Answer / samy

DC supply gives constant , steady magneticflux.
but AC supply gives an alternating flux. so in alternators
field give to dc supply.

Answer / saeid

The answer is because these alternators are synchronuse
generators. The philosophy of electrical synchronuse
generators is based on DC excitation in the rotor and
synchronuse vontages in stator.

However,there are some application for induction generators
such as wind generators that have AC excitation like other
induction motors.

As a reslut, if alternator is synchronuse generator it has
to have DC excitation and if alternator is induction
generator it has to have AV excitation.

Answer / a.kossay

THATS BECAUSE IN GENERATORS THERE IS A TURBINE WICH CAUSE
ROTATION TO THE ROTOR SO WE ALREADY HAVE ROTATING ROTOR SO
THERE IS NO NEED FOR A ROTATING FIELD, AND IF SO IT WILL BE
HARD TO CONTROL THE SPEED OF THE FIELD

Answer / ajay kumar yadav

no ripple factor in DC excitation but there is ripple factor
in AC

Answer / xengso0nax

alternator is sync. machine which not self start,so we need
dc excitation to get const field

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