Question

why DC current is applied to provide low exciation in the file pole?​

Answer 1

$\huge \fbox \red {✥Aɴsᴡᴇʀ✥}$

In the alternator, as the rotor conductor changes its position as the rotor rotating so constant flux is to be produced in the rotor winding and it is possible only when the DC supply is given to the rotor winding.As we need constant flux that is why DC is used instead of AC for excitation of Synchronous Alternator.

Answer 2

Some of the answers seem to be for synchronous motors but the question is about synchronous alternators.

Excitation in an alternator is the current applied to the rotor to make it a magnet with fixed polarity. As the rotor turns, the magnetic field is applied successively to the stator windings, causing a positive, then zero, then negative then zero etc. magnetic flux in the stator windings. The changing flux produces the generated voltage in the stator windings and the flux reversals are due only to the rotation of the rotor. If we apply AC excitation voltage to the rotor, then its magnetic field will alternate due to the excitation and the stator windings will receive alternating flux varying both with rotation and AC excitation. The resulting generated voltage is hard to predict but it won't be the nice three-phase AC that you want. The rotor must be excited with DC.

However another type of generator is the Doubly-Fed Induction Generator, often used on wind turbine generators. It does have AC power applied to windings on its rotor. It allows a generator to run at a range of speeds depending on the momentary speed of the wind while still being able to generate 60 Hz or 50 Hz power for application to the grid. I suggest that anyone interested in DFIG’s search specifically for their information and description.

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