On measuring the internal resistance of a DC motor

[fishandchips]

Hello, I opened up the motor and found that it consists of three sectors. I plugged in the probes of a multi-meter at the areas indicated by the arrows to measure the internal resistance. I got 0L. I tried two more multi-meter and got the same zero reading. Am I measuring the internal resistance correctly? It could be that the resistance is so small that all three multi-meter do not have resolution high enough to do the measurement correctly.

I also used another approach by letting the motor turned and suddenly stalled it. Then, measured the voltage across the motor and the current in series with the motor. Using Ohms law, I calculated R =V/I. I did 10 runs and took the average of 10 Rs to get the internal resistance.

https://bbqbbq2bbq.smugmug.com/My-First-Gallery/i-fpFQz8H/A

[ebclr]

You measured the static resistance, In real life when running the Coal contacts need to be considered

[fishandchips]

Thanks. Usually, how many percent different between the static and dynamic resistance? Are they almost the same? Do you have a better way to measure the resistance than the second approach I mentioned?

[Zero999]

As mentioned above, you haven't included the contact resistance of the brushes but I doubt that's significant.

It's quite normal for a very low voltage motor to have such a low winding resistance, a typical multimeter won't be able to read it.

The winding resistance is only useful for working out the stall current. When the motor is running, it generates a voltage proportional to the speed, which counteracts the applied voltage, thus lowering the current.

[yada]

You measured the static resistance, In real life when running the Coal contacts need to be considered

What are those? Graphite brushes? Whats the difference in static and dynamic?
Did you know a motor is generating the most torque when its stopped? Opposite of a car engine.

[ebclr]

You should be measuring terminal resistance (resistance from one lead wire to the other lead wire) for IR compensation. You mention that you are measuring "internal resistance" which is often exclusive of the lead wires and brushes (i.e., it is measured from bar to bar on the armature). There are 2 ways to measure terminal resistance.

First, you can lock the rotor of the motor and apply voltage to the terminals. You increase the voltage until the current is at 25% of the rated full load current. Using that voltage and current, determine resistance using Ohm's Law. Repeat this a number of times at different locked rotor positions and then take the average resistance.

The second method requires only 2 measurements but it is a little more complicated test setup. This method requires you to back drive your test motor at a slow speed. Typical speeds are 30 to 100 RPM. While the motor is being back-driven, apply a voltage to the terminals (again, until the current reads 25% of rated full load current). Use Ohm's Law to calculate resistance. Repeat this test by either switching the lead wires or switching the direction of rotation of the back-driven motor. Then average the 2 measurements.

The locked rotor method is probably easier to setup but the dynamic test will give you better repeatability.