Flux Density / Voltage

[ptitchard]

Hi All,

I have just finished building a duel rotor, axial flux, 3 phase alternator.

Eight 1" dia x 1/4" neos per 1/4 thick steel disc and 6 coils of 250 turns 22swg wire. Six foot three bladed prop

I have spare a further 16 magnets is it worth stacking these ontop of the existing magnets or perhaps spacing them between the magnets ( This option would mean less than a 1/8" clearance between magnets so maybe a bit close)

Would this increase voltage output as it must increase the flux density and by approximately how much

At present tested ac open circuit voltage between phases 100 RPM 4V,350 RPM 15.5V,625 RPM 25.2V

Regards

Paul

[Flux]

Adding extra magnets between the existing ones is not viable.

If you stack another magnet on each existing one you will see a considerable increase in flux. Depends on how thick your stator is, but you may see about 70% increase in output voltage.

Is this a good thing? possibly yes!

I have no idea what voltage you are expecting to work at, but taking your 350 rpm figure you have 15.5v ac. You say between phases which I assume is line voltage as phase volts are between a line and neutral. That will give you a dc voltage of 15.5 x 1.4 = 21.7v

Cut in at 12v will be 12/21.7 x 350 or 193rpm. Probably quite nice for 6ft.

At 24v it will be 387, rather fast.

If you want to run at 24v then adding the extra magnets will bring your 24v cut in speed within reason.

You have thin wire that I suspect makes it a non starter for reasonable output at 12v.

You will almost certainly do far better with the added magnets and a 24v battery.

If you want to stick to 12v you will see no benefit from the extra magnets unless you alter the stator connections, it will stall very badly and if you pull it out of stall you will probably fry it.

I have just run your figures with the limited data I have and to be perfectly honest I see no hope of using a 6ft prop with the present magnets and 12v.

With the extra magnets and 24v it may take 6ft if you can make it furl very early and keep things below 150W.

If you are in a low wind area and can protect the thing in high winds then you may be ok. The truth is that you have too little magnet for a 6ft prop.

Far better to start again with larger discs make it 16 pole 12 coil and use the larger diameter to your advantage.

Flux

[ptitchard]

Thanks Flux for the detailed response.

70 % increase is definately worth it and shall reconfigure for 24 Volt operation after adding the extra magnets.

I live in a low wind urban area so I cannot get the height or the size of some of the great mills on this site so mine is really a garden ornament in comparison.

I suppose power output is like a car. Its far easier to start with a large engine (magnets) than try to super tune a small engine to get the same proformance.

Because of the disturbed wind flow around the buildings I may build a VAWT for my next project and a larger sized alternator as you suggested.

Things I now have a better understanding of during the construction of this project, the bigger the magnets the better, keep the stator thin to reduce the airgap, use larger diameter wire and balance the size of the prop to the alternator.

Thanks again and keep up the entertainment we all get from reading the board.