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Can I acquire the shaft position of my motor using my VFD

  1. Mar 24, 2017 #1
    The project I am working on is to design the control system for a shock dynamometer. I am using Labview and communicating to the VFD using Modbus. The shock dyno will look a little something like this;
    Screen Shot 2017-03-24 at 11.37.44 AM.png

    So one of the main objectives in my project is to produce a plot as seen below;

    The force will be data coming from a load cell.

    The Displacement will be the linear displacement of the shock being tested (the compression/rebound). To clarify what I mean, I have included the figure below;
    Screen Shot 2017-03-24 at 12.35.54 PM.png

    My question is: Do you think I would be able to acquire the shaft position from my motor using my VFD?

    I called AutomationDirect (the manufacturer of the VFD) and asked them this question and they told me no considering the motor I am using (3 phase) is not a servo motor but I just wanted to double check with the forums to make sure. I tried reading the encoder feedback registers through Modbus and moving the shaft by hand to see if any values changed but no luck. I'm guessing I may have to implement some kind of proximity sensor in order to acquire the data for displacement. Due to the design of the dyno, I don't think I would be able to attach an external rotary encoder to the shaft of the motor. Any suggestions?

    In case it helps, below is a picture of the nameplate on the motor and also the link and model of the VFD I am using.

    motor nameplate.jpg
    VFD: DURApulse GS3-21P0 AC Drive
    VFD Manual: https://cdn.automationdirect.com/sta.../gs3m/gs3m.pdf

    Thank you.

  2. jcsd
  3. Mar 24, 2017 #2


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    Is the main concern the shaft position or the position of the slide? There are many encoders that will report linear position.
  4. Mar 24, 2017 #3
    This looks like an induction motor, so there is no telling where the rotor position is at any particular instant.
  5. Mar 24, 2017 #4


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    Gold Member

    The answer you have been told from mikeholt is correct as well as in post #3 of this thread.
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