An investigation of inverter circuits for variable speed single phase induction motors.

The prime object of the thesis is to examine a number of different thyristor inverter circuits in order to obtain a guide to the most suitable system for single-phase induction motor speed control. A search of the literature reveals a large number of papers and patents on inverter systems. In order to be able to compare these meaningfully the thesis proposes a classification system and a list of 'goodness factors'. Inverters are first classified into bridge or push-pull. Within each of these groups further subdivision is then possible based on the method of voltage control used, and on the commutation system. Any classification group of inverters may be modified to enhance one particular property of the circuit. Those dealt with in this thesis are modifications to increase or reduce voltage boosting, operation from an auxiliary d.c. commutation source and high frequency inverters. Following a theoretical comparision of various typical inverter circuits, including computer generated harmonic analysis of the load waveforms, the performance of the bridge and push-pull inverters are studied experimentally. In order to cut down on development work required this study is limited to eight different inverter combinations. Generally the results are predictable from theoretical considerations of inverter operation and waveform analysis. The design of inverter circuits are also considered in the thesis and reference curves produced by a computer. The thesis concludes that although several different inverter systems may be used to control the speed of an induction motor, a push-pull motor is generally undesirable apart from use in special circumstances such as low supply voltages. Of the bridge circuits, that using d.c. voltage variation as a method of changing the fundamental output would give a relatively simple system for single-phase motors. A patented modification to enable this to be obtained is described