Motor Starting Auto-Transformers reduce the inrush current by stepping down the applied voltage during the start-up operation of induction and synchronous motors.
Direct-online starting of a motor usually generates voltage flicker and mechanical stress to the equipment. To avoid this, the motor must be started at reduced voltage, which can be achieved by using an auto-transformer. The auto-transformer greatly reduces the high starting current with moderate starting torque.
Auto-transformers are typically designed with voltage taps of 50%, 65% and 80% of the rated voltage. In contrast to reactor based motor starters, the motor voltage is fixed. Thus, additional reduction in starting current is achieved using the same starting torque.
The motor is connected to the taps of the auto-transformer during the reduced voltage starting period. With lower starting voltage, less current is drawn by the motor, which therefore develops less torque then if it were connected to the line voltage.
- Motor terminal voltage is less than line voltage (by transformer ratio)
- Motor current exceeds line current (by inverse of transformer ratio)
- Starting torque is reduced by the square of the motor terminal voltage