Star-Delta Starter

STAR –DELTA STARTER

o     In this method, the stator phase windings are first corrected in star and full voltage is connected across its free terminals.

o     As the motor pickup speed, the windings are disconnected through a switch and they are reconnected in delta across the supply terminals.

o     The current drawn by the motor from the lines is reduced to 1/3 as compared to the current it would have drawn if connected in delta.

PROOF:

·         Thus, by connecting the motor windings, first in star and then in delta, the line current drawn by the motor at starting is reduced to one third as compared to starting with the windings delta connected.

Reduced Torque due to star connection:

In induction motor, torque developed is proportional to the square of applied voltage. As the phase voltage is reduced to (1/√ 3) times that in star-connection, the starting torque will be reduced to one third. To get full torque in the motor, it must be switched over to delta connection.

·         In making connections for star-delta starting, care should be taken such that sequency of supply connections to the winding terminals does not change wile changing form star-connection to delta-connection. Otherwise the motor will start rotating in opposite direction, when connections are changed from star to delta.

·         Star-delta starters are available for manual operation using push-button control. An automatic star-delta starters uses time-delay relays (TDR) through which star to delta connections take place automatically with some pre-fixed time fixed keeping in view the starting time of the motor.

Derivation: