AC Excitation for Alternators

You should have a look on Faraday's laws.easily available on google.

BTW, if you you use an AC excitation, there is no need to rotate the armature as it is going to give you some voltage due to transformer action.

sir, can u tell more about self sustaining electromagnetic generator?

Yes Sir! someone has invented it few hr back, you can read more about it here (5th post by guest)

http://cr4.globalspec.com/thread/43835

Be a good student, don't treat others as your student.We have no obligation to answer you with theoretical explanation/derivation.

I never said you are obliged to that!

I think it is could be a good interview question -- provide of course it has something to do with the job (I would not use it for interviewing HR personnel!). There should be no "wrong" answer, if it is supported with reasonable theory. The question (in its two parts) would be more appropriate for finding bright design engineers than for selecting electricians.

Of course, AC can be rectified to DC for excitation purposes, as is often done. Also, an alternator could be designed to operate with a three phase field (fed through slip rings) "rotating" in one direction supplemented by physical rotation of the rotor. You can imagine all sorts of unusual output waveforms based on various AC excited fields. The likelihood that an AC field of almost any sort, moving past some arrangement of coils would produce no current whatsoever is small, I think.

If the second part of the question (Why can't it be AC?) were deleted, then you should get very straightforward answers that test knowledge more so than the ability to think outside the box. Any industrial electrician should be able to answer the first part, because is is testing for a basic knowledge of electrics.

if u r familier with 1 phase motor working u will get d answer then

when we use dc supply for d exciation system it produces a fixed rotating magnetic field which swips through the conductors and produces +iv & -iv cycles in each phase

and u wil get d single phase or 3 phase supply as u want or as d generator u r using

for d exciation of u might want to use 1 phase ac supply system but its nature is alternating one undergoing +iv &-Iv cycles and we consider 50 cycles of such a nature for a very short time this may produce emf and currents in stator but will be opposite in diren and magnitude also and may tend to cancle each other so dc exciation is mostely used

if u use 3 phase ac system for this purpose but for this one u have to give very concentric winding on rotor to produce synchronously rotating field

quaries -parimal.pathak85@gmail.com

call me MASTER

Exciters are like a Permanent-Magnet so only DC current can produce Permanent-Magnet 's properties not an alternating magnetic-field

If you supplied AC excitation to an automobile type alternator (wonder where you would find AC in your car) of cource you would have some output, It would be direct (DC) because output rectification is applied inside an alternator but the output voltage amplitude would be a fast changing curve of twice the exciting frequency and directly proportional to the absolute value of the exciting momentary current and the alternators speed . And of course would not be of much usefullness and you would ruin the remaining rotor magnetism that allows the alternator to self-start excitation (ability to start generating without an initial power source to excite it)

Regards.

Your answer seems to be of some consideration.

But he may be considering to see what will be the result in Lab, not in a car.

But the Core of Exciter need to be of good Retentivity, so slow in demaetizing & the magnetizing in the opposite direction.

Will it be possible to have this cycle of magnetizing / demagnetizing @ 50 / 60 Hz ?

Hve you confirmed your reply?

Alternator rotors are usally made of mild steel which of course have very weak but also very stable PM qualities. But that's only about self demagnetizing. Applying AC has a very random effect on its PM ability.The core keeps the magnetic polarity it had the moment the AC current was cut off. Could be OK , could be reverse, could be zero. And if slowly decreasing AC amplitude was applied it would be fully demagnetized.

Yes to your concept.

I like to add:

to magnetize fully the waveform applied is a slowly decreasing to zero volt is needed

A badly drawn hand-sketch: