20 Foot Page 2 - Stator
The first step in making the stator mold was to start with a square piece of plywood (we used Baltic birch, its strong and smooth). That stator will be 28" in diameter so the plywood mold bottom was 32inches square if I recall. I drew circles on it at 16inches in diameter and 22 inches in diameter. This shows the path of the magnets and the centers of the coils need to be centered over that. I also divided the circles into 15 parts. Since we have 20 magnets we need 15 coils if we're to stick with what seems to be the 'standard' of using 3 coils per 4 magnetic poles. Once I've drawn all those lines I can draw out what seems an appropriate coil shape (pictured above).
I used the same coil winder for this machine that I did for the last 17' diameter turbine we built. These coils are a bit larger though, so I drilled 4 holes and used nails for 'pins' that determine the inside of the coil.
There is our test coil. It needed 110 windings of #14 gage wire to be the right size and thickness.
It's pretty close to what we planned for.
Pictured above we've glued the test coil to a piece of plywood, and drilled out the plywood so that we can bolt it to the stator bracket. Now we can adjust it between the magnet rotors and get some data off it so we know how to wind the actual coils for this 48V machine.
I could make a pretty good guess what to expect based on the older 17' wind turbine, because we used the same magnets and the coils were similar in shape. As it turned out, our test coil (with 110 windings) gave up 10 volts @ 60 rpm. (actually somewhere between 9.4 and 9.8) We're measuring AC volts here. Peak AC volts (that which is important to know when we're rectifying things to DC) is very close to 1.4 x RMS AC volts (that which our meters show us). So - the goal at 60 rpm for 1 phase is about 33.6 Volts. Weve got 10 coils in each phase, and we know that the sum of the output of 5 coils x 1.73 must equal 33.6 Volts @ 60 rpm. 33.6/1.73 = 19.42 Volts, so the sum of 5 coils in phase with one another must equal about 19.42 Volts. 19.42/5=about 4 Volts. So we want about 4 volts AC from each coil @ 60 rpm. Turns out about 44 windings is appropriate here. I was hoping we could wind with 3 strands of #14 in hand, but it's looking like the best we can do here is 3 strands of #15 to get 45 windings per coil (throwing in that extra winding for rectifier loss). Not very scientific but lots of fun and it should come out close. If it doesn't we'll either open or close the airgap (we have plenty of room to do either I think).
Pictured above is the winding setup - 3 rolls of #15 gage wire on a stick in the vice, and our hand crank coil winder clamped to the work bench.
There are all 15 coils wound up and placed in their approximate position around the bottom of the mold. You can see they fit pretty tightly together, there isn't much room for more copper here. All in all, about 25 pounds of wire in this stator.
There are 5 coils in series per phase. The series connection between the coils is made as pictured above. It's tricky to hook up 6 strands of wire neatly - so we cut little pieces of copper pipe and crimped that over the connections. Then we soldered them.
There is one phase hooked together, and taped to the bottom of the stator mold in exactly the right position.
Pictured above all the coils are first taped in the right position with duct tape, and then we went between each two coils and glued down a small square (about 2 inches square) of fiberglass fabric with cyanocrylate glue (Super Glue). This holds everything together pretty well so we can handle the stator before we cast it.
There is our stator all finished except for casting. We used lots of zip ties around the inner diameter to hold all the connecting wires tightly in place.
There the mold is finished up. The cavity is 28 inches in diameter, the island in the middle is 11 inches diameter, and the mold is 5/8" deep. We caulk wherever one piece of wood is screwed to another to assure that resin can't run between cracks. It also makes the finished casting come out much easier.
Here we are almost done with the casting process. This stator took about a gallon of resin if I recall. There is fiberglass fabric on both sides, and lots of shredded/rolled up fiberglass scraps around the inner and outer diameter to give it extra strength. We used polyester resin which works fine, though lately I prefer Vinyl Ester. The Vinyl Ester seems to hold up to higher temps, it sets more slowly, it shrinks less and I believe it's stronger. I always worry when doing a large casting with polyester - I've usually had a few cracks come up in larger castings. Usually nothing serious - but I've seen other folks have to start all over when their polyester cracked badly.
The lid for the mold was just the scrap that we cut out from the mold cavity. We clamped it with C clamps and put lots of weights on it.
The stator came out of the mold fine with only a few minor cracks. I patched those with epoxy. Pictured above we've centered the alternator head over the stator, and we're drilling through the holes in the stator bracket into the stator.
Now we're assembling the alternator to make sure the stator works as we hoped it would. Adam is slowly lowering the top rotor with 3 jacking screws.
There the alternator is assembled for testing. The cutin speed is about dead on what we were shooting for - 65 rpm. This thing is quite incredible if you spin it up and short the stator out - it slams to a stop immediately.
The next step is to finish up the metal work. We put that off because we didn't want a huge thing in our shop all winter long, we figured it'd be best to build/test the alternator and then let the machine start getting big and heavy!