Answer / deadzg_devil

Y-D starter is used in starting of IM.
The main purpose of using this topology is to reduce the starting current of IM.
As in Y the voltage will be 1/root 3 time line voltage therefore current drawn will be less as compared to in D connection.

When the IM motor attain 75% of it's rated speed at this the connection is changed to D topology to attain full speed.

Answer / apoorv

it is start delta starter. in star connection ,
phase voltage is 1 by root 3 of line voltag and phase current
ids equal to line current.and in delta its vica versa
so at output we get , both reduced current and voltage
doe to reduce starting current.

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