Originally Posted by
Cain
Anyone got a suggestion on those tester devices guys are using? Not saying I would grab one now, but would be curious to see what they go for or at least know what are good numbers to look for on a motor if someone is selling one with ratings given.
Originally Posted by
Rodarbal
As in multimeter? Any decent one will do to read the DC resistance in milli ohms from coil to coil. I don't recall off the top of my head what the lowest allowable coil to coil resistance is for the Novak 25.5s at USVTA-sanctioned races.
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))."