Electric motor controller with bypass contactor

A three-phase motor (10) is operated by a controller having sets of thyristors (16,17,18) which when triggered switch electricity from each phase (A,B,C) of a supply to the motor. The motor (10) is started by triggering the thyristors (16,17,18) to be conductice during successively greater portions of each cycle of the AC power. In order to reduce the cooling requirements for the controller, a contactor (15) is connected in parallel with the thyristors (16,17,18) to provide apply power to the motor (10) once it reaches full speed. When the controller receives a signal to stop the motor (10), the contactor (15) is deactivated and the controller goes through a sequence to synchronize the thyristor triggering process to the phase of the AC power supply. The synchronizing process provides a smooth transition from contractor control back to the thyristors control of the electricity application to the motor (10). Once the thyristor control resumes, the controller executes a triggering routine which decreases the voltage applied to the motor.