Investigating unknown stepper motors

[AlainB]

Hi,

Here is a method (absolutely not guaranteed to always work) of finding an approximation of the specifications
of Nema 23 stepper motors, presuming they are older ones, from obsolete printers or alike.

a) - Find out the resistance per phase of the motor.
b) - Presume the consumption to be 5 watts for short motors and 7 watts for the long ones.
c) - Find the square root of the resistance multiplied by the consumption; this will give you the voltage.
d) - Divide the presumed consumption by the found voltage; this will give you the amperage.

Not guatanteed!!;)

I did test many old Nema23 motors that I have with known spec. and the consumption is very often near
this 5 or 7 watts. I did not test any of the newer high torque motors or motors of different size.

Let's try it!

This is a HP Laserjet III motor.

From my multimeter I read a resistance of 3,9 Ohms per phase and since it is a long motor, I will presume
a consumption of 7 watts per phase.

So Square root of ( 3,9 * 7) = 5,22 volts
and 7/5.22 = 1,34 Amp.

The spec. written on the motor: 5,2 volts/phase and 1,4 A/phase.

Perfect fit for this one. But it is not always as near. However, I doubt that it would be possible to harm a motor using
these calculations.

For the torque, I know from the web that this particular motor is rated at 100 oz/in. This is the holding torque. I dont know
the metric equivalence ( maybe 70n-cm ) or if it is when only one phase is energised. But it would be easy to find out a
good approximation by tying a rod to the shaft of the motor and moving a weight until the shaft move.

Please take everything that I said with a grain of salt. I am really not an expert!

Alain