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Parallel control lines for two VFD's

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  • Parallel control lines for two VFD's

    This question is almost tailor made for J-tiers

    Presently have a VFD for the standard spindle motor on my bridgeport cnc. The 0-10V control line comes from a mesa electronics interface board. I just ordered a 24,000 rpm 1.5kw air cooled spindle with matching hungyang vfd to use as a auxillary high speed spindle for engraving and such.

    I am thinking of using the 0-10V speed control output from the interface board and paralleling it to both VFD's speed control input. Only one VFD would be given a run command, the other would sit powered but idle. I am thinking the 0-10V inputs on common VFD's are probably reasonably high impedance and I could get away with using the same control voltage to both.

    Calibration isn't a issue, I would have a separate configuration file for the cnc controller for each spindle setup. Again, both VFD's would be powered but only one would get a "run" command. The run fwd , run rev etc lines would be isolated outputs from the control board to each vfd, no control lines shared except the 0-10V speed control line.

    The 0-10V control voltage is sourced from the control board, the 10V supply output on the drives is not used.

    Sound like a reasonable config?
    Last edited by Sparky_NY; 04-09-2020, 10:10 PM.

  • #2
    Maybe.

    GENERALLY, the control input portion is independent of any other wiring. It would be good to verify that the grounds are in fact independent, not connected to anything else.

    If that is the case, and the two use the same control voltage range, then you "should" be able to parallel the inputs as long as the "commons" are connected together. You would, as you mention, ignore the control voltage outputs. But make sure the external voltage is well regulated, and noise-free.

    Impedance? WEll that depends. What I have run into has generally been in the range of 20 to 30 k ohms for an input. Half that for the two in parallel. You would want to make sure that the source is 10x lower in impedance, which means a control pot that is certainly not over 2K. Take half the highest allowable control pot as the highest usable for the two VFDs together (if the same maker)

    Yes, you need both powered, the impedance may drop if the input is not powered, AND the unpowered one could severely limit the control voltage to the other one.

    If both are same type, and the common connections are floating, not connected to anything inside, Then I give it a good chance. With different makers or models, you will have to check more closely to be sure the inputs are compatible.

    If other control inputs are also paralleled, That needs more looking. Different units may have different settings for "low" and "high" levels for other inputs, or may have interactions that I cannot predict.

    Even there, it would probably work as long as the polarity of inputs is the same; i.e. "start" is a low or high for both, "stop" ditto., etc. That can often be programmed.
    Last edited by J Tiers; 04-09-2020, 10:30 PM.
    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.

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    • #3
      Appreciate the quick reply ! VFD manufacturers of course do not publish schematics for public consumption so much of this subject is a best guess proposition. Your background working on these makes your "best guess" much better odds than average.

      Thinking this over with the morning coffee I may have another/better solution. The hunyang has a RS485 port also available for control and my linuxcnc control supports RS485 spindle controls. That would make ALL interfacing to the hunyang VFD digital via the RS485 serial method. The first/original vfd for the main spindle can remain as-is with no wiring changes, just not enabled via the software in the controller. NO shared lines on the two VFD's. This is looking like the best approach. Linuxcnc controller software even has a driver available specifically for Hungyang spindle control via RS485 making it a pretty easy proposition.

      Considering the hunyang vfd and its high speed spindle will be something rarely used, it may be nice to de-power the hungyang primary power when not needed. I have some 50A solid state relays around, what are the pros/cons of using a SSR to switch the hungyangs primary on/off ??????? If there are issues using a SSR, a simple DPST switch of suitable ratings on the primary power wouldn't be objectionable either.
      Last edited by Sparky_NY; 04-10-2020, 09:02 AM.

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      • #4
        That seems like it has fewer questions than the other approach. The RS485 are clearly isolated.

        SSR is OK for switching, as long as you don't expect it to be a disconnect (and you already know that). A mechanical switch is fine too, of course, unless there is a reason to use a controller for it.
        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.

        Comment

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