Point well taken. I should have elaborated a bit more on why I concentrated at the higher frequencies.
Robert > Lightning detectors function (among other frequencies) > > down to 10 kHz. My opinion is that most of > > the lightning energy is below 1 MHz. > > > > > > Regards, > > Ing. Gert Gremmen > > > > g.grem...@cetest.nl <mailto:g.grem...@cetest.nl> > > www.cetest.nl > > > Kiotoweg 363 > > 3047 BG Rotterdam > > T 31(0)104152426 > F 31(0)104154953 > > > > Before printing, think about the environment. > > > > > > Van: emc-p...@ieee.org [mailto:emc-p...@ieee.org] Namens Fred Townsend > Verzonden: Friday, November 06, 2009 11:52 AM > Aan: m...@sfo.com > CC: emc-pstc@LISTSERV.IEEE.ORG > Onderwerp: Re: [PSES] Discharge capacitors > > > > Robert it is difficult to validate your models without knowing more > about them but they are suspect when they fail to uphold empirically > derived practice that has existed for longer than field solvers. You > certainly stick your neck out when you say: > > > > "CONCLUSION: > Round is always better." > > I can think of numerous exceptions, exceptions that may prove the rule. > In fairness many of the exceptions are caused effects not considered by > your models but are real world, nevertheless. ( I thought this > discussion was about best practice.) For instance if one is terminating > in a, PCB or just a tab, round tube requires a round to rectangular > conversion. How may that conversion effect corona? As you said nature > doesn't like 'pointy' stuff. > > It would be folly to characterize the problem as purely one of > inductance. We like to model lightning as pulse but we know lightning is > often a damped sinusoid. Where does the sinusoid come from? > > We know that if hit a LC circuit with an impulse function, the circuit > will ring at its natural frequency. Could this be the source of the > sinusoid? If we suspect there are resonance effects, then clearly we > must address capacitance and L/C ratios and transmission line effects as > well. Clearly your 2D field solver isn't going to do that. > > Finally in your models, isn't there going to be significant energy below > 10 MHz? My AM broadcast radio certainly thinks there is whenever > lightning strikes nearby. > > Why do you show reactance for ribbon and resistance for tube? Is that an > error of missing 'j's or is this truly apples and oranges? > > Best regards, > > Fred Townsend > DC to Light > > m...@sfo.com wrote: > > 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 > > > > g.grem...@cetest.nl <mailto:g.grem...@cetest.nl> > <mailto:g.grem...@cetest.nl> > > 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 <emc-p...@ieee.org> 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 <emcp...@socal.rr.com> Mike Cantwell <mcantw...@ieee.org> For policy questions, send mail to: Jim Bacher: <j.bac...@ieee.org> David Heald: <dhe...@gmail.com>
