"Jon Elson" wrote in message
...
Existential Angst wrote:

Awl --

How do they determine motor or generator efficiency, in industry?
Seems to me the only fundamental way to do this would be to have the
motor
lift a weight, calculate mgh, and compare to ExI x power factor x time.

Then, for a generator, have the motor drive a generator, measure ExI
through a resistor, compare.
Kind of. But, you can calculate mechanical power with a Prony brake.
You either put the motor housing on a bearing and have an arm press on a
scale, or have the brake system on bearings and press on the scale.
RPM times torque (the weight on the scale) can be converted easily to
power.

How would a diy-er determine motor/generator efficiency?

If the generator is bolted directly to the engine, it is really hard.
If you can get the two separated, then the Prony brake system can be
cobbled
together with really simple stuff, like bathroom scales.

For a small one, you can run a DC motor (using it as a generator) with a
variable resistance, with the motor base attached to an arm, and the other
end of the arm attached to the scale. You don't have to worry about how much
power the generator is producing. It's the torque, measured by the scale,
that matters. Then measure rpm and you've got it.

This is done for small electric motors and such.


A related Q:
Why don't dynomometers measure generated current through a resistance,
rather than calc'ing torque x rpm? Seems to me you would have better
control of the load with I^2xR, and varying R.

Small dynos are often electrical, and that makes load control and reading
power easy. When you get to big dynos, like for car or racing engines,
then you have the problem of what to do with hundreds of KW of heat.
Water dynos have the same problem, of course, but the water can be cooled
with a water-water heat exchanger. A big resistor could also be cooled
with water, and the load could be controlled by varying the generator
excitation.

Measuring output of the dyno brings in complications of the *dyno's*
efficiency, etc. It's not accurate without a lot of calibration.

Just running the dyno as a brake -- like the water brake on some dynos or
the friction band on a classical Prony brake -- gives a much more accurate
result.

That's for measuring absolute values of horsepower. If you just want
relative values, you can just measure the output of the dyno.

--
Ed Huntress