Why Starters are used in Induction Motors ?

To answer Why is starter necessary for a three-phase induction motor ? consider the equivalent circuit for a 3 phase induction motor at any slip S. Only one phase winding is considered for simplicity . You may have observed, it is very much similar to that for a transformer with variable resistance representing a mechanical load being the only difference. In fact, an induction motor is a rotating transformer.

Now, when an induction motor is started, its slip is equal to 1. This reduces equivalent circuit for the induction motor equivalent to a short circuited two-winding transformer.

Mechanical load= r₂(1-s) / s = 0

By transformer action, the secondary current is reflected to

primary as

I₁^’ = N₂I₂/N₁

Because I₂is very high so I₁^’ will be very high.

This shows that at starting the induction motor stator line currents will be very high. This high current is not good. It causes objectionable voltage drops in the power supply lines feeding the induction motor. These large voltage drops cause undesirable dip in the supply line voltage, consequently the operation of other equipments connected on the same line is effected considerably. This is also the reason for momentary dimming of lamp and tube-lights in homes at the instant a refrigeration motor starts.

So, to limit the starting currents starters are used for induction motors.