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Long magnets
By Simen, Section Mechanical Posted on Fri Apr 25, 2008 at 09:47:30 AM MST
Advice needed on coilshape and rotordesign.

I've finally got hold on some N40 magnets (for free!), but they're pretty long compared to most of what are used here.
They are roughly 4,25 x 1 x 13/16 inch (108x25x20mm), and i have 34 of them.
Should i just wind 'standard' wedgeshaped coils, the size of the magnets? Would the extra length of wire cause losses, or would that be compensated for with the length of the magnet? (Would i require fewer turns, compared to 1x2inch magnets?)
And some rotorquestions...
I have to limit the size of the blades, due to neighbours etc., so 10 feet diam. would be absolute maximum. Preferably not over 7 feet.
Should i go for a single-rotor with 12 or 16 magnets, or would a dual-rotor be ok?
Everything over 200W in a good breeze would be nice... ;)
If i built a dual-rotor with 32 magnets; how large would the blades have to be?

Long magnets | 14 comments (14 topical)

Re: Long magnets (3.00 / 0) (#1)
by Flux on Fri Apr 25, 2008 at 05:38:27 AM MST

"If i built a dual-rotor with 32 magnets; how large would the blades have to be?"

Larger than your 10ft limit.

You could build it dual rotor but I can't see any logic unless you insist on using all the magnets.

I would also leave the coils roughly rectangular, you could make them wider on the outside but I suspect it will only gain you increased resistance.

I don't have time to look at it now but I would guess that 8 pole dual rotor would take your 10ft prop. Just go for simplicity rather than the ultimate best compromise for volume of magnet. I would guess that disc diameter will be governed more by room at the centre of the coils than any other factor, especially if you use the standard hub arrangement with threaded rods.

I will try to look at it later but your geometry will be way off anything I have tried.

Flux

Re: Long magnets (3.00 / 0) (#2)
by snowcrow on Fri Apr 25, 2008 at 05:54:44 AM MST

Sounds like those magnets would be more suited for a 18' wind turbine. If its noise and not visual size that is the concern, the larger turbines have lower RPM's and are less noisy than a smaller ones.

It might be better to work a trade with someone for the right size magnets, with magnet wire as well, if your sure. Magnets that size are really hard to come by for FREE!!!

Here are some links for a 10' turbine magnet rotor and a 17' turbine magnet rotor to give you a better idea of the size difference in magnet rotors and magnets:

http://otherpower.com/magnetrotors.html

http://www.otherpower.com/new17page2.shtml

Blessings, Snow Crow

Re: Long magnets (3.00 / 0) (#3)
by electrondady1 on Fri Apr 25, 2008 at 06:24:40 AM MST

so how would the neighbors handle two 10 footers?

[ Parent ]

Re: Long magnets (3.00 / 0) (#4)
by Simen on Fri Apr 25, 2008 at 07:02:24 AM MST

I can't see the rotor diameter below 12" in any case, so 8 pole would fit that nice.
There is no goal using all magnets; just make som practical amount of power for starters.

I know these magnets aren't ideal for my use, but the right Neo's are difficult to come by here in Norway. (These come from somebody experimenting with building a powerful speaker.. :D ) Actually, ideal for me, would be some 3-7' windgen's. :)

And size; it's not just my neighbours, but i think there are some restrictions set by the local goverment too, but i haven't looked into that yet... ;) Also, there's a limit on space to put up a tower, and what i can build (and afford) myself. :)

Re: Long magnets (3.00 / 0) (#6)
by Flux on Fri Apr 25, 2008 at 10:33:59 AM MST

It looks as though 8 pole dual rotor will be fine for 10ft. You will need discs about 14". You could go down to 8ft with a wider air gap or less turns.

To use blades less than 8ft you will need to go single rotor and I make no predictions what will happen but again you will probably have to stick to 8 pole unless you want an awkward winding.

Flux

[ Parent ]

Re: Long magnets (3.00 / 0) (#5)
by wooferhound on Fri Apr 25, 2008 at 08:50:00 AM MST

Here are some links where others have asked the same question.

http://www.fieldlines.com/story/2005/8/1/45755/84087
http://www.fieldlines.com/story/2004/1/11/213814/974
http://www.fieldlines.com/story/2003/4/16/9422/88616

W o o f -={(

Re: Long magnets (3.00 / 0) (#7)
by Simen on Fri Apr 25, 2008 at 12:54:54 PM MST

Thanks Wooferhound... Guess i should have searched a bit first... ;)

Longer magnets = more wire exposed to the flux; but will # of turns go down with 4" magnets compared to 2" length? (assumed that rotordiam., # of poles and cutin speed@volt are kept the same?)

And thanks Flux; seems like i'll go for 8-pole. I think i'll try with a dual disk, with magnets on one of the disks only as a start. (Makes it more flexible for me that way. ;) )

Any suggestions to diskthickness? I saw 3/8" was recommended?

(hmmm... Strange to have TOO much power available as a problem...) :D

Re: Long magnets (3.00 / 0) (#8)
by Flux on Fri Apr 25, 2008 at 01:40:13 PM MST

Yes the number of turns will go down with the long magnets if you keep poles and cut in speed the same. Probably fairly close to half.

I didn't think of a single rotor with a spinning return disc. That should put you nicely into the 6 or 7ft range with 8 poles. It may even manage 8ft in a low wind area but I would certainly not try bigger.

3/8 thick discs will be fine, in fact 5/16 will do. Magnetically you could go to 1/4 on the one with no magnets but I wouldn't risk it mechanically.

For this arrangement keep your stator to no more than 1/2" thick.

Yes it is strange to have too much magnet. Up to a point it doesn't matter if you make the alternator too powerful and efficient, you can always add resistance in the line, but beyond a certain point it becomes wasteful and pointless. Once you have something rugged enough not to burn out there is no point is using more material and having to lift extra weight in the air.

Flux

[ Parent ]

Re: Long magnets (3.00 / 0) (#9)
by zeusmorg on Fri Apr 25, 2008 at 03:32:28 PM MST

Just wondering, wouldn't those magnets be more suited to a radial design?

Re: Long magnets (3.00 / 0) (#10)
by RogerAS on Fri Apr 25, 2008 at 04:43:14 PM MST

Hi,

I would think these long magnets might be best used in a motor conversion. If you can find a low rpm AC motor with an armature about the same length you might want to think about that. Just a thought.
Roger AS
9 Years off-grid & counting
[ Parent ]

Re: Long magnets (3.00 / 0) (#13)
by vawtman on Fri Apr 25, 2008 at 06:59:35 PM MST

Ya they would but it may be over his head at this time.Jerry's mention of halving them seems like a plan.

[ Parent ]

Re: Long magnets (3.00 / 0) (#11)
by Lumberjack on Fri Apr 25, 2008 at 05:19:03 PM MST

all I am going to say is.....

I am so jealous right now....

Re: Long magnets (3.00 / 0) (#12)
by Jerry on Fri Apr 25, 2008 at 06:33:51 PM MST

I built my first dual rotor machine from diamond/cophin shaped magnets. I think they were around 3" long.

I used my $60 HF wet tile saw and cut them in half. They cut easy and were a better shape.

If I had your long magnets I would cut them in half.

PS that littel 8" alt did over 1600 watts at 80 MPH for most of an evening without any burnt coils.

JK TAS Jerry

Airheads Page

[ Parent ]

Re: Long magnets (3.00 / 0) (#14)
by Simen on Tue Oct 14, 2008 at 12:30:21 AM MST

Just a small update...

I've cut 4 of the magnets in two, resulting in 8 2" long magnets. I've got hold of a wheelhub and 2 12" steeldisks, which i've given to a friend of mine who are a mechanical wiz. (I'm not... ;) )

I've also carved a 7' 3-bladed prop. out of 2"x6" boards.

I'll write a story of it when it's near completion... :)

Long magnets | 14 comments (14 topical)

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