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Measuring Stepper Motor Current Consumption

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  • Measuring Stepper Motor Current Consumption

    I am using a do-more BRX (BX-DM1E-18ED13) PLC to drive an integrated stepper motor (STP-MTRD-17038E). I am using the latest release of Do-More designer.

    Is there a simple and accurate way to measure current consumed by the stepper motor?


  • #2
    Considering the nature of the current flow through a stepper, I'd think a digital oscilloscope would be the best way (with a shunt or current sensor). A high end DMM might work as well, a true RMS converter "should" work, but as the wave form could be significantly different that what is usually measured with a DMM I wouldn't be 100% confident. If you are using one of AD's linear supplies with the big filter capacitor one might try measuring the current before the capacitor which should smooth things a lot.

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    • #3
      Originally posted by Tinker View Post
      Considering the nature of the current flow through a stepper, I'd think a digital oscilloscope would be the best way (with a shunt or current sensor). A high end DMM might work as well, a true RMS converter "should" work, but as the wave form could be significantly different that what is usually measured with a DMM I wouldn't be 100% confident. If you are using one of AD's linear supplies with the big filter capacitor one might try measuring the current before the capacitor which should smooth things a lot.
      Do you recomend any current sensor for this? I will try measuring the current at the stepper driver

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      • #4
        I have used the DCT100-42-24-F set to the lowest scale of 0-50A on the 70vdc input to a STD-DRV-80100 to get current consumption.
        Made about 7 wraps through the core to up the current to the range of the sensor and then scaled. ( You have to wrap the dc wire in the correct direction )

        I was able to get the current feedback, but in my application the stepper was turning a spindle that could not jam without breaking.
        The current changed fast enough while running that the unit could not slow down fast enough.
        Wound up going to a servo with torque control.

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        • #5
          I dont believe any standard current sensor AD sells and analog input will be fast enough to really "see" peaks and startup surges. These devices you wrap coils of wire around tend to be quite slow, and analog inputs usually have a decent bit of filtering used so they arent so susceptible to noise. An oscilloscope would see them because it is much faster, but I doubt most could interface to a PLC in a way you would want and the cost would be very high, higher than just using a servo anyway.

          Perhaps there is a stepper drive out there that already comes with a current reading output? I would imagine that some sort of sensorless vector reading like a VFD has could be done if someone were to design it. All that is doing is measuring the amperage draw based on known output frequency (which stepper drive also knows) so theoretically it should be possible.

          Or maybe you could create your own circuit board with large enough shunt resistors to do this. The voltage should be "constant" right? So if you ran the output lines with shunt resistors you could get a power reading from them. As long as you set your analog input fast enough with absolute minimal filtering it might work.
          Last edited by MikeN; 10-01-2019, 04:50 PM.

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