How to measure stepper motor

It's hard to stay up to date when you are floating in a sea of technical jargon. However, it's imperative that engineers know the terminology associated with the areas in which they work. And that's especially true if their assignments take them outside their engineering discipline. For example, a mechanical engineer specifying a stepmotor should understand the associated mechanical and electrical terms.
Stepper motors need the right current if they are to work correctly. Without it, the motors can overheat, miss steps, and even freeze in their tracks. Yet the one electrical specification that most confuses all engineers, from the recent graduate through seasoned veteran, is the rating for stepmotor current. No doubt this happens because stepmotor-current ratings come in many forms such as amps/phase, amps RMS, average current, and even amps peak current.

Basics of stepper motor torque

Stepper motor holding torque is one of the main specs of any stepper motor. It is a simple indication of the "strength" of the nema 23 stepper motors.

Stepper motor torque is usually measured in oz/in or ounces per inch. The picture above shows what that measurement means, and a method of actually measuring it. If the motor can HOLD a weight of 100oz on a 1 inch radius pulley it is said to have a stationary "holding torque" of 100oz/in and is therefore sold as a 100oz/in motor.

The picture above shows the same 100oz/in motor but with a more sensible measuring system. Increasing the pulley radius gives greater leverage for the weight, so we can use a smaller weight, and also gives a lever length (10") that is easier to make and will measure more accurately.

As with all leverage ratios increasing the lever length means decreasing the weight accordingly, so instead of 100oz/1" we use 10oz/10" at 10:1, (the result is still 100oz/in).

Measuring stepper motor torque

To measure the holding torque the leadshine m542 does not need to rotate, so the pulley can be replaced with any simple lever.

I used a plastic ruler. The lever distance was 25cm and used the hole that was already on the plastic ruler. Balancing the lever with a simple counterweight can be done using a piece of string. This also compensates for the weight of the measuring cup. I used a flat plastic food cup and 3 strands of fine wire. Everything was glued together in seconds using hot melt glue which can be easily "broken apart" afterwards.