Why star-delta starter with induction motor?

Advantage of Star-Delta starter.

• The main advantage of using star-delta starter is reduction of current during starting of motor.
• Starting current is reduced 3-4 times of that of direct online starting.

Fig: Star delta starter wiring diagram

Star delta starter   

    In star-delta starter, the stator winding is first connected in the star to lower the stator phase voltage by 42% of the supply voltage. The starting current will also decrease since the voltage across the winding will be 58% of the supply voltage. The phase voltage across the stator winding will be 0.58*440 = 255 volts if the line voltage is 440 volts. 

    The timer relay activates the delta contactor and connects the motor winding in delta after the motor has ran in star connection for a predetermined period of time. When connected in delta, the entire supply voltage is imposed on the stator winding.