Author Topic: 2hp induction motor conversion - how much neo magnet required?  (Read 16828 times)

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hiker

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Re: 2hp induction motor conversion - how much neo magnet required?
« Reply #33 on: November 22, 2015, 12:17:02 AM »
Few years back...their was talk of over saturation from using to big of a neo mag. On motor conversions...so how do you know how much mag is enough or to much ? From what I recall it kind of shorts out the magnetic field a it travels over the lams..maybe flux can jump in ? 
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mab

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Re: 2hp induction motor conversion - how much neo magnet required?
« Reply #34 on: November 22, 2015, 02:52:18 PM »
...so how do you know how much mag is enough or to much ? 

That's exactly what I want to know too;  ;D maybe Zubbly & others have done calculations to determine how much mag to add or they just had a feel for it, but I don't truly understand how Zubbly and others have chosen the mag's they used.

Is it a case of calculating the Flux in the motor at full load amps and selecting mags (and gap) to match this? (my head's hurting already).

presumably if you do massively over mag the motor you will wast energy heating iron? But I need enough to get 800w+ from a 2200w motor, ideally at 700-900rpm - I guess I should look at Sparwebs numbers again, and I could do a crude extrapolation but I don't know if I'd get it anywhere near right.

Hope my old genny's bearings will hold on a bit longer.  :)

SparWeb

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Re: 2hp induction motor conversion - how much neo magnet required?
« Reply #35 on: November 24, 2015, 04:17:50 PM »
It's OK.  Took me a long time to sort it out, too.
My humble opinion is:
Lots is good.  More is better.  Too much is just enough.  :]

What would be more helpful, though, is an idea how to maximize the money spent on magnets when they're so expensive.  Get the most bang for the buck.

I've never used magnets thicker than 1/2" for example.  Using thicker magnets would increase the flux, but my guess is only slightly, and the practical problem of fitting a rectangle on the outside of a cylinder makes thicker and thicker magnets a losing battle.

The other dimensions left, the length and circumference of the rotor, are pretty good for you, since the rotors on older motors was much bigger than they are now.  I just did a sketch of your 5.5 diameter rotor.  I can only fit 27 1/2" x 1/2" magnets around the circumference, not 28 like was said before.  Spacing out 24 magnets equally around the circumference fits better.  Too bad because 28 would cog much less than 24.

The rotor looks long enough to fit 3" of magnets, so I'd look at fitting 3 rings of them around, staggered a little to reduce cogging.
Back to KJ Magnetics...   1" long x 1/2" wide x 1/2" thick...  Uhhhkk   sorry   500 bucks.

So this is the point where we get motivated to make it more affordable!

What would happen if you used cheaper 1/4" thick magnets instead?  The funny ones I found earlier are much cheaper, and because they're thinner you can go back to fitting 28 around the circumference.  90 mags x 3.77$ is about 340 dollars.  Their website knows I'm in Canada, so it's probably gouging me - check for yourself if you're in the US and you might have a much better price.   (It's called the hoser discount, for a reason.)

There's half the magnet thickness, but thankfully there isn't 1/2 the flux.  It's lower than it would be with 1/2" thick magnets, but we've also gone up to 28 magnets around the ring.  We also haven't decreased the gap space.  Back when I was learning about conversions of my own, I found that these factors compensated quite a bit for less magnet.

http://www.sparweb.ca/3_Gen_MoCo/Baldy.html

If this is acceptable, then the total volume of magnet is...
28 x 3 x 1 x 0.5 x 0.25 = 10.5 cubic inches.
That's pretty good for 2HP

My GE motor conversion (3HP) had a magnet volume of 12 cubic inches.
On my Baldor motor (3HP), I managed to cram on 15 cubic inches.

I was very happy with the performance of the GE, and I'm still happy with the Baldor conversion.  I'm sure you'll be satisfied if you can get 10 cubic inches of magnet on your Brooks motor.
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joestue

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Re: 2hp induction motor conversion - how much neo magnet required?
« Reply #36 on: November 24, 2015, 05:27:18 PM »
 N45 Neodymium Magnets 3 inch x 1/2 inch x 1/4 inch Thick NdFeB Bar Rare Earth Magnets
Magnetized through the 1/4" Thickness

4.20 each if you buy 30 of them. at magnet4less dot com
Rare Earth Magnets 4" x 0.5"x 0.25" Thick Neodymium NdFeB Block Magnet N42
Magnetized through the 0.25" Thickness

4.15 each...
the 1 inch wide magnets are cheaper by volume but the airgap volume will be increased as well.
« Last Edit: November 24, 2015, 05:33:07 PM by joestue »
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mab

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Re: 2hp induction motor conversion - how much neo magnet required?
« Reply #37 on: November 29, 2015, 02:41:07 PM »
Aha! fieldlines is back  :)

OK, I went away & thought about what you've said - can't say I fully understand yet, but I'll be guided by your wisdom. So I'll have a rethink on which magnets to use. Just to be clear the rotor dimensions I gave are for the 3Hp, not the 2hp referred to in the title, so I guess I should go for 12c.i. or more if I can manage it.

I'm in the UK so I doubt those US suppliers will work out cheaper after adding shipping even if they ship over here.

although if it works out cheaper to buy 2x1x1/2 " mags for an axial flux I wonder if that'll give me a more efficient generator anyway (or at least easier for limited machining skills - maybe).

Thanks again :)
m