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Old 11-06-2013, 06:14 AM   #5434
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Originally Posted by howardcano View Post
There are very few multimeters that can measure low resistances accurately enough.

I use a homebuilt 1A current source along with a normal, run-of-the-mill DVM to make these measurements.

One can also use a power resistor and LiPo battery as the current source, with the resistor being about 1 ohm per volt to give about one amp. This is much cheaper than buying a dedicated milliohmmeter, though not as accurate. It works fine for comparing motors.

At the moment, there is no ROAR rule regarding motor resistance. (If resistance is measured, it is only used as a flag to indicate that a motor might need further examination.) I have written resistance rules, but they have not yet been ratified. Here is the suggested rule for 25.5 motors:

"The resistance between terminals A to B, A to C, and B to C must be at least 102.1 milliohms at 25C, when measured over a range of 20 to 30 degrees C and multiplied by a correction factor of (1-((T_measured-25)*0.00393))."
Thanks for chiming in, Howard! Understood. And the quick check is for weeding out folks who like to plop in 21.5 stators (at an otherwise 25.5 location).
RJ Labrador
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