Conditions for self-sustained oscillations in feedback control of AC motor speed by power electronic controllers

Speed control of AC induction motors by power electronic controllers is considered. The basic method of speed control is obtained by application of variable frequencies to the motor. Certain parameters are subject to modification by the user in the field. A model of the resulting operation is shown to contain a specific nonlinearity. In feedback applications the interaction of the motor dynamics with the nonlinearity of the controller may result in self-sustained oscillations in speed of the closed-loop system. The frequency and amplitude of such oscillations are determined by use of a describing function for the nonlinearity combined with a simplified linear model for the motor dynamics.