Under low wind you want all the power you can get - because there's almost none there. Cogging is the "detent"-like effect of the magnets irregularly attracting the iron core through the coils (when present). It's like the way an old mechanical TV channel selection knob sticks to the selected channel, or like rolling a log over a series of low humps. It can be mitigated by shaping the cores correctly and/or staggering the magnets appropriately (to smooth out the "humps"). Cogging (if severe) may make the rotor "stick" when it's calm and not start up until the wind is blowing moderately fast.
Because the genny has to be spinning at a minimum speed ("cutin speed") to do any charging, cogging is not a problem if it's mild, as long as the rotor starts spinning below the wind necessary to spin it up to cutin. But if it cogs too much you lose the opportunity to pull some power when you need it the most.
As the wind speeds up the power goes up fast: Ideally it could go up with the cube of the speed, but for the way the simple mills we build here work, it only goes up roughly with the square. That's not a problem, since the divergence gets significant mainly at high wind speeds, when you have all the power your genny can handle anyhow.
The limits on mills are twofold:
- The genny heats up - with heat proportional to the square of the current - so it will melt down or otherwise be damaged if you pull too much current. With batteries connected it will try to feed them and fry.
- If the blades spin too fast the mill will tear itself apart. The genny keeps the speed down somewhat by loading them. But you can't do that at high speeds because it would fry - and unhooking it in high winds would let the blades spin even faster, leading to immedate self-destruction.
So the trick is to tweak the mill so it "furls" - dropping power collection at the mechanical level in high winds. Typical mill here does that by having the tail pivot up and the mill turn away from the wind, using the offset-shaft, angled-tail-pivot trick. Some commercial mills have the blades designed so they do an aerodynamic "stall" - which works but makes it sound like a helicopter. But in addition to the noise, getting that right is tough, while adjusting a tail is trivial. Other systems include adjustable air brakes and a host of other things. But the pivoting tail is simple, reliable, and very easy to fabricate.
Does that clarify it?
