Introduction: 7 Foot Axial Flux Wind Turbine

This instructable documents the process of my single rotor wind turbine. It is built with inspiration from Hugh Piggot and the folks at This is my first attempt at building a wind turbine, and I will try to include the mistakes I've made along the way so that other first-time builders can avoid them! Since this entire project has a budget of just about $1000, it is meant to be able to be completed by both newcomers to wind energy as well as those who don't have a lot of money to throw around. I also tried to use as much recycled material as possible, so a large portion of this turbine is made from a junked car and metal that was going to be thrown out by fabricators.

When I first began this project I had a decent grasp of electricity and electronics, which was helpful. I knew hardly anything about wind turbine design, but picked it up quickly with the use of several books and websites.

The books and works that have been most influential in the building of this wind turbine include:
-Windpower Workshop by Hugh Piggot
-Windpower by Paul Gipe
-Arc Welding Instructions for the Beginner by The James F. Lincoln Arc Welding Foundation as well as many helpful individuals on the Otherpower forums

If you want to know more about electricity or electronics, the book Teaching Yourself Electricity and Electronics by Stan Gibilisco is a great learning book and valuable resource to have around.

Here is a video of the final project.

(The whole thing about the modern day Don Quixote... yeah, I don't think it's right either.)

Step 1: Deciding on Size

Since this is my first wind project, I decided to work on something that is a bit easier to construct than a high-output "dual rotor" design. I also didn't have a lot of metal fabrication shops available to me, so I was looking for something that required little fabrication.

A Volvo brake rotor has become popular among homebrew windpower enthusiasts because of its wide availability and its relative ease of modification. Volvos have a reputation for being long lasting and their rotors are not much different. A trip to the junkyard landed me with a $20 Volvo 340 strut (rotor, spring, and everything).

A single rotor design only has one set of magnets that spin in front of a stator coil. This makes construction easier and less dangerous, since you don't have to use "jacking screws" to bring together two magnetic disks. This is dangerous because if you slip, you could very well break your hand or finger. With a single rotor design, there is much less chance of this.

One thing that many people don't realize about wind turbines is that the blades of a particular turbine are matched to its generator. This is all based on what is called "Tip Speed Ratio" or TSR. By matching the correct blade diameter to your generator, you ensure that the turbine will start generating at a certain wind speed. By having the right size blades and generator, the correct RPM and torque will be produced to generate the maximum amount of power safely (that is without overheating or over-speeding the turbine). Matching the blades to the generator is a very important aspect of designing your windmill, and many other aspects of the machine are based on the blade diameter.

For this particular design the blade diameter should be around 7 feet.

Step 2: Modifying the Strut

Most homebrew wind turbines are designed to mechanically turn out of high winds. This is called "furling" and is very important to protect the wind turbine in high winds. Do not underestimate the amount of power in a strong gust of wind, as it can seriously damage a wind machine. To have the turbine turn out of the wind, the generator and blades are mounted off to the side of a yaw bearing. We mounted our generator about 6 inches away from the center. This is overkill for this size generator, but we'll be playing it safe as this will cause the machine to furl earlier.

In addition to mounting the generator at an offset, the tail will be mounted to specification to allow the generator to turn while the tail stays straight.

We removed the spring and the inner cylinder of the Volvo strut with a cutting torch. We also removed the rotor (to be modified later) and the piece of metal behind it. Then, the remaining piece of the strut was cut in half and welded onto the side. We also added a piece of metal to give another place to attach the stator to.

Step 3: Rotor and Stator

One of the more technically challenging and labor intensive parts of building a wind turbine is winding and casting a stator coil. A very thorough knowledge of permanent magnet generators is needed to calculate how to wind an optimal stator. If you are determined to do it, you can simply follow the example of someone else's successful generator. Again, this being my first shot at windpower, I didn't want to risk winding the wrong thing, messing up the wiring, or ruining the casting, so I bought one from They make lots of different stators for a variety of magnets (rectangular and round) and voltages. Most axial flux wind turbines use a three phase stator for more efficient generation.

Since this is a single rotor design (stators on Otherpower are usually made for dual rotors) we will only be generating half the stated voltage. So, for this project I wanted 12 volts and had to buy a 24 volt stator. Otherpower actually is now offering a kit for a single rotor wind turbine that is pretty much the same as this one. It's still cheaper to fabricate your own, though.

The stator is made for a 12 inch rotor, but the Volvo brake rotor is only 10 inches. So, we were forced to add on to the outside edge. This was probably not the best solution, but it should work fine. I would recommend getting a 12 inch steel disk from a CNC water jet if possible, which would be a lot cleaner and provide a better surface to mount blades.

We used a piece of 1/2 inch plywood mounted to the strut and then mounted the stator to the plywood. Make sure there is clearance for the stator coil to clear the wheel bearing.

Step 4: Magnets

I used twelve 1 x 2 x 0.5 inch rectangular magnets from (very reasonable prices). On the back of the rotor, I also used an aluminum plate to keep the magnets separated (this plate is also available from Otherpower). We used heavy duty epoxy to ensure that the magnets won't fly off at high RPMs. Be VERY careful when you put these magnets into place - they have enough power to severely injure or break your fingers! I found that a little caution and a rubber hammer were invaluable tools.

The magnets are placed in alternating polarities on the back of the rotor. We used a small magnet to test each magnet's polarity.

I left the magnets exposed, but I am planning on a possible renovation soon. I should've cast the magnets in some sort of resin to prevent corrosion. There is plenty of good information on the internet on how to do this, and it's very similar to casting a stator coil.

Once all the magnets are on the rotor, remember that the whole disk is now one gigantic magnet. Do not set it on a workbench with metal shavings or magnet-sensitive materials!

Also, you may notice that the pictures are a bit out of order (the rotor is painted already) but I am trying to lay this out in the most logical order... not necessarily the order we did it in!

Step 5: Tail

The area of the tail vane and the length of the tail boom are both dependent on the diameter of the windmill. This ensures that furling takes place at the right wind speed. The tail boom should generally be about as long as the radius of the turbine (one blade length). In this case, it should be around 3.5 feet. In reality we made it quite a bit longer by mistake. It still seems to find the wind alright, but I think it will effect furling.

As for the area of the tail vane, it can be determined by a simple equation: D(squared) / 40 = Area of the Tail Vane (where D is the diameter in your unit of choice). This equation is given by Hugh Piggot who has many years of small wind turbine experience.

The shape of the tail vane really has very little effect on the mill as long as the area is correct, so feel free to get creative. I spent some time with an air grinder and cut out a neat looking tail, but it's not entirely necessary. What does matter is the angle at which the tail is mounted. It needs to be at about 45 degrees from directly behind the center post on the opposite side of the generator. In addition to that, it should be mounted at a 20 degree angle from perpendicular - the pictures I've included illustrate this nicely. This may need to be adjusted slightly depending on when you want to mill to furl and how heavy the tail vane is.

The tail is made of electrical tubing and brazed together. We put a lot of supporting pieces in the middle of it to strengthen it. The tail vane is made of stainless steel and it is riveted onto the tubing.

Step 6: Blades

Personally, I am not a very good carpenter. My woodworking skills are pretty basic and making your own blades requires some pretty good attention to detail. So I decided to save myself the time and effort of making blades and just buy a ready made set.

There are lots of guides and tutorials for blade-making online. Hugh Piggot even offers a nice table of useful figures for different blade diameters in Windpower Workshop.

Now, there is a lot to consider when buying blades. First and foremost, consider the power you are taking from the wind. This is based mostly on the diameter of your wind turbine blades. A wind turbine steals energy from the wind by slowing it down as it passes through the blades and makes the rotor turn. This is all laid out mathematically by the Betz' Limit. These calculations show the maximum amount of energy that is possible to be extracted from the wind, though they do not account for losses in generator efficiency... they also assume that you have infinite blades which all have no drag, and if you can hook me up with some of that, I'd be very interested :).

It isn't so simple as bigger blades equals more power, though! No, no, no. What is most important is that you match the blades to your generator. Generators are designed to start producing usable electricity (the voltage you want) at a certain RPM. This is where the Tip Speed Ratio comes in. Basically, you want to have blades that provide that RPM at the most typical wind speed you'll be experiencing. Sound complicated? It is, kind of. Usually the Tip Speed Ratio (TSR) is an indicator of efficiency, but beware of websites claiming abnormally high TSR's! I would recommend using it as a rule of thumb.

There really is a lot to blade making as well as blade theory, but that should give you some good general insight into what you're trying to accomplish. As for me, I bought my blades from a website called Magnet4less (no 's'). At first I actually bought the wrong size blades thinking I could get away with a bigger diameter, but alas, physics wins again (Some of the pictures may have these first blades in them). I eventually folded and bought the 6.8 ft. diameter blades. However, I would NOT recommend buying these blades again. They have terrible reviews from wind power people around the country and with my personal experience they look like the angle of attack just isn't enough to spin up high RPMs.

This is the part where I suggest another retailer but, to tell you the truth, I really don't know of one. Homebrew wind turbines enthusiasts seem to be mostly (but I could be corrected) out of luck for high quality universal blades with different diameters. I suppose you could use a pair of replacement blades from another commercial turbine assuming they are to your needs.

Other references!

For making wooden blades:

If you're interested in making blades out of PVC, I'd recommend using Google for instructions. I don't recommend PVC blades for a turbine like mine, though.

Step 7: Tower and Yaw Bearing

Now that most of the construction of the actual wind turbine piece is complete, you'll have to decide on a place to put the beast! Obviously a good spot with lots of wind and few obstructions is optimal. Most DIY wind power enthusiasts recommend that your tower be 30 feet above the nearest obstruction within a 300 foot radius. In other words, pretty much as high and in the most open area as you can get it.

Selecting a location and tower is difficult, there is no doubt about it. It's hard on your wallet and your time. The biggest concern when putting up a tower should always be safety. This is a homebrew turbine and could run into problems that we can't quite predict, so a little extra safety is always good. Be aware that if your blades fail at high RPMs if they will fly quite a distance and could pose a threat to nearby houses or cars.

A basic run down of the tower options available.

-Monopole tower
No guy wires, requires a lot of concrete and are almost always very expensive.
-Tilt down pipe or lattice tower
Used by the majority of the DIY Wind Power community because of cost and ability to tilt the machine down for maintenance. Uses guy wires and a hinge at the bottom of the tower and can be welded together with the correct pipe.
-Lattice tower
Another choice is to construct a lattice that does not tilt down but still has guy wires. The problem with this is that it must be climbed to do anything to the windmill. Many people do not feel safe doing this and many people have been hurt from falling off stationary towers like this. Use extreme caution and a safety harness to work on top of one of these towers.

I was fortunate enough to receive a free 30 foot lattice tower from an airport. It was once used for a rotating beacon and should be strong enough for my small wind turbine. I had to add to the top of it (about 5 feet) to attach the wind turbine with enough clearance for the blades to move around when the machine furls. For support, we welded gussets onto a piece of 2 inch pipe and attached it to the top of the tower.

Then I took a piece of schedule 80 pipe to a machine shop and had it turned down on the lathe so it would act nicely as the yaw bearing. We leveled it on the tower and placed some set screws in so it could be welded in place. This piece is important because, if it comes loose, the whole turbine could fall off or the blades could hit the tower! YIKES!

Welding was completed with a DC arc welder. I'm still not terrific at welding by any means, but it's a great skill to learn. Before I made the welds on the tower, I did some practicing on some scrap pieces I acquired from a metal shop. That's another thing you find while trying to learn to weld - people in metal shops are usually quite friendly and willing to help out. If you're thinking about learning but are lacking material to practice on, ask around - what many might consider junk could be very useful to you.

Step 8: Controller

At first I was going to build my own controller, as I have shown in the pictures below. The project shifted locations, though, and I decided to charge a battery instead of heat water, so I decided against it. I did have a rough idea of what I was going to do, but I think not making the controller on my first turbine was probably a good decision. Things can get dangerous if your controller fails and lets your turbine run away at reckless RPMs. Also, believe it or not, electrical permits prefer stuff that's UL listed... who'd have thought that?

Instead of heating water, I decided to charge a 12 volt battery that would be hooked up to a light. This cut down on costs, logistics, and other restrictions that made it a bit easier to install. I settled on using two Xantrex C40 charge/dump controllers. One regulates the charge coming in and the other watches the charge level of the battery to ensure it is not overcharged (it can't do both at the same time). I have a 12 volt dump load that is a water heating element, rated for 600 watts.

I used a three phase disconnect switch as the "brake" on the wind turbine. When the switch is closed, all three phases of the wind turbine are shorted out and it will slow itself down. I also used a 750 watt modified sine wave inverter to run the light. The light has a photoelectric eye on it and is wired up to turn on at night.

Step 9: Testing Jig

Before programming the controller, it is important to see what kind of power output you can achieve with your generator. This will help you determine what RPM is optimal for you to run at and how that affects the pitch of the blades.

Now, a lot of people have a lathe, a tachometer and a bunch of other fancy tools to test their generators. Well, I am a bit less fortunate and do not have those available to me. So, instead, I made a simple jig to spin up my generator using an electric drill.

I cut a notch in a small piece of wood and shaped it as a trapezoid to avoid all the holes for the bolts. On the other side, I screwed through a hole in the metal into the wood to hold it in place. The other two planned attachment places were too close to the edge to take a screw, so I drilled a couple of small holes and used small zip-ties. Surprisingly, this setup is pretty solid. The piece of wood doesn't act like it's going anywhere and those zip-ties are nice and snug.

Now, with all of these funny angles and stuff, it's hard to find the exact center of the rotor. I did the best I could to eyeball it and I drilled two more long screws partway into the wood on either side of the center. Then use an allen wrench with a T-handle on it that is long enough to touch these two screws. These screws will take the torque from the drill to spin up the generator.

Step 10: Generator Testing

To gather data on the output of the generator, you'll need a few things. Mainly a couple of good multimeters, a test load, and some way to read the RPM.

I attempted to use the equipment in my physics lab and Datastudio to measure the frequency of the AC current, then I tried to work backwards to figure out the RPM of the generator. If I remember correctly, I had to divide by 6 to get the RPMs. This method, however... was extremely inaccurate. I kept getting readings that were very off. Despite the pretty graphs the equipment was generating, I would not recommend trying to use this method for measuring RPM.

Many people use fancy bicycle tachometers with good results. Some bicycle tachometers can be found for under $20. Luckily my physics teacher is an avid bicycle rider and had a box full of old bicycle computers that he let me try out. I found one with an RPM counter and zip tied a neodymium magnet onto one side of the testing jig. Then I used a ring stand and a clamp to hold the sensor in place. Now I could read the RPMs fairly easily, although they didn't update immediately this method was far more accurate. I would highly suggest if you want accurate readings to get a decent tachometer or a bicycle computer with an RPM setting.

I was also able to use Datastudio to measure the DC voltage coming off the generator. The equipment only measured up to 10 volts DC, so I used a simple voltage divider to scale down the voltage being read by the computer. This allowed the computer to read 1/12 the voltage being produced. So the actual voltage on the graph was 12 times voltage read by the computer. See pictures for the schematic. The voltage across R2 should be 1/12 the voltage of the generator, this is shown in the graph.

Also, I needed to flatten out the DC coming off the rectifier. To do this I used a big capacitor in parallel to flatten out the voltage spikes, giving a more accurate DC voltage reading. You have to watch a lot of things when you spin up the generator. It's hardest to measure voltage, current and rpm while still holding onto the drill.

For my test load, I used six 18 ohm 25 watt resistors in parallel for a total of 3.6 ohms.

Step 11: Base

Since my tower was 36 feet tall and in a triangular lattice structure (18 inches per side), I knew I would need a big sono tube to support it. After googling, reading, and asking a certified wind turbine installer, I settled on a 2 foot diameter, 6 foot tall sono tube. It's not a bad idea to consider the soil you'll be digging into, as some places may need more of a footing just to be sure.

Luckily for me, since this project was being funded, we were able to talk the local concrete place into doing it all for free. They even dug a hole and centered a piece of pipe in the casing for us.

Also, consider how you want to attach your tower to the base. My first thought was to have tree bolts sticking up from the base (one for each leg), but the wind turbine installer suggested something different. He suggested we put a piece of plate or something on the bottom with a hole in it, then have one solid pipe or piece of bar stock in the base and slide the tower down over. This does a couple of things. Firstly, it allows the tower to move, which is nice, and secondly, it reduces stress on the concrete base because the pipe is centered on the casting.

Step 12: Guy Anchors

I decided to use three guy anchors, one for each corner of the tower. These anchors should be evenly spaced and as close to equidistant from the tower as you can get. I chose a 22 foot guy radius, meaning the distance from the center of the base to the guy anchor hook was 22 feet. This figure was based off of some other towers I had read about as well as the suggestion of the wind turbine installer.

It's a good idea to get a friend to help you put in the anchors, since pouring concrete by yourself isn't much fun. The anchors we used were 4 foot long screw anchors (but we didn't actually screw them in). The backhoe dug the holes and then we put in the anchors with concrete. I screwed the anchors in slightly to keep them in place while we poured the concrete. I used four 80lb bags of quickcrete in each hole. This was simply an estimation; I know very little about concrete and felt this was plenty (and I am pretty sure it was). I let the quickcrete dry and then backfilled the holes. It is a good idea to let the concrete settle for at least a day before backfilling the holes.

Step 13: Guy Wire

The guy wire we used was second-hand from a local telephone pole installer. It's the same stuff you see guying down telephone polls all over the place. I think it's technically 3/16th of an inch thick, but I'm not certain. The wire is overkill for my tower, but we got it for free so I figured it's better to be safe. We didn't use traditional cable ties on the wire. It came with a bunch of twist hook pieces that wrapped around the wire and then twisted on. They were made to work specifically with this size and type of cable. Look at the pictures to get a better idea of what I used.

The tower is guyed at 6 places, three wires at 20 feet and then 3 more at 30 feet.

If this is your first time putting up a tower like this, it is nice to have someone around who has done it before. A lineman from Central Maine Power was able to help erect the tower and rig the guy wires on the day on installation. It was tremendously helpful to have someone with experience.

Step 14: Assembly

Before you get too trigger happy, assemble the turbine on the ground once more before you raise it. Check to see if the tail will run into the blades, if the blades are too close to the tower, if the rotor and blades are balanced and anything else that might not look quite right.

I assembled the whole turbine in the garage and made sure my tail wouldn't hit the blades by adding two 3 inch bolts that will stop the tail if it whips around. This has not been a problem, though, since most of the tail movement is very slow and gentle, but you never know when you'll get a tremendous gust of wind.

Make sure the top of the wire is water proof and the electrical connections are solid.

Step 15: Tower and Electrical Work

For the type of tower I have, I decided it would be best to assemble it all on the ground and use a bucket truck to lift it up. Having a few friends around to help is a very good idea and during the tower installation it helped a lot. We had 4 people working on preparing the tower. We all worked on different pieces, a couple of us put together the tower, I bolted it together. We ran conduit and wire and attached one end of the guy wires to the tower while it was still on the ground. This saved a lot of time and gave us a lot of eyes for when the tower was in the air.

We had to be careful to leave enough wire at the top of the tower to allow the turbine to yaw back and forth. I've read that wind turbines rarely will do a 360 degree turn, but it's a good idea to allow enough slack just in case. All of this installation was performed to electrical code because we had the electrician on hand to help us.

We attached the guy wires to the tower before it was raised and set the generator on top. Since the generator just sits on top of the tower, the tower needs to be up at an angle while it's on the ground or it will slide off (hence the saw horses). I used some lithium grease we picked up at the hardware store to grease the yaw bearing and the tail bearing.

The third picture shows the top of the tower all rigged to the bucket truck (which you can't see) and ready to be lifted up.

Step 16: Excuse Me While I Kiss the Sky

Now the tower is lifted and shifted into position. We lowered it down onto the base and attached the first set of guy wires. We then tightened them up with a come along, going evenly around the tower and ensuring that it was level. Once the guys were attached, it was time to attach the blades and tail. I was fortunate enough to get a ride in the bucket truck to do this. It could also be done with a safety harness and someone who wants to climb the tower, but it would take a bit longer and be a lot more treacherous.

I made sure I had the electrical brake on while I assembled the turbine. The last thing I wanted was the machine to take off with only two blades attached! Luckily, the weather held out and it was a nice calm day. I was able to attach the blades and tail with no major problems.

Sadly, I have no pictures of the tower being put in place because I was too busy helping.

After everything was up, I drove a ground rod in the soil next to the tower and ran a piece of wire to the tower to electrically ground it. Driving a ground rod is hard work, luckily I had help. Then I covered the wire and ground rod with a bit of dirt to make it look better.

Step 17: Finish Up the Wiring and Test It All Out

This was a big project. Once everything is up you can sit back for a minute and relax. It is awfully satisfying to see your creation spin up in the wind. The first day I put mine up, I didn't see it spin at all, so don't get discouraged if you see no immediate results. It is often said that once you put up a wind turbine, there are two weeks of dead calm weather afterwards.

Haven't gotten a chance to get any real power readings yet because the turbine is installed about 6 hours from where I live. I'll be making a trip up there soon though to make some adjustments and hopefully get some data. I am shooting for around 500 watts (I've read of people with similar designs getting about that much). As long as I get any usable power out of it, I'm happy. It's been a long project for me: I started constructing the turbine in July of 2008 and it got put up in May 2009.

I'd encourage anyone who is interested in wind power to do their own project, it will help you learn a lot and is very rewarding. The view from on top of the tower is an added bonus.

Step 18: Project Updates

This turbine was installed quite a distance from where I live so it was hard for me to remotely monitor the site. It would have been nice to live near it so I could tweak things and monitor it's performance regularly. I also never got around to running wiring to the house which would've enabled me to put the power to better use. However, the cost of big running large wires was pretty prohibitive. Also I had some trouble with the light I used, the electrician installed a bulb that required a ballast and I burned up an inverter trying to drive it. Be warned that those modified sine wave inverters you can buy at your local auto parts store don't like to drive inductive loads. 

The biggest problem was with the welding on the tail section. As you can see from the pictures it wasn't very well supported and overtime the support pin for the tail fatigued and failed. The tail fell off and got bent up on impact but luckily nothing was damaged and no one was hurt. I would recommend a more robust support solution for the tail. 

I started building another machine but got distracted during college and it's sitting in my shed at home. Fast forward 4 years and one electrical engineering degree and now I can really make some improvements!

Thanks for all the interest in this instructable, even though it is quite dated I do my best to keep up with the questions and help anyone along.