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Old 02-06-2010, 01:14 AM
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Originally Posted by wingnutt View Post
Jjw – I am curious as to the value of measuring the DC resistance of a motors coils. Rm I can understand, since that can be used to help predict the performance capabilities of a motor in use. Other than checking for shorts or to make sure you didn’t miss a turn in rewinding, I cannot see where the value is useful, but would like to know if I am missing something.
A motor coils DC resistance is not the same as Rm and shouldn’t be used for performance prediction. I recently built a milliohm tester, using Lucien’s LM317 current source plans. On my most recent rewind, the DC resistance (Rdc) is 0.047 Ohm and the calculated Rm (at operation voltage) is 0.113 Ohm. Other motors I tested had the same discrepancy between Rdc and Rm.

Please understand that I really am curious and not trying to be contradictory at all. I just don’t want to not miss out on something.
Hi... For the purposes of estimating DC motor performance, Rm (or terminal resistance) is the motor coil DC resistance... I typically measure this using a constant current source and feeding this current through the coil, measuring the voltage across the coil, then using ohms law to calculate resistance.

When you say you calculate Rm, I can only guess that you are running the motor at some known volltage, current and load conditions and then back calculating the coil resitance... all completely valid.

The fact that you find different numbers than a measured DC coil resistance is probably due to other losses seen in real world motors... magnetic (or iron) losses (eddy current and hysteresis) and mechanical losses (friction and windage)...

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