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Thread: May I ask a question about the single vs two phase stuff

  1. #21
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    [QUOTE=Russs57;1930348]https://www.allaboutcircuits.com/tex...power-systems/

    "I suppose it is because I learned far more about transformers building tube audio amps. There it is quite common to use a center tapped transformer to either.....output two identical signals, 180 degrees apart......or to sum to signals 180 degrees apart into one "phase"."

    At the risk of starting a semantics war............

    You do NOT get 'two identical signals, 180 degrees apart' with a center-tapped transformer. To do that, the transformer would have to shift one signal in TIME, uniformly, across the audio spectrum.

    What you DO get is two signals OF OPPOSITE POLARITY. Which, if you are looking at a perfect sine wave, is INDISTINGUISHABLE from one '180 degrees apart' from the other.

    Thank you, soapbox is now available, let the fun begin !

  2. #22
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    Quote Originally Posted by Russs57 View Post
    https://www.allaboutcircuits.com/tex...power-systems/

    The following may be at the heart of what he was going on about. Note that the author admits it can just as easily be depicted as two sources 180 degrees apart. After reading the article I am finding it hard to call him dead wrong, but I would have before.

    I suppose it is because I learned far more about transformers building tube audio amps. There it is quite common to use a center tapped transformer to either.....output two identical signals, 180 degrees apart......or to sum to signals 180 degrees apart into one "phase".
    If you have been dealing with audio signals, then you know that with any waveform more complex than a pure sine wave, inversion and phase shift do not look the same.

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    Point taken. Historical names are “phase splitter/inverter”. One could also get into a semantics battle regarding polarity of AC waveform.

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    Quote Originally Posted by Russs57 View Post
    ...One could also get into a semantics battle regarding polarity of AC waveform.
    I think what is confusing for many people on a standard split-phase system is that if you scope L1 to neutral and L2 to neutral, with neutral as the common reference, you get opposite 'polarity'. However, if you view them as two sections of transformer winding in series, and you scope L1-L2, L1-N, N-L2, you see all three waveforms with the same polarity in sync, because you have measured them all with the same orientation to the primary source.

    The beginner may intuitively think that using neutral as the common reference is the way one should look at it. But the more advanced person realizes that this is just a choice, among other possible choices, depending on what you're trying to demonstrate or achieve.

  5. #25
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    Quote Originally Posted by jaggedben View Post
    I think what is confusing for many people on a standard split-phase system is that if you scope L1 to neutral and L2 to neutral, with neutral as the common reference, you get opposite 'polarity'. However, if you view them as two sections of transformer winding in series, and you scope L1-L2, L1-N, N-L2, you see all three waveforms with the same polarity in sync, because you have measured them all with the same orientation to the primary source.

    The beginner may intuitively think that using neutral as the common reference is the way one should look at it. But the more advanced person realizes that this is just a choice, among other possible choices, depending on what you're trying to demonstrate or achieve.
    And there is where the discussion breaks down every time.
    If you don't think too good, don't think too much.

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    If you don't reverse the leads, there is no 180 degree shift. If you 'keep the red on the right' the scope will show both 120's and the 240 'in phase', will it not?

    If you keep the same lead on the neutral and move the other lead from one side to the other to measure 120 volts, then to be accurate you should also be switching them to read 240 volts, which will give you your 180 shift not on just the 120 volt sides, but the 240 volt side as well, would it not?
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    Quote Originally Posted by electrofelon View Post
    Careful - this topic is probably more likely to end up closed than a topic with "gun" in the title.
    It's not the topic that ends up closing these threads, it's the way the opposing sides end up being so nasty to each other.

    The smartest guy in the room is usually a solo act. Here at the Mike Holt Forum we are blessed with having all the smartest guys in the room at the same time. Some are better at sharing the room than others.
    If you don't think too good, don't think too much.

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    Quote Originally Posted by ActionDave View Post
    It's not the topic that ends up closing these threads, it's the way the opposing sides end up being so nasty to each other.

    The smartest guy in the room is usually a solo act. Here at the Mike Holt Forum we are blessed with having all the smartest guys in the room at the same time. Some are better at sharing the room than others.
    Well said
    Ethan Brush - East West Electric. NY, WA. MA

    "You can't generalize"

  9. #29
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    Quote Originally Posted by Gary Glaenzer View Post
    You do NOT get 'two identical signals, 180 degrees apart' with a center-tapped transformer. To do that, the transformer would have to shift one signal in TIME, uniformly, across the audio spectrum.

    What you DO get is two signals OF OPPOSITE POLARITY. Which, if you are looking at a perfect sine wave, is INDISTINGUISHABLE from one '180 degrees apart' from the other.
    Code references based on 2005 NEC
    Larry B. Fine
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  10. #30
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    I agree, this is a thread about terminology, and 'phase' has many meanings.

    One of the points made in the other thread was about the 'hexaphase' transformer. This is a set of 3 single phase coils, all with center taps, with the center taps joined to make a 'star'. Each of the output terminals of such a transformer is 60 degrees out of phase from the adjacent ones.

    In the literature on high phase order motors they came up with a term to distinguish phase counting when you had 'phases' that were 180 degrees apart. They used 'hemi-phase'. So using that terminology the hexaphase transformer is a 3 phase transformer with 6 hemi-phases brought out.

    Similarly a center tapped transformer is single phase with 2 hemiphases brought out.

    Don't know if this helps or is just an extra word to be pedantic with

    -Jon

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