Thank you for your reply. You make a good point: the importance of matching 'design to function'.
These analyses were purposely done on 'isolated' conductors, because the original intent was to make a conductor between two points - a lightning simulator. You make a great point in talking about the 'extra' fields involved when the conductor is used to make a coil and reappears near itself. However, I have NEVER gotten full conductivity in a flat foil. If the thin dimension is equal to the skin depth, for the wide dimension all the carriers bunch up to the edges equal to the thickness of the foil, and (again) most of the middle section is wasted. For a coil made from a round conductor, the carriers all bunch along the conductors at the minimum radius to the center of the coil, which means the outer sides of the conductors are wasted. Just have to match shape to function. Robert > The setup is rather artificial in that it compares isolated conductors. > The situation flips around for proximal conductors due to proximity > effect. Try modeling a coil with a foil winding versus round wire. Flat > adjacent surfaces perform better than adjacent round ones because the > current bunching is less pronounced in the foil. Also note the huge > difference in area efficiency. As you mentioned, for small skin depth > compared to diameter most circular cross sectional area is electrically > wasted. Foil of suitable thickness (not too thin and not too thick for > the frequency of interest) will carry current throughout the entire cross > section, although there will be higher current density at the foil edges > than down the center. Round is not a panacea, and in certain situations > can be distinctly inferior. > > Orin Laney > > On Thu, 5 Nov 2009 17:27:34 -0800 Robert Macy <[email protected]> writes: >> Flat rectangular is NOT better than a round tube if each have the >> same >> circumference. >> >> The round tube has lower inductance than a flat ribbon conductor of >> comparable circumference. Makes sense, because Nature abhors >> 'pointy' >> stuff. >> >> This statement is based upon a comparison analyzing the two >> structures >> using finite element analysis, femm 4.2. >> >> Given: >> solid ribbon copper conductor, 1 inch wide and 120 mils thick >> copper tube, 0.75 inch outside diameter and 120 mils wall thickness >> >> The two circumferences are approximately the same. >> Rectangle 2 * (0.12+1) = 2.24 inches >> Tube pi * 0.7 = 2.20 inches >> >> Which is a better conductor? >> >> Assume infinitely long, straight conductors. Assuming most of >> lightning >> energy is significant between 1MHz to 100MHz, calculate each >> conductor's >> characteristics at 10MHz and at 100MHz using 2D finite element >> analysis. >> >> Method: place each conductor in a 24 inch diameter metal 'tube' to >> provide >> return current and represent infinity. Note: I also used single >> conductors in free space with current return at infinity. Values >> changed, >> but the conclusions did not change. Use a 12 inch length to >> reference >> values per ft. Mesh was set to be fine near the surfaces of the >> conductors, so that even with hgih frequency currents inside the >> conductors were accurately represented. Skin depth was more than 3 >> nodes. >> >> From results, the current as a function of depth into the conductors >> matched expected values. Plots of current/eddy currents verified >> mesh was >> of suficient density for these calculations. >> >> TABLE RESULTS: >> >> Ribbon - Solid mesh 69,406 >> 10MHz 210 nH/ft 0.00347 W >> 13.2j ohm/ft 83 milliohm/ft >> 100MHz 210 nH/ft 0.0115W >> 132j ohm/ft 0.15 ohm/ft >> >> Tube - Hollow mesh 90,233 >> 100MHz 203 nH/ft 0.00239W >> 12.8 ohm/ft 69 mOhm/ft >> 100MHz 203 nH/ft 0.00732 >> 128 ohm/ft 0.12 ohm/ft >> >> It was interesting to note that with a wall thickness of more than 10 >> mils, at these frequencies the metal was doing nothing but >> physically >> supporting the outside layer. >> >> CONCLUSION: >> Round is always better. >> >> Robert >> >> > As a rule of thumb, a conductor >> > >> > has an (self)inductance proportionally inverse with >> > >> > it's circumference (if fact the shortest way the magnetic field >> lines >> > will take). >> > >> > Big flat conductors always perform better then round ones, as they >> > >> > have the highest circumference per kilo. >> > >> > Litz and silver(gold) coated conductors do contribute to the >> > >> > real part of the impedance (=resistance) properties only. >> > >> > Litz by increasing the conductive surfaces so reducing >> > >> > the resistance increase caused by the skin effect. >> > >> > >> > >> > >> > >> > >> > >> > Regards, >> > >> > Ing. Gert Gremmen >> > >> > >> > >> > [email protected] <mailto:[email protected]> >> > >> > www.cetest.nl >> > >> > >> > Kiotoweg 363 >> > >> > 3047 BG Rotterdam >> > >> > T 31(0)104152426 >> > F 31(0)104154953 >> > >> > >> - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <[email protected]> All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc Graphics (in well-used formats), large files, etc. can be posted to that URL. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <[email protected]> Mike Cantwell <[email protected]> For policy questions, send mail to: Jim Bacher: <[email protected]> David Heald: <[email protected]>
