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Wind pirate's 24 magnet machine
By scoraigwind, Section Homebrewed Electricity Posted on Sun Oct 03, 2004 at 04:16:30 PM MST
The original thread is buried down in history now.

Hi
Regarding the discussion thread ?'s For DanB a few days ago. I am working on that web page and I'll try to remember to announce it here when I have it up on my site. It shows the nirvana alternator machine being assembled with blade balancing and such and gives the data for how it performed. But a course is just starting here in Wales and soon we'll be having too much fun building windmills to spend much time on the computer.
Cooling is my biggest anxiety as we have discussed. But it hasn't melted yet! If it does I might try using high temperature resin (heat resisting resin). I recently found out that my supplier does this and it makes sense here. I would not make major design changes for this problems (such as use laminations - yuck) I'd rather use more, bigger magnets if I have to. But so far it's just an anxiety and not a problem.
Returning to WindPirate's project, I have run the numbers for it and here is what I predict.

magnets
coils
turns per coil
Coils connected in series/wye(star)
96 volt cut in at 66 rpm
that's pretty slow but an 18 footer would do it OK.
However I can only get #16 wire in there (how do you get dual #14?) and the efficiency is not great and I would not push it much beyond 1 kW if you want to avoid melting it. I reckon about 64% efficiency at 1 kW and this means dissipating about 500 watts in the winding. It gets hot and and it gets wasteful.
If you can do something to let the voltage rise a bit during the increased in wind (using heaters in series would do it) then you an operate at a better tip speed ratio and get more power (albeit heat power) in the stronger winds.
I am not clear what size blade rotor and I am not clear why you don't want to have a 48 volt battery (more usual than 96 if you want an inverter to run off it).

Wind pirate's 24 magnet machine | 8 comments (8 topical)

Re: Wind pirate's 24 magnet machine (3.00 / 0) (#1)
by DanB on Sun Oct 03, 2004 at 06:42:47 PM MST

Thanks for the followup on this one Hugh!

For those who missed it.. the origional thread is here: http://www.fieldlines.com/story/2004/9/29/111141/841

"Cooling is my biggest anxiety as we have discussed. But it hasn't melted yet! If it does I might try using high temperature resin (heat resisting resin). I recently found out that my supplier does this and it makes sense here. I would not make major design changes for this problems (such as use laminations - yuck) I'd rather use more, bigger magnets if I have to. But so far it's just an anxiety and not a problem."

I have the same worries...! I've not seen a 'problem' yet - except when I test them on the ground the heat sort of seems astounding and it worries me. After seeing my 14' machine run in overspeed for a while last week, and sending most of its power through a single phase (and eventually blowing the rectifiers) I feel more comfortable.

"Returning to WindPirate's project, I have run the numbers for it and here is what I predict.

24. magnets
18. coils
135. turns per coil

Coils connected in series/wye(star)
96 volt cut in at 66 rpm
that's pretty slow but an 18 footer would do it OK.

However I can only get #16 wire in there (how do you get dual #14?) and the efficiency is not great and I would not push it much beyond 1 kW if you want to avoid melting it. I reckon about 64% efficiency at 1 kW and this means dissipating about 500 watts in the winding. It gets hot and and it gets wasteful."

I use 2 strands of #14 for a 12 volt machine (34 windings per coil). I must be off or missing something, I was figuring about 135 windings per coil of #17 for a 48 volt machine, and double those windings with #21 for a 96 volt machine with cutin around 75 rpm for perhaps a 15' machine.

"I am not clear what size blade rotor and I am not clear why you don't want to have a 48 volt battery (more usual than 96 if you want an inverter to run off it)."

Yes... Im unclear on the 96 volt issue as well.. It seems uncommon. At 200', 48 volts should be fine for a machine this size I would think.

Re: Wind pirate's 24 magnet machine (3.00 / 0) (#2)
by Flux on Mon Oct 04, 2004 at 02:22:37 AM MST

Dan
I don't get this at the moment, can we confirm a few things.

Your magnets fit 16 on a 14"dia with 8" hole. How thick are they?

For 24V you have 48 turns per coil #14 wire, how many in parallel one or two?

How thick is your stator?

Magnet rotor diameter is 14"

Wind Pirate is proposing to use 24 magnets on 24" dia?

At the moment I don't see how Hugh gets 135 turns for 96v or how you get 135 for 48V,

lets get the figures straight first, I think we are using different figures somewhere.

Flux

[ Parent ]

Re: Wind pirate's 24 magnet machine (3.00 / 0) (#3)
by DanB on Mon Oct 04, 2004 at 06:01:32 AM MST

Hi Flux - in my figures I was talking about the 1" X 2" blocks, not the large ones I used on other machines. I think Wind Pirate is considering using the smaller 1" X 2" magnets - it would be more economical.

I've built machines using 12 of the 1" X 2" blocks before - and I get 12 Volt cutin around 150, with 3 coils in series per phase, wired in Star, using 34 windings per coil.

So I'd figure for 48 volt cutin at half that speed (75 rpm), using 24 magnets per rotor, wed need to double the number of windings - so about 68 windings per coil, or.. about 136 windings for 96 volts @ 75 rpm, and I guess I would try about #17 wire but #16 would be better if it fit!.
not sure how I came up with double that figure before... I made a simple miscalculation somewhere. Going from what I've seen in the last couple alternators I made, Hugh's prediction makes sense to me - what I said in my earlier post seems to be off by a factor of two.

[ Parent ]

Re: Wind pirate's 24 magnet machine (3.00 / 0) (#4)
by Flux on Mon Oct 04, 2004 at 06:48:26 AM MST

Thanks Dan, I assumed that Wind Pirate was doubling up on your big magnets.

Yes Hugh's figures make sense now. I haven't had a detailed look to see what wire will fit, but the turns make sense.

Flux

[ Parent ]

Re: Wind pirate's 24 magnet machine (3.00 / 0) (#5)
by wind pirate on Mon Oct 04, 2004 at 06:58:56 AM MST

Thanks Guys for all the help. I reconfigured the rotor so that now I figure I can get 32 magnets (2" x 1" x 1/2") on the 24" rotor. My CAD drawings also indicate that if I use 24 coils 3/4" in depth, and 3/4" legs, I can get that all to fit (coils touching side by side).

With that in mind, the coils (have to mock one up first)will be 135 turns of single #15 wire, 8 coils per phase, 3 phase. I'm not sure of voltage or output, so I'll just have to mock up 1 coil and a couple of magnets and see what I get at various RPMs.

As far as the prop diameter, I figure between 14 and 16 feet in diameter.

The rotors are going to be "spoked" to aid in cooling and less weight, and I'll probably use 5/16 mild steel.

Thanks again to all of you for sharing your knowledge. I'm still in the planning stage, so anything you can add is greatly appreciated.

Thanks

Wind Pirate

Re: Wind pirate's 24 magnet machine (3.00 / 0) (#6)
by Flux on Mon Oct 04, 2004 at 08:24:00 AM MST

32 magnets look good. I think you will need about 70 turns and at a quick look #15 may fit.

Will have a better look later.

Flux

[ Parent ]

Re: Wind pirate's 24 magnet machine (3.00 / 0) (#7)
by scoraigwind on Mon Oct 04, 2004 at 01:11:18 PM MST

That will be paking them in tight. I recommend a little more space between them to get the best value. You could use a bigger rotor disk?

I still don't really know what size blades you plan nor what battery voltage. I can work out the output given enough information . Your stator must be way too thick if you can get that size wire in there. Putting one or two magnets and one coil willl not tell you much.

I'd say that 5/16 is too thin especially if yo uplan to cut such big holes in it.

I hope this helps but I am fairly confused (as usual). Our course is going well and we have started winding coils. Four blades half made and several people now know how to weld. The sun is shining now after a night of wind and rain.
Hugh Piggott http://www.scoraigwind.co.uk
[ Parent ]

Re: Wind pirate's 24 magnet machine (3.00 / 0) (#8)
by wind pirate on Mon Oct 04, 2004 at 01:45:40 PM MST

Thanks Hugh - Someday I hope to be at your course, sounds like a really good time and a great learning experience.

The coils (designed in CAD - not actual) are 4.5" tall, .75" leg width, internal diameter of 1.25" at the top and 1" at the bottom, and .75" thickness (my last stator was .5" thick). I was going to make this one .75" thick and use a 1" air gap between the magnets of each rotor.

Question - will the additional .25" distance for the air gap be worse than using less turns per coil? I think that the magnetic strength decreases with the square of the distance or am I wrong? (I could do less turns and make the stator .5" thick).

For the steel plate, I also looked at the "spokes" and was concerned that the magnets would warp the plate. I think I'll have to either make the spokes wider (more than the 1" I had planned) and perhaps go to a thicker material maybe .375". That brings another question to mind. For the magnet backing plate, how much material should I leave above and below the magnet(axially)to get the best effect? Currently, I am only allowing .25" above the magnet and .5" below.

Voltage: I think you all are correct, I should go for a 48 Volt system. I was thinking that if I went higher voltage I could step it down before converting it to DC.

So it looks like I'll shoot for 48 volts, 32 magnets per rotor, 24 coils - 8 per phase using 15ga wire, 14-16' blades, and hope for high output, no meltdown, and a reliable system.

Still need to figure the number of turns / coil, but looks like I can go with 75 - 80 and be okay.

Wind Pirate

[ Parent ]

Wind pirate's 24 magnet machine | 8 comments (8 topical)

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