for reducing the motor starting current,initially motor
will take 4to 5 times of full load current,If we use star
delta it will reduce 1.732times of its torque as well as
current will reduce,motor will get safe
At the starting motor needs high torque to reach rated speed
so it will required more flux to get that torque hence it
takes more current from the supply, but the winding
resistance is too low to withstand this current.
we know that
in star phase current = sqrt(line current)
In delta line current = phase current
So current is reduced by sqtt(3) times in star so motor is
first connect to star and then delta
actually the main concept is to reduce the starting current.
initially stator windings are connected to the bus through
star connections it reduces the stating voltage to 1.732
times n hence the starting current.
after sometime ,when max.speed is achieved it has been delta
connected and the voltage applied at the terminals is equal
to phase voltage.
Basically star delta starter is used to reduce the starting
current.When u'r connections are in star, the voltage is
reduced by 1/1.732 times.When the motor attains 80% of
speed, connections changed to delta.
for an induction motor at starting current is very high
speed is small so initially reduced current is applied.for
star connected winding phase voltage is root3 times the line
voltage and for delta current is root3 times the line current
so initial current can be reduced by using delta and then
star during starting of the motor