Inverter

Chapter 1 INTRODUCTION

1.1 Introduction An inverter is an electrical device that converts direct current (DC) to alternating current (AC); the converted AC can be at any required voltage and frequency with the use of appropriate switching, and control circuits. The inverter converts the DC electricity from sources such as batteries, solar panels, or fuel cells to AC electricity. Inverter is one of the converter families which are called DC to AC converter. It converts DC power to AC power to a symmetric AC output voltage at desired magnitude and frequency .Inverter is widely used in industrial applications such as variable speed AC motor drives, induction heating, standby power supplies and uninterruptible power supplies. The DC power input of inverter is obtained from the existing power supply network. It can be a battery, photovoltaic, wind energy, fuel cell or other DC sources. In this project, two phase inverter was designed. Using this inverter capacitor   phase splitting motor can be driven easily. In this type of motor a capacitor was used in the secondary winding but the capacitor is not a suitable solution of this  problem. The value of Capacitance can not be adjusted and it does not not always  provide a good output speed of the motor. On the other other hand, a two phase induction motor readily provides the necessary two phases which is exactly 90 degree out of phase and as a result provide a better output speed of the motor. The speed of this type of motor can be easily kept under control using the two  phase inverter rather than using a capacitor.

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1.2 Literature Review Previous works on inverter have been studied and used as a continuous support of reference throughout this thesis. Many researchers have been studying and analyzing types of switches that can be used in inverter. All switching strategies mostly concentrate in term of reducing the power losses, reduce the total harmonic distortion and increasing the efficiencies of the inverter. The power semiconductor devices such as the diode, thyristor, triac and power transistor are widely used in power applications as switching devices. Two types of power transistors used for switching devices are Bipolar  Junction Transistor (BJT) and Metal Oxide Semiconductor Field Effect Transistor (MOSFET). Both of power transistors have a different characteristic where, MOSFET have faster switching speed and BJT have higher capability. The important criteria of power transistors in circuit applications also depend on the parameters of rating, conduction losses, switching losses, switching times, control strategy and finally are cost.

Sinusoidal pulse width modulation (SPWM) [3] is widely used in power  electronics to digitize the power so that a sequence of voltage pulses can be generated by the on and off of the power switches. The pulse width modulation inverter has been the main choice in power electronic for decades, because of its circuit simplicity and rugged control scheme. SPWM switching technique is commonly used in industrial applications. SPWM techniques are characterized  by constant amplitude pulses with different duty cycle for each period. The width of this pulses are modulated in order to obtain inverter output voltage control and to reduce its harmonic content. Sinusoidal pulse width modulation or SPWM is the most common method in motor control and inverter  application. Conventionally, to generate the signal, triangle wave as a carrier  signal is compared with the sinusoidal wave, whose frequency is the desired 2

frequency. The proposed method used in this design is to replace the conventional method with the use of Atmel microcontroller [4,5]. The use of the microcontroller brings flexibility to change the real-time control algorithms without further changes in hardware. It will reduce the overall cost and has a small size of control circuit for the single phase full bridge inverter.

A variable-frequency drive controls the operating speed of an AC motor by controlling the frequency and voltage of the power supplied to the motor [1, 2]. An inverter provides the controlled power. In most cases, the variable-frequency drive includes a rectifier so that DC power for the inverter can be provided from main AC power. Since an inverter is the key component, variable-frequency drives are sometimes called inverter drives or just inverters.

Baharuddin Bin Ismail [3] presented the microcontroller-based converter system design for 3-phase BLDC motor drives. The aim of this research is mainly to develop a converter system which can be used to drive 3-phase BLDC motor. This project also implements several PWM switching schemes to generate  pulses for the inverter to drive a 3-phase BLDC motor.

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1.2 Objective of the Project The aim of this project is to design a two phase inverter which can be controlled using microcontroller. The interest of using microcontroller (ATMEGA 16L) in this research is to produce proper design of the control signal with flexibility. The importance of the proper design of control signals with powerful switching (by MOSFET) is to reduce the harmonics and power losses of the inverter  output voltage. This inverter can be used to drive motors. The speed and torque of the motor can be adjusted using this inverter by changing the frequency. This can be easily done by the controller circuit designed using microcontroller. This inverter is designed in such a way so that it can work under high voltage and current, and also at high frequency.

1.3 Thesis Outline This thesis consists of seven chapters including this chapter. Chapter 1 presents an introduction about inverter, literature review and objective of this project. Chapter 2 discusses about the approach and method of this project. This chapter  includes single phase inverter topology, rectifier circuit design, block diagram of the project. Chapter 3 explains the development of the whole project in details providing the schematics of the whole circuit and actual PCBs of both the control circuit and the power circuit. Chapter 4 presents the results and wave shapes Chapter 5 concludes the thesis with a discussion, future work and a conclusion.

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Chapter 2 Approach and Method

2.1 Inverter Topology There are two circuit topologies commonly used in inverter circuit. Half bridge and full bridge configuration are the main topologies used in low and high  power applications. For certain low power application, the half bridge may suffice but the full bridge is more convenient for adjustment of the output voltage by pulse width modulation techniques.

2.1.1 Half Bridge Inverter The power circuit topology and output example for half bridge inverter is shown in Figure 2.1. The inverter circuit consists of two controlled static switching elements. The switching elements can be transistor, MOSFET, IGBT and extra. The switching elements are labeled S1 and S2 and each of switches has an anti parallel diode. It is evident from the presence of the diodes that the switching devices S1 and S2 need not have the capability to block the reverse voltages. If  the switching element is power MOSFET, there may not be a need to use the anti-parallel diodes because the devices structure has an anti-parallel diode.

The basis operation of half bridge inverter can be divided into two operations. If  switch S1 turned on for period of T/2, the instantaneous output voltage across the load equal to V DC/2. If switch S2 turned on for period of T/2 to T, the instantaneous output voltage - V DC/2 will appear. The switching strategy for  switch S1 and switch S2 must be designed to make sure both switches not turn on at the same time. If that happens, it is equivalent to a short circuit across the 5

DC input, resulting in excessive current and possible damage to the switching elements.

Figure 2.1.1: Schematic diagram of two phase half bridge inverter topology.

2.2 DC Input Voltage A full-wave rectifier converts the whole of the input waveform to one of  constant polarity (positive or negative) at its output. Full-wave rectification converts both polarities of the input waveform to DC (direct current), and is more efficient. However, in a circuit with a non-center tapped transformer , four  diodes are required instead of the one needed for half-wave rectification. Four  diodes arranged this way are called a diode bridge or bridge rectifier.

Figure 2.2: Wave Shapes and Circuit diagram of a full wave rectifier.

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In this project, an auto-transformer was used to step down the 220 V AC supply to 110 V AC supply. A rectifier circuit was also build to convert the 110v ac supply into dc supply using heavy duty diodes. This dc supply was then used as the input source of the power circuit which was then converted into AC supply in order to run the induction motor.

Full bridge rectifier circuit

Half bridge rectifier circuit

Figure 3.3: Full bridge and Half bridge rectifier circuit.

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2.3 Approach Two phase variable frequency inverter consists of full bridge diode rectifier is fed from 110Vac power supply; the rectifier bridge is used to convert the AC supply voltage to a 155.56Vdc voltage. The output voltage of the rectifier   bridge is smoothed using a capacitor which helps to remove its ripples. Then the fixed DC voltage is fed to the half bridge inverter, which receives the DC voltage and converts it to AC voltage with variable frequency to feed the motor  under control. The microcontroller has been programmed to vary the frequency of the PWM signal that controls the frequency of the voltage applied at the gate drives, and as a result of this we can control the frequency of the inverter. The inverter consists essentially of four power MOSFETs, this inverter converts the DC link voltage into an adjustable three-phase AC voltage. The PWM control scheme used to control the inverter output frequency, by modulating the on and off times of power MOSFETs.

Full bridge AC Power  Supply

Rectifier 

Inverter  DC Power  Supply

Circuit

Motor  AC Voltage

Control Circuit

Figure 2.3: Block Diagram of the system.

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2.4 Designing of the Control Circuit In this project, Atmega16L was used to produce 4 square waves. These waves were then reconstructed using four opto-couplers. The output waves of the optocouplers were amplified using BJTs. Four isolated DC power supplies were used to power up the opto-couplers and BJTs. These four waves were then fed into the Gate of four MOSFETS in the POWER CIRCUIT in order to control them.

MICROCONTROLLER  Signals

A

B

Opto-Coupler 

BJT

To the Gates of the Power  MOSFETs in the Inverter Circuit

Figure 2.4: Block Diagram of The Control circuit

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Chapter 3 Experimental Setup 3.0 Introduction In this chapter the procedure of designing a two phase inverter using the microcontroller is explained in details. The PCB design and the schematic diagram of the control circuit and the power circuit are also shown.

3.1 Microcontroller Programming

Figure 3.1.1: Pin Configuration of ATMEGA16L

The ATmega16 is a low-power CMOS 8-bit microcontroller based on the AVR  enhanced RISC architecture. By executing powerful instructions in a single clock cycle, the ATmega16 achieves throughputs approaching 1 MIPS per MHz allowing the system designer to optimize power consumption versus processing speed. 10

The Microcontroller was the heart of this project because it was used to produce the control signals. The control signals need to be very precise and accurate in order to obtain a proper AC signal at each phase of the inverter. The dead time needs to be controlled very precisely. There is a possibility of overlapping  between ON period switch pair (S1 and S2) or (S3 and S4) pair in half bridge two phase inverter. It is important to avoid the short circuit of DC bus. This dead time was controlled precisely using this microcontroller. Dead time period must be suitable to avoid the problem of damage the switch and harmonic  problem. If the dead time is too short it will damage the switch and if dead time is too long it will increase the total harmonic distortion. The microcontroller Atmega16L has a built in 8 channel, 10 bit ADC and 32 registers connected directly with the ALU. The port A was used for ADC. The Port C was used to produce the four control signals. The C code was written using CodeVisionAVR software and using the same software the C code was converted into HEX file. This HEX file was then uploaded into the microcontroller using USB programmer which is a USB in-circuit programmer  for Atmel AVR controllers as shown in the figure below. The C code was shown in the appendix.

Microcontroller  ATMEGA 8L

Crystal

Figure 3.1.2: USB Programmer  11

3.2 Opto-isolation In electronics, an opto-isolator, also called an optocoupler, photocoupler, or  optical isolator, is an electronic device designed to transfer electrical signals by utilizing light waves to provide coupling with electrical isolation  between its input and output. The main purpose of an opto-isolator is to  prevent high voltages or rapidly changing voltages on one side of the circuit from damaging components or distorting transmissions on the other  side. Optocouplers typically come in a small 6-pin or 8-pin IC package, but are essentially combination of two distinct devices: an optical transmitter, typically a gallium arsenide LED (light-emitting diode) and an optical receiver such as phototransistor or light-triggered DIAC. The two are separated by a transparent barrier which blocks any electrical current flow  between the two, but does allow the passage of light as shown in the figure  below. Optocouplers are essentially digital or switching devices, so they are best for transferring either on-off control signals digital data. Analog signals can be transferred by means of frequency or pulse-width modulation.

Figure 3.2: Optocoupler PC817

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In this project, four opto-couplers were used to isolate the four signals between the high voltage side and the low voltage side. The microcontroller typically operated at 5 volts. But the control signals need to be amplified because the minimum voltage necessary at gate of the MOSFET is 10 volts. So these four  control signals need to be amplified in order to turn on the MOSFETs.

3.3 Gate Drive Amplifying Circuit

The amplification of the control signals was done using BJTs. A  bipolar junction transistor (BJT) is a three-terminal electronic device constructed of doped semiconductor material and may be used in amplifying or  switching applications. In electronics, a common-emitter amplifier is one of  three basic single-stage bipolar-junction-transistor (BJT) amplifier topologies, typically used as a voltage amplifier. In this circuit the base terminal of the transistor serves as the input, the collector is the output, and the emitter is common to both (for example, it may be tied to ground reference or a power  supply rail), hence its name.

Figure 4.5.3: Basic NPN common-emitter circuit (neglecting biasing details).

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Each output of the opto-couplers was connected with a BJT. Each BJT was supplied with 12 Volts from a power supply. Four power supplies were used for  four BJTs. This was done in order to make isolation between each control signals. This was very important because any interference between the control signals can ruin the output of the inverter. The outputs of the BJTs were connected with the gates of the Power MOSFET. These power MOSFETs (UTC 4N60) require at least 10 Volts at gates to turn on.

The UTC 4N60 is a high voltage MOSFET and is designed to have better  characteristics, such as fast switching time, low gate charge, low on-state resistance and have a high rugged avalanche characteristics. This power  MOSFET is usually used at high speed switching applications in power  supplies, PWM motor controls, high efficient DC to DC converters and bridge circuits.

3.4 Total Circuit Diagram

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Figure 3.4.1: Schematic diagram of the Inverter.

The schematic was designed using the software- Proteus Design suit as shown in the figure above. The schematic was then run and analyzed for the desired output. The code for ATMEGA 16 L microcontroller was written using CodeVisionAVR software. After obtaining suitable output, the design was implemented in the bread broad for the first time. When the practical output matched with the theoretical output then the PCB fabrication of both the control circuit and the power circuit was prepared.

3.4.1 Control Circuit The Microcontroller ATMEGA16L produced four signals through its Port A. These four signals are named as Signal A, Signal

, Signal B, Signal . All

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these signals are in the shape of a square wave. Signal

and Signal

are the

inverted outputs of Signal A and Signal B respectively but Signal B is 90 o shifted of Signal A. Then these signals are inputted into the four opto-couplers.

16 MHz Crystal

12V Re ulator 

5V Re ulator  BJT NPN

Microcontroller  ATMEGA 16L

Optocoupler 

Figure 3.4.2: Control Circuit The opto-coupler (pc817) was used to isolate the low voltage side and the high voltage side of the Control Circuit. The optocoupler is an electronic device designed to transfer electrical signals by utilizing light waves to provide coupling with electrical isolation between its input and output. Four separated  power supplies were used to power up the opto-couplers and the transistors. The transistor used to amplify the signals is BJT (ISC1815). The four BJTs  produced four square waves oscillating between 0V and 10V. These signals were then connected with the gates of the MOSFETS in the Power Circuit. The BJTs were powered by four isolated power supplies and a regulator was used in order to ensure that a constant 10V DC was supplied all the time.

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3.4.2 Power Circuit The power circuit is designed as a Half Bridge Inverter. There are two half   bridge inverter circuits for the two phases- Phase A and Phase B. MOSFET 1 and MOSFET 2 controlled the output of Phase A and MOSFET 3 and MOSFET 4 controlled the output of Phase A as shown in the figure below. The Signal A and the Signal

from the control circuit are inputted to the gates of MOSFET 1

and MOSFET 2 respectively in order to produce an alternating current (AC) for  Phase A. Similarly The Signal B and the Signal

from the control circuit are

inputted to the gates of MOSFET 3 and MOSFET 4 respectively in order to

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 produce an alternating current (AC) for Phase B. This is how two AC signals are produced to run a split phase induction motor.

MOSFET 1 MOSFET 2

Capacitor 

MOSFET 3

400V,1000uF

MOSFET 4

Figure 3.4.3: Power Circuit

Chapter 4 Results 4.0 Introduction In this chapter the results of this project is mentioned in details. The problems and difficulties that were faced during the design and experimental setup of this  project are also mentioned.

4.1 Results 18

In this project the inverter was built successfully. A split phase AC induction motor was run from a dc supply of 110 volts. The motor rotates at 1450 rpm measured by a Tachometer. A voltage drop of 65 volt was found across each  phase and the neutral. The control signals were produced in such a way so that the switching of the four MOSFETs was smooth enough to produce an alternating signal in each of the phases.

4.2 Different Waveforms The control signals are shown below

Signal A

Signal B

Figure 4.2.1: Signal A and Signal B.

Signal A or  Signal B

Signal

or 

Figure 4.2.2: Signal

and Signal .

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The output wave shapes for each phase is shown below.

Alternating signals at Phase A or Phase B

Figure 4.2.3: Output wave shapes for one phase

Chapter 5 Conclusion 5.1 Discussion In this project an inverter of robust architecture was built. It has the capability to convert DC into HVAC. Four separated power supply was used to produce the four control signals which was then inputted to the ground of four heavy duty MOSFETs. These MOSFETs can withstand very high current and heat sink was used to prevent excessive temperature rise. The microcontroller was powered from another power supply. For safety purpose in the power circuit a fuse of 5A 20

current was placed. However many problems were faced while building the control circuit and the power circuit. The PCB of control circuit was prepared for four times. To prepare the isolation  between the high voltage side and the low voltage side optocoupler was used. The high voltage side was designed very carefully so that the four signals have complete isolation. BJT is a temperature sensitive device, with temperature its operation varied. But the BJTs that were used in this project can withstand wide temperature range without changing its output. The control circuit was made with lots of flexibility like any component can be replaced at any time if found faulty. Sockets were used to place the components in the PCB board so that they can be replaced at any time. The control circuit is a very important part of this  project since it controls the switching of the MOSFETs in the power circuit. So very cautiously the control circuit was designed. The PCB of power circuit and the rectifier circuit was prepared meticulously  because these boards need to endure very high current. Soldering lead was  poured all over the copper path of both the circuits as shown in the figure  below. The POWER MOSFETs used in the circuit can withstand very high voltage and current, and capable of fast switching. However the power circuit was prepared successfully in the end.

Figure 5.1.1: Power circuit (bottom layer)

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Figure 5.1.2: Full bridge and Half bridge rectifier circuit (bottom layer)

Another difficult and tiresome job was to find an induction motor operating at 110V @ 50 Hz in this part of the world (Asia Minor) since over here the supply line voltage is 220V @ 50 Hz. So all the equipment used in this country are rated as 220 V @ 50 Hz. For this reason an auto-transformer was used to step down the voltage as explained in Section 6.4. The auto-transformer has the following ratings: 100 W, 110 V @ 50 Hz, 5~10 A. That is why it can be used easily to power up the power circuit and run the motor. All the components used in this project were of very high quality and best in the local market. This equipment can withstand high voltage and current with less switching time (in case of the MOSFETs). To cope with the heat in the electrical component heat sink was used where necessary. Fuse was also used in the power circuit to prevent high current flow. Necessary precautions were taken at the places where adverse outcome can take place. All these were done to make a robust architecture of this inverter which can cope with wide range of  environments. This project can be extended further as will be explained in the next section.

5.2 Future Work  The main objective of this experiment was to control the two phase inverter  using the microcontroller was done successfully. The split phase induction 22

motor rotates at 1450 rpm. The output wave shapes were shown in page 19. The  project can be extended further and can be modified like the frequency of the output wave can be varied in order to vary the speed of the motor. With this microcontroller (ATMEGA 16L), the output signal of each phase can be varied as necessary. To do this, the program of the microcontroller needs to modify. This can be easily done using the Code Vision AVR software to build the  program and using the USB programmer to burn the program into the microcontroller. The control signals can be modified further in order to get a sinusoidal ac output from the inverter instead of square wave. The square wave produces more harmonic contents in inverter output compared to sinusoidal wave. This can be done by pulse width modulation switching technique. In future this technique can be implemented by modifying the control circuits and the C code used for  microcontroller. Power inverters are increasingly becoming a must-have in many aspects of  human lives. Power inverters need to be designed more precisely so that it can  perform efficiently in order to fulfill our needs. So in future more research should be done on power inverter.

REFERENCES

1. A.M. Khaled & M.E. Abozaed, Microcontroller Based Variable  Frequency Power Inverter., vol. 2, Hong Kong: 2010, pp. 1–4.

2. M.N. Norkharziana, Design of a microcontroller-based converter for 3 phase brushless dc motor drives., 2009, pp. 23–32.

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3. B.B. Ismail, Design and Development of Unipolar SPWM Switching   Pulses for Single Phase Full Bridge Inverter Application., 2008, pp. 7–  10, 24-48.

4. B. Ismail, S. Taib & A.R. Mohd Saad, “Development of control circuit for single phase inverter using atmel microcontroller .”, in International Conference on Man-Machine System, 2006,

5. Dustin Bailey, Jason Horner, Daniel Martin, and Min-chiat Wee , “12 VDC / 120 VAC POWER INVERTER”, Department of Electrical and Computer Engineering, Mississippi State University.

6. Bedford, B. D.; Hoft, R. G. et al. (1964). Principles of Inverter Circuits.  New York: John Wiley & Sons.

7. Wikipedia “Inverter (electrical)”, Internet: http://en.wikipedia.org/wiki/Inverter_(electrical), 16 Nov 2011 [Nov 20, 2011].

8. D. Bailey, J. Horner & D. Martin, 12 VDC / 120 VAC Power Inverter, Mississippi State University: Department of Electrical and Computer  Engineering, 2004, pp. 1–15.

9. A.F. Zaidi, A. Muhida & A.M. Zaidi, “Bridge Circuit Rectifier Design”,  Development of Microcontroller Based Inverter Control Circuit for   Residential Wind Generator Application., Malaysia: 2009, pp. 55–70. 24

10.Obasohan I. Omozusi, “Dynamics And Control of a Battery Inverter  Single-Phase Induction Generator System”, M.A. thesis, Tennessee Technological University, USA, 1998.

APPENDIX 25

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