For starting of an induction motor, star/delta starting can be considered equivalent to an autotransformer starter with the ratio of

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PGCIL DT Electrical 22 Aug 2021 Official Paper (NR III)

Answer 1

In the star-delta starting method of induction motor, a two-way switch is used to connect the stator winding in star while starting and in delta while running at normal speed.
Star-Delta starter in a three-phase induction motor is used mainly to reduce the starting current.
In the star-delta starting method of induction motor, the starting current is reduced approximately by 1/3 times as compared to the original.
The starting torque will 1/3 times that it will be for delta connected winding.
During starting the stator winding is star connected, so that voltage over each phase in the motor will be reduced by a factor of 1/√3 of that would be for delta connected winding.
Hence a star-delta starter is equivalent to an auto-transformer of ratio 1/√3 or 57.73 % reduced voltage.