Answer / abid ali khattak

As you well known the DC is direct current which doesn't
change its direction. therfore we always using the DC
excitation for Generation due to continues induced magnatic
field for proper excitation.

Answer / anish

i think its b'coz we need constant poles at the ends of the
field(i.e. rotor) so as to lock the rotor wid the rotating
magnetic field.hence DC is used.if AC is used it ll create
NORTH n SOUTH poles alternately on each side of the rotor

Answer / rvg.msceb

The basic requirement of an alternator is considered to be
as a sine wave generator. for getting pure sine wave, the
excitation to be of DC with low ripple factor.
in commercial generators, pure sine wave cannot be achived,
because of the core property of the material.

The excitation can of AC too. If we provide ac excitation,
the armature reaction will be more, and there by more core
loss, leads to heating up of core materials.The harmonics
disturbance also having consider effect in the power loss.

Answer / ashokreddyreguri

Rotor of the alternator is given dc supply so constant
magnetic poles are formed.These constant magnetic poles
produce constant flux.The rotor of the alternator is
rotated by using prime mover,so constant poles are
rotated,so these rotating magnetic poles produce rotating
magnetic field.When this rotating magnetic field cuts three
phase stator conductors produces three phase supply.

Answer / sangeetha.np

When it is of Ac it behaves like a transformer by means of
the induction principle as there are two windings ie
armature and field windings. So in order to produce a
constant mag field we go for DC.

Answer / ketan

when the rotor is rotating via a prime mover.. dc excitation
is given so that constant poles are generated on the rotor
and if ac excitation is given, the poles developed will be
alternating and hence the frequency of the current developed
will be much higher and hence, an ideal Rotating Magnetic
Field(RMF) wont be developed which is generated(for a
3-phase) iff dere's a 120* time phase difference n 120*
space phase diff. is dere. and also due to ac current
flowing dere will be an self induced emf as well
contributing to further losses.

Answer / tapan desai

Because due to ac supply the
torque produce in rotor is in
pulsating nature so filed is
produce also in pulsating
nature so that neglect each
other so for provide constant
torque and field we give dc
supply or dc excitation to
rotor with help of slip ring.

Answer / manoj kumar sahoo

DC which does not change its direction & magnitude .So dc
excitation work properly in alternator without change the
pole.And AC change its direction in 50 c/s. Therefore AC
excitation is not suitable for the alternators.

Answer / b.t.ramakrishna rao

a current carrying conductor placed in a magnetic filed experiences a force
if d.c excitation is provided a constant magnetic field is produced hence the rotor conductors can experience a force.
where as if a.c excitation is given an alternating magnetic field is produced which results zero net magnetic filed,hence the rotor conductors fials to experience a force

Answer / kavya

since the electromagnetic torque produced is zero for a ac
excitation., we always prefer dc which does not change

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