That's a bit simplified but generally right.
You have designed your prop for a tsr of 8, so ideally it needs to run so that its tip speed is always 8 times the wind speed, you would then work under ideal conditions.
In practice it is not usually possible to achieve this over the whole wind speed range. A prop designed for tsr 8 will work with good efficiency down to about tsr 6.
Below this the output will start to fall and below perhaps tsr 5 it will fall very rapidly. This is when the angle of attack goes over 12 to 12 degrees, this results in a serious reduction in lift and is what we call stall, it also causes aircraft to crash.
If your generator makes more power at a given wind speed than the propeller can supply at the ideal speed you run into this problem. It usually happens when the cut in speed is low and the generator is very efficient. Things work well at cut in but the output fails to rise rapidly with wind speed.
You don't give a lot of information to make any judgement about your machine, was it furling at 30A for instance. It may not be as windy as you think, but a machine of that diameter ought to be capable of much more than 30A into a 12v load so you may well be reaching stall.
If your rectifier is at ground level a very simple test is to disconnect one ac lead from the rectifier and see what happens. If the machine speeds up and produces more power it is a fair indication that you have stall.
Usually the simplest cure is to increase the clearance between the magnet rotors as suggested above. This will make the alternator have to run at a higher speed to produce the same power and the blades will operate at a better tsr.
Flux
