Hi
Didn't see this until tonight. I spent most of the weekend outside, tilting a tower up and down. Don't get excited: radio tower in this case
ULR is on the right track; we can do a lot of rough sizing with our thumbs... rule of thumbs I mean.
The thickness of the magnet disk needs to be enough to carry the magnetic field inside and not let it leak out. Half the thickness of the magnet is the minimum I believe. Adriaan is not wrong; there will be some losses if you use the minimum, but not a lot and what I see from Windy's writing, it seems that 3/4" thick mag's on 1/2" thick plate is well above the minimum and works OK.
Keeping the gap between magnet disks to the minimum allows space for the stator and a clearance to prevent rubbing and hopefully not much else. The physical space clearance you allow between the stator's faces and the magnets spinning is determined by how stiff you think the rotor disks are, and how confident you are with your hub, etc. Maybe you can tolerate 1/8" physical space, maybe less maybe more. Consider that these turbines are big gyroscopes, and as they pivot to face left and right, the things want to pitch up and down. The pitch is a pretty substantial force, don't underestimate it, especially on a 20' rotor.
Critical question that affects the gap: will the blade hub be attached to the alternator, or will the alternator be driven remotely from the blades? If you attach the blades to the alternator (the way the Dan's do), then these pitching loads will be like trailers going over speed bumps and you'll need more than 1/4" gap. If you attach the alternator to a driveshaft taken off the blade hub (the way Midwoud is doing it), then the pitching moment doesn't warp the rotors, and you may be able to get the gap down to 1/8" or less. Below that is dependent on your fabrication skill.
The next question is how thick the stator can be... that's where Windy's question really comes from. I take it you're still considering possibilities designing your stator, and if it's coming out to be fairly thick, then let's look at this carefully. Are you getting the phase resistance we worked out for you a few months ago?
My guesstimate for magnet spacing:
magnet thickness = 0.75 inch
2x magnet thickness = 1.5 inch = maximum space between opposing magnet faces
stator thickness = 1.5 inch
Air space left for clearance (stator centered between magnet faces)
(1.5in - 1.0in) / 2 = 0 inch = no clearance
So let's try something else, using the minimum clearance that I think you could get away with if you are really careful.
1.5in - 2*(0.125in) = 1.25 in = maximum thickness you should make your stator.
So if you go back to the spreadsheet and try to get the thickness of the stator down to 1.25" then let's see what else happens. Does some other parameter go out of whack?
Here's another look at the big picture:
http://sparweb.ca/2_Gen_Ax/AXIAL_FLUX_HowItWorks_V4.pdfMy one contribution to the internet that's been plagiarized a thousand times. I googled it, and this time it turned up in a dozen spots. The original is way down the list. All these websites aggregating others peoples' stuff get more hits.