Star-Delta Starter in Induction Motor

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.

Star-Delta Starter
Star-Delta Starter

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 VL/√3 (VL 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.

VL = Line voltage

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

ISYL = Starting line current with stator winding in star connection

For Star connection

    \[ Line \, current = Phase \, current \]

    \[ I_{SYP} = I_{SYL} \]

V1 = Phase voltage

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

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

Ze10 = Standstill equivalent impedance per phase of motor referred to stator

    \[ I_{SYP} = \frac{V_1}{Z_{e10}} = \frac{V_L}{\sqrt{3}Z_{e10}} \]

    \[ I_{SDP} = \frac{V_L}{Z_{e10}} \]

For delta connection;

Line current = √3 x Phase current

    \[ I_{SDL} = \sqrt{3} I_{SDP} = \frac{\sqrt{3}V_L}{Z_{e10}} \]

Now,

    \[ \frac{ I_{SYP}}{ I_{SDL}} = \frac{\frac{V_L}{\sqrt{3}Z_{e10}}}{\frac{\sqrt{3}V_L}{Z_{e10}}} = \frac{1}{3} \]

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.

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