SANG GIDEON KIPCHIRCHIR

Transcription

1 University of Nairobi MICROCONTROLLER BASED THREE PHASE INVERTER Project Index: PRJ 12 By SANG GIDEON KIPCHIRCHIR F17/2161/24 Supervisor: Dr.-Ing. W. Mwema Examiner: Mr. Ogaba Project report submitted in partial fulfillment of the requirement for the award of the degree of Bachelor of Science in ELECTRICAL AND ELECTRONIC ENGINEERING of the University of Nairobi Submitted on 2 th may, 29.Department of Electrical and Information Engineering

2 Dedication This project is dedicated to my parents Mr. and Mrs. Koech who have been my source of inspiration all my life. Your love, care and support throughout my life means the world to me. ii

3 ACKNOWLEDGEMENT First and foremost I would like to thank God for bringing me this far. Most gratitude goes to my project supervisor, Dr.-Ing. W. Mwema for his unrelenting advice and guidance in the design and implementation of the project. I would also like to thanks my cousins Samson and Gilbert for moral and financial support. Lastly I would like to take this opportunity to give special thanks to all my classmates. Your continual support and trust in my abilities has not gone unnoticed iii

4 ABSTRACT Use of photovoltaic systems to generate electricity in homes and businesses is becoming increasingly popular, as the cost of conventional electric energy increases while the costeffectiveness of solar power systems improves. While much attention is paid to gradual improvements in the efficiency of solar cells, steps must be taken to improve the efficiency of the power conversion electronics of the system. Solar electric systems incorporate inverters or power control units that transform the DC electricity generated by the solar cells into AC to run appliances or sell to a utility grid. Inverters convert DC battery power to standard AC power. The AC power produced can run regular AC appliances, including TVs, computers, microwaves and power tools. This project presents a design that will attempt to convert 12 V DC power to a three phase 12 V AC power at 5 Hz. The design is based on CMOS logic inverters made up of power MOSFETS and a microcontroller. Simulation is carried out and actual implementation done. From the laboratory measurement, the inverter is seen to generate a three phase 118 V AC at 47 Hz. The discrepancy in frequency of oscillation from the design value can be attributed to the execution time and propagation delays of the microcontroller and other components. iv

5 Table of contents Dedication... ii ACKNOWLEDGEMENT... iii ABSTRACT... iv 1. INTRODUCTION Objective The need for inverter circuit Recognition of previous work LITERATURE REVIEW Amplifier type sine-wave inverter The saturated switch The Voltage driven inverter... 4 Preloading the inverter... 7 Using feedback diodes The current driven inverter THREE PHASE INVERTER conduction conduction Control of inverter output voltage Reducing of harmonics of the inverter output PERFORMANCE PARAMETERS (1) Harmonic factor of nth harmonic (HF n ) (2)Total harmonic distortion, THD (3)Distortion factor, DF (4)Lower-order harmonic, LOH CHAPTER 3: INVERTER DESIGN The power MOSFET switching circuit Gate drive signals Switching circuit CHAPTER 4: IMPLEMENTATION v

6 4.1 Gate drive circuit CHAPTER 5: RESULTS OBTAINED AND ANALYSIS CHAPTER 6: CONCLUSION AND FUTURE WORK Conclusion Recommendation for future work APPENDIX... 3 APPENDIX A. CIRCUIT DIAGRAM FOR THE IMPLEMENTATION OF THE PROJECT... 3 APPENDIX B. 1 MICROCONTROLLER ASSEMBLY CODE APPENDIX B 2. AVR microcontroller hex file for the code developed APPENDIX C: Three phase inverter conduction modes conduction conduction mode of operation APPENDIX D: DATASHEETS ATMEL 8-BIT MICROCONTROLLER DATASHEET IRF954 P-CHANNEL MOSFET ELECRICAL PROPERTIES IRF83 N-CHANNEL POWER MOSFET ELECTRICAL PROPERTIES Electrical Characteristics of the voltage regulator used to power the microcontroller REFERENCE vi

7 1. INTRODUCTION 1.1 Objective The project aimed to come up with specification, design and implementation of a microcontroller based three phase inverter that can work with a solar power panel. In the design proposed, 12 V DC from the power supply is used as the input. 1.2 The need for inverter circuit When it is required to provide AC power for a load from a DC supply as the only source of power for example in the case of solar power, there is need for conversion of the available DC energy to AC. Most industrial and domestic application utilize AC energy hence the need for the conversion. The design proposed in this project can be described by the block diagram of Figure 1.1 the switching signals are generated by the microcontroller while CMOS logic inverters are used for switching. DC BUS VOLTAGE THREE PHASE CMOS LOGIC INVERTER THREE PHASE LOAD GATE DRIVE SWITCHING SIGNAL Figure 1.1 Block diagram of the proposed design of the inverter 1.3 Recognition of previous work Most power inverters available in the market for domestic purposes are single phase. This means only single phase machines can be run on such inverters. To cater for low power three phase 1

8 machines, there is need for the design of three phase inverters. This project tries to solve this problem by converting DC voltage to three phase AC. This project is organized in six chapters. Chapter one gives a general introduction, project objective and the need for power inverter. Chapter two gives the theory and background information concerning power inverter. The principles of operation of both single phase and three phase inverters are outlined here. The performance parameters are also described. Chapter three describes the system design. Operation of CMOS logic inverter and how it is used to realize a power inverter is described in this chapter. A single phase simulation of an inverter is described and the results explained. Chapter four explains the actual implementation of the three phase inverter using CMOS logic inverters. The gate drive circuit used in implementation is described in this chapter. Chapter five gives the results and analysis of various waveforms obtained at different stages in the implementation of the project. The waveforms were edited using picture editing software for clarity. Chapter six gives the conclusion and recommendation for future work on this project. 2

9 2. LITERATURE REVIEW An inverter circuit is used to convert DC power to AC power. This conversion is achieved either by transistors or by SCRs. For low power and medium power output, common MOSFETs and BJTs transistors are suitable but for high power outputs SCRs and high power transistors such as IGFET are used. For low power self oscillating, transistorized inverters are suitable but for high power output, driven inverter are more common than self oscillating ones [1]. Moreover for multiphase ac output, driven inverters must be used. The driven inverters have better frequency stability because a separate master oscillator is used for the purpose. For inverter applications, transistors have the following advantages over SCRs: Higher switching speed Simplicity in control circuit Higher efficiency and greater reliability This is mainly due to the fact that SCR inverters require extra circuit to turn SCRs off, moreover additional complex logic circuits may be required to prevent false triggering and provide proper commutation timing. SCRs can handle much higher load current than BJTs and MOSFETs thus, for high power output, SCRs become more desirable than the transistors. Inverter circuits may be divided broadly into two classes namely: [1] 1. Amplifier type sine-wave inverter 2. Saturated switch type square wave inverters 2.1 Amplifier type sine-wave inverter Transistors are used as amplifiers operating in a non saturated condition. The efficiency of this type of inverter is generally low because of high power dissipation in the transistors. Another problem is the crossover distortion in class B and C push-pull circuit. These circuits are suitable for low power outputs where load power factor and load regulation are not important and efficiency is not a criterion. 2.2 The saturated switch The saturated switch type inverter has high efficiency because transistors or SCRs are operated as switches that are either in fully saturated conducting mode or in cut off blocking mode. The 3

10 losses in the semiconductor device are thereby reduced considerably consequently improving the efficiency and power output as compared to an amplifier type circuit using transistors with same rating. These inverters can be classified into two groups namely: Voltage driven inverter Current driven inverter The Voltage driven inverter A voltage driven inverter is defined as any inverter in which the circuit connects to a DC voltage source through semiconductor switches directly to the primary of a transformer. This is illustrated in Figure 2.1. Driving circuit S 1 i s1 V1 Y T1 LOAD S 2 i s2 Z Figure 2.1 Basics scheme Voltage-driven Inverter In Figure.2.1, S 1 and S 2 are semiconductor switching devices which open and close alternately at regular intervals depending on the desired output frequency.v1 is a DC voltage source. When S 1 is closed, the entire source voltage appears across the transformer primary between X and Y. The saturation voltage drop of the device is small and is generally neglected. S 1 remains closed for certain period of time after which it is opened and S 2 closed. S 2 remains closed for the same period of time during which the supply source is impressed across transformer primary between the point Y and Z. S 2 then opens out and S 1 closes. Thus an alternating voltage is generated across the primary of transformer and delivers power to the load through the secondary. Since the direct current supply is impressed directly on the primary of the transformer, the output waveform of the inverter is always a square-wave irrespective of the type of load and load power factor. The transformer primary current is not always a square-wave since it depends on the type of load and the load power factor. 4

11 Types of load Resistive load The resistive load poses no major problem to the inverter. The voltage waveform is a squarewave and since current and voltage are in phase the current waveform is also square-wave. This is as illustrated in the Figure 2.3 (v) (v) (A) Figure 2.3 Voltage and current waveform for a resistive load Each semiconductor switch conducts for 18 and the magnitude for current depends on the load demand. The power delivered by each semiconductor device is and current waveform is a square-wave whose area is proportional to the power delivered. Inductive load An AC source when operating on a power factor load delivers power to the load in one halfcycle and receives power from the load in the next half-cycle. In static inverter the actual power source is DC and if it has to operate on a power factor load, it must be capable of delivering power in one half-cycle of the inverter and receiving power in the next half-cycle. In voltage driven inverter, the transformer voltage is always a square-wave since it works in sequence with the driving circuit and consequently the current must shift in phase. Therefore in some part of the voltage waveform, power is delivered to the load and the inverter must be capable of receiving power and delivering it to the source during the other part of the voltage-wave, or this power 5

12 must be dissipated on the load side of the inverter. The voltage and current waveforms for a purely inductive load are shown in Figure 2.4. (v) (v) (A) Figure 2.4 Voltage and current waveforms for a purely inductive load When the load voltage and current are both either positive or negative, power is absorbed by the load. But when the load voltage and current are in anti-phase, power is delivered by the load. In a voltage-driven inverter, the semiconductor devices should pass current as soon as they are switched on, that is S 1 should begin to conduct in the normal direction as the voltage crosses zero, but due to the inductive load, the current does not change direction instantaneously and continues to flow in the negative direction. This means that the inductive nature of the load attempts to force a reverse current through the devices. However, the semiconductor devices are unidirectional and block the required reverse current. Again the interruption takes place when the load current is at its peak. This sudden stoppage of current causes a very large reverse voltage spike to develop on the transformer primary. This reverse voltage is theoretically of infinite value which can destroy the devices. Switch S 2 would also face the same consequence when it tries to conduct at π. This problem can be overcome by providing a path for the load current to flow during the device switching period. There are two ways to make it effective namely: 6

13 1. Preloading the inverter 2. Using feedback diodes. Preloading the inverter Inverter preloading involves connecting a resistance in parallel with the load. This provides a path for the stored energy in the inductive load to dissipate itself and improves the load power factor. This method reduces the efficiency of the inverter to a large extent and because of large power dissipation across the resistor, the inverter size has to be greatly increased. Using feedback diodes This method provides a bypass path for the current across the switching semiconductor devices. This is done by connecting diodes across the semiconductor switches as sown in Figure 2.5 is1 Driving circuit S1 S2 S 1 S 2 i s2 is2 i s2 D1 D 1 V1 Y DC voltage source D2 D 2 Z T1 LOAD Figure 2.5 Circuit in a voltage driven inverter with purely inductive load and feedback diodes The diodes are referred to as feedback or free-wheeling diodes. When seen from the direction of the load the feedback diodes operate as rectifiers permitting reverse energy to flow from the load to the source. Consider the situation when switch S 2 is closed and S 1 is open, as S 2 is opened and S 1 is closed, the current through S 2 becomes zero abruptly but the energy in the inductive load tries to force current in the same direction. This creates a high surge voltage due to L if no path is available for the current to flow. To avoid this situation, diodes D 1 and D 2 are connected across the switches S 1 and S 2 respectively as shown in figure 2.5, the transformer acts as a source and excess voltage greater than the supply source forces current through the voltage source V 1 and through the diodes D 1. This continues till the transformer voltage becomes equal to or less than 7

14 the supply voltage. So long as D 1 is conducting S 1 is reverse biased by the voltage drop of D 1 and cannot conduct. As soon as current flowing through D 1 becomes zero, S 1 begins to conduct if it is still closed. The same phenomenon occurs in the reverse cycle when S 1 is opened and S 2 is closed. The average current through the supply source is zero since no active power is consumed by the purely inductive load. Capacitive load A capacitive load creates a similar problem to an inductive load for the voltage driven inverters. The voltage becomes a square wave while the current waveform changes considerably due to capacitive loading. The voltage and current waveforms of the transformer primary for purely capacitive load are shown in figure 2.6 e π π π ωt I ωt Figure 2.6 Voltage and current wave form for capacitive load Each time the semiconductor switch begins to conduct, large current spikes appear in the transformer primary because the square wave voltage of the transformer secondary supplies power to reverse-charge the capacitor through the very low impedance presented by the transformer winding and the reflected saturation resistance of the semiconductor switches. This current continues to flow till the charge across the capacitor builds up sufficiently. Due to these large current peaks, the losses in the inverter suddenly rise to a large value lowering the efficiency. Moreover the high value of exceeds the safe limiting value of the semiconductor 8

15 devices and permanently damages them. This problem is overcome by incorporating some resistance in the circuit to limit the peak current but this increases the size of the inverter. Motor load The voltage driven inverter does not operate satisfactorily on motor loads. At the time of startup, power requirements of a motor may be several times more than required in the normal operation. This extreme transient condition may continue for several seconds depending on the angular acceleration of the motor s rotor. Moreover, the power factor of the motor at this condition becomes extremely low and may be of the order.2 lagging. Even with a power factor improvement capacitor connected across the motor, the low transient power factor during startup cannot be compensated. To cater for transient power, the rating of the semiconductor devices and the transformer should be adequately increased and properly protected. Alternatively the motor inrush current could be restricted to a minimum value by inserting a current-limiting resistor in series with the motor. The loop response should be compatible with the motor, otherwise there will be hunting. That is sudden application or removal of motor load may generate oscillations which may continue indefinitely if a proper damping arrangement is not provided The current driven inverter In a current-driven inverter, the current is held at a constant value and fixed in phase with the switching time and the voltage waveform depends on the type of load. This means that a constant current is forced to flow through the semiconductor switch and the transformer primary for a full conduction period irrespective of the source-voltage waveform, type of load and power factor. A current-driven inverter is shown in Figure 2.7. T1 Driving circuit S 1 LOAD - S 2 + L1 C1 C L 1 L 2 L2 9

16 Figure 2.7 Schematic diagram of a current driven inverter The circuit of a current driven inverter is quite similar to that of the voltage driven inverter except that the supply source is a constant-current rather than a constant-voltage source. The constant voltage source can be converted to a constant current source by inserting a large choke L (theoretically of infinite inductance) in series with it. This choke, usually referred to as a feedback choke and must be sufficiently large to maintain a constant current flowing through the circuit under all conditions. The current waveform is a square wave irrespective if the type of load and power factor. Usually, the DC supply is a battery which should have sufficiently low impedance so that power can be drawn and fed back by the inverter whenever required. In practice however, all the reactive power cannot be dumped properly into the power source particularly when there are other equipments operating on the same DC bus. This would cause a large ripple current to appear on the same bus bar and cause interference with the operating of other equipment. To overcome this difficulty, an LC filter is always provided across the battery source as shown in Figure 2.7, L 1 attenuates the ripple current while C 1 serves to reduce the impedance of the dc source and is capable of delivering and receiving power during operation on a power-factor load. The LC filter in conjunction with a battery may be used either in a constant voltage or constant current inverter. The operation of a current driven inverter with various loads is shown with various loads is shown with the help of the waveforms in figure 2.8 I (A) π π π ωt a V L ωt b V L ωt c V L ωt d 1

17 Figure 2.8(a) Current waveform, (b) Load voltage waveform at a purely resistive load, (c) Load voltage waveform at a purely inductive load, (d) Load voltage waveform at a purely capacitive load. For a square-wave current, the voltage across a resistive load is a square wave in phase with the current waveform. For a purely inductive load the voltage is spiked and for a purely capacitive load, the voltage is triangular. The spiky nature of load voltage on a purely inductive load is unsuitable for practical purposes. A triangular waveform on a purely capacitive load means that voltage changes from positive to negative alternately in each half-cycle duration. In the same half cycle, power is delivered and received from the load without being transferred through the inverter. This shows that the current-driven inverter has the capability to handle power factor load without interrupting the semiconductor devices. 2.3 THREE PHASE INVERTER A three phase inverter may be regarded as three single phase inverters and the output of each single phase is shifted by 12 with respect to each other. The three single-phase inverters can be connected in parallel as shown in Figure 2.9 to form the configuration of a three phase inverter. The transformer primary winding may be connected in Y or delta. The transformer secondary is normally connected in Y to eliminate triple harmonics (n = 3, 6, 9..) appearing on the output voltage. [2] Inverter 1 A D T1 a Inverter 2 B E T2 b Inverter 3 C F T3 c n Figure 2.9 Block diagram of a three phase inverter 11

18 A three-phase output can be obtained from a configuration of six transistors as shown in Figure 2.1. Q1 Q3 Q5 V1 g1 D1 g3 D3 g5 D5 a b c Q4 Q6 Q2 v2 V2 g4 D4 g6 D6 g2 D2 Figure 2.1 three phase inverter formed by three single-phase inverters Two types of control signals can be applied to the transistor: 18 conduction 12 conduction conduction Each transistor conducts for 18. Three transistors remain on at any instance of time. When transistor Q1 is switched on, terminal a of Figure 2.1 is connected to the positive terminal of the DC source. When transistor Q4 is switched on, terminal a is connected to the negative terminal of the DC source. There are six modes of operation in a cycle and the duration of each is 6. The gating signals are as shown in Figure The transistors are numbered in the sequence of gating the transistors. That is 123, 234, 345, 456, 561, and 612. The signals are shifted from each other by 6 to obtain three phase balanced voltages. The load may be connected in Y or delta. For a delta connected load, the phase currents can be obtained directly from line to line voltages. Line currents are determined from phase currents. 12

19 For a Y-connected load, the line-to-neutral voltages must be determined to find the line currents. g1 g2 π 2π 3π ωt g3 ωt g4 ωt g5 ωt g6 ωt Vab ωt π 2π 3π ωt Vbc π 2π ωt Vca π 2π ωt Figure 2.11 Gating signal Waveforms for and 18 conduction conduction In this type of control the, each transistor conducts for 12. Only 2 transistors remain ON at any instance of time. The gating signals are as shown in Figure The conduction sequence of the transistors is 61, 12, 23, 34, 45, 56 and

20 g1 g2 π 2π 3π ωt g3 ωt g4 ωt g5 ωt g6 ωt Van ωt π 2π 3π ωt Vbn π 2π ωt Vcn π 2π ωt Figure 2.12 Gating signal for 12 conduction 2.4 Control of inverter output voltage There are many applications in which it is necessary to control the output voltage of the inverter. Two such application are a stabilized AC or DC voltage source from a battery whose voltage varies during discharge, and an AC motor control system, in which a constant voltage-tofrequency ratio has to be maintained to avoid saturation of the motor. In both cases, control of inverter output voltage is necessary [1]. The output voltage of the single-phase inverter is roughly square wave with amplitude approximately equal to the DC supply voltage. Therefore the output is proportional to the input voltage. 14

21 The common methods of output control are: Controlling DC input voltage Controlling AC output voltage Pulse width modulation If inverter is supplied from an AC source through a rectifier, the input to the inverter can be regulated by means of an induction regulator, variac or a controlled rectifier. If the supply is DC, it can be regulated by shunt or series regulator or chopper using time-ratio control method. The pulse width modulation can be applied for both types of inputs. 2.5 Reducing of harmonics of the inverter output The inverter output waveform may vary depending on the application and the circuit used. In most cases an AC load requires sinusoidal output but the majority of the inverter produces square wave voltages. Therefore appropriate means are used to alter the waveforms of the inverter output to a more or less sinusoidal wave shape.[1] Harmonic attenuation can be achieved by the following methods: Resonating the load Using LC filter Using pulse width modulation Using Polyphase inverters. 2.6 PERFORMANCE PARAMETERS The output of a practical inverter usually contains harmonics therefore, the quality of an inverter is usually evaluated in terms of the following performance parameters [2]. (1) Harmonic factor of nth harmonic (HFn) This is the measure of individual harmonic contribution and is defined as: HF n = 15

22 Where V 1 is the RMS value of the fundamental component and V n is the RMS value of the n th harmonic component. (2)Total harmonic distortion, THD This is the measure of closeness in shape between a waveform and its fundamental components. It is defined as: TDH = (3)Distortion factor, DF It is a measure of effectiveness in reducing unwanted harmonics without having to specify the values of a second order filter. DF indicates the amount of harmonic distortion that remains in a particular waveform after the harmonics of that waveform have been subjected to a second order attenuation. DF = The distortion of an individual (or nth) harmonic component is defined as: DF n = (4)Lower-order harmonic, LOH This is that harmonic component whose frequency is closest to the fundamental frequency, and its amplitude is greater than or equal to 3% of the fundamental component. CHAPTER 3: INVERTER DESIGN 3.1 The power MOSFET switching circuit A power MOSFET is a voltage controlled device and requires little input current. It has a high switching speed and time on the order of nanoseconds and is used for low power high frequency converters. Figures 3.1(a) and (b) shows the switching circuits used in the DC to AC inverter designed.a 16

23 CMOS switch was implemented using power MOSFETs. Two sets of CMOS MOSFET circuits are used and are controlled by the anti-phase signals generated by the microcontroller. Q1 L Q3 N-MOS OFF A T1 B +12 V Q2 P-MOS OFF H Q4 Figure 3.1(a) switching circuit used to implement the inverter In the case when the gate inputs of transistors Q1 and Q3 are L level signifying volts, and the inputs of transistors Q2 and Q4 are H level signifying 5 volts, transistors Q1 and Q4 are turned ON while transistors Q2 and Q3 are OFF. Therefore, the electric current flows through the direction of A to B on the primary coil of the transformer as shown in figure 3.1(a). 17

24 Q5 P-MOS OFF H Q6 A T2 B +12 V Q7 L Q8 N-MOS OFF Figure 3.1(b) Switching circuit used to implement the inverter Considering when the input of transistor Q5 and Q6 are H level and the input of transistors Q7 and Q8 are L level. Transistors Q6 and Q7 are ON while transistors Q5 and Q8 are OFF. Therefore, the electric current flows through the direction of B to A on the primary coil of the transformer as shown in Figure 3.1(b). This is contrary to the case in Figure 3.1(a). To produce an ac signal, current is made to flow in one direction for half a period then reversed in the next half period. The duration of the period determines the output frequency. 3.2 Gate drive signals The gate drive signals were generated by the AVR microcontroller. 18

25 The ATtiny26L AVR microcontroller was chosen as the most appropriate source of gating signal because it has the following characteristics: It has an internal oscillator with frequencies ranging from 1 MHz to 8 MHz Most of its instructions are single clock cycle execution therefore executes faster. It has an on chip RAM of 128 bytes. It is programmed by connecting some of its pins directly to some pins of the computer parallel port. It is readily available and cheaper than most microcontrollers. The desired output frequency is 5Hz hence a period of.2 seconds equivalent to 2, microseconds. To obtain the three phase square wave AC signal, the three phases must be 12 out of phase as shown in the figure 3.2. Figure 3.2 Three phase square waveforms From the three phase waveforms drawn in Figure 3.2, it can be observed that for every one sixth of the period, one of the three waveforms will either be changing from high to low or from low to high. To achieve this, a delay of one sixth of the period corresponding to 3333 microseconds was created so that after 3333 microseconds one pin of the microcontroller would be cleared then another pin set and the delay subroutine executed. The default frequency of the microcontroller used is 1 MHz according to the Manufacturer s datasheet. This implies that one machine cycle is 1 microsecond. In creating a delay of

26 microseconds, two 8-bit registers were used to create software loops. This was done by loading the registers with a value 3333 and decrementing the value while monitoring the content of the register. When the value is zero, then the microcontroller clears one pin and sets another pin and the value loaded to the registers and decremented again [4]. R1, Y1 and B1 in figure 3.2 are taken as the pin from the microcontroller that switches the waveform from zero to a positive value while R2, Y2 and B2 switches the waveforms from zero to a negative value. The pins are connected to the coinciding gates R1, R2, Y1, Y2, B1 and B2 of Figure 3.4. The sequence of switching ON and OFF various pins is of the microcontroller to achieve a three phase square-wave waveform is as shown in the flow chart of Figure 3.3 The loop will continue indefinitely as long as power is connected to the device. 2

27 START CLEAR R2 AND SET R1 THEN LOAD VALUE 3333 TO THE REGISTER DECREMENT THE VALUE IN THE REGISTER NO IS THE VALUE ZERO? YES CLEAR B1 AND SET B2 THEN LOAD VALUE 3333 TO THE REGISTER DECREMENT THE VALUE IN THE REGISTER NO IS THE VALUE ZERO? YES CLEAR Y2 AND SET Y1 THEN LOAD VALUE 3333 TO THE REGISTER DECREMENT THE VALUE IN THE REGISTER NO IS THE VALUE ZERO? YES CLEAR R1 AND SET R2 THEN LOAD VALUE 3333 TO THE REGISTER DECREMENT THE VALUE IN THE REGISTER NO IS THE VALUE ZERO? YES CLEAR B2 AND SET B1 THEN LOAD VALUE 3333 TO THE REGISTER DECREMENT THE VALUE IN THE REGISTER NO IS THE VALUE ZERO? YES CLEAR Y1 AND SET Y2 THEN LOAD VALUE 3333 TO THE REGISTER DECREMENT THE VALUE IN THE REGISTER NO IS THE VALUE ZERO? YES Figure 3.3 Flow chart showing implementation of three phase wave forms by the microcontroller 21

28 3.3 Switching circuit The switching circuit for each phase consists of two CMOS logic inverters with their gates driven by two anti-phase signals from the microcontroller. Figure 3.3 shows three phase design of the inverter where the gate drive signals are generated by a microcontroller. The design is based on the saturated switch approach where high efficiency is achieved because transistors dissipate very little power. 12 V 9 Q1 Q5 Q9 R1 1 Y1 18 B1 2 Q3 Q6 Q1 Q2 Q7 Q R2 2 Y2 19 B Q4 Q8 Q T1 T2 T3 IRON_CORE_XFORMER IRON_CORE_XFORMER IRON_CORE_XFORMER Figure 3.4 Switching circuit of a three phase inverter The circuit of Figure 3.5 shows the simulation of a single phase inverter using MULTISIM POWER PRO Edition version simulation package developed by NATIONAL INSTRUMENTS. A simulation package which could incorporate a microcontroller could not be found therefore a signal generator was used in place of a microcontroller with input frequency of 5Hz and the voltage is 1V peak-to-peak. The output of the signal generator was split into two in creating anti-phase signals, one signal was passed through a logic inverter and the other passed through a buffer. This was necessary to ensure that both signals had the same propagation delay. An oscilloscope was connected to the output of the transformer. The output of the simulation is as presented in Figure

29 XFG1 V1 12 V Q1 5 U2B 2 IRF954 41BF_1V U1C 7 V2 12 V 4 Q3 IRF83 Q2 IRF954 3 V(p-p): 14.1 uv V(rms): 11.4 V V(dc): 11.4 V I: 4.83 A Probe1,Probe1 I(p-p): 384 ua I(rms): 4.83 A I(dc): 4.83 A Freq.: 5. khz 1 T1 IRON_CORE_XFORMER 8 A + _ XSC1 Ext Trig + _ B + _ 49BCP_1V Q4 IRF83 Figure 3.5 Single phase inverter simulation circuit t -2.45V Figure 3.6 Results of simulating the single phase inverter of Figure 3.5 From Figure 3.6, it can be seen that the output of the inverter is a square wave of output voltage 4.9V peak-to-peak centered at zero volts. The output voltage depends entirely on the transformer ratio. A step down transformer was used for the simulation. 23

30 CHAPTER 4: IMPLEMENTATION A microcontroller based three phase inverter was implemented using CMOS logic inverter. IRF954 PMOS and IRF83 NMOS power MOSFETs were used in the actual implementation of the CMOS logic inverter. These set of power MOSFETs were chosen because of the following reasons; They have freewheeling diodes internally connected between their drain and source They are also affordable They are locally available An LM785 voltage regulator was used to power the microcontroller. Its input voltage was 12V from the laboratory power supply and the output was a stable 5.1V. 4.1 Gate drive circuit The output of the microcontroller was a square wave of voltage 2.2V. This voltage could not drive the gates for the CMOS logic inverter because the threshold voltage for the MOSFETs is 4.5V. Power MOSFETs have large stray capacitance between the gate and source. The effect of this is that the gate voltage must first charge the capacitance before the gate is turned on. For efficient switching of the MOSFETs, the gate drive voltage need to be in the range of 1-2 V depending on the device rating. A simple BJT buffer circuit shown in Figure 4.1 was used for the gate drive. connection from the microcontroller output pins R2 R1 1kΩ Q1 connetion to the MOSFET gate 1kΩ BC17BP Figure 4.1 Buffer circuit used to drive the CMOS logic inverters The circuit in the Appendix A.1 was connected and the outputs at different points observed in the oscilloscope and recorded. 24

31 CHAPTER 5: RESULTS OBTAINED AND ANALYSIS Different waveforms were obtained at different stages in the implementation of the project. These are shown in Figures 5.1 to 5.6 Figure 5.1 shows a square wave obtained from all the pins of the microcontroller that were to be used. The voltage is 2.2 V at a frequency of 47 Hz. The desired output frequency was 5 Hz. Figure 5.1 Square waveform produced by the microcontroller pins The waveform of Figure 5.1 is desirable since the delay created sets a pin to high then call the delay subroutine after which it clears the pin then calls the subroutine. The duty cycle is thus 5% since the same delay subroutine is implemented upon setting a pin to either high or low. This sequence is continued indefinitely as long as power is connected to the microcontroller. Figure 5.2 shows the anti-phase square waveforms generated by the microcontroller pins to be connected to the three transformer primary coils. The waveforms have been coloured and one waveform shifted downwards in position on the oscilloscope for clarity. It can be seen that both waveforms have the same frequency and duty cycle of 5%. Similar waveforms could be obtained for the other two phases. 25

32 Figure 5.2 Anti-phase signals that drives the CMOS logic inverter gates Figure 5.3 shows the 12 phase difference between the red and the yellow phase. The waveforms were obtained after connecting one channel of the oscilloscope to the output of the buffer connecting pin R1 from the microcontroller and the second channel was connected to the output of the buffer connecting pin Y1 from the microcontroller. The two waveforms can be seen to have a phase difference of 12, same frequency and duty cycle of 5%. Figure 5.3 Waveforms showing the 12 phase difference between red and yellow phase 26

33 Figure 5.4 Waveforms showing the 24 phase difference between red and blue phase Figure 5.4 shows the 24 phase difference between the red and the blue phase. The waveforms were obtained after connecting one channel of the oscilloscope to the output of the buffer connecting pin R1 from the microcontroller and the second channel was connected to the output of the buffer connecting pin B1 from the microcontroller. The two waveforms can be seen to have a phase difference of 24, same frequency and duty cycle of 5%. Figure 5.5 Waveforms showing the 12 phase difference between yellow and blue phase 27

34 Figure 5.5 shows the 12 phase difference between the yellow and the blue phase. The waveforms were obtained after connecting one channel of the oscilloscope to the output of the buffer connecting pin Y1 from the microcontroller and the second channel was connected to the output of the buffer connecting pin B1 from the microcontroller. The two waveforms can be seen to have a phase difference of 12, same frequency and duty cycle of 5%. The output voltages obtained at the output of each transformer was 118 V AC at a frequency of 47 Hz. From the design the desired output voltage was 12 V AC at 5 Hz. The difference between the two values of frequency can be attributed to the use of different components with unequal propagation delay. The three phases of the inverter implemented gave same values in terms of voltage and frequency. The current that the inverter can draw from the source will depend on the load to be driven. 28

35 CHAPTER 6: CONCLUSION AND FUTURE WORK 6.1Conclusion In this project an attempt has been made to come up with a three phase inverter that is suitable for low power applications. The design was simulated and actual implementation carried out from which 118 V three phase AC was generated from a laboratory 12 V DC power source. The frequency of the output voltages was 47 Hz. The desired output frequency was 5 Hz the difference can be attributed to execution time and propagation delay of the various components used. An attempt was however made to take care of these factors by manipulating the value loaded to the registers that created the software loops in the microcontroller. After several attempts a frequency of 47 Hz was achieved. 6.2 Recommendation for future work The implementation of this project is not conclusive. A lot is still to be done to increase the output power.the following recommendations are suggested for better performance, 1. To obtain a proper sinusoidal ac power output, advanced means of harmonic reduction should be employed. These includes: staircase modulation, stepped modulation, harmonic injection modulation and trapezoidal modulation [3]. 2. To ensure high switching speed of order of 1 nanoseconds, a proper charging and discharging circuit should be provided to every CMOS logic inverter gate. 3. The output frequency can still be improved by loading the registers in the microcontroller responsible for creating delay with different values until the desired output frequency is achieved. 29

36 APPENDIX APPENDIX A. CIRCUIT DIAGRAM FOR THE IMPLEMENTATION OF THE PROJECT Figure A.1 shows the Circuit diagram used in the implementation of the project. The circuit was designed using MULTISIM POWER PRO developed by NATIONAL INSTRUMENTS. The power source used was a laboratory power supply of 12 V DC. 27 V1 12 V U1 LM785KC LINE VREG VOLTAGE C1 COMMON 33nF 16 C3 1uF AT tiny26l C2 1nF R3 1kΩ 17 R4 1kΩ 1 Q14 BC17BP Q1 IRF954 2 Q3 IRF83 R5 1kΩ 19 R6 1kΩ 5 Q15 BC17BP Q5 IRF954 6 Q6 R9 1kΩ IRF83 BC17BP IRF83 23 R1 1kΩ Q17 9 Q9 IRF954 1 Q1 2 R1 1kΩ 15 R2 1kΩ 3 Q13 BC17BP Q2 Q4 R7 Q7 R8 R12 1kΩ 1kΩ IRF954 IRF954 IRF Q16 Q8 1kΩ IRF83 BC17BP IRF83 7 R11 1kΩ 25 Q18 BC17BP Q11 Q12 IRF83 T1 T2 T IRON_CORE_XFORMER IRON_CORE_XFORMER IRON_CORE_XFORMER Figure A.1 Microcontroller based three phase inverter circuit The transformers used in Figure A.1 are 12/24V step up centre tapped transformers. 3

37 APPENDIX B. 1 MICROCONTROLLER ASSEMBLY CODE The code below was loaded on the ATtiny26l AVR microcontroller for creating the gate signals. Six pins on port A of the microcontroller were used. Pins and 1 generated gate anti-phase drive signals for the red phase, similarly pins 2 and 3 generated anti-phase signals for yellow phase and pins 5 and 6 generated anti-phase signals for blue phase. To achieve three phase waveforms, a value equals to a sixth of the period should be used to create the delay. For an output frequency of 5 Hz a delay of 3,333 microseconds should be created. However, this value could not be loaded directly on the registers of the microcontroller because there are many components that the signal passes through introducing their delay. After several trials a frequency of 47 Hz was obtained after loading the register with value equal to INCLUDE "tn26def.inc".def mp = R16 main: rjmp main ;Includes the tn26 definitions ;file ldi mp,low(ramend) ;Initiate Stackpointer out SP,mp ldi mp,xff ; 8 Ones into the universal register out PORTA,mp ; and to port A (these are the pull-ups) ldi mp,xff ; 8 Ones to the universal register out DDRA,mp ; and to the data direction register ldi mp, out PORTA,mp AGAIN: CBI PORTA, SBI PORTA,1 RCALL DELAY CBI PORTA,5 ;clear pin of port A ;set pin 1 of port A ;call delay subroutine ;clear pin 5 of port A 31

38 SBI PORTA,4 ;set pin 4 of port A RCALL DELAY ;call delay subroutine CBI PORTA,2 ;clear pin 2 of port A SBI PORTA,3 ;set pin 3 of port A RCALL DELAY ;call delay subroutine CBI PORTA,1 ;clear pin 1 of port A SBI PORTA, ;set pin of port A RCALL DELAY ;call delay subroutine CBI PORTA,4 ;clear pin 4 of port A SBI PORTA,5 ;set pin 5 of port A RCALL DELAY ;call delay subroutine CBI PORTA,3 ;clear pin 3 of port A SBI PORTA,2 ;set pin 2 of port A RCALL DELAY ;call delay subroutine RJMP AGAIN ; jump to label AGAIN to repeat the sequence. ; delay loop generator DELAY: LDI R21, $B ;load register 21 with value decimal 11 LOOP: LDI R22, $DD ;load register 22 with value decimal 221 LOOP1: DEC R22 ;decrement value in register 22 by 1 BRNE LOOP1 ;branch to loop1 if value in register 22 is not equal to zero else ;proceed to the next instruction DEC R21 ;decrement the value in register 21 by 1 BRNE LOOP ;branch to loop if value in register 21 is not equal to zero else ;proceed to the next instruction ; ============================= RET ;return to the main program. 32

39 APPENDIX B 2. AVR microcontroller hex file for the code developed. The hex file for the microcontroller program used in the implementation of the project is given below. This is the obtained after conversion of the assembly code in Appendix B1 and is the value loaded in the microcontroller.the software used to load the hex code to the microcontroller was WINAVR developed by ATMEL. :22FC :1CFEDDBFFEFBBBFEFABBE 1 :11BBBD898D99A1DDD98DC9ADDDA981D :12DB9AADD998D89A7DDC98DD9A4D8 :13DB98DA9A1DEDCF55E6DED6A95F1F7D6 :C45A95D9F751E5A95F1F78955 :1FF 33

40 APPENDIX C: Three phase inverter conduction modes 18 conduction There are three modes of operation in a half cycle and the equivalent circuits are shown in Figure C.1. The output waveforms for the line voltages are shown in Figure C.2 R R c c V1 V2 V3 b R b R a R c R MODE 1 MODE 2 MODE 3 Figure C.1 V an 2V s /3 V s /3 π 2π 3π Vbn V bn 2V s /3 V s /3 π 2π 3π Vcn V cn 2V s /3 V s /3 π 2π 3π 34

41 Figure C.2 Phase voltage for 18 conduction During mode 1, for ωt R eq = R + R = i 1= = V an = V cn = = V bn = = During mode 2, for ωt R eq = R+ i 2 = = V an = i 2 R = V bn = V cn = = During mode 3 for, ωt π R eq = R + i 3 = = V an = V bn = = 35

42 V an = = The instantaneous line-to-line current voltage, V ab, in Figure C.2 can be expressed in a Fourier series, recognizing that V ab is shifted by 3 and the even harmonics are zero. V ab = (a) V bc and V ca can be found by phase shifting V ab by 12 and 24 respectively. V bc = (b) V ca = (c) From equations (a), (b) and (c) it can be noticed that the triple harmonics n=3, 6, 9, would be zero in the line to line voltages. The line to line RMS voltages can be found from, V L = (e) = =.8165 From equation (a) the RMS nth component of the line voltage is V LN = (f) Which for n=1, gives the fundamental line voltage. V L1 = The RMS value of the line-to-neutral voltages can be found from the line voltage Vp = = = 36

43 With resistive load, the diodes across the transistors have no function. If the load is inductive, the current in each arm of the inverter would be delayed to its voltage. The transistors must be continuously gated since the conduction time of transistors and diodes depends on the load power factor. For a Y connected load, the phase voltage is Van = with a delay of 3. The line current i a for an RL load is given by: i a = Where θ n = tan conduction mode of operation There are three modes of operation in one-half cycle and the equivalent circuits are Y connected loads. During mode1, for ωt, transistors 1 and 6 conduct, Van = Vbn = Vcn = During mode 2, for ωt, transistors 1 and 2 conduct, Van = Vbn =, Vcn = During mode 3 for, ωt π, transistors 2 and 3 conduct Van =, Vbn =, Vcn = The line to neutral voltages can be expressed in Fourier series as V an = V bn = 37

44 V cn = The line voltage between line a and b is V ab = V an with a phase advance of 3. There is a delay of 3 between turning off transistor Q1 and turning on transistor Q4. Thus there should be no short circuit of the dc supply. At any time there, two load terminals are connected to the dc supply and the third one remains open. Since the transistor conducts for 12 the transistors are less utilized as compared to that of the 18 conduction for the same load. 38

45 APPENDIX D: DATASHEETS ATMEL 8-BIT MICROCONTROLLER DATASHEET. 39

46 4

47 41

48 42

49 43

50 44

51 IRF954 P-CHANNEL MOSFET ELECRICAL PROPERTIES 45

52 IRF83 N-CHANNEL POWER MOSFET ELECTRICAL PROPERTIES 46

53 Electrical Characteristics of the voltage regulator used to power the microcontroller. 47

54 REFERENCE [1] P C SEN, POWER ELECTRONICS, 1987, TATA McGRAW-HILL PUBLISHING COMPANY LIMITED, printed by Rajkamal Electric Press, B 35/9 G T Karnal [2] MUHAMMAD H.RASHID, POWER ELECTRONICS AND CIRCUITS DEVICES APPLICATIONS, SECOND EDITION, 1993,Prentice-Hall, Inc, One Lake Street, Upper Saddle River, New Jersey 7458, U.S.A. [3] John G. kassakkian Martin F. Schlecht George c. Verghese, Principles of power Electronics, 22 by Prentice-Hall(pearson education Inc.) New Jersey USA. [4] Dhananjay V. Gadre, Programming and customizing the AVR microcontroller, 21, by the McGraw-Hill companies, Inc. New York [5] [6] 48