at the time of starting of induction motor speed is zero,so
back emf is also zero,as back emf is zero & rotor coil is
sorted(in case of squirrel cage rotor),starting current is
about 5 to 6 times higher then the rated current.so because
of this high voltage drop gets created in the supply line &
equipment connected to this line will experience this.so to
reduce the high current generally we use star delta
starter.at the time of starting we use star connection so
the voltage got reduce then at the time of run we use delta
starters also increase the starting torque & improves the
consider a equation ( kvl form in starting period )
at starting time,
the back emf is zero,
In case the R is small mean,
eg) v=220 Volt, R=.5 Ohm sub this and get I=440 Amps,
that reason we increase the starting resistance of the
motor using starter.
T=FLUX*I*POWER FACTOR, as time of starting s=1
so reactance is very large in comperision to
resistance(normaly is very low) for constant load(and
inertia) the power factor(theta=reactance/resistance is
large) is very less(flux is constant) so starting current
is very large so Starter is used to reduce 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.
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.