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Regarding AC distribution & DC distribution

Discussion in 'Electronic Design' started by Jayesh, May 24, 2006.

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  1. Jayesh

    Jayesh Guest

    Hi can anyone plz help me out in comparing between high freq AC
    distribution line with the DC distribution line as nowadays DC is only
    used esp. for spacecraft systems or others too such as fuel cell
    vehicle. So i would like to know what are the disadvantages &
    advantages of AC distribution over Dc distribution as research seems to
    have stopped in this in round abt 1995.

    Plz help me out

    Thank you
    jayesh
     
  2. Simply:
    With dc, you cant use transformers and the voltage will decrease over
    distance.
    With ac, you can use transformers to step up the voltage and with higher
    voltage on the lines they have to conduct less current, and this is why the
    wigh tension wires are much smaller. Dc would require very big wires to
    transfer the same power that AC high tension wires do. The power transfered
    by the ac lines is the voltage x current. Think of it like a see saw, if
    voltage raises - the current lowers but the power stays the same. In
    essence that is why we use ac current over dc. Hope I didnt get you to
    confused :)
    JTT
     
  3. Jayesh

    Jayesh Guest

    thanks for your reply but i guess voltagexcurrent applies for Dc too
    and nowadays we find Dc distribution only if you see in spacecraft so i
    am very curios why so why not ac line
     
  4. I beleive HVDC trasmission is also popular these days. In fact, we
    have one underwater DC transmission line here in AU:
    http://tdworld.com/news/SEIMENS-PTD-Austalia/
     
  5. Tim Williams

    Tim Williams Guest

    Because you almost always use DC at the endpoint.

    IIRC, big planes like 747s use 400Hz AC, higher than the line frequency used
    everywhere else to save weight, but AC instead of DC to improve
    transmission.

    I don't know why you think something is exclusive to spacecraft.

    And BTW, you will gain *much* when talking to people if you SPELL and
    CAPITALIZE properly... this isn't some teen chat room.

    Tim
     
  6. linnix

    linnix Guest

    Because the power is coming from low voltage DC solar panels anyway.
    Real Spacecrafts are usually too small for high voltage transmissions.
    USS enterprise is not real!!! We are not building spacecrafts bigger
    than
    a bug (volkswagen) yet.
     
  7. engineer

    engineer Guest

    I have doubts that AC v DC is really an issue for low power spacecraft.
    As someone said, there is a transmission over long distance advantage
    that AC has over DC.

    One distinct advantage that DC power has over AC is both the reach
    (radiated extent), and the change in electro-magnetic field intensity,
    as it interacts with things in the envroniment. Some say that high
    levels of low frequency emf (electro-magnetic fields) can cause
    cancers. To date this hasn't been disproven.. or so I think... but it's
    generally held that only ionizing radiation (wavelength on the order of
    the width of a water molecule and shorter) is understood cleary to
    cause cancers. If DC power distribution were better-done underground,
    maybe the landscape would look nicer than how it's done today.

    On small scales, RF and microwave power distibution systems could be
    feasible.. lord know for what application, maybe a primary power
    collection center with remote modules or capsule for some interstellar
    expedition? Some lunatics got press time for proposing that we beam
    microwaves down from space (from solar collectors).. but it's doubtful
    that could happen for environmental reasons.

    One thing that is useful about AC is that with electromagnetic
    coupling, the power is moved without a continuous conductive
    connection, which would have some advantage in a lightning strike.

    One could come up with a longer list.. and argue this or that. In
    systems that use batteries, voltages can be converted up or down by
    various switching and storage techniques, more efficiently than other
    means, such as "voltage regulators". It doesnt really make sense to
    convert a battery (or solar) DC souce entirely into AC to in turn be
    converted back to DC for use by DC circuits- taking the losses twice...
    however I'd reserve some doubt, that it may be possible in some vehicle
    that such AC power is more efficient for a some type of motor.

    It seems in the realm of possibility that there may one day be an AC
    battery, but not in the near future. Maybe technology would evolve to
    that some mix of DC, AC, RF, Microwaves, and Light would comprise the
    'power pipes' of the future.

    Advantage and disadvantage depends on the context.. most people like to
    narrow it down to one context that is useful to them, with one rule of
    thumb to apply.
     
  8. There is some very basic stuff (relating to tramways) at:
    http://www.bathtram.org/tfb/tT42.htm
     
  9. Tim Williams

    Tim Williams Guest

    "Paging Commander Laforge, deck 13 junction B7 for plasma conduit repair"

    :)

    Tim
     
  10. Terry Given

    Terry Given Guest

    DC transmission is used to counter transmission-line effects in AC power
    distribution systems. Multiple AC generators need to be precisely
    synchronised to prevent circuilating currents; when transmission lines
    become an appreciable fraction of a wavelength (1000km or so), this
    becomes very, very difficult. So HV rectifiers create HVDC, its shipped
    long distances as DC, then a giant inverter generates AC again.

    Megavolt DC links are common, and the power electronics is pretty neat too.

    Cheers
    Terry
     
  11. No it did NOT.
    The ~100 years of usage of both styles produced a lot of practical
    solutions like:

    TV syncronisation over whole countries or continents to line frequency
    easly later replaced by (new, better, improved) electronic cirquits.

    Hydraulic power generators in Sweden having their power converted to HV
    DC for 2000Km transfer to the "needy" inhabited and industralised
    region, reconverted to AC for "normal" distribution. On this length of
    line the magnetic fields in 3-phase AC made the dual change cheaper.

    Heart patients in High Intensity rooms are connected to measuring
    instruments insulated from mains cirquitry by RF coils so NNNNOOOO
    galvanic contact exist for ANY amount of leakage.

    And so on and on.
    Create a definition of requirements for power distribution and some used
    before solution would pop up.

    Have fun

    Stanislaw
    Slack user from Ulladulla.
     
  12. Don Bowey

    Don Bowey Guest

    There are some high voltage DC transmission systems.

    Go back to google and do a better job.
     

  13. Twice as much power can be sent over a HVDC line for a given breakdown
    voltage.



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    Don Lancaster voice phone: (928)428-4073
    Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
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    Please visit my GURU's LAIR web site at http://www.tinaja.com
     
  14. Genome

    Genome Guest

    AC transmission implies AC generation.

    This may well make your spacecraft spin in the opposite direction. You might
    compensate for the spin up to power the targeting systems but compensating
    for the load dump when you fire makes aiming your X-ray laser a bit
    difficult.

    DNA
     
  15. Pooh Bear

    Pooh Bear Guest

    Probably weight ! Eliminating transformers saves weight so useful for
    spacecraft. 400Hz AC is typically used in aircraft ( also saves weight compared
    to 50/60 Hz ).


    Graham
     
  16. Pooh Bear

    Pooh Bear Guest

    Doesn't the shuttle qualify as a spacecraft ?

    Graham
     
  17. Pooh Bear

    Pooh Bear Guest

  18. Terry Given

    Terry Given Guest

  19. Terry Given

    Terry Given Guest

    lots of LASCRs in series to form a unidirectional switch, 4 of these per
    half bridge. all driven directly by fibre optics (Light Activated SCR)

    the (LA)SCR failure mode is short-circuit, so each switch has far more
    LASCRs than are strictly necessary. I'm unsure as to the exact number,
    but ISTR its about 2x as many LASCRs as required, so half can fail and
    the unit keeps operating. this means PM can be done when convenient,
    rather than when an LASCR fails.

    Cheers
    Terry
     
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