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Forum Index : Windmills : Power Vs Revs Vs Windspeed
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| RossW Guru  Joined: 25/02/2006 Location: AustraliaPosts: 495 |
Hi all. Finally got some decent wind today (I know why too: because I wanted to lower the mast!) - anyhow, it was great to actually see more amps come out of the windgen and solar panels than the daytime load in here (I have way too many computers running - even though laptops, base-load is still over 500W). I was watching the current (and therefore pretty close to power) comming out of my windgen, while sitting in the bunker beside it, and listening to the RPM of the turbine above. It was quite interesting that the current/power shot up much faster than the props RPMs changed. I'd hear the wind blowing faster, and immediately see an increase in output current, but it would take a second or so before the prop sped up, and several seconds before it had stabilised at the new wind speed. Here's the instantaneous wind vectors for the hour:  Clearly then, the output is not just a function of RPM. On thinking about the physical processes, I *THINK* (but would appreciate some feedback) what's happening is that at some arbitary wind speed, the prop is running at X RPM, producing a certain amount of electrical power and the load at those volts means a certain current - which would slow the prop thus lower volts etc, until the system becomes stable. When the wind picks up, there is more POWER (torque) available to the turbine, so even though the RPMs are the same, it's got more "grunt". (The turbine will be going slower, if the wind picks up there will presumably be more wind velocity drop across the blades and since power extracted is proportional to air density, swept area, and delta_air_velocity... more power even without increasing prop speed). The turbine, batteries and volts/amps will settle to their new equilibrium at some higher RPM where it will sustain the current until the wind changes again. Does this make sense, or am I completely missing the mark here? |
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Gizmo Admin Group  Joined: 05/06/2004 Location: AustraliaPosts: 5019 |
Hi Ross Yeah it makes sense but I think its wrong. The generator output is governed by its load and RPM. If the load stays the same, and the RPM stays the same, then the output current stays the same. If the wind increases, but the RPM has remained steady, then the load must have increased proportional to wind speed. The only way to confirm your finding would be to measure the generator RPM. Its possible that while it sounds like it didn't speed up, it in fact did. A slight increase in RPM can have a big effect on current. I know with my windmill the current guage is all over the place, even though the windmill seams to be running at the same RPM. I think eyes and ears are not accurate enough, you would need a RPM meter. I like the graph. What system do you use, looks pretty flash. Glenn The best time to plant a tree was twenty years ago, the second best time is right now. JAQ |
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| RossW Guru Joined: 25/02/2006 Location: AustraliaPosts: 495 |
That was my original thought also, but I've watched it a bit now, and remain unconvinced it is that simple. I think I might hook the cro up and try to catch it, although the timescale will make it difficult. I may have to make a frequency to volts converter first, so I can quickly see frequency as a DC level on one channel, and current (voltage across the shunt) on the other. I wrote it myself, I can't find any reference to wind-rose displays in this format. I was trying to find something to show me wind trends (particularly wind shifts) without damping or averaging it to the point I could no longer see gusts, or how far it was swinging. In the end, I settled on taking the velocity and angle information from my weather station every 5 seconds and plotting them over a variable timescale (1hr, 4hr and 24hr currently). They can make very interesting viewing. They update every minute, here's the 24-hour one which at the moment is interesting, but may not be by the time anyone else looks at it :)  |
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| KiwiJohn Guru Joined: 01/12/2005 Location: New ZealandPosts: 691 |
Hmmm....I dont think the alternator could produce more power without increasing RPM. My best guess is a combination of the eye being quicker than the ear and suprisingly perceptable amount of time it takes for the change in sound to get down from the top of your mast. But then I might be wrong as I dont really know..  |
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