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Do I need a feedback card to acquire motor RPM from a VFD?

  1. Nov 22, 2016 #1
    The VFD I plan on using;
    DURApulse GS3 series AC drive: https://www.automationdirect.com/static/specs/gs3drives.pdf
    Model: GS3-22P0

    The feedback card accessory for the VFD: https://www.automationdirect.com/static/specs/gs3fb.pdf

    Hi everyone, I would like to know if I need this feedback card to obtain RPM of a motor that is being wired using the VFD specified above. I plan on controlling a 3 phase motor with the program Labview. I will be using MODBUS to communicate between the VFD and Labview.

    The end goal of my project is to plot the force of a load cell with respect to an increasing motor RPM so I was wondering how to get the RPM feedback from the motor. What exactly is the main purpose of this feedback card? In what kind of applications would this card be needed?
  2. jcsd
  3. Nov 22, 2016 #2


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    Gold Member

    You can make a closed loop controller.
    The feedback is subtracted from the feed forward signal, which results in a speed ( or acceleration or position ) error.
    This error is integrated, increasing or decreasing the torque, until the speed is accurate ( error = 0 ).
    A digital controller will give you a speed, that is as accurate as the crystal controlling the clock frequency in your controller ( measured in parts per million ( ppm ) ).
  4. Nov 22, 2016 #3


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    Staff: Mentor

    No, it comes with multiple configurable inputs and outputs. You can get a 0-10V speed output, which you can translate into RPM.
  5. Nov 30, 2016 #4
    Depending on the precision required, If you know the design slip factor of the motor, (based on load, read AMPS) Frequency output of the VFD may be close enough.

    Some VFD's offer "conversion" calculations to display "motor RPM" based on similar input.
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