Star-Delta Starter in Induction Motor

A star-delta starter is a very common type of starter and is widely used compared to other types of induction motor starters.

The star-delta starter is used for the squirrel cage induction motor that is designed to run on delta-connected stator winding.

The connection diagram of a start-delta starter for a three-phase induction motor is shown in the figure below.

The stator winding is connected in star connection at the time of starting (switch s is connected at START position).

When the motor reaches 80% of rated speed, the changeover switch s thrown quickly to the RUN position. And the stator winding is connected in a delta connection.

During starting condition, the stator winding is connected in a star connection. Hence, the starting current is reduced to one-third as compared to the delta connection.

At the time of starting, stator winding is connected in star connection. Therefore, each stator phase gets a voltage of V_L/√3 (V_L is line voltage).

Since the torque developed in the motor is directly proportional to the square of the voltage, the start-delta starter reduces the starting torque to one-third of that obtained by direct delta starting.

Calculation of Star-delta Starter

At the time of starting, the stator winding is connected in a star connection. Therefore, the voltage across each phase winding is equal to 1/√3 times the line voltage.

V_L = Line voltage

I_SYP = Starting current per phase with stator winding connected in star connection

I_SYL = Starting line current with stator winding in star connection

For Star connection

V1 = Phase voltage

I_SDP = Starting current per phase by direct switching with stator winding connection in delta connection

I_SDL = Starting line current by direct switching with stator winding connection in delta connection

Z_e10 = Standstill equivalent impedance per phase of motor referred to stator

For delta connection;

Line current = √3 x Phase current

Now,

Therefore, in star-delta starter, the starting current from the main supply is one-third of that with direct switching in delta.

Also, the starting torque is reduced to one-third of the starting torque obtained with direct switching in the delta.