PMA Geometry Question

[CraigM]

Anyone have an idea on how the geometry of this PMA works out?

I'm guessing the two adjacent coils are wired in series along with one other coil pair per phase. Maybe six phase and three of the rectifiers aren't shown, possibly on the backside of the stator plate?

My best guess is 26 for number of poles but I'm not sure if the poles are N,S,N,S or N,N,S,S,N,N,S,S.

Is it a single rotor or dual rotor?

Is the pole / coil relationship configured this way to reduce cogging as much as possible?

I ran across this photo on the Homepower website with the caption under the photo saying "This axial PM alternator has magnets on its rotor that spin past stationary coils face to face." Photo Courtesy Ian Woofenden. I researched Ian Woofenden and found a reference on Hugh's site, scoraigwind.com but couldn't find anything more about this PMA.



I sketched this out in CAD trying to scale the photo and came up with this.



Thanks,
CM

[Bruce S]

CraigM;
My thoughts after looking it a couple times.
That's probably a 3 phase unit two coils per section.
That could stay with the 3-4 standard configuration along with the 3 rectifiers you have there. Since I can only see those and the red & Black wire coming out I'd say thats it.
The little round spaces on the other parts looks more like where the magnets would be week magnet with a known pole side would help tell which way it is setup.
A different pic with the full mag side face would be easier to count, but using the mirror of the pic gives me 24 poles, which stays with the 3 phase 4 pole setup.
 You can drop him an email from his website, he normally answers mine with in a day of two.
Interesting setup, where did you score that pretty item from?
Bruce S

[tecker]

Looks like counter wound

[CraigM]

Bruce,
This is just a photo I found while looking at the website Homepower.com and it's puzzling the heck out of me. I'll agree after closer inspection that it's definitely 3 phase. With 12 coil pairs that tells me it has 4 coil pairs per phase. Using the 3-4 (coils to poles) standard means 12 coils would require 16 poles to equal a 3 phase machine. Or maybe in this case 16 poles with two N-N or two S-S magnets per pole equaling 32 individual magnets.

This is where when looking at the rotor it doesn't add up. The photo shows nearly exactly half of the rotor and what's shown is 14 poles. If you draw a straight line through the center of the rotor it appears to intersect the last two poles in the photo... so what isn't visible would be 12 poles or 26 poles total.

24 poles would make this function more like a single phase machine. Which if that was the case this machine would cog terribly. Who knows, maybe it does?

This is just a puzzle that I'm trying to solve. I understand 3-4 geometry, overlapping coils and many other coil/pole configurations but this one has me stumped.

I did find a contact email for Ian Woofenden so the simple answer would be to ask the source.

Thanks much,
CM

[CraigM]

Just had to draw it out. 28 poles works for a 3-4 coil/pole arrangement using the double coils shown.

Tecker if poles are arranged N-S-N-S you're correct, counter wound would be required.

Still looks like this design would cog rather badly, but it's interesting. I've wondered if there is a way to skew magnets on an axial design such as this similar to what is done with reworked electric motors to minimize cogging.



Puzzle solved,
CM

[Flux]

I suspect the combination is chosen to reduce the effect of cog.  If it was built 12 pole single phase the cog would be too grerat to start up. By using a non integer relationship between poles and coils the reluctance is not the same for each coilk /magnet position so the cog force is very diluted and by a clever combination of magnet size compared with coil core size it may be possible to keep it to a reasonable figure.

It looks as thopugh the coils are in pairs because they are wound on c cores.

My guess is that the coils are split into 3 groups and each seems to have its own single phase bridge. When rectified this will work out well enough but I suspect that it is not true 3 phase and using a conventional 3 phase rectifier would not work well.

I suspect you can regard each phase group as being nearly single phase with a bit of phase dispalcement equivalernt to short pitching on a conventional motor coil. The phase relationship between phase groups would need someone better at maths than me to untangle, but as it is rectified it really doesn't matter one bit, it is enough to give the advantages of polyphase and more important it will keep cog reasonable.

Just regard it as a development of the Ford Model T flywheel alternator. It is single rotor.

I think you can regard it as 3 phase for the fundamental of the waveform but it will most likely not be a nice clean near sine wave that tends to happen with ironless machines. This is where the separate rectifiers come in.

Flux

[CraigM]

Flux,

Thank you for the reply, as always your wisdom is greatly appreciated.

Thanks again,
Craig

[jlt]

I think the gen is from a marlec or rutland wind turbine. They were about 3ft machines.