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Thread: 4-20ma controls

  1. #1
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    I find myself working on process chillers more frequently.
    One brand is controlled by a PLC with Danfoss 4-20ma sensors and valves.

    Low preasure and High preassure controls are thru the PLC and are 4-20ma.

    When recharging the unit from an empty condition, any suggestions on a work around for the low preasure sensor until the preasures get back into operating range?

  2. #2
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    4-20ma with what supply voltage? (5 or 10V or 24)

    A sample chart:
    ----------------------------------------
    | Percent of | 4-20 mA | 1-5 V |
    | measurement | signal | signal |
    ----------------------------------------
    | 0 | 4.0 mA | 1.0 V |
    ----------------------------------------
    | 50 | 12.0 mA | 3.0 V |
    ----------------------------------------
    | 100 | 20.0 mA | 5.0 V |
    ----------------------------------------



    [Edited by sysint on 12-20-2004 at 07:47 PM]

  3. #3
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    24

  4. #4
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    just to add

    replace the 250 ohm resistor with a 500 ohm and you will rescale your 0 - 5 vdc signal to a 0 - 10 vdc signal.

    remove the resistor and you have a straight 4-20 Ma signal.

    Allen

  5. #5
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    Originally posted by wolfdog

    When recharging the unit from an empty condition, any suggestions on a work around for the low preasure sensor until the preasures get back into operating range?
    On the interface is there an option to change the low pressure setting ? If there is just lower it to a point where the chiller doesnt alarm while you are charging it. You should be able to do this on the high side as well (except raise it). Put them back to original after you are done.
    Since the chiller is empty you might want to weigh in your charge according to factory specs. This should put you in the pressure range and stop any annoying alarms or protection devices that might keep you from starting it up. good luck and nice diagram sysint

  6. #6
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    This chiller does not allow for changing the preasure settings.

    I have been dialing in about 75% of span with a Fluke process meter.

    I just wanted to see what bright ideas others might have and support the new area.

  7. #7
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    You have to watch out for the max resistance you can dump into the line before impedance is too high. I think it is around 1000 ohms. (typically) -EDIT How about 1200 since I did the math.

    [Edited by sysint on 12-21-2004 at 07:43 AM]

  8. #8
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    I don't suppose you have access to the PLC program? I work with PLCs on a daily basis and use the PLC programming software to help in me troubleshooting. You can force inputs/outputs to do watever you want.

    Getting the programming software might seem overkill, but I would think that as more PLCs are used in the field, having a way to monitor the ladder logic would be a great benefit.

  9. #9
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    It would be nice if the PLC people would allow a timer override on a jumped input to ignore the LP switch for a preset time.

  10. #10
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    Hey wolfdog how about an Altek Simulator? If you can disconnect the switches and use it as an input

    http://cgi.ebay.com/ws/eBayISAPI.dll...sPageName=WDVW

    Im currently looking for a 0-10VDC one or possibly a Transmation 1040

    Sorry didnt see your Fluke comment
    Quote Originally Posted by MatrixTransform View Post
    very soon it is you that will be pwned

  11. #11
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    I have calibrators by Ronan and Transmation.

  12. #12
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    Lift the 4-20 ma input to the system and put a 4-20 ma loop calibrator in and set it to a setting that will allow operaition of the unit. Just fool it with the calibrator.
    John

  13. #13
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    Originally posted by sysint
    It would be nice if the PLC people would allow a timer override on a jumped input to ignore the LP switch for a preset time.
    Most PLC programs have the ability to "force" and output.

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