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I have an unknown DC motor with specifications I was testing. I used a Prony Brake setup and scale to evaluate torque, a tachometer for RPM, and multimeters for voltage and current.

I get a linear graph relating torque and RPM. However, when I physically stalled the motor and checked the stall torque, it was much lower than what the linear graph expected! Almost 1/2 the expected value!

Is there a physical reason to explain this? This outlier is pretty consistent through 4 trials. Is it possible it's due to the Prony Brake? Or perhaps something within the motor? Back EMF or something due to friction? I'm at a loss as to why such a data occurred, and not only once but multiple times!

Thank you.

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  • $\begingroup$ Were you getting the expected stall current? If the power supply was in current limit, that would limit the stall torque. Same if brush resistance had increased through wear. If you had the expected stall current but lower torque, has the magnetic field been reduced, e.g. after overheating? $\endgroup$ – Brian Drummond Mar 7 '17 at 12:22
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Torque should be proportional to current, whether stalled or not.

Measure the current at stall. Is it what you expected? It should be just the applied voltage divided by the DC resistance of the motor coils that are switched in.

Also consider that there can be significant ripple on the torque over a rotation. This is especially true for mechanically commutated motors. Try testing the stall torque and current at multiple points over one magnetic rotation. Note that there may be multiple magnetic rotations in one physical rotation. The multiple is the number of "poles" the motor has.

Back EMF has nothing to do with your observations. This is effectively a voltage in series with the motor that is proportional to speed. When stalled, the speed is 0, so the back EMF is 0.

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