Chris, that is interesting. I presume you are talking about an air gap alternator? To what do you attribute this change?
I've never really looked into what causes it, and it's most noticeable with 12 volt generators where you may have 10 or more rpm/volt at cut-in. I just looked at the data from the last one I tested and here it is, copied and pasted, unedited, out of my notes when I was doing it:
hand test with test coil-
2.0 volts @ 135 rpm --- 13 rpm/volt
running on test stand with test coil-
7.2 volts @ 450 rpm --- 12 rpm/volt
14.9 volts @ 850 rpm --- 11 rpm/volt
The above was using 2 x 1 x .5 N42 bars and winding the test coil with 13 AWG wire.
I have notes like this from over 20 different generators I have built and they all exhibit the same thing - the faster you turn it, the rpm/volt ratio decreases.
Why dou you think that an ideal variable gap alternator would produce a constant volage/rpm? If the voltage reduces as rpm increases as you claim, wouldn't that require that the gap is reduced as is being proposed rather than increased as rpm rises?
Typically, what I have seen in current homebrew designs is generators that cut in at a reasonably low wind speed and put the rotor into stall in higher winds above 20 mph where it should be really spinning and start to shine. I have tested some of these generators I've built at various air gaps to see what it actually does to the overall performance, and I've found that this phenomenon where the rpm/volt ratio starts to decrease at higher speeds is partially responsible for putting the rotor into stall.
The approximate amount of amps your stator is going to deliver at any speed can be figured by subtracting the clamp voltage from the open voltage and dividing that result by the resistance of the winding in ohms.
If you put a generator on the test stand and graph this, which I have done with an 11" diameter 12 pole 9 coil 12 volt wired wye, I found that opening the air gap by roughly .050" for every 100 rpm increase in speed kept the rpm/volt ratio constant and provided a non-linear power curve that made it possible for the rotor to keep spinning in its optimum TSR range (6-7 for the rotor I was testing). If I left the air gap at .650" a 10 foot rotor would cut in nicely at around 6.5 mph wind speed, but at that air gap the generator needed to run at 430 rpm to deliver its peak continuous power and the rotor could not do it - the fastest it could spin it was 333 rpm and by the time the wind was blowing at 28 mph that 10.6 foot rotor was in hard stall running at a 4.5 TSR and 333 rpm and that's all it would do.
Opening the air gap by .150" made all the difference in the world - that 10 foot rotor popped right up to 460 rpm like a charm and the generator put out about 27% more power - BUT - it got hot.
So there is some basis for a variable air gap system and I don't downplay that effort at all. What the problem ends up being is that changing the air gap actually reduces the efficiency of the generator at higher speed (gives you a higher rpm/volt ratio). It also drastically increases the efficiency of rotor that was previously running severely stalled. Now what you get is a rotor that's putting out way more power than the generator can handle.
The most common method to deal with this is to wind with heavier wire or more strands in hand. But there's limits there - you get harmonics problems when you wind with multiple strands and you get eddy problems when you use overly big wire.
There's another way to do it, and wind the stator using actually smaller wire - switch the thing to delta at the proper speed. The latest one I have cuts in at 72 rpm with a 10.58 (I call it a 10) foot rotor. At 10 mph wind speed it delivers about 8.5 amps. At that point it's turning at 132 rpm and it gets switched to delta. There's a slight delay while the rotor spins up and it goes up to 210 rpm - 8 TSR - and it puts out the same ~8.5 amps that it did in wye. As the wind picks up it only gets better. That machine will deliver 880 watts on a 12 volt battery charging system, continuous @ 28 mph and I don't furl it until just about 40 mph where it puts out slightly over 1.3 kW.
The key is in the fact that for any given winding in wye, the resistance of that same winding is roughly 1/3 in delta and the ampacity of the winding is double in delta what it is in wye. That machine can easily deliver 1 kW of power to a 12 volt bank and it doesn't get hot. What gets hot is everything else - wiring and rectifiers specifically. You start pushing 65-70 amps continuous on a good wind day and you'll find out good your rectifiers are.
So I think I understand what joseba1 might like to try to accomplish with a variable air gap here. And that's the only reason this thread caught my eye - it's kind of in sync with some of the stuff I've tested. I will never sit here and tell him not to do it - he might come up with a way to make it work. I'm just offering some of the experiences I've had in testing along the same lines. And I think if you talk to Ed Lenz you'll find that long ago he did some of the same testing I've done more recently and came to the same conclusion I did, which is that the REAL answer lies in using a delta winding if you want ultimate high wind performance.
--
Chris
