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Inductor from extension cord?

  1. May 1, 2014 #1
    We used to buy these cheap outlet strips that came with really long wires, wound up. They said to unwind them before use because they caused fires. I thought it was inductance. But now that I think about it, there's not just a single conductor with AC in there. There's also ground and neutral conductors. So, wouldn't those conductors get rid of the energy that the 'hot' conductor is puting into a magnetic field? Also the 'core' of these 'inductors' was simply more extension wire, ran in a loop through the first, outside loop and perpendicularly.

    Was it an inductor, or not?

    Thanks
     
  2. jcsd
  3. May 1, 2014 #2

    berkeman

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    No, not an inductor. Because the Hot and Neutral are run together, there is no net current in one direction down the cord.

    The fire warning may just have been because the wire/cable does not dissipate heat as well when it is all wound up. You shouldn't be running enough current in the cable anyway to get much of a temperature rise. That would be a bad design.
     
  4. May 1, 2014 #3
    I've never seen one catch on fire myself, but I've seen pictures. They are the cheapest made in china stuff that sometimes literally falls apart coming out of the package. I thought it was heat from the wire resistance as well. I just wondered if there was a reactive component to that.

    Thanks
     
  5. May 1, 2014 #4

    sophiecentaur

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    Cables are rated for Current under free air conditions. Coiling them up in a confined space seriously limits their heat dissipation and the appropriate de-rating should be applied for tightly coiled cable. Manufacturers publish specs to deal with this sort of thing, I believe (and also for routing cables in house insulation materials, these days).
    Coiled mains extensions are, effectively, bi-filar wound, so no inductance to speak of.
     
  6. May 1, 2014 #5

    davenn

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    yeah,
    I have personally seen several that have melted to an extent that the cable could not be uncoiled
    some serious heat buildup

    cheers
    Dave

    EDIT ...
    thank's sophi ... that's the explanation/term I was looking for :smile:
     
  7. May 1, 2014 #6

    sophiecentaur

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    They mostly have thermal cut-outs incorporated, these days. Annoying if you forget to uncoil the extension lead and plug in a heater - but a lot safer.
     
  8. May 1, 2014 #7

    berkeman

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    Bifalar winding is used to minimize leakage inductance in pulse/comm transformers, I believe. It does nothing to influence magnetizing inductance, which I think is what the OP's question is about.
     
  9. May 1, 2014 #8

    berkeman

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    Unless there is another meaning for bifilar winding, which is certainly possible. At its core, bifilar means to wind 2 conductors together in a magnetic arrangement, so I guess if the 2 conductors carry opposing currents, then your statement is correct. I'm just used to the comm transformer design context, where the 2 wires are primary and secondary windings, so they carry currents in the same direction.
     
  10. May 2, 2014 #9

    sophiecentaur

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    I was using the term 'bifilar' in the way I learned it, years ago. An extension lead on a drum is not wound as a transformer. Low inductance resistors are wound ('bifilar') the same way as the conductors on a cable drum.
    The transformer in an RCD is also wound 'bifilar', to cancel the effect of balanced currents in L and N.
    [edit: reading around this, it's clear that bifilar winding is done to achieve either of two things so we're all correct, in our particular contexts]
     
    Last edited: May 2, 2014
  11. May 2, 2014 #10

    sophiecentaur

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    I just spotted this and, although it could be looked upon as 'true', there is no more significance in the Field caused by current in the L conductor than that caused by the N current. As it happens, there is no 'Energy' because there is no resultant Field - and it's Fields that add vectorially and not Energies. Any current that flows in the E conductor, due to leakage, could be in either direction; the important thing is that the sum of the three currents is zero (unless there is another path to Earth) .

    The material of the drum etc (what would be the 'core' material is not important because the H field is zero). Just as well, because drums and frames are often made of steel for strength.
     
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