# Relating Stepper Motor Current To Torque

I'm looking to measure the torque generated by a stepper motor by relating it to winding current during steps. Has anyone done anything
like this or can anyone suggest some references?
I reckon the main problem will be change in winding resistance with temperature - is this correct?
Cheers TW
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For any high-performance stepper drive, the coils have to be fed from a constant-current source. It makes the point of winding resistances moot. The winding resistance only sets a requirement for minimum voltage-compliance of the current sources. In any reasonable system, a more significant part of the compliance requirement comes from the back-EMF of the motor (caused by rotation speed).
If the current sources are implemented as direct series-pass linear elements, the power dissipation in the current sources usually is many times the motor power, so in a system with more than a few watts of power, the current sources have to be built as switchers.
HTH
Tauno Voipio tauno voipio @ iki fi
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Ted Wood wrote:

[...]
How were you going to measure torque? I have an idea to build a small dynomometer - one of those projects that will probably never happen. I have the equipment, but putting it together is a challenge.
This involves using a good brushless DC servomotor and digital drive. The drive needs to be able to operate in a torque mode and provide output - probably in the form of an analog out or streaming data that corresponds to the various drive parameters - speed, torque, etc.
Hook the motor to be studied up directly to the torque drive. Create estimates of the viscous and dynamic drag of the servomotor. Adjust the torque of the servo to observe the curves of the motor under study.