This is sort of an interesting chart we made up yesterday - based upon the tests we did last week. Interesting to think about matching the alternators to the blades.
Alternator 1 uses the 1" x 2" x 1/2" blocks (N40 grade) and the coils are wound with 140 turns of #17 gage wire. The cutin speed as tested was 140 rpm. The resistance of this stator is around 3Ohms, so you can figure how much power is consumed overall - the curve only reflects what we get out - the power into the shaft is always more, especially at higher outputs. This one is about 50% efficient at 700 Watts.
Alternator 2 uses teh 2" diameter x 1/2" discs (N40 grade) and the coils are wound with 110 turns of #15 gage wire. The cutin speed of this one was around 130 rpm. Resistance in this one is about half (1.5 ohms or so) of alternator 1 - so at any given output there's about half the heat in the stator.
'Betz' (the blue line) is based on a 10' diameter blade running at TSR 6. So that is the power curve of a 'perfect' 10' diameter blade. To get wind speed you can divide rpm by 16.8.
Of course our blades are not nearly as good as they could be, so the 'betz' curve there doesn't reflect reality. The TSR of the blades is also bound to change at different wind speeds because it's impossible to perfectly match the power curve of the blades with the alternator (maybe you could with an electronic MPPT controller. In order to maintain the right TSR all the time, the power produced by the alternator (Both useful power and wasted power) would have to exactly match the power curve of the blades.
The power curve of the alternator is linear. It slumps off at the higher end on the graph, that's because of the wasted power (heat in the stator) is not shown there. The power curve of the blades is related to the cube of rpm and windspeed. So a good match is tricky and we shoot to line things up best we can between about 6 and 25 mph or so. (an intelligent MPPT controller should be able to match things up nearly perfectly I suppose)
Any time the power produced by the alternator (again, we have to consider power wasted in the stator and the line) is less than the power produced by the blades, then the blades will run at a higher TSR, and if the power produced by the alternator is less than that of the blades, then the blades will run at a lower TSR - if it's too low, the blades will stall.
When we consider the power wasted in the stator, then Alternator 1 is a pretty good match to the blades - assuming our blades are somewhat less efficient than the betz limit, and so long as we furl at 600 - 700 Watts output or so. This graph seems to agree with the sort of performance we've seen from those machines.
Alternator 2 would stall the blades and as it is - this would happen in very low winds because the cutin speed is too low. It should be a pretty good match though if we open the airgap and add about 1 ohm to the line I figure. Some don't like the idea of a wide airgap, but I sort of like the wide mechanical clearance - it's a bit wasteful of magnets but adds a safety factor I think. The other safety factor in this alternator is greatly reduced heat in the stator. Instead, we'll have that heat in a resistor, between the rectifiers and the battery.
This is a 1 Ohm 1KW resistor which should be a good start in matching alternator #2 to a 48V battery. Of course, if it were a 24 V machine we'd use a .25 ohm resistor and at 12 Volts it'd be 1/4 of that (about .06 ohm). For a lower voltage machine one might want to build the heavier alternator just to make up for line loss - use the line as the resistor.
This is all sort of simple stuff but I thought I'd post it because I think the heavier alternator is a bit safer for 10' machines. It might also be useful to some people building new machines to understand why you cant stick a big blade on a tiny alternator. Getting the alternator matched to the blades is the trick with all this stuff. |
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