Thanks Thanks:  0
Likes Likes:  0
Needs Pictures Needs Pictures:  0
Picture(s) thanks Picture(s) thanks:  0
Results 1 to 3 of 3
  1. #1
    Join Date
    Feb 2006
    Location
    Perth
    Posts
    27,757

    Default VFD current waveforms and values

    I did a bit more testing with the CRO and various ammeters on the VFD I'm using for my compressor.

    Firstly just to review the input power waveform with the motor running
    Compressor Control Circuit-compressor-jpg

    Now with the VFD on, but not running the motor
    The signal is of course much lower but it's the same basic waveform as when the motor is running - the larger peak is now just very narrow.
    The current clamp is connected to the CRO and the DMM at the same time
    The DVM readings need to be multiplied by 100 to give actual current) so the VFD appears to be drawing ~0.4A without running the motor
    A back of the envelope RMS determination of the current from the CRO waveform gives 0.45A so they agree with each other within the resolution of the DVM - so they should as their current signal comes from the same sensor (i.e. the current cable clamp).
    VFD current waveforms and values-nooutputvfd-jpg

    Now I place the current clamp around one of the phases of the output of the VFD and this lets us look at the output of the VFD with the motor running (not shown before) see pic below.
    The first thing one notices is how much cleaner/simpler the signal is.
    Even though there are small gaps in the signal it's basically sinusoidal so RMS determination of the signal is MUCH easier and then more accurate
    So now the CRO, DVM and Panel meter can be compared.
    NB: The panel meter is still on the input side of the VFD while the Current clamp (as read by the DVM and CRO) is now on one of the output phases of the VFD
    VFD current waveforms and values-pmandccmimage-jpg

    Making comparative current readings is difficult because motor load changes rapidly as the compressor fills the current changes but by taking a series of photos in time as the compressor filled the tank I was able to simultaneously record the currents as registered by the CRO, DVM and Panel meter (PM)

    The PM current is the total single phase input current to the VFD, while the CRO/DVM are reading a single phase current so assuming the 3 phases are equal (something I need to test) to compare the PM to the others requires the CRO current values to be firstly determined as RMS (so convert peak to peak to RMS) and then for both the CRO and DVM, the values to be multiplier by the square root of 3.

    Then taking the CRO values as most reliable I calculated the % differences of the current as read by the DVM (blue diamonds [legened on the graph below as CCM*SQRT[3)] on graph) and PM (red squares)
    All values now agree within +/-10%. One of course would expect the Blue diamonds to agree with the CRO as they are being sensed by the same current clamp, and they all do, except one value, so I am now pretty happy that the current clamp with the DVM seems to be working correctly. The PM is now also not that far out, certainly good enough to use as an indicator of what is going on.

    VFD current waveforms and values-pmandccmcurrents-jpg

    My remaining concern is now what is going on with the other panel meter as it seems to be reading significantly lower than the panel meter referred to in this post.
    Attached Images Attached Images
    Last edited by BobL; 6th September 2018 at 09:05 PM.

  2. # ADS
    Google Adsense Advertisement
    Join Date
    Always
    Location
    Advertising world
    Posts
    Many





     
  3. #2
    Join Date
    Jun 2010
    Location
    Canberra
    Posts
    769

    Default

    Interesting to see the difference between true RMS measurements and the panel meter, although I'd have expected the difference with the panel meter to be more consistent as the duty cycle of the waveform increased with current, but clearly not - perhaps there's something else that influences the panel meter's reading.

    The other thing I noted was the standby current of that drive - 96W or so. At a vineyard I work with, all the circuits are monitored with hall-effect current sensors, including the irrigation pump's 3ph circuit - it's about a 5kW or so Danfoss Aquadrive hooked up to a Grundfos pump - that standby current on that unit is only 40W.

  4. #3
    Join Date
    Feb 2006
    Location
    Perth
    Posts
    27,757

    Default

    Quote Originally Posted by RustyArc View Post
    Interesting to see the difference between true RMS measurements and the panel meter, although I'd have expected the difference with the panel meter to be more consistent as the duty cycle of the waveform increased with current, but clearly not - perhaps there's something else that influences the panel meter's reading.
    The problem with something like a compressor is getting a constant current because the load continually changed so some of this could be due to where the meter is in its reading cycle.
    The CRO reading is also far from precise. If I was to put uncertainties on the graph it would be +/- 5% for the CRO reading and that would cover all bar a couple of outliers.

    The other thing I noted was the standby current of that drive - 96W or so. At a vineyard I work with, all the circuits are monitored with hall-effect current sensors, including the irrigation pump's 3ph circuit - it's about a 5kW or so Danfoss Aquadrive hooked up to a Grundfos pump - that standby current on that unit is only 40W.
    Yep I though it was rather high as well. I will check the other HY units and see what these are like and if it is an issue of these being on the cheap/low budget side of things.

Similar Threads

  1. Triton values
    By gnu52 in forum WOODWORK - GENERAL
    Replies: 3
    Last Post: 19th July 2010, 10:50 AM
  2. Timbers and values
    By benupton in forum TIMBER
    Replies: 2
    Last Post: 29th April 2010, 09:33 AM
  3. trade values
    By old mate in forum WELDING
    Replies: 5
    Last Post: 19th January 2010, 07:41 PM
  4. Australian values and culture???
    By Clinton1 in forum NOTHING AT ALL TO DO WITH WOODWORK
    Replies: 46
    Last Post: 13th October 2006, 09:31 PM
  5. Australian Values
    By Phil Spencer in forum NOTHING AT ALL TO DO WITH WOODWORK
    Replies: 34
    Last Post: 6th May 2006, 01:22 PM

Tags for this Thread

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •