Sizing the magnet rotors.

[TheCasualTraveler]

     Since my last post I have been researching small mills, cut in and stall and have concluded that I need to focus on making as large and efficient a prop as possible. Perhaps 7 foot but more likely 6 foot. Also with light winds I will keep cut in low, 100 to 150 and not worry about stall since it appears that is an issue for a mill putting out a good amount of power and I think any alternator I build will be putting out only a small fraction of it's possible power and stall is something I may not be lucky enough to see.

     Now my next question, how big to make the magnet discs? Somewhere in my reading I saw a page with a link to a graph sizing magnet rotors to props. It turned out to be a broken link and no amount of searching has been able to find that chart. So I'm wondering just how big is reasonable. Bigger means heavier and slower startup, but it could also mean more momentum and smoother operation as well as the magnets moving faster. Several times I've read where Flux has written that the inner 1/3 of the prop/ blade does little so looking at it as far as wind being blocked or disturbed perhaps that could be the limit for magnet rotor size. Seems awfully big, a 24 inch rotor with a 6 foot prop. But what are the guidelines?

     So, two questions,

1.    Has anyone seen, or know of a graph for sizing magnet rotor to the prop?

2.    Can anyone give me input as to what rule of thumb you use for sizing your magnet rotor*, given a set diameter prop?

*(I realize the size has to fit the number of magnets, but I plan to pick the number and size of magnets after I decide on the largest practical magnet rotor size.)

Thanks again,

[Flux]

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