| As per my last posting I'm trying to build a larger 14' diameter wind turbine.
Again, the steel rotors for the alternator are 14" diameter, and the magnets are such that 16 of them form a ring 14" outer diameter and 8" inner diameter, so the magnets are 3" tall, and about 2.5" wide at the top. I'm using 12 of them, so there is about a 3/4" gap between the magnets.
I wound up one coil with 120 windings of AWG 14 wire - I knew this wouldn't be right, but I figured that by poking this one coil between the magnet rotors on my lathe I could get a pretty good idea exactly how many windings I needed per coil to get an appropriate cutin speed, and once I knew that I could figure the thickest possible wire that could be used to fit that many windings in the limited space available. I figure for a 14' prop, it should start charging (cutin) at around 75 rpm if I'm running with a reasonably slow tip speed ratio. This should start charging between 6 and 7 mph I think with if it runs at a TSR of around 6. The lathe is great for this sort of thing, it makes it very easy to hold a single coil in exactly the right space and gives me a range of rpm to play with. I welded 4 nuts to the back of the back rotor so that I could get this thing in the 4 jaw chuck on my lathe... the rotors are 14" diameter, my lathe has a 14" swing.
I figured from my tests that, if it's wired in Star, which I was planning, that 32 windings per coil would be just right. As it turned out, 31 windings of 3 strand of AWG 14 wire worked out perfectly, so I went with that... close enough. Pictured above I've layed out the areas where the coils must fit on plywood. This will later become the bottom of the mould.
Above you can see the coil winder in the front, and in the back there is a stack of 3 spools of magnet wire from which I wound the coils. Doing it this way is much easier than trying to wind with really heavy gage wire. I also suspect that with very heavy wire, in a dual rotor machine like this - we might actually get eddy current losses within the thick copper wire. This should help. The other option would be to just wind my coils with 93 windings of 14 gage, and rather than wire my coils in series - I could've wired them in parallel. But - past experience has shown that wiring coils in parallel creates losses, because certain windings are slightly out of phase with others and this allows current to flow between the coils when they are wired in parallel. So I believe series is the best way to go - and at low voltage, the current becomes so much that I have to either use very heavy wire, or a bundle of finer wire. I've opted for the bundle...
In this picture all 9 coils are wound and the mould is ready (in the background). I'm out of nice large pieces of fiberglass mat, so I've cut lots of little squares and strips to work with. This worked out fine.
Above is the stator. It actually came out OK, but not great. Kind of lumpy on the back side and a bit thicker than I'd hoped. The stator is about 5/8" thick. I was concerned that this extra thickness would compromise my airgap and raise the cutin speed above that which I'd hoped for... meaning I'd either have to make a new stator, or use a smaller prop. (or run with a 14' prop and a high TSR which is an option I would probably not feel comfortable with... too fast.)
Yesterday I got the whole thing together. My total airgap at this time is around 7/8", but cutin speed is right at 75 rpm... just about perfect I think! It was interesting assembling it. When I had only the back rotor on with the stator, it cogged pretty badly against the 3 bolts that hold the stator. This is typical - and usually goes away almost completely when the 2nd rotor is placed on. Of greater concern however was that not only did it cog - but it had a lot of drag on it! I was fairly certain at that point that there must've been currents flowing between the parallel strands of wire in each coil, in which case I'd have had to start over - probably build a higher voltage machine and use transformers or something. As it turned out, after the top rotor was placed on - all this drag, and all the cogging disappeared. So I figure the drag must've been due to eddy currents in the bolts that hold the stator. Kind of impressive at any rate, because the drag was very significant, I'd not have thought that 3 bolts could've been responsible for so much - but they must've been, because it's fine now. Lots of fun - tomorrow I'll hopefully find some nice boards in town to start carving on! |
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