(ET) Ideas in an ET-ish context.

[Max Hall <mhall maxmatic com>]

I have assembled a two-bicycle electrical generating station for a bike event at a local children's science museum. (fellow ET-er Joel P and I gave them an E-15 a couple of years back.)

The goal:

to quantify the work produced by pedalers on this contraption.

Why I'm stuck:

My plan to generate AC directly was flawed; the threshold rate of rotation for the generator head is so high that no kid is going to be able to get the thing to produce useable power. (There seems to be a self-excitation dimension at work... crank faster and faster and the AC voltage creeps up from 0 through tenths of a volt, then, when you're really cranking, it bursts up to 70V and then climbs up from there.) Of course, AC would have been sweet, because I could use my Kill-A-Watt watt-hour monitor to quantify the output very easily!

What I'm thinking right now:

I have a nice permanent magnet gen-e-motor I can use for the job; that would be fine. The catch? Voltage out is not constant, so my new idea for measuring total work produced is a no-go.

That new idea for measuring total work produced was this: IF voltage were constant and only current varied in the output loop, the the power produced is proportional to current in the output circuit (I*V). Thus, a little dc motor, whose no-load (virtually speaking) output speed is only voltage dependent could be run by putting it across a shunt resistor (like the current sensor in our beloved tractors). The duration of the rotation is the duration of the pedaling; and the duration times power (P*t) is the work produced. A little "metering disk" on the motor's output shaft could show the work produced. (Like a kW-h meter on your house, but a DC version.)

SO:

1. HOW could I easily measure the electrical work produced even with non-constant voltage? (The little dc-motor would make a brilliant time integrator of the power output! I-squared-R is a little harder to capture... yes?)

2. any better ideas? (Other than lifting mass... despite how cool that would be ("Mom! I lifted one ton one foot in 45 seconds!"), It's hard to set up and a little more dangerous. BUT I'm not completely stuck on electrical output, either.)

Please "Reply Sender" since this is only tractor-tangent. Thanks, in advance, for any thoughts you have! 

Best to all you clever people,

-Max

-- 
www.maxmatic.com