Hi everyone, I have not checked in for a while but I see this board is still humming along!
I first encountered this stalling problem back in 2003 shortly after I started to use Neodymium magnets. Before that time I always tried to design alternators so the machine would cut in at low windspeed and blades would run at their best speed in low winds. As the wind got stronger, the speed of the alternator would rise pretty fast allowing the blades to continue running at a nice speed. Ideally the blade speed should be roughly proportional to windspeed so as to keep the blades at their most productive. This worked out pretty well using the older ferrite magnets because the alternators were less efficient than they are now. They needed more speed to produce more volts to overcome the internal resistance of the many turns of thin wire that I had to put into them to cut in at low speeds. The maximum power was rather lower and the braking torque was pathetic but I have to say that there are quite a few of those older ferrite alternators still working around here and they do the job cheaply and without corrosion headaches.
So back in 2003 I designed my 8 foot turbine to cut in at about 150 rpm and it worked beautifully in light winds. But with the new strong magnets and low resistance windings, the speed did not change much at all as the wind got stronger and so the blades stalled. Stalling is when the blade speed is too low compared to the wind speed and so the angle that the wind hits the blade is too large and it fails to get a good grip of the wind. Blades are nice and quiet but output is pathetic. (Higher battery voltage helps to counteract this by allowing the blades to run faster.)
Anyway at that time I posted some suggestions for fixing the stalling issue here http://www.scoraigwind.com/axialplans/update.htm
There are basically 4 options:
"1. One is to increase the alternator rpm. This can be done by increasing the space between magnet rotors so that there is a larger gap each side of the stator. Or wind the stator with fewer turns in each coil. This prevents stall but it will also mean a higher cut in rpm, resulting in some slight loss of performance in low winds (around 3 m/s or 7 mph). These are both good, simple solutions.
2. You can also increase the range of speed of the alternator, by increase the circuit resistance. Using a longer or thinner cable is one way to do this. This allows you to put the machine further away without excessive cost. Choose cables sizes (on page 40) for 30% loss at 500 watts. Or use a resistor (12V=0.15ohms, 24 volts=0.6 ohms, 48 volts = 2.4 ohms). These values are only suggestions. The resistor needs to be able to handle 500 watts when the machine output is 700 watts. Maybe you could use it to heat water? It may seems crazy to burn off power, but it improves the blade efficiency over the whole range, has very little impact on efficiency in low winds and is a relatively simple solution too.
3. Or you can put a larger diameter set of blades on the machine say 2.7 metres/9 feet. Scale up all the dimensions (length, width, drop and thickness). Or use the dimensions lower down this page. I only recommend going up to 9 feet if you seriously want a larger wind turbine. I have tried it and it seems to work well. But there may be extra loads on everything due to the larger rotor.
4. Finally you can use some sort of electronic voltage converter between the input from the windmill and the battery. This is only for electronic wizards. this promises to be the only 'real' solution but it is still under development and I worry about the consequences for cost and reliability."
I basically adopted the first option and designed the machine to cut in at 200 rpm and have not really had a problem with stalling since then. However it can happen if the battery voltage is rather low and somebody makes a turbine with a very small air gap, and uses short thick transmission wiring, and is not aware of the potential problem.
The issue that you have with burning out the stator, Volvofarmer is really not an essential consequence of getting good power output. The furling tail can and should work well to protect the turbine when it is running at a good speed for power production. It is true that a machine that stalls will protect itself better from overload, but that doesn't mean that it has to stall in order to furl properly. You can make adjustments to the tail hinge angle and the tail weight and get a good furling behaviour. If it's not working out then increase the offset of the alternator on the frame and reduce the tail hinge angle and make sure that the tail is large, and both light and strong.
I hope this helps.