Renewable Energy Questions/Discussion > Wind and Hydro

Induction Motors for Wind Generators

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bvan1941:
oztules,
I believe you, (and I think m12ax7) have mentioned merits of Induction Motors. I have done some reading and am intrigued with its ability to protect itself from overspeed when used as a generator. 

 "self-limiting power generation" set by the back EMF building up as speed increases past the useful rpm range seems to be a real asset not discussed on the Forums except in passing statements.

oztules obviously has the ability to fabricate numerous kinds of WT's but has chosen this type of unit.
 Could you enlighten us some more on the merits and limitations?
Bill

oztules:
Busy day here Bill, but I will answer this fully perhaps tonight.

.......oztules

bvan1941:
YOU Bet !
Bill

oztules:
ok, armed with too much red wine we'll see how this goes. :o

It's not induction motors persay.... it is armature reaction that current limits at a temperature that keeps the stator in a  safe operating region.

We'll start from the start.

If we have a  changing magnetic field near a conductor, it will induce an EMF... so nothing new here.

It's an EMF not current.

When we put a coil of wire near a changing field, we will induce an EMF in the coil, because we have linked the coils with the changing field.

We can draw current from this coil and this is where it starts to complicate things...... because if we draw current in the wire the first thing that happens when the electrons start to move is they create a magnetic field of their own. The more current we draw (more electrons on the move), the more the magnetic field emanating from the coil.... just like an electro magnet.

The field created in this way will be in opposition to the field that is creating the EMF in the first place. If we think about this, this means that the field made by the coil is now opposing the magnetising field that created it in the first place. It will try to repel it, but as it does so, it denatures the magnetising field....... now we can see that we can get to the situation, where the magnetising field has been so messed up that it can no longer induce any more EMF into the coil, as the coil is repelling it. It gets to a situation where there is a stand off, and this is where the current limits. Nothing to do with back EMF, but with the ampere turns in the coils.

All alternators suffer this problem, but any design "encourages " this to a more or less degree.

If the magnets are neo, and very very strong, they will take a lot of repelling before this situation occurs, and the alternator may end up overheating towards destruction before the limiting situation  occurs.....

Axial flux generators are (at the levels we use them) a long long way off limiting when they are over hot. This is two fold.

1. We generally use very very strong magnets.
2. We have very large air gaps, and the coil legs  are wide, and so it is difficult to repel the very strong field over the very large air gap, as the repelling field is diffuse and having to operate over a large gap.

In a generator with very small gaps (<1mm), and a lot of iron, we can focus the repelling field.... just as we used the steel to focus the inducing field... now the repulsion will be directly across the gap between the teeth and the magnets. So now when we repel, we stand a good chance of the stand off occurring at a level where the alt is not burning up, but the current can no longer increase. It just cant get enough (or more) field into the coils because of the interference from the back MMF (magneto motive force) of the coils.

If the AWP had neos, it could be lighter, and smaller drum, but that also means our cooling is retarded, and the massive neo field we have to overcome to current limit will probably mean too late to save it... so it is all down to design.... again.

Now induction conversions (as per Zubbly) seem to allow the current limit to come on before destructive heat does.... so we win. If you manage to cram more magnet in (than seems to have occured in the forums so far), maybe you will get the current up high enough to heat damage them, but i have not heard of this so far..... mostly they complain that when they short the winding in good wind.... the mill runs away, or speeds up even though shorted..... in fact it exacerbates it.

The AWP is really just an induction conversion with the magnets on the outside, but still with a tiny air gap, it uses weak magnets, and so achieves it's current limiting fairly early for it's size and so can run shorted all day, overloaded all day, over sped all day... you get the picture. It cannot self destruct. If it had Neo, it may actually overheat, as the massive increase in magnet field will be much harder to overcome and may be too late to save the stator from over heat.

As a rule of thumb, when ever you have ferrites and steel teeth, you will probably current limit safely. Long salient teeth even more so ( the F&P mentioned on the backshed forums) If you replace the ferrites with neos, chances are you will have an increase in power, but at the expense of safe running.
Generally the increase is not nearly as much as the folks that do it expected, as the steel was probably saturated with the ferrites, so any increase they see will probably be effectively air gap increase.... meaning that the neos no longer see the steel itself, and the steel behaves as air , not steel after saturation.

In the induction conversions this happens too, and it would be a fair guess that most of the teeth depth in the converted motor will be saturated, this will push back current limiting a bit, but you also have very good conduction of heat out of the coils because of the steel mass.

So yes, if you design the thing well, then you can have a mill that self limits the current, and if you pick your blade profile you may get that near when the blades will self limit due to the wash of the proceeding blade... solidity I think it's called.... then you won't need furling.

The AWP furling is rubbish ( on this one anyway), so it runs flat out in high winds with no ill effects at all.... and as we are in the roaring forties, that is frequently for weeks at a time. It can sit for days with the amp meter wound off the clock. It does not care.

So, if you purposely use current limiting due to armature reactance, you can have a bullet proof machine ( the blades need to bolted on well)

I had hopes that Chris would find current limiting with his ferrite units, but it appears that the diffuse field created does not cause this to happen, or he has not driven them hard enough yet. I don't know for sure.

If he did get current limiting, he may be able to introduce more R into the stator and get better low - mid performance, and stable high end, but it may not too. :( My hunch is not.... but I don't know... speculation is cheap.

As the current limit is current induced in the first place, transformers can make a huge difference to output. The AWP limits at around 7-8 amps I think (over the three phases), so the current in the 1mm wire is very modest.... but we can increase the EMF with rpm..... so if we place a transformer in the mix, we can get 40 amps, and pull the voltage back from the 200-500v back to 50-60v. The high frequency (30 poles) helps keep the transformer size down to a modest size to handle 2kw easily and cool as well.


Does that give you an idea as to how and why it happens Bill?


.............oztules

Edit:
Saturation is interesting. It seems that moving electrons create the magnetic field... always. This means the electrons orbiting atoms or shared  covalent bonds (molecules) will exhibit magnetic flelds. Both from their spin, and orbit. They group into domains of atoms,and this domain will exhibit a N and S pole. In unmagnetised steel, the domains are randomly arranged, and cancel out on a macro scale. When we place a strong field, we re-arrange these domains, and the steel becomes a magnet with a N and S pole.

So it would seem that when we have used up enough photons from the field to line them all up, the remaining photons, can no longer interact with the steel, and pass straight through it without interference. When Chris did a video showing the paper clips sticking to the rear of a plate with magnets on, these clips were held on by the field that had passed straight through the steel. Placing other bits and pieces of steel about the place, used up some of these free photons, and there were not enough left to completely saturate the disk any more, they had found an alternate way home.

Interestingly, I don't know what a magnetic field is, nor even electricity... does it flow in the wire or outside the wire...

Oh well, back to the drinking after that I think.

tomw:
Thanks. Oz;

Interesting explanation.

BTW both electricity and magnetism are simply "magic".  8)

That is my story and I am sticking with it.

Zubbly [R.I.P.] alluded to this a few times when trying to explain that a conversion would produce X amps and do that safely into any reasonable voltage. For the same machine it would obviously need higher RPM.

In terms of our discussion it was that it would push the same amps into a 12, 24 or 48 volt  battery equally within the thermal capacity of the motor. Plus the mass of steel wicks away a lot of heat.

He used NEO's and my little 1 meter unit does exactly as he predicted.

It can push the same current into a 12 volt and a 24 volt bank. Which is interesting because it means the unit is producing 2X the power at 24 it does at 12 volts.

Supports your transformer theory quite well, in fact!


Tom

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