"Stall" is "aerodynamic stall".
It occurs when the angle of attack is such that the air flowing over the heavily-curved (downwind) side of the blades doesn't stay "attached" all the way to the trailing edge. Instead it lifts away, allowing air to come in from the trailing edge and raise the pressure there, reducing the "lift" force on the blades. (There's a positive feedback effect as the replacement air stream comes in faster than the main air stream can suck it out and the "sucking out" slows the main airstream, leading the detachment to quickly move forward over a significant part of the blade to a region of different angle.)
In an airplane this occurs on the top of the wings when it tries to climb too steeply and at too low an air speed. The loss of lift causes the nose to drop and the airplane to dive, until the falling and angle change gets the airspeed up and the airstream reattached to the top of the wings. Then the plane can level out (if it hasn't started spinning...) This is what makes toy gliders do a series of scallops if the wings aren't adjusted correctly for stable flight.
In a wind turbine the angle of attack is the angle of the APPARENT wind at the blade. (Apparent wind is the vector sum of the actual wind and the blade's motion.) Putting load on the shaft slows the blade, which raises the angle of attack. Do this sufficiently and the blade starts to stall. This happens at two times:
- At startup, when the blade is stopped. You have to have enough excess of torque over shaft load when operating as a DRAG rather than a LIFT turbine to get you spinning, get the wind attached, and get the resulting wind-produced torque up. (Fortunately that's easy with a magneto turbine and a battery charging load, which doesn't significantly load the shaft until the voltage is up to the battery voltage. It's tougher with resistive loads, which load the shaft as soon as it turns. But set your cutin too low, even with a windcharger, and you may find your blades never get out of stall and up to operating power.)
- At high winds (IF the alternator is strong enough to load the blades sufficiently). The battery's load clamps the output voltage, which puts an increasing load on the prop as the wind continues to speed up and the charging current increases. This causes the mill's RPM to increase more slowly than the wind, raising the angle-of-attack on the blades. IF (BIG IF) the alternator is strong enough this eventually cause the blades to stall, reducing the torque and slowing the blades further in a positive-feedback loop, until the blades are turning slowly as a drag turbine.
Some commercial mills are designed to furl by stalling (rather than by turning away from the wind using a pivoting tail, air brakes, centrifugal brakes, pitch adjustment, or some other mechanism.) But getting that tuned right is a tough design job - with thrown blades if you get it wrong on the high side. And your mill runs away if you lose your load in a high wind. But the big problem, IMHO, is that when a blade stalls it sheds vortices downwind, turning much of the lost power into sound. And a TINY amount of power is a LOT of sound! A stall-furling wind turbine sounds like a helicopter landing on your head. Not nice for family and neighbor relations. (The furling tail design is much more peaceful and easy to tune. B-) )
