Old 02-06-2010, 03:49 AM
  #37  
dirtybird
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Join Date: Feb 2010
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Originally Posted by MustangMan View Post
In the simplest sense Rm is the DC resistance of the windings between any two phase leads. Because it is substantially less than 1 ohm in any of our model motors it's a little difficult to measure with a standard multi-meter. The trick to getting a good measurements is to push a significant DC current, like an amp or two, through the winding and measure the voltage drop with a meter capable of millivolt resolution. The current must be known accurately as well. Knowing both the current and the voltage drop you can compute the Rm with Ohm's law ( R = V / I ). Note that you must measure the voltage drop at the point where the motor wires exit the motor, or very close to it to get an accurate measurement. Otherwise you'll be including the resistance of your test harness wiring in the measurement, which you don't want to do. Commercial instruments designed for this purpose use "4-wire" measurements rather than 2 wires like a standard DMM uses to measure resistance in order to eliminate the resistance of the measurement leads from the problem.
I have an adjustable lab power supply(Mastec 5020) that I can connect to two motor leads in series with a 10 ohm resistor and adjust to 1 amp within +/- 0.1V. I need the resistor or the PS thinks its output is shorted. I then verify the current with the clamp on meter. I then read the voltage at the two leads with a millivolt meter. The MV reading then converts directly to milliohms.
The Sears meter has a 200MV range. The Harbor Freight $3 meter also has a 200MV range. Both meters agree.
The resulting reading is not the coil resistance. The coil resistance is 3/2xRmeasured if the motor is delta connected. It is 1/2x Rmeasured if the motor is Y connected.
The measured resistance I get agrees with the MFG spec for the Scorpion and Eflight motors but not for the Turnigy.
The Turnigy runs with a lot of vibration in the no load test. I suspect its bearing wont last long.
The Eflight is very smooth running but it is an inrunner.
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