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.



  • I agree with this assessment your looking to get more speed you either need to spec the motors appropriately or consider other option. Example running a motor rated at 50v on a 24v circuit isn’t going to suffice for proper speeds. Many of the issues come from properly choosing the components you need. If you want to run external drivers simply utilize driver motor combinations designed for each other. One of the easiest ways to go about driver choice is to learn the specifications of your steppers and consider what drivers are used in conjunction with them. Many of the chines steppers rum poorly at lower rated voltages. For example using a 80v max rated motor with a 36v driver which is commonly seen when purchasing package motor driver combinations. Many of those 36v drivers also have a rating of 24v - 50v max so running a 48v power supply is often more productive. That said you can not overcome the limiting factor of majority of steppers. Most cannot operate at higher than 1000 rpm at max voltage. Also consider are you using a belted drive, ballscrews, or leadscrews? There is a common misconception that a larger stepper will provide more speed and that is simply untrue. In fact I would further suggest that oversizing should have other purposes unrelated to overall speed where speed may be an equivalent option between choice of motors. In other words you’re likely increasing torque and not total rpm which is inherently unnecessary in 3D printers. We have a project currently underway to build a high speed moderately large format printer (380x380x500). To get higher speeds a simple choice to utilize integrated driver servos with a speed of 3000rpm and max unloaded speed of 4200rpm. The x and y will maintain a Nema 17 sized (40mm) Motor so we are not oversizing and cranking up the torque where a gantry is light weight and torque unnecessary. These servos will operate off external drives and expansion board. For the z axis we chose a high speed Nema 23 to integrate a single operation versus dual motors. We also chose high speed and relatively low 2nm or .57nm max rpm torque. With the oversized 1/2” milled Precision bed and 4 linear rails and blocks to maintain rigidity the overall mass had been increased therefore requiring the need for a Nema 23 sized frame where a typical over the counter printer would not need this size motor as the beds and guides in general are lightweight. These are factors you should consider as well the good advice in reconsidering your build specifications. You may also want to reconsider the heatbed depending what your intentions I’m printing are as stated it is either a relatively small printer or undersized heater. Larger heaters can heat up faster and maintain better consistency so as long as your not reaching temps that could warp your print bed. The other question you need to ask is what speeds are you trying to achieve and what your existing design is that perhaps some negligible changes could resolve your issue. If you have high voltage motors than use external drivers on those motors and operate them at 80% efficiency. One of our CNC machines utilizes Nema 34 framed Motors. It came factory with a 60v system and 80v drivers to find out that the max voltage of the motors is slightly over 105v suggesting they are clearly under rated and explained the issues we had in speed and step loss why it was under performing. Raising voltage to 75v was the natural choice without changing drivers still only running over 70% efficiency the improvement was night and day as a 15v increase made an entire different piece of equipment. I know this post is older however for future builders I wanted to elaborate and empathize additional research is necessary in a case like this and that typically larger steppers will have very little improvement on overall speed where force and load isn’t issue and additional torque unnecessary. Summary: properly matched hardware/voltage/drivers/motors is key in peak performance of your 3D printer.



  • This is almost the exact question I want to ask.

    I’m impressed by the Duet board. My dilemma is that I want to build a very large printer using what I know from my othet cnc router and plasma build.
    The size is going to be - 6x6x4 ft cube around 1800x1800x1200mm.
    I don’t need large X motor as it only need to move the print head, Y motors might need to be tandem to run the gantry, Z needs 4 motors to lift the entire frame. What is the most powerful stepper I can run from the board? Can I pin out from the existing XYZ to external drivers ( ie Gecko Drive )?
    Also,
    It will need a large heated bed, can I run 4 heat bed from the board?


  • administrators

    See https://duet3d.dozuki.com/Wiki/Using_external_stepper_motor_drivers for how to connect external stepper drivers. You may find the external stepper driver breakout board useful, see the shop.

    For a bed of that size, use AC mains voltage silicone heaters, controlled from the Duet using one or more DC-AC SSRs. See https://duet3d.dozuki.com/Wiki/Choosing_a_bed_heater#Section_Bed_heater_driven_using_a_Solid_State_Relay.


 

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