MICROPROCESSOR BASED PWM INVERTER SYSTEM AND ITS USE FOR CONTROL OF INDUCTION MOTOR

Transcription

1 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

2 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.

3 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)

4 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

5 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-

6 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-

7 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 -

8 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 Objectives of The Thesis Major Contribution of The Work Presented Chaptecwise Presentation of The Thesis 9 CHAPTER-2 REVIEW OF PWM INVERTERS AND CHOICE OF A SUITABLE TECHNIQUE 2.1 General Classification of PWM Inverters Based on Input Source Comparison Between Current Source inverter (:2SI and Voltage Source Inverter (VSI) Merits and Demerits of CSI and VSI 22

9 2.4 PWM Techniques Single pulse modulation Multiple pulse modulation ON-OFF modulation periodic modulation Choice of The Modulating Signal For PWM Inverter Analog Integrated Circuit for PWM Inverter Analog Circuit Limitations Conclusion 46 CHAPTER-3 ROM BASED CONTROLLER FOR PWM INVERTER SYSTEM 3.1 General Merits of digital Control Circuits Predetermination of Firing Schedule Fourier Analysis of Three-Phase Sinusoidal PWM Inverter Program for Computation of Angular Coordinates Line-to-Line Output Voltage Selection of Carrier Ratio 'I' Selection of.modulation Index 'M' Formation of Look-up Tables Digital Controller Design Consideration for the ROM Based Controller Clock Frequency Selection 76

10 3.6.2 Phase Sequencing of Trigger Pulses Frequency Divider Inverter Frequency Selector Decimal Rate Multiplier Decimal to Binary Counter Address Counter Phase Address Generator Selection of The Memory Device-EPROM Phase Decoding of Trigger Pulses Delay Time Generator Mixer and Amplifier Experimental Results Industrial Utility of the ROM Based PWM Modulator Design of a ROM based UPS System Conclusion 108 CHAPTER-4 MICROPROCESSOR BASED PWM TECHNIQUES 4.1 General PWM Switching Strategies and Their Implementation Single-Pulse Modulation Sinusoidal Pulse Width Modulation Natural Sampling Technique Delta Modulation Regular Sampling Techniques 118

11 4.3 On-Line and Off-Line Microprocessor Based PWM Control Schemes Single Processor Scheme Double Processor Scheme The Proposed Modified Natural Sampling Strategy Generation of Sinusoidal Reference Waveform Generation of the Triangular Carrier Signal Synchronisation of the Sinusoidal Reference Waveform With the Carrier Signal Voltage Control in PWM Techniques Program for Computation of the Point of Intersection Salient Features of the Proposed Method Formation of Look-Up Tables for Three-Phase PWM Modulator Program for Triggering SCRs and Delay Time Generation For the Proposed Three-Phase PWM Modulator Merits of the Proposed Technique Experimental Results Conclusion 162 CHAPTER-5 HARMONIC CONTENTS IN MICROPROCESSOR BASED PWM MODULATORS AND LOCUS OF CIRCULAR FLUX IN MNS STRATEGY 5.1 General Effect of Angle of Deviation on Harmonic Distortion Angle of Deviation in Regular Symmetrical -x-

12 Sampling (RSS) Technique Angle of Deviation in Regular Asymmetrical Sampling (RAS) Technique Angle of deviation in Modified Regular Sampling (MRS) Technique Angle of Deviation in Modified Natural Sampling (MNS) Technique Summery of Comparison of Various Microprocessor Based Strategies Effect of Point of Synchronisation in MNS Technique Instantaneous Voltage Vector And Circular Flux of PWM Inverter Locus of Circular Flux in MNS Technique Conclusion 204 CHAPTER-6 DESIGN AND FABRICATION OF POWER CIRCUIT 6.1 General Power Circuit Choice of Three-Phase Power Circuit Commutation Circuit Auxiliary Impulse Commutated Inverter Design of Commutation Circuit Effects of Optimum Switching Lag Time Selection of SCR Rating and Their Protection Selection of SCRs dv/dt Protection Over-Current Protection, di/dt Protection 222

13 6.4.4 Choice of the Fuse Ratings SCR Gate Protection Interfacing Circuit Experimental Results Conclusion 228 CHAPTER-7 MNS MODULATOR USED FOR CONTROL OF INDUC"ION MOTOR 7.1 General Modes of MNS Modulator Variable-Voltage Fixed-Freciency Operation Fixed-Voltage Variable-Fregaency Operation a Variable-Voltage Variable-Frequer.,:y Operation Switching Over From One Mode of Operaticol to the Other Soft-Start and Soft-Stop Operations of Induction Motor Reversal of Induction Motor Speed By Software Control of Induction Motor Using Speed F?ec. Back :. Speed Sensing Mechanism 24S 7.7 Experimental Results J. Interpretation of Results Conclusion 299

14 CHAPTER-8 CONCLUSION AND FURTHER EXTENSIONS OF THE WORK 8.1 Conclusion' 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