Installing sensors to monitor current and rpm

  1. I would like some assistance to select the right sensor/transducer to monitor current from an inverter to the spindle motor it feeds. If it is possible to get RPM signals from the inverter please advice on how to go about it. Please suggest the most suitable sensor/s to achieve both current and RPM (if possible), if not, just current.

    Inverter: Mitsubishi Transistorized Inverter (FR-A500-3.7K)
    Inverter output voltage: 200 to 240 VAC 3-phase (u,v,w terminals pg 12) 50/60 Hz

    Attached is some technical info for the inverter. Pages 220, 12, 41, 2 & 36 may be of considerable interest. Page numbers mentioned here are the page numbers printed at the bottom of each page in the attached technical manual for the inverter. Thank you for taking the time to help.

    Attached Files:

  2. jcsd
  3. measuring RPM is best done with a Hall Effect sensor or something optical. It is very simple to do. Trying to do it by analyzing the current waveform? Ugh. I would have trouble trusting it, even if you can do it.

    And your pdf doesn't work for me. It also seems a bit small for a 220+ page technical manual. I think it did not upload completely.
    1 person likes this.
  4. vk6kro

    vk6kro 4,058
    Science Advisor

    The rotational speed of a motor can't be derived directly from its current.

    However, if you painted one side of the motor shaft white and the other side black, you could reflect light off it into a photoelectric sensor and this would give a stream of pulses out as the black and white sections reflect less and more light respectively.

    If you have access to EBay, the next step is easy. Buy a non contact optical tachometer.

    Measuring current in a three phase motor circuit has to be done carefully and professionally. Get an electrician to supply and fit a suitable meter.
    The device supplied will probably monitor the current in each phase.
    1 person likes this.
  5. Baluncore

    Baluncore 3,305
    Science Advisor

    I believe that parameter registers 52, 53, 54 and 158 can be programmed to select display of output current.

    You know the output frequency that you have programmed, so you know the synchronous speed of an induction motor, if that is what you are using. An induction motor will have a slip below synchronous speed that is proportional to current. Once you have calibrated your motor RPM to current you will be able to use current as a proxy for slip and from that compute RPM.
    1 person likes this.
  6. Last edited: Jun 18, 2014
  7. I have added a dropbox link for the inverter tech docs Sir. Please check out the pages I mentioned in my first post for reference to help suggest a suitable sensor to monitor current based on the demands of the motor.
  8. So in otherwords the synchronous speed = RPM (after calibration) ?

    Or is there an equation that relates both entities?

    Is there a relation to power and rpm?
  9. Baluncore

    Baluncore 3,305
    Science Advisor

    Are you using an induction motor? An induction motor has a slip, usually quoted in %.
    Slip varies between about 1% at no shaft load to about 10% at full load.
    The number of poles in a motor counts the N and the S poles, so the number of pole pairs, or field cycles, is half that.
    Induction motor theoretical synchronous speed in RPM = Hz * 60sec * 2 / poles = 120 * Hz / poles.

    For 4 poles at 50 Hz, synchronous speed would be 1500 RPM.
    Typically, with 4% slip that becomes the 1440 RPM speed rating on the plate.
    1440 RPM / 1500 RPM = 0.96 * synchronous speed = 0.04 slip = 4%.

    Slip is proportional to torque on the shaft. Therefore, for a fixed voltage, also to power and hence current.
    See the graph here;
  10. This makes lots of sense, thank you. Yes its an induction motor. So frequency is the only variable here. How do I obtain the variable frequency? Do you think I can obtain that from the inverter which powers the motor?
  11. Baluncore

    Baluncore 3,305
    Science Advisor

    You control the speed of the motor by programming the frequency produced by the inverter.
    Do you have a front panel on the inverter? or did the inverter have an external PLC?
  12. vk6kro

    vk6kro 4,058
    Science Advisor

    If you want to measure rotational speed of a motor, then you need a tachometer.

    Here (just as an example) is a tachometer:

    As you can see, they are available, delivered from China, for around US$10. This is probably a lot less you could build one for, even if you had some way of calibrating it.

    Current is measured with an AC ammeter. If the currents are large, or you want to monitor the current at a distance, you can use a current transformer followed by a suitable meter.
  13. Unfortunately it doesn't have a front panel but there's a PLC display on the machine.
  14. Please ignore the last last post. Auto correct got me there.

    Answer to your question is Yes, there's an external PLC on the machine on which the inverter is installed. Is it possible to pick off output frequency & current signals from the PLC via a DAQ to a computer for RPM computations like you suggested?
  15. Baluncore

    Baluncore 3,305
    Science Advisor

    If you have a display with up/down buttons on the VFD, then you can program it's parameters to show the motor current on the VFD.

    You will need to get the interconnection diagram that shows how the PLC is interfaced with the VFD inverter. That will show how you might get the analogue? parameter for display.
  16. Yea the VFD (Inverter tech manual attached) has a display as well as the tool the inverter is installed on outputs current and RPM readings on an inbuilt display. So I was wondering if there is a way to pick those analog or digital signals from the inverter or the machine (machine technical manual attached) then route it through a DAQ to a computer for analysis. Meanwhile I'll dig around for the connection diagram between the PLC and VFD). I checked the attached manual but didn't see anything promising. Please feel free to check as I may have skipped it without knowing. Thanks for all your effort and time Baluncore.

    Attached Files:

  17. Baluncore

    Baluncore 3,305
    Science Advisor

    Note; It seems that your .pdf attachments are actually .html links to the files.
    Download, then rename the attached filename.pdf files to filename.html to download the files.
  18. Baluncore

    Baluncore 3,305
    Science Advisor

    Page B33 of the 841 Technical Reference shows analogue speed control voltage and current monitor signals on the inverters connecting to the ANLG1 module. See attached image of part of that page.

    When you pick off the the voltages at one end of the screened twisted pair cables you will be able to digitise and record those parameters with an external recorder.

    Attached Files:

  19. Yea they are originally PDF files but I uploaded them to dropbox and shared the link on here because I couldn't upload them directly on here (Physics forum). If you click on it, it should download to your computer.
  20. Baluncore

    Baluncore 3,305
    Science Advisor

    Re: My post #17 attachment. Have you looked at page B33 of the 841 Technical Reference ?
    I believe that should now have solved your problem.
    1 person likes this.
  21. Yes Sir, thanks much for all the help. Hopefully I can successfully get the data routed to my computer.
    Well this is the main reason I'm hassling with this machine. Spindle current for the Z1 axis goes into
    over-current mode when it reaches 12.1 amps or more causing it to burn wafers occasionally. The pre-set parameter value is set to 12. I don't know why this is happening but I figured monitoring the current while in operation over time might help shed some light. Any ideas why this is happening? or suggestions on what to check for or look into? Thank you
Know someone interested in this topic? Share this thead via email, Google+, Twitter, or Facebook

Have something to add?

Draft saved Draft deleted