Auto transformer starter

This starter produces high efficiency and have the option to be adjusted to obtain the necessary torque and in accordance with the system.It very efficiency stater but the disadvantage is it very high cost to setup control panel stater.

It can improved torque ratio is achieved and starting current is typically 3 times Full Load Current , depending on the voltage taps selected.Typically it have taps at 50%,65% and 80% voltage.

During start-up, the motor is connected to the auto transformer’s tapping. This means that the motor starts up with a reduced voltage and a correspondingly low current.


What is auto transformer starter?

The auto transformer reduces the current in the mains supply line further and in accordance with its ratio.The technique for connecting an Auto Transformer stater is via magnetic  contactor and connect the motor to taps by mean of the contactor.

When motor accelerate to full speed,the tap of contactor was open and disconnecting motor from transformer and other contactor close to connecting  the motor to the supply.When the motor has almost reached its rated torque, the star connection on the transformer is opened.

The transformer’s partial windings act as chokes in series to the motor windings, and therefore, like the uninterrupted star delta connection, the motor speed does not drop during switch over.

After the main contactor has been switched in, the motor windings are applied to the full mains voltage.Finally, the transformer is disconnected from the mains circuit.

For more detail information,please refer to AUTO TRANSFORMER HANDBOOKS :



    I appreciate your topic abt autotranformer starter but one more question
    Suppose I want to use either auto transformer starter or rotor resistance starter which one is the best and WHY?

  2. shakiru says

    How many cable will connect to the motor when using star delta starter with auto transformer? Is it 6 cable or 3

  3. How many timers are use in auto transformer contrrol wirring

  4. Christian Musonza says

    do we need shunt reactors on the sending side of high voltage line and receiving side also?


  1. mer info says:

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