Generators and Motors
Generators
Are rotating electrical machines that convert mechanical energy input to usable electrical energy
Main Parts
Yoke Pole and Pole Shoe Field Winding Armature Commutator Brushes
Main Parts Yoke
Field Winding
• Support the field coil and spread the flux over large area
• It is cylindrical in shape to which even number of poles is bolted
Main Parts Pole and Pole Shoe
It is cylindrical in shape to which even number of poles is bolted
Armature • A cylindrical core • Made of sheet steel laminations and insulated from each other by a thin layer of paper and varnish to reduce iron loss
Main Parts Commutator
Cylindrical in shape and consists of segments of hard drawn copper . A mica strip insulates each segment from each other. Windings of armature are terminated on it Brushes
• Used to connect the external circuit to the armature
General Types of AC Armature Winding
Lap Winding Type of winding which coil end are connected to commutator segments that are near to one another.
Wave Winding Type of winding which the coil ends are connected to commutator segments that are of some distance from one another; nearly 360 degrees
Parallel Paths For Lap Winding
a = mP
For Wave Winding
a = 2m
a = Armature current paths for both DC motor and DC generator M = plex or degree of multiplicity of the winding P = number of poles
Parallel Paths Winding
m
Simplex
1
Duplex
2
Triplex
3
Quadruplex
4
General Voltage of a DC Generator (EMF) Voltage across the armature of the DC generator AC not DC Commutator
Commutation
responsible in converting the generated AC voltage in the armature to DC
the reversal of current in the coil passes through the brush position
General Voltage of a DC Generator (EMF)
Eg =
ZPΦN 60a
-8
X 10 V
Eg = generated voltage/induced voltage Φ = flux per pole, lines or maxwells N = speed of rotation of the armature; rpm Z = number of active conductors a = number of parallel paths
Armature Reaction
When the generator is loaded, the armature conductor carries current and hence current carrying conductors produce a magnetic flux of its own which affects the flux created by the main poles.
Effects of Armature Reaction
Field Strength in the gap is weakened under the leading pole tips and strengthens under the trailing pole tips.
Magnetic field of the machine is distorted.
Compensating Windings
Neutralizes the cross-magnetizing effects of armature reaction
Connected in series with armature such that the current in it flows in opposite direction to that flowing in armature conductors directly below the pole shoes
Interpoles
Does not reduce armature reaction
A better method of providing commutating field.
Types of DC Generators
Separately Excited DC Generator
Self Excited Generator
Shunt
Series
Long Shunt
Compound
Short Shunt
Separately Excited DC Generator
The field winding is energized from an external DC source is called “exciter”. The exciter maybe a battery or another DC generator.
Ia = IL If = V/Rf Eg = VT + Va + Vbc Eg = VT + Ia( ra + rbc )
Self Excited DC Generator
The field winding is energized by its own armature( shunt, series or compound)
Shunt Generator
Ia = ISH +IL If = VT/RSH Eg = VT + Va + Vbc Eg = VT + Ia( ra + rbc )
Self Excited DC Generator
Series Generator
Ia = IS = IL Eg = VT + Va +Vs + Vbc Eg = VT + Ia( ra + Rs + rbc )
Self Excited DC Generator
Compound Generator Long Shunt
Ia = ISH = IS = IL ISH = VT/RSH Eg = VT + Va + VS + Vbc Eg = VT + Ia(ra + rbc + RS)
Self Excited DC Generator
Compound Generator Short Shunt
IS = IL Ia = ISH + IL Eg = Va + Vbc + VSH Eg = VT + Va + VS + Vbc Eg = VT + Ia(ra + rbc) + IsRs
Self Excited DC Generator Where: Eg = generated voltage/induced voltage of the generator Ia = armature current; A IL = load current / line current; A ISH = shunt field current; A IS = series field current; A ra = armature winding resistance; Ω Rbc = brush contact resistance; Ω RSH = shunt field resistance; Ω RS = series field resistance; Ω RL = Load resistance; Ω
Self Excited DC Generator VT = load voltage / terminal voltage; V VSH = shunt field winding resistance drop; V VS = series field winding resistance drop; V
Va = armature winding resistance drop; V Vbc = brush contact resistance drop; V
Losses in DC Generator
Copper Loss
Losses due to current in the various windings of the machine Armature copper loss Field copper loss Brush contact loss
Losses in DC Generator
Iron Loss
Magnetic or core losses Hysteresis Loss Eddy Current Loss
Losses in DC Generator
Mechanical Losses
Air friction of rotating armature Bearing friction Brush friction
Alternators
Machine designed to generate alternating curves Operating Principle
When the rotor rotates, the stator conductors are cut by the magnetic flux, hence they have induced emf produced in them. Because of the magnetic poles are alternately N and S poles, they have induced an emf and hence current in the armature conductors, which first flow in one direction and then in the other. Hence an alternating emf is produced in the stator conductors whose frequency depends on the number of poles moving past in a conductor in one second and whose direction is given by Flemings’s Right hand Rule.
Alternators
Frequency of the Generated Voltage
P(rpm) f =
Hertz 120
Where: F = frequency, Hz P = no. of poles rpm = speed of rotation
Alternators
Generated Voltage of an Alternator
E = 4.44NΦkdkp x 10-8 V
Where: E = total generated voltage, V N = no. of turns per coil Φ = flux per pole, maxwells kd = distribution factor kp = pitch factor
Note: For full winding, kp = 1 For concentric winding, kd = 1
Alternators
Effects of Various Types of Load on the Alternator Terminal Voltage
Resistive Loads Inductive Loads Capacitive Loads
Alternators Resistive Loads Incandescent lamps, heating devices or loads with unity power factor 8% to 20% drop in terminal voltage below its no-load value
Inductive Loads • Induction motors, electrical welders, fluorescent lighting or loads with lagging power factor. • 25% to 50% drop in terminal voltage below the no-load value
Capacitive Loads • Capacitor devices or special types of synchronous motor or loads with leading power factor. • Tend to raise or increase the terminal voltage of the alternator above the no load value.
Motors
Rotating electrical machines that convert electrical energy into mechanical energy It has a reverse operation with generators The presence of back emf causes the armature current to automatically changes with the increase of load on the motor. If there is no back emf, the armature may take very high current and winding may be damaged (like during starting, the current is high
DC Motors Utilizes DC energy as input to produce mechanical actions Counter EMF of Motors
ZPΦN Ec = 60a
-8
X 10 V
Where: Ec = back emf or counter emf,V P = no. of poles Φ = flux per pole, lines or maxwells N = speed of rotation of the armature; rpm Z = number of active conductors a = number of parallel paths
Types of DC Motors
Series
Shunt
Compound
Shunt Motor
Its field winding is connected across the armature Nearly constant or adjustable Medium starting torque Used for fan, blower, pump, grinder, etc
Note: To reverse the direction of rotation, interchange the brush or reverse the connection of the shunt field Never open the field circuit while the motor is operating for it will “race” or “run away”
Shunt Motor
IL = Ia + ISH ISH = Vs/RSH Ec = Vs – Va – Vbc Ec = Vs – Ia(ra + rbc)
Series Motor
The field winding is connected in series with the armature Variable speed High starting torque Used for elevators, crane, conveyor, hoist, gear drive, etc.
Note: To reverse the direction of rotation, interchange the brushes Never start this series motor without load or remove the load while operating for it will “race” or “run away”.
Series Motor
IL = Is = Ia Ec = Vs - Va - Vbc - VSF Ec = Vs – IL(ra + rbc + Rs)
Compound Motor
Variable-speed or adjustable speed Has a series and shunt field coils similar to compound generator High starting torque Used for elevators, conveyor, crane, milling machine, punching machine, etc
Torque Developed in the Armature
Ia x Ec T = 9.55 (
)N-m N
Where: T = torque developed, N-m N = speed of rotation, rpm Φ = flux per pole, weber
K = proportinality constant
T = kIaΦ
Mechanical Power Output
2πNT HP =
33,000
Where: HP = horsepower
2πNT HP =
44,760
Speed Regulations
Percentage rise in the speed of the motor when the mechanical load is removed
%NR =
NNL - NFL NFL
• Where: • NNL = no-load speed • NFL = full-load speed
x 100%
Synchronous Motors
An Arc machine that operates at synchronous speed and converts electrical energy to mechanical energy
Parts of Synchronous Motors Stator Houses 3 phase armature windings in the slots of the core and receives power from 3-phase supply Rotor Has a number of alternate N and S poles. The rotor poles are excited by an exciter, which is a DC generator, mounted on the rotor shaft
Characteristics of Synchronous Motors Under-excitation the field excitation that the back emf is less than the applied voltage. The motor has lagging power factor Normal Excitation Operates at almost unity power factor Over-Excitation Operates in leading power factor
Uses of Synchronous Motors
Used where a constant speed is required Used in power factor correction in the factories
Induction Motors Principles of Induction Motors
When a 3-phase supply is applied to the stator, a rotating magnetic field is produced. This rotating magnetic field produces induced emf in the rotor windings that cause induced current to circulate.
Induction Motors Principles of Induction Motors
By Lenz’s Law, the induced current tends to oppose the action producing it and therefore circulate in such a manner that a torque is produced. However, the rotor not rotate as fast as the rotating magnetic field.
Speeds of Induction Motors Synchronous Speed
The speed at which the rotating flux rotates
Rotor Speed
• Actual speed of the motor • It cant be calculated but it can be measured using tachometer or speedometer
Slip
• The difference between the synchronous speed and the actual speed
Types of Induction Motors
Squirrel Cage Motor Used where low power needed and speed control is needed
Slip Ring Used only when high starting torque is required
Advantages of Induction Motors Simple in construction, robust and almost unbreakable
Requires minimum care and maintenance
Good power factor
High efficiency
Self-starting
Disadvantages of Induction Motors Speed cannot be varied without loss of efficiency Has inferior starting torque Speed decreases with the increase load
Converters and Rectifiers
Methods of Converting AC to DC
Motor-Generator Set
An AC and DC generator mechanically coupled; AC motor can be synchronous or induction motor
Rotary Converters
Single machine with one armature and one field Combines the function of a synchronous motor and DC generator
Methods of Converting AC to DC
Motor Converters
Rectifiers
consists of ordinary slip ring induction motor coupled both mechanically and electrically to a DC generator
Converts AC to unidirectional current by virtue of permitting flow of currents in only one direction
Applications of Generators and Motors
Amplidyne
Trade name for rotating amplifiers Quick response DC generator, output of which is controlled by a very small field power Power amplifier; most suitable use as an exciter in a closed loop control system
Applications of Generators and Motors
Brushless Generators
Generator employing silicon rectifiers as static commutation devices Aircraft generator
Applications of Generators and Motors
Dyna-motor
Rotary transformer A composite machine having a single magnet frame but two separate armature windings, one acting as a generator and the other as a motor, and independent commutators
Applications of Generators and Motors
Rototrol
A single-stage rotating amplifier relying on the use of positive feedback
Magnicon • Trade name for rotating amplifiers with cross field excitation
Applications of Generators and Motors
Magnetohydrodynamic Generator
Converts thermal energy into electric by breaking a stream of hot ionized gas Plasma hydrodynamic generator
Applications of Generators and Motors
Electrohydrodynamic Generator
A stream of gas is ionized, the positive ions being carried away by the stream while the electrons are collected by an electrode ring causing a current to flow through a wire between the ring and a collecting grid
Applications of Generators and Motors
Metadyne Generator
Rotating amplifier Similar to the nature of amplidyne
Applications of Generators and Motors
Motor Converter
An induction motor and a synchronous converter mechanically and electrically coupled Converts AC to DC
Applications of Generators and Motors
Motor Generator
A converter consisting of an AC motor directly coupled to a DC generator No electrical connection between the two machines
Applications of Generators and Motors
Static Converter
A converter based on electronic devices of the semiconductor, mercury arc or gaseous type, usually in combination with a transformer
Applications of Generators and Motors
Thermocouple Generator
Thermal-electrical conversion device
Transformers
It is an AC device that transfers power from one circuit to another without rotating and change of frequency
Transformer Construction
Core Type
The windings are placed on outside of the core
Transformer Construction
Shell Type
The windings are placed on inside of the core such that the magnetic circuit completely surrounds the winding
Parameters of Transformer
For Ideal Transformers
Pp = PS
Equivalent Circuit of an ideal Transformer
Parameters of Transformer Voltage Ratio
Current Ratio
a=
ES
1
ZP a² = ZS
=
NP NS
NS
IP =
= a
Impedance Ratio
EP
IS =(
NP NP NS
)²
Parameters of Transformer Where A = ratio of transformer/ turn ratio EP = primary line (impressed) voltage ES = secondary line (impressed) voltage NP = no. of primary turn NS = no. of secondary turn IP = primary line current IS = secondary line current
Losses in Transformers
Iron Losses
Hysteresis
Eddy Current
Copper Losses
Losses in Transformers
Hysteresis
Eddy Current
Heating loss due to the collision of iron’s magnetic particles when it aligned to the external magnetic induction
Loss due to eddy current(eddy currents are currents circulating around the magnetic core of the transformer
Condition for Maximum Efficiency
Copper Loss = Core Loss
Transformer
Open Circuit Test or No-load Test
Determine the no-load loss or core loss
Transformer
Short Circuit Test or Impedance test
Equivalent impedance, leakage reactance and total resistance of the transformer as referred to the winding in which the measuring instruments are placed
Rated or Full-load copper loss
Autotransformer
Has only one winding which performs the function of both primary and secondary winding. These transformers are used as regulating transformers where only a small variation of voltage is required
Electrolysis and Batteries
Electrolysis
The conduction of electric current through the solution of an electrolyte together with the resulting chemical changes
Important Terms Anode
plate or electrode connected to the + terminal
Cathode
plate or electrode connected to the - terminal
Ions
The electrolyte gets chemically decomposed
Anions
Ions having + charge
Cations
Ions having - charge
Faraday’s Law of Electrolysis First Law The mass of an ion set free by a current in the process of electrolysis is proportional to the quantity of charge that has passed through the electrolyte W α Q α It
W = ZIt
Where W = mass of ion liberated I = current in amperes t = time in seconds Z = a constant value that depends upon the nature of substance
Faraday’s Law of Electrolysis Second Law When the same current passes through several electrolyte for the same time, the mass of various ions deposited at each of the electrodes are proportional to their chemical equivalents
m1 m2
=
E1 E2
=
Z1 Z2
Where m1 & m2 = mass of ion deposited or liberated E1 & E2 = chemical equivalent weights (atomic weight / valency) Z1 & Z2 = electromechanical equivalent
Applications of Electrolysis Electroplating Depositing a thin layer of precious metal (silver, gold) over an inferior metal
Extraction and Purification of Metals
Battery An assembly of voltaic primary and secondary cell
Primary Cells
Secondary Cells
Chemical action not reversible
Also known as accumulators or storage batteries
Acid Cells
Uses acid as an electrolyte
Alkali Cells
Uses alkali as an electrolyte
Local Action The continuous dissolution of the zinc rod even when the cell is not connected to the external circuit
This is due to impurities present in commercial zinc. The impurities form small tiny cells, which are short circuited by the main body of the zinc rod
Can be minimized by using amalgamated zinc
Polarization The collection of hydrogen bubbles on the surface of the copper plate Effects of Polarization The bubbles act as insulators and hence increase the internal resistance of the cell Sticking H2 ions on the +Ve plate exert repulsive force on the other H2 ions coming towards the Cu plate. Minimized by surrounding the cathode by depolarizers, which oxidizes H2 bubbles as soon as they are produced
Charging the Battery Process of reversing the current flow through the battery to restore the battery to its original position 5 Types of Charge
1. 2. 3. 4. 5.
Initial Charge Normal Charge Equalizing Charge Floating Charge Fast Charge
List of Batteries and their Corresponding Output Primary Alkaline Mn02
1.15 V
Carbon Zinc
1.5 V
Electrolyte
2.8 V
Leclanche
1.2 V
Li-organic
2.8 V
Magnesium
1.5 V
Manganeses dioxide (alkaline)
1.5 V
Mercad
0.85 V
Mercury
1.2 V
Mercuric Oxide
1.35 V
Silver Oxide
1.5 V
Solid
1.9 V
Zinc-Air
1.1 V
Zinc-Chloride
1.5 V
List of Batteries and their Corresponding Output Secondary Edison
1.2 V
Lead - Acid
2.1 V
Manganese Dioxide (alkaline)
1.5 V
Nickel - Cadmium
1.25 V
Nickel - Hydrogen
1.2 V
Nickel - Iron
1.2 V
Silver - Cadmium
1.05 V
Silver - Zinc
1.5 V
Zinc - Chloride
2.0 V
Zinc - Nickel Oxide
1.6 V
Most Commonly Used Cells Primary Type
Voltage (V)
Remarks
Carbon - Zinc
1.5
used for flash lights and toys; low cost and low current capacity
Zinc - Chloride
1.5
higher current capacity
Manganese Alkaline
1.5
hydroxide electrolyte and high current capacity
Silver Oxide
1.5
hydroxide electrolyte
Lithium
2.8
long life, high cost
Most Commonly Used Cells Secondary Type
Voltage (V)
Lead Acid
2.1
wet electrolyte
Silver - Zinc
1.5
rechargeable dry cell, high current capacity
Silver - Cadmium
1.05
rechargeable dry cell, high efficiency
Nickel - Cadmium
1.25
rechargeable dry battery
Remarks
Review Questions
Review Questions 1.
A 4-pole DC generator with duplex lap winding has 48 slots and four elements per slot. The flux per pole is 2.5 x 106 maxwells and it runs at 1500 rpm. What is the output voltage?
a.
60 360 225 120
b. c. d.
Review Questions 2. Find the frequency in kilocycles per second in the armature of a 10 pole, 1200 rpm generator? a. b. c. d.
100 1000 10 .1
Review Questions 3. What is the voltage regulation when the full load voltage is the same as no-load voltage assuming a perfect voltage source? a. b. c. d.
100% 10% 1% 0%
Review Questions 4. In DC motors, the emf developed which opposes to the supplied voltage a. b. c. d.
Residual emf Coercive emf Induced emf Counter emf
Review Questions 5. What will happen to a DC series motor when its load is removed? a. b. c. d.
the motor will stop the motor speed remains the same the torque remains the same the motor will over speed
Review Questions 6. The armature of a DC generator is laminated to _________. a. b. c. d.
Reduce the bulk Provide passage for cooling air Reduce eddy current losses Insulate the core
Review Questions 7. Which of the following helps in reducing the effect of armature reaction in DC generators? 1. Interpoles 2. Compensating Windings a. b. c. d.
1 only 2 only Both 1 & 2 Neither 1 & 2
Review Questions 8. The loss in DC generator that varies with the load is ___________. a. b. c. d.
Copper loss Eddy current Loss Hysteresis Loss Windage Loss
Review Questions 9. Magnetic field in a DC generator is produced by _________.
a. b. c. d.
Electromagnets Permanent Magnets Iron Core Steel Laminations 1 only 2 only 1 & 2 only 1,2,3 & 4
Review Questions 10. In DC generator, the cause of rapid brush wears maybe _________.
a. b. c. d.
Severe sparking Rough commutation surface Imperfect contact Slots disorientation 1,2 & 3 only 1,2 & 4 only 2,3 & 4 only 1,2,3 and 4
Review Questions 11. Which of the following components of a DC generator plays vital role for providing direct current of a DC generator a. b. c. d.
Dummy coils Commutator Eye bolt Equalizer ring
Review Questions 12. Find the voltage regulation of a generator when full load voltage is 110 V and the no load voltage is 120 V. a. b. c. d.
1% 9.09% 90.9% 10%
Review Questions 13. Where does voltage generated in a DC generator depends?
a. b. c. d.
Field resistance Speed Flux Field current Armature resistance 1,2 and 3 only 2 and 3 only 2,3 and 4 only 1,3 and 5 only
Review Questions 14. Generators are often preferred to be run in parallel because of ___________.
a. b. c. d.
Great reliability Meeting greater load demands Higher efficiency 1,2 and 3 1 and 2 only 1 and 3 only 2 and 3 only
Review Questions 15. DC generator preferred for charging automobile batteries is __________. a. b. c. d.
Shunt generator Long shunt compund generator Series generator Any of these
Review Questions 16. The purpose of providing dummy coils in a generator is __________. a. b. c. d.
To reduce eddy current losses To enhance flux density To amplify voltage To provide mechanical balance for the rotor
Review Questions 17. Which of the following generating machine will offer constant voltage on all loads? a. b. c. d.
Self excited generator Separately excited generator Level compounded generator All of the above
Review Questions 18. A DC generator works on the principle of a. b. c. d.
Lenz’s Law Ohm’s Law Faraday’s Law of Electromagnetism Induction None of the above
Review Questions 19. With a DC generator, which of the following regulation is preferred? a. b. c. d.
100% regulation Infinite regulation 50% regulation 1% regulation
Review Questions 20. The purpose of an amperite regulator a. b. c. d.
Power regulation Loss regulation Current regulation Voltage regulation
Review Questions 21. The only purpose of a DC generator that has been modified to function as an amplidyne is to a. b. c. d.
Serve as a booster Serve as a regualtor Serve as a meter Serve as power amplifier
Review Questions 22. A simple method of increasing the voltage of a DC generator is ________ a. b. c. d.
Increase the length of the armature Decrease the length of the armature Increase the speed of rotation Decrease the speed of rotation
Review Questions 23. A 4-pole lap wound armature has 120 slots and 4 conductors per slot. The flux per pole is 50 mWb and it generates 240 volts. Find the speed. a. b. c. d.
1200 rpm 800 rpm 600 rpm 300 rpm
Review Questions 24. The power stated on the nameplate of any motor is always the ________ a. b. c. d.
Gross Power Output power at the shaft Power drawn in kVA Power drawn in kW
Review Questions 25. A DC motor is used to ___________ a. b. c. d.
Generate power Change mechanical to electrical energy Change electrical to mechanical energy Increase energy put to it
Review Questions 26. A DC motor is still used in industrial applications because it a. b. c. d.
Is cheap Is simple in construction Provides fine speed control None of the above
Review Questions 27. Carbon brushes are preferable to copper brushes because
a. b. c. d.
They have longer life They reduce armature reaction They have lower resistance They reduce sparking 1 and 2 only 2 and 3 only 3 and 4 only 1 and 4 only
Review Questions 28. The field poles and armature of a DC machine are laminated to ________ a. b. c. d.
Reduce the weight of the machine Decrease the speed Reduces eddy current Reduce armature reaction
Review Questions 29. Steam turbo alternators are much smaller in size than water turbine alternators for a given output. This is so because _________.
a. b. c. d.
Steam turbo alternators are built with smaller capacities Steam turbo alternators run at high speed Steam turbo alternators have long rotors 1 & 2 only 1 & 3 only 2 & 3 only 1,2 and 3
Review Questions 30. When the speed of a DC motor increases, its armature current ________ a. b. c. d.
Increases Decreases Remains constant None of the above
Review Questions 31. The amount of the back emf of a shunt motor will increase when __________ a. b. c. d.
Load is increased The field is weak The field is strengthened None of the above
Review Questions 32. The speed of a DC motor is ________ a. b. c. d.
Directly proportional to the flux per pole Inversely proportional to the flux per pole Inversely proportional to the applied voltage None of the above
Review Questions 33. The torque developed by a DC motor is directly proportional to ______ a. b. c. d.
Flux per pole x armature current Armature resistance x applied voltage Armature resistance x armature current None of the above
Review Questions 34. The speed of a _______ motor is practically constant a. b. c. d.
Cumulatively compounded Series Differentially compounded Shunt
Review Questions 35. ________ motor is a variable speed motor. a. b. c. d.
Series Motor Shunt Motor Cumulatively Compounded Differentially Compunded
Review Questions 36. What do you call an electromagnet with its core in a form of a magnetic ring? a. b. c. d.
Paraboloid Solenoid Toroid Motor
Review Questions 37. The working principle of a transformer is ________ a. b. c. d.
Self induction Static induction Mutual induction Dynamic induction
Review Questions 38. A type of transformer that is protect technicians from deadly electrical shock is called a/an ______ a. b. c. d.
Absorber transformer Step down transformer Step up transformer Isolation transformer
Review Questions 39. What is the typical use of an autotransformer? a. b. c. d.
Toy transformer Control transformer Variable transformer Isolating transformer
Review Questions 40. Synchronous motor is capable of beig operated at _________ a. b. c. d.
Lagging pf only Unity pf only Leading pf only All of the above