal manner due to the movement of the free electrons. Like P-type silicon, it can also flow due to the migration of holes.
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Chapter 15
Semiconductor Devices
P-N JUNCTION DIODE Both P and N-type silicon conduct electricity at different rates, depending on the amount of doping. Both types function as resistors and conduct in both directions. The N-type material contains mobile electrons and an equal number of positive ions, which provide an overall neutral charge. The P-type material similarly contains mobile holes and an equal number of negative ions. Each part is initially neutral. If a junction is made by joining a piece of P and N-type material together, electrons will only flow in one direction through the junction, from N to P.
When the two materials are placed together, some of the free electrons in the N-type material cross the junction and fill the holes in the P-type material close to the junction. As the free electrons cross the junction, the N-type material becomes depleted of electrons near the junction and the holes in the P-type material become filled, depleting the holes near the junction. The region where the holes and electrons become depleted is known as the depletion layer. Depletion Layer
This leaves the N-type material with an excess of positive ions and the P-type material with an excess of negative ions near the junction. The material close to the junction is in a charged state. The N-side is positively charged and the P-side negatively charged, which is known as a diode. This is an electronic one-way valve and is represented by the symbol shown below.
The anode is the negative side of the diode, which is associated with the P-type material. The cathode is the positive side, which is associated with the N-type material. If voltages, known as bias voltages, are applied across a diode, it behaves differently depending on the polarity of the power source. When the positive terminal is connected to the N-type material, the diode is reverse biased and no current flows (i.e. it is in a non-conducting state), as shown in the following diagram. Electrics 15-4
Semiconductor Devices
If the negative terminal is connected current flows (i.e. it is in a conducting positive terminal attracts electrons in terminal similarly attracts the holes depletion layer, as shown below.
Chapter 15
to the N-type material, the diode is forward biased and state), as shown above. If the diode is reverse biased, the the N-type material away from the junction. The negative in the P-type material, increasing the thickness of the
If the diode is forward biased, electrons are attracted from the N-type material across the depletion layer to the positive terminal and the holes are attracted to the negative terminal, as shown below.
A forward biased diode acts as a closed switch and a reverse biased diode as an open switch.
Electrics
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Chapter 15
Semiconductor Devices
USE OF DIODES Diodes in their basic forms are used for rectification (or conversion) of AC into DC, for example, in a battery charger circuit, as shown below.
4 3
1 2
The diodes offer an easy path for currents to flow in one direction and offer a high resistance path in the opposite direction. During the positive cycle (1), current flows through diodes 1 and 3, whilst diodes 2 and 4 are switched off. The reverse occurs during the negative cycle, producing a DC output. The following special types of diode exist:
ZENER DIODE This is a special type of diode, which consists of a reverse-biased silicon P-N junction and is represented by the following symbol.
This type of diode is designed to operate normally when it is forward-biased, but unlike a conventional diode, it will also operate when high reverse currents are applied. When the reversebias voltage reaches a set value, the Zener diode will break down, and thermal avalanche occurs. When this happens, one electron gains sufficient energy to knock others out of the valence band, causing a rapid increase in current flow through the diode, as shown in the following diagram. This typically occurs from 4 to 75 V, depending on the design.
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Semiconductor Devices
Chapter 15
Zener diodes are used to provide a fixed reference voltage over a range of input voltages and to precisely regulate or stabilise the output from a power supply, as shown below.
VARIABLE CAPACITANCE (VARICAP) DIODE In this type of diode, the depletion layer situated between the P-N junction acts like the dielectric in a capacitor, whilst the P and N materials act as its plates. When the diode is reverse-biased, the depletion layer widens and gives the effect that the plates of the capacitor have moved further apart, reducing the capacitance value. Conversely, if the reverse bias voltage reduces, the capacitance value increases. It is possible to vary the capacitance of this diode simply by altering the magnitude of the reverse bias voltage. This is the method commonly used in radio tuners using DC, rather than using a mechanical variable capacitor. A variable capacitance diode is represented by the following symbol.
BI-POLAR TRANSISTORS Transistors are made up of a sandwich of P and N-type materials. They can be used as relays, switches, or variable resistors. The two configurations of bi-polar transistors are PNP and NPN, as shown below.
Electrics
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Chapter 15
Semiconductor Devices
The three layers of a bi-polar transistor are the emitter, base, and collector, where the arrowhead depicts the flow of conventional current. The base is extremely thin and has fewer doping atoms than the emitter and collector. A very small voltage or current applied to the material in the centre of the sandwich (base) can control a much larger current flowing through the complete device, allowing a transistor to act as an amplifier.
OPERATION OF A PNP BI-POLAR TRANSISTOR If the transistor is reverse biased by connecting across two power sources, the positive terminals of each attract electrons in the N-type material away from the P-N junction. The negative terminals similarly attract the holes in the P-type material, as shown below.
This increases the thickness of the depletion layer between the different layers and the transistor does not conduct. For the transistor to operate, the emitter-base junction has to be forward biased, whilst the collector-base is reverse biased, as shown in the following diagram.
The positive junction of the emitter battery (Ve) repels the holes in the P-type emitter toward the P-N or emitter-base junction and crosses through into the lightly doped N-type base. The majority of the holes (approximately 95%) do not combine with electrons in this region and pass directly to the P-type collector. The holes are then rapidly neutralised with electrons from the negative terminal of the collector battery and are swept away from the collector. For each hole, which is neutralised by an electron, a covalent bond near the emitter electrode breaks down, and an electron is released to the positive terminal of the emitter battery. This in turn produces a hole, which quickly moves through the material from left to right. A small number of holes (approximately 5%) also combine with electrons in the N-type base material and are lost. The major charge carriers in a PNP bi-polar transistor are the holes. A very small emitter-base current (Ib) causes a large emitter (Ie) to collector (Ic) current to flow, but in all cases of operation:
Ie = Ib + Ic 15-8
Electrics
Semiconductor Devices
Chapter 15
OPERATION OF A NPN BI-POLAR TRANSISTOR An NPN transistor conducts like the PNP transistor with the emitter-base junction forward biased and the base-collector junction reverse biased. This is achieved by reversing the battery polarity, as shown below.
Electrons are repelled from the negative terminal of the emitter battery (Ve) and flow toward the positive terminal of the collector battery (VC). The electrons are forced into the emitter junction. Since the P-region base is only lightly doped, the majority of the electrons (approximately 95%) diffuse through the base and reach the collector junction. A small amount of the electrons (approximately 5%) combine with the holes in the P layer and are lost as charge carriers. For every electron that leaves the collector, one electron enters the emitter junction, maintaining a continuous flow of electrons from left to right through the transistor. The major charge carriers in an NPN junction transistor are, therefore, the electrons.
DISADVANTAGES OF DIODES AND TRANSISTORS Diodes and transistors share several key features (e.g. too much current causes a transistor to become hot and burn out, like a diode). This is because semiconductors have a negative temperature coefficient and can go into thermal avalanche (i.e. one electron gains enough energy to knock others out of the valence band), causing an increase in current flow through the transistor. If a transistor overheats, it does not operate properly. Engineers sometimes use a freezing spray to locate a failing component in a circuit. If the PNP transistor is to conduct, the emitter must be connected to a positive voltage and the collector to a negative voltage. If the base is connected to a voltage that is more positive than the emitter, a small current flows into the base. The flow of current then causes a large current to flow between the other two connections (emitter and collector).
Electrics
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Chapter 15
Semiconductor Devices
TRANSISTOR APPLICATIONS If the base of an NPN transistor is earthed (0 V), no current flows from the emitter to the collector, and the transistor is switched off. The transistor operates as an open switch, as shown below.
If the base emitter’s forward bias voltage is gradually increased, the emitter collector current, which is much higher, follows the same variation as the smaller base current, and the transistor acts as an amplifier. This explanation applies to a transistor in which the emitter is the common connection for both input and output, which is known as a common emitter. Transistors can also be used in either the common base mode or the common collector mode.
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Semiconductor Devices
Chapter 15
INTEGRATED CIRCUITS Integrated Circuits (ICs) are manufactured by combining transistors, diodes, and resistors on a small piece of silicon. The complete device is known as a chip and can contain a few or many thousands of transistors.
THE ADVANTAGES AND DISADVANTAGES OF INTEGRATED CIRCUITS The advantages of ICs are that they: ¾
Are extremely small and light
¾
Consume little power
¾
Can operate at high speed
¾
Are extremely reliable
The disadvantages of ICs are that they: ¾
Are easily damaged by high voltages or currents
¾
Cannot be repaired
The advantages, however, outweigh the disadvantages and IC chips are extensively used in the aviation industry.
TYPES OF INTEGRATED CIRCUITS ICs are grouped into the following categories: Analogue (or Linear) ICs are typically used in the manufacture of amplifiers, timers, oscillators, and voltage regulators. They amplify or respond to variable voltages and produce outputs. Digital (or Logic) ICs are typically used in the manufacture of microprocessors and computer memories. They normally respond to two discrete voltage levels (or gates) representing ones or zeros, and act as electronic switches to produce outputs.
Electrics
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Chapter 15
15-12
Semiconductor Devices
Electrics
INTRODUCTION Logic gates are represented diagrammatically and their logic inputs are shown on a truth table. Logic gates may also have more than two inputs, which increases the decision making capability of a gate and also increases the number of ways of connecting one to another to form advanced digital logic circuits.
LOGIC CIRCUITS NUMBER SYSTEMS The decimal number system requires ten different numbers (0-9) and ten discrete voltage levels. It then repeats itself by going into 10s, 100s, and 1000s, etc. This system can be typically used to represent the position or groundspeed of an aircraft. The binary number system uses numbers that are to the base of two, as shown below. 2
6
64
2
5
32
4
2
16
8
2
3
2 4
2
2 2
1
2 1
0
Binary Number Decimal Equivalent
In digital electronic applications, binary numbers are used as codes to represent decimal numbers, letters of the alphabet, voltages, and many other forms of information. For example, a simple switch can be assigned a binary value 0 to the OFF position and a binary 1 to the ON position. Alternatively, the polarity of a DC switching circuit can be altered so that a (+) indicates a binary 1 and a (–) indicates a binary 0. An alternative method is to vary the mean voltage in a circuit, causing it to increase by a pre-set increment for a binary 1 and to decrease by a similar increment to achieve a binary 0. The latter method is the most common. The voltages used for this purpose vary between manufacturers but are normally in the range from +5 V to +12 V. They are also designed to use either positive or negative logic. Positive logic is where a Logic 1 voltage is more positive than a Logic 0 voltage, and negative logic is where a Logic 1 is more negative than Logic 0. Other possible numbering systems are the: ¾ ¾ ¾
Electrics
Octal system, in which the numbers are to the base 8 Hexadecimal system, in which the numbers are to the base 16 Duodecimal system, which is based on the figure 12 (e.g. the clock, and is used on a daily basis)
16-1
Chapter 16
Logic Circuits
BINARY REPRESENTATION Digital computers are electronic units, and in electronics it is a relatively easy procedure to operate circuits in such way as to encode them in a binary format.
BASIC LOGIC GATES The following basic gates exist: AND Gate This type of gate is represented by two switches connected in series and requires two Logic 1s (A & B) to produce an output (Q), as shown below.
OR Gate This type of gate is represented by two switches connected in parallel and requires only one Logic 1 (A or B) to produce an output (Q), as shown below.
NOT Gate A single switch represents this type of gate where the input signal (A) is inverted to provide an output (Q), as shown in the following diagram.
16-2
Electrics
Logic Circuits
Chapter 16
NAND (Not or Negated AND) Gate This type of gate is represented by two switches connected in parallel and requires only one Logic 0 (A or B) to produce an output (Q), as shown below.
NOR (Not or Negated OR) Gate This type of gate is represented by two switches connected in series and requires two Logic 0s (A or B) to produce an output (Q), as shown below.
Electrics
16-3
Chapter 16
Logic Circuits
EXCLUSIVE OR Gate This type of gate is a combination of NOT and NAND gates and requires only one Logic 1 (A or B) to produce an output (Q), as shown below.
ADDER AND SUBTRACTER CIRCUITS Adder circuits are used to add binary digits (1s and 0s) together and subtracter circuits are used to subtract binary digits. These circuits are used in computer systems to carry out basic arithmetic functions. When carrying out addition functions, it is always necessary to carry a digit to the next adjacent higher order (e.g. 011 + 100 = 111 or in decimal terms 3 + 4 = 7). Conversely, in a subtracter circuit, it is necessary to borrow a digit from the next adjacent lower order column (if applicable) (e.g. 111 – 011 = 100 or in decimal terms 7 – 3 = 4). A half adder circuit is capable of adding two digits but is unable to carry a digit to the next order. It is necessary to join two half adder circuits together to form a full adder circuit in order to satisfy this requirement. A half adder electronic circuit consists of a combination of AND, OR, and EXCLUSIVE OR gates, as shown below.
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Two Stage Adder Circuit A two stage adder electronic circuit similarly consists of a combination of AND, OR, and EXCLUSIVE OR gates, as shown below.
AB + CD
Example The following table can be established using the above circuit by inputting a series of 0s and 1s.
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A (21)
B (20)
0 0 1 1 1 1
1 1 0 0 1 1
+
C (21)
D (20)
0 0 0 1 1 1
0 1 1 0 0 1
= C12
(2 )
S1 (21)
S0 (20)
0 0 0 1 1 1
0 1 1 0 0 1
1 0 1 0 1 0
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DIGITAL LATCH AND FLIP-FLOP CIRCUITS These circuits both use a combination of logic gates to perform basic memory functions for computers and their peripherals. A typical latch circuit is the RS latch circuit, as shown below, which retains the output signal even after the input signal has been removed. A
Logic 0
B
Logic 1
On initial power up with a positive supply at ‘S’ (Set), output ‘A’ would be at logic 0 and output ‘B’ would be at logic 1. If the positive input is switched from ‘S’ to ‘R’ (Reset), the logic states at ‘A’ and ‘B’ would reverse. A flip-flop circuit is similar to a Latch circuit, although the output is changed if a trigger pulse is applied to the circuit, as shown below.
A
Clock Pulse CP B
This circuit has three inputs and two outputs, with the S and R inputs identical to the Latch Circuit. The circuit switch time is controlled by inputting a Clock Pulse (CP), which simultaneously changes over the output signals, ‘A’ and ‘B’ at a specified time interval. This arrangement is particularly useful in computers when several memory circuits are being used simultaneously, since if the outputs are changed out of sequence, the entire memory may become invalid.
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INTRODUCTION The modern aircraft is highly dependent on the digital computer, which governs almost every facet of its operation.
COMPUTERS ANALOGUE COMPUTERS Analogue computers are non-programmable and deal with infinite continuous values rather than discrete ones. They use digits from 0 to 9 and operate as a mechanical computer using a rotating gear or wheel to represent different values (e.g. if the wheel is between 0° and 10°, it represents 0 or between 11° and 20°, it represents 1). The analogue computer suffers from friction between the moving parts and mechanical wear. The speedometer in a car is an everyday example of an analogue computer. It is attached to a sensor that counts the revolutions of the road wheels and, using an assumed wheel radius, calculates the distance covered since the last reset. It adds this to the distance at the start of the run and indicates the total distance the car has covered since the beginning. It also uses the distance per unit time to provide an indication of speed. The speedometer is a calculating machine, which uses a data input, and by carrying out a calculation it converts the input into another form of information; speed via a moving needle and distance as a digital read out. Analogue computers are still widely and effectively used, although they suffer from the following limitations and shortcomings: ¾ ¾ ¾
They are specific to a particular role and a separate computer is required for different applications. They use moving parts. They tend to be bulky and heavy.
DIGITAL COMPUTERS A Digital computer is also a calculating machine, but instead of using synchro and gears, different voltages are used to represent the digits from 0 to 9. For example, 0 – 0.9 V would represent the digit 0 and a voltage from 1.0 – 1.9 V would represent the digit 1, etc. This machine uses actual high-speed arithmetic to do the necessary calculations typically using a decimal number system. It is also possible to convert decimal values into digital values or to convert analogue values into binary code. Everything that a digital computer does is based on one operation, which is represented by the ability to determine if a switch or gate is open or closed. That is, the computer can recognise only two states in any of its microscopic circuits (i.e. an on/off, high voltage or low voltage, or in the case of numbers, 0 or 1). It is equally valid to reverse the process and produce an analogue value from a digital process using binary arithmetic.
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Computer Technology
The speed at which the computer performs this simple act, however, is what makes it such an essential element of the modem technology aircraft. Computer speeds are measured in megahertz or millions of cycles per second. A computer with a clock speed of 133 MHz is capable of executing 133 million discrete operations every second. Digital computers are also normally integrated with other systems on an aircraft, via signalinterfacing devices such as analogue-to-digital (A/D) converters and digital-to-analogue (D/A) converters. The input interface converts analogue data into a digital format and the output interface converts the digital data into an analogue format. The processing speed of a digital computer and its calculating power are further enhanced by the amount of data that is handled during each cycle. If a computer checks only one switch at a time, that switch only represents two commands or numbers. For example, ON would symbolise one operation and OFF would symbolise another. By checking groups of switches linked within a single unit simultaneously, the computer is able to increase the number of operations it can recognise during each cycle. For example, a computer that checks two switches at one time can represent four numbers (0 to 3) or can execute one of four instructions at each cycle, one for each of the following switch patterns: OFF-OFF (0), OFF-ON (1), ON-OFF (2), or ON-ON (3). When digital computers were first introduced, they were capable of checking eight switches (binary digits) or bits of data during every cycle, or a byte, which contains 256 possible patterns of ONs and OFFs (or 1s and 0s). A computer uses a standard information format that consists of a group of bits, or a word, which equates to: ¾ ¾ ¾
An instruction Part of an instruction A particular type of datum (e.g. a number, a character, or a graphics symbol)
The pattern 11010010, for example, might be binary data (in this case, the decimal number 210) or it might tell the computer to compare data stored in its switches to data stored in a certain memory chip location. The total list of recognisable operations or patterns, which a computer is capable of, is called its instruction set.
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COMPUTER ARCHITECTURE The physical components of a computer are known as hardware. A digital computer is not a single component machine but is made up of the five distinct elements, as shown in the following diagram.
The programmes used in a computer are known as software.
INPUT DEVICES Input devices are the means by which a computer is fed with the information required for problem solving and consist of the following typical hardware: ¾ ¾ ¾ ¾ ¾ ¾ ¾
Keyboard Scanner Touch sensitive screen Speech recognition Mouse Joy stick Data from sensors
As long as the data is identifiable, the computer’s processor is able to recognise it and accordingly routes it along the appropriate internal buses or data lines. These form a network of communication lines that connect the internal elements of the processor and lead to external connectors linking the processor to the other elements of the computer system. The following types of CPU buses exist:
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¾
A control bus consists of a line that senses input signals and another line that generates control signals from within the CPU.
¾
The address bus is a one-way line from the processor that handles the location of data in memory addresses.
¾
The data bus is a two-way transfer line that both reads data from memory and writes new data into memory.
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CENTRAL PROCESSING UNIT The Central Processing Unit (CPU) may consist of a single chip or a series of chips that are able to perform arithmetic and logical calculations. It can also control the operations of the other system elements. A microprocessor is a miniature CPU chip, which incorporates additional circuitry and memory. CPU chips and microprocessors consist of the functional sections shown below.
The CPU receives input data and uses that data to carry out specific instructions, from which an output is derived. Typical input data might be wind velocity and direction or even the distance to run to a destination. The CPU then carries out calculations on this data using the following parts to give output data, such as TAS or time to run to the next waypoint. A central control unit coordinates the functions carried out in each section of the computer via a communication link or data transfer bus. The control unit decodes or reads the patterns of data held in a designated register, or temporary storage area, and keeps track of any instructions. The register also holds the location and results of these operations. The control unit translates the pattern into an activity, such as adding or comparing. It also indicates the order in which individual operations use the CPU and regulates the amount of CPU time that each operation may consume. Memory is normally divided into either volatile memory, which is lost whenever the computer loses power, or non-volatile memory, which remains in the system until it is over-written with new data. The main types of internal memory are: RAM (random access memory) is volatile memory. The data deposited in it is lost whenever the power is turned off or alternative states are written in. ROM (read-only memory) is non-volatile memory and normally contains data that has been inserted on the chip during its manufacture. The ROM typically contains start-up details and mathematical formulae, which are maintained even after the power has been switched off. Replacing the entire chip is the only way to change the instructions on ROM. PROM (programmable read-only memory) is non-volatile, but unlike the ROM can be reprogrammed once only, with the chip still fitted in the aircraft’s computer. EPROM (erasable programmable read-only memory) is also non-volatile and can be reused indefinitely. It can be totally erased and then reprogrammed with the chip still fitted in the computer. 17-4
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Arithmetic and logic unit (ALU) chips give the computer its calculating capability, allowing both arithmetical and logical calculations using a combination of digital logic circuits. These circuits are used to make specific true-false decisions based on the presence of multiple true-false signals at the inputs. The signals may be generated by either mechanical switches or by solid-state transducers, which are combined together to form an integrated circuit (IC)
OUTPUT DEVICES The output devices enable the user to see the results of the computer's calculations or data manipulations. The most common output device is the video display screen, which is a monitor that displays characters and graphics on a cathode-ray tube (CRT), or television-like screen, which is usually small. Portable computers commonly use liquid crystal displays (LCD) or other forms of screen. Examples of such screens are the EFIS and ECAM displays on modern aircraft. The standard output devices include printers and modems. A modem links two or more computers by translating digital signals into analogue signals so that data can be transmitted via telecommunications. Outputs may also be in the form of signals that are sent to the operating devices and are typically used to control the engines or automatic flight control system on the aircraft.
STORAGE DEVICES Computer systems can store data internally (in memory) and externally (on storage devices). External storage devices may physically reside within the computer's main processing unit or externally to the main circuit board. These devices store data as electrical charges on a magnetically sensitive medium such as an audiotape, a disk coated with a fine layer of metallic particles, or as an imprint on a laser readable disk. The most common external storage devices are floppy and hard disks. Floppy disks can contain from several hundred thousand bytes to well over a million bytes of data, depending on the system. Hard or fixed disks cannot be removed from their disk-drive cabinets, which contain the electronics to read and write data onto the magnetic disk surfaces. Hard disks can store from several million bytes to a few hundred million bytes. CD-ROM technologies, which use the same laser techniques that are used to create audio compact disks (CDs), also provide storage capacities in the range of several gigabytes (billion bytes) of data.
OPERATING SYSTEMS An operating system is a master control program, which is permanently stored in the memory. It interprets user commands and requests various kinds of services, for example, to display, print, or copy a data file; list all files in a directory; or execute a particular program. Different types of peripheral devices, such as disk drives, printers, communications networks, and so on handle and store data differently from the way the computer handles and stores it. Internal operating systems are usually stored in ROM memory, and are developed primarily to co-ordinate and translate data flows from dissimilar sources, such as disk drives or co-processors (processing chips that perform simultaneous but different operations from the central unit).
PROGRAMMING A program is a sequence of instructions that tells the hardware of a computer which operations to perform on the data. Programs can be built into the hardware itself, or they may exist independently as software. In some specialised computers, the operating instructions are embedded in their circuitry; as in the Flight Management System (FMS). Once a computer has been programmed, it can only do as much, or as little, as the software instructions enable it to do. Software in widespread use includes a wide range of applications programmes and instructions to the computer on how to perform various tasks. Electrics
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INTRODUCTION Air carrier operations ideally require uninterrupted communications when: ¾
Contacting Air Traffic Control to ensure a safe flow and separation from other traffic and to be kept up to date with conditions along the route and at the destination. Being able to communicate is also essential in the event of any incident that might endanger the aircraft or those on board.
¾
Communicating with the Company concerning people, freight, or maintenancerelated items
¾
Assisting other aviators
In 1994 for example, 99% plus of such communications were achieved by voice, using either short range VHF or HF for long distance communications. This situation has now dramatically changed, and information is now digitised and routed over data links including satellites, with printouts available as required. The antenna map below shows the typical equipment installation in a modern jet transport aircraft for communication purposes.
Aerial Locations - Communications Only HF Couplers SATCOM ATC 1 & 2
ATC 1 & 2
HF 1 & 2
VHF 1
VHF 2 VHF 3
Many equipment manufacturers produce communications and navigation equipment, so the following descriptions are generally representative of the many and various models that are available.
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HF and Satellite Airborne Communications
AIRBORNE COMMUNICATIONS LONG RANGE COMMUNICATIONS (UP TO 4000 KM) At present, when flying over 370 km (200 miles) from land, aircraft use HF transceivers, which are linked with unreliable propagation characteristics. Such HF installations are usually duplicated, with one set used for ATC purposes and the other for company messages. HF communications are also used in areas where VHF communications are not possible (e.g. sectors over the oceans or over sparsely populated continents such as Africa). Since HF communications rely on skywave propagation, it is essential to use the correct frequency (i.e. at night, the frequency needs to be reduced to maintain the skip distance). A typical HF radio control panel is shown below.
The frequency range covers the part of the spectrum between 2.8 MHz and 24 MHz (and very often between 2 and 30 MHz) in 1 kHz steps. There is also a facility for AM and LSB operations. However, USB is the standard operating mode. Similar to other receivers, a squelch control cuts off background noise in the absence of ground transmissions. The power output is approximately 400 watts on voice peaks, which gives ranges greater than 3700 km (2000 nm) in good conditions.
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An audio selector panel is situated at each crew station and enables switching between the various radio devices.
SHORT RANGE COMMUNICATIONS (UP TO 450 KM) Most communications are accomplished on VHF frequencies when over land, except possibly for remote or sparsely populated areas, where HF must be used. Like HF, operations are in a simplex mode (i.e. transmission and reception are not possible simultaneously). Commonly, a VHF control unit displays two frequency readouts. Each is controlled by its own selector knob. A transfer switch is used to select one VHF frequency as active, whilst the other is at standby. A light over the frequency window shows which frequency is active.
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The radio also has a fine-tuning (filter) facility and incorporates features such as: An automatic volume control (AVC) that maintains the receiver output signal at a given strength and automatically reduces the receiver gain if the signal becomes stronger. An automatic frequency control (AFC) that keeps the receiver tuned to the selected signal irrespective of any slight wandering of the transmitted frequency. Connection to the microphone(s) and the headset/speaker is through individual crewmembers’ audio selector facilities. The transceivers are remotely located and connected to vertically polarized blade antennas. An aircraft may be equipped with as many as three identical VHF transceivers, which operate in the frequency range 118 MHz to 137 MHz. The International Aeronautical Emergency frequency of 121.5 MHz also lies within this band. The frequencies are channelised at 8.33 kHz intervals. The type of transmission is AM, and uses an output power of approximately 25 watts. The range is quasi-optical and is typically 407 to 460 km (220 to 250 nm) at jet cruising levels.
SELECTIVE CALLING (SELCAL) SYSTEM The Selcal system is designed to relieve the flight crew from continually monitoring the communication channels and is operative on HF and VHF radios. The ground station transmits a four-tone audio signal, and if the aircraft radio is tuned to the same radio frequency, the four-tone signal is routed to the decoder circuits of the Selcal unit, which over-ride the setting of the squelch control. When the transmitted tones match the pre-selected aircraft tone combination (Selcal code), an intermittent light on the Selcal indication panel and a two-tone chime visually and audibly alert the flight deck crew.
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A SELCAL Code consists of a 4-letter group (e.g. HMJE). A registrar of SELCAL codes makes an assignment to the air carrier from 10 920 possible combinations. The carrier in turn assigns a 4letter group to each aircraft in the fleet to set on the decoder unit. The system is not used on VHF air traffic channels, because immediate response to control instructions is essential. The system operates on two separate channels that may be switched to any one of the available transceivers. Operational Check System operation should be checked by calling the ground station and requesting a SELCAL check using the code set in the decoder. Operation of both SELCAL units is checked simultaneously if receivers are on and selected to the same frequency. An intermittent light and two-tone chime indicate proper operation. Operation continues until the SELCAL light cap is pushed or until a microphone is keyed to transmit on the appropriate HF or VHF system. Either action resets the system for the next call.
SATELLITE COMMUNICATIONS (SATCOM) The deficiencies of VHF and HF over oceans and unpopulated areas may be overcome by the use of satellites for air/ground communications. An internationally owned co-operative called Inmarsat (International Maritime Satellite Organisation) maintains a number of geostationary satellites in orbit, which amongst other functions provide operational services and passenger telephone facilities to aeronautical users. In support of the space segment, there is a requirement for a number of ground stations linked to terrestrial communication networks. The British Telecom station at Goonhilly, in Cornwall, England, is one example. Groups of such stations band together to furnish near global coverage, and they contract their services to the various airline users. This enables passengers to directly dial out-going calls in flight from pay phones in the cabin. There is also a choice of voice or data, and the wide use of printers favours the data format. Other groups of ground earth stations support either Sita or Arinc, either of which can accept or distribute air traffic network (ATN) messages as well as company messages. Those airlines favouring the use of the Sita network (generally non-US carriers) use ground earth stations located in California, Western Australia, France, and Quebec, as shown on the next page.
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SATELLITE AIRCOM (SITA) Messages to and from the satellites are relayed to airline offices over the existing Sita network. US carriers favouring the Arinc network use another similar group of ground earth stations for the same function. It is the intention that both networks shall be mutually supportive. The flight crew may well be unaware of the service provider, since they merely log-on to the visible satellite at the start of a sector; and any subsequent receipt and dispatch of information is normally done automatically. The aircraft-to-satellite link is accomplished on L-Band channels between 1530 MHz to 1660.5 MHz. The satellite-earth link is also able to use C-Band frequencies of 4000 MHz upward via large steerable terrestrial dishes.
L - BAND
C - BAND
PRIVATE NETWORKS (AOC - AAC)
PUBLIC SWITCHED NETWORKS (AAC, APC)
ATS AOC
GES
CAA DISTRIBUTION NETWORK
ACC OR OCEANIC CENTRE GES AOC AAC APC CAA ACC
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TOWERS
SAR, MET, AIS, ETC.
AFTN (CIDIN)
Ground Earth Station Aeronautical Operation Control Aeronautical Administrative Communication Aeronautical Passenger Communication Civil Aviation Administration Area Control Centre
AFTN CIDIN SAR MET AIS
OTHER NETWORKS
Aeronautical Fixed Telecommunication Network Common ICAO Data Interchange Network Search and Rescue Meteorology Aeronautical Information Services
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To operate high-speed data and digitised voice, a high-gain directional antenna must be installed on the aircraft. This antenna is steered electronically from knowledge of satellite position and aircraft position derived from the aircraft’s flight management computer. An option also exists to use low-gain antennas, which are significantly cheaper than the high-gain variety, but this precludes voice link-up and operation is restricted to low-speed data transfer. Each aircraft may be individually addressed by its 24-bit unique transponder mode S code and is able to download FMC information and engine/performance related information on request.
LIMIT
OF C OVER AGE
GEOSTATIONARY SATELLITE
EQUATOR
AREA OF NOT TO SCALE
COVERAGE
OF LIMIT
RAGE COVE
LATITUDE LIMITATION OF GEOSTATIONARY SATELLITE
One shortcoming of the geostationary satellite is the inability to cover polar areas. The limit is 81½ degrees north and south at sea level, which is increased by 2 or 3 degrees for high flying aircraft. 80° is most commonly quoted for the purposes of JAA examinations.
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