With modern magnet materials there is no need to use very large diameter magnet discs but using the old ferrites the little Marlec machine has a very large alternator for its size. The large inertia is no handicap to it despite the crazy worries about it here and the constant desire to use aluminium.
For a low wind area all you need to do is match the prop to an alternator that is efficient aver the wind speed range you are looking at. If you are efficient enough to hit stall at about twice cut in wind speed then you have probably cracked it. Going even more efficient would be detrimental unless you adopt some of converter.
What you must avoid is an alternator that is not efficient enough to fully load your prop.
Are you playing with pvc blades? ( I forget who is doing what).
For respectable wooden blades then a cut in of about 160 rpm is about as low as you should go and that is probably a bit low to use without a converter. PVC blades may need to run somewhat slower.
As a rough guide for a low wind area where you are not interested in the higher winds I would start with a prop of tsr6. choose a cut in speed so that it cuts in at about tsr7 in a 6mph wind. Make the alternator efficient enough to hit stall at about 15mph. That will bring your tsr down to about 6 at 10 mph ( general operating region). At wind speeds above about 15 mph your tsr will drop below 4 and you will stall and that will help with furling for the occasional high wind that you need to still protect against.
Dual rotors about 10" diameter with 8 magnets per rotor of the 2 x 1 x 1/2 or metric 46 x 30 x 10mm will meet this requirement very adequately with a stator about 1/2" thick.
Turns and wire size will be determined by your chosen prop. You could go to a bigger and more efficient alternator but keep the same cut in speeds, you will hit stall earlier and you would likely benefit from added line resistance.Don't aim for lower cut in speeds and add the resistance at that point. First starting point is prop above working tsr at cut in so that you spend as long as possible on the peak of the curve.
If you cut in too early you start below the peak of the power curve.
That is the best you can do for a given size prop and wind speed region, the very best may not be much above what you already have, but I don't have your wind speed figures and most likely you don't either ( accurately at hub height).
Flux
