An autotransformer starter reduces inrush current by using a transformer in
the line just ahead of the motor to step down the voltage applied to the motor
terminals. By reducing the voltage, the current drawn from the line is reduced
When the setting time on the timer has expired, the autotransformer is bypassed.
The 1S contactor drops out, the run contactor closes, and the 2S contactor
opens, proving full voltage to the motor.
The ABB autotransformer starter is a closed transition type, meaning that the
motor remains connected to the line during the entire acceleration period.
The transformer has three taps which provide 50%, 65% and 80% of full line
voltage. At delivery, the transformer is connected to the 65% tap; the inrush
current will be reduced to 42% of normal; and the starting torque will be reduced
The autotransformer starter can be used for any squirrel-cage motor.