Why star delta starter is preferred with induction motor?

one of the main reason is to reduce the starting current which is 5 to 7 times of maximum current.

if the motor is started with star connection the phase voltage get divided by √3 times(phase voltage= line voltage/√3). since the voltage is reduced, the starting current is also reduced.

once if it reaches the rated speed, the connection will be changed to delta, but now the motor will not draw much current like in the time of starting.

if the motor is started with delta connection, the line voltage will be equal to phase voltage, which tends to draw more current at the time of starting.

Thanks for invitation

actually, this way not using with high power motors, it is common for low power motors

however, it is prefered because it is cheap way and simple

it is depending on changing the voltage which applied on the motor coils to reduce starting current

note: not all motor can be work with this way

motor shall have facility to change connection from star to delta

that is mean the rated parameters of motor shall be for delta connection

Because, starting current of Induction Motor is high and Delta starter offers lesser starting current thus reduces copper losses and other associated losses.

There are a lot of method for motor starting the widest use is Delta/Star, due to the following:- 

* Cheapest and simplest.

* The starting current reduces to 1/3 against the current in DOL.

* Cheaper maintenance.

The motor starting current is reduced by 3-4 times Of current of Direct online starting. Hence the starting current is reduced and the voltage drops are reduced during the starting of motor in distribution systems.