# Cupla Generator Q's.... efficiencies

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

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Existential Angst wrote:

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.

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.

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.
Jon
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Energy losses in electromechanical systems all eventually resolve into heat.
Obtain an accurate measurement of how much heat is generated by the system, and subtract that amount from the total input energy--the result is your actual transation efficiency.
IOW, if it acts like a giant space heater or the motor generator set gets so hot that you could cook a friggen egg on it then unless it's cold in the shop or you have eggs to cook......
FWIW: where electric space heating is used, there's no advantage to using compact flourescents except perhaps during summertime when it will cut down on the need for air conditioning.
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Indeed. And reversible electric ceiling fans are a shop's best friend....
The only way I can see anyone really assessing true motor efficiency is with mgh, and a true wattmeter -- or an oscilloscope. Shaft hp is a measure of physical work/power, so it must somehow be measured, and compared to total energy input. Possible in a diy shop, I spose, but not in mine.
I spose I could call up Baldor and ask them how they determined their top motors are 98% efficient.
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EA

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As a practical matter, this is almost impossible to do. That's why real-world dynamometers have relied on mechanical measurements of torque and direct measurement of rpm, for many decades. They still do, although the acceleration type has become popular for small motors, up to 50 hp or so. Cheap computers make them very practical.
But among standard types of dynos, used for measuring horsepower at a fixed motor speed, even the ones driving generators usually just use the generator as a brake. If you measure electrical output, and use it as a proxy for shaft horsepower, allowing for losses, you have to calibrate the thing, frequently, against a mechanical dyno that works on the Prony brake principle.
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Ed Huntress

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Ed Huntress wrote:

Good to have you back, Ed...
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Richard Lamb

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Gee, thanks, Richard.
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Ed Huntress