Stepper motor torque constant measurement

Thread Starter

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.
 

shortbus

Joined Sep 30, 2009
9,372

Sensacell

Joined Jun 19, 2012
2,999
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
Hmmm this article is NOT saying you will get reduced dynamic slewing torque from the motor.
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
1,302
Hmmm this article is NOT saying you will get reduced dynamic slewing torque from the motor.
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.

1649667118153.png
 
Last edited:

Sensacell

Joined Jun 19, 2012
2,999
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
1,302
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.

More information about microstepping in attachment Faulhaber_AN015_EN.pdf.
 

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