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Author Topic: Generator calculations.  (Read 6501 times)

Offline Fester

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  • Posts: 40
Generator calculations.
« on: October 24, 2012, 01:10:25 PM »
Calculating number of windings for a given voltage( what i could find on the web):

"Lets change this formula so we can use it to determine the number of turns we need for our coil. We know we are going to need around 14 volts to charge 12 volt batteries. We will change the 14 volts to a negative 14 volts to account for Lenz's law.
So with the use of a little algebra we get.
N = -1 * (-V/ change in (( tesla * area meters squared)/ seconds))
Now lets plug in the numbers.
V = 14.
Now lets figure the tesla. You will need to know how much Gauss or tesla you permanent magnet is rated for. This will give you something to work with because each winding will have a distance from the magnet that will determine the gauss or tesla that, that winding will be cutting through. The shape of the magnet plus the design of your coil and what your using to draw the flux through the coil will all determine the average gauss or tesla you will use in this formula.
I have a rare earth magnet rated for 6325 Gauss so I'm going to cut that in half and use the number 3162 Gauss just to get an idea of how many windings I'm going to need to get my 14 volts. I'm cutting this value in half because the 6325 Gauss figure is taken very close to the magnet, the windings of my coil will, on average, be farther from the coil.
10,000 gauss = 1 tesla
 3162/10000 = .3162 tesla
Tesla = .3162
Now lets figure the meters squared. My magnet is 2" by 1". So I need to convert meters square to inches square. One square meter is 1,550.0031 inches square. My magnet is 2 inches square.
1,550.0031 = one meter square
 2/1,550.0031 = .00129 meters squared (more or less)
Meters squared = .00129
Lets figure 5 turns per second, that gives us 300 RPM. If we do a good blade design we might be able to get 300 RPM in 3 to 5 mile winds.
5 turns per second gives us one turn every .2 seconds
 seconds = .2
This gives us the formula
N = -1 * (-14/ (( .3162 * .00129)/ .2))
 N = -1 * (-14/(.000407898/.2))
 N = -1 * (-14/.00203949)
 N = -1 * - 6864.5
 N = 6864.5
We get 6864.5 windings we need for our generator to give us 14 volts.
Of course with that many windings we are going to have to lower our estimated value of tesla we have, because with that many windings we are going to have to in crease the average distance the windings are from the magnet, which will mean that we need even more windings. To make this thing work we are going to have to increase the RPM's or the number of magnets.
You will probably have more then one magnet in your design and more then one coil. So this number divided up between a number of coils and magnets can become more manageable."

I could use a little clarification on a few things..
1) Would i double the surface area( in the equation) of the magnet for a axial flux, on a 9/12 set up would it be area of 24 mags or would it be the area of 12 since there is still one field between them technically?
2)Im 99.9 percent certain that on a 3 phase set up, the voltage from the 3 coils on the same wire will add. So if i set a target of 4 volts per coil, i should get 12 volts for the phase. But i could be wrong, and would need to be corrected.
3) On a 3 phase set up (with the way one coil coming on/coming off/ and completely off)... lets for instance say each phase is 12v, would the total output at peak be 36v or would it still be 12v because of the phase cycles.

I've never built a generator before. And im trying to get a grasp of some of the mathematics behind it versus just copying someone elses work. Im not even really that knowledgeable on electricity either, but one way to learn is to dive in