I never thought of skin effect as an advantage where current carrying is the point of interest. It reduces the effective current carying cross section as a function of frequency. It does mean that a thin foil can shield well for high frequency (low current relative to the cross section).
A far as concentric conductors, you just look at the cross section of each. On Fri, Apr 15, 2016 at 4:53 PM, Jim Walls via EV <[email protected]> wrote: > I have seen it both ways. > > Jim Walls > > > > > ---------------------------------------- > From: "Cor van de Water via EV" <[email protected]> > Sent: Friday, April 15, 2016 1:49 PM > To: "Electric Vehicle Discussion List" <[email protected]> > Subject: Re: [EVDL] Aluminum cables > David, > I think there is a little confusion between "individual wires" and > cable. > What I typically see is a steel core (for example 1/4" dia) with 6 alu > wires > wound around it, each another 1/4" so total cable dia is 3/4" and to the > electric field it looks like a wire that is 3/4" dia with the core 1/4" > steel and the "skin" of alu is 1/4" thick so this captures almost all of > the skin-effect current (6mm versus 8mm) > Hope this clarifies, > > Cor van de Water > Chief Scientist > Proxim Wireless > > office +1 408 383 7626 Skype: cor_van_de_water > XoIP +31 87 784 1130 private: cvandewater.info > > http://www.proxim.com > > This email message (including any attachments) contains confidential and > proprietary information of Proxim Wireless Corporation. If you received > this message in error, please delete it and notify the sender. Any > unauthorized use, disclosure, distribution, or copying of any part of > this message is prohibited. > > -----Original Message----- > From: EV [mailto:[email protected]] On Behalf Of EVDL > Administrator via EV > Sent: Friday, April 15, 2016 12:57 PM > To: Electric Vehicle Discussion List > Subject: Re: [EVDL] Aluminum cables > > On 14 Apr 2016 at 14:31, Jim Walls via EV wrote: > > > Because of skin effect, the outer portion of a wire is carrying most > of > > the current, so having a steel core does not affect the current > > carrying capability much. The stuff I have seen has individual wires > a > > little under quarter inch in diameter with the steel core being about > > 1/3 of the diameter. > > So the AL part would be !/4" / 3 == 1/12" thick or 2mm thick. Under > Skin > Effect Wikipedia (not always reliable) says that at 60Hz skin depth is > about > 8.5mm. So I would think that here skin effect wouldn't apply much if at > all, > and the entire conductor would be carrying the current roughly equally. > But > then I'm not an engineer. > > As that hobbyist-not-engineer I would also think that the effective DC > or LF > AC resistance of the wire would be equivalent to two resistors in > parallel, > with the resistances of the core and the shell proportional to their > respective cross-sectional areas and to their materials' respective > resistance per unit of length. (But how would this be affected by the > fact > that they are in contact all along their lengths?) > > A 6mm dia conductor would have a total cross sectional area of 28.26 > mm^2. > If the steel core is 2mm dia that is 3.14 mm^2, so the AL area is much > larger, 25.12 mm^2. > > I found the resistance of 2/0 carbon steel wire listed as 0.00212 ohms > per > meter. That is a 67.43 mm^2 cross section. A steel cross section of > 3.14mm^2 would then be 0.0455 ohm/m. > > The resistance of 2/0 AL wire is 0.00042 ohms/meter, so an AL cross > section > of 25.12mm would be 0.00113 ohms/meter. > > Thus, ignoring skin effect and assuming that the conductors being in > contact > the whole length has no effect, the effective resistance of a 1m length > of > this 6mm steel core cable should be 1/(1/R1 + 1/R2) == 0.00110 ohms (1.1 > m- > ohm). > > For comparison, a copper wire of the same size would be 0.000598 > ohms/meter > (about 0.6 m-ohm). That's roughly half. > > BUT, I think I recall reading that an important limitation in long > distance > power transmission is the tensile strength of the material, and that > copper > would need to be larger than necessary for the power itself just to > support > its own weight. It would be more expensive, too. Someone correct me if > my > memory is off here. > > > Cables are made from a bunch of these wires. What I was holding was a > > bundle about an inch and a half in diameter. > > So, in a stranded cable, does skin effect apply to the entire twisted > bundle, or to each strand individually? > > If the former, then you're not getting any skin-effect advantage from > the > AL's conductivity over the steel's, because they're intermixed rather > than > having an actual steel core. It might be just the resistances in > parallel. > > If the latter, uh, then it would appear that skin effect doesn't help > much > there either because of what I said above. > > But I might be totally wrong about this. I really don't understand skin > > effect very well, sorry. > > David Roden - Akron, Ohio, USA > EVDL Administrator > > = = = = = = = = = = = = = = = = = = = = = = = = = = = = = > EVDL Information: http://www.evdl.org/help/ > = = = = = = = = = = = = = = = = = = = = = = = = = = = = = > Note: mail sent to "evpost" and "etpost" addresses will not > reach me. 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