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Hard cable

  1. Mar 1, 2006 #1

    wolram

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    We have an inverter driven 3kw motor at work that is the main drive for
    a piece of kit we have, it drives via a 300-1 or so gear box, you can turn
    the whole thing with one finger via the fan on the motor, the problem is
    the 3ph cable to the motor hardens an breaks down, it is the recomended
    cable to use (screened) and correct conductor size, the motor hardly gets
    warm in opperation, and the cable only hardens at the motor end, any one
    have an idea? if you do i can supply free triffles, cheese cakes, for a year.
     
  2. jcsd
  3. Mar 1, 2006 #2

    berkeman

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    Can you post a picture? Also, what voltage is the 3-phase power typically?
     
  4. Mar 1, 2006 #3

    Bystander

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    Ozone. Curing things is a bit trickier --- or, maybe just not possible. Had several vacuum systems that used to "eat" power cables, usually to the roughing pumps, and same power cable used for diffusion pump inches away was fine, occasionally vice versa. Went to "new cables every 6 mos." to avoid troubles --- didn't really "solve" the problem, just dealt with it. Picked up non-rubber insulation (vinyl? Just about universal on 10-20 amp power cords these days) and got much longer service.

    "Ozone" was the "folk" explanation --- might actually be high voltage from inductive flyback at "unfavorable" load conditions devulcanizing (hardening) the rubber jackets. Check with your electrician(s) about hanging bypass capacitors in the circuit, or tweaking power factor.
     
  5. Mar 1, 2006 #4

    wolram

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    WHOA, Bystander, i know that these things can deflect voltage back to
    the inverter, giving twice the output voltage, the motor does live in a damp
    enviroment, but i am not at your level of understanding, how can changing
    the power factor help?
     
  6. Mar 1, 2006 #5

    Bystander

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    Tweak the power factor and cut the flyback voltage. Or, "soften" the spikes a bit with bypass capacitors.

    This is assuming that the speculation about voltage spikes breaking down rubber insulation is even close to a mechanism for what happens.
     
  7. Mar 1, 2006 #6

    dlgoff

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    You want to make sure the cable connections are very good i.e. as low resistance as possible. Is there a torque specification for the connector bolts?
     
  8. Mar 1, 2006 #7

    berkeman

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    Pictures. We all want pictures. We want to help! (and learn too...)
     
  9. Mar 2, 2006 #8

    wolram

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    There may be a spec for nut torque but it is not in the paper work we have.
     
  10. Mar 2, 2006 #9

    wolram

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    I doubt if a picture would help, the cable looks the same along its lenght
    It is PVC covered cable by the way not rubber.
     
  11. Mar 2, 2006 #10

    wolram

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    It is 240v 3ph.
     
  12. Mar 2, 2006 #11

    berkeman

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    When you say that the cable hardens and breaks down, what do you mean? Is the insulation or the metal conductors that harden and break down? What is the failure exactly?
     
  13. Mar 2, 2006 #12

    wolram

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    On the inverter we get a motor over volt message, when we test the cable
    ( disconnected from motor) between conductors we only have a few meg
    ohms resitance, it seems the insulation is breaking down, the first motor
    had been on the machine for 5 years, we mistakenly sent it for a rewind
    only to find it was a cable fault, that cable has been replaced, but after
    only 8 months is showing the same fault.
     
  14. Mar 2, 2006 #13

    wolram

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    There is a second cable that goes to this motor, same junction box, that
    used to opperate a clutch brake, it is not connected at the motor end as we no longer use clutch brake, this cable is still supple.
     
  15. Mar 2, 2006 #14

    berkeman

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    Do you think the ozone suggestion might be valid? If so, just aim a fan from a clear air area at the end of the cable at the inverter. That should keep the ozone away from the insulator/cable.
     
  16. Mar 2, 2006 #15

    wolram

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    I not sure about ozone, all i can say is the room is at a constant 10c and
    is all ways damp, is this good conditions for ozone production?
     
  17. Mar 2, 2006 #16

    berkeman

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    I think it's arcing that produces the ozone, but I don't know much about that. It sounds like a refrigeration application (10C and damp) -- is the cable getting wet too?
     
  18. Mar 2, 2006 #17

    wolram

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    The cable is in the bowles of the machiene, so it is well protected from any
    washing that takes place every night, but i guess it could get damp at times
    from evaporation of cleaning water.
     
  19. Mar 2, 2006 #18

    wolram

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    I have found this,

    At high temperatures the material becomes soft so that conductors which are pressing on the insulation (eg at bends) will 'migrate' through it, sometimes moving to the edge of the insulation. Because of this property the temperature of general purpose P.V.C. must not be allowed to exceed 70°C, although versions which will operate safely at temperatures up to 85°C are also available. If p.v.c. is exposed to sunlight it may be degraded by ultra-violet radiation. If it is in contact with absorbent materials, the plasticiser may be 'leached out' making the p.v.c. hard and brittle.

    i do not know of any absorbent material ?? sun light, no chance.
     
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