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Using magnets we already have for a larger rotor | 10 comments (10 topical, editorial)

Re: Doubling magnets for a larger rotor. (3.00 / 0) (#6)
by finnsawyer on Mon Nov 5th, 2007 at 09:24:09 AM MST
(User Info)

When the rotor advances the magnets from being over one coil to the next the change in flux will be the same for the two cases. With the 16 magnet layout the time it takes at a given RPM will be 3/4 as long. This would translate into 1/3 more average voltage per coil if the coils had the same number of turns in the two cases. Also, there will be 1/3 more coils. This would result in 16/9 times the voltage per phase if the number of turns didn't change. Of course the coils won't have the same number of turns as the spacing gets smaller. Or will they? It's up to you. Reducing the wire size increases the resistance per unit length of wire, but reducing the coil size has a strong effect on the total amount of copper used per coil and hence the resistance per coil since both are proportional to the area of the coil minus the area of the central hole. If you know the wire size and area of the coils for the 12 magnet 9 coil case you should be able to predict the effect of different coil sizes and wire sizes in the 16 magnet 12 coil case.

In looking at your numbers it appears your diagrams are not to scale, that you could go to 24 magnets with 18 coils (1.8" x 1.2" magnets doubled on a 20" diameter rotor). Your aspect ratio is rather severe (3 to 1), but with the 20 inch rotor you should do ok. Of course, you are showing the magnets right to the edge of the rotor, which would have the coils extending past the stator rim. You need to sit down and do some analysis of your own.
GeoM

Re: Doubling magnets for a larger rotor. (3.00 / 0) (#7)
by Flux on Mon Nov 5th, 2007 at 10:08:21 AM MST
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I never checked whether the things were to scale. You could go up to 24 magnets on a 500mm disc if you want to. I would certainly do that rather than stacking 2 magnets in line.

24 poles would give more alternator capacity than you need without doubling up on magnets.

I can't help much more without knowing the pump and motor characteristics.

Flux

[ Parent ]

Re: Doubling magnets for a larger rotor. (3.00 / 0) (#8)
by finnsawyer on Tue Nov 6th, 2007 at 08:16:31 AM MST
(User Info)

With a single row of magnets he frees up more space, enough so that it looks like he could go to 28 magnets with 2.4 inch wide coils (0.6 inch wide coil legs) or very nearly so. Doubling the magnets doesn't look like that good an idea.
GeoM
[ Parent ]

Re: Doubling magnets for a larger rotor. (3.00 / 0) (#9)
by cslarson on Tue Nov 6th, 2007 at 11:07:34 AM MST
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you all are absolutely right. 24 (or as you say maybe even 28) would work as a single ring. i was going to post that last night, but our internet got cut off (as it does at 11pm every night). i think i was hoping to get away with a fewer number of coils. anyway, today i found out i'd have to order a new batch of magnets for this alternator and can specify any dimension i want. i'd like to decrease the magnet rotor diameter and use fewer, larger magnets. maybe 16 of 60x40x15mm on one of the magnet rotors that would now be 400mm rather than 500mm.

the induction motor conversion idea just won't leave my head, either. there seem to be pluses and minuses to both ideas. i think the axial flux (when finished) is elegantly simple, weather-proof, and efficient. but the idea of buying a motor, modifying it slightly, and away you go... rather than winding all those coils. then there's the cost. looks like the magnets alone for an alternator like this are around $250, not to mention the copper. so, today i picked up a 3-phase, 1400rpm, 3kw induction motor. it was about $130. i think i may end up making the axial flux alternator, and playing around with the induction motor, too.

thanks very much for the help.

[ Parent ]

Re: Doubling magnets for a larger rotor. (3.00 / 0) (#10)
by cslarson on Tue Nov 6th, 2007 at 09:40:50 PM MST
(User Info)

ha! so it looks like we paid way too much for that motor. the guys are taking it back and will pick up one from Pakistan, which are supposed to be better quality...

[ Parent ]

Using magnets we already have for a larger rotor | 10 comments (10 topical, 0 editorial)

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