# Can I use stall torque to calculate torque constant?

I'm using torque control for my project, and dc motor manufacturer doesn't give me the number of their motor's torque constant and motor resistance. I can use multimeter to measure the motor's resistance, but I'm not sure about torque constant. I wonder if I can use equation Kt = torq/current with torque is max stall torque, and current is the current at max stall torque.

No, because the stall current is much higher than nominal. The torque constant notably drops on high currents.

• Is there any way that I can calculate torque constant with my multimeter? Sep 8, 2021 at 19:49
• No. You do need a scale and a current source. Sep 8, 2021 at 19:50
• The reason why the torque constant drops is that the core becomes magnetically saturated
– jpa
Sep 9, 2021 at 5:46

Does the manufacture give you the back emf constant? The torque constant and the back emf constant share the same units of Weber/radian. Thus if you have the back EMF constant, you can perform a unit conversion to get the torque constant.

"The SI units for the torque constant are newton metres per ampere (N·m/A). Since 1 N·m = 1 J, and 1 A = 1 C/s, then 1 N·m/A = 1 J·s/C = 1 V·s (same units as back EMF constant)." (https://en.wikipedia.org/wiki/Motor_constants)

• I think this is the best answer. Sep 9, 2021 at 5:29
• And back EMF constant is easier to measure, if you have a tachometer or other way to measure the motor speed. Then just apply constant voltage and measure speed.
– jpa
Sep 9, 2021 at 5:45
• Thank you for your answers. They don't give me back emf either, and I have been looking for a way to measure my motor's back emf Sep 10, 2021 at 16:59
• @Uchiwuwu, no-load speed and voltage will get you pretty close to the back EMF constant. No-load speed / Rated voltage = Kv (approximately). Sep 11, 2021 at 22:15

In general, maximum ratings are different from functional ratings and mixing them in calculation leads to errors. E.g. if you have a transistor with a beta of 100 and 1A maximum collector current, you certainly can't expect to find the corresponding base current as 1A/100, because beta changes with current. And maximum base current will be different yet, probably tested at 0 collector current.

In case of motors, "torque constant" also changes with current. It can only be considered constant in a certain working range, and a stalled motor driven with maximum current is probably outside that range.

In addition maximum stall torque could be derived from mechanical constraints (what bearings/gearbox can withstand), while maximum stall current could be derived from magnet properties (larger currents could kill the permanent magnets), so finding a ratio between the two is simply meaningless.

Torque constant is often not specified because you get the back-EMF constant (in RPM/V or rad/V·s), and you can calculate the torque constant from it as

If the internal motor friction is low, Kv can be easily found by measuring the RPM on a free-running motor driven by a known voltage.

What you can do is spin the motor at a known RPM somehow. Perhaps using a cordless drill to drive the motor mechanically. While it is spinning, measure the motor open circuit voltage using your volt meter. Measure RPM using a low-cost tachometer (you can buy one from amazon or ali-express or what have you).

You can calculate Kv (the voltage constant for the motor). It is just the RPM divided by the voltage you just measured.

Now go look at Real Magnetics's answer and read the wikipedia page. You can convert Kv into rad / sec /volt. Then invert it to get volt-seconds / radian. At this point, it is actually identical to Kt, the torque constant.