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Current & Power Supply

Discussion in 'CNC Mills/Routers' started by danny73, Jan 10, 2018.

  1. danny73

    danny73 New
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    I'm trying to understand the power requirements. I have three nema 17 steppers that draw 2A/phase. So from my research, i would need at a minimum a 12A power supply. Other sources on the intertubes says i would only need 6A supply. I have tuned three drivers to 750mv reference so i can run my steppers at 1.5A. That was with a 12v supply. My dilima seems to be when i'm place my multimeter in between the power supply and my cnc sheild while running three steppers at the same time, i'm only draw at most 620mA from the power supply ([email protected]). My current rating barely change between the 12v supply and the 20v. Under a load, current barely changed. my current setup is an uno with grbl, cnc sheild, A4988 drivers, and nema 17 steppers.

    Why am I drawing 620mA of current from my supply instead of something closer to 12A when all three steppers are running? Even under a load, the current needle barely moved.
     
  2. Joe Santarsiero

    Joe Santarsiero OB addict
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    Take 2/3's the total stepper current to determine minimum PS capacity. You only need a 3A supply if set to 1.5A for three motors. 1 phase is active at a time. Drivers limit current. Steppers draw most current at a powered standstill.
     
  3. Kevon Ritter

    Kevon Ritter Veteran
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    As Joe stated, the voltage really has nothing to do with the current. Voltage allows current to travel quicker which ultimately gives a quicker response.

    I've seen the 2/3's statement plenty, but simply adding them up will give you a safe range. The auxiliary electronics aren't pulling much current.
     
  4. danny73

    danny73 New
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    So if I use the appropriate gcode to enable all three steppers to hold their position (no free movement), I should get a reading on my multimeter around 3A then from power supply?
     
    #4 danny73, Jan 11, 2018
    Last edited: Jan 11, 2018
  5. David the swarfer

    David the swarfer OpenBuilds Team
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    no, your volt meter will never tell you the truth for stepper current because the drivers are pulsing the power on/off at high frequency. (except 1x microstepping and even then you have to do the right stuff)

    read this for the details
    https://www.geckodrive.com/gecko/images/cms_files/Step Motor Basics Guide.pdf

     
  6. danny73

    danny73 New
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    Thx David, but I've seen and read this before. I can see the current jump from 0 to about 380mA and hold while motor is stepping and back to zero when done. So I can see the current, just nothing even remotely close to 1.5A. I would expect to see more than that being drawn from the power supply regardless of what the drivers are doing. I had my set up under load with all three moving and everything is working without the power supply even heating up which I would also expect or at least see some of the smoke trying to escape from it.
     
  7. Joe Santarsiero

    Joe Santarsiero OB addict
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  8. danny73

    danny73 New
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    I was able to lock the stepper by changing $1=255 in UGS. So I'm assuming that current is flowing through one of the phases, but I'm showing meter at 107mA. When $1=25 (default) I'm at 15mA and stepper is not locked. Still can't figure why I'm so low on amperage. I can't even get close to 1A.
     
  9. David the swarfer

    David the swarfer OpenBuilds Team
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    it is because the driver is pulsing the motor at high frequency (usually about 15khz) and the voltmeter does not know how to read that correctly as it is expecting DC as in from a battery.

    the voltmeter is digitally sampling the current and depending on that frequency and the frequency of the PWM in the stepper driver, you will get more or less current reading, pretty much never correct. it will only be correct if both sampling frequencies are exactly the same AND the sample is during an on period, exactly, AND the on period is long enough to settle (due to inductance the wave form is quite spiky and non linear).

    modern stepper drivers always power BOTH windings, even in single step mode. I believe the video explains this.

    I suggest you turn up the power pot on the driver is small increments and run the motor for a few minutes at each increment until either the motor or the driver gets too hot, then turn it down a bit. measuring the current is a fools errand.
     

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