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
I2 is 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.
