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3 phase sewage scraper bridge with 24VAC control issues

  1. May 15, 2016 #1

    I'm having a lot of issues where I work with a sewage scraper bridge chattering it's drive contactors when being called to run, the coils for
    the contactors are switched on their neutrals from an Allen Bradley logix processor. The control circuit is 24v AC. I've spent a few hours on this over the past week with little success. I appear to be getting large volt drops on to the coils of the contactors, down to 13v in places. I've replaced both the control transformer and the Allen Bradley processor which has made no difference.

    It's worthy of note that a new guy has started and installed polarity sensitive beacons on top of the control panels to signal when the bridge is running. Depending on where he's tapped into the supply, could this cause the issue, I've not been back to look yet to see if this is the problem, could it be likely?

    Here are some drawings






    Hoping you guys can give me some pointers.

  2. jcsd
  3. May 16, 2016 #2


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    What do you mean by "polarity sensitive" beacons? If the controls are ac then there is no polarity.
    Could the use of dc(?) beacons be causing an issue? quite possibly. I would disconnect the beacons and see.
    If so get 24vac beacons.

    Do the contactors for the scraper lift , and lower chatter only or are there more that chatter?
    Last edited: May 16, 2016
  4. May 18, 2016 #3


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    If there are voltage drops on the coils work back from there to the power source to see where the drop is introduced.

    Bad cable joint?
    Wire gauge too small?
    24v supply overloaded?

    PS: Also check for problems with any common return (0V or earth). It might not be the 24V "coming down" but the return/earth "coming up".
    Last edited: May 18, 2016
  5. May 18, 2016 #4

    jim hardy

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    Analytical Troubleshooting training courses teach :
    'if it used to work and now it doesn't work, something has changed.
    Things can change by normal wear and tear,
    or they can be changed by somebody tinkering around.
    If somebody has been tinkering , that's the first place to look. '

    What wires got moved?
  6. May 19, 2016 #5


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    Tinkering has brought on several tears over the years, for me. Very good point.
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