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Different between 1 phase voltage and 3 phase

Discussion in 'Electronic Basics' started by yakobiana, Sep 23, 2003.

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  1. yakobiana

    yakobiana Guest

    Hey:
    1- What is the differnce between 3 phase voltage and 1 phase?
    2- Whay for strong machine that needs a lot of power i need 3 phase voltage
    and what is the formula to calculate how much power 3 phase voltage can
    supply?
    3- Can i create 3 phase electrical plug point by myself?
    4- Whay not use only DC voltage instead of AC?

    Thank you.

     
  2. in message
    A single phase system is a power distribution system whereby only 2 wires
    are used and only one AC voltage is delivered to a load. Three phase power
    distribution includes 3 (or 4) wires and three different AC voltages are
    supplied to the load. The different voltages are offset from each other by
    120 degree phase shifts. You really should attend class or read your book
    to get a full feeling for this concept as it is incredibly important. They
    will continue to talk about three phase for a long, long time, so it
    behooves you to spend the effort to learn this.
    Umm, your question doesn't make sense to me. I'm sure your book could be of
    help, however. You want a power formula, most likely for a balanced load
    case, but I hesitate to give you one as it isn't exactly clear which one you
    want and then I would be essentially doing your homework.
    No, not without a power converter of some kind. Switch mode power supplies
    can be made to do just about anything, and a single phase motor mechanically
    driving a three phase generator could create a three phase power system, but
    there are very few uses for doing so.
    DC voltages are inherently hard to increase or decrease efficiently and with
    always on reliability. I.e. you need a switch mode power supply to do so.
    AC can simply be sent through a transformer to change voltage. Because of
    I^2*R losses, high power distribution occurs at high voltage to minimize
    current.

    Howard Henry Schlunder
     
  3. With single phase AC (1 pair of wires) there are two times each cycle
    when there is no power being transferred to the load. Three phase AC
    (3 wires) convey a constant flow of energy at all times, because the
    three waves overlap to smooth the peaks and holes in each phase. This
    is similar to how a 6 cylinder engine provides smoother torque (3
    pushes per turn) than a two cylinder engine (one push per turn).
    Because of the holes in the energy flow from a single phase source,
    the average must be made up with larger peaks of energy. This causes
    pulsations in the torque of single phase motors. 3 phase motors turn
    with smooth torque and since the delivered energy is shared over 3
    wires, the current is less in each wire (for a given voltage).
    Not very easily. You need a system that absorbs energy during the
    peaks of the line voltage, and supplies this energy to the other 2
    phases during the low voltage parts of the line wave. This can be
    done with rectifiers and capacitors that store the energy as DC, and
    use inverters to produce the 3 waves, or you can store the energy with
    an LC resonator and phase shifter (very load dependent) or as
    rotational momentum by driving a single phase motor that has extra
    windings that act as a 3 phase generator, or that is coupled to a
    separate 3 phase generator. The first method is what is used in
    commercial 3 phase variable speed drives that operate off single phase
    sources.

    http://web1.automationdirect.com/store/Shopping/Catalog/Drives
    AC motors do not need brushes, and AC is easier to distribute because
    transformers allow the voltage to be changed as needed. The switch
    gear is also cheaper for AC, because the voltage zero crossings make
    the arcs easier to extinguish. Opening a 100,000 switch that is
    carrying a thousand amperes DC is a big deal.
     
  4. Jim Large

    Jim Large Guest

    It's a lot easier than you think. Google for "rotary phase
    converter." Basically, it's just a three phase motor with a
    big flywheel on it, and wired up in the right way. The
    motor needs to have about the same horsepower rating as the
    motor that you intend to power. And (since I've never built
    one) there's probably some other details that I'm omitting.
    Used milling machines and lathes and etc. end up in more
    hobbyists homes than you might think. I used to know a kid
    in high school whose family had a fairly complete machine
    shop in their basement. Don't know if they had any three
    phase machines, but that google search will turn up numerous
    cookbook examples of how to use an old motor as a phase
    converter.

    -- Jim L.
     
  5. JeffM

    JeffM Guest

  6. Baphomet

    Baphomet Guest

    3 phase minus 1 phase = 2 phase :) Sorry, I couldn't resist.
     
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