Magnet question

[Capt Slog]

I've seen on a few posts the mention of "doubling up magnets" and I'm not sure what you mean by this.  Is it putting another magnet rotor facing the first? or literally stacking thin magnets to make thick ones?

Also.

Just before writing this I had an idea and wondered if it would work.  If wedge shaped magnets are hard to come by, could they be replaced by two disc magnets placed on the rotor to look like a figure '8'?  The magnets would lie along the axial line and the smaller of the two would on the inner radius.  Not a true wedge I know, but the end result would be a shape that tends to fill a similar arc.  Rectangular magnets might be used to do the same job, again, bigger ones on the outside.     Or am I missing something??

[electrondady1]

you got it.

[DanB]

Yes, normally it refers to 'stacking' the magnets.

I think rectangular or even round magnets work quite well and there's little point in putting together weird assemblies of smaller magnets to try to get a 'wedge' shape.  Wedge shaped magnets are neat - but there's very little increase in power/area of magnet with them vs rectangular or round magnets.

[Capt Slog]

It's just that I can easily obtain some sizes and shapes, whereas others have to come from further afield and therefore become even more expensive due to shipping.    I was surprised to see that you say there is very little to be gained from the wedge shape, it seems a lot of effort for a small benefit.

Actually, the nice wedge shaped ones are a bit overkill when I only want to make something that is going to be 4 to 6 foot diameter.

[Flux]

Unless you are working with slotted steel cores or with overlapped coil windings the wedge shape will not make a lot of difference.

Rectangular magnets loose a bit of flux near the centre where they are a bit close.

Round ones are a bit close near the centre of the circle, but the area in which excess leakage occurs is small.

I wouldn't be too fussy about the shape. If you have to use lots of little ones then there may be some point in arranging them in a rough wedge shape, but you will do better with large ones of less than perfect shape than with many tiny ones that approximate to the ideal shape. The outer 10% or so of any magnet will not be at full strength, so the bit on circular ones that is too close will loose you very little. What you loose will likely be more than made up by the shorter windings and lower resistance.

With slots or overlapped coils you can't gain from this shorter turn length and it matters more to have flux linking the whole area that you are forced to use.

Flux

[Capt Slog]

Slotted steel cores?  Where would these go?

Overlapping coil windings I can visualise though   :-)

[Flux]

Slotted steel core would be used with a single rotor axial. It is the equivalent of a motor core in a radial machine.

Have a look at Windstuffnow, Ed has examples of slotted cores and overlapped coil windings.

Flux

[Capt Slog]

The slots are in the backing plate behind the coils, right?

I was hoping to avoid doing this if possible, because I don't want the cogging effect I believe it produces.

The small wind generator I have at the moment cogs a lot, and most of the time its stationary despite having a breeze which should run it.