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Dinges
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Posted: 10 January 2009 at 6:04am | IP Logged Quote Dinges

(Ok, let's test the bandwidth of Glenn's new server with a few MB of animated GIFs . Apologies in advance to users with low bandwidth connection).

Here are the final axial flux animations. I expect these will be the last I've made as I think all the basic variations on the theme are covered by now. I've run 5 extra cases. One where the return rotor is a flat plate, without any magnets on it. Next simulation is of a plain steel return plate but with 'dummy' steel blocks on them, the blocks having the same dimensions as the magnets. Then, an example of a triple-rotor axial flux, an idea one occasionally hears about.

But I think the most interesting simulation is this one. Instead of varying airgap, I varied spacing between magnets whilst keeping airgap constant (at one magnet thickness, 12.7mm). The results seem to suggest that spacings above about one airgap wide seem to yield little to no benefit; as spacing is increased, the flux, instead of shorting to its neighbouring magnets, starts to fold back on its own magnet or steel rotor plate.

Considering that 'normal' magnet spacing is considered to be about one magnet width (that would be 25mm in this case), there seems to be potential for improvement here in axial flux construction, at least from a magnetic point of view. It's similar to what Gordon did with his axial flux generator, where magnet spacing also was about the same as airgap. There's of course still the other matter that we need space for the coils, which also becomes smaller as magnet spacing decreases. But from a purely magnetic-circuit perspective, it makes little sense to have spacings much larger than the airgap, as there's no reward in more airgap flux:


http://picasaweb.google.com/motorconversion/AxialFluxFEMMSim ulations#5289268036848722690


The other new animations:


http://picasaweb.google.com/motorconversion/AxialFluxFEMMSim ulations#5288967311555894498 (Vawtman, the above case is nearly identical to your radial flux cartwheel generator)



http://picasaweb.google.com/motorconversion/AxialFluxFEMMSim ulations#5289043782055815058



http://picasaweb.google.com/motorconversion/AxialFluxFEMMSim ulations#5289225266561397682


The final animation is of Gizmo's idea to mount the magnets elevated on protrusions:


http://picasaweb.google.com/motorconversion/AxialFluxFEMMSim ulations#5289160456097816914


Glenn, as you can see, I ran the simulations and made a little animation of your idea to try to improve the axial flux. However, as the animation only shows the flow of the flux lines it's not easy to determine absolute value of flux densities to compare your design with the normal axial flux. So, I ran two more simulations (for the specific case of an airgap of exactly one magnet thickness, i.e. 12.7mm) to gather the hard numbers. As peak flux densities turned out to be exactly the same for both cases I did another check and integrated the flux (area of integration is 25mm wide and exactly halfway in the airgap). It turns out that even then there is no difference between the normal axial flux design and yours... Not what I intuitively expected (says more about my intuition than anything else, I suppose...). See numbers below:

Normal axial flux generator:
Average B.n = 0.67953 Tesla

Gizmo's axial flux generator:
Average B.n = 0.680348 Tesla

Absolutely no difference between the two apart from noise.

It looks like the conventional dual rotors (with magnets on both) are actually a pretty good design. One thing I've concluded from doing all these simulations and watching the animations is that it will take a pretty clever guy to make improvements to the magnetic circuit of the axial flux...

One final disclaimer of all these simulations and animations: they're only an approximation of reality and don't take all factors into account. They basically show only the magnetic flux as it is when there is no generation going on. As soon as real generation starts, the coils create their own, counter-acting magnetic fields, which influences the magnetic field the permanent magnets generate, in essence 'pushing it back'. So, anyone reading this, keep that in mind when interpreting the simulations and drawing conclusions. They show only a fraction of what is really going on inside a generator. Consider it to be an approximation (at best).

Peter.


Edited by Dinges on 10 January 2009 at 9:31am



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oztules
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Posted: 10 January 2009 at 8:13am | IP Logged Quote oztules

Quote:
But I think the most interesting simulation is this one. Instead of varying airgap, I varied spacing between magnets whilst keeping airgap constant (at one magnet thickness, 12.7mm). The results seem to suggest that spacings above about one airgap wide seem to yield little to no benefit; as spacing is increased, the flux, instead of shorting to its neighbouring magnets, starts to fold back on its own magnet or steel rotor plate.


In reality, it is more likely that the airgap for that magnet size will be 19mm. The stator would be 13mm min (strength and winding room) with perhaps a few mm either side.

Using your analogy, that would put the inter magnet gap up to about .75 magnet width.... which is about the range I would expect.. (3/4 to 1 magnet width).

Thanks once again for your tireless work in this femm business.


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

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Posted: 10 January 2009 at 1:40pm | IP Logged Quote wdyasq

It sure is nice when someone does work you have been wondering about. Thanks Peter.

I have been contemplating a 'wave wound' axial flux alternator. This style of winding should give twice as many 'cuts' of the magnetic field as 'simple coils' and allow a thinner stator. This all should result in more voltage at lower speeds.

I need to do some calculations as to proper magnet spacing. I plan on using the 1 x 2 x 1/2" magnets. the array of magnets that will give proper 'flux change' and sufficient capacity still needs to be 'guessed' and a prototype made.

Peter's work has been inspiring. Some folks get so inspired they commit suicide.

Ron

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Dinges
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Posted: 11 January 2009 at 3:13am | IP Logged Quote Dinges

Whiskey wrote:
The magnetic rotor was approx 2 1/2ft in diameter and its magnets were fixed horizontal, not vertical like most home brew wind turbines are built. The magnets were approx 150mm long and 50mm wide and 20mm deep.

Hm. That would be about 110 cubic inch of magnetic material?! That's a lot. Are they NdFeB or ceramic magnets?

Whiskey wrote:
The air gap was very wide, 5mm at least.

For a 2.5ft (750mm) diameter rotor, I don't consider 5mm airgap to be excessive to provide the required mechanical clearance. Of course one could reduce it, but it'd require greater accuracy in machining and may turn out to be less reliable in the long term, when (not if) the bearings start wearing out. Especially in an environment where regular preventive maintenance isn't carried out (where is that done anyway... usually things get looked after when they have become defect and ignoring the situation is no longer an option).

Whiskey wrote:
I will look into extracting some pictures of the magnetic rotor is any of you want to see it.

Of course we want to see it Whiskey... Any chance to get a closer look at a professionally built windturbine is welcome.


Oztules wrote:
In reality, it is more likely that the airgap for that magnet size will be 19mm. The stator would be 13mm min (strength and winding room) with perhaps a few mm either side. Using your analogy, that would put the inter magnet gap up to about .75 magnet width.... which is about the range I would expect.. (3/4 to 1 magnet width).


I agree, the 12.7mm airgap is a bit on the smallish side when compared to current reality. The main reason for using an airgap equal to one magnet thickness in those simulations is for scaling and comparison purposes.

However, I disagree with you where you state a magnet spacing in terms of magnet width (though I realize I've done the same myself... Hm.); it's more correct to state it in terms of (desired) airgap. When airgap varies, optimal magnet spacing varies too, more or less independent of magnet width. I think in this particular case I would settle myself for a spacing somewhere around 2/3 magnet width, which, if I'm not mistaken, is a tad closer than is commonly suggested. Resulting in more magnetic material in the generator for a same given physical dimension, thus resulting in a more powerful generator (and one with lower resistance; you'll love that part ).


Ron, I'd suggest to put out your idea in the open for discussion and criticism, but your call. The more eyes have a look at it, the more relevant criticism you may receive. After all, it's possible that you (and I) have missed something. Unless you're too insecure for that, of course...

BTW, have you ever considered a career in politics? Your natural ability for 'double speak' (where things can be interpreted in various ways, according to how each listener wants to interpret it) would render you a natural candidate for such a career. Because I'm still not sure whether your remark was a compliment or the opposite...

Peter.

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Posted: 11 January 2009 at 5:46am | IP Logged Quote whiskey

Hi Peter,


I have not worked out the magnetic area, but will trust your figures as a fellow engineering type :)

The magnets are painted black and it is difficult to tell what material they are constructed with. If I had to guess I would think NdFeB and certainly lower than N40 grade. N35 would be closer in my opinion.

A 5mm air gap to coil spacing in my experience is light years. I aim for 1mm most of the time and only wider on larger machines where there is less rotor support available. Some home builders like a wide air gap to help reduce or eliminate stall at higher wind speeds. I dislike to build a 90% efficient alternator and open up its air gap and reduce the efficiency due to poor blade design. The minimum air gap for maximum energy production is the aim of any alternator, and a blade must also be designed for the alternators power curve, or should be. CNC machining is a must for tighter tolerances and I am fortunate to have access to such machines.

The Proven blades are something else, made from some plastic flexible material and does not instill confidence in me at all if I were an owner. The coils I think could be interesting if we could get a look inside the goop they are embedded in. Toroidal coils appear to be used and that is not a common way to wind coils for wind turbines.

I have taken a movie of the machine and will see can I extract a few stills from it and post them here.

With regards your comment on Ron, we all know he is really a yankie dressed up as a Texan and as such he has a way with words. Words to confuse, jest and offer sense. Having exchanged many chats with Ron on IRC I would qualify my statement of Ron and say he is one of the good guys. Sorry Ron, I know you said no agreeing, but the truth must be told ;)

Whiskey



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vawtman
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Posted: 11 January 2009 at 5:54am | IP Logged Quote vawtman

http://www.fieldlines.com/story/2007/11/25/18599/255Flux reply
Peter the a.k.a cartwheel generator was never intended to be efficient

What do you make of fluxs'comments ?
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Dinges
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Posted: 11 January 2009 at 8:31am | IP Logged Quote Dinges

whiskey wrote:
Having exchanged many chats with Ron on IRC I would qualify my statement of Ron and say he is one of the good guys.

...you sure we're talking about the same Ron?... The one I often see in IRC doesn't fit that description very well... And he seems to get some infernal pleasure out of abusing poor little innocent Dutch boys. Not sure why he has this irrational hatred of the Dutch. We're only people too, you know.

Though I must admit... sometimes he even makes sense and almost looks human. But that's as near to a compliment as I will go! And if anyone asks me, I'll deny I said it.

Admit nothing, deny everything, make counteraccusations...

Peter.
(who discussed the idea Ron had with him, and thinks it's worthy of wider attention)

edit: and without sarcasm this time... yes, Whiskey, I agree, he's definitely one of the good guys.

Edited by Dinges on 11 January 2009 at 10:05am
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Posted: 11 January 2009 at 9:09am | IP Logged Quote Dinges

Vawtman,

My previous remark wasn't intended to say your generator would be inefficient; it was only intended to point you to the similarity of that simulated situation to your generator. I recall you were wondering at the time what airgap you should decide upon for your generator, and that simulation may be of help to you in that respect. It doesn't show absolute flux values though, but this one does: http://picasaweb.google.com/motorconversion/AxialFluxFEMMSim ulations#5277567403450056930. There are more images there for other airgaps (5-7-9-11-13-15-17 etc. mm).

Your generator may not make maximum effective use of magnets but that's compensated for by the large amount of magnets (54) you are using. So I expect it'll still be a powerful generator.

I hadn't seen at the time the discussion you linked to, but Flux is one of the better people to receive advice from. His posts provide a wealth of information and education. But I'm telling you nothing new there.

Just keep in mind that when he talks of 'total flux per pole' in that equation it's not flux density (T) he's talking about, but total flux (in units of Wb, Weber); 1Wb = 1T * m^2. To use Flux's equation you'd have to multiply the flux density (T) with (approximately) the surface area of your magnet, i.e. 25*50 = 1250 mm^2. Keep correct units in mind: 1250mm^2 = 1.25*10^-3 m^2.

Multiply the area of 1.25*10^-3 m^2 with your desired flux density (T; 1T = 1Wb/m^2) and you'll get the total amount of flux in units of Weber as output. That value can then be used in Flux's equation to calculate EMF.

Peter.

edit: on 2nd thought... I'm not sure the area of 25*50mm is entirely correct. It may be more, depending on the size of your coil that's exposed to the flux. You might also have to take some of the flux in account that's wavering out, beyond the width of the magnet. Hard to say. I guess there's only one real sure way to find out what voltage you'll get, and that's by winding a test coil...

Edited by Dinges on 11 January 2009 at 9:18am
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Posted: 11 January 2009 at 10:43am | IP Logged Quote GWatPE

Dinges wrote:


Just keep in mind that when {he "FLUX"} talks of 'total flux per pole' in that equation it's not flux density (T) he's talking about, but total flux (in units of Wb, Weber); 1Wb = 1T * m^2. To use Flux's equation you'd have to multiply the flux density (T) with (approximately) the surface area of your magnet, i.e. 25*50 = 1250 mm^2. Keep correct units in mind: 1250mm^2 = 1.25*10^-3 m^2.

Multiply the area of 1.25*10^-3 m^2 with your desired flux density (T; 1T = 1Wb/m^2) and you'll get the total amount of flux in units of Weber as output. That value can then be used in Flux's equation to calculate EMF.
..


Hi Dinges,

I am trying to equate this paragraph to measured values from my mill.

I measure emf of 0.2VAC at 300rpm,55Hz. I have 22 magnets per rotor, 11 pole pairs, dual rotor. I have 17turns per coil and 11 coils in series[187turns/phase].

I know you did a FEMM simulation for 3mm airgap, 12.5mm x 25mm x 25mm blocks, but I can't seem to find it. I think you calculated approx 0.7T for my AxFx alternator.

According to formulae for AxFx, dual rotor.

emf = 4 x 0.7 x 0.000625 x 55 x 187 = 17.99V

The measured emf is approx double this. Is this related to a x2 factor with dual rotor and the effective flux area doubled?

Gordon.





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Dinges
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Posted: 11 January 2009 at 12:18pm | IP Logged Quote Dinges

Hello Gordon,

I've never seen or used that equation myself before, but since it came from Flux I assume it's correct. Actually I've never tried to calculate voltages in any way, I've only used test coils. I know Steven F (Sparweb) was working on the problem for motorconversions and I know FEMM should be capable of doing it too, just that I haven't figured out yet how to as I've only played with the magnetics part, not the electrical part yet.

Gordon wrote:
The measured emf is approx double this. Is this related to a x2 factor with dual rotor and the effective flux area doubled?

No, that can't be it. The double rotors are already taken into account because of the higher total flux the two of them create.

In your case the numbers obviously don't add up. Some things that pop into my mind:

- the factor '4'; Flux says it's really 4.4 for RMS, but since the output is a bit triangular, '4' would be closer. Either way, it's a fudge factor, related to waveshape. I recall your generator having pretty rectangular waveshapes. If we compare the Crest factor of a triangle to a square wave (http://en.wikipedia.org/wiki/Crest_factor) the difference is about a factor 2....

- are you sure that the voltages you measured were RMS values; i.e. measured with a true-RMS meter?

- finally, one other factor could be the area of integration. I initially thought it would have to be the area of the magnet but I don't think this is correct anymore. What area should we take then? Coil area? Some intermediate value?

Obviously, the only really accurate way to calculate EMF is by using Vind=-d(flux)/dt. But that equation is useless in practice. Probably the only real practical way is to use a test coil.

Just some thoughts that pop to mind. Flux is probably the best suited person to ask though. I'm not an expert on the matter, just stumbling along and trying to learn in the process.

Peter.

Edited by Dinges on 11 January 2009 at 12:37pm
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Posted: 11 January 2009 at 1:53pm | IP Logged Quote GWatPE

Hi dinges,

I guess at he end of the day when equations [probably empirically determined on a particular machine] are presented, that attempt to predict measured behaviour, that utilise a linearising factor, that these equations will not necessarily be applicable to another machine. In an unloaded alternator the magnetic field strength, number of turns and frequency will determine the peak voltage. The coil shape determines the output waveform shape. The RMS voltage is a calculation, used to simplify power output calculations.

The area of the coil that passes within the magnetic field in my alternator is 1.25x the area of a magnet face. The coil shape, combined with the magnet configuration that produces a square wave type output maintains the peak emf for a longer part of a cycle, and essentially fattens the sine wave to the square wave. In my case the peak emf, and the RMS are almost the same.

In my case, the actual winding would predict a higher magnetic field strength. I would assume that the test would be unloaded emf.

Hi wdyasq,

On a side note, a wave wound stator could have similar output waveforms to my design, but the requirement to make the coils for a full stator in one stage might be a big ask. My mill has 187 coil turns per phase, for nom 24V output at 250rpm, with 3mm magnet spacing. The wire I used is very thin and soft, so this was achievable with individual pole coils. The minute you opt for narrow magnet spacing, enginering and electrical problems become abundant. 1mm wire introduces substantial eddy current loss at modest rpm with 3mm magnet spacings and Neo magnets.

Gordon.

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Posted: 12 January 2009 at 4:43pm | IP Logged Quote SparWeb

Hi Peter,
Nice looking animations. Did you notice while you were doing them that you need an "extra" magnet on either end to get a consistent result in the middle? At one time I resorted to adding about 3 on each end of these "cross-sections" just to be sure.

I wouldn't ask you to do so, but for argument's sake you could have tried a "halbach array".

Or is that the secret piece you've withheld for you own nefarious purposes?!?



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