Answer / iamdon

No.....AC voltage cannt be used in field winding of rotor
of synchronous machines for following reasons....

1)AC produce poles on rotor whose strength is varying
sinusoidally.It is undesirable because the magnetic locking
between two fields in this case are not achieved troughout
the cycle.At current zero of AC there is no locking at
all.So synchronism will be lost.

2)Field winding is designed for DC. It has higher
resistance.According to Lenz law emf is induced in field
winding.so current flowing through through winding will be
very low.Pole strength is very weak.

Answer / santosh kawade

No.
In synchronous generator or motor we cannot use ac voltage
for field winding.

Synchronous generator/motor works on magnetic locking
principle hence we have to create constant magnetic poles
on rotors.

In case of generator these magnetic pole create voltage in
stator winding as per the speed of rotar.

And in case of motor these magnetic poles gets lock with
stator rotating magnetic field.

Santosh Kawade
9224 69 3116

Answer / rajasekhar

we cant use ac voltage in place of dc for exitation of field
winding because for every cycle the polarity of the field
winding will be changing so there will not be constant poles
on the rotor

Answer / shelja singh

no we cant use AC voltage.if we use this then it will no
longer be called synchronous generator ,it will be called
induction generator as there will be two mmf waves and there
interaction will be same as that in an induction generator.

Answer / vino kumar

In case of synchronous machines we have to maintain a
constant frequency.to maintain constant frequency the rotor
of syn.machine should be rotate with same speed of stator
rmf.so finally to achieve magnetic locking between stator
rmf and rotor ,the poles on the rotor should not be vary ,it
should be constant .the ac voltage changes the poles for
every half cycle.
so to maintain fixed polarity poles on rotor and to achieve
magnetic locking the rotor should be excited by dc supply only.

Answer / nitin nawaghare

IF YOU apply AC source then it will act as a transformer and
work on mutual inductance like transformer. if you rotate
its rotor then it generate the power with higher frequency
(Harmonic.

Answer / sujeet kumar

If we supply 3 phase ac and there is relative motion between
stator rotor flux then only emf will be induced.

Answer / binay agarwal

no, if we supply the ac current in rotor side, then non-
uniform ac will generate, which is not good.

Answer / pavan

it can be used to generate ac current if the frequency of
the ac voltage supplied to the rotor and the frequency of
the rotation of the motor are different. but i am not sure
if the generated power will have only a single harmonic.

Answer / percy

yes ac can used

if it has frequency of 25 hz then synchronous speed will be
half of the actuall i.e. dc is applied

some how we have to rotate field flux at synchronous speed

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