I'm trying to measure the resistance of a brushed DC motor, as part of information I need for a motor simulation model. I tried connecting a multi-meter across the motor terminals and measuring resistance, but if the shaft rotated a little I would get a different reading.
How do I measure the resistance of a brushed DC motor? Thank you, I realize this is kind of a dumb question.
The DC resistance is only a small part of the motor's useful parameters. It will give you an idea of what the initial inrush or "stall" current will be.
Don't allow the shaft to turn while measuring the resistance.
The more important value is the back-EMF when the motor is running. To measure this you need to know the RPM that the motor is turning when you measured it. One way is to turn it at a known RPM (maybe with a battery drill or something) and measure the voltage it produces. That will give you RPM per volt, which is extremely useful when you want to know how fast it will turn when powered by a specific voltage.
Depending on the size of the motor, you can generate some dangerous voltages this way. Don't hold the test leads on the pins by hand. Use alligator clips or more permanent connectors. You also have to keep your fingers and loose clothing away from the rotating parts. Take the extra time to set it up properly, maybe with a wooden jig clamped in a bench vise.
measuring resistance, but if the shaft rotated a little I would get a different reading.
The brushes and commutator can be oxidized or damaged and make a temporarily high resistance contact, so you should rotate the motor shaft to several different positions before taking resistance readings.
Take the lowest reading you get as the best estimate. The stall current will be the motor supply voltage divided by that resistance, and a suitable motor driver must be able to handle that current.
Yep! Or at least be able to limit the current to protect itself or the motor.
If the resistance varies significantly with the rotor in different positions either:
Its a 3 or 5 pole rotor (ie a small motor) - this is normal, the variation is significant.
Or its a multipole (12+ or so commutator segments) motor with burnt out windings
on the armature.
Or the brushes are worn-out/broken