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AC vs DC

  1. Jun 20, 2010 #1
    Why is it that AC is superior to DC in long distance power transmisson. I was thinking it has something to do with the resistance of copper wire but if this is the reason then why is that? Or, why is copper loss not a factor with AC current? Thank you!
     
  2. jcsd
  3. Jun 20, 2010 #2

    berkeman

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    Welcome to the PF, Tregg. :smile:

    https://www.physicsforums.com/showthread.php?t=410588

    .
     
  4. Jun 23, 2010 #3
    It's because by increasing the voltage, you're decreasing the current in the wires, which means the volume of electrons scraping against the side of the wire also decreases, so less energy is lost over long distances.
     
  5. Jun 23, 2010 #4

    berkeman

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    The electrons don't really "scrape along the side of the wires", but the rest of what you posted is correct.
     
  6. Jun 23, 2010 #5

    vk6kro

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    A wire transmission line will have some fixed maximum current rating and, at that current, it will drop a certain voltage.

    This applies equally to AC or DC.

    Suppose this voltage drop is 50 volts and the current was 1000 amps.

    If the voltage entering the line is 120 volts, the output will be 70 volts at 1000 amps or 70 KW.
    The input was 120 volts at 1000 amps, so the input power was 120 KW.
    This is a very poor efficiency of 58 % but it also provides an output of only 70 volts at that load. And, the line voltage would be very dependent on load.

    If the supply was DC, there is only one thing you could do to deliver 120 volts to the consumer. You would have to feed 170 volts into the line and keep adjusting this, depending on the load. If you got it wrong, all the light bulbs and most other equipment could be destroyed by getting too much voltage.

    If the supply was AC, you could use a transformer to increase the supply voltage to 3300 volts. Now the input power is 3300 volts at 1000 amps or 3300000 watts and the output power is (3300 - 50) * 1000 or 3250000 watts. This is an efficiency of 98.5% and 46 times as much power out as before.
    You still have 3250 volts to deal with, but a transformer will convert this back to 120 volts again.
    These transformers are expensive but a negligible cost compared with the cost of the transmission lines. Using AC instead of DC gives you the opportunity to do this

    Skin effect is not a factor at 50 Hz or 60 Hz. The skin depth at 60 Hz is 0.3 inches so there is no effect on conductors of less than 0.6 inches thickness.
     
  7. Jun 23, 2010 #6
    AC is technically not superior to DC transmission. The reason we in the US have AC lines is the cost of DC line plus other benefit like simple 2 way transformers. In general the skin effect is minimal at 50 or 60Hz. The high voltage transmission lines can range from 765KV to 138KV after the substations it can be from 26KV to 4KV. These high voltage levels are difficult to generate as a DC voltage. Once they are up in the KV range they would then need to be switched down to a user level of 120V or 240V.

    Back when the transmission lines where first put up there was either no way or no cheap way of doing DC at those high voltages. The advantage of going with DC is that you do not have the electric fields that the AC power lines generate and less talk from the lines.

    The main advantage of AC power lines is the cost of going from high voltage to low and back from low to high.

    2nd advantage is everything is naturally isolated.

    3rd more recent advantage is that when the consumer wants to co-generate or have a micro generator they can very easily send that power back onto the grid. In a DC system there would need to be a switcher capable of going from 240 / 220 back to something like 4KV in the best case. The power companies would not want to incur that kind of cost just because someday someone may want to send power back out on the grid.
    L
     
  8. Jun 24, 2010 #7

    sophiecentaur

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    The transformer is a brilliant concept in electrical engineering - analogous to the lever in mechanical engineering. We couldn't do without either of them.
     
  9. Jun 24, 2010 #8
    yes they sure are in fact did you know that the analysis of a 3 phase induction motor can be though of as a transformer with 2 airgaps and a shorted secondary?

    L
     
  10. Jun 24, 2010 #9

    mgb_phys

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    Other advantages of DC are that you don't have to have the grids in-phase at each end so useful for international links.
    You only have to design insulation and switches for the max DC voltage not the peak AC (which is 40% more than the RMS voltage)
    And you don't have large reactance losses for buried or underwater cables.

    But, as you say, even today it's much cheaper to step up/down high power AC than DC
     
  11. Jun 24, 2010 #10
    Yes, as you say, I suppose if you consider the sea water as a secondary it would kind of "short" things out so to speak. L
     
  12. Jun 24, 2010 #11

    sophiecentaur

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    I could believe anything about
    1. Induction motors
    and
    2. Equivalent circuits
     
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