Yelp!!! (Induction Generators...)

Hi! New to Usenet Groups so plz don't mind the inhibitions... the thing's this... I was trying to do some study in Induction Generators so I go to all these web pages, Wind Power... captive generation... etc etc... ok cool... but what about the principles... so I download a few IEEE papers, read a thesis or two... yaa they have the mathematical model (dq0 or per-phase etc etc)... the simulation even works... wonderful,

But all this time, the thing which's being irritating me is the basic principle... what I could find till now is dis... just like in Induction Motors, the rotor follows the synchronously rotating mmf, in IGs its the opposite... the rotor is ahead and the mmf of the generated electric field is following it. This is fine, and can be visualized when we connect an IG to grid and rotate the prime mover wid speed greater than the grid frequency... BUT... the problem I have is in self-excited IGs,

They say that u require a reactive power source (usually a charged capacitor or smthing)... dis is not ENUF!!! How does one explain the building up of voltage... I mean how can I visualize it? There is no external excitation to rotor... so how's the induction phenomenon causing the rotating rotor to induce voltage back into the stator windings.... it just doesnt make any sense....

I mean in synchronous generators, we can justify that yes.. dere is a rotating magnetic field (due to excited rotor) viz. cutting the stator coils and emf's being induced... In DC generators, its pretty much the same thing as excitation source (or atleast remenant magnetic field) is there helping the field windings... but what the heck is going on in IGs.... Smone having anyy idea.... plz plz plz help me..... my project's dependent on it.

Study English composition first so my eyes do not glaze over when I read your post.

Bill

-- Fermez le Bush--about two years to go.

some of the posts reminds me of the stewardess in "Airplane" asking if anyone on the plane can translate Jive.

Induction generators with no 'grid connection' are pretty lousy. The thing is, the amount of slip between the rotor speed and that of the rotating MMF is highly variable. So with no other frequency base, the output frequency of the IG varies greatly with load.

As far as voltage buildup, first you start with a cap. It discharges through a phase of the stator. This temporary magnetic field induces a current in the rotor. The rotor field induces a voltage in the stator.

*IF* there is a load connected, then some more current flows in the stator winding and the process builds. If there is no load, then the capacitor alone *may* be charged slightly and the process may still build. But this will depend a lot on the size of the cap versus the reactance of the machine.

Suffice to say, the build up is very problematic. A load that carries enough current in the stator, but not too much. The frequency of the stator will be a function of the total impedance (internal as well as load) and the speed of the rotor.

I can't really see how an isolated IG would be of any use to anyone. Voltage output highly uncontrollable, frequency output also uncontrollable. Thus, power output very uncontrollable. Depending on the prime mover being used, a lack of control on loading may result in overspeed.

daestrom

>

Hello

You have already had several useful suggestions/explanations.

The best paper of which I am aware describing what goes on is :-

The process of self excitation in induction generators by J M Elder et al IEE Proc Vol 130 Part B No 2 March 1983 pages 103-108.

This mentions the point that has been missed so far "residual magnetism". This can provide the initial kick to get things started.

My main interest in this area was in understanding what happened when an induction generator running in parallel with the public supply system became separated from it at a point remote from the generator, with load connected between the generator and the break.

John

Sorry I did not make the situation that I was describing clear enough. One normally has capacitors adjacent to the generator for power factor correction. These and the system capacitance can mean that if the connection to the main system opens several miles away, the generator can continue to generate at an undefined voltage and frequency. There can be customers connected to the system between the point it is open and the generator, and they may not appreciate the voltage and frequency that they receive.

John

"Seems to me" that a UPS could be used with an IG. The UPS would provide some "reactive power" but the IG would provide the "real power." A portion of the "real" power would be use to provide replacement juice for the batteries in the UPS.

Some years ago I was told that larger aircraft use IGs with inverters for frequency stabilization. The IGs are rotated by a "roughly" constant output speed hydraulic "transmission."

Also "seems to me" that IGs would be a natural for "supplemental" power while on the grid. Basically, they would slow down your meter when the wind is blowing. If you could count on the IG to stop generating when the grid goes down you would not need to protect against "back feeding."

When it comes to commercial aircraft to a good approximation, cost isn't much of a problem. But (pulling numbers out of you know where) paying $20k for a "box" that flies is down in the noise. But more than $2k for a home use "box" may be way too much.