Yesterday was a fun day working on my new wind turbine.
At the end of the 2nd days work we'd got the magnets down and glued in place. We started yesterday by building a new coil winder. Pictured above is the coil winder and a paper cutout of what seems a logical shape for the coil. The magnets are 3" tall, 1.5" wide. I could be wrong.. I like to make the hole in the coil a bit smaller than the magnet. This compromises the inner windings a bit, but I believe it optimizes the outer windings and cuts down on resistance some. I've also got the magnets somewhat crowded on the rotors, so in order to fit a reasonable amount of copper in this stator I need to do this. The hole in this coil is about 2.5" tall and about 1" wide at the top. Before building the coil winder, I did get the 2nd magnet rotor mounted to the machine, with an airgap of .9". Poking around right between the rotors with a Gaussmeter, it's clear that the strongest flux is found in an area smaller than the magnet (about 1/4" from the edge of the magnets field density starts dropping off). It was also interesting, when testing with the gaussmeter - to see how badly the leakage from magnet to magnet was hurting me. Not too badly... the flux near the inner diameter of the 'magnet ring' was only slightly less than it was near the middle. Also, with these magnets crowded closely, there is a moment of 'cancellation' when we have 2 poles over 1 leg of the coil - but it's very brief and it only happens on the inner part of the coil, not over the whole thing. I don't believe it'll hurt me too badly. (though I do wish I'd started with 18" diameter rotors instead of 16")
I wound a test coil of the proper size (so that 12 will barely fit in the stator). I used #13 wire, 79 turns. When starting a new alternator design like this, I dont worry about the number of windings or the wire gage for my test coil. We simply shoot to wind one the right size... and then test it. From testing 1 coil we can figure pretty close how many windings and what gage wire we'll need in the end. I did start with #13, because I figured it would be close - but I could've used anything. At this point I've decided to build this machine for a 48 volt system (which doesnt exist yet). The coil winder is 1/2" thick - the stator will be 5/8" thick. I tend to wind the coils a touch thinner because often times they seem to expand a bit when we take them out.
I had planned to cast the test coil in resin so I could poke it between the rotors - but didn't have the patience to wait for glue to dry. We made a plywood bracket and superglued the coil in there. It fits two of the stator brackets on the wind turbine, so it should hold in there tightly and I should be able to put some load on the coil.
Pictured above we have the test coil mounted with only 1 magnet rotor on the machine. At 70 rpm we see 2.4 VAC.
There's a picture from the back of the machine with the test coil in there. You can also see the resistor I used to test it (described below)
Watch your fingers!!! Tom and George are lowering the front magnet rotor. It's not quite as dangerous as it looks here - there are nuts to stop the front rotor and hold the airgap at .9", and we have 3 jacking screws.
There we have the front rotor on. At 70 rpm now, we see 5.4 Volts AC - which, is just about perfect for 48 volt cutin. So my test coil is right in there. I probably have room for a couple more windings, I could make the coil winder slightly thicker... so in the end I'll probably wind the coils with about 80 turns, and I'll probably use 2 strands of #15 wire (equiv to #12). I can also bring in the airgap a bit tighter if needed. One other test I did... I wired this single coil up to a big resistor (somewhere around 1 ohm I think) and cranked it hard by hand. At about 104 rpm, under load, I had 6 Volts AC and 6 Amps into the resistor (36 Watts). Im not sure if I'm figuring all this right - but I think that with 12 coils wired in 3 phase Star configuration... I should be close to 400 watts @ 100 rpm. I'll know more when I build a stator and bolt this alternator up to the rear axle of my truck.
So we took it all back apart, and set it up outside. Pictured above we're checking the stops to make sure the blades cant hit the tail. On this one, we've welded a 'bracket' to the side of the tail boom that will collide with the yaw bearing before the tail gets near the blades. We'll probably back up the 'normal' position stop with a cable so that we need not worry too much about the tail bearing getting bent or cracked.
Here George is cleaning up the tail with some sand paper. This pipe has lots of nasty black coating on it. This is just before we welded on the stop for the furled position so you dont see that there.
So there's the machine all finished up except for paint, a stator, and assembly. Total weight we figure will come in right around 200 pounds, not counting the blades. The magnet rotors are 45 pounds each (estimated after we cast resin on them). We've got nearly 25 pounds of magnets in this alternator! I figure it'll take about 15 pounds of copper in the stator. The tail is a bit over 40 pounds.
Here I'm spraying on a nice coat of self etching primer. I really wanted to get to this point yesterday - it was unseasonably warm (about 65 deg F) for January in the mountains. Painting inside my small shop is no fun.
Putting on a nice shiny coat of green acrylic enamel!
There it is, all painted up - ready for the next fun filled Monday to come along at which time we'll hopefully get the stator done.
I made the magnet rotors yellow. I think this is the first time I didn't use a single Volvo part! (but if you look real close you can see one of the lucky Volvos that still has 4 wheels in the reflection). Lots of fun.. lots of progress for only having 3 days into it. Can't wait for Monday.
