Star-delta starter for an
induction motor is explained below. But, before we get to that it is worth to
study Why is starter necessary for a three-phase induction motor ?
By
reading above link you have seen that an induction motor takes high starting
current. In order to limit this starting current to reasonable values, star
delta starter is used commonly.
This
method is used for motors with stator winding designed for delta operation and
is connected in star during the starting period. When the machine is up to
speed, the connections are changed to delta. For the connections, a special
switch called triple pole double throw (TPDT) switch is used.
Lets see how current changes with these connections.
Only one phase is considered for analysis.
Let
VL be the line voltage. Then per phase motor starting current Ist.y
with stator winding in star is given by
(Ist.y)ph= VL /
(√3 ×Zsc)
With
stator winding in star, Ist.y is also the starting line current with
star-delta starter i.e
(Ist.y)L=
VL / (√3 ×Zsc)
If
the stator winding were delta connected, then with direct switching, the per
phase motor starting current Ist.d would be given by
(Ist.d)ph=
VL / Zsc
And starting
line current would be equal to (Ist.d)L=
√3 ×Ist.d
Comparing
the line currents in above two connections, we get
starting
line current with star-delta starter is only 1/3 times the starting line
current with direct switching. Thus, starting current is successfully minimised.
Lets see how torque changes with these connections.
The
torque equation for an induction motor is directly proportional to the square
of the phase voltage
i.e Tr ∝
Vph2
when stator is star connected, Vph= VL /
√3
So
that (Tr) y = K(VL2
/ 3)
when stator is delta connected, Vph= VL =
voltage with direct switching
So
that (Tr) d = KVL2
From above two equations,
(Tr) y = (Tr) d
/ 3
So, The star-delta starter also reduces the starting torque
to one-third of that produced by direct switching in delta. This is the
disadvantage of wye delta starter method.
