MICROPROCESSOR BASED PWM INVERTER SYSTEM AND ITS USE FOR CONTROL OF INDUCTION MOTOR By MEHRAN MOTAMED EKTESSABI Thesis submitted in fulfilment of the requirements for the award of the degree of DOCTOR OF PHILOSOPHY 0 i La DEPARTMENT OF ELECTRICAL ENGINEERING INDIAN INSTITUTE OF TECHNOLOGY, DELHI February, 1989
Certificate Certified that the thesis entitled "Microprocessor Based PWM Inverter System And Its Use For Control Of Induction Motor", which is being submitted by Mehran Motamed Ektessabi in fulfilment for the award of the Degree of Doctor of Philosophy in Electrical Engineering of the Indian Institute of Technology, Delhi, (India), is a record of the students own work carried out by him under our joint supervision and guidance. The matter embodied in this dissertation has not been submitted for the award of any other Degree or Diploma. (C.M.Bhatia) Associate Professor Department of Electrical Engineering 4 (SILamba) Professor & Head of Department of Electrical Engineering Indian Institute of Technology New Delhi, India.
ACKNOWLEDGEMENT I am deeply indebted to my supervisors, Prof. S.S.Lamba and Dr. C.M.Bhatia from whom I received,continuous encouragement, guidance and advice. I wish to express my sincere thanks to both of them. I am also thankful to Prof. J.Nanda Ex. Head, and Prof. S.S.Lamba the present Head of the Electrical Engineering for permitting me to avail the laboratory facilities required for this research work. I also acknowledge the co-operation extended to me by the Pcwer Electronics laboratory staff members Mr. Chand Singh, and Mrs. Arun Gupta, and Post Graduate Machine laboratory staff Mr.R.P.Sharma, and Work-Shop staff Messers R.N.Singh, and Darshan Singh. I also acknowledge the help extended to me by my coresearcher Mr. T.M.George and all the other friends, from time to time during the completion of this work. cnt e)6.. (M.M.Ektessabi)
ABSTRACT Dedicated digital control techniques for three-phase PWM inverter used for AC drives have been reported in this thesis. In order to illustrate the advantages of the digital PWM technique over that of analog technique, a ROM based controller for three-phase PWM inverter has been designed and developed. In this controller the preformed look-up tables containing firing instants of the inverter SCRs are stored in an EPROM for constant V/f (constant torque) operation upto the rated 50 Hz frequency, and constant Horse-Power operation above this frequency with constant input voltage at its rated value. The range of frequency control of this modulator is designed to be from 5 Hz to 75 Hz, in Steps of 5 Hz each. Based on this technique a modulator for a 50 Hz, 1 KVA Uninterrupted Power Supply (UPS) has also been designed and fabricated. Since the microprocessor based modulators for PWM inverter systems are more flexible because of their ability to use the software programs, a new strategy very simple and versatile in nature, has been proposed in this thesis. This New Strategy named as Modified Natural Sampling (MNS) has been implemented using a very simple eight bit microprocessor so as to illustrate its simplicity. The newly proposed technique uses only
integer arithmetic for development of its software, thereby avoiding the problem of using a coprocessor. The technique obviates the solution of complex transcendental equations as is normally the case with Natural Sampling technique. -A vivid study of the harmonic contents using the main frame computer (ICL-2960) for the proposed technique and the other commonly used microprocessor techniques such as Regular Symmetrical Sampling (RSS), Regular Asymmetrical Sampling (RAS), and Modified Regular Sampling (MRS) is done. This comparison as reported in the thesis demonstrates the advantages of the proposed strategy over the other existing strategies. Next a thyristor based 10 KVA three-phase auxiliary impulse commutated inverter based on pole voltage modulation has also been designed and developed for the experimental purpose. Through suitable softwares developed and implemented using the proposed microprocessor based modulator, it is possible to obtain an independent control of the inverter output voltage and frequency so as to achieve four different modes of operation; such as Fixed-Voltage Fixed- Frequency (FVFF), Variable-Voltage Fixed-Frequency (VVFF), Fixed-Voltage Variable-Frequency (FVVF), as well as -iv-
Variable-Voltage Variable-Frequency (VVVF). These operations along with the other flexibilities as provided by the proposed PWM technique paved the way to have a more flexible AC drive system. The existing microprocessor based PWM techniques published so far, have their own unique merits and demerits, but the MNS based system reported in this thesis provides the advantages listed as under: 1) Simplicity for the microprocessor implementation of the MNS modulator. 2) Lesser hardware requirement, and hence a better reliability. 3) Lesser memory size requirement. 4) Lower harmonic contents as compared to the other microprocessor based PWM techniques. 5) Use of only integer arithmetic, and avoiding the solution of transcendental equations make the method suitable for any kind of simple microprocessor systems. 6) Easy reversal of Induction Motor phase sequence is possible through software. 7) Flexibility to synchronise the reference and the carrier signals to any desired point, so as to obtain the desired output voltage waveform at low carrier ratios (I<6) is also possible: -v-
8) Prediction of locus of circular flux in an induction motor fed by MNS inverter is possible. 9) Software based generation of optimum value of commutational delay in MNS modulator causes higher magnitude of fundamental output voltage. 10) Since the commutational delay in MNS modulator can be set to its optimum value, the generation of narrower PWM pulses is possible. 11) The MNS modulator is a universal one and can be used with a variety of power devices with different ratings. 12) The, different types of inverter operation such as FVFF, VVVF, VVFF, and FVVF are also possible. 13) Soft-start and soft-stop operations of induction motor fed by MNS inverter can be easily achieved. The experimental investigations made on the ROM based as well as MNS based" inverter systems reported in this thesis have yielded encouraging results in terms of harmonic contents in the output voltage, level of fundamental output voltage for the same value of the modulation index, and overall control of inverter system as compared to the existing systems already published. It is hoped that the newly suggested strategy will be accepted in industry for application in control AC drives. -vi -
CONTENTS Page Certificate Aknowledgement Abstract Contents List of Figures List of Principal Symbols List of Abbreviations ii iii vii xiv xxii xxiii CHAPTER-1 INTRODUCTION 1.1 General 1 1.2 Objectives of The Thesis 5 1.3 Major Contribution of The Work Presented 7 1.4 Chaptecwise Presentation of The Thesis 9 CHAPTER-2 REVIEW OF PWM INVERTERS AND CHOICE OF A SUITABLE TECHNIQUE 2.1 General 12 2.2 Classification of PWM Inverters Based on Input Source 13 2.3 Comparison Between Current Source inverter (:2SI and Voltage Source Inverter (VSI) 14 2.3.1 Merits and Demerits of CSI and VSI 22
2.4 PWM Techniques 25 2.4.1 Single pulse modulation 25 2.4.2 Multiple pulse modulation 27 2.4.2.1 ON-OFF modulation 29 2.4.2.2 periodic modulation 31 2.5 Choice of The Modulating Signal For PWM Inverter 37 2.6 Analog Integrated Circuit for PWM Inverter 38 2.7 Analog Circuit Limitations 44 2.8 Conclusion 46 CHAPTER-3 ROM BASED CONTROLLER FOR PWM INVERTER SYSTEM 3.1 General 47 3.2 Merits of digital Control Circuits 48 3.3 Predetermination of Firing Schedule 50 3.3.1 Fourier Analysis of Three-Phase Sinusoidal PWM Inverter. 52 3.3.2 Program for Computation of Angular Coordinates 54 3.3.3 Line-to-Line Output Voltage 56 3.3.4 Selection of Carrier Ratio 'I' 57 3.3.5 Selection of.modulation Index 'M' 65 3.4 Formation of Look-up Tables 69 3.5 Digital Controller 73 3.6 Design Consideration for the ROM Based Controller 76 3.6.1 Clock Frequency Selection 76
3.6.2 Phase Sequencing of Trigger Pulses 77 3.6.3 Frequency Divider 79 3.6.4 Inverter Frequency Selector 79 3.6.5 Decimal Rate Multiplier 80 3.6.6 Decimal to Binary Counter 84 3.6.7 Address Counter 84 3.6.8 Phase Address Generator 85 3.6.9 Selection of The Memory Device-EPROM 88 3.6.10 Phase Decoding of Trigger Pulses 89 3.6.11 Delay Time Generator 89 3.6.12 Mixer and Amplifier 95 3.7 Experimental Results 97 3.8 Industrial Utility of the ROM Based PWM Modulator 103 3.9 Design of a ROM based UPS System 104 3.10 Conclusion 108 CHAPTER-4 MICROPROCESSOR BASED PWM TECHNIQUES 4.1 General 111 4.2 PWM Switching Strategies and Their Implementation 113 4.2.1 Single-Pulse Modulation 114 4.2.2 Sinusoidal Pulse Width Modulation 116 4.2.2.1 Natural Sampling Technique 117 4.2.2.2 Delta Modulation 118 4.2.2.3 Regular Sampling Techniques 118
4.3 On-Line and Off-Line Microprocessor Based PWM Control Schemes 128 4.3.1 Single Processor Scheme 129 4.3.2 Double Processor Scheme 129 4.4 The Proposed Modified Natural Sampling Strategy 130 4.4.1 Generation of Sinusoidal Reference Waveform 131 4.4.2 Generation of the Triangular Carrier Signal 133 4.4.3 Synchronisation of the Sinusoidal Reference Waveform With the Carrier Signal 134 4.5 Voltage Control in PWM Techniques 135 4.6 Program for Computation of the Point of Intersection 137 4.7 Salient Features of the Proposed Method 139 4.8 Formation of Look-Up Tables for Three-Phase PWM Modulator 148 4.9 Program for Triggering SCRs and Delay Time Generation For the Proposed Three-Phase PWM Modulator 150 4.10 Merits of the Proposed Technique 154 4.11 Experimental Results 156 4.12 Conclusion 162 CHAPTER-5 HARMONIC CONTENTS IN MICROPROCESSOR BASED PWM MODULATORS AND LOCUS OF CIRCULAR FLUX IN MNS STRATEGY 5.1 General 165 5.2 Effect of Angle of Deviation on Harmonic Distortion 166 5.2.1 Angle of Deviation in Regular Symmetrical -x-
Sampling (RSS) Technique 168 5.2.2 Angle of Deviation in Regular Asymmetrical Sampling (RAS) Technique 172 5.2.3 Angle of deviation in Modified Regular Sampling (MRS) Technique 172 5.2.4 Angle of Deviation in Modified Natural Sampling (MNS) Technique 176 5.3 Summery of Comparison of Various Microprocessor Based Strategies 179 5.4 Effect of Point of Synchronisation in MNS Technique 188 5.5 Instantaneous Voltage Vector And Circular Flux of PWM Inverter 195 5.6 Locus of Circular Flux in MNS Technique 200 5.7 Conclusion 204 CHAPTER-6 DESIGN AND FABRICATION OF POWER CIRCUIT 6.1 General 206 6.2 Power Circuit 206 6.2.1 Choice of Three-Phase Power Circuit 210 6.3 Commutation Circuit 211 6.3.1 Auxiliary Impulse Commutated Inverter 212 6.3.2 Design of Commutation Circuit 215 6.3.3 Effects of Optimum Switching Lag Time 218 6.4 Selection of SCR Rating and Their Protection 221 6.4.1 Selection of SCRs 221 6.4.2 dv/dt Protection 222 6.4.3 Over-Current Protection, di/dt Protection 222
6.4.4 Choice of the Fuse Ratings 223 6.4.5 SCR Gate Protection 223 6.5 Interfacing Circuit 224 6.6 Experimental Results 226 6.7 Conclusion 228 CHAPTER-7 MNS MODULATOR USED FOR CONTROL OF INDUC"ION MOTOR 7.1 General 231 7.2 Modes of MNS Modulator 232 7.2.1 Variable-Voltage Fixed-Freciency Operation 235 7.2.2 Fixed-Voltage Variable-Fregaency Operation 238 7.2.a Variable-Voltage Variable-Frequer.,:y Operation 240 7..3 Switching Over From One Mode of Operaticol to the Other 241 7.4 Soft-Start and Soft-Stop Operations of Induction Motor 244 7.5 Reversal of Induction Motor Speed By Software 245 7.6 Control of Induction Motor Using Speed F?ec. Back 248 7.6.:. Speed Sensing Mechanism 24S 7.7 Experimental Results 252 7.7. -J. Interpretation of Results 289 7.8 Conclusion 299
CHAPTER-8 CONCLUSION AND FURTHER EXTENSIONS OF THE WORK 8.1 Conclusion' 300 8.2 Further Extensions Of The Work 305 REFERENCES 308 APPENDIX-A EFFECT OF POINT OF SYNCHRONISATION A-1 APPENDIX-B DESIGNED HARDWARE CIRCUIT FOR A SINGLE-PRASE UP:i SYSTEM B-1 APPENDIX-C RATINGS OF INDUCTION MOTOR AND THE D.C. GENERATOR SET C.1 Ratings Of The Induction Motor C-1 C.2 Ratings Of The DC Machine Coupled To The Induction Motor C-1