Re: Steppers

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.


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