In a 3 phase squirrel cage induction motor why does rotor
rotate?
will 3 phase transformer rotate like a motor if it is
placed on bearings and powered up? give reason
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Answer Posted / ranjeet mukherjee
In 3phase induction motor,when 3 phase supply is given,a
rotating and constant magnetic field is developed in the
air gap between rotor and stator.This magnetic field is cut
by rotor bars,so an emf is induced in the rotor bars.As the
rotor is short circuited from both end so a current will
flow in the bars.The direction of this current will be such
that it will oppose the cause which is producing it(Lenz's
Law).Here in this case the cause is the relative speed
between the rotating magnetic field and the standstill
rotor.So the rotor starts rotating in the direction of the
magnetic field so that it cathes the speed of the magnetic
field and reduce the relative speed between them.
Although the principal of working of 3phase induction motor
and transformer are same but in case of transformer there
is no relative speed between primary and secondary windings
magnetic field.Moreover both the windings are placed in
same core and there is no air gap between the magnetic
fields of the windings.Both the flux are in same core.So it
is not possible to rotate a transformer even though it is
placed in bearings.
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