Stepper motor current



  • Hello!

    I read the awesome Duet documentation and I have some questions about choosing a stepper motor.

    I am looking at moons nema 17 motors (almost 40 size) and followed the calculations in the "Choosing and connecting stepper motors" article for three different max current rates variants. For 1A the peak back emf due to inductance plus the approximate back emf due to rotation (200mm/s 16T pulley) ends up being more than 24V. As I do the calculations for the 1.5A and 2A variants, the voltage drops way down (I should noted I used 80% of current for the peak back emf calculations.)

    So here's my question. Why wouldn't I want to go with the 2A motors? I see a lot of i3 printers all linking 1A motors running 24V.

    Will the higher current motors generate more heat? That's the only thing I can think of, but then I read this from that same article:

    "So the usual practice is to set the motor current to no more than about 85% of the rated current. Therefore, to get maximum torque out of your motors without overheating them, you should choose motors with a current rating no more than 25% higher than the recommended maximum stepper driver current."

    And for Duet 2 it says "Duet 2 WiFi and Duet 2 Ethernet (maximum motor current 2.4A) => Stepper motor rated current <= 3.0A"

    So why wouldn't I just go with the 2A variants and run them at 50% of their rated current (1A)?

    Thank you for helping me understand this. Hopefully it can help others.


  • administrators

    For the Duet WiFi/Ethernet, 2A motors are a good choice.

    For a Prusa i3 or similar Cartesian printer using 1.8 deg motors, travel speeds are normally not much higher than 100mm/sec, so lower current motors are OK especially when using 24V power. The cheaper Trinamic drivers can't handle much more than 1A without generating a lot of heat, so there is a trend towards lower current motors and 24V power for that style of printer.



  • @dc42 hi! Thank you. But I'm confused. If I build an i3 style printer with the Duet 2, the Duet 2 has the TMC drivers on it, and anyone using the Duet 2 (let's say for a coreXY printer) would be using the same drivers, wouldn't they? Again, I apologize, I'm just trying to learn here. I also noticed that the documentation says the following:

    "Unless you will be using external stepper motor drivers, choose motors with rated current of at least 1.2A, and at most 2.0A for the Duet 0.6 and Duet 0.8.5, or 3A for the Duet 2.
    Plan to run each stepper motor at between 50% and 85% of its rated current."

    So the nema 17 for 39.8mm has four variants based on current: 0.4A, 1.0A, 1.5A, and 2.0A. I did the math and obviously the higher the rated current the lower the voltage needed. But what I don't understand is when it says this:

    "Therefore, to get maximum torque out of your motors without overheating them, you should choose motors with a current rating no more than 25% higher than the recommended maximum stepper driver current."

    Which part is causing the heat? From my understanding of electronics, heat is generated by current (too many electrons trying to rush down a path that's too narrow causing friction which is heat.) I'm guessing that the difference between those variants of motors is that they are using larger gauge wires to allow more current?

    If that's true, then what is the downside to running the 2.0A rated current variant? The only thing I can think of (but don't know if it's true, and it's why I'm asking all these questions trying to understand and can hopefully help someone else out in the future lol!!) is if I run a 2.0A motor at a lesser amperage (let's say 1.0A) that the holding torque and other performance characteristics wouldn't be as good?

    Hah, again, thank you in advance 🙂


  • administrators

    @ringo1508 said in Stepper motor current:

    @dc42 hi! Thank you. But I'm confused. If I build an i3 style printer with the Duet 2, the Duet 2 has the TMC drivers on it, and anyone using the Duet 2 (let's say for a coreXY printer) would be using the same drivers, wouldn't they?

    Yes. But i3 style printers are normally small (around 200x200mm) and not especially fast because of the moving bed. That's why I said the maximum speed for that style of printer was typically not much greater than 100mm/sec. Whereas CoreXY printers are typically larger and faster. So motor back emf tends to be more of an issue with CoreXY printers than with i3-style printers. It's even more of an issue with large delta printers using 0.9deg motors.

    "Unless you will be using external stepper motor drivers, choose motors with rated current of at least 1.2A, and at most 2.0A for the Duet 0.6 and Duet 0.8.5, or 3A for the Duet 2.
    Plan to run each stepper motor at between 50% and 85% of its rated current."

    So the nema 17 for 39.8mm has four variants based on current: 0.4A, 1.0A, 1.5A, and 2.0A. I did the math and obviously the higher the rated current the lower the voltage needed. But what I don't understand is when it says this:

    "Therefore, to get maximum torque out of your motors without overheating them, you should choose motors with a current rating no more than 25% higher than the recommended maximum stepper driver current."

    Which part is causing the heat? From my understanding of electronics, heat is generated by current (too many electrons trying to rush down a path that's too narrow causing friction which is heat.) I'm guessing that the difference between those variants of motors is that they are using larger gauge wires to allow more current?

    Stepper motors generate heat due to the resistance of the winding. You can run stepper motors at full rated current, but then they will probably get hotter than you want. That's why we suggest you run them at 85% of rated current or less.

    If that's true, then what is the downside to running the 2.0A rated current variant? The only thing I can think of (but don't know if it's true, and it's why I'm asking all these questions trying to understand and can hopefully help someone else out in the future lol!!) is if I run a 2.0A motor at a lesser amperage (let's say 1.0A) that the holding torque and other performance characteristics wouldn't be as good?

    The only downside of using 2A motors instead of 1A motors is that the stepper drivers will get hotter for the same performance. For example, using 1.7A (i.e. 2A motor run at 85%) will generate more heat in the drivers than using 0.85A (1A motor run at 85%). For the Duet WiFi and Duet Ethernet this is unlikely to be a problem, because at 1.7A (i.e. 85% of 2A) the Duet doesn't normally need to be fan cooled unless it is in a confined space. For the Duet Maestro, the 1.0 or 1.5A motors would be a better choice. The 1.5A would allow a higher maximum speed than the 1.0A before the torque starts to reduce, but the drivers would need better cooling.

    HTH David



  • @dc42 so awesome! Thank you! I understand now. My Duet 2 Wifi should be on it's way sometime soon. Cannot wait!


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