Stepless starting of wound-rotor induction motors

This paper gives experimental and calculated speed-torque-current curves for a slip-ring induction motor with an automatically controlled secondary impedance to obtain stepless starting with low current, high sustained torque, and low full-load slip. A saturable reactor in the secondary circuit is supplied with a control current from a direct-connected exciter on the motor shaft. Since the control current is proportional to speed, the secondary reactance varies smoothly from a high value at starting to a low value at full speed.