Answer / madhu

basically in star connection line current is equal to phase
current,it gives high starting torque. after motor reaches
pick up speed we changed to delta.because in delta line
voltage is equal to phase voltage& line current is root 3
times the phase current.if we run in star in running time
the power losses are more due to current is high. thats why
we changed star to delta.

Answer / marikkani.m

The purpose of starter is to start the motor and to limit
the starting current.we are starting the motor in y-
connetion during this impedence of two windings put across
the line voltage hence the starting current gettinh
redeuced
but the motor rated capacity (out put torque)can only be
obtained when itis changed to delta connection this time
the voltage is incresed bu 1.732 time to meet the torque
(torque is direct proportion to square of supply voltage .

Answer / velusamy

There are two reasons

1.To get high starting torque, motor runs at star for
predetermined duration and then changed to delta.

2.If the motor runs continuously , the capacity of the
motor is reduced by root 3 times (Capacity of motor
rating/1.7324) and hence motor will fail if the OL setting
is not reduced to 1.7324times of motor full load current).

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