Efficiency of a motor

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Sounds ok but is there a prof way of measuring torque as this will look a bit Heath-Robinson. ie can I buy some sort of torque device to put on teh shaft?

H.

What sort of budget/accuracy do you need? How large a motor?

If this is on a budget, you can measure torque by rigging the motor mount up such that it pivots about one end of a lever with the other end resting on a bathroom scale. This wil depend on how you will be applying a mechanical load to the motor (a water pump and some sort of metering valve works well).

RPM can be done with a paint mark on the shaft, a photodiode and misc other components and a frequency counter.

Look for "torque sensor" or "torque transducer".

"HardySpicer"

Bit similar to Hfe in a BJT - it varies with every damn thing else !!

Consider that you can measure efficiency very simply if you know the input power and the rate of heat loss. The difference of the two is output power.

For an AC induction motor ( you mentioned PF ) the copper loss dominates and that can be found from I squared R, given that I is an rms value. Add a nice guestimate for iron losses and windage and you have it.

..... Phil

I think it was Joule who put paddles in water and measured the temperature rise.

George H.

Strain-gauge based torque transducers are available, and will likely be the most costly of the three instruments you mention (and will require the most intrusive installation).

You can measure torque with a Prony Brake, and maybe something like a Kill-O-Watt to measure power, albeit I don't know if the KOW deals with power factor.

Good Luck! Rich

The Kill-a-Watt does measure power factor; also volts, amps, watts, VA, Hz, KWH and hours.

"Robert Baer"

By simple phase displacement of the proper way.

..... Phil

Sampling, I(t), V(t), and T. Throw in a little computing and it all falls out.

"krw" "Phil Allison"

But how do YOU know that is what the unit does?

.... Phil

Yes.

A friend made a dyno for a small electric motor by driving a slip belt on a polished drum, with the two ends of the belt attached to spring scales. One of the spring scales was movable to adjust the tension in the belt until the motor was running at the RPM of interest, and the reading on the balances shows the force at the radius of the drum. Very simple, he knocked it up in 20 minutes.

Clifford Heath.

This apparatus, in a physics lab, is called a 'prony brake'. It would be fascinating to know the power output for (for instance) my air power tools as a function of speed and air pressure; maybe it's time I built one.

Thanks for the suggestion!

lots of dynos for cars is just a big drum with a know moment of inertia, logging the speed when accelerating and a little computing gives torque and power vs. speed rolling to a stand still gets you loss from transmission etc.

-Lasse

Unfortunately, this requires a specific acceleration for a specific force. You can't apply a specific force while maintaing a constant rotational speed.

That's not an issue for cars; the drum is sized according to the mass of the car, so torque causes acceleration as it would when driving on the road.

Thanks for telling me the name! The adjustable mounting was made using one of those adjustable clamps. Hang the fixed jaw from the frame, and the spring scale from the other jaw, and you can tighten the brake using the hand-grip. Very easy.

I know that those exist, just the same any shaft torque is exactly mirrored in frame and mount torque. If you can instrument the frame and mount, guess which is easier to measure.

Maybe he wrote the code in it.