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7.5 hp motor conversion
By windy, Section Homebrewed Electricity Posted on Sat Jan 05, 2008 at 02:58:58 AM MST
Magnet placement on rotor

I have a question concerning the placement of the 24 magnets on the rotor. I am using the original rotor, turned down and machined flat to accept the 1" wide x 2" long x 1/2" thick grade N42 magnets. There is an 1/8" hole through the thickness of the magnets which I will use to hold the magnets in place. The stator will be skewed. The rotor is 4.5" long with 12 rows of magnets. It will be a 12 pole conversion. Now my question. Is there anything to gain by mounting the ends of the magnets tight together and leaving a 1/4" on either end of the rotor, or would it be better to keep the magnets out to the ends of the rotor and keep a 1/2" space between the magnets at the center of the rotor? Hope this makes sense! Any comments would be greatly appreciated.

7.5 hp motor conversion | 5 comments (5 topical)

Re: 7.5 hp motor conversion (3.00 / 0) (#1)
by Flux on Sat Jan 05, 2008 at 02:10:07 AM MST

It will probably make little difference except on the skew angle required to get minimal cog.

From the electrical point of view then keeping the magnets together and away from fringing areas at the end of the core pack would seem logical, although mounting may be a little easier with a gap between magnets. If you leave a gap then keep it small enough to keep the magnet shorter than the core length.

Flux

Re: 7.5 hp motor conversion (3.00 / 0) (#2)
by dinges on Sat Jan 05, 2008 at 02:38:00 AM MST

"If you leave a gap then keep it small enough to keep the magnet shorter than the core length."

Why, Flux ?

I was faced with the same decision a while ago; drew up a diagram and settled on the scenario at the top, where I can fit 50% more magnet in (though only 30% of it will be used effectively), even though they will extend slightly beyond the stator.

http://www.anotherpower.com/gallery/dinges/magnet_problem_blue_monster

This will mean the magnets will extend slightly beyond the stator. I don't see how this would be a problem, but maybe I'm missing something ?

Here's a rendered drawing of it; the magnets extend a little, but nothing too dramatic:

http://www.anotherpower.com/gallery/dinges/10hp_illustration_2

Peter.

[ Parent ]

Re: 7.5 hp motor conversion (3.00 / 0) (#3)
by Flux on Sat Jan 05, 2008 at 03:07:18 AM MST

No Peter you are not missing something, you are looking at a different case.

Logically the magnet will be the same length as the core, that will give you most flux, but even then you will not be using the very ends of the magnets as effectively as the flux will fringe beyond the core. If you have shorter magnets than the core then keep them from the edge and avoid this loss.

If you are looking at adding more magnet then you will gain. All the magnet inside the core will be active and you may even gain a little from about 5mm sticking out either side. The remainder will contribute nothing. There can be some minor ill effects from extending the field beyond the core especially on large machines with thick conductors but you can ignore this.

Unless you could add at least half a magnet it wouldn't hardly be cost effective to buy the extra magnets. Same thing about increasing magnets beyond about 2/3 pole pitch by reducing gap between them, you will gain something and if the chosen magnets force you to do it, fine. If you don't have to do it then keep the most effective pole arc and save money.

Flux

[ Parent ]

Re: 7.5 hp motor conversion (3.00 / 0) (#4)
by dinges on Sat Jan 05, 2008 at 03:15:45 AM MST

Ah ok, you were talking about using the magnets as cost-effective as possible. For a moment I worried there might be other ill effects of having the magnets extend beyond the stator.

Thanks for putting my mind back at ease again.

Peter.

[ Parent ]

Re: 7.5 hp motor conversion (3.00 / 0) (#5)
by windy on Sat Jan 05, 2008 at 11:58:13 AM MST

Flux

Thanks for the reply. I will do as you suggested.

windy

[ Parent ]

7.5 hp motor conversion | 5 comments (5 topical)

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