# Measuring the spec of stepper motors

Hi All,

Is it possible for me to measure the spec of stepper motors without any prior information?

Here is the situation. I brought a 3D printer a year ago, which has four cheap ordinary stepper motors. The working temperature of the stepper motors is way too hot, at least 110 degrees C. I suspect the reference voltage is way off from what it suppose to be. When I check the stepper motors, there is no labels or any relevant information on it. Then, I asked the vendor for the spec of the stepper motors. ...

The vendor told me that the labels are deliberately removed, so that they can keep the spec of the stepper motors as a COMPANY SECRET, not a chance to tell me. WTF!!! (sorry for being rude) The vendor goes on and told me to do stupid things, e.g. adjusting reference voltage blindfolded, sending the machine back for maintenance at an EXCEPTIONALLY HIGH CHARGE, buying some new stepper motors from them... They even told me that they are adjusting the reference voltage arbitrarily regardless the stepper motor spec. Mmm... that's probably why the reference voltage is way off the chart.

Anyway, how to measure the spec of stepper motors without any prior information? Can anyone help me out?

Yours faithfully,
Gary

The amperage rating of a stepper can be determined by measuring the diameter of the wire used in the coils. Given that you can't do that without ruining the stepper you have to go by heat; if it's too hot to hold your hand on it for a few seconds then you're feeding it too much amperage. Your risk here with too much amperage is that you melt the insulation off the coils.

For the resistance of the coils grab a multimeter.

Knowing the amperage and resistance you can then calculate the rated voltage using Ohm's law. But the rated voltage is a useless spec anyway; in practice the stepper is run at a high voltage (~40V is typical) to get optimal speed out of it while current regulation (using high wattage resistors or preferably a "chopper drive") is used to prevent the overheating problem described earlier.

For the inductance of the coils again grab a multimeter. The inductance is a useful spec because it determines the appropriate peak voltage to use; beyond that peak voltage you don't gain more speed but rather everything starts turning into heat. That inductance can be used in the formula 32 * sqrt(inductance) = peak voltage. Lower inductance is generally preferred to avoid having to use high voltage power supplies.

....so what it all really comes down to is heat. If the motor is too hot -- if you can't hold your hand on it -- back off the amperage.

Hi,
I have a similar problem, unlabeled steppers without any hint whatsoever about manufacturer and or specs.

The amperage rating of a stepper can be determined by measuring the diameter of the wire used in the coils. Given that you can't do that without ruining the stepper you have to go by heat

I do have access to the coils, what wire diameter would correspond to what ampereage?

I also found the advice to just apply a current of 100mA to one coil, wait and see if it heats up and then increase the current in little steps, until there is a noticeable heat production to find the current rating. Any thoughts on that?

As the four steppers drive two very short linear axis and two goniometers that are probably precise to µm's, I'm not really interested in reaching the highest possible speeds by increasing the voltage beyond repair, I just want to get them to work without destroying them.

Saluti!

I also have Steppers without specs. What I notice is that the stepper starts to produce noise even when not stepping if I allow higher current.
Is a stepper supposed to produce noise when actively holding position but not stepping? Or does this indicate too high current?

If the current is too high, the stepper suddenly starts stepping "back" a few steps even if it should turn "forward".

Robert

I do have access to the coils, what wire diameter would correspond to what ampereage?

If you disassemble a stepper motor you will irreversibly weaken it. With respect to the current capacity of a given wire size you should google for an "ampacity chart".

I also found the advice to just apply a current of 100mA to one coil, wait and see if it heats up and then increase the current in little steps, until there is a noticeable heat production to find the current rating. Any thoughts on that?

Yes, that's the basic idea. Or more to the point you should increase the current to where your drive system is running reliably; no need to use more current to create unneeded torque.

robvoi:
I also have Steppers without specs. What I notice is that the stepper starts to produce noise even when not stepping if I allow higher current.
Is a stepper supposed to produce noise when actively holding position but not stepping? Or does this indicate too high current?

If the current is too high, the stepper suddenly starts stepping "back" a few steps even if it should turn "forward".

A singing stepper is normal when using a chopping driver. The driver is pulsing current through the coils and that causes the coils/laminations in the motor to vibrate and you'll get that high pitched noise. If you've ever walked past a power transformer and heard it humming it's the same thing -- just at a lower frequency. Winding coils in a motor or transformer is a bit of an art form because of this.

If your stepper is making incorrect steps I would look at how your drivers and motors are wired up. A broken/damaged lead or bad connection on the stepper or bad grounding of the driver.

Chagrin:

I do have access to the coils, what wire diameter would correspond to what ampereage?

If you disassemble a stepper motor you will irreversibly weaken it. With respect to the current capacity of a given wire size you should google for an "ampacity chart".

I believe that's no longer a problem - modern rare-earth magnet hybrid steppers are fine.
Older permanent magnet motors used magnet alloys with low permanence that were
very easy to demagnetise. Such alloys lose magnetisation if hit with a hammer too...

MarkT:

Chagrin:

I do have access to the coils, what wire diameter would correspond to what ampereage?

If you disassemble a stepper motor you will irreversibly weaken it. With respect to the current capacity of a given wire size you should google for an “ampacity chart”.

I believe that’s no longer a problem - modern rare-earth magnet hybrid steppers are fine.
Older permanent magnet motors used magnet alloys with low permanence that were
very easy to demagnetise. Such alloys lose magnetisation if hit with a hammer too…

agree

old day standard is < 85 C/185 F, Now even motor OEM indicate “Operate the motor keeping the motor surface temperature at 100 C or lower.”

However if you need to recover motor model for other purpose (repair parts), you could do reverse lookup.

sonnyyu:

Get Width/Height(mm), Resistance(?/Ø), Inductance(mH/Ø), MASS(g), “Number of Pin”, Length(mm), then reverse look up to find out motor model. (fuzzy look up and start with same size (Width/Height) motor from Ebay first)

if you have no LCR meter, Using an Arduino to measure inductance