External Stepper Drivers for Duet WiFi



  • Hello Sir,

    I want to run stepper motors on external stepper motor drivers to suffice the current. Please suggest which stepper motor drivers to use?
    Thank you!


  • administrators

    I can't advise on which external stepper drivers to use, but others here may be able to tell you of their experiences. But first, are you sure that you need more current? The modern Duets can drive Nema 23 motors if you choose them carefully, i.e. 2.5A to 3.0A rated current, and 24V power to the Duet.



  • I need around 4.5A for each stepper motors to drive. Can duet wifi provide total of around 13.5A of current for steppers motors ?
    Bed heater requires around 8A. So total current becomes 21.5A. Will duet wifi provide this current?


  • administrators

    The Duet WiFi is currently limited to 2.4A per motor. As motors are usually run at between 60% and 85% of their rated current to avoid overheating them, this makes it suitable for motors up to about 3.0A rating. It could drive your 4.5A motors at up to 53% of rated current, which is rather low.

    Have you checked whether you can get motors of sufficient torque with a rating closer to 3.0A? If so then changing the motors may be an option. Otherwise you need external drivers.

    The bed heater current rating for the Duet WiFi is 18A, so a 8A bed heater is on problem.

    Your 8A bed heater rating suggests that your printer is rather small; but the motor current rating suggests a very large printer with heavy moving parts. Are you sure that you haven't over-specified the motors?



  • Yes sir. We are sure for that. I am asking for higher rated motors and external motor drivers since we are not getting enough speed with 12kg/cm2 motors with rated current of 2A. We need higher speed. Can you help in that?



  • Hello sir,
    Can we run 4.5A rated current stepper motors with external TMC6600 Stepper drivers in duet wifi? Will it damage the board?


  • administrators

    @jaydevdave:

    Yes sir. We are sure for that. I am asking for higher rated motors and external motor drivers since we are not getting enough speed with 12kg/cm2 motors with rated current of 2A. We need higher speed. Can you help in that?

    Limited speed may not be due to lack of low-speed torque, it may be due to using high inductance motors with insufficient supply voltage. What is the specification of your existing motors, what is your steps/mm, and are you using 12V or 24V power?

    You can use the spreadsheet linked to at the end of https://duet3d.com/wiki/Choosing_stepper_motors#How_to_work_out_the_power_supply_voltage_you_need to work out whether the speed is being limited by the supply voltage.

    There is a wiki page about using external stepper drivers.



  • Specification of existing motors -

    Torque - 12kg/cm2

    Rated Current - 2A

    Inductance - 4.1 mH

    Ball Screws Steps/mm - X and Y - 320 Steps/mm

    • Z - 640 Steps/mm

    Power Supply - 24V 20A


  • administrators

    You are using ball screws, which give you a high steps/mm, so you need to move the X and Y stepper motors very fast. Also, if your 12kg.cm torque figure is correct (it's very high for a Nema 17 motor) then there is a large back emf due to motion.

    I have assumed that the motors are 1.8deg, the steps/mm you quoted is at x16 microstepping, and Cartesian geometry. The spreadsheet predicts that the torque will start to reduce at between about 50 and 70mm/sec.

    The problem you have isn't lack of motor torque (you need less torque with your ballscrews than a traditional belt-driven XY system would need), it's the supply voltage being insufficient for your choice of high torque motors with moderate inductance coupled with a high steps/mm.

    Here are some options for you:

    1. Use your existing motors with external drivers and a driver supply voltage of at least 48V (the Duet is limited to 25V, that's why you would need external drivers).

    2. Use lower torque lower inductance motors, preferably also with a higher current rating (not more than 3.0A), driven from the Duet. Use the spreadsheet to check the performance with alternative motors.

    3. Change the XY ballscrews to ones with double the lead, so that your steps/mm is halved.



  • Got to say it. Amazing David.
    I always learn something new here.



  • If you run out of options and still must use eternal drivers, I suggest trying Gecko drivers. Chinese external drivers tend to run pretty rough in my experience.

    But I'm doing extremely well with NEMA23 motors directly from a DuetWifi and Eth models with the 2660 drivers. It looks like I happened to use motors in the range recommended in this thread.


 

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