Star Delta Starting

Star-delta starting of three-phase induction motors

The problem of each three-phase asynchronous motors is its high starting current, which can be about 7 to 10 times the rated current. Therefore, big 3-phase induction motors from 4 kW should only start with starting aids. Star-delta starting (in short YΔ-starting) is one possibility.

In the European supply network, the voltage phase to phase is 400 V. That means, the winding of this induction motor must be rated for 400 V to apply YΔ-starting.

Here, the three-phase motor starts in star, then after a certain time, the motor is switched to delta:

3 Phase Induction motor Star-Delta starting circuit

3 Phase Induction motor Star-Delta starting circuit

Problem of YΔ-starting: The torque in star is 1/3 of torque in delta. Perhaps the motor cannot start with full load torque. Also, consider that starting with reduced power can lead to a longer acceleration time.

3 Phase Induction motor – Star-Delta characteristic curves

3 Phase Induction motor – Star-Delta characteristic curves