All - I’m wondering if I am going crazy here. Most likely, I am misunderstanding something…
I have a surplus brushed PMDC (permanent magnet direct current) motor. All specs I can find on it say it’s stall current at 12 volts is around 25 amps. So far so good.
I found a couple of different methods to find the stall current of a (mostly) unknown brushed PMDC motor.
The first was to take the terminal resistance (resistance of the coils of the armature, via the commutator), then apply Ohm’s law for the working voltage to get the current at stall.
When I measure the terminal resistance (several times, rotating the shaft before each measurement), I get an average of around .45 ohms. At 12 volts working voltage, this calculates to around 26 amps (I = V / R).
The second method I found was to use an adjustable power supply, so I tried that, too. According to this method, I take my adjustable power supply, and set it for 3 volts (a fraction of the working voltage). I hook up the motor, and it turns on drawing some small amount of current (no-load current at 3 volts). I grab the shaft, and stall it out. The current when it is stalled is 1.32 amps.
You are supposed to then figure out the resistance of the motor by:
Resistance (R = V / I = 3 volts / 1.32 amps) is 2.27 ohms.
Then use that resistance, and your actual working voltage, to figure out the stall current. So in my case, at 12 volts, current at stall (I = 12 volts / 2.27 ohms) should be about 5.3 amps.
What am I doing wrong, or misunderstanding, or both…since these numbers for the resistance and current are not anywhere close to each other (and only one of them is close to what specs I can find)?