Answer / raul

When the dc supply is given to the rotor windings, then the
polarity of the north and south poles will be fixed in the
space and time, so a sinusoidal magnetic field/flux will be
set up in the air gap which in turn will cut the stator
conductors and hence as per the lenz's law, there will be
induced emf and consequently induced currents in the stator
conductors.

But if ac supply is provided to the rotor windings, then
the polarity of the north and south poles will not be fixed
and it will keep on varying as per the sinusoidal curve, so
a sinusoidal field/flux will not be created in the air gap,
no flux cutting of the stator conductors and hence no
current in the stator windings. That is why the rotor
windings are not provided with the ac supply.

Answer / guest

in case of ac supply the current will vary
alternatively,its value varies from zero,+ve max,zero,-ve
max .so u will get zero magnitude of magnetic field for one
cycle.so u cant cut stator with flux,so there is no induced
emf in stator. finally no output from stator.but in dc
exicitation u will get net magnetic field.and u will get
induced emf.

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