Induction motors with modulated terminal voltage

Doctoral Thesis


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University of Cape Town

Following the development of high speed solid state switches for the forced commutation of current in AC power circuits it has become feasible to apply forced commutation techniques to the frequency and voltage control of induction motors as variable speed drives. The availability of these methods raises questions regarding their effect on the performance of the motor and the generation of harmonic currents in the electrical supply. After the initial use of numerical simulation in the study of these systems it became apparent that this approach was not suitable for the evaluation of steady state performance factors. This thesis sets out to establish an analysis of the induction motor under these conditions which leads to the algebraic formulation of the performance of the system in the steady state. This approach can be used in any situation where the electrical supply to an induction motor is modulated by switching techniques. The analysis has been applied to chopper modulation of the supply voltage and also to a phase-stepping frequency convertor which incorporates chopper modulation. Experimental results are included to illustrate the validity of the analytical method.

Bibliography: pages 292-294.