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daveames
Newbie

Joined: 19/09/2009
Location: United States
Posts: 30

Posted: 02:39pm 09 Sep 2011

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Hi BobMann,

That's a nice looking machine you have there! curious if you have any power output data on it yet?

Guessing at the size of the thing but bet it's good for close to 200 watts in a stiff breeze. Well done!

This stuff is a blast.
cheers, dave

BobMann

Senior Member

Joined: 30/06/2011
Location: United States
Posts: 134

Posted: 03:45pm 09 Sep 2011

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Hello no Guessing needed the chart above is on the money. my RPM to wind speed is very close to 1 to 1 ratio

post to pack up data.
Perry wrote:
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I wouldn't normally hound someone for data but you keep plugging this and posting on multiple threads at multiple forums that people should buy your system. So is it out of line to ask for some performance data? What charts have you provided? I guess I missed them. All I have seen is Steve's predictions and models based on no testing. Maybe I am wrong?

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Perry, you have so little faith in my modeling ability. :-)

Just to get all the info out there, this is how I figure out the power (voltage, or current) from Bob's Smart Drive.

1) OCV = RPM * 1.0 (If I ever get more careful measurements, that 1.0 might be adjusted a little, but I think it's very close.)

2) Take a guess at R (resistance) of the alt, say 10 ohms or so.

3) Current (I) = (OCV - Vload) / R

4) Calculate R based on the current: R = 7.5 * (1 + .1 * I + .03/5*I^2 + .03/5 * I^3)

The R you calculate and your initial guess will be different. Go back to Step 2 and Guess at an R somewhere between your initial guess and the results of #4. When your guess and the #4 results are close (0.1 ohms or so), you are done.

This is the method I used to generate my graphs.

Bob recently gave me two current measurements. The real results and the results of my model are as follows:

81 RPM Measured short ciruit current: 4.69A Predicted: 4.76A Error: 1.4%

103 RPM Measured short circuit current: 5.10A Predicted 5.29A Error: 3.7%

For a model that I came up with in 45 minutes or I'd say that's a pretty good fit!

Steve from VAWTS.net

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