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Electro-magnet myth?

  1. Mar 11, 2009 #1
    Hey, I've been running into a bunch of lighting and sound techs that keep trying to tell me that leaving power cable coiled and not figure 8-ed will cause the wire in the cable to heat up to higher levels than the amp load on the individual wires (so much that it is dangerous to the life span of the cable) due to the magnetic field produced by the electricity running through the coils. I don't think this is true at all, but it has been a while since I took a physics of electricity class to come up with a reasonable argument other than to say that I don't remember magnetic fields causing extra heat in the coils that actually make up the electro-magnet, and furthermore wouldn't the neutral leg in the cable actually negate most, if not all, of the magnetic properties of the coil? Another aspect to this is the notion that the magnetic field is strong enough to cause distortion in speaker cable or the pick-ups of guitars, which seems a little more plausible if there is a sizable magnetic field created as both speak-on cable and pick-ups are notoriously touchy when it comes to electric noise.
     
  2. jcsd
  3. Mar 11, 2009 #2
    That's a little "out there". So coiling the power cord increases inductance, resulting in higher dissipation due to maybe- skin effect? I don't know what's being implied here. Maybe you can ask them for a detailed scientific explanation of what is happening when the power cord is coiled.

    The power cord, I presume has 2 conductors, hot & neutral. They are in close proximity and coiling them does not increase the inductance by much, not the transverse inductance anyway. The common mode inductance increases when the power cord is coiled, which is a good thing as far as noise goes.

    I would ask the person(s) making such claims if they've heard or measured this difference, or if they are just repeating what they've been told. I'm not discrediting anyone, but merely asking for elaboration. There may be an underlying reason not explained to you.

    As far as power cords coupling noise into the guitar pick-ups, I've always seen techs keep power cords away from low level guitar wires, and if necessary to cross them, they do so at right angles to minimize crosstalk. Maybe a figure 8 makes this easier.

    Elaboration on their part might clear things up.

    Claude
     
  4. Mar 11, 2009 #3

    Vanadium 50

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    I don't think this has anything to do with electromagnetism. If you have a lot of air around your cables, it can carry heat away. If instead of air, you have other cables, this cooling is less efficient.
     
  5. Mar 11, 2009 #4

    mgb_phys

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    Exactly there are wiring specs based on how much current a given cable size is permitted in a conduit with other cables, buried in a wall, or in air.
     
  6. Mar 11, 2009 #5
    A coil of wire has inductance if the current in it creates a magneic field. You have both the current in the hot wire and the current in the neutral (return) wire, so the currents cancel out and you have no net inductance. But the wire will heat up just from its resistance, and amp load on individual wires, and lack of cooling from ambient air flow.
     
  7. Mar 12, 2009 #6
    This is somewhat complicated, but nonetheless easily understood.

    A "coiled" power cable does indeed have an inductance effect. However, the power supply unit at the exit-end of the the power cable usually compensates through electronics; thus delivering clean power.

    The greater problem is the fact that most power cables are not shielded, and those cables lying close to data cables can cause problems, especially if the data cables are not shielded and grounded(or only minimally so).

    In short, I would not worry so much(if at all) about how the power cable(s) are laid; rather their proximity(closeness) to the data cable(s).

    Keep the two at least 1-foot away from each other. More is better. 3-6/ft is really good.
     
  8. Mar 13, 2009 #7
    In practice, coiled power cable will heat itself more, even to melt....and straitened will be less warm....
    Ideal for fire is intercrossing of two cables....

    For signal flow, it is best always to be properly massed....to have proper el cage-shield....(good example is high voltage air cables + two leading cables as shield....)

    It is unbelievable how easy you can keep S/N and disturbance out of signal and cable....
     
  9. Mar 20, 2009 #8
    Thanks a lot guys!
     
  10. Mar 20, 2009 #9
    Coiling a power cable, if the cable includes both the current in the hot wire, and the equal returning current in the neutral wire, does not increase the inductance. Inductance is the energy stored in magnetic fields per unit current (actually (B/I)2).

    One example of a cable that depends critically on inductance is a coaxial cable such as RG-8. In RG-8, there is energy stored in the magnetic field between the inner and outer conductor, and none outside. Coiling the coax cable does not change this inductance. The stored energy in inductance can be written (with permeability u = 1)

    (1/2) L I2 = (1/2) u0 integral [ B2 dV]

    where L = inductance, B = magnetic field, and the integral of magnetic field is over volume.

    This leads to

    L = (u0/I2) integral [B2 dV]

    So if the value of the integral does not change, the inductance does not change.

    In the case if the power cable, although the hot conductor and the neutral conductor are not concentric, the magnetic fields are still local to the two conductors (a pair). But because there are some fringe fields, there could be some cross talk to adjacent cables. To minimize this, the two conductors are twisted together to form a twisted pair, a configuration often used in data cables..

    But the noise in power cables (fringe fields), due in part to SCRs and Triacs causing spikes in the current, can couple to adjacent data cables, so they should be kept apart.
     
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