Increasing rotor diameter

[stag]

There have been previous discussions on increasing rotor diameter so that magnets are moving faster over the coils,and it it has been agreed that there is no advantage in this,providing all other things remain equal( same no of mags and coils etc). SO! what would be the effect if I had,say, 24 magnets,and instead of building a dual rotor alt with 12 mags on each, and 9 coils, I built a larger diameter, single rotor machine with 24 mags and 18 coils( with core behind). I would have doubled the number of poles and coils, and increased the speed of mags over coils,but probably have the same amount of flux, or less??. As there is nothing new under the sun, this probably has already been tried, and you guys will have the answers. Opinions please.

[Flux]

I think you gain under favorable circumstances, but there are factors to consider.

I wouldn't try this trick with thin magnets but if you are thinking of the 2 x 1 x 1/2 style of magnet then it should be a winner.

The main considerations will be mechanical, to work you need a thinner stator but you still need a safe mechanical clearance each side which becomes a much bigger factor with small gaps. You need a stronger set of discs as the bending force will be a lot higher and you need them to run concentric and this could be deamnding at a much larger radius.

If you can make strong, concentric discs ( probably need machining true ) then you should gain a fair bit.

Basically you need the stator about half the thickness, this with a safe mechanical gap will get you something approaching the same flux per pole. Doubling the frequency is a great help but you loose out with the thinner stator in that you need to use thinner wire so the resistance gain is not as great as you might expect.

I done a rough check assuming 1/2" thick magnets and it looks as though with the same cut in speed you can virtually halve the resistance.

If you can live with the mechanical requirements and can tolerate a heavier alternator it looks as though it is a useful move.

I wouldn't attempt it with magnets thinner than 1/2" and it would be better with even thicker magnets so that the mechanical limitations are less of a factor.

You are right , the peripheral speed is irrelevant, but in this case you are doubling the frequency ( angular velocity).

Flux

[stag]

 Thanks Flux.  Might give it a try. Would it be a suitable idea to use on a VAWT which has less potential RPM.

[Mary B]

Might need to band the magnet rotor too, at the higher rpms you may be pushing the ability of glues to hold them down

[ChrisOlson]

SO! what would be the effect if I had,say, 24 magnets,and instead of building a dual rotor alt with 12 mags on each, and 9 coils, I built a larger diameter, single rotor machine with 24 mags and 18 coils( with core behind)

I've done that before and it works pretty good.  The effect is a more powerful generator with the same amount of magnetic material.  For the core I just used a mild steel disc on the other side of the stator with no magnets on it.

[stag]

 Chris. Is the mild steel disc/core a fixture on the back of the stator and therefore part of it, or is it a rotor without the magnets. I envisaged the former,as to build two rotors would seem a needless complication.

[ChrisOlson]

Chris. Is the mild steel disc/core a fixture on the back of the stator and therefore part of it, or is it a rotor without the magnets. I envisaged the former,as to build two rotors would seem a needless complication.

It is a rotor with no magnets on it.  If you make it stationary, or make a stator with an iron core, you get eddy currents in your core.  It is too complicated to build a core lamination that will not have eddy current problems.  If the core rotates with the mag rotor, it still completes the magnetic circuit and does a fine job as a core so you get the flux linked thru the coils, but no eddy current problems.

The downside to using your 24 magnets and going to a 24 pole/18 coil generator is the diameter vs using the same 24 magnets in a 12 pole configuration.  If you use the N42 2 x 1 x 1/2" bar mags you will get roughly 70% of the flux in the air gap that you get with dual mag rotors.  But doubling the frequency more than makes up for it.  So I have found it makes better use of the magnetic material for the tradeoff of a larger, heavier generator.

On build hints:
Like Flux said, don't try it with magnets thinner than 1/2".  Your results will not be as good.  And when you increase the diameter of the rotors it's probably going to require machining to get their axial runout +/- .12mm or so.  Do not try it with a trailer hub design.  The inherent "play" in tapered roller bearing sets, and the difficulty in keeping them adjusted, will limit how tight you can run the air gap without getting rotor/stator contact.  Use a shaft-based design with radial ball main bearings and it will work fine and you can run very tight air gap with no danger of bearings getting out of adjustment and causing problems with rotors rubbing the stator.

When I have built these, rather than changing the 12 pole, 24 magnet, 9 coil design to 24 pole - I have gone with 20 pole 15 coil with 20 magnets.  The internal resistance of the stator comes out about the same as the 12 pole/9 coil design.  And with the higher frequency you get less "ripple" on the DC output at lower rpm's which works excellent with both the Classic 150 and the Morningstar TS-MPPT60-150 MPPT controllers.  Which, BTW - the TriStar controllers from Morningstar now have programmable wind curve capability so they can be used on wind power applications.