Measurement of gear DC motor characteristics

[fishandchips]

You can also determine the internal resistance by stalling the motor with full applied voltage and measuring the current.  Again, you need to take both measurements simultaneously and be quick about it.  The ammeter is getting hot!  (probably)

For determining the internal resistance, what is the different between using this method vs. measuring the resistance of the motor by connecting the two motor terminals to the multi-meter probes when the motor is still (i.e. without the applied voltage)?

When people talk about measuring the "internal" resistance, do they mean I have to take apart the motor? Is it valid if I just measure the resistance by connecting the probes to the two terminals coming out from the base of the motor?

[fishandchips]

I connected a small motor hub to the gearhead of the motor. Then, I measured the voltage, current and speed of turning using a tachometer. I am not sure if the added weight/inertia due to the 12g hub could be neglected in the calculations. Here is the data:

Supplied voltage: 11.1V
Measured voltage across the motor: 10.2V
Measured current in series with the motor: 1.2A
Speed (with the gearhead and hub): 78RPM (8.17 rad/s).

I also removed the gearhead and measured the voltage across the motor and current in series with the motor while the motor was stalled. I did 10 runs using an applied voltage of 5Volt (higher voltage caused smoke). Using R = average of measured V divided by average of measured I, I obtained a resistance of 0.1 Ohms. Is this the resistance of the coil inside the motor?

Based on these data, can I calculated the Back EMF constant, Torque constant and Voltage caused by the Back EMF? I believe the Back EMF constant is the same as the torque constant.

Any advice appreciated. Thanks.

[rstofer]

I would expect the meter measured resistance to be the same as the current derived resistance.  Is it?

[fishandchips]

I would expect the meter measured resistance to be the same as the current derived resistance.  Is it?

Am I doing something wrong? By measured resistance, do you mean connecting a multi-meter to the two terminals of the no-load motor and measure the resistance? By current derived resistance, do you mean stalling the motor and measure the current and voltage. Then, use V/I to derive the resistance?