# Relating Stepper Motor Current To Torque

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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

Re: Relating Stepper Motor Current To Torque
Hi, winding resistance will make no difference. Torque is proportional to
current, it drops off with motor speed. You should be able to get a graph for
your motor from the data sheet.

Re: Relating Stepper Motor Current To Torque

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

Re: Relating Stepper Motor Current To Torque

[...]

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.