Understanding small DC motor curves (torque, efficiency etc)

I have a small DC motor/gearbox unit and the spec sheets is shown below. All* the units are measured at the gear box output, so 100 RPM would be 3,000 at the motor.

I'm using it as a generator so am putting mechanical power (which I have calculated) into it, and, using a resistor as a load, getting electrical power out (measured with an oscilloscope).

In order to see how things are working, theory Vs practice, I need to know how efficient my motor/gearbox is.

Firstly I understand these curves are designed for use as a motor, so I'm A) using the motor as a generator, and B) driving the gearbox from the wrong end. So comments on what affect this will have are welcome.

Anyway, to get the efficiency from the supplied curves, I'm doing this:

A) drawing a horz line across from my speed (75rpm) in the top right chart

B) where that intersects the speed line (green, blue?) draw a vert line. Gives me about 50 oz-in

C) on the bottom left chart go vert from the 50 oz-in mark

D) where that intersects the efficiency curve look across horz to the c 18% figure.

Am I doing this correctly?

• bar the torque vs power curve which shows input power

• beware a resistor may not always be the ideal load. I also suggest you plot rpm vs efficiency once you take your experimental measurements. since you are measuring it, just take those motor specs, throw them out, and use what you observe in your measurements. There's insufficient data on the separate pieces to predict. Friction in gearboxes matters and is dependent on direction driven; for example a worm gear with sufficiently fine pitch cannot be backdriven.
– Abel
Commented Feb 3, 2022 at 10:11
• Thanks. The gearbox is regular spur gears, motor shaft bevel, rest straight cut. Could you expand on the resistor issues please? I get what you say about experimenting, that's what I've been doing. My RPM is largely determined by the load (resistor) so I was hoping the curves may help eliminate some of my unknowns (mechanical, electrical losses). Commented Feb 3, 2022 at 12:28