| Summary: | The introduction gives consideration to the aspects of communication principles, power converters, PWM modulation techniques and the application of LSI intergrated circuits as regards their effect on the design of variable speed AC drives. Chapter 1 includes background material where several types of variable speed A.C. drives are compared. The theoretical possibilities of various PWM schemes show that certain techniques offer improved harmonic spectrum properties, more suited to the wide range speed control of induction motors. Chapter 2 introduces alternative approaches to the analysis of sinuisoidal or natural PWM and the regular sampled PWM waveforms. The latter waveform is analysed by expressing the function in terms of the time domain; conversion from the time to the frequency domain is then obtained using a Fast Fourier Transform (FFT) technique implemented on the 32K PET microcomputer. Chapter 3 describes a successful analogue rig designed to generate several, alternative PWM waveforms which were used to drive an induction motor via the McMurray bridge inverter. Practical performances of the differing PWM waveforms were evaluated, using this equipment and the regular sampled, asymmetric PWM waveform was shown to have some operational advantages - it was this technique which was then given further consideration for microprocessor implementation. Chapter 4 outlines the top-down, structured design philosophy necessary for the efficient successful realisation of the hardware and software of a microprocessor system. The detail of the complete unit is given in Chapter 5 which describes the hardware and software of a microprocessor design used to provide the switching drive to the induction motor via the inverter. Several sotware algorithms are described which, when run by the control program in the appropriate sequence, produce the three phase PWM output waveform. An analysis of results is provided in Chapter 6 which shows that both the analogue and microprocessor rigs can operate an induction motor over wide speed ranges. The micropocessor realisation was far more accurate in its emulation of the Regular Sampled Asymmetric PWM waveform and controlled the motor over a 100:1 speed range. The concluding chapter summarises the problems, state-of-art and further applications of the microprocessor implementation whose unique properties are hardly obtainable with other, more conventional forms of practical realisation.
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