Self-excitation of induction motors compensated by permanently connected capacitors and recommendations for IEEE Std. 141-1993

Abstract

Self-excitation of induction motors compensated by permanently connected capacitors is investigated in this paper. Theoretical analyses of self-excitation phenomenon are carried out by using some simplified equivalent circuits, and a hybrid mathematical model in ABC/dq axes, respectively, in steady state and transient state. An unusual operating condition about water pumping stations is reported, in which water within the pipeline may drive the motor in the reverse direction at speeds higher than synchronous, when a supply interruption coincides with a check-valve failure. In order to prevent the motor from dangerous overvoltages due to self-excitation, it is recommended to connect a simple static protection circuit consisting of a resistor in series with a thyristor switch between any two lines of the motor. Critical resistance boundaries, which will lead to loss of excitation, and demagnetization of the rotor core are determined separately as a function of operating speed. A suitable resistance value chosen in the safe design area constitutes a reliable protection mechanism against self-excitation.