Otherpower 7 footer air gap

[brokengun]

I know on the 10 foot duel rotor design the air gap can be anywhere in the 3/36 to 1/16th of an inch ballpark. Is this the same for the single rotor 7 footer design? I am trying to get my rotor at about the right place and I'm unsure weather or not I should back it off more than 1/8th of an inch from the stator.

Thanks.

[veewee77]

Shoot me if I am wrong here, but don't you measure air gap to be the distance between the magnets not the distance between the coils and the magnets? 1/8 gap between coils and magnets is pretty tight. Unless you are very good at making the stator, you'll likely have a rubbing condition at some point in the near future even if it don't rub right now.

If it were me, and I wanted longevity, I'd go for a slightly larger gap and not have the rub worry, even sacrificing a watt or two in the process.

Just don't make it so large that you can't shut it down in extreme winds.

JMHO - YMMV

Doug

[Ungrounded Lightning Rod]

The air gap (in a coreless axial-flux design) is the distance between the pole pieces on the opposing rotors.  The nomenclature is confusing.  But the non-coil parts of the stator (and the coil parts when it's not moving) are essentially "air" as far as their effect on the magnetic field strength is concerned.)

The clearance gap is the space between one pole piece and the stator.

brokengun:  Are you concerned with mechanical clearances or electromagnetic design?

[brokengun]

Sorry for the confusion with the term air gap, I mean the distance between the magnets on the rotor and the stator. I am mostly concerned with electromagnetic stuff, I will make sure the magnets don't hit the stator before I put my machine in the air.

Basically I wanted to know if I needed a larger air gap than the 10 footer, because I should be running at a higher RPM because of the smaller blade diameter. I'm going to run some bench tests and see how this works, but in practice I thought I should have a slightly larger air gap.

[Flux]

I wish we could put this air gap thing to rest, it is the distance between opposite magnets on a dual rotor. In motor conversions and conventional machines it is the gap between rotor and stator. In a single rotor axial machine the air gap is not defined, it is presumably the mean leakage flux path between adjacent magnets and not much may link the stator if you get the stator too thick.

For a single rotor you are using the magnets very badly and I suspect you will need to keep mechanical clearance as small as you can from the reliability point of view. I think that design is short of flux anyway so large clearances will probably make it run away.

Flux

[DanB]

The 7' machine has me slightly puzzled lately and Im not sure quite what to tell you. The first one e made involved N35 grade magnets and the mechanical clearance (what you are calling airgap) between magnets, and stator) was reasonable at about 3/32.   With N42 grade magnets I find it can be as much as an 8th or more to keep the cutin reasonable at about 200 rpm.  We made one with smaller, rectangular magnets, still a single rotor and it seems to work pretty well, better than I'd expected, the gap is about 1/16th (A bit tight) but the cutin speed is fine.  It does run kind of fast thoughl.

[brokengun]

I've got some stronger magnets in the N40 range someplace, so I'll back it off a bit more. I'll let you know how I make out when I make a diary entry or something. Maybe I'll make some pretty graphs if I get ambitious.

But at the current air gap it just feels like it needs too much torque and my bench tests have all been at like 150 RPM which I think is a bit low.

[DanB]

yes, 150 is quite low for a 7' turbine, you should be looking for at least 200 rpm cutin or more (probably not more than 250).

[Dave B]

I do not read what type of blades you have. If it's in there some where and I missed it I apologize. The profile of your blades can make a very large difference in your cut-in speed and therefore overall performance of your machine. Just a thought to be sure to compare apples to apples when dealing with so many variables.  Dave B.