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Tach from VFD?

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  • Tach from VFD?

    I have a Hitachi wj200 VFD on my mill, works good. I've been wanting to add a tach and was going to go with a hall sensor one but I think the VFD might be able to provide this data. I've searched through the manual and am having trouble determining if this is doable and if so what I would need for an output display (digital preferred but analog meter ok too)

    To complicate it more I was thinking about getting a TouchDRO rig for my cheap scales because I don't like the little displays that came with them. I wonder if the RPM data from the VFD can feed into the TouchDRO too?

    If anyone is able to share some experience with all this it would be greatly appreciated.

    Thank you.

  • #2
    If you look on page 3-8 you will see d-007, which is "scaled output frequency".

    You put in a number that the frequency gets multiplied by. I think it must be whole numbers, integers.

    For any one setting of whatver belts there may be, this can give you approximate rpm.

    If the motor directly drove the spindle, and it was a 1725 rpm motor, you would multiply the frequency by 29. so 60 x 29 = 1740, and 30 x 29 = 870. If there is a belt ratio of 3:1 reduction, then you would have to divide the 29 by the ratio, giving as the best number probably 10. Then, 60 Hz would show up as 60 x 10 = 600 Actual for that is more like 1725/3 = 575, so error is not too bad.

    And, if you had a step UP, you would multiply by the ratio, so a step-up of 2:1 would show as 60 x 58 = 3480 (actual would be 3450, again not too bad.

    But every change of belt will change the speed factor. You might just leave it on Hz and relate to the nominal speed by proportions..... if the nominal speed is 575 rpm, then 30 Hz would be nearly 300 rpm, etc.
    CNC machines only go through the motions.

    Ideas expressed may be mine, or from anyone else in the universe.
    Not responsible for clerical errors. Or those made by lay people either.
    Number formats and units may be chosen at random depending on what day it is.
    I reserve the right to use a number system with any integer base without prior notice.
    Generalizations are understood to be "often" true, but not true in every case.


    • #3
      I do this with my Automation Direct VFD on my machines. I checked my mill and it is dead on with in 2 or 3 RPM using one of those laser tachs.
      Grantham, New Hampshire


      • #4
        I do this with my SJ100 to display the speed on the VFD. It means that I can't do this and change the belt position but I'm happy with the compromise.


        • #5
          I use the scaled output on my WJ200 via the remote mounted head option (vfd is in a chip proof box on the back of the mill).

          The WJ200 has a 1000 step PLC built in. You might be able to program it (easy.. just like a spreadsheet) to read a 4 position switch (assumes you have 4 belts positions) and change the constant.

          The VFD also puts out frequency dependent 0-10v. You can feed that to a dirt cheap panel meter through a 4 position switch with simple resistor dividers to give a voltage proportional to frequency and belt position.
          Last edited by lakeside53; 03-29-2017, 01:17 PM.


          • #6
            Most VFD's I've work with had an analog output. A resister and a panel meter plus a stock-on Corel Draw meter scale and you have a tach you can plant in line of sight. Tacks are handy but a load meter is handier.

            I can eyeball speed by tool and chip behavior. Load can be a little tricky unless you have motor or gear noise corresponding to a musical pitch. 5% motor slip at full load is almost exactly a mucical half step. Got a good sense of pitch.

            Point is, your senses can be trained to where you're less dependent on read-outs and instrument readings. That doesn't mean you can bypass numbers altogether - just saying: Use all thetools you were born with.

            Ahem! Back to the OP: Look in the manual on your VFD. Even the basic consumer grade VFD's are talented gadgets. There's almost sure to be a scaling factor, analog output, or some means of connecting a simple display.


            • #7
              For load I use the amps display function of the VFD. Not so much for "absolute load' ; the chips etc tell you that, but for tool wear. I tend not to thrash my machine but a 50% increase in current for same feed/feed tell me it's time to look at or change tooling. A sudden jump means I just crumbled a tip/edge!