# Stepper motor torque constant measurement

#### Amirmkr

Joined Apr 8, 2022
1
Hi, I have an MST342C02 stepper motor (200 step/rev and 4 phases) with SMD42C2 Driver. I have a converter which gives 48VDC 12.5A from 110VAC. I have Teensy 4.1 as the microcontroller and using Accelstepper library. The motor is connected Bipolar Parallel. Some of the specs are as follows:

(Driver)
Holding Torque = 9 Nm
Running Torque (low speed) = 7.2 Nm
Phase Current (A) = 9.5 (parallel), 4.7 (series)
By looking at the driver manual, for a 4 phase motor in parallel the max current is 1.41*(Nominal current per phase). So for my configuration we would have 1.41 * 9.5 = 13.4 A .

I set resolution at 10 meaning 2000 ministep/s = 1 rev/s. Since I use Teensy with 600MHz clock speed, I have no problem at 5rev/s or 300 RPM. This speed in motor datasheet graph is 7 Nm. I run the motor at 300 RPM with PWM 255 that gives the max torque. The Measured current at such speed is 1.48 A. Also the measured holding current is 0.4A. I searched the net for the torque constant and some said it's (Holding Torque)/current = 9/13.4 = 0.67 but this doesn't give the 7 Nm at 1.48A.

#### Sensacell

Joined Jun 19, 2012
3,442
There are many complex factors that will alter the motor torque output, you need to measure it.

Joined Jul 18, 2013
28,669
A stepper motor requires the constant rated plate current applied throughout the RPM range.

#### shortbus

Joined Sep 30, 2009
10,045

#### Sensacell

Joined Jun 19, 2012
3,442
I might not be understanding you but.... If your after full rotation of the motor, why are you using mini(micro is the common word used) stepping? Using anything other than a full step causes less torque output in a stepper. Read this link -
https://www.machinedesign.com/archive/article/21812154/microstepping-myths
It's implying that the detent or holding torque is not constant, you get "rubbery" static positions as you micro-step, but the performance slewing through 1000's of steps will not be diminished.

It will run more like an AC synchronous motor driven from a sinusoidal drive, with less torque ripple caused by the discrete step-wise drive you get when running in full step mode.

#### Danko

Joined Nov 22, 2017
1,834
It's implying that the detent or holding torque is not constant, you get "rubbery" static positions as you micro-step, but the performance slewing through 1000's of steps will not be diminished.
It will run more like an AC synchronous motor driven from a sinusoidal drive, with less torque ripple caused by the discrete step-wise drive you get when running in full step mode.
The angle between the rotor poles and stator poles in a synchronous motor is known as the Torque angle.
More angle - more torque. But in microstep mode stepper motor torque angle is limited by full step/microsteps relation.
Therefore table below is true for dynamic torque too.

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#### Sensacell

Joined Jun 19, 2012
3,442
The angle between the rotor poles and stator poles in a synchronous motor is known as the Torque angle.
More angle - more torque. But in microstep mode stepper motor torque angle is limited by full step/microsteps relation.
Therefore table below is true for dynamic torque too.

View attachment 264801
This does not make any sense from the perspective of general performance observed in the wild:

If a motor truly outputted only 0.61% of it's nameplate torque when operated at 256 micro-steps per step, the whole concept would be useless.
Motors would need to be over specified by so much, it would be absurd. If I needed 10 gram-CM of torque, I need a 1600 gram-CM motor?

While this is true for static positioning applications, when you are slewing at speed, the inertia of the rotor and it's load average out the torque.
At each multiple of N steps, it puts out 100% torque. There is zero difference between micro stepping 0, 256, 512 , 768 and whole steps 1,2,3,4...
At these points the conditions are identical.

The torque will be less when micro-stepping, but the article cited states "significant impact of the incremental torque/microstep as a function of the number of microsteps/full step"
This is only partially relevant in the most applications- The key word is "Incremental"

#### Danko

Joined Nov 22, 2017
1,834
The torque will be less when micro-stepping, but the article cited states "significant impact of the incremental torque/microstep as a function of the number of microsteps/full step"
This is only partially relevant in the most applications- The key word is "Incremental"
Thanks, understood.
Seems these numbers from 100% to 0.61% simple are amplitude of torque vibration.