A Variable Speed Induction Motor Using Thyristors in the Secondary Circuit

This paper describes a closed-loop control system for an ordinary wound-rotor induction motor, which makes use of thyristors on the secondary side as phase-controlled ac switches operating at slip-frequency. It is shown that such phase-controlled thyristors allow the torque of the machine to be varied from zero to values bounded by the normal torque/speed characteristic of the basic motor, and when overall speed control is applied the motor can be given shunt-type torque/speed characteristics.