Alternator Materials

[gorfad]

Hello,

I'm currently working on a class project to design and construct a vertical axis wind turbine, and I'm looking specifically at the alternator.  The alternator will be a 1-disc axial alternator with 12 neo-magnets.  For the stator I was going to use an aluminum plate, however, then talk started about potential problems with eddy currents in the aluminum causing the whole system to act like a break.

So, I was wondering if anyone knows what effect an aluminum plate, holding the windings, would have on the alternator preformance.

Thanks

Sam

[Flux]

About as useful as running your VAWT under water.

[ghurd]

Sam,

Make that underwater in a pond. Not in a river.

Have a look at Ed's site. This one will help.

His coils and wiring is different than most, but it shows what to do.

http://www.windstuffnow.com/main/alt_from_scratch.htm

Good luck!

G-

[DanB]

yes - the eddy currents from the aluminum plate will kill you.

for fun though... don't take our word for it.  Build your magnet rotor - mount it to bearings and then give it a spin and put a piece of aluminum plate near it/see what happens!  It should be quite obvious to you then.

[richhagen]

in brief,

Current is induced in any conducting material exposed to a changing magnetic field.  That current produces a field that opposes the original, inducing drag on the rotor of your vawt in your case, and creating heat in the aluminum.  This robs your system of energy which you would like to convert to useful electrical energy.  You would not want to place anything that conducts current in the area which is exposed to oscillating magnetic fields (unless it is rotating with the magnets such that it does not experience a significant change in the magnetic field strength from the permanent magnets) except for your coils of magnet wire, which are shaped to take advantage of this and direct the generated current and voltage for a useful purpose.  Anything else results in a loss of power.  Rich Hagen

[ghurd]

Great explaination Rich!

Expanding on what Rich said, the aluminium is like coils that are shorted out.

For a good test of the concept, take a PM brushed motor. Spin it.

Next short the wires together and spin it.

'That' is what we are talking about.

Mounting the coils on plywood would be far better than aluminum.

G-

[Ungrounded Lightning Rod]

Best would be to mount the coils above a spiral of scrap lamination metal (so the magnets "see" the edge and "slide along it" as they rotate).  This will loop back the mag field to the next mag pole, minimizing your gap and thus maximizing the field from the magnets, and only steal a small fraction of your power heating itself.

Of course you won't be able to find long strips.  So use the longest you can, orient them edge-on to the magnets and long-side along the magnet path, and stack them like making a wall with thin bricks (so the boundaries don't line up) until the stack is as wide as the magnet poles.  The rest of your coil support should be as non-metallic as possible:  Wood, epoxy, fiberglass.  Screws as far as possible from the magnets as possible - if any must be close use brass (to avoid cogging) and point them along the field rather than across it (to avoid eddy current braking).

A disk of non-laminated magnetic material (or laminates oriented incorrectly) will also increase your mag field but will have similar eddy-current braking problems as a disk of conductive material - unless it rotates with the magnets.  (This is what dual-rotors are about:  Not "dragging" the field through the return-path metal and thus avoiding eddy current braking and losses.)

Return path metal will be STRONGLY attracted to the magnets so you'll need to fasten it down VERY well.

[dinges]

Have you ever tried rubbing a magnet (pref. a strong one, like Neodymium ones) over a piece of aluminium?

The best illustration of eddy currents you will ever have. Like putting a spoon in pot of honey and trying to stir.

Peter.