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Scott-T and 2 Phase

Discussion in 'Electronic Basics' started by Randy Gross, Sep 29, 2005.

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  1. Randy Gross

    Randy Gross Guest

    Greetings,

    I was gathering information on the Scott-T transformation when I ran
    across this discussion:

    http://www.integrate-oz.net/forum/printthread.php?t=672&page=2&pp=10

    in it was stated that residential 2 phase isn't two seperate phases but
    one 240v single phase split into two 120v circuits.

    The Scott-T description states that 2 phases 90 degrees apart are used.

    Is this "split phase 240" a norm in residential or intermittent?

    Is residential "2 phase" suitable for the Scott-T connection?

    rg
     
  2. Dan Hollands

    Dan Hollands Guest

    All Residential Power I have ever heard of is what you call "split phase
    240" i.e. 2 phases 180 degrees apart.

    I don't think the Scott T will work with this

    Dan

    --
    Dan Hollands
    1120 S Creek Dr
    Webster NY 14580

    www.QuickScoreRace.com
     
  3. Residential power in the US is single phase from a single transformer.
    It is often referred to as 2 phase, because the center tap of the
    distribution transformer secondary is grounded, so the two ends of the
    winding swing in opposite directions, 180 degrees apart, but there is
    no way to derive other arbitrary phases from this output using only
    transformers.

    A Scott-T transformer is two single phase transformers connected
    together so that they can deal with true 3 phase power. The end of
    the winding of one transformer is connected to the center tap of the
    winding of the second transformer. Two of the 3 phases are connected
    across the center tapped winding and the third phase is connected to
    the remaining end of the winding whose other end is connected to the
    center tap. The voltage of the non center tapped winding is square
    root of 3 times the voltage of the center tapped one.
    Something like this, where W represents windings

    1-WWWW-+-WWWW-2
    |
    W
    W
    W
    W
    |
    3

    The voltages across the two windings are 90 degrees apart, but the
    voltages phase to phase are 120 degrees apart. From the outside, it
    acts like a delta or Y, 3 phase transformer, but uses only 2 cores.

    You can connect 240 volt residential power as phase 1 and 2, but there
    is no third phase to use as phase 3.
     
  4. I found a tutorial on transformers that has mention of the Scott-T
    about 3/4 of the way down:
    http://www.ibiblio.org/obp/electricCircuits/AC/AC_9.html
     
  5. Randy Gross

    Randy Gross Guest



    Thanks for clearing up the 2 phase issue. I took it at face value that
    the 220 was individual, I should have looked deeper. My oversite!

    I can see that there will not be a 3 phase to 2 phase transformation
    using the Scott-T but, will the Scott-T transform the 2 phase
    residential to 3 phase seeing that the phases are 180 degrees apart?

    rg
     
  6. Randy Gross

    Randy Gross Guest

  7. The Phantom

    The Phantom Guest

    No, it won't. As John said:"there is
    no way to derive other arbitrary phases from this output using only
    transformers."

    It's a trigonometry problem. When you add two sine waves of different phases, you get a
    resultant sine wave of different amplitude and phase, depending on just how much of each
    sine wave you add; *except* in the case where the two sine waves you're adding are either
    zero degrees or 180 degrees from each other. Then the resultant will be zero or 180
    degrees from the two you added; that is, no change in phase angle, or 180 degrees from one
    of the inputs.

    I once had a discussion about this with one of my technicians, and finally convinced him
    it couldn't be done with just transformers.

    However, it can be done (in a sense) with inductors and capacitors. You can build
    networks that can change phase, but they are load sensitive. In other words, if your load
    were constant, it could be done with L-C networks.
     
  8. Randy Gross

    Randy Gross Guest



    After researching the Scott-T, I wondered why the connection wasn't
    utilized more extensively in residential shop applications. Now I know
    that 2 phase is not two true phases so, all is not lost. I've gained
    another chip in the game.

    Thanks to all,
    rg
     
  9. Jasen Betts

    Jasen Betts Guest

    In america it is...
    according to what you say, not unless you can arrange a 90 degree phase shift.

    what can you tell us about "Scott-T"? I only found a patent which suggests
    to me that it's only of interest to its inventor.

    Bye.
    Jasen
     
  10. I've seen it used industrially to substitute as a cheap Y transformer.
    You can even produce an equivalent of a grounded center Y by having
    a tap off center on the winding that connects between a phase and the
    center tap of the other winding. The windings are not as equally
    balanced as a true Y, but from the outside, you still have three ends
    for 3 phases and a center node to connect ground.

    The Scott-T is just one permutation of the many possible, to change
    phase angles using two or more transformers.
     
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