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 motor usually generate voltage flicker and mechanical stress to the equipments. The motor must be started at reduced voltage, which can be enabled by an auto-transformer. The auto-transformer greatly reduces the high starting current with moderate starting torque.
They are designed with the typically voltage taps of 50, 65 and 80 percentages of the rated voltage. In contrast to reactor based motor starters, the motor voltage is fixed and more reduction in starting current is achieved with the same motor starting torque.
The motor is connected to the taps on the auto-transformer during the reduced voltage starting period. With the lower starting voltage, the motor draws less current and 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