Homebrewed Electricity > Wind
Please discuss stalling and adding resistance to the line.
Volvo farmer:
I've installed new round-magnet rotors and a "heavy duty" stator on my DanB 10 footer. I had burned out the previous stator in a pretty intense wind storm last fall. I think this stator is wound with 55 turns of #15. I think my old stator was wound with #14 but I'm not sure how many turns.
So I got to watch this thing in pretty decent ~20mph wind yesterday, and the performance is dismal. This is a 24V machine and I was seeing 6-8 amps sustained with peaks of 10-12 amps. It also seems to fly much slower than my old machine with the 1x2 rectangular magnet rotors and old stator.
So I have read a lot of Flux's ideas on this subject and my understanding of the dilema is this:
I can have a very well behaved machine that operates mostly in stall and doesn't make a lot of power. The alternator is too powerful for the blades and the blades won't turn fast enough to break free of stall. Because of this, I would likely never burn out another stator and furling isn't all that important.
Conversely, I can have a machine that performs well in light winds, that the blades will break out of stall early and operate in lift. This machine needs to furl early because the blades will turn very fast in heavy winds and build lots of heat in the stator, and maybe burn it out.
Now, I think I can improve my dismal performance by adding resistance to the line, but my understanding is that as I move this direction, I also move towards a direction where it is easier to burn out a stator... So it's a balancing act. However, resistance in the line is better than resistance in the stator, because some of the heat will go into the line. Please feel free to correct my thinking if I have it wrong.
So... My system is as follows: 24V, danB 10 footer, about 200 ft from the turbine head to the batteries, all #8. My questions are: What is an easy way to add resistance to the line? What range of resistance should I be experimenting with? How do I know when I have it right?
Last question... What does the air gap have to do with this, and should I play with that? or just add resistance to the line?
Thanks! ;D
ChrisOlson:
--- Quote from: Volvo farmer on April 24, 2010, 05:24:02 PM ---Now, I think I can improve my dismal performance by adding resistance to the line, but my understanding is that as I move this direction, I also move towards a direction where it is easier to burn out a stator
--- End quote ---
I got a 13 footer that (did) the same thing. I tried adding resistors to the line and that definitely woke it up but I didn't get any more power to the batteries - all I got was hot resistors. So I took the resistors out and threw them in the junk drawer. Then I opened up the air gap until it started to fly. Now it makes power - into the batteries. I think I ended up at about .850 air gap and she starts folding up at 1,400 watts and gets fully folded up at 1,600-1,700.
Opening the air gap didn't make one bit of difference at low wind speeds, except it maybe cuts in a 1/2 mph later is all. With the air gap opened up it spins up quicker and makes about the same power as it did before, running in heavy stall at under 15 mph. Above 15 mph it's a power pumper now.
It's also a lot noisier than it was before - those blades sing at 20-25 mph wind. One bad thing about opening up the air gap is that it doesn't shut down very well if I short it. But it has run in 40 mph winds with the air gap opened up and is well-behaved so I'm too worried about it.
--
Chris
ChrisOlson:
--- Quote from: Volvo farmer on April 24, 2010, 05:24:02 PM ---What range of resistance should I be experimenting with? How do I know when I have it right?
--- End quote ---
Here's what I did:
I took a 2 x 6 about 24" long and drilled three holes in each end of it and put bolts thru the holes. I stretched common, everyday hardware springs between the bolts, end to end on the board. I hooked the three legs from the turbine to one end of the board at the tower base junction, and hooked the underground line to the other end with jumper wires.
Then I let it fly. The resistance end to end on the springs was about 1.7 ohm (the smaller the wire diameter in the springs the more resistance you'll have) and that was too much. So I moved the jumper wires up the springs about 3/4's of the way and that's where it seemed to fly the best. Going closer to the end where the drop cord was hooked (less resistance) put it back into stall, going the other way from about 3/4's of the way took power away from the batteries and got the springs really hot.
It's a cheap, quick and simple variable three-phase resistor that you can adjust "on the fly". If you find the exact place where it seems to work the best, just measure the resistance from the start of the spring to where you hooked on with the jumper wires and that's the resistance you need for a permanent resistor setup.
And BTW - use leather gloves when moving the jumper wires - you'll get a little "tingle" if you happen to touch two legs at once with bare hands if the turbine is running balls out. Don't ask me how I know but it was hard to crack open a Bud Lite for about two hours after that ordeal without getting suds all over the place.
--
Chris
Flux:
The first thing is to get the cut in speed right to get the best match in sensible winds. If cut in is too slow you will stall at all speeds.
If you open the air gap too much it will hit the low end performance but the first thing is to open the gap just to the point where your cut in becomes marginal for the lowest winds you get useful power from. It is probably pointless cutting in below 7mph as there is little energy available.
Opening gap doesn't effect electrical efficiency, you are just moving the alternator up on the blade curve where it should be if you got the design right.
Once you have chosen your optimum cut in for your wind area then all that you described applies. If it still stalls then you will get it out by adding resistance. The resistance does affect system efficiency so don't use it as a means of dealing with too low a cut in but once the cut in is right it is your only trick to avoid stall for a simple scheme. It doesn't cause much loss in the 15mph region if you get the cut in right and it will make it far better in the high winds.
Remember that the final temperature is a function of stator current and that is not affected by the added resistance. You will have gained though compared with an under powered alternator where the matching resistance is part of the stator itself and consequently would have been wound with thinner wire.
I would prefer to add the resistance if needed to get the better higher wind performance, once it is out of stall you can prove it is furling and get the furling right and have the best of everything.
As Chris pointed out, if you go up in cut in speed too far for the size of alternator by opening the gap you can no longer guarantee it will remain stalled in the high winds. If it won't brake to a stop in very high winds I suspect it will pull through stall when running in some critical wind speed which you won't have checked the furling at.
Flux
Volvo farmer:
Thanks to both of you for the replies. I will attempt to raise the cut in speed by increasing the air gap first. I will also post back with my results. Thanks again for the advice!
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