Because the voltage in star connection is Vph=VL/square
root of three.
But in delta connection Vph=VL.
So in star connection is only for starting a motor.
But to run a motor through a delta winding. Becaus then
only full voltage given to motor and it will run in full
because instart for motor, we need high current ,for that
purpose we use delta instead of star. Another reason is
that delta connection control the harmonics and increas the
motor life, also we can use star but by using star the
efficency of moter will decrease.
all the above is not seems to be correct.. the real thing
is in star IL=Ip so we are going for star at the time of
starting. after the machine attains rated speed we are
changing into delta becuase the machine cannot develop
rated TORQUE of the motor in star but it can develop in
delta since in delta VL=VP. that's why its changed to delta
instead of running in star. V is directly proportional to
flux is inturn proportional to torque.
motor running in delta it can be in star beacuse star
connected motor has taken voltage root three times and
current high that the cost is high threrfore motor runngin
in delta but in industres star motor connection also
working its motor rating low kw.
When a delta motor is connected in star, the voltage across
the windings is reduced by the square root of three
resulting in reduced flux in the iron, this will reduce the
magnetizing current, and will also reduce the torque
capacity of the motor
Simple 3 phase motors can be connected in either STAR or
In star the maximum power is reduced but the starting
current is lower than for Delta
In Delta the motor gives maximum power but the starting
current is often too high and the overload trips out.
To overcome this a star/delta starter is fitted
All 6 wires from the motor windings are connected to the
starter and the starter functions are as follows :-
When start button is pressed the motor is automatically
connected in star and the start current is acceptable. At
the same time a timer is initiated and after a
predetermined time the starter automatically changes from
star connection to delta. The motor now runs in Delta until
This is a fully automatic starter, many years ago manual
changeover star/delta starters were more common.