The application of thyristors to induction motor drive systems

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Virginia Tech


This thesis provides a comparison of three different power circuit arrangements of thyristors for controlling the speed of an induction motor through control of the stator voltage. The output voltage harmonic structure is provided for each circuit along with a comparison of the motor torque-speed characteristics. Means for coping with the motor heating problem, which results from adjustable voltage control, are discussed and curves are provided to evaluate the additional heating which results from the non-sinusoidal stator voltage.

The principles of phase control are developed for single phase circuits and expressions are developed for calculating the input power factor which can be less than unity, even when the load is a resistance. These principles are then extended to three phase circuits where expressions for determining the phase sequence of the harmonics are developed. The applicability of superposition to the qualitative analysis of the effect of stator voltage harmonics upon motor performance is demonstrated, and the problems encountered in the quantitative application of superposition is discussed.