link to Home Page

icon Axle Math

Here is where the math comes in, it's pretty straight forward stuff. All formulas that we will use here are in meters, we can convert them into inches when we are finished. The formula to compute the diameter of any home made wind generator comes from Hugh Piggott a world leader and author involved with home made wind power. This approach is actually a little backwards as we are starting with only one variable the generator.

Here it is,

Diameter in meters = (P/Cp*(24.5*TSR*G/RPM)^3)^0.2


P = Output in Watts
Cp = Power coefficient <Cp>: Default = 0.2
TSR = Tip speed ratio <TSR>: Use 7 here
G = Gearing up ratio <G>: Use 6 here
RPM = generator rated r.p.m. <RPM>: Use 2000 here

The other integers in the formula are " Constants " that are not changed for horizontal generators. All of the variables that I have used are conservative ,as they should be for a home made wind generator. Because the material for fabricating efficient blades will be difficult to find I have used a tip speed ratio of 7 for a three bladed hub that will still be fairly efficient if using rather crude blades. Power in watts = 14 volts X 40 amps (low end production to assure power production at low level wind conditions) = 560 watts. So our formula now reads.

= (460/0.2(*24.5*6*9/2000)^3)^0.2
= 3.66 meters x 39.37
=144.09 inches for the rotor diameter.

Now we have a rotor diameter, in a day or two we will size our blades area for optimum performance under certain wind conditions. The wind generator I have described above has now been in operation for a little over three months requiring no repairs and no maintenance. Just a note: for those of you having trouble with integers, and don't have a scientific calculator, download Sicyon V1.5 and use the arithmetic function. I don't remember the web site but it should be easy to find.

Offered by Jay.