why are use star delta startor.

Question Posted / gautam jangra

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why are use star delta startor. ..

At the time of starting motor draws very high strating
current due to very small armature resistance & absance of
back EMF.
To restrict high starting current,at starting stator is
connected in star & after 5-15sec(depending upon motor use
purpose)delay is connected in Delta with the help of star-
delta starter.
current in star connection: i=(V/1.732)/Z
current in Delta connection: i=V/Z
After 5-15sec motor pick-up it's rated speed & devolp back
EMF.

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why are use star delta startor. ..

Answer / pratheesh

if an induction motor starts directly online, it will
take very high starting current(Approximately 6-8 times
of full load current). For small motors, this starting
current is not much. But if the motor is large one, it
will draw a very huge current thats how a voltage
fluctuation(Sags) will be created in the electrical
system in which motor is connected.This voltage sags is
not at all desirable and it may affect the other
equipments connected to this same electrical system. And
also this huge current may damage the motor itself.

So by connecting the DELTA wound motor in STAR, the
applied voltage to the motor winding is reduced by
1/1.173 thats y the current drawing by the motor reduces
to 1/3.

After 6-10 seconds(Motor will reach almost its rated
speed within this time) the star connected is removed
and will be connected as delta. So the motor can carry
full load in delta.

There are other kinds of startes like, Auto transformer
starter, Resistance starter etc.