A starter is needed for a three phase induction motor
because at the time of starting, if induction motor is
started directly, it will draw large amount of current
which causes damage to adjoining equipments.
Thus, a starter is needed in order to limit the starting
current.After the motor has started at reduced starting
current and hence reduced voltage, the connections are
diverted towards the mains supply so that now, the motor
can run at higher starting current and voltage.
Four types of starting methods are employed:
2.Auto transformer method
3.Primary resistor starting.
Low rating upto 7 H.P motors can be started directly.
at the time of starting slip S= 1, so the rotor resistance
which depends on slip i.e.
R2(1-S)/S will be equzl to "0",
i.e. rotor will act as short circuit.
hence initailly motor will draw heavy amount of current.
So in order to reduce this inrush current(5 to 7 times of
rated current), we apply less input voltage at the time of
starting so that input curent will be low(as in case of
star connection) & after some time or specified time input
will be fed in Delta connection.
if u understand this then ok other wise read more books in
starter is reqired for the purpose of reduce starting
current. starting current reduce in starting period so that
starting torque is increase. another thing is that if
starter is not provided than high amont of current flow in
starting beacuase of that it may be chances to damege
itself and alose its adjoining m/c.
Starter is used to reduce starting current.starting current
is usually large because of inertia,charecteristics of
motor winding(at starting slip=1,rotor resistance low
implies less back emf).So at starting we use starter .for
example.STAR DELTA starter.by using a starter at least the
switching is improved which is much important for safe and
longer life time use.simply a DOL can be used for lower
rated motors (up to 07 HP)in this case.for higher rated
motors (up to 15 HP)we can use a star delta connection.a
delta connected motor takes 1.7 times line current than a
star connectd motor.think ! at starting the current is
almost halved .Now a days we use drives (power electronics
products which not only takes care of starting of a motor
(improving power factor) also controls it as perrequirement.
when the induction motor starts.slip i.e "s" has a value
1...and hence the internal resistance value becomes
0..since resistance is directly proportinal to (s-1)....as
the resistance becomes 0..according to ohms law current
becomes very high.....tha value of current is about 5 to 6
times the full load curent......therefore the motor must
accelerate to high speed ,,which is not possible at "load"
condition.....and hence the motor may burst due to
overheating.............to prevent the motor from bursting
by reducing the value of current we use a STARTER....