Jeff Nyman

I just bought some broken 3D printers at a surplus sale in order to salvage the stepper motors from them. I got 9 steppers for $6.00 US. None of them have any model numbers or anything. I have found that in turning the spindles with my fingers that some of them are much stiffer to twist. Like the magnets are much stronger in them. Am I correct in assuming they would have more torque than the others? They all look to be the size of Nema 17 motors.

Robert Benward

Where do you stand with your project?  Do you have anything built?

Nema17 are 42mm square as measured on the face.  Do you have a stepper motor driver you can test with?  Stepper motors can be low voltage or high voltage, high resistance or low resistance winding.  They also may or may not be bipolar.  You can use a low voltage motor on a higher voltage driver (e.g. 5V on a 12V system) if you control the current flowing through the windings.  Voltage breakdown is not a issue at these low levels.  If you had the part numbers you could check the torque charts.

How many wires coming out of each motor?  Do you have a resistance measurement of one of the windings? 

The correct stepper motor can be had for $15, with a planetary gearbox for under $30.   Send a picture of what you have.


George Cushing

The 17s are NEMA 1-5/8" spec. if you don't have a metric caliper handy.

Jeff Nyman

I am in the process of acquiring all the parts. I bought a MiniESP3 from George which came with some LV8729's which I didn't know about so I also bought some TMC2130 stepstick SP1 drivers. And These steppers-
I also got these steppers in the bunch I bought, which do have model numbers but they don't "feel" as strong when you twist them by hand.
They are all 42mm. I have not measured any resistance yet. I'll have to read up on how to do that.

Robert Benward

So, at least one of them has a part number.  Here is a datasheet for a very similar model.  The PN is KV4239-T2B801 or 802 vs yours which are B012.  Based on current for the entire list of motors, these look like they will work OK, just watch the current in your settings.  If you want the exact data sheet, you will need to contact the manufacturer to confirm they are in the right current range.

To measure the windings, you need a DVM.  They are cheap.  You should not be without one.  If you want to spend more, let me know.

A cheap one:


Jeff Nyman

I have a pretty decent multimeter. I just need to read how to measure the windings.



You have four wire stepper motors.  You just need to identify which colors are paired with each other.

The simplest method which does not require any meter is to separate all the ends of the wires so none are touching each other.

1. Turn the motor shaft with your fingers, noting the amount of torque required.

2. Short any two wires together.

3. Repeat step one, if the torque is greater, the wires that are shorted are a winding pair.  If not, swap one of the wires that are shorted with another color and retest the torque.

You should now have identified a pair of wires that forms one of the motor's windings.  The other remaining two wires are for the other winding.

You now have enough info to attach this to a driver.  (Don't worry about +/- identification of the wires, as this will only affect the rotational direction of the stepper motor.)

If when your entire setup is connected you determine that when the stepper motor is asked to move the scope west, it actually moves it east, reverse one of the winding connections on the stepper, or modify the config file to reverse the rotation direction in software.

Hope this helps,


On Thu, Aug 5, 2021 at 9:35 AM <jeff@...> wrote:
I have a pretty decent multimeter. I just need to read how to measure the windings.

Robert Benward

Simply take your meter and put it in the "ohms" mode.  It might have the upper case "Omega" symbol instead, but unfortunately symbols are not included in this message service so I can't show you (google it).  Put your meter lead on one motor wire, and then look for continuity on one of the three remaining wires. The one that has continuity is the other end of the winding.  The remaining two wires are the other winding.  Don't worry about the + or - on the windings now.  Now that you have a winding/wire pair identified. 

Now, for low resistance readings, you have compensate for the meter lead resistance.  Short the leads on your meter.  Jiggle or rub the probe tips (or alligator tips) and record the minimum reading.  Now measure the winding.  Again jiggle or rub the motor wire along the probe tip, and hold tightly with your fingers.  Record that reading.  Subtract the shorted lead reading from the winding reading, that is your winding resistance, hopefully within 1/2 or 1/4 of an ohm.

For much higher readings, like 20 ohms and up, you don't need this shorted lead reading.  That shorted reading is only for really low resistance measurements.  Alternately some meters may have a four wire mode, for super accurate low resistance measurements, but that is a lesson for another day.  Most are using their meter to separate out parts, 10 ohms or 100 ohms, 4.2K vs 4.7K, shorted or not shorted, etc.  I have several 6 digit meters around the house.  I cannot tell you the last time I used a 4 wire mode.  Arlen's suggestion is excellent if you don't have a DVM, or even it the meter is not within arms reach.  As he and I have suggested, polarity is not important right now, it can be dealt with later.

Now that you have the winding resistance, you can backtrack to the model you have.  Again, I think most of those models on that page were all below 2A.  The voltage listed is usually given as a data point for the current measurement, but most of those motors will be run at higher voltages, and in fact, the torque graphs show them powered at 24V.  Breakdown voltage at these low levels are of no concern.  As long as you have current control (you do; most modern stepper drivers have it, by potentiometer or by software) you are in good shape.  

If you have a small power supply, you can test your stepper.  Just push 1A through one winding** (use constant current mode) and confirm the holding torque is way up there compared to unpowered .  Do not leave it on for long, it will get hot.  Note that Onstep has a guiding and a slewing current setting.  This is so you do not dissipate power and generate unnecessary torque when not needed during guiding, but the power and torque is there when you slew.

**this is an alternate way of measuring the winding resistance, pump current through the winding and measure the voltage across it, then use Ohm's law.

Let us know how you make out.


George Cushing

NEMA 17 motors with 1.8° step angle start about $8 on Amazon and Aliexpress ($3 plus $5 postage). Five 0.09° step angle NEMA 17s are starting at $43 shipped or about $7 a unit.

George Cushing

Set the DMM to Ohms.
connect the probes to any two wires.
If there is no reading, move one probe to another wire. 
If there is no reading, move the probe to the remaining wire. 
Record resistance.

Usually Red and Blue are the ends of one coil and Black and Green the other.
Polarity in such cases is 
Black A+
Green A-
Red B+
Blue B-