Star-delta starting

 Star-delta starting

The stator winding of the motor is designed for delta operation and is connected in star during the starting period. 

When the machine is up to speed, the connections are changed to delta. 

The circuit arrangement for star-delta starting is shown in Figure.

The six leads of the stator windings are connected to the changeover switch as shown. 

At the instant of starting, the changeover switch in thrown to "Start" position which connects the stator windings in star. 

Therefore, each stator phase gets V/V3 volts where V is the line voltage. This reduces the starting current. 

When the motor picks up speed, the changeover switch is thrown to "Run" position which connects the stator windings in delta. 

Now each stator phase gets full line voltage V. 

The disadvantages of this method are

(a) With star-connection during starting, stator phase voltage is 1/3 times the line voltage Consequently, staring tongue is (1/5) or 1/3 times the value it would have with A-connection. This is rather a large reduction in starting.

(b) The reduction in voltage is fixed.

This method of starting is used for medium-size machines (upto about 25 H.P.).