“Marmelosa”, 11/203, Mambakkam Road, Medavakkam, Chennai- 600100, India

044-22771709, 22771516


ASTA-certified CTs


Auto Transformers for Motor Starting Duty

Technical Section

For the starting of Motors, on-line starting is a simple and inexpensive means. While it provides maximum starting torque andacceleration, it has two important limitations:

  1. Starting current is high, usually 5 to 8 times normal full load current. This can cause an excessive temporary voltage drop on power lines which causes lights to flicker or may eveninterrupt the service.
  2. Some loads cannot stand the high starting torque.


Various types reduced voltage starting are used, mainly on    motors 25 HP or larger, to overcome this limitation.

Auto transformer starting

The major advantages of this method of starting motors are:

  1. Best for hard to start loads.
  2. Adjustable starting torque.
  3. Used with any standard motors.
  4. Less strain on motor.


Autotransformer – Starting uses an Autotransformer to reduce the line voltage during the starting period. The Autotransformer Voltage taps provide choice of three different starting currents and torques, depending on the load requirements. In this system, the first step is connected one of the reduced voltage taps which results in a reduced starting current and torque. After a pre-set time delay, the second step applies full rated voltage to the motor for normal operation. For the same value of line current drawn during the starting period, motors with Autotransformers type starters deliver the maximum starting torque when compared to other type of starters.

Specifications: In order to completely specify the auto-transformer, the following information is required.

Both LV and MV auto-transformers are offered.