Author Topic: Why 1 vs 2 rotors? (Read 3258 times)

Grant MacLeod · « **on:** June 20, 2006, 04:17:44 PM »

Hello everyone

I've got Hugh P.'s axial flux windmill instructions and am looking at windmill plans in it and elsewhere. I'm curious why some plans use 1 rotor and others have 2. I've seen some discussion in the plans about this but the answer is vague about the magnetic circuit not being complete if only 1 rotor but that it doesn't matter that much. I learned magnetic theory some years ago but it never got practical enough to be helpful here

I've seen the formulas related the number of coils and the flux rating of the magnets contributing to the voltage output for a given speed. What does 1 rotor do to this vs the usual (it seems) 2.

Thanks

Grant MacLeod

Flux · « **Reply #1 on:** June 20, 2006, 10:38:00 AM »

If you want to make best uss of your magnets you use a dual rotor.

Sometimes it is coinvenient to use common magnets and adapt them to a design without too much consideration of best use. It may be cheaper to use a standard magnet than use something more ideal but less common and more expensive.

With a dual rotor the flux crosses from one magnet rotor to the other and is virtually uniform in the gap. If you remove one rotor the field close to the magnets will be similar but as you go away from the magnet face the field falls off rapidly. If you use thin coils the results can be satisfactory but you will not benefit from coils that are thicker than the magnet thickness. Even then the field is not uniform and it is mostly trial and error, you can do little in the way of calculation.

Flux

zap1 · « **Reply #2 on:** June 20, 2006, 10:38:13 AM »

I'm also curious with a 2-rotor setup. Can I simply use a steel plate to replace the second magnetic rotor so that the first magnetic plate and the steel plate rotate together?

What are the advantages and disadvantages?

Thx

ghurd · « **Reply #3 on:** June 20, 2006, 10:51:09 AM »

I would guess some of it is related to parts at hand and limited machine tools, too.

It is quite simple to make a single rotor that 'functions'. From poorly assembled junk.

It seems many tinkerers make a single rotor for fun, get the bug, then quickly move on to a dual rotor.

G-

Flux · « **Reply #4 on:** June 20, 2006, 10:51:29 AM »

Yes you can. It doesn't make a lot of difference if you only put magnets on one disc. It is virtually the same except that you can regard it as having magnets of half the length. 1" magnets on one rotor with a plain disc would be similar to 1/2" magnets on 2 discs.

The field is less uniform with the magnets on a single disc so the result will be slightly worse but this may be compensated for to some extent by the better utilisation of the material in the thicker magnet.

The set up is far superior to a single rotor with no return path.

If you use the same magnets for a dual rotor and a single rotor with return disc, then doubling the number of poles will likely give you more output, but at the cost of larger rotor discs and a thinner stator.

Flux

theTinker · « **Reply #5 on:** June 20, 2006, 11:22:24 AM »

exactly what i am doing

Ungrounded Lightning Rod · « **Reply #6 on:** June 20, 2006, 11:36:18 AM »

Some tweaks to the above.

- Note that the steel disk has to rotate with the one with the magnets, or, if it's stationary, be made of "soft iron" (such as silicon steel) laminations in a "bullseye" pattern edge-on to the mag field. Otherwise it will act as a BIG magnetic brake. The rotating version is better than the laminated stationary version.

- You can focus the magnetic field somewhat by putting pole pieces the size and shape of the magnets (or thicker) on the steel plate. However, it's still somewhat better to split the magnetic material evenly between the two plates. The total thickness of the magnetic material you need depends on the air gap you want to drive your field through. Unless you get a big price savings by using half as many double-thick magnets, you're ahead to use single-thick magnets split between the two rotors.

Grant MacLeod · « **Reply #7 on:** June 20, 2006, 01:27:48 PM »

Interesting. Hugh P's plan call for $158 USD worth of neo magnets. These being the cadillac of magnet I'm assuming. Since presumably these are good because they have a high magnetic field (flux) for a given size, is it possible for a guy (still in the just dinking around stage) to use cheaper magnets but use more turns in the coils to get much the same output?

Grant MacLeod

Flux · « **Reply #8 on:** June 20, 2006, 01:42:02 PM »

The only serious alternative to neo is to use ceramic ( ferrite) magnets.

The machine will be much bigger and heavier for the same output and will contain much more copper. If that is not an issue then it can be done.

Look at Hugh's site and you can find a design that you can download as a PDF. It is from some work he did with ITDG.

If you want to build something with ferrite magnets and can find the bits, Hugh's brake drum windmill plans will give good results but not everyone can find suitable parts.

Flux

Grant MacLeod · « **Reply #9 on:** June 20, 2006, 01:46:56 PM »

Ah yes. I did have a look at that. Very nicely put together. Those plans do call for the ceramic magnets as I recall.

Grant

rpcancun · « **Reply #10 on:** June 20, 2006, 09:15:15 PM »

I'm sure i missed this in the postings but I have to ask.

On a dual rotor config, are the magnets opposing s vs s

or are they attracting n wants s??

Thx

The Crazy Noob · « **Reply #11 on:** June 21, 2006, 02:35:29 AM »

attracting so that the flux make a straight line form one pole to the other. that's allso why you put the jacking screws before the second rotor (the rotors attract with huge forces)

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