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Stupid question of the day....

Discussion in 'Electronic Basics' started by AllTel - Jim Hubbard, Jul 30, 2005.

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  1. Alexander

    Alexander Guest

    If you connect Au to Cu and put a Current through it, for best results AC,
    the Cu starts corroding at the transistion from Cu to Au. This is always the
    case when putting to metals together, the greater the difference between the
    metals the faster the corroding will be.
     
  2. Alexander

    Alexander Guest

    This is also the reason why silver and gold contacts should never be
    soldered with normal Sn63Pb37 solderwire
     
  3. John Fields

    John Fields Guest

     
  4. --
    Tzortzakakis Dimitrios
    major in electrical engineering, freelance electrician
    FH von Iraklion-Kreta, freiberuflicher Elektriker
    dimtzort AT otenet DOT gr
    Yes, but also in voltages >=15 kV there's a signifigant skin effect, that's
    why all transmission conductors are constructed with a steel *core* and an
    *aluminium* outer sheath, because the current tends to flow on the skin of
    the conductor.I mentioned corona discharge, to bring into evidence the very
    strong electric field around the conductor in very high voltages.
     
  5. Alexander

    Alexander Guest

    Op [GMT+1=CET], hakte John Fields op ons in met:
    You're right it needs also an electrlyt which is most of the times present.
     
  6. John Larkin

    John Larkin Guest

    Not nil. Do the math.

    John
     
  7. DBLEXPOSURE

    DBLEXPOSURE Guest

    Firstly, Aluminium is Al not Au. Au is gold. You are speaking of
    aluminium and coper?

    Galvanic Corrosion Is possible when Al and Cu are in contact with one and
    other. If I recal correctly a dialectric such as water needs to be present.
    Cathodic protection, (electric current) can be used to slow or stop this
    proccess. I Imagine reversing the polarity may speed it up. Aluminium is
    the "Less Nobel" of the two metals so I would imagine that it would be the
    one to corrode.
     
  8. Alexander

    Alexander Guest

    Op [GMT+1=CET], hakte DBLEXPOSURE op ons in met:
    Correct I also added the remark of the diëlectricum to the discussion.
    And your remark about Aluminium is correct, however as stated in some
    applications I have seen an Copper core and an Gold (aurum) shell. And since
    the combination gold-copper is worse then the well known combination
    aluminium-copper.

    But at least ThanX for confirming my statement and not saying its not true
    without giving a reason as someone else did.
     
  9. TokaMundo

    TokaMundo Guest

    Did you even look at that number? That is 8.5 mm!

    No? OK. So for all practical purposes that do not have 20 cm wire
    involved (ie any normal residential application) there is NO skin
    effect! Where in everyday life does a person use wire that has a
    diameter greater than 8.5 mm that would present anything other than
    100% current density in the conductor? The wave is just too slow for
    anything other than full propagation. Hell, even a 25kW transformer
    won't see any difference.

    In ohms per foot DC or AC at 60Hz the value is the same for all wire
    diameters that have a gauge number.

    Before any difference could even be noted, the wired diameter would
    have to be over 16 mm.

    What part of the word negligible, or not of any effect do you not
    understand?

    http://www.bcae1.com/trnsfrmr.htm

    If you'll read down the page to the skin depth calculators, you'll
    see that the ohms per foot (the only value that matters here) is the
    same for any gauge wire put into the calculator fields.
     
  10. TokaMundo

    TokaMundo Guest

    Your logic varies.
    Tell us, guru don, where does 100kHz get used as an AC power line
    frequency?
     
  11. TokaMundo

    TokaMundo Guest

    It's called galvanic reaction.

    The Navy seems to think it's real. Does that make you an idiot?
     
  12. TokaMundo

    TokaMundo Guest

    Wrong! Skin effect is a function of frequency, not voltage. The
    reason YOU think there is a significant skin effect is likely due to
    the method at which your "15kV" source was generated.

    That being likely a high frequency switcher or oscillator.

    No, it isn't. FREQUENCY is the reason. The other reason steel
    cores are used is due to the fact that transmission lines are
    typically hung from between two points separated by some great
    distance such that the wire itself needs to be able to support the
    weight of itself. The steel cores are there to increase the tensile
    strength of the overall transmission line. This is a reference to
    high frequency RF transmission lines. For power lines, steel is used
    for the same, tensile increasing reason. The cladding is used to
    reduce the ohms per foot of the strand. At 60Hz, however, the skin
    effect hovers near NIL.
    That is 100% dependent upon frequency.

    http://www.bcae1.com/trnsfrmr.htm
    Corona is a function of voltage and the capacity for air to ionize.
    How much corona is on a wire in oil that carries a high voltage AC or
    DC? Same wire in air?

    Electric fields increase with current in a wire. Look at a quarter
    shrinker. Low voltage, high current, huge field.
     
  13. TokaMundo

    TokaMundo Guest

  14. Owamanga

    Owamanga Guest

    Wow, and I thought it was a Mexican beer that tastes so bad you have
    to add lime to it before consumption.
     
  15. John  Larkin

    John Larkin Guest

    Not only did I look at it, but I calculated it, and typed it.
    I just bought a building that has 3-phase, 800-amp service, and skin
    effect certainly has affected the sizing of the main feeder wires. And
    I work with people who build gigawatt 60 Hz power plants and jumbo-jet
    400 Hz power systems. That's my "everyday life."

    In big AC transmission lines, there's a complex optimization involving
    wire weight, tensile strength, ohmic losses, skin effect, corona
    losses, wire cost, and tower spacing/cost.
    Not so. At 0.85 cm depth, current density is down to 1/e (ie, only
    0.37 of) the surface density. That's pretty significant.
    I'm an engineer, so I consider something to be "negligable" if I can
    demonstrate, quantitatively, that it doesn't matter enough to affect a
    system. 37% is therefore worth a second look.

    To speak in your style, what part of "doing the math" do you not
    understand?

    John
     
  16. TokaMundo

    TokaMundo Guest

    And they have the nerve to call it a "specialty beer" here in So
    Cal.

    Just so the idiots can charge more for less!

    Talk about misnomers!

    Give me a Sam Adams or Chimay Gold ANYDAY!

    Bass Ale... anything!

    Sheesh Coors' "Killian's Red" is better than that Mexican crap!
     
  17. TokaMundo

    TokaMundo Guest


    0.85cm is 8.5 mm. That means that the wire has to be bigger than
    that number as a radius before the current flow anywhere else besides
    the entire wire.

    So, again, if the wire is less than 17mm in diameter, there will be
    no skin effect involved. You may note some other effects, but skin
    effect won't be one of them.
     
  18. TokaMundo

    TokaMundo Guest

    No shit.
    Your application of your "math" is what needs a second look.
    What part of "you did the math wrong" do you not understand?

    Before it would make a difference, the wire will have to be pretty
    big (over 17mm diameter) , and before it will make a 37% difference,
    it would have to be bigger still! Real simple math, there.
     
  19. DBLEXPOSURE

    DBLEXPOSURE Guest

    Your welcome, I thought you deserved a respectful reply...
     
  20. TokaMundo

    TokaMundo Guest

    No shit.

    For one thing, they are primarily designed for high tensile strength
    as they have to stay mounted through all weather and environmental
    conditions.

    After that, their resistance is an issue as the primary material has
    to be steel for the tensile forces involved. They usually get clad in
    Aluminum as copper is too costly for such long runs, and the losses in
    using aluminum are little in comparison. This is also the reason that
    high voltages are used in long haul transmission lines. The loss
    over 2000 feet of line with 120 volts on it is significantly different
    than the loss over 2000 feet of line with 20,000 volts on it.

    Corona will become a problem as that line voltage is raised. At
    that time line spacing becomes an issue.

    Tower spacing is a function of the terrain being traversed. Line
    spacing ON a given tower design is a function only of the voltage that
    is proposed to be carried, and the total number of conductors.

    Skin effect, in these high tension line realms is only an issue if
    the idiots that made the wire didn't know how deep to make the
    cladding. If the wire is clad to thinly, there will be more loss as
    the steel is more resistive, and the wire will heat more as well.
    If it is clad too thickly, an unnecessary cost is introduced.

    This is specifically because the skin depth is so deep at this
    frequency, NOT due to it being a thin depth! So in power line cases,
    the effect is an issue of how deep the cladding is, not how thin.

    In RF transmission lines, which are typically nickel or silver
    plated, it becomes a cost issue, and claddings are made as thin as
    possible for a given application frequency. These cases are where one
    will see hollow conductors, or plated tube or solids. This is where a
    Litz configuration or plated conductor will assist one in design of a
    circuit.

    At 60Hz, a high voltage step up transformer will have some transfer
    efficiency number. At switching frequencies, the same transformer
    design (wire turn count wise) will operate better if the primary, and
    or secondary have litz wire used in them as the effective resistance
    of the winding will be reduced at the higher frequencies.
     
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