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Skin effect and hollow conductor

Discussion in 'Electrical | Electronics | Communications' started by lal, Dec 3, 2012.

  1. lal

    lal Moderator

    Engineering Discipline:
    Electrical
    When current flows through a conductor in AC mode, it does not make use of the full cross section of the conductor available. The core of the conductor does not carry any current, rather only the surface and a little portion below it (skin depth) would take part in conduction. Skin depth is the thickness at which the current density is reduced by 63%. Thus a solid conductor in effect acts as a hollow one, increasing the effective resistance offered to the current flow. The amount of conductor cross-section used for current flow is dependent on the frequency. Higher the frequency, lower will be the cross-section used and hence higher resistance for the same conductor. Thus the resistance offered to AC is higher than the DC resistance by some factor. I've seen AC resistance to be taken 1.6 times DC resistance for 50 Hz AC supply (though some sources say it is 1.1 times or higher).

    Why don't we use hollow conductors when it will save a lot of material in long distance transmission? Saving material means less cost too. Or is it not feasible to make hollow conductors for such long distances?
  2. Jeffrey Samuel

    Jeffrey Samuel Star

    Engineering Discipline:
    Electronics
    Nice thinking friend

    Let me quote you an example here Let us say a cable of 50 mm Dia and due to skinning the effective area used for transmission is say 43 mm. Secondly let us take a hollow material as you say having an exterior diameter of 50 mm and the interior diameter of 45 mm. Therefore the effective width of the conductor is just 5 mm

    Now we know the resistance offered is inversely proportional to area of cross section So you decide which conductor will have HIGHER RESISTANCE.

    That is the only reason why we prefer core conductors than hollow ones.

    I want you to prove my theory other wise Buddy cos I am just stating my opinion That can not be 100% true
  3. Naga teja

    Naga teja Certified CEan

    Engineering Discipline:
    Electrical & Electronics
    yes we can use hollow conductors.but some factors dominate the use of cored one rather than hoollow one.one of such factor is mechanical strength,which is very important consideration in transmission lines.
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  4. A.V.Ramani

    A.V.Ramani Mentor

    Engineering Discipline:
    Biomedical
    At low frequencies skin effect is not very prominent. At 50 Hz it is 30 mm for copper, which means that skin effect comes into play above 60 mm diameter for round conductors If one uses a bus bar (that is rectangular section) this can be easily by passed by having a thickness less than the skin depth.
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  5. solar morpher

    solar morpher Certified CEan

    Engineering Discipline:
    Electronics
    Ever wonder why we don't use single core cables for power transmission but always a multi stranded cables. This reduces skinning effect as the conductor is effectively not a cylinder but more pieces having rectangular cross section. This can help in reducing skin effect to great extent.
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  6. lal

    lal Moderator

    Engineering Discipline:
    Electrical
    Well, till date I thought a stranded conductor was used because it gave better mechanical strength and flexibility. I found a multi-stranded cable called 'Litz Wire' used to reduce skin effect. but in that each strand is insulated from the other. In that, the conductors are woven and twisted in a particular patten to reduce skin effect and proximity effect. I doubt how an ordinary multi-stranded conductor can reduce skin effect as all the conductors together act as one conductor ultimately.
  7. lal

    lal Moderator

    Engineering Discipline:
    Electrical
    Sir, I read the skin depth is only 8.5 mm for copper at 60 Hz here in a wikipedia page. And when I calculated for 50 Hz, I got the skin depth as 9.3 mm for copper and 11.6 mm for aluminium.

    So I think its safe to conclude that we do not use hollow conductors for transmission of AC for the reasons that skin effect is not prominent at low frequencies and making flexible hollow conductors is a tough task.
  8. lal

    lal Moderator

    Engineering Discipline:
    Electrical
    Actually its the opposite, due to skin effect 43 mm gets unusable. That means only the outer portion of the conductor conducts which effectively make the solid conductor act as a hollow one.
    This page may be useful
  9. A.V.Ramani

    A.V.Ramani Mentor

    Engineering Discipline:
    Biomedical
    You are right. I made a mistake. Instead of 9.22 I read it as 29.22 mm for Cu at 50 Hz. Still a bus bar of rectangular section less than twice the skin depth will work.
    Between solid and hollow core conductors, if the diameter is more than twice the skin depth, a hollow core conductor (but with the same cross sectional area) will work better than the solid core. It is necessary that the cross sectional area is same for a proper comparison.
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