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Old 07-16-2013, 12:28 PM   #1
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Default Measuring motor resistance

A on road racer I met recently told me that he measures the resistance of his 17.5 motors to see which one is best. Lower is better he said.

How do you do that? Can you use a voltmeter with resistance settings? Thanks for any help.
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Old 07-16-2013, 12:52 PM   #2
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I think you need a milliohm meter, which is a pretty specialized bit of equipment.
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Old 07-16-2013, 12:54 PM   #3
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Old 07-16-2013, 01:14 PM   #4
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Do a web search for the MMS Complete system -
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Old 07-16-2013, 01:22 PM   #5
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I use a standard handheld DVM with a 200mV (0.1mV resolution) range in conjunction with a simple 1A current source that I made myself. It's not calibrated to NIST standards, but is pretty close. Since I only use it for comparison purposes, it works fine.

You'll need to measure the motor temperature too, since the resistance increases with temperature.
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Old 07-16-2013, 04:53 PM   #6
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Old 07-17-2013, 06:15 AM   #7
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where would you measure on the motor?
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Old 07-17-2013, 07:15 AM   #8
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Quote:
Originally Posted by Nathan Wilson View Post
where would you measure on the motor?
Across the terminals, A, B, and C. That will give you three numbers:

A to B
A to C
B to C

I use an average of these three numbers, and compare it to the average of another motor measured with the same test equipment. As you have mentioned, lower resistance = more power.

The motor can remain connected to the ESC, since the stator has a much lower resistance than the ESC MOSFETs have in their "off" state. Make sure the battery is unplugged!

Use a thermometer to determine the motor temperature during the resistance measurement, then correct the resistance reading back to a reference temperature (traditionally 20C or 25C). (An infrared thermometer, like that used to check maximum motor temperature when setting gear ratios, will do nicely.) The correction factor I use is -0.393% per degree C.
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Old 07-17-2013, 12:04 PM   #9
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Sounds like Howard built the same thing I did years ago for brushed motors (and still use for brushless). Parts usually available at Radio Shack (if you can get their attention away from the cell phone accessories aisle )

A circuit schematic is attached. It only puts 1/2 amp into the motor (brushed motors would try to turn at 1A), for 1 amp change the resistor to 5 ohms (or 2-10's in parallel).
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Measuring motor resistance-motorcopbl.gif  
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