Is measuring the internal resistance of a brushed DC motor with a multimeter the correct way of doing it? The resistance seems to vary while the motor is being turned by hand; however, the resistance will somehow settle within some (reasonable) range eventually.
Is this the same resistance in IR compensation of speed control? I am controlling the speed of a DC motor without a speed sensor.
I measured the resistance of 30 brushed DC motors of the same model from the same manufacturer with a multimeter. However, the reading ranges from 3.x ohms to 6.x ohms and one of them even goes up to 12.x ohms. The inductance reading by an LCR meter reports 10.x mH to 12.x mH. Is this normal or more of a problem with the manufacturer? I previously assume that the characteristics of the same model should somehow fall in the same ball park.
Thanks in advance.
Brian
Best Answer
Measuring with a multimeter is OK, but what you need to do is stop turning the motor. When you do, the motor acts as a generator and the result (it's called back-EMF) messes up the multimeter ohms circuit. Even better, fix the shaft in place and put a few volts (say, 10% of normal operating voltage) and measure the current. Then compute R = V / I.
You would expect fairly consistent R and L values from the same model motor, but, as indicated above, turning the shaft while you take the reading will give you false results.