I decided to give homemade wind generators a try again. I was so close back in 2001 and 2002, the making power part of it went great, it was simply a durability issue. The generator doesn't need to produce a lot of power, but it needs to be there all the time. For my next foray into wind generators, I decided to keep it small, see if I could make 40 watts or so in 12-15 mph winds and have it survive the days of extreme high winds and thunderstorms that frequent Northwest Iowa.
Most of what I had left over from my first attempts at building wind generators I had sold. I did have one used trailer axle stub and hub in my inventory, as well as a good selection of already wound coils. I still had some magnets, but not enough of the kind I needed for an axial, dual rotor unit. I've had my eye (or bookmark) on CMS Magnetics for a few years. They seemed to offer a great selection of high quality magnets at good prices. I searched their inventory for the best deal on mid size magnet suitable for this generator. I found a 1" diameter by 3/8" thick magnet that looked like it might just work. It's a little small, and that concerns me, but at $1.50 each it's worth a try.
I had a set of 10 coils, looks like about 60 turns of 15 gauge flat magnet wire in each. They were already soldered together and ready to use, I had glued the windings together with epoxy, so there wouldn't be any changing them. So I ordered 20 of the 1" magnets, that'll give me a single phase alternator, 1 pair of magnets over each coil.
For the frame of the generator I scrounged up some used 1" x 3" x 1/8" steel tube. I wanted this generator to yaw out of the wind by tail furling. To do this, you need to offset the generator head from the tower yaw bearing. I wasn't sure exactly how much to offset, but looking at other wind generators to get an idea, I decided to go with a 4" offset.
The initial frame welded up.
I drilled a 1.25" hole through the front tube and inserted the axle shaft into the tube, I gave it about 2-3 degrees of upward tilt to keep the blades farther away from the tower. Once the front was welded, I did a plunge weld into the back and into the shaft, securing it firmly in the tube. The front tube is 12" across, I drilled 4) 3/8" holes in the ends for the stator mounting threaded rods. The back tube has the tail pivot mounted to it, the yaw bearing will mount between the tail pivot and the front tube. You can really see how much offset it has.
Here's a shot looking down on the frame.
This is the first time I've tried this type of tail pivot/furling design. But from what I've read, it works great. The pivot mounts to the frame tilted back 20 degrees. What this does, is it uses gravity to always try to bring the tail back down, in the middle position. Due to the offset head, the generator will always have a force trying to turn the head out of the wind, the weight of the tail tries to keep the tail in the center behind the generator. If the wind gets too strong, the weight of the tail is overcome and the tail stays parallel to the wind, but the generator head turns out of the wind. Increasing the tail weight keeps the generator pointed into the wind at higher speeds, decreasing the tail weight has the reverse effect, causing the unit to turn out of the wind sooner.
So what's with the funky 45 degree angle of the whole thing? If we didn't have some extra force to hold the tail steady, the whole unit would shudder back and forth as the tail moved on its pivot. To get that extra holding force, we angle the pivot off to the opposite side of the offset by 45 degrees. That way the the tail is heading downhill all the time, and we can again use gravity rather than a spring to do our work. Of course, you need a stop on the pivot to hold the tail in position. Next picture will show that.
Close up of the pivot.
The tail pivot is tilted back 20 degrees and to the side 45 degrees, 2 pipes just slip over each other to form the pivot. To make a stop, I cut a length of pipe down the middle and welded it to the upper pipe so it hits the bracket to keep the tail from pivoting too far out. This allows the tail to pivot one direction only (towards the offset head) and gravity always tries to bring it back down to the stop I welded on the pivot.
I used 1" schedule 40 pipe on the bottom of the pivot and 1.25" pipe on the top. I couldn't find any seamless pipe, so I had to chuck up the 1.25" pipe in my lathe and bore out the weld. The tail boom will be welded on the upper pipe.
Here are the 2 rotor plates with the magnets just stuck on for now.
The rotor plates are 3/16" sheet steel. I had a couple of good size pieces laying around, so I used them. But, it's not necessarily an easy job making good, round disks out of sheet metal. Luckily, I was access to a large metal shear, so I took the sheet to a friends shop and sheared the plates into octagons (or close to it). Then I drilled a 3/8" hole in the centers, ran some ready rod through, bolted them together and chucked up the ready rod in my lathe. Then I could draw the plates against the chuck and turn them down to final size. I have a 15" swing lathe, so I can do this. You might have to have someone laser cut them... You really don't want to use a torch here, the plates need to stay flat and heating them may warp them.
Once the outside disk was turned down, I chucked up the full disk in the lathe and bored out the center holes. The disks are now 10" in diameter with a 2.42" hole in the middle. All that was left was to drill out the 4 holes for the hub bolts. I drew the bolt pattern out on paper, transferred it the plates, center punched and drilled the 4 holes.
The stator mold ready for casting.
The stator mold is a 3/4" plywood base with 1/2" plywood cut out to form the mold. I used a latex caulk to seal around the edges, then painted on two heavy coats of PVA mold release. You could use wax or something else as a mold release, but I've got PVA and it works great. Just make sure you use something good or you'll never get your stator out of mold!
The coils freshly embedded in polyester resin.
Now here's where I almost made a huge mistake. I just about took it for granted my coils were wound properly, since they were already soldered together and had been used to bench test some magnet rotors in years past. This is just a plain old single phase alternator, and I'm certainly no genius when it comes to alternating current, (In fact, it almost seems more of a black art to me at times.) But I decided to check the direction of the windings on the coils. If memory served me, the coils needed to alternate, one wound one way the other the other way. I was pretty sure in the past, what I had done, was just flip over every other coil. But I found 3 coils in the set of 10 were in backwards! So I managed to cut the leads, switch the coils around and solder them back in proper positions. No wonder this setup didn't work well, with 3 coils in backwards, that would have essentially canceled out 3 other sets of coils, making this a 4 coil alternator rather than a 10 coil alternator...
I knew the casting of this stator wasn't going to be pretty. The coils were soaked in epoxy and set like rocks. I wasn't going to be able to compress them into the 1/2" mold like I would have wanted to. But I would make do, and hope for the best. I drew out the pattern in the mold where each coil must be placed, along with the path of the magnets. Then I placed a coil in its proper position and used duct tape to hold it in place. I considered using some quick set epoxy to hold the coils in place, that way I could remove the tape and add some Fiberglas cloth to reinforce the stator, but I really didn't have the height to work with. So this stator is not as strong as I would have liked, but since it's on a rather small machine, I think it will hold up the way it its.
When I cast polyester resin like this, I only use about 1/3 the recommended amount of hardener. When the resin is cast in a thick layer like this, it heats up and sets too quickly, causing it to crack. They say an entire can of resin will eventually cure if just one drop of hardener is added to it. I'm not going to try just a drop, but you get the idea. Plan on the cure taking a couple hours or more, high ambient temperatures accelerate the cure too. So if you're working in hot weather, reduce the curative even more. In cold weather, increase the curative slightly.
Before I started pouring the mixed resin in the mold, I made sure I had a supply of sticks, boards, etc. to level the mold.
Here's the mold closed up.
In a perfect world, the mold lid would be drawn down on the coils to compress them as much as possible. A bolt through the center is an easy way to compress the coils.
Several hours later the resin had cured.
You're looking at the good side of the stator, the bottom. The top side's not so pretty, but it will work. It's just over 1/2" thick with a few wires from the coils on the top side sticking up out of the resin. I'll have to increase the air gap some to make sure I don't have contact between the rotating magnets and the stator wires.
The alternator loosely assembled for the first time.
The moment of truth was at hand, how would the alternator perform on the bench? Everything was loose and the air gap very wide, so I didn't expect great performance. For the first test, I just short the 2 output wires and gave it a spin. It barely turned, that was a good sign. So I hooked up the AC leads to my voltmeter and gave it another spin, this time it spun freely and the volt meter ran up to over 15 volts. Another great sign, over 15 volts spinning by hand meant the unit would have a low cut in speed. Next test was to run the output wires through a bridge rectifier to convert the AC to DC, I spun it up again and still got 15 volts, now DC. I switched the meter to the high amp setting, then gave the alternator another spin. With much resistance to turning the meter read over 5 amps DC. Wow, this little guy was doing much better than expected. The last test was to connect a 12 volt lead acid battery, and see if I could spin it up enough to get a charging current into the battery. Sure enough, with a quick spin of the rotor the meter read about 1.1 amps charging current into the battery.
I have a feeling I'm going to get a bit more than 40 or 50 watts out this wind mill in modest winds. I still need to be careful though, and not get greedy. There's going to be a lot of internal resistance in the coils, if I try to push too much current through it I'll turn the stator into a heater and melt things. I'll just have to work with the furling system to make sure it turns out of the wind before it's making too much current. The blades I used in the past were great low wind blades, since they have an aggressive angle and are not tapered they have a lot of torque and start in very low wind speeds. I was going to go with 5' blades, but I may just go with 6' blades instead. The theory, or plan for this wind generator is to produce useable power in low to modest wind speeds, and turn out of the wind sooner rather than later.
I think I underestimated the power of these magnets. My largest generator used 1.5" magnets by 3/16" thick, I'm not so sure these 1" by 3/8" magnets aren't just as powerful. Of course, I ran two sets of 9 coils on that generator, so it would be capable of at least twice the power of this one.
One of the magnet rotors prepped by grinding the surface to give the epoxy a better grip on the surface.
A rotor with epoxy curing.
After carefully adjusting the magnets on the rotor, I trimmed the outside edge in tape and made a round dam in the middle. I used melted wax to seal the aluminum ring to the disk. This is a high grade epoxy and should adequately hold the magnets in place. I did add some fiberglass cloth pieces for extra strength, but I doubt that was needed. The second rotor was done the same way, it's important to think through the positions of the magnets on the disk. They need to be in exactly the same place on each rotor in relation to the bearing hub bolt holes, but the magnets need to be mirrors of each other on the 2 rotors. That is, a north pole needs to match up to a south pole so the magnetic flux is drawn through the stator when installed.
Here's the start of the blade set.
I making these blades the same way I made them years ago. I was very pleased with the performance then, so I really didn't see any reason to change them. These are straight profile blades, no twist or taper. The stock is 1" x 4" clear pine, each blade is 3' long for a rotor diameter of 6'. To get the angle needed, I set my table saw at 6 degrees tilt and ran the end of each board in about 8". In the picture above the bottom board has had the 6 degree cut made, I used a fine saw to slice off the excess, as you can see in the top board above. Next was to cut the 120 angle cuts in the base of the board, these were made on a power miter saw.
The next part is to make the rest of the board into a "wing" profile. I used a power hand planer to create the rough shape of the air foil, then a belt sander to do the final forming of the profile. A power palm sander was used to finish sand. I cut an 8" disk from plywood to form the hub of the blade rotor. I pre drilled the 4 bearing hub holes in the plywood disk.
The 6 degree angle of these blades is a compromise. A greater angle is usually used at the base of each blade, and a lesser angle at the tip. I recall reading one of Hugh Piggott's documents were he stated the a blade without twist or taper suffers very little drop in performance. From my past experience I agree with that, and an added benefit is that the full width blade with the slightly steeper angle gives you better starting performance. The real reason I don't make twisted and tapered blades, is that I'm no master wood worker. It's much easier to make a good profile that's continuous along the blade length. I think I spent about 2 hours making this set, compared to perhaps a day or two to carve an elaborate set. The inexpensive cost of these blades comes into play too, less than $9 makes a very useable blade rotor.
Here's the rotor attached for initial testing.
I used 3 screws near the points at the base of each blade to loosely hold the blades to the plywood hub, then I laid the assembly flat on the workbench and measured between each pair of blades, adjusting the blade angles slightly so they were all evenly spaced. More screws were added from each side to secure the blades to the plywood hub, the 4 bolt holes were then drilled through the blades.
In the picture above I'm checking to make sure each blade travels through space on the same plane. I just used a ruler set to almost touch the end of the blade, one blade was about 1/2" off, so I adjusted the bolts on the plywood hub to bring all the blades in the same plane. I also checked to make sure all the blades were running at the same length, which they should be, and were.
At the same time I tested for balance. With a free turning rotor, you can easily check for balance by gently starting the rotor turning, and watching where is stops. If it's out of balance, it will stop with the heavy blade at the bottom. You then need to add some small weights to the rotor to bring it into balance. This is the first blade set I've made where I couldn't find any weight imbalance at all, so I didn't need to add any counter weights.
The tail section after welding.
The tail support is a 1" pipe, 34.5" long. I ground the pipe to "more or less" fit over the tail hinge pipe. I added a 1" x 1/8" length of angle iron for extra support. Two flat straps were welded on the pipe to bolt the tail vane onto the support structure.
I should also note, I added another stop point to the tail hinge to prevent the tail from swinging into the blades during full furling. So there's one stop keeping the tail from going to the right (from behind), and another stop to keep it from going too far to the left. The tail stop is set to give the tail about 10 degrees offset right of square. That helps keep the generator straight into the wind when you take into account the yaw offset.
After a couple of coats of paint on the blade rotor, and after fabricating a yaw bearing from 3" pipe, I was ready to install the unit on my roof top pole for some testing.
Here's the generator installed on the roof pole.
The roof pole was 2.5" schedule 40 pipe, so I picked up an 18" length of 3" schedule 40 pipe for the yaw bearing. I didn't want to weld the 3" pipe on permanently, so I welded the pipe to a 1/2" thick plate, then made some flat straps to bolt the yaw pipe to the frame. The tail was just a scrape piece of 1/2" particle board, it won't be on there long. I'll pick up a thinner and lighter piece of plywood and paint it for the final tail.
I decided to make the run to the battery pack directly from the alternator output as AC, placing the rectifier on the battery side of things rather than at the generator head. Since this is a single phase alternator I only needed two wires either way, and it made more sense putting the rectifier inside.
The first few hours it was up, the winds were very light and I was seeing charging current into the batteries of 0 to over 5 amps. I was using the 10 amp setting on my multimeter. Early in the morning I heard the wind pick up, and decided I better get the multimeter out of line, or I'd burn it up. So I installed a 30 amp meter I've had since my first time off grid.
Here's the amp meter reading some 28 amps going into the battery. The 35 amp rectifier on the right is mounted to an aluminum plate as a heat sink.
By mid morning the wind had done a 180 degree shift and was now blowing all the way across the roof. To my surprise, the power output of the generator increased in pretty much the same wind speeds. Some of the increase may have just been the repacked bearings setting in. But I think the flow across the roof is less turbulent too, allowing for better power output. Either way I'm putting out way more power than my little battery pack can absorb, and more power than I can use at the moment. I'll have to decide if I want to buy a shunt controller or build one. I've seen them sold for as little as $35, so I may just go that way. For now I'm shutting the alternator down by shorting out the AC leads. In winds around 10 to 15 mph it pretty much stops the rotor blades.
I'd like to see how the furling mechanism works in higher winds. But I'm afraid my small battery pack may not have enough resistance to keep the alternator pulled down in high winds. That, and the fact I don't have enough loads at the moment to help pull the batteries down. I may have to wait with gathering high wind data until I get a larger battery pack. For now I'll continue to let it run in modest wind speeds and see how it holds up. As for power output, I can already see it's going to do a lot better than I originally expected.