Reducing Eddy Currents

[brokengun]

I've been working on a single rotor design (like the Dan's 7 footer) and currently I have no laminates behind the stator coil. I am worried about this inhibiting performance (although I'm unclear on how much it actually will inhibit performance).

Laminates are usually placed in a loop on these side in a loop around the back of the stator. Why is this? I know it has to do with magnetic circuits, which I don't  completely understand. Basically, I've already built the backing of the stator and I was wondering if there was any way to reduce the eddy currents in a single rotor design without having to route a hole in my already made backing.

If I put pieces of steel behind the wooden backing, will this help? Can I put laminates behind the backing plate assuming it's not too far away from the coils (how far is too far)?

Basically, I am looking for a realitively quick and dirty way to get make sure eddy currents aren't bogging my whole machine down. I've got limited time to put it up (it's a bit of an experiment that I need to finish before I leave school).

Any insight would be much appreciated and if Flux has anything to say about this, I would be very appreciative.

[Flux]

I don't really understand your question.With a single rotor with the coils self supporting or on something non conducting such as wood there is no eddy current problem.

As you don't say what your backing is I can't help much, but if it is wood, plastic , resin or similar there is no issue and you don't need to take any precautions. If the backing is conducting then unless it rotates with the magnet rotor you will get eddy loss and it could be serious.

The idea of an iron circuit at the back of the stator is to complete the flux path and direct the most flux lines through your coils. If this iron circuit is stationary it needs to be made of good grade magnetic steel and it needs to be laminated in the right direction to break the eddy current path. For your case I advise you not to go this route as a suitable core material will cost more than it is worth and trying to use cheap materials such as crate banding will give very poor results.

The single rotor is a cheap and cheerful design that makes poor use of your magnets but it doesn't suffer eddy loss and will make a good low wind machine. Just expect only about 1/4 the power of a dual rotor.

Using a dual rotor completes the flux path but as the whole thing is rotating together there is no eddy current issue. It doesn't matter a great deal if you don't put any magnets on the second disc as long as it rotates. If you leave the magnets out then expect about half that of a dual rotor rather than 1/4 without the second disc as far as power goes.

Hope this helps, if not try to explain better what your backing is.

Flux

[brokengun]

Well yes, my backing in wood, but the nacelle is made of metal and there is a piece of metal tubing and a couple small pieces of 1/4 inch place behind the wood where it mounts on.

Here is my backing piece, I mounted the stator directly onto that.

And here is a better view of what is behind the wooden backing.

I just didn't understand why the otherpower design used laminates behind their coils unless they had a metal backing, but I don't think they did. http://www.otherpower.com/wardalt.html Why did they use laminates here?

[Janne]

Hi, the laminates work for a return path for the magnetic flux. You could think this kinda of a dual rotor configuration with the second rotor replaced by the laminations.

The backing iron needs to be laminated to prevent any eddy currents. In dual rotor machine it can be solid, as both of the disks are turning.

[Flux]

OK I see your problem. I don't have much idea even now how close this steel is to the magnets, if the stator goes onto the plywood you look as though most of this stuff will be over an inch away.

There will no doubt be some eddy currents induced but probably the bigger issue may be with some cogging which may delay start up.

I have only built one single rotor machine and I didn't have a lot of trouble but I had about 1.5" clear from virtually everything except the stator mounting brackets. These did cause detectable cog but not enough to delay start up. If you look at Hugh's single rotor design he has gone to some length to avoid this issue. I can't remember what they did on the Otherpower single rotor ( forget the early machines with laminated core behind the stator).

I suspect that you will not have enough problems to prevent it working but things may not be as nice as they should be. Certainly I can't see any real way to do much about it at this stage, your steel bits seem vital structurally and you can't go cutting slots in them. Similarly a solid steel plate behind the plywood will be a disaster so don't try that.

Fortunately it looks as though the solid heavy steel bits are about inside the main field region of the magnets and fringing prevents the magnet field being very strong much behind the stator so my advice is try it and it will probably be ok.

Flux

[brokengun]

I've heard that using brass hardware reduces the effects that might cause the cogging your talking about.

Here is the stator mounted on the plate, with typical bolts. If I were to replace these with brass would there be a noticeable difference that would actually matter?

[Flux]

Yes brass or non magnetic stainless would prevent any cog from the bolts, it would have no effect on any attraction to the steel mounting parts beyond the plywood.

Also going to brass or stainless does nothing for eddy loss. With your little bolts I strongly suspect that it will not matter much but if the cog is considerable you could change to brass. A lot depends where they are in relation to the magnets and it will be worse if the number of bolts is the same as the number of magnets.

Flux