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During starting the torque requirment is very high and the starting current will be very high. A Star-Delta starter is used for starting motors to reduce the starting current. In star connection only phase voltage (0.707 X Line voltage) comes across each winding. hence during starting the current is lesser compared to applying Line votage across a winding. So you need a power supply source of lesser current capacity to start the motor while using Star-Delta starter than when using a direct on line DOL starter. But after starting, your motor needs to take load and for meeting the torque requirment, Delta is needed. Thats why these Y-D starters chage over to delta connection once sufficient speed is reached.
