Consider the simplified view of a 3 phase induction
motor shown below.
The stator hosts a three phase winding distributed
symmetrically on its inner periphery.
This stator winding is energised from a
three phase supply.
The rotor also hosts a 3 phase winding on its
periphery. But, the rotor winding is not energised from any source and is short-
circuited on itself.
Three phase Induction motor working principle
(1) When the 3 phase stator winding is energised from a
3 phase supply, a rotating magnetic field
is produced which rotates around the
stator at synchronous speed.
(2) The rotating magnetic field cuts the rotor
conductors, which as yet, are stationary. Due to this flux cutting, emfs are
induced in the rotor conductors. As rotor circuit is short circuited,
therefore, currents start flowing in it.
(3) Now, as per Lenz's law ,
"the direction of induced current
will be such that it opposes the very cause that produced it "
.
(4) Here, the cause of emf induction is the relative
motion between the rotating field and the stationary rotor conductors.
Hence,
to reduce this relative motion, the rotor starts rotating in the same direction
as that of the stator field and tries to catch it but, can never catch it due
to friction and windage and therefore emf induction continues and motor keeps
rotating.
Thus, principle of 3 phase induction motor
also explains why rotor rotates in same direction as the rotating field and why induction motor is self starting.
When rotor winding is short-circuited with no
resistance in series, it is called a squirrel cage induction motor and
when rotor winding is shorted through a
resistance in series, it is called slip ring induction motor.
Whatever be the type, working principle basically
remains the same.
