Unknown motor specs - how to guesstimate?

  • So I've been trying to get an Tiertime Up! Mini 2 to play nice with a Duet Maestro and for the most part its been okay.

    The last hurdle is what parameters to use for the motors, I'm getting a lot of phase X disconnected on drive N and so on, but to me the motors seem to be driving just fine; at least with respect to lost steps.

    All I know is that they are 0.9 degree step, around 23ohm and were driven at 19V, and somewhere in the range of 60-100mm/s I think. There are no marking or ratings on the motors.

    But I don't know the inductance or the old current. Short of ordering a inductance meter, are there any quasi scientific / cheap&easy ways to derive the specifications and or settings to use. I might try and ,measure the current of the old board.

  • Moderator

  • have you updated your firmware to at least 2.02?

    is that 23 ohm resistance per phase? that seems very high.

  • administrators

    23 ohm phase resistance is indeed high. To estimate the specifications, look for another Nema 17 (if that's what they are) 0.9deg bipolar motor of the same length as your motors, for example here https://www.omc-stepperonline.com/nema-17-stepper-motor?mfp=149-step-angle[0.9],148-bipolar-unipolar[Bipolar]. Then use the following scaling laws:

    • Inductance scales approximately with resistance
    • Rated current scales inversely with the square root of resistance

    The message phase X disconnected on drive N means that there wasn't a chopper event for a significant period of time. Possible causes are:

    • Unsuitable stealthChop settings. I suggest you run the drivers in spreadCycle mode while you are debugging this.
    • Motor current set too low. But this is more likely with low inductance motors.
    • Supply voltage too low for the motor current and speed you are trying to use. If the error messages only occur at high speeds, and you use spreadCycle mode, this is the most likely cause. You need to increase supply voltage (maximum is 28V for the Maestro), reduce maximum speed, or reduce motor current. Are you already using 24V power?

  • About 23omhs pr coil, yes (ranging 22-25 between the different motors, average around 23). And firmware is 2.02 as the latest stable release yes.

    Sorry, should have included current settings. And thanks, will review the specs.

    25V (belived that was max) , tried currents between 600 - 1000mA, not very scientific testing but couldn't notice much difference.

    I see I've also gotten my bearings wrong in the documentation, I was under the impression Duet had drivers that didn't support stealthchop and that I was using spreadcycle. Will review and post config when i get home.

  • administrators

    25V is the maximum recommended VIN for the Duet WiFi/Ethernet. It's 28V for the Maestro.

    The drivers on the Meastro do support stealthChop. The default setting in firmware 2.02 is stealthChop at low speeds, switching to spreadCycle at higher speeds.

  • looking at steppersonline. all the motors that have phase resistance in that range are 0.3-0.4A motors

    so try a current of 200 or 300.

  • I started off with 600mA as a best guess from looking at Rsense on the old board and DRV82-something-something datasheet. But I'll try lower currents and try to measure the reference voltage and verify the old max current.

    Yeah, I was set straight with respect to which drivers were on the maestro, unsure why I thought it was the 2660. I will do some poking and prodding and see what happens.

    Thanks for the input so far!

  • @veti said in Unknown motor specs - how to guesstimate?:

    looking at steppersonline. all the motors that have phase resistance in that range are 0.3-0.4A motors

    so try a current of 200 or 300.

    Sure enough 300mA current prints without warnings, but feels like the sound level increased despite keeping speeds, accelerations and other settings the same.

    I ordered an inductance meter so might revisit this when I get the full specs for the motors, but thanks for your inputs so far!

  • @bearer said in Unknown motor specs - how to guesstimate?:

    I ordered an inductance meter

    hope that it was not more expensive than new steppers with know spec 🙂

  • Hehe, I ordered the cheap $15 multi purpose LCR/ERS/Diode/Transistor tester thing just because that was clearly missing in the otherwise (un)healthy collections of meters.

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