Stepper Torque

If you could know many of the internal parameters of the motor, you could calculate the anticipated torque from the applied voltage and current. But since you don't know even know if these are inductive or permanent magnet motors, it's not worth trying to calculate this. It will be better to just measure the variety of torque values. I seriously doubt that a stepping motor from a printer can be damaged by any stalling action you apply to it. Keep in mind that the available torque will change with the both the current applied and the running speed of the motor.

Hi,

if you do the test with the same current as the motor is rated you will not burn anything.

If you have only the voltage this is different as at speed the emf-voltage U_ind is counteracting the supply-voltage U_batt so the current is I = (U_batt-U_ind)/R_coil.

If you have only the maximum allowable housing temperature, then make a test and put some current through the coils, low enough so nothing will be damaged. Measure the housing temperature and the electrical input data, and calculate temperature rise divided by electrical power. This is the thermal resistance of coil to housing. Then scale up the input power until you are at your wanted or allowed housing temperature (by calculation or experiment).

A string is not too good as force transmitter as usually too flexible lengthwise, I prefer a lever with an adjustable weight at adjustable distance.

Have in mind that this torque you want to have is sinusoidal with the angle between rotor and magnetic field vector (not seen in this test). So a force or a torque sensor would be best but your arrangement will do all you need.

At speed the available torque is different depending on the motors resonances and the drive circuit that is used.

This can be better checked in the start-stop-mode if the motor can follow with its torque and rotor-inertia to stepwise changes of drive frequency.

Last test: run at slowly rising frequency with or without external load until the motor stops.

Be careful with calculating and measuring internal power loss as the coil resistance will go up with temperature with near 0.4%/°C.

I did something similar with stepper-motors extracted from 1985? IBM HDDrives (then reputable 20MB!).

Have success!

RHABE

Put a small drum (sewing thread spool or similar) onto the shaft, with it horizontal. Attach string to it, let it go to floor, where it will attach to one end of light chain piled in a heap. Run the motor, which will lift the string, and the chain; as it continues, it will be supporting more and more chain, until the lifted portion's weight times the lever arm equals the stall torque. If you already know the weight per foot for the chain, a tape measure will quickly provide the weight by measuring from floor to top of chain when the motor stalls. Quick, simple, repeatable (or was when I used the method to test slip torque of a miniature clutch 35 years ago! I can probably still lay hands on the very spool I used . . .). I had a cardboard strip with height marks in ounces next to the string, so that the instant it slipped, I could read the weight and shut off power, letting it unspool and be ready to go again. Sash chain proved suitable for my test; bead chain worked but I needed about 4 strands of it in parallel to get reasonably quick test without mounting the clutch 10 feet in the air.

Provided you stay within the rated voltage/current/timing of the motor, even holding the rotor still should not damage it.

You should also consider as to whether you want to "step" or "Microstep".......that changes the torque and the smoothness of operation and the frequencies where the motor just stays put!!!!

A good chip will give you all the options you need, I personally prefer the newer Allegro chips for ease of use and easy interfacing....if you want chip numbers just ask me again....