What are the advantages of star-delta starter with induction motor?

     A star delta starter is the most commonly used method for the starting of a 3-phase induction motor. In star delta starting an induction motor is connected in through a star connection throughout the starting period. Then once the motor reaches the required speed, the motor is connected in through a delta connection.

     The main advantage of using the star delta starter is the reduction of current during the starting of the motor. Starting current is reduced to 3-4 times of current of Direct online starting. Hence the starting current is reduced, and the voltage drops during the starting of the motor in systems are reduced.

     The star delta starter is used to start induction motors. At the time of starting the motor, the rotor is at a standstill and the slip (between the stator's magnetic field and rotor) is large which causes the large inrush of armature current (which is 6–7 times the rated value). This large current can damage the stator windings and burn the motor.

     To avoid this situation, we use a star delta starter. At the time of start, motor connections (stator connections) are made in star mode, so impressed voltage is reduced by 1/√3 (Phase voltage = Line voltage/√3) which reduces the starting current. Once the rotor is at 80–90% speed, the centrifugal switch operates changing the connections from star to delta mode (full line voltage is impressed).