Gone by the wind

[fritzblitz]

I have got two windmills running .They are 100 meters  away from my house on a hill .

One of them was used one I bought 6 years ago. Then I did not know anything about all that stuff  an had never heard anything about "otherpower" .

In the meantime I discovered "otherpower" and soon I began  to build up my own windmill with the help of Hugh Piggott s  booklet. Since 2 years, this little windmill works great and survived two heavy storms with more den 140 km/h windspeed. without any trouble, while the other mill went to the ground.

I think this happened because the furling system of the used machine did not work correctly.  When having a closer look I found that the tail had the 20 degree inclination at the tail piviot, but the  35 degree angle did not exist. In all the time it was running, I never rally saw it furl.

At approximately  more than 500 RPM it could not be stopped by short circuit and  ran away. At normal windspeed up to 25 m /sec. it produced 40 amps at 28 volts. The other data I measured of this windmill were:

Propeller 2 blades 3.0 meter diameter

Chord at root      235 mm

Chord at tip         120 mm

Angel Root          11 degrees

Angel tip                 5 degrees

Thickness at root  55 mm

Thickness at tip     31 mm

Offset                    120 mm

Tail length             3.0 meter

Tail weight            35 kg

Well , next day after the storm I found the mill wrecked, but the generator was ok. I tested it and found it was not bad at all:

Generator modified Siemens motor with permanent magnets,3 phase,

Open voltage at 650 RPM  190 volts

Ohm stator       2.7 ohms

When I turn the shaft, I can feel 32 times the magnet resistance.

So I decided to rebuild the windmill using this generator and avoiding the faults it had before .

For that reason I have some questions to the board, I am sure you have got the right answers for me.

I am experienced in making blades and I want to make a 3 blader. 7 to 8 TSR. What would be  the right diameter for the prop?

Is there any formula or rule of thumb for the length and weight of the tail with 20 + 35 degrees of pivot inclination (I made the boom extensible in order to adjust it to the correct length)?

Thank you for your support so far!

Fritzblitz

[Flux]

Without the 35 deg angle that thing should have furled early and should have had trouble keeping into lower winds.

I can't help feeling that the cause of failure was too small an offset on the alternator and you don't seem to have changed that. I suspect it will fail again.

Your 2 blade prop seems to have worked well enough so I would convert to 3 blades by keeping the same angles but using 2/3 the chord width on the blades to keep the same solidity.

Unless you increase the offset I suspect that you will need a considerably reduced prop diameter but that will mess up the performance.

A reactance limited motor conversion will require more alternator offset than a stall limited air gap machine. The fact that you could not stop it with a short almost certainly means that it was reactance limiting in high winds.

Still doesn't explain why the tower came down, guy wires must have been too small. Proper furling would reduce tower load but you should have made it strong enough anyway.

Flux

[RobC]

Flux is right you have almost no offset and its probably not going to furl properly. The offset should be more like your other machine. RobC

[fritzblitz]

Thanks for your opinions which, however, leave me somewhat bewildered.

The offset is 120mm, that is 4% of the rotor's diameter. For a PMG that would be the right size. Is that different for a  motor conversion? And if yes, what size of the offset do you recommend? And to the length of the blades, what do you think?

Well, the weight of the rotor blades was more than 30 kilos, perhaps it is possible that the centrifugal forces kept them from furling?

[Flux]

Thanks for giving the dimensions, it is difficult to judge these things from pictures.

I agree that 120mm should just have been enough but it does depend a lot on the blades and how they are operating in relation to stall.

You have the thing well overhung over the front of the yaw point, I don't know exactly what effect that has. In terms of direct thrust it makes no difference but what effect it has on the seeking force is a different matter. I have never tried anything this far forward.

I have no reason to believe that the weight of the blades has any effect.

You may find that it works ok if you keep the prop down to about 8ft with your present arrangement but that will reduce the power a lot below the 10ft blades.

I see that you over hung it a long way to clear the lattice tower, my guess is that with it so far overhanging you need to increase the offset.

If you can manage it without too much trouble I would be tempted to add 25mm to the offset and try again with the 10ft as that seems to have suited the alternator.

Flux

[bigkahoonaa]

I find it concerning that your mill would not stop after shorting.   You must have very high wind in your area, and furling seems to be important.  An 8 to 10 feet diameter blade seems ok.  What kind of mechanism do you have for furling?