Figuring out the stall current of a brushed PMDC motor...

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)?

Your first method is correct, except you want to use the lowest resistance found in Ohm's law calculation. Paul

For measuring very small resistances you need to do a 4-terminal (aka Kelvin) measurement, since otherwise the resistance of the meter leads and internal switches will dominate the result.

for sub-ohm resistances I use a bench supply and program a fixed current, and measure the voltage across the load with a multimeter. For instance programming 100mA and measuring 35mV would mean the resistance was 0.35 ohms. A naive measurement with a multimeter in resistance mode can often add 0.1 to 0.5 ohms on top of this due to stray resistances in the meter itself.

Note that to measure the stall current the rotor must be locked solid - otherwise back-EMF will ruin the measurement.

Paul_KD7HB: Your first method is correct, except you want to use the lowest resistance found in Ohm's law calculation. Paul

Ok, thank you - I might have to try MarkT's suggestion of voltage measurement, too.

But what about the other method I outlined? I found numerous examples of people mentioning it...why does it not yield a consistent answer with the first method (at least "in the ballpark")?

MarkT: For measuring very small resistances you need to do a 4-terminal (aka Kelvin) measurement, since otherwise the resistance of the meter leads and internal switches will dominate the result.

I figured that this might be the case, but had forgotten about the method you mention; I'll have to look into that again...

MarkT: for sub-ohm resistances I use a bench supply and program a fixed current, and measure the voltage across the load with a multimeter. For instance programming 100mA and measuring 35mV would mean the resistance was 0.35 ohms. A naive measurement with a multimeter in resistance mode can often add 0.1 to 0.5 ohms on top of this due to stray resistances in the meter itself.

Note that to measure the stall current the rotor must be locked solid - otherwise back-EMF will ruin the measurement.

I'll have to try this method out, too. My supply will allow me to set a current limit...should I set the voltage to something low (say 1 volt) or something closer to the operating voltage of the motor (ie - 12 volts)?

Thanks.

The motor voltage rating is irrelevant for the test. Only the resistance matters.

The output voltage only needs to be high enough to supply the programmed current into the total resistance. Start at 2-3 volts without the motor connected as this should be sufficient to get to 100ma. Connect the motor and you’ll see the supply voltage will drop to a very low value. The actual voltage will be what is required to generate the programmed current limit into the total resistance - which is the motor resistance plus the resistance of the leads and all other voltage drops in the loop.

If you don’t get the current you set the current limit for, increase the voltage. Once you’ve reached the current limit, measure the voltage across the motor with your DMM, don’t use the displayed value on the supply as that reflects the total resistance, not just the motor.