Beware multi-pole magnets

[stag]

My friend  collected some round magnets for the construction of an axial flux machine. These were taken from broken hazard lights of the type used on small trucks ,here in the UK. They are held onto the roof of the vehicle by magnetism. They appeared quite powerful,but the output of the turbine was very dissapointing. I ,then noticed the way that iron filings had arranged themselves on a spare magnet, and it was apparent that each magnet had 6 poles,NSNSNS. This messed up the pole to coil ratio explaining the lack of output. New magnet rotors were made using microwave mags and he now has a decent 12v mill. So, watch out for those multi-pole mags.

[Flux]

Yes,holding and lifting magnets tend to have poles in places that make them no use for axial generators.

These were probably ferrite magnets and if big enough they may have been useable in pairs. The Rutland wind turbine has magnets like this but much bigger.

Flux

[joestue]

if you have enough of them you can re-magnetize them with a 1T field in whatever orientation you want. you would need two neodymium magnets of equivalent thickness and area, with a thick steel plate wrapped around them, then just pass the magnet through the field.

does anyone know if those microwave magnets are C5 or C8 magnetized to max field? it seems to me they aren't fully magnetized but i don't have any way to test that.

[Flux]

I have never tested one, they don't seem obviously less powerful than other ceramic magnets of that size ( bearing in mind the hole).

Magnetrons do have a precise cut off field and radar devices have accurately magnetised magnets, below maximum for stability and to get the precise field.

I would suspect that domestic magnetrons can be made using just the minimum magnet material but it may still be necessary to do a little adjustment if the thing is too far cut off.

Flux

[stag]

Flux. Yes,I guess a pair of round, multi-pole magnets would make a gen, with just one large mag making a complete magnet rotor. Being a bass player, I have used 18" speakers. Maybe the mags from these might be suitable, if a source of scrap speakers could be found.

[Bruce S]

discarded micro-wave ovens will have better magnets than speakers. AND they're more plentiful
 

[stag]

 Yes! I have built 2 successful mills using microwave magnets . My friend thought he might do better with the bigger lamp mags without realising they had multiple poles.