B Field approximations & Drilling Holes in Stator for cooling

[Chendy]

Hi. I have read this very forum pretty extensively before asking these questions - thanks to all those who have posted, it has been very useful to me.

B Field--------

I understand this complex function of various things: air gap, rotor metal thickness, Strenth of magnets, thickness of magnets.....

I have two 40 by 20 by 5 mm N42's with a separation distance (air gap) of 20 mm.

From reading the posts I recal that after a distance of the length of the magnet the B Field = aprox 0.6 ?? but within the range it is a linear ramp to another number (I saw 1.2 T)

What is this second B Field for the scenario above & how does it change if i have one 40 by 20 by 5 mm N42 facing one 40 by 20 by 10 mm N42? (one thicker magnet)

- assume thick steel rotors

Stator cooling-------

I am thinking to drill small holes in the core of the coils after they are cast to let heat out - As this region vill be the hottest. I dont think it will make much difference structurally, but I am also not sure it will do anything thermally either. What do you lot think?

I am too poor to be buying aluminium powder, but I was think of wrapping some shreds of aluminium foil in the cores of the coils. The only negative I can think of is eddy currents, but if I only put them in the cores, will these be significant?

- - - - - - - - - - Finally - - - - - - - -

I am making a 8 magnet dual axial flux generator, with 6 coils for 3 phase delta. I am using 4x1mm steel sheets for each rotor & airgap of 20mm, using awg 22 wire. I am hoping to get 300+ watts out

Doing preliminary caclutions found on forum, for OC voltage of 14 & cut in speed of 250 rpm, assuming B field = 0.6:

The calculation say I need 100 turns per coil.

Does this all sound right? any help appreciated

Chendy

[Flux]

For 5mm thick magnets you will see a gap flux density of about .6T with a gap somewhere under 10mm. At 20 mm it will be way below this. The fall is proportional to the reciprocal, the curve is a hyperbola if you start looking at big gaps. It's reasonable linear in the region where gap is about total magnet length.

With one magnet 5mm thick and one 10mm thick you will see .6T somewhere below 15mm gap.

At 20mm you may be down round .4T but magnet spacing and leakage flux gets very variable with large gaps.

With two thin magnets keep your stator under 10mm thick. With one thick and one thin magnet keep it under 15mm thick if you want a good operating point.

I don't think the holes will make any difference.

I doubt that you will get away with shredded foil unless it is very finely shredded, you will be far better with talc or marble flour or even sand than risking aluminium, I doubt that you could get if finely divided enough to work.

Don't use .6T for your calculations with 20mm gap with thin magnets, it may be nearer .25T with 2 x 5mm thick magnets.

Wire as fine as 22 gauge is about wet string for 12v and I don't think you will get your 300W at any realistic speed or efficiency, but I haven't done any calculations.

Someone else was playing with a similar sized thing with small magnets and thin wire and it didn't work out too well.

Flux

[blueyonder]

 clever folk on here yeh. i am going to drill meny holes in my stotor