yet another coil question

[Capt Slog]

Hi,

 "the holes in the centre of the coil should be about the same size as the magnet"

I've seen this written so many times that it is becoming a mantra, but I'd like to know why.  It's mentioned in the excelent thread about 3-phase (friday?) and also  in the question about 1" copper pipe.

It would seem logical that as long as a leg of a coil is over one pole when it's other leg is over the opposing pole, then the size of the hole is irrelevant.  So, what am I missing?

[Flux]

You can think of this in many ways but probably the easiest way is to forget legs and think in terms of flux linkage. If all the flux from a magnet links all the turns of a coil then that is about all you can expect. If the hole in the coil is smaller than the magnet then this can't happen. You can only link the flux from the area of the magnet over the hole totally effectively. It doesn't mean that it doesn't work, it just means that you have turns contributing little to volts and still contributing full resistance.

Another way to look at it is to think that a coil has two sides ( forget the ends, they only complete the circuit) The sides on one leg are adding volts in one direction and those on the other in the opposite direction. When one leg is over a N pole and her leg is over a S pole the whole thing adds.

If you reach a situation where sides in one leg are over the same magnet pole as sides in the other leg then the volts cancel. If you make the hole smaller than a magnet then this situation arises. It just means that the whole winding has moments of not being fully effective. You will almost certainly gain volts up to the time where the hole disappears but you reach a point where the gain is very small. If you want volts alone for some special device such as a charging source for an ignition system it may make sense, but for real power you are adding the cost of copper and resistance which will make it a poor thing to do.

Flux

[Capt Slog]

"Another way to look at it is to think that a coil has two sides ( forget the ends, they only complete the circuit) The sides on one leg are adding volts in one direction and those on the other in the opposite direction. When one leg is over a N pole and her leg is over a S pole the whole thing adds."

I can see that bit, it makes sense.  

But is there a reason why the hole can't be bigger than the magnet, as would be the case if the magnets are further apart?  I can see that this uses more wire than is necessary and so ups the resistance.

Principally though, we're aiming for a coil spacing and size that allows it to cover two magnets? one per side.

[Flux]

You can make the hole bigger than the magnet but you are then wasting space that could be used to wind with thicker wire to keep resistance down.

It is all a compromise. It is further complicated with axials where you don't have a fixed gap between magnets. Unless you use wedge shaped magnets you have more gap on the outside. You tend to find that you have wasted space on the outside and it is all jammed up at the centre. Many compromise by making the holes a bit small at the centre where you loose little flux. Others go with radial coil sides where the hole is big at the outside. I suspect that it adds more resistance with little return. I strongly suspect that keeping the coil the same shape as the magnet is a good compromise.

Why do people worry so, this is something of a second order effect. Dan has made many good machines with holes a bit small and I am sure many have used radial coil sides with rectangular or circular magnets.

One day someone will write the computer program to sort out these things and all machines will be perfect and still someone will want to do it another way.

The single phase case was sorted out by Ferranti, Mordey and others about 120 years ago, it's a pity they didn't have a go at doing it 3 phase, I would love to know whether they went with overlapped coils or ended up with the single layer winding. They didn't work at variable speed and rectify to charge batteries so the conclusions may have been different anyway.

Flux

[alibro]

Has anybody ever tried to make coils without the top and bottom ends? I know that sounds daft but would it be possible to have two thick rings of copper going around the inside and outside of the stator. The magnet wire would join them (a bit like railway sleepers). I know this would only give single phase but if it worked it would do away with the useless bits of the coils.

I guess it would be hard to get any Volts but look at all that current

Maybye it wouldn't practical to do it with only two rings but I'm just thinking out loud is there any way to reduce the losses in the non useful bits of the coils?

Alibro

No dear I'm going out to do some work, not to play.

[Stonebrain]

(blank)

[Stonebrain]

You mean like my racastator?

Just don't know how to wire it.lol

[alibro]

Not really. I was thinking more of using the outside and inside heavy wire like bus-bars with thiner insulated wire running between them, but on reflection it probably wouldn't work as the voltage generated would be very small. I guess it would be the same as having coils with big fat single windings, ie not much good.

Ah well, just a thought.