Star-Delta Starter is a very widely used type of motor starter. The Star Delta Starter is used to reduce the starting current and the starting torque.
When the motor is started, there is a risk of heavy inrush currents if the line voltage is applied across the motor windings. This is because there is no back-emf to oppose the applied voltage. Thus, the three windings of the motor are initially connected in star. Thus the voltage across each individual winding is the phase voltage.
The current is thus reduced. This also reduces the torque.
Thus, heavy motors can be started with less disturbance to the power system. Once the motor has acquired sufficient speed, the winding connections are changed to Delta. By now, the sufficient back-emf would have developed to keep the currents within limits.
The Delta connection enables the motor to develop higher torque. The motor windings are in delta during normal operation. Interlocks are provided to prevent short circuit during the changeover between the star and the delta connections. The star-delta starter can be used where the load is connected to the motor after start-up. This starter cannot be used for motor which have to be started along with the load as the initial starting torque through this method is zero.