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Can two small magnets be one large one?
By Paulm, Section Homebrewed Electricity Posted on Thu Nov 25, 2004 at 11:19:24 PM MST
Small magnets

I'm collecting magnets for my motor conversion but free ones don't come as large as the nice ones we could purchase. If the smaller magnets are stuck together by their oposite poles do they become as strong as a larger one if it was all on piece? They seem to. I was amazed to see these neodymium magents moving other nearby magnets at 4 or 5 inches away!

Can two small magnets be one large one? | 7 comments (7 topical)

Re: Can two small magnets be one large one? (3.00 / 0) (#1)
by Flux on Fri Nov 26, 2004 at 01:46:42 AM MST

Yes you can stack small magnets, it is a problem keeping them in place while you stick them, as they have a natural tendency to fly apart. As well as stacking them side by side you can also stack them end to end to increase thickness, this is easier as they attract.

The nett result will not be as good as a single magnet of the same size, as the fringe regions on the outside of magnets seem to be less effective, but as long as you do not go to extremes its practical.

Flux

Re: Can two small magnets be one large one? (3.00 / 0) (#2)
by tecker on Fri Nov 26, 2004 at 06:07:51 AM MST

Check the posts here many topics have writ ups on stacking magnets but the dynamics of stacking magnets are not linear for best results save up and pop for the larger magnets.Prototyping this way is a good idea though. I've been smacked a few times by a loose set .

Re: Can two small magnets be one large one? (3.00 / 0) (#3)
by baggo on Fri Nov 26, 2004 at 08:10:06 AM MST

I experimented some time ago with a small dual rotor genny which originally used 5/8 inch diameter by 1/8 inch thick magnets. I eventually doubled up on the magnets by putting a second one on top of the originals to make them 1/4 inch thick. This actually increased the output by about 40%. As has been mentioned though, multiple magnets will not be as effective as a similar sized solid one.

John

Re: Can two small magnets be one large one? (3.00 / 0) (#4)
by test lab guy on Fri Nov 26, 2004 at 08:19:49 AM MST

Hey I'm new, but I want to help. Can you provide a bit more info? The size of magnets available and the size you need for your motor conversion? Your question was "If the smaller magnets are stuck together by their opposite poles do they become as strong as a larger one if it was all on piece?"

When stacked in this manner you are producing a long bar magnet with a pole on each longitudinal end and nothing to write home about happening on the other 4 sides.

If, however you are wanting to glue these down on the rotor, stacking them lenghtwise along the rotor to build a long N or S pole, then you will need to stack them as Flux had mentioned above, and they will want to push apart.

If the magnets are glued to a flat steel surface, the attraction to that surface will help alot to hold them in place, but also the epoxy will be slippery and allow the magnets to slide apart.

That is my $.02. Tell us more about the motor you are working with, hp, rotor diameter, number of slots, are you going to rewind...

Re: Can two small magnets be one large one? (3.00 / 0) (#5)
by Paulm on Mon Nov 29, 2004 at 04:58:29 AM MST

The motor is a 1/3 hp 1725 rpm, 120 vac. I'm planning on using 4 magnets. Thin or thick? Thin if using one, thicker if using two stacked.

[ Parent ]

Re: Can two small magnets be one large one? (3.00 / 0) (#6)
by test lab guy on Mon Nov 29, 2004 at 07:15:19 AM MST

I seem to recall in Mr. Lampola's paper that thin magnets are more prone to demagnetization during short circuit conditions. Of course a shorted generator would never happen on purpose, but if it did...

Sounds like you are modifying a 4 pole motor to make a 4 pole PM gen. How well do your magnets cover the rotor? Someone jump in here if I am wrong, but I am thinking that the magnets will reinforce each other as the rotor iron below the magnets gets more saturated. This should occur with wider magnets that wrap around to make wider poles, there has to be an optium width, I am not sure what it is.

Cogging will be an issue since the magnets will align with the stator slots. Count your slots, that will be the number of cogs per rev.

Lampola described a PM rotor that used poles = slots/1.5. For a 36 slot stator, thats 24 poles. Here he cogging is 72 per rev and much lower amplitude. Also the stator needs to be rewound for this configuration.

Skewing the stator iron (rarely possible unless you can disassemble and restack) or skewing the magnets, either one, by the amount of one stator slot-opening width will again reduce the cogging. For the 36 slot, 24 pole example, 144 cogs per rev, very low amplitude.

What I am wondering is if you skew the magnets by one slot-opening width will your cogging amplitude be reduced as well, resulting in 2x number-of-slots cogs?

I'm in over my head now (maybe even earlier). Some one jump in and bail me out

TLG

[ Parent ]

Re: Can two small magnets be one large one? (3.00 / 0) (#7)
by test lab guy on Mon Nov 29, 2004 at 09:39:43 AM MST

I assume that you are using a 4 pole PM rotor such that you can use the 4 pole stator as wound. If this is the case AND the stator has 3 phase windings, you may want to consider making an 8 pole PM rotor and reconnecting (not rewinding) the stator coils such that each coil in each phase group is connected Start to Finish to Start to Finish, etc., much like a consequent pole winding.

If this idea works, this will yield a lower speed machine.

TLG

[ Parent ]

Can two small magnets be one large one? | 7 comments (7 topical)

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