by using starter, we can control starting current becus, at
the time of starting current is very high. It may damage
stator winding so we have to us starter to control starting
current.

Starter means at the time of starting our motor takes more
current due to low emf in starting so its function is to
limit the starting current. we know in starting motor takes
5 to7 times current of rated current.
for starting induction motor we use three method which
are further giving
1.Direct on line starter.
2.AutoTransformer.
3.Star-Delta.

As we know at the time of starting an induction motor the rotor is stand still hence the slip is 1.if slip is 1 the rotor induced voltage is maximum hence drives maximum current in the rotor circuit.This rotor current current has impact on stator current(As per LENZ's law).This is the reason why an induction motor takes high current at starting(About 5 to 7 times the full load current).

This high current causes voltage fluctuation,unwanted power loss & unwanted stress to the system or equipment.So it has to be reduced by using different starting methods.

There are two possible ways to limit the starting current
1.Reduce the applied voltage.
2.Increase the rotor resistance.

A reduced voltage is applied at the starting,and the
voltage is increased to the rated value,when the motor has
picked up speed.the reduced voltage is obtained by using a
starter.

the current in an induction motor has the equation

i=sE/(r^2+(sx)^2)^0.5
at starting s=1
so sE=E
SO MAXIMUM EMF IS applied in the rotor circuit.hence maximum
current.to limit this high current a starter is used

R2 is D.Proportional to (1-s) { s = (Ns-N)/Ns) } . Same as
a transformer, Sy current determines the primery current.
So at starting R2=0 as ( N=0 and s=1). So Sy current is
very high as much as 5 times than F.L current. Hence as per
Transformer action py current also is so high. By reducing
the applied voltage or introducing resistance in the rotor
ckt this ckt interference can be reduced . FOR WHICH WE
USE A STARTER.

Starter is mainly used inorder to reduce the HIGH STARTING
CURRENT and to protect the electrical equipments.at the
time of starting the slip=1;speed=less; hence the rotor
resistance value equals '0' due to the relation r2(1-s)/s.
there is no back emf,such that it draws a high starting
current.It may lead to damage the equipments in motor.hence
to avoid this starter is used.main types of starters are
1.Direct On Line starter.
2.Auto Transformer starter.
3.Rotor Resistance starter.
4.Star - Delta starter.

the armature current of amotor is given by Ia=(V-E)/Ra.
thus Ia depends upon E & Ra,if V is kept constant. when a
motor 1st starts , the armature is stationary so the back
emf is 0 and Ra is very low so Ia is very large. this
current will spark in brushes, damage windings &
commutator. that's it.