This has come up several times recently, if Gordon from Wales is watching this should help him as well.
It has been covered several times but as always it is difficult to find.
I don't want to go too deeply into this, but some idea of aerofoil operation is needed.
Wind approaching an aerofoil section at an angle, the wind will split into 2 parts and some of the air will flow over the top surface and some will flow underneath. The wind flowing over the top is speeded up in relation to the other part and there is a force known as lift and in the case of a plane it is the bit that lifts it off the ground. This lift also is the bit that produces power in a windmill blade but it is a vector component that produces torque.
The lift increases with the difference in angle between the blade and the wind ( angle of attack) up to about 15 degrees, but there comes a time when the air flow separates from top surface and turns into a turbulent wake. This is the stall point and lift drops drastically with disastrous consequences for planes.
It really needs diagrams to explain all this, but I don't have time for that now.
For a wind turbine to work satisfactorily the angle of attack needs to be less than about 12 degrees and it is common practice to aim for about 5 deg where the ratio of lift to drag is highest.
If you can imagine the wind approaching a stationary blade the angle of attack will be not much under 90 deg and the thing will be highly stalled ( that is why thins are slow to start).. Now when it is spinning with its tip speed several times that of the wind, the relative wind to the blade doesn't look anything like before. It seems to the blade as though the wind is coming in at a small angle to the blade.
The angle of attack that we want is the difference between the physical angle of the blade( pitch)and the relative wind angle.
To keep this angle of attack constant, the blade rotational speed must increase directly with wind speed so that the tip speed ratio stays constant.
For every blade there is an optimum tip speed ratio depending on its geometry , angle and width and normal blades here are intended to work with tip speed ratios between about 5 and 7.
If we think of a blade designed for a tsr of 7, it will work best at 7. at 6 or 8 it will work quite well, but things are falling off the peak when you get down to about tsr6 and below that you are approaching the region where the angle of attack is up towards the 15 deg. Any further slowing will bring the thing into stall and below tsr5 the thing will stall and power will drop very rapidly.
If you can follow that then we are in a better position to follow what is happening
with different loads on a windmill blade.
If the alternator doesn't load the blades enough they fly fast, you have a high tsr and you don't extract good power. If you load correctly they fly at design tsr with optimum power. If you overload they fly slowly and you approach stall and the power falls rapidly.
We have seen that ideally the rotational speed should follow wind sped. This would imply a load that tracked the energy in the wind and blade load should rise with wind speed cubed.
The air gap alternator has an output power that is almost exactly linear, so that its input power is a bit nearer speed squared if you include the losses.
The result is that if you get the light load right it doesn't load enough in high winds and the prop runs away .
If you get the high wind load right then you have too much load in low winds and the blades stall. If bad enough it will never pull out of the stall and get to the correct high wind load.
We should have a load such that the prop speed tracks wind speed, but the best compromise with simple loading seems to be to accept a 2;1 speed increase from a 3:1 windspeed increase. If we cut in at8 mph then we would want the alternator speed to double by 24 mph.
We balance things by cutting in at a high tsr say tsr8 for our chosen prop with tsr7.
It's tsr will fall rapidly as the wind picks up and we are stalling to some extent in the 12 mph region. Tsr continues to fall right up to rated power and if you have it right you will be close to stall at tsr5 at your furling point.
How do you achieve this?
Choose your cut in speed for about tsr8, that gives you the cut in speed, relative to prop diameter.
Now the way in which the alternator loads with speed depends on its resistance, so if it has a high resistance it will not provide enough load at higher speed. If the resistance is too low then it will stall the blades in high winds.
Normally the design is chosen so that the winding resistance is what you need. if resistance is too high you need more or bigger magnets that work with less turns and let you use thicker wire.
If you are drastically too powerful then you can save by using less magnet. If you are only a little too powerful it will be better to add a bit of extra resistance in the line. This does the same thing but keeps heat from the stator.
Most people err on the slow side at cut in, but trying to get too much from low winds will bring you into stall early and reduce power in higher winds, so if you hit stall then the first step should be to raise cut in speed as far as you can without spoiling the low end too much. This is done by increasing the air gap.
When you have got he best cut in and you are still staling then think about adding resistance.
If you err on the side with too little magnet and too high a resistance then there is not a lot you can do except perhaps to opt for a smaller prop.
I realise this that got very long and perhaps over complicated in places, but even if you haven't followed everything you should get most of the implications of stall.
If you can follow most of it then it may be worth looking at Hugh Piggot's site at the blade theory he has, with the diagrams that make it simpler to follow.
Flux
