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Can anybody give me an explanation for giving DC supply only to alternator field? What will happen if AC power is given instead of DC?

The shaft of the alternator turns, which provides the "relative motion" required to induce emf into the stationary windings. You don't need the field to vary as well. If you did vary the field (say by applying 60 hz), you would get a complex waveform generated rather than a frequency directly related to the shaft speed of the alternator.

(OK, OK. for the purists, it really isn't *THAT* complex a waveform. It's just an additive function, but it would sure look pretty complex to the poor bloke trying to figure out what in blue blazes was wrong.)
--
Michael Batchelor
www.IndustrialInformatics.com

Hmmm....

What prompts you to ask?

Personally, I've never thought about why we use DC for creating the rotating magnetic field of a synchronous generator (alternator). It would seem that if a magnetic field can be created with DC or AC, it would seem that either could be used. Now that I've considered it, how would one apply AC to the field windings? With slip rings? With a commutator?

Now, to consider how one would produce a varying magnetic field strength as required for synchronous generator operation with AC, one would need to be able to control the AC current and voltage, and for that one would likely need some kind of variable AC source, which would likely use some kind of variable DC source as is currently used for most large synchronous generators.

But, I feel certain that some engineer or physicist or professor has considered this, and done some research on the feasibility and the cost-effectiveness of using AC to produce the rotating magnetic field of the synchronous generator rotor. And that those people have come to the conclusion that for one reason or another it's not any more or less cost-effective than the current method of using DC. It may even be significantly less cost-effective, or simply not feasible for one reason or another (what frequency would one use? how would it interact with the stator frequency?). Because one would think that if it were feasible and more advantageous (cost-effective) for one reason or another it would be done.

Interesting question, nonetheless.

Responding to Achumman's 20-Nov (12:33) question... This subject was covered in Thread # 1026226790 (http://www.control.com/thread/1026226790) titled, "Alternators!"

Regards, Phil Corso (cepsicon@aol.com)

You would get some really interesting waveforms.
Intuitively, your output waveforms would be
amplitude modulated with the frequency applied
to the field. Depending on the frequencies and
rotational speed and phasing, your output would
vary wildly, even reversing phase.
Thats why they don't do that.

Regards

cww

If you want steady magnetic field you are using DC. But in case of AC the field strength will change in same frequency as that of AC 50 HZ or 60 HZ. Finally we will get variable field, hence variable EMF at output. The waveform will not be complex as it seems to be. But it will be distorted and will be resultant of both armature and field MMF.

Thanks...
Naresh