A radial flux generator demo

[Ken Sachs]

As I was surfing you tube last night I came across a bunch of “windmills” (they were really wind spinners since they didn’t do anything but spin) made from bicycle wheels.

They just used soda cans, even packing tape, across the wire spokes to get a crude airfoil and it spun in an upside down front fork. A quick, easy, neat toy.

But it made me think. I know there are things called radial flux generators where they spin magnets on a wheel inside a ring of pick up coils to generate electricity. A big advantage of this design is that all the wiring is on the stationary outside ring. No need for brushes.

I also know there are 26 inch and 27 inch bicycle wheels.

They seem to like pretty close tolerances between the wheel and ring.
But then I saw one using horseshoe magnets and what looked like 1/8 inch tolerances.
There is something called “cogging” they seem to worry about at low wind speeds also.
It seem having fewer magnets lowers this problem, but at a cost in power of course.

That is about all I know.

So, what do we think?
Would there enough power from a 26 inch inside wheel to spin magnets inside a 27 inch ring?

Are there any sites I can get practical building hints on radial flux building?
There is a lot on axial flux but everything on radial seems to be thesis papers.

This would be a science project at the 8th grade level. As much scrounged, and thrift store, parts as possible for the magnets, coils, etc, etc, etc.

If I could light a tail light or charge a cell phone, it would be a success.

[Rob Beckers]

Hi Ken,

Welcome to the forum!

Radial flux alternators are more or less the standard for commercial permanent magnet alternators. That's what most commercial small wind turbines use. It's the other type, axial flux, that is popular with the do-it-yourself crowd. Reasons are for the most part that axial flux are easier to fabricate (both types have their windings stationary, so no brushes). For the radial flux alternators, to get good flux density you need to add laminates to the stator. That's part of the problem. Here is a picture that shows the inside of a Skystream alternator (a 2kW turbine with radial flux PMG):

1009

The wheel with spokes in the picture holds the magnets. You can just see them stick out at the outside. The black part are the windings, they have a metal core made out of laminates.

Cogging is where the magnets on the rotor of the PMG have 'preferential' positions, where they have strong attraction to (usually) the laminates. So when you turn the alternator axle it feels a bit like there are 'bumps'. For wind turbines the problem is that it makes it harder to get the rotor started, since it has to overcome that force.

Cogging can be fixed with the proper placement of the magnets (they are often installed at an angle), and/or a better profile for the laminates so the force between iron and magnets doesn't change (much) when the magnets move around the stator. Some designs inherently have no cogging, axial flux PMGs being one of them. The radial flux alternator of the Skystream is another type that is inherently free of cogging, despite the laminates in the core of the windings.

Fun stuff! If you are interested in building permanent magnet alternators be sure to take a look at axial flux types. They really are easier to make.

-RoB-

[Dale Sheler]

There are several commercially built machines that put the magnets on a ring around the blades, I can't recall any names right now but it seems like one is 8' in diameter.

[Ken Sachs]

Thanks for the info.

A big part of this project it the "coolness factor" of using the bike wheels.
It is not meant to be very practical.
Just a "get the juices flowing" type thing.

I did see a you tube on a guy converting a ceiling fan.
Would this be a radial flux?

Also one called a Haitian that used horse shoe magnets.
Well named since the tolerances seemed to be in the 1/8 inch range.
It real light on the theory, and thus adaptability to other designs, though.
It seemed the horse shoe magnets got away from the need for the tight fit between the wheel and the ring?

[Chris Olson]

Thanks for the info.
It seemed the horse shoe magnets got away from the need for the tight fit between the wheel and the ring?

If I would understand the orientation of the horseshoe magnets, it would put a north and south pole on each side of the coils, similar to an axial design, which would eliminate the need for core laminates and eliminate cogging, assuming no iron cores are used in the coil centers.

With a pole on both sides of the coil, yes, you can run much larger air gap and still get decent performance. Iron core machines, whether they be radial or axial, depend on very close tolerances to get the proper flux linkage thru the core.

I'm sure you could get some power out of a bike wheel but it wouldn't be much due to the extremely small swept area. It does present an interesting concept, though, for a science fair project or similar.

Yes, a ceiling fan motor is a radial design. One axial design that was used commercially is Gordon Proven's toroidal generator. Gordon's design uses an iron core, but with huge ferrite magnets on both sides with like poles facing each other thru the core. I personally prefer the axial design over radial for raw performance. Eliminating the laminated or iron core gets rid of a lot of problems inherent in iron core generators such as reactance limiting, cogging, and eddy losses in the core. However, as Rob noted, most commercial designs are radial simply due to the fact that that they're cheaper to build from a materials standpoint, and typically more compact and lighter weight than a comparable axial design.
--
Chris