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About 3-phase AC

Discussion in 'Electronic Basics' started by Jack// ani, Apr 7, 2005.

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  1. Jack// ani

    Jack// ani Guest

    Hi there,

    In 3-phase AC wiring, if phase to neutral voltage is 110V, then why is
    phase to phase voltage 220? I know phase difference between any two
    phases differ by 120 degree, so they should add up to give something
    less than 220V! It should sum up to give 220V if the phase difference
    were 0 degree or 360degree!

  2. The mathematical impossibility you ask about does not
    happen. What makes you think it does?
    Many people do not distinguish 0 and 360 degrees
    for continuous sinusoids.
  3. Lord Garth

    Lord Garth Guest

    The phase to phase voltage in the USA is 207 VAC which is phase to ground x
    120 x 1.732 = 207.84 You can find this voltage supply running the light in

    Houses get one 240 volt phase which is transformed to provide 2 outputs of
    120 volts
    that are 180 degrees apart, with respect to ground.

    Three 240 volt phases implies the phase to phase voltage from this circuit
    is 415.68 volts.
  4. Andrew Holme

    Andrew Holme Guest

    They would *add* to give 110V, but I suspect you're talking about the
    potential difference between phases. Using the cosine rule, the PD between
    phases is:

    sqrt(110*110 + 110*110 - 2*110*110*cos(120)) = 190.5
  5. Jack// ani

    Jack// ani Guest

    I think you got something wrong, or I didn't expressed it correclty!
    Say you have two AC sources of 110V, now if I put them in series they
    should add up to give 220V if their instantaneous phases are same(0 or
    360) or if they are 180 phase out they should sum up to zero. I think
    these two AC sources are just like two phases of 3-phase AC supply
    which are 120degree phase apart. And they should give a voltage less
    than 220V when summed up.

    Any Help...Thanks
  6. I'll go with that set of alternatives.
    Your above statements are consistent with phasor
    arithmetic as I understand it, as long as "just like",
    "less than", and "summed up" are interpreted in a
    way most favorable to your understanding.
    You'll need to describe your issue more specifically
    to get any help with it, I believe.
  7. Here in bulgaria a 3 phase AC is 220 V phase-ground and 380 V phase-prase
  8. In North America, line voltage is 120 volts to ground. With three
    phase power this gives you 208 volts between phases.

    If the phase difference between two circuits is 0 degrees, you will
    measure zero volts between them.

    In normal residential wiring, we have two wires that are 180 degrees
    out of phase - this gives 240 volts between "phases" (some people
    object to using the term "phase in this situation...)

    Although the electrical distribution system as a whole is three-phase,
    individual homes are fed from the secondary of a single phase
    transformer. The secondary of the transformer is center-tapped, with
    the tap grounded to form the neutral conductor. There is 240 volts
    between the ends of the secondary.
  9. Bob Eldred

    Bob Eldred Guest

    It's a very simple geometric relationship. If two lines or vectors are 120
    degrees apart and are of equal length from their common, crossing end, the
    distance between the tips of the lines is 2*sin (120/2). = 2*0.866 = 1.732.
    Now, if the line lengths represents 120 volts from the center or crossing
    point to the tip, the tips must be 120 * 1.732 apart = 207.8 Volts. OK
    class, for homework, prove the geometric relationship. BTW it can be proven
    without trigonometry.
  10. John Larkin

    John Larkin Guest

    One common but bizarre US wiring practice is to have a 240 volt
    line-to-line delta three-phase system in which one side of the
    triangle is center-tapped and is neutral. So 120 single-phase is
    available for regular outlets, 240 single-phase is available for
    things that need it, and 240 line-to-line is available for three phase
    loads. That's fairly common in small commercial buildings. The leg
    opposite the neutral is call the "bitch leg" or the "stinger."

  11. Bob Eldred

    Bob Eldred Guest

    I can't see how that can be a delta. I think what you have described is a
    six phase "Y", center neutral as usual. It's 120 Volts out any leg and 240
    Volt center tapped any leg to it's stinger. And, 208 Volts leg to leg. It's
    also 120 Volts from any leg to the adjacent stinger. It's not all that
    bizarre and can be created from any three phase system with a transformer.
  12. John Larkin

    John Larkin Guest

    What I described is this:

    / \
    / \
    / \
    / \
    / \
    / \
    / \
    / \
    / \
    / \
    / (gnd) \
    / | \
    | | |
    | | |
    120 N 120

  13. Bob Eldred

    Bob Eldred Guest

    Why would anybody do that? What is the voltage from A to C? A to N?, What
    is the relation of N to ground. Clearly this kind of a bastard connection
    would be prone to gross unbalance or to parasitic voltages on the neutral.
    If its 240V leg to leg, with N in the center of one leg, what is N to B for?
    What is the voltage B to C? Furthermore if its 240 Volts three phase on all
    legs, what equipment uses it, most are 208? Since 120 Volts is only
    available from one leg with a neutral how is power distributed in a building
    say to lighting or to other 120 Volt circuits maintaining some semblance of
    balance between the phases. I'm not saying you have never seen this but I
    never have and it can't be very common.
  14. John Larkin

    John Larkin Guest

    Google "three phase stinger" or some such.

  15. Guest

    It's still common in the US where houses are mixed with small motor
    installations. That unexpected 208-to-neutral is(was) a problem on
    water-well controls when you needed 120v for the control-circuit.
    WAde H
  16. Lord Garth

    Lord Garth Guest

    Around here, all delta circuits are being replaced with a wye configuration.
    The utility has standardized on this...then all the other power utilities
    on the scene so who knows !
  17. Bob Eldred

    Bob Eldred Guest

    Well, that certainly makes more sense than some sort of a nutty asymetrical
    grounding scheme. Oh well, based on the above comments, I guess anything is
    possible. What does the NEC have to say about it?
  18. Don Kelly

    Don Kelly Guest

    Your two single phase 110 sources in series will give 220V or 0 depending on
    connection. This is the Edison or 3 wire single phase system in the 220V
    case (and something useless in the other case) which is a common North
    American configuration. In Europe, where it is not used, it is called a 2
    phase system and if one defines an phase system as having n voltages to
    neutralThere are two basic forms of a 3 phase connection -star in which
    there is a common neutral or delta where there is no neutral. .You mention a
    voltage of 110V to neutral so the corresponding voltage between lines is

    In a star connected 3 phase system the phase and line currents are the same
    but line to line voltages are greater than phase to neutral voltages by the
    factor of root(3). In a delta the phase and line to line voltages are the
    same but the line (external) currents are root(3) times the phase (internal)

    There are advantages and disadvantages to both and advantages for 3 phase
    over single phase (2 or 3 wire), particularly in rotating machines and
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