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Axial flux turbine MK 2

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joestue:
interesting, i found another document on optimum ratios for air gap machines
link: http://jfgieras.com/RJW_IEEE_MAG_2005.pdf

for trapezoidal magnets the optimum magnet thickness 1, while the air gap is 1.56. so for half inch thick magnets that is a 3/4th gap.
optimum magnet width is .78 to .72 pole width to arc width.
the optimum inner diameter to outer diameter ratio is .7
so this means for one inch wide magnets, you need 1.33 inch long sides of the inside diameter polygon. so for 16 poles that's 6.77 inch inside diameter.
optimum magnet length would come out to 1.5 inches, for a 9.67 outside diameter... so 2 inch long magnets aren't eating up too much...
however, the difference in area between 16 trapezoids and 16 rectangular magnets is quite large.

I find it quite convienent that the numbers in this linked document are almost identical to iron cored machines, with the exception that iron cored machines can have larger magnet coverage, 80% is a minimum, with the actual number selected to minimize cogging.

i would be willing to bet that the optimum magnet separation is much closer for rectangular magnets, for 16 poles and 1 inch wide magnets perhaps as low as .25 inch gap on the inside corners, so that would be 6.36 inside diameter.

karlb:
Thanks Joestue,
Ordered the magnets last night, they were on offer so I decided to go for 20 pole 15 coil instead of 16 pole. So 40 magnets! Total cost £50
Should take two weeks to come so I will build other parts in the meantime.
The coil winding space on a machine with 4:3 magnet:coil ratio is 1/6 magnet width + 2/3 inside magnet gap, regardless of number of poles.
In my earlier post I assumed an inside corner magnet gap of 0.5xmagnet width was best, so with magnet width 10mm and gap 5mm I should get coil width 5mm.
If you're suggesting a 0.25xmagnet width gap then I will get a winding space of 3.33mm which doesn't seem like much.
If anyone else can confirm this then that would be good.

Not sure where your 1.56 air gap is coming from. In the paper you linked to they found that for maximum efficiency with magnet thickness 11.12mm the stator thickness was 17.5mm and clearance 2.5mm, making a total gap of 20mm, scaling this to a magnet thickness of 10mm, the total gap should be 17.8mm

karlb:
On the parent site http://www.otherpower.com/magnetrotors.html they build their 10 footer with a gap of 1xmagnet width on the inner edge. So there seems to be a lot of different gaps being used here.
Does anyone know for certain what the optimal is?

electrondady1:
i  have always used a spacing of one magnet width on the inside
but  the trend is to a tighter spacing .
the important ratio is the one between spacing on the rotor and the air gap between rotors. you want the space between that mag rotor surfaces to be closer than the spacing so the flux travels through the copper rather than side ways to an adjacent magnet.
so the mag spaceing dictates the thickness of your stator and the number of turns you can fit in there.

electrondady1:
Karl if you can wait until your mag rotors are finished you can do  test coils.
that will take the guess work out of your stator.
 

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