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10-19-2009

How I Built a Wind Generator in My Backyard for $150. Part 2.

Tools Required

Surprisingly, building a simple wind generator only requires very basic hand tools, and if you are desperate you won't necessarily need all of them. I used...

  • Jigsaw (or a hacksaw and a lot of determination)
  • Drill
  • (2) Drill Bits (1/2", 7/32")
  • Tape Measure
  • Crescent Wrench
  • Pipe Wrench
  • Protractor (to measure angles for the hub)
  • Sandpaper (various grits)

Parts Required

I wanted to be as minimal as possible with my design (I'm a poor college student), so I took the already simple designs from around the web and made them even simpler. All of the parts are available at any local home improvement or hardware store, and the entire setup can be constructed in as little as a weekend. Many of the parts you may already have lying around, and lots of substitutions can be made (instead of 1" steel pipe for the tower, you could use an antenna pole for instance). Here are the parts I used to build my generator...

  • 10" x 14" Steel Sheet
  • 10" x 1/4" Steel Nipple
  • 1-1/4"Floor Flange
  • 36" x 1" Square Tubing
  • 1/2" Bore Circular Sawblade (for hub)
  • 5/8" x 1/2" Arbor (to attach sawblade to motor shaft)
  • (2) Metal Straps
  • 8" x 4" PVC Pipe
  • 30" x 8" PVC Pipe (6" pipe works well too)
  • A DC Permanent Magnet Motor (preferably Ametek 30V or 260V 5A treadmill motor)
  • (8) 1/4" Bolts (with washers and nuts)
  • (2) 1/4" Sheet Metal Screws
  • 10-40 Amp Diode (the bigger the better)

All of the above parts (with the exception of the motor), can be picked up in a single stop to any large hardware or home improvement store. For the motor, the most popular types are old tape drive motors manufactured by a company called Ametek. The key is to finding a motor that puts out the highest voltage per RPM. For instance, the Ametek is rated for 30V at 325 RPM, making it excellent as an electricity generator. However, pretty much any permanent magnet motor with a good volt/RPM ratio will do. Keep in mind that if you want to generate useful electricity, you will need to produce at least 12V to charge deep cycle batteries or run an inverter. My setup can easily achieve 300-400 RPM in a pretty average wind (for Oklahoma). These instructions assume an Ametek motor with a 5/8" shaft, but can easily be adapted to other motors (search ebay for "wind generator" and you will get a listing of lots of good motors).

Blade Construction

Arguably, the most important part of a wind generator are its blades. A lot of people like to carve their own blades out of wood or composite materials. However, for the rest of us, it's quite easy to make a good set of generator blades from common PVC pipe (and the efficiency isn't too bad either). A 2-3 foot section of either 6" or 8" PVC pipe will do the trick. Before we go any further, here are a few blade theory quickies...

  • The longer your blades are the more "swept area" you have to gather energy from and easier your blades will spin in low winds, but the slower your rotation speed will be
  • The tips of the blades always spin faster than the base, therefore one needs to take into account the "tip speed ratio" (TSR) when designing blades (there is a reason why old farm windmills will spin all year long at 40RPM)
  • The power that can be extracted from the wind increases by the cube of wind speed (something like P=k*v^3 k=constant of wind generator, v=wind velocity)
  • According to the Betz Limit, only about 59.3% of power can be extracted from the wind (so in reality P=.593*k*v^3, assuming k accounts for mechanical inefficiencies in the generator motor)
  • The higher you get the generator off of the ground, the more wind it will be exposed to (the general recommendation seems to be 25-50ft., but I've had decent results at just 12ft.)

Cutting the blades for this machine is very simple. You will need to cut your PVC pipe into 3 sections, two 150 degree sections and one 60 degree section (I've attempted to illustrate this VERY APPROXIMATELY in my favorite CAD program--and by CAD program I really mean MS Paint). The red lines are cut marks. You will want to use a good tape measure and possibly some construction paper or newspaper to mark everything before you cut. The 150 degree angles will result in wide blades that start up in lower wind speeds, however this will lower the shaft turning speeds. In practice, you will find that the optimum angle could be anywhere from 75-150 degrees. The best idea is start out with a wide set of blades that you can always thin out later if you need to. Remember, measure twice and cut once!



After the blades are cut, I like to go ahead and smooth out all of the edges. If you want to follow aerodynamic theory, you can round the angled (leading) edge and flatten the straight (trailing) edge, but in practice I haven't seen this make much difference with PVC blades. So, you should end up with something roughly like these... 

To be continued ...

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