Control circuit of automatic star delta starter

The working of automatic star delta starter is explained here. The control circuit of the automatic star delta starter consists of three contactors – main contactor, star contactor and delta contactor. It also required one timer for switching of motor from star to delta operation.

Working of control circuit of automatic star delta starter

The working of control circuit of automatic star delta starter is explained in following steps.

Push on button
The star contactor coil gets supply through phase R – OFF push button – ON push button – T NC – star contactor coil – D1 – phase Y. The star contactor becomes energized.
The timer coil gets supply at the same time.
As soon as the star contactor ON, its auxiliary contact S1becomes NC and S2 becomes NO.

Auxiliary contact S1: From NO to NC

Auxiliary contact S2: From NC to NO

The main contactor coil gets supply through phase R – OFF push button – ON push button – S1 – main contactor coil M – phase Y. The main contactor becomes energized.
The main contactor auxiliary contact M1 becomes NC.

Auxiliary contact M1: From NO to NC

Now, the main contactor gets supply through auxiliary contact M1.
The timer contact becomes NO as per timer specific time setting.

Timer contact T: From NC to NO

As soon as the contact of timer becomes NO, the star contactor de – energizes. It is all auxiliary contactor change its position means NO to NC and vice versa (Initial condition)

Auxiliary contact S1: From NC to NO

Auxiliary contact S2: From NO to NC

The coil of delta contactor gets supply through phase R – OFF push button – ON push button – M – S2 – delta coil – Y phase.
The main contactor and delta contactor remain is in energized during running condition of the three-phase induction motor.