Wide speed-range operation of a chopper-controlled induction motor using asymmetrical chopping

The paper describes the operation of a slip-ring induction motor with a thyristor-rotor-current chopper in an asymmetrical mode. A composite torque/speed characteristic is obtained, partly from the conventional balanced characteristic of symmetrical operation, and partly from the single-phase rotor characteristic. A symmetrical component approach to the system solution is given and the system is shown to give shunt-type torque/speed characteristics as a motor or a generator in the speed range from 50% to 75% of synchronous speed. System efficiencies are comparable with those obtained in symmetrical chopper operation above 75% of synchronous speed. An appropriate selection of symmetrical and asymmetrical chopper operation is shown to effectively double the speed range of the symmetrical chopper system. Closed-loop speed control of the variable mark/space ratio chopper can provide steep shunt-type, torque/speed characteristics over a speed range from 50% of synchronous speed to normal full speed with a simple, cheap, 2-thyristor current chopper.