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oztules

Guru

Joined: 26/07/2007
Location: Australia
Posts: 1686

Posted: 11:35am 23 Jan 2009

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Gordon, this

Quote The first point relates to the number of phases that you can readily fit into a single stator. Generally, the more phase coils that you have, the wider the air gap that you will need to accommodate them, and I imagine that the 4 phases that Gordon has used would be about the practical limit. A wider air gap means a less efficient use of your magnets.

had me confused as well.

Don, if you look at those two radial photo's I pasted, you should see that the interleaved coils are all in the same plane as well. There is no sacrifice of gap distance, just winding space as it has to be a function of distance of a single coil width.. ie #phases/width per coil hole diameter = coil leg width.... the gap is unaffected.... as per Gordons explanation. (unless you want the same turns/volt as a 3ph unit, then winding space will have to be accommodated by going higher and forcing the gap up, but that is only if you keep to the same wire gauge as the 3 phase version... which would alter the power of the alternator anyway.

3 phases is all that is necessary really for batt charging. Rotation is silky smooth with a 3phase axial, you can feel no magnet / coil cogging at all under load. There is no "normal" iron related cogging because of the design.

You will hear the hum of the coils going under torque stress no matter how many phases you have.... they all get belted and give a sigh of relief as the magnet passes them by.

I am pretty certain there is nothing to be gained by going beyond three phases.... apart from building difficulties for no real positive affect. (everyone would be doing it... particularly the commercial boys)

[quote]I do wonder about the possibility of eddy current losses in a steel intermediate rotor disc from the alternating fields set up by the stator coils. Anything conductive, such as aluminium or carbon fibre, might well have the same problem. Maybe fibre glass would be best for the intermediate rotor spiders, as the centrifugal forces in a low speed alternator at least are not huge. [/quote]
If you have magnets on the intermediate plate, the back MMF of the coils will be swamped by the magnets MMF, and not cause any more problem than in a normal axial flux.

[quote]On the Otherpower site there was some concern expressed as to the effect on flux density of having more than one air gap per pole. Some seemed to think that having two gaps would result in only half the flux density in each, or somesuch. [/quote]
I think this was in reference to that figure 2 chapter 3.1 axial flux etc. from a university in Finland. It has only 1 set of magnets for two stators with plate steel backing disks disks. I don't follow the design and description, because it is terrible in all aspects. They have, they claim plate steel fixed against the stators for both mag attraction through them (fixed against the back of the stator), and cooling... the eddy currents must be horrific... the professor must be on something good I think.

[quote]While I would not have expected this with multiple magnets, flux density was also of concern to me, as, should the flux vary significantly across the air gaps in a stack, then the voltages produced by the stator coils would vary, and cause problems. [/quote]
In Delta, I would worry...a lot. Circulation currents would be a problem. In Star, I would not loose any sleep, as it is only the vector products being delivered to the rectifiers. Any variations during a rotation would amount to a hill of beans. The EMF varies between + and - emf each cycle, and between phases. No-one will notice if we are a little low anywhere between the phases as the vector addition of all these constantly changing voltages is what we see. The output emf may fluctuate a tiny bit... but output volts will be far removed from the emf anyway over cutin.

Be wary with ribbon, as those who have tried it, have given it the thumbs down. Eddy currents (I know they shouldn't because of orientation) cause problems with excessive heating... with no load even. (Danb tried this .... once)

It makes no sense I know, but failure in the field is worthy of notice.

After that long winded drivel, what I was trying to say was that if you get two identical standard axials and put them on the same shaft you get twice the power. If you gather up all those coils and magnets and use a single disk of twice the diameter to accommodate all the mags and coils, keep the gap the same, you get twice the power again... better use of magnets and copper to get an extra 100% increase at the same rpm... for nothing more than bigger disks.

..........oztulesEdited by oztules 2009-01-24

Village idiot...or... just another hack out of his depth

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