Generator Field

A

Thread Starter

achumman

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?
 
M

Michael Batchelor

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.
 
C

Curt Wuollet

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
 
I was recently conacted off-forum with two questions related to this thread:

1) What is the alternator's output waveform equation?

2) What would the alternator's waveform be if the AC field frequency and alternator's rpm were equal?

If anyone is interested in my reply, contact me off forum.

Phil Corso, cepsicon[at]AOL[dot]com
 
>Can anybody give me an explanation for
>giving DC supply only to alternator
>field?

I'm not an electrical whiz, so I try to keep it simple. DC current is used in the field (rotor) to make an electromagnet instead of using a permanent magnet. The desire is to have a rotating magnetic field.

Using a permanent magnet like one on the refrigerator holding Johnny's homework up would be A) too large and B) not allow the ability to increase or decrease the field strength to control voltage.

I tell operators and non super technical people that the purpose of the excitation system and rotor is to just create a big electromagnet that spins. When put in those terms they seem to get the big picture.

 
I will gladly provide on-forum answers to questions asked off-forum, about supplying an alternator's field-winding with AC excitation, rather than DC!

I only ask that a reader, not encumbered with a PtP's mind-set, have the fortitude to ask, here on-forum!

Regards, Phil Corso, [email protected]!
 
A

akhil kangath

alternating current is varying one...so there is a chance to emf induction even we did not rotate the shaft....is this possible???
 
akhil kangath,

Yes, there would be induction if the shaft were not rotated when AC was applied to the rotor, but voltage doesn't do anything unless you put in a Volkswagen and push it down the street.

Power is amperes. Generators convert torque (applied to the generator rotor by the prime mover driving the generator--a steam turbine, or a gas turbine, or a reciprocating engine, or a hydro turbine, etc.) into amperes.

The formula for electric power is (basically):

P (watts) = V (volts) * I (current)

The voltage of an alternating current system is supposed to be relatively constant (at any point in the system), so the power of an AC system is directly related to the current flowing in the system. And, generators convert torque to amperes; motors convert amperes to torque. Electricity is just a way for converting torque into a medium that can be transmitted and distributed over wires and then reconverted back into torque--or useful work--at the other end of the generator. The prime mover driving the generator is actually doing the work of all the motors connected to the generator by the wires and transmission and distribution system.

DC is the most efficient method of developing generator terminal voltage--but voltage, by itself, doesn't do a darned thing.
 
S
Do you think both ac and dc supply can be used for excitation of field in alternators? u are saying DC is economical to use when compared AC as it uses other variable devices?

to get Ac why we are giving AC what need is fulfilled? if Ac is given there will be induced emf without any rotation.

sir please clarify me
 
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