Electrical motor-Necessity of Starter in Induction Motor (Basics of starter)

"The starter is a device which basically used to limit high starting current by supplying reduced voltage to the motor at the time of frequency"

In 3-phase Induction motor the magnitude of induced emf (E2r) in the rotor circuit depends on slip of induction motor.

Means induced emf decides the magnitude of the rotor current.

At start speed of IM is zero.
N=0;
So slip is at its maximum value i.e.unity

So magnitude of induced emf in rotor at start is very large. As rotor conductors are short-circuited large induced emf in rotor circulates high current through rotor at start.

This condition is similar to transformer when secondary of transformer is short-circuited. Such transformer when excited by rated voltage ,circulates high current through short circuited secondary. So primary also draws very high current from supply.

Similarly in a 3-phase IM when rotor current is high, stator draws a very high current from the supply. This current can be of order of 5to 8 times load current at start. This damages winding of motor.

To avoid this it is necessary to limit current drawn by motor at start.Such a reduced voltage is applied only for short period and once rotor gets accelerated , full normal rated voltage is applied.

So by this way motor starts and winding of motor get protected.