View Full Version : Investigating unknown stepper motors

13-02-2009, 10:43 PM
Having acquired a selection of stepper motors(and a few servos) over the last few years I've been interested in diy cnc, I realise I would have quite a problem identifying them. Internet searches rarely locate the exact model as so many seem to be specials rather than standard versions.
More to the point, discovering what use I might put them to, based on their capabilities suggests a need for some standard approach, using readily available kit, that would enable anyone in this position to do likewise.
What comes to mind is something like a basic set of instruments - a couple of meters, a simple pulse generator, a set of weights and bits of string etc,etc.
With this I would hope to be able to work out what max voltage/current might be used, and what sort of torque might be available at different pulse rates.

Is this a reasonable proposition, or has anyone actually spelt out a method along these lines ?


13-02-2009, 11:41 PM
As far as choosing volts/amps the starting point will be winding resistance and inductance. A good multimeter with a 0 - 10ohm and 0 - 10mH ranges would do the trick here. Coil voltage isnt an issue, steppers are rarely run at the rated voltage, its current thats the key thing.

Estimating the maximum current allowable is harder. I believe the best solution is to energise the motor in the locked state (2 windings) and measure the temperature rise, gradually increasing current and allowing the temperature to stabilise The maximum RMS current allowable is reached when the case temperature gets to 60degC or so.

This might also help... how to estimate torque from an unknown stepper from Mr Gecko himself...

1) Step motors are essentially 'constant power' motors. That means the product of torque times RPM is constant at higher speeds, meaning torque is the inverse (1/x) of RPM.

2) What matters is power because it's what gets things done. Power is torque times RPM. Since motor power is a constant at higher speeds, all you have to do is measure power accurately at a single speed to computationally derive what your torque will be at any speed. How do you do that?

2a) What you need is a paper towel, a pair of channel-lock pliers (we call them mole-grips) and a multimeter.

2b) Dismount the motor. Take a paper towel and fold it over as many ways as you need to until it is about 2" long, 1" wide and at least 0.25" thick (50mm by 25mm by 13mm).

2c) Fold the 2" (50mm) length around your motor shaft ('U' shaped). Wet the paper towel very slightly if more than 50W is expected from the motor.

2d) Place your channel-lock pliers around the folded towel on your motor shaft. The pliers will be the brake calipers, the towel will be the brake 'shoe'. The towel will dissipate the motor's mechanical power (by boiling the water used to wet it it) while protecting the shaft from being gouged by the pliers.

2e) Connect only one motor (the test motor) to the driver. Set your multimeter to 'DC Amps' and put it in series with your power supply to the driver.

2f) Run your test motor up to 1,000 or so RPM. Do it without the paper towel on the shaft. Measure the power supply current and write it down as 'Amps no-load'.

2g) Place the towel and channel-lock pliers on the motor shaft. Very gradually apply pressure to the plier handles; watch the current as you do, it will increase. Keep increasing pressure while watching the current until the motor stalls. Catch the reading at the instant of stall. This may take several tries until you can sneak up to that point accurately.:-) Mark that current as 'Amps stall'.

2h) Subtract 'Amps no-load' from 'Amps stall'. Multiply the result by your power supply voltage; the result is your motor mechanical power output in Watts (Watts = VDC * (Istall - Ino-load)).

2i) The results from (2h) allows you to compute torque at any speed where the result is less than the holding torque (low-speed torque) of the motor. The general equation is:

in-oz = Watts * 1351 / RPM. If you prefer Nm, divide in-oz by 141 to get torque in Nm (Nm = 9.58 * W / RPM). If the test is carefully done, the accuracy will be within +/-5% of what you would get from a dynamometer.

14-02-2009, 02:09 PM
Thank you Irving2008. The para 2h is a key piece of information that I've never come across.
Not only credit to Mr Gecko, but to you too, for knowing your source material.
I think it's the "re-distillation" of knowledge into a consumable form that so appeals to me in such a forum as this.


14-02-2009, 06:19 PM
Thank you Irving2008. The para 2h is a key piece of information that I've never come across.
Not only credit to Mr Gecko, but to you too, for knowing your source material.
I think it's the "re-distillation" of knowledge into a consumable form that so appeals to me in such a forum as this.

You're welcome.

I was playing around with the idea of automating the process... a 'pulley' on the stepper with some form of encoder to determine stall point, a brake shoe on the pulley that can be tightened by a small stepper and an analogue to digital converter to read current and voltage... and a simple program to pull all the info together.

27-02-2009, 02:05 PM

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!