I recently bought a CNC mini-mill at an auction. It is fitted with steppers on the three axis. The X and Y are the same, and the Z is a bit smaller (physically). All the markings were removed from the motors and they are painted flat black. Yet another reason to hate proprietary hardware. The machine was originally designed to use a custom controller which is missing.

My question is, can I figure out what voltage the motors were intended to run at? Does the coil resistance offer any clues? I have a good quality multi-meter, and I have already reverse engineered the wiring. I don't want to fry the motors though.


If you use a good driver circuit that has current limiting and chopper features, it really won't matter. Basically you can slowly crank up the current on these drivers while checking the stepper motor temperature and usually you can operate them at a voltage MUCH higher then they were intended with the right circuit. I am currently in a CNC build project and I am about to do that very thing. You can follow me on my Blog. WWW.MENDINGTHINGS.COM

Good luck.

  • \$\begingroup\$ I like this suggestion but I think it represents a "plan B". I would rather figure out the actual specs (or as close as possible). \$\endgroup\$ – jkopel Feb 17 '10 at 3:09
  • \$\begingroup\$ I understand. I was going to do this anyway so I set up a test and posted a video on my blog. This is actually done very often in industrial applications. However if you don't use a stepper driver with chopper features then all bets are off. As for the driver needing to be oversized, I think you can see the little EasyDriver that is rated at 750MA is not straining at all and for $15 you can't beat it. Good luck with your project. \$\endgroup\$ – Mark B Feb 18 '10 at 3:46
  • \$\begingroup\$ I forgot to mention that I am betting your steppers are much larger and you would most likely need a larger driver than this but maybe not. \$\endgroup\$ – Mark B Feb 18 '10 at 3:53
  • \$\begingroup\$ Nice post Mark, and thanks! My steppers are quite a bit larger then the ones you are using, but I am going to get one of the EasyDriver boards and give it a try. \$\endgroup\$ – jkopel Feb 21 '10 at 18:19

The total weight of the motor and the coil resistance might be a good guide. See if you can find some specs on the web and match them to your motor. I have done similiar things with rechargeable batteries matching chemistry and weight. I also think that Mark B's advice is pretty good, but suggests an over sized driver.


Steppers get marked like other motors, but they aren't operated like other motors. Your motors, depending on size, might have two or three heavy coils in parallel. This drives the resistance down to 1 or 2 ohms. At 100% duty, this gives them a nameplate rating of 3 or 4 volts. But, in actual operation, you don't use 100% duty. When you are running, the coils are at 50% duty or lower. When at rest, you usually switch them off or go to a 10% holding current. So to run them at high speed and/or high acceleration, the packaged drives use a high voltage (somewhere between 24 and 90 volts) and a current limiting resistor. This overcomes the internal inductance to give a snappy response. It's not plan B, it's just the way they work.

So, the critical part of the nameplate is the temperature rating. It's usually around 120F. If you can keep your hand on it for a half second or more, then you're fine.

But acceleration and deceleration is important, too. You usually can't just jump in a full speed. So you best bet might be to find a mill control box that has the drives and a control computer that does the profiling that you want.

  • \$\begingroup\$ you meant "half a minute", right? \$\endgroup\$ – Dave Mar 9 '11 at 16:27
  • \$\begingroup\$ Nope. Half a second. When you get into industrial sizes and drives, steppers get hot. There is no way you're going to leave your hand on 120F for half a minute. \$\endgroup\$ – Ron Mar 11 '11 at 14:15
  • \$\begingroup\$ Ok, I was just wondering if the rule of thumb was meant to be like ICs, and thought maybe steppers shouldn't get that hot. But if their nominal temp can get up to 120, then I understand. I've not run large industrial steppers before. \$\endgroup\$ – Dave Mar 11 '11 at 19:38

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