Why do we prefer Ac over DC?

Discussion in 'General Electronics Chat' started by salmanshaheen_88, May 6, 2010.

  1. salmanshaheen_88

    Thread Starter Active Member

    Mar 5, 2009
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    I asked this question from my professors one of them told me that for DC supply we need pure copper transmission line which is very expensive, other told me that the transmission line fact isn't important that much the only reason that we can't use DC that it can't be step up or step Down. I was thinking that as DC has negligible losses as compared to AC but the only disadvantage is that we can't step up or step down it so why don't we produce the desired voltage and current in power station and supply it to for home or industry???
     
  2. Externet

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    Nov 29, 2005
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    Some imprecisions...
    DC can be stepped up or down. That is what DC to DC converters do.
    DC suffers of same losses than AC in an equal transmission line.
    The DC or AC choice at the beginning of electrification of cities over a century ago had already its mini-war between Mr. Edison and Mr. Tesla

    http://en.wikipedia.org/wiki/War_of_Currents

    The wiring gauge for distributing electricity is dictated by the voltage on them ( its resistance ) Boils down to $$

    http://www.strongerwesterngrid.com/background.html

    http://www.projectsmonitor.com/NEWPROJECTS/siemens-ag-indian-arm-wins-contract-from-adani-power
     
    Last edited: May 6, 2010
  3. Jaguarjoe

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    Apr 7, 2010
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    I didn't think DC-DC convertors existed 125 years ago other than maybe m-g sets. It would be enormously impractical to have a DC-DC convertor or an m-g set do any high power level shifting. Manufacturing plants littered with DC brush motors would be a maintenance nightmare compared to the simple 3 phase AC induction motor.
    Wire size is determined by current not voltage. Did you mean voltage drop?
     
  4. Externet

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    Nov 29, 2005
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    For a given power transmitted, choosing a higher voltage takes a smaller wire gauge. (Of course because the current gets then smaller)
     
  5. pilko

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    Dec 8, 2008
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    The Prof that told you it's all about step up /step down was correct.
    Energy is transported from power stations at very high voltage ( one million volts in some cases) The reason for high voltage transmission is to enable lower current to be transmitted for the same energy. Energy =2nd root 3.V.I.Cos Phi.
    The lower current means lower line loss and therefore small diameter transmission lines can be used.
    This high voltage is then easilly converted to lower voltages for industrial use and domestic use by the use of transformers.

    pilko
     
  6. nsaspook

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  7. ShockBoy

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    I'm more than likely wrong here, but I was under the impression that AC does not flow through the long distances at all, but makes use of the electrons already in the wires. Pushing and pulling at whatever desired voltage.
     
  8. t_n_k

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    Mar 6, 2009
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    If there was no current flowing in the conductors you wouldn't have to size the conductors for the required current carrying capacity. You appear to be under a misapprehension.
     
  9. mik3

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    Feb 4, 2008
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    In both DC and AC the electrons used are the electrons in the wires and equipment. Electrons are not generated but just moved around by electric forces. In DC, if say one electron is located at the positive terminal of the source, it will flow until it reaches the negative terminal and so on (provided the circuit is closed). In AC, electrons vibrate around their initial position prior the application of the AC voltage. The distance they travel depends on the frequency of the voltage and other physical factors of the wires etc.

    Personally, I think that there is the possibility that some electrons travel around the whole circuit in an AC circuit because the distance traveled in the positive cycle is not equal to the distance traveled in the negative cycle. However, the total net effect is an AC current.
     
  10. nsaspook

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    This is a pretty good answer.
    http://amasci.com/miscon/speed.html
     
  11. ShockBoy

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    So for a typical AC current in a typical lamp cord, the electrons don't actually "flow," instead they vibrate back and forth by about a hundred-thousandth of an inch.

    So I was right for once...:)

    It's kinda like a a tsunami; major force through a conductive source, using the source's makeup as the road. Current, that is.
    Electrons reside in everything; how can anyone say that there is a river of electricity anywhere, at any given time, powering any given lightbulb?
     
  12. Norfindel

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    Mar 6, 2008
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    V = I * R. Whatever the cable resistance, there will be less voltage drop and power loss in the cable if you keep the current down. Transformers are reliable, simple, efficient devices to change the ratio of Voltage to Current.
     
  13. nsaspook

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    As usual it's not that simple. A major reason for going to DC for long distance power transmission is GIC effects. We have been in a solar minimun but are now heading back to more activity.

    http://engineering.dartmouth.edu/spacescience/wl/res/ae/biblio/molinski00.pdf
    http://150.162.19.200/congressos/PowerTech/papers/460.pdf
     
  14. Norfindel

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  15. nsaspook

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    Sure, you could use it for free energy.:) The damage is mainly limited to transformer saturation so that won't happen with DC commutation.
     
  16. Ron H

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  17. retched

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    Question about skin effect.

    And I answered it. It was about using tubes.

    they do, And the steel is in the center of the AL cables for strength and is immaterial for conduction.
     
  18. Norfindel

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    I don't know. Couldn't the induced current go in the opposite direction?
    Anyways, using switching supplies to regulate DC would introduce pulsating currents in the wire, which has it's own share of problems. A switching supply is much more complex than a transformer, also, and a lot of stuff can fail more often.
    It looks easier to try to block that almost-DC induced current with capacitors, so that they don't reach the transformers.

    It's not by inertia that we continue to use AC. It's cheaper, and reliable.
     
  19. BMorse

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  20. nsaspook

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    A battle royal. Somewhere Edison is smiling.
     
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