Moving the gantry damage the stepper drivers when powered off?



  • Does moving the gantry in x and y damage the stepper drivers due to back EMF? I know this was a problem with the rambo and smoothie boards. Is this also an issue with the duet?



  • The stepper motors are not damaged by manually moving the gantry. Stepper motors are simply coils, that creates a magnetic field as current flow through it. By varying the current through each of the coils, it can push the magnetic core to create rotation - there is no electronics inside a stepper motor.

    What can happen, if you move it too fast, is that generated voltage may become higher that what the board can handle and then there could be damage. As long as you don't move it too fast, it should not be a problem (I move my gantries around by hand very often, and have not had such as issue).


  • administrators

    @injoi9000 it depends on the motors and how fast you move them. The stepper drivers are rated for 28V so it's possible to exceed the voltage rating. If you have to move the axis by hand, move it slowly. If you need to do a lot of movements (to manually test alignments or something) then unplug the motors.



  • Ok thanks! So yea I'll just move them slowly ☺



  • I move motors with the printer powered down constantly, and have never had any issues. Other people report damage. I have a few theories for this:

    1. All my printers have screens (Vicki, LCD, PanelDue, whatever). If the motors generate a meaningful amount of power, the LCD backlight will come on and then there’s a load on the system to keep the board capacitors from overcharging too much. If your motor is generating a lot of power, it will act as a very strong brake, so the problem becomes self-limiting.
    2. Some board designs have diodes for PSU reverse polarity protection. With such a diode, I think power generated can’t escape the board. Without these diodes, the motor EMF can charge up the PSU capacitors, so you get much more safety cushion before anything overloads.
    3. Maybe people are using motors with bad specs (too many coil windings) and that generates more voltage for a given motor RPM. Frankly, it is HARD to hit a motor RPM that will generate 28v of back-emf to overload something.

    Just a few theories. I don’t know what the truth of the matter is.

    By the way, there ARE some add-on PCBs for Pololu-socket drivers that include zener diodes and flyback diodes to try to prevent motor EMF board damage.


  • administrators

    @rcarlyle said in Moving the gantry damage the stepper drivers when powered off?:

    1. Some board designs have diodes for PSU reverse polarity protection. With such a diode, I think power generated can’t escape the board. Without these diodes, the motor EMF can charge up the PSU capacitors, so you get much more safety cushion before anything overloads.

    A reverse polarity protection diode won't help here. You would need something like a large Zener diode across the supply rails instead.

    By the way, there ARE some add-on PCBs for Pololu-socket drivers that include zener diodes and flyback diodes to try to prevent motor EMF board damage.

    I'm not aware of any that include zener diodes. Waterott does these https://www.watterott.com/en/SilentStepStick-Protector but as all modern constant current stepper driver chips have integral flyback diodes anyway, I doubt that they make any difference.



  • @dc42 I think the one with zeners I’m remembering was just a homebrew board someone was trying to sell. I agree that flyback diodes are redundant with the driver H-bridge body diodes, although I have seen CLAIMS that additional external flyback diodes provide better suppression. (Lower voltage drop or conducting resistance or something, I don’t know.)


 

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