What is Auto Transformer Starter?

Auto transformer Starter

What is auto transformer ?

An autotransformer is an electrical transformer with only one winding. The term “auto” refers to a single coil acting alone, not to any kind of automatic mechanism.

In electrical engineering, the Korndorfer starter is a technique used for reduced voltage soft starting of induction motors.

The circuit uses a three-phase autotransformer and three three-phase switches. This motor starting method has been updated and improved by Hilton Raymond Bacon.

Operation of AutoTransformer Starter:

Auto transformer starter
image source: wikipedia

AT : stands for Auto transformer

M : stands for motor and 1,2 and 3 is starter switches

To start the motor. first, switches 1 and 2 are closed. This supplies the motor with a lower voltage from the autotransformer. The lower voltage limits the input current to the initially stationary motor, which accelerates. The torque of the motor is also lowered.

The motor continues to increase its speed until the motor torque and the load torque balance each other and a steady speed is achieved.

At this stage switch, 2 is opened and momentarily the motor is supplied by even lower voltage because of the windings of the autotransformer act as inductors connected in series with the motor.

This time is short – just enough to disconnect the switch 1 and engage switch 3, which connects the full voltage to the motor. Further increase in speed begins and motor reaches its full rated speed.

At this point the “soft start” is ended and motor can work under full load. The autotransformer is no longer required and is de-energized by opening switch 1 .

The motor is supplied directly from the three-phase network. To stop the motor, switch 3 is opened.

Advantages of Auto Transformer starters

  • Its efficiency is more when compared with the conventional one.
  • Its size is relatively very smaller.
  • Voltage Regulation of autotransformer is much better.
  • Lower cost.
  • Low requirements of excitation current.
  • Less copper is used in its design and construction.

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