High performance induction motor controller using thyristor voltage feedback

The voltage across the nonconducting thyristor is an indirect index of the back EMF developed. Hence this parameter was used to obtain information on motor operation, thus resulting in a closed loop control scheme. The voltage across the thyristor, being an indication of the slip variation, has also been used for sensing load changes, in addition to sensing the end of soft start. The corrective action in case of load changes, for optimal operation is implemented based on a relationship developed between the voltage across the thyristor and the current drawn by the motor.