A Current-Source Inverter in the Secondary Circuit of a Wound Rotor Induction Motor Provides Sub- and Supersynchronous Operation

A control system is described in which a current-source inverter is connected between the secondary winding of a three-phase slip-ring induction motor and the ac mains supply to give a variable speed sub- and supersynchronous operation. Control techniques developed in an earlier work were used to ensure fully stable operation over a very wide speed range with either driving or braking torque. Means are provided for power-factor control to compensate for the lagging power-factor effect of the inverter when operating into the large secondary electromotive forces (EMF) at high slip values. Experimental results are presented for a speed range from standstill to approaching twice synchronous speed for both driving and braking torque. There are no instabilities and the developed torque even at low frequencies near synchronous speed is steady with motor derating due only to the quasi-square wave current in the secondary circuit.