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Bryan1

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Joined: 22/02/2006
Location: Australia
Posts: 1464

Posted: 10:42am 19 Jul 2009

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Hiya Guys,
I just did a quick price check on 2" round neos from applied magnets in the US.
I can get 2"x1/2" N42 by 25 off for $344 including shipping
now these mags have a pull of 110lb's

I can also get 2"x3/8" N42 by 25 off for $251 including shipping. These mags have a pull of 90lb's.

Now would that 20lb less pull be a big power reduction on a dual axial flux genny as for that $90 odd saving it would pay for the epoxy for the molding. I know the general rule is go for the strongest magnet you can get but most of the genny's that are made using 2x1x1/2" N42's only have a pull of 75lb's.

Cheers Bryan

p.s. those prices are Oz$ as I used the xe.com convertor from US$

oztules

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Joined: 26/07/2007
Location: Australia
Posts: 1686

Posted: 11:47am 19 Jul 2009

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Bryan.... buy the biggies. (1/2").

The pull is not the real thing here, the flux gap is... and the winding room/space that is required to get the lowest resistance.

As a general rule, if you add the depth of both magnets (1/2" + 1/2")and multiply by 3/4, then this is about the best compromise for the air gap. So 1/2 inch ones will give you 1" thick of depth, and so a gap of 3/4".. say 19mm.

If you leave 2mm on each side for air clearance, that leaves you with only 14mm for winding space. Take out say 1.5 to 2mm for glass matt covering the coils, and you get about 12mm for the coil width.

If your after a powerful unit, you will need all of this to get the best thickness wire in there for the turns you need.

Suppose you go for the 3/8 ones. First, you have less flux in the gap, and second if you do the same calcs, you will find you are in trouble. The airgap and glass cover does not change... so you will blow at least 6mm for that, ....

So 3/8 + 3/8= 6/8 (3/4") for the magnet depth.
Now 3/4 X 3/4 =9/16" for the total gap width.
We subtract the overhead (clearance both sides and glass over the coils) of a fixed 6mm (say 1/4") and it looks like this 9/16-4/16 = 5/16" ..... pretty thin coils... and they will require more turns per volt than the bigger ones.

As you can see it is diminishing returns. On anything 8' and over ... don't go less than 1/2 thick magnets, or you will be losing badly for the lousy 90 bucks.

The magnets are not the dearest part of the project, the tower tends to be.... but they will give the the chance for the best returns and the most flexibility (can go to larger blades at lower rpm .... but only if you have the magnet to do it)

Mine will handle a 4m prop very easily. For an off gridder like you, I would not skimp on the magnets.

Just my opinion.... but born out from testing... and with the help of the Dutchman's Femm things, it appears to be born out in theory as well.

All the best with it. It is good to see you head in this direction. F&P's and induction conversions are for hobbyists. If you need power to live, then the axial is the obvious choice..... and if you need power to live, then $90 is not a big deal in the scheme of things.

Warning .... use good epoxy paint/ powder coat to seal the steel disks. Then a fine rough up with sand paper and then good epoxy (west systems etc) to seal them in. Magnets (neos) do get cancer...any chance they get, and it can't be stopped. It is of the utmost importance to seal them from the elements forever.

And just as important......Don't Play with them. You will scratch them, and once the nickel coat is breached...it's all down hill..... experience talking... with about 16 cancerous magnets to show for it... stupidity in the extreme.... just don't do it.... I've done it for you.

And all the usual warnings about how scary strong they are.... it's all true.

Oh, and the rectangle magnets.... don't do it if your contemplating anything over 8'. You will end up with a burn't stator. I know they use them for 10 footers in the USA, but they also have a swag of burnouts. They are just too weak for anything over 600-700w continuously if you design for stall limited operation (stator R will be far too high). If you have poor wind and only fluky gusts to 1kw or more, then OK, but a storm will kill them unless the furling is excellent. Big magnets mean low resistance... which means much more room for safety... and I mean disproportionately more.

............oztules Edited by oztules 2009-07-20

Village idiot...or... just another hack out of his depth

windlight
Guru

Joined: 03/03/2007
Location: Australia
Posts: 331

Posted: 12:16am 20 Jul 2009

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Bryan have you looked at ed lenz's wedge magnets?

http://www.windstuffnow.com/main/

Allan

"I like this place and willingly could waste my time in it" - (Act II, Scene IV).

mrdd
Newbie

Joined: 21/07/2009
Location: Australia
Posts: 5

Posted: 10:28pm 24 Jul 2009

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oztules said Bryan.... buy the biggies. (1/2").

The pull is not the real thing here, the flux gap is... and the winding room/space that is required to get the lowest resistance.

As a general rule, if you add the depth of both magnets (1/2" + 1/2")and multiply by 3/4, then this is about the best compromise for the air gap. So 1/2 inch ones will give you 1" thick of depth, and so a gap of 3/4".. say 19mm.

If you leave 2mm on each side for air clearance, that leaves you with only 14mm for winding space. Take out say 1.5 to 2mm for glass matt covering the coils, and you get about 12mm for the coil width.

If your after a powerful unit, you will need all of this to get the best thickness wire in there for the turns you need.

Suppose you go for the 3/8 ones. First, you have less flux in the gap, and second if you do the same calcs, you will find you are in trouble. The airgap and glass cover does not change... so you will blow at least 6mm for that, ....

So 3/8 + 3/8= 6/8 (3/4") for the magnet depth.
Now 3/4 X 3/4 =9/16" for the total gap width.
We subtract the overhead (clearance both sides and glass over the coils) of a fixed 6mm (say 1/4") and it looks like this 9/16-4/16 = 5/16" ..... pretty thin coils... and they will require more turns per volt than the bigger ones.

As you can see it is diminishing returns. On anything 8' and over ... don't go less than 1/2 thick magnets, or you will be losing badly for the lousy 90 bucks.

The magnets are not the dearest part of the project, the tower tends to be.... but they will give the the chance for the best returns and the most flexibility (can go to larger blades at lower rpm .... but only if you have the magnet to do it)

Mine will handle a 4m prop very easily. For an off gridder like you, I would not skimp on the magnets.

Just my opinion.... but born out from testing... and with the help of the Dutchman's Femm things, it appears to be born out in theory as well.

All the best with it. It is good to see you head in this direction. F&P's and induction conversions are for hobbyists. If you need power to live, then the axial is the obvious choice..... and if you need power to live, then $90 is not a big deal in the scheme of things.

Warning .... use good epoxy paint/ powder coat to seal the steel disks. Then a fine rough up with sand paper and then good epoxy (west systems etc) to seal them in. Magnets (neos) do get cancer...any chance they get, and it can't be stopped. It is of the utmost importance to seal them from the elements forever.

And just as important......Don't Play with them. You will scratch them, and once the nickel coat is breached...it's all down hill..... experience talking... with about 16 cancerous magnets to show for it... stupidity in the extreme.... just don't do it.... I've done it for you.

And all the usual warnings about how scary strong they are.... it's all true.

Oh, and the rectangle magnets.... don't do it if your contemplating anything over 8'. You will end up with a burn't stator. I know they use them for 10 footers in the USA, but they also have a swag of burnouts. They are just too weak for anything over 600-700w continuously if you design for stall limited operation (stator R will be far too high). If you have poor wind and only fluky gusts to 1kw or more, then OK, but a storm will kill them unless the furling is excellent. Big magnets mean low resistance... which means much more room for safety... and I mean disproportionately more.

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

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