Star - Delta Starter Working

Star - Delta Starter 

To start a large induction motor we use star - delta starter because large induction motors(cage type) with delta connected stator when started directly on line it produces large starting current surges which causes fluctuations in voltage on supply line. To prevent this we run induction motor at reduced voltage at the start of induction motor and later on after getting require speed we operate induction motor at full supply voltage for this purpose we use star delta starter.

Working Principle Of Star - Delta Starter:

As discussed above to reduce fluctuations in voltage on supply line we need to reduce starting surge currents so we need to reduce the voltage at the start of induction motor for this purpose we firstly connect the stator windings of induction motor in star connection and later on we connect the stator windings in delta to operate at full supply voltage. See the below circuit diagram how the star - delta starter is connected to induction motor.

Connection diagram of star - delta starter:

star delta starter control circuit diagram

During start position of switch the stator windings are connected in star as shown in the following figure.

Now the induction motor gradually picks up it's speed. When the speed becomes 80 percent of its rated speed then the switch moves to run position as a result stator windings get connected in delta as shown in the below figure.

Theory And Calculations Of Star - Delta Starter:

To understand how voltage is reduced by star connection of stator windings see the below calculations.

In star connection the phase voltage is 1/√3 times of line voltage as the torque is directly proportional to square of voltage applied the torque is reduced to 1/3 times than the torque produced by starting with direct delta connection.

Let,

VL be the line voltage.

V1 is the phase voltage.

Istyp be  starting current per phase when the stator windings are connected in star.

Istyl is the starting line current when the stator windings are connected in star.

IstΔp is the starting current per phase by direct switching with the stator windings connected in delta

IstΔl is the starting line current by direct switching with the stator windings in the delta.

IscΔp is the short circuit phase current by direct switching with the stator windings in the delta.

Ze10 is the standstill equivalent impedance per phase of the motor, referred to the stator

In star connection,  line current will be equal to phase current.

In delta connection, the line current is equal to the √3 times of the phase current. so we get,

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

This shows with star delta starter, the starting torque is reduced to one-third of the starting torque obtained with the direct switching in the delta.

Here,

IflΔp is the full load phase current with the winding connected in delta. But

So the above equations show how voltage and torques are reduced with star delta starter than direct delta starting.

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