Three phase motor speed control - is for motors of less than one horse power and uses triac connected into one phase only

摘要

The Triac is connected into one phase only of the supply to the motor which is governed by a normal circuit breaker. The angle of trip of the Triac can be manually or automatically controlled by a potentiometer. When the motor is first switched on with two phases only connected, the motor is stationary but as the controlled Triac opens the third phase is progressively fed. The Triac may be switched out of circuit when the operating speed has been achieved. This control is limited to motor of 1 HP or less but overcomes the difficulties of small resistor control units especially when the run-up time of the motor is appreciable.