| This weekend we had a pretty strong front come through our part of Colorado. While I didn't have an anemometer set up, I would agree with the weather people (this time) who mentioned gusts of 70mph. For a while it seemed like it was pretty sustained winds of around 50 mph.
Since I had installed a number of untested "features" on the windmill, I decided to leave it running and test them in strong winds. Afterwards I lowered the tower and examined the results. This is what I found:
- The Pipes furling system. I had been concerned that the furling system didn't seem to do much, but that was in rather low wind, below 30mph. In the strong wind it furled well. In fact it maxed out several times while I watched it, so it does indeed furl nicely when it should.
- 3 blade, 7' diameter rotor. I had carved these up a few months ago using fir and gluing a 1" board and a 2" board together to get enough depth for the root section twist. I used the newer type polyurethane glue for the first time. I didn't see any stress lines showing on the painted surfaces and so it appears to have held together well at pretty high rpms.
- The 24v 194rpm DC motor as generator. I've wondered what output I could really get out of this. With my current setup it appears the answer is at least 20 amps (27 volts, so 540 watts) when the wind is really going. So far the bearings are holding up.
- The slip rings. Here is where it fell apart. Watching the ammeter during the storm I was seeing surges to 20 + amps and then down to 0. I know the rotor was moving fast, so I was confused until I examined the slip rings. I had a joint in each ring which was slightly uneven. At some point in the evening, one joint was lined up with a brush, which straddled the (small) gap and started sparking. The heat build up in the brush melted the plastic brush holder and very effectively glued the brush into position. The ring next to the joint was heavily carbonized and some of the copper ring was melted near the joint. I used two brushes per ring. One ring getting glued off the ring shouldn't have killed output, but it did. The other brush on that ring was a little stiff in it's holder, and could have been off the ring at times. It did have a lot of carbon build up on that side of the ring too, but I didn't see any other melted copper. The other brushes appeared to have no problems.
I believe the slip ring unit would have worked fine if...
a) a thin copper lining surrounded the brushes in the plasitic holder. This would make them slide easier and keep hot plastic from adhereing to the brush. Leads from a lining to the terminals might have helped keep the resistance down and the heat lower.
b) a jointless ring had been used. I was unable to find a copper or brass pipe the right size or I would have done this in the first place. A smoothed over solder joint may have helped.
Next project ...
I have a set of two thin nylon blades (8' diameter) to try out. It should be fun to see how these work on the 24v-164rpm motor. I'm concerned about startup with only two blades and also the oft mentioned yaw jerkiness two blades are reported to have. Until I beef up the slip rings, I'll go with dangle wiring to test this one out.
Chuck
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