Measuring Motor Resistance Value "R"

[mclovin]

Hello All,
As the subject says how do I measure the motor resistance? Does it vary for brushed and brushless? Somehow I don't feel it is as simple as just whiping out the ole multi-meter....or is it? I thried that on my brushed WE BD36 and read values that we all over the place. I suspect that the brushes are interfering somehow but I'm not sure.

Cheers.

 

[swbluto]

Your old multimeter's resistance measurer is only good for values down to somewhere around 1 to 10 ohms and above. A typical 1000 watt motor will have a resistance less than 3/10ths of an ohm, so there's your problem. What you need is to use "the four wire method". Or, basically, generate some small current through the motor(If you have a power supply, great, just use the constant current controls. If not, then.. uhhhhh... you could build a circuit to generate the current source, but that may be more work than you're looking for) so that it won't rotate(A brushless won't rotate) and measure the voltage drop across the wires. Use the formula R = V/I ( resistance = voltage divided by current), and you have your answer. A multimeter can measure small voltages very accurately, so that's why this method is so accurate. Or conversely, generate a small voltage, measure the voltage drop and measure the current and calculate the same way.

With a brushed motor, doing this might cause the motor to rotate, but if that's the case, just reduce the current so that it won't. With a brushless motor, there are three wires with each combination of two being an independent phase. Basically measure the resistance of each combination(There'll be three) and that'll be the resistance of each phase of the 3-phrase motor. I'd take the average measurement to be the "resistance of the motor".

 

[mclovin]

Thanks, swbluto.

 

[rkosiorek]

for good resolution you need to be able to pass 10A or so. a good stable source is a couple of fully charged "D" NiCads in parallel. use a known resistor 0.1R 10W or so. wire it in series with 2 of the BLDC motor phase wires. quickly measure the voltage drop across the resistor and the motor.

the resistance will be in the same ratio. or if math isn't you thing - Rmotor = (Rresistor * Vmotor) / Vresistor.

tough to do on a brushed motor unless you take it apart and measure at the winding. the reading will be affeccted by the resistance of the brushes.

when doing this with a BLDC motor it will step or cog to a fixed position. usually a fraction of a turn. once it cogs it lock into a fixed position until the circuit is broken.

the resistor is meant not only as a measurment reference but also to limit current to a safe level.

rick