Answer / hitesh parmar

If A.C. excitation is provided in the alternator, the flux
produced will be alternating and accordimg to faraday's law
of magnetism back emf will be induced in the rotor, which in
turn causes a A.C.current to flow and as we know whenever a
current carrying conductor is placed in a magneti field
experience a force thus the torque will be produced in the
opposite direction to that of rotor rotation and the 2nd
reason is that in will produced a alternating poles that we
don't want so we are using D.C excitation only

Answer / mohankumar n

to avoid super imposition of rotor & stator frequency. From
MOHANKUMAR N Eectrical Engineer Power plant.Sankeshwar
Belgaum (D) Karnataka State.

Answer / alok verma

dc excitation is used for the alternators coz,DC IS CONSTNT IN MAGNITUDE ND IT HAS LINEAR GRAPH AND THERE4 THERE WILL BE MAX CUTTING OF FLUX...
(YAAR jisko jo maan me aa raha hai likh raha hai,so ah ikha karo)

Answer / krish

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 / rakesh

Alternator works on the principle of dynamicaly induced emf
which is induced when a constant flux is cut by a rotating
conductor , So dc exitation produce constant flux THUS DC
IS USED FOR EXITATION

Answer / swapnil

if we use ac exitation it will creat alternatly changing
flux and pole will also be changing with flux in case of dc
it will produce constant flux and constant pole as rotor is
rotating sin wave will be genrated in genrator if ac will
be given for exitation sine wave distortion will take place.

Answer / hitch

1-the dc current produce constant field
2-the ac produce more eddy currents means temprature rise
3-ac produce harmonics more than dc

Answer / tanaya

In case of dc excitation the
reluctance is provided by the
magnetic field is lesser than
the ac excitation.,also dc
excitation provide a uniform
magnetic field ,thats why dc
excitation is considered.

Answer / roy

AC itself produces alternating flux ,so If u gives AC to the
field coil the generator will give transformer action.there
is no need of rotating it.
In case if we rotate the field ,constant field will not be
produced.For every cycle of rotation ,there will continuous
change in the pole wrt 3 wdg of armature,result in irregular
waveform.we cannot get pure sinusoidal 3 phase supply.

so to overcome from above, dc excitation is given so that
constant field strength will be maintained.

Answer / unknown

When ac supply will have given to rotor side then synchronous machine becomes induction machine fed from stator side ,so the frequency of voltage or current that is produced at stator terminal is "SF" i.e very less than 50 ,so we can't get pure sinusoidal waveform.

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