A variable-speed induction motor using thyristor chopper

Description of a closed-loop control system for an ordinary wound rotor induction motor, which makes use of thyristors in the secondary side as pulsewidth modulated dc switch operating at a suitable preset pulse repetition rate, is presented. The slip frequency rotor voltage is rectified and fed to a thyristor chopper, a variable ON time of which results in a variable speed. The ON time is controlled by a strong feedback loop from speed signals, so that the set speed remains nearly constant irrespective of load variations. It is observed that torque of the machine varies widely with ON time depending on the two possible extreme conditions: 1) when the thyristor switch is in parallel with the source (rectified rotor voltage) and external resistance; and 2) when the switch is in series with the source and external resistance. The scheme may be adopted as a brushless drive system.