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Thread: Power Distribution At Residential Level Understanding

  1. #21
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    Quote Originally Posted by Gary Glaenzer View Post
    Sorry, no, that is backwards.

    They are of opposite polarity, which in this case is indistinguishable from a 180 degree phase shift.
    The blue part is helpful. The red part is semantics.

    L1-n and L2-n are 180 degrees apart.

    L1-n and n-L2 are 0 degrees apart.

    You can define the polarities either way you want, so it's a matter of stating which way you are defining it.

  2. #22
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    Quote Originally Posted by hbiss View Post
    No offense, but you are an electrical engineer and you have to ask about the phase relationships of a center tapped winding of a transformer?

    -Hal
    Electrical engineering is a wide and varied field. My concentration in engineering school was in device physics and I spent 23 years in semiconductors before moving to solar about ten years ago. I didn't know crap about three phase power, transformers, or the NEC at first. I've learned a few things since then.

  3. #23
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    Quote Originally Posted by hbiss View Post
    No offense, but you are an electrical engineer and you have to ask about the phase relationships of a center tapped winding of a transformer?
    Sorry, Hal, but I have to say that I find that offensive. We don't question each other's qualifications here. Otherwise someone would be saying similar things to electricians (e.g., you are an electrician and you don't know how to bend conduit?). An EE degree normally will not include any course work related to the NEC, and most will not touch power systems distribution or analyses. My own introduction to 3-phase design, from the academic world that is, was a senior-level course that I took during my masters degree program, 12 years after I received my BSEE degree. I learned 3-phase systems from my work environment, not from school.

    Charles E. Beck, P.E., Seattle
    Comments based on 2017 NEC unless otherwise noted.

  4. #24
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    Quote Originally Posted by roger View Post
    I'll just say this: If you put a glitch on a waveform at its maximum, shift it by 180 degrees, replot it and superimpose it on the original, the two glitches are at the same voltage but not at the same time. If you invert the glitched waveform, replot it, and superimpose it, the glitches are at the same time but not at the same voltage. It's a distinction without a difference for a single frequency sine wave.

  5. #25
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    Quote Originally Posted by ggunn View Post
    Electrical engineering is a wide and varied field. My concentration in engineering school was in device physics and I spent 23 years in semiconductors before moving to solar about ten years ago. I didn't know crap about three phase power, transformers, or the NEC at first. I've learned a few things since then.
    doesn't everyone take ckts 1 and 2?
    kvl, kcl, thevenin, etc, applied to ac/dc
    at most abet schools regardless of concentration the first 4-5 courses are the same
    ckts 1 and 2
    basic machines/power
    basic ss devices (lol mine included tubes)
    fields

  6. #26
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    Quote Originally Posted by charlie b View Post
    ... you are an electrician and you don't know how to bend conduit? ...
    Could happen, if all your work was residential/light commercial (NM & MC) or industrial. (rigid conduit assembled with straight segments and pre-fab elbows)

  7. #27
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    Quote Originally Posted by Ingenieur View Post
    doesn't everyone take ckts 1 and 2?
    kvl, kcl, thevenin, etc, applied to ac/dc
    at most abet schools regardless of concentration the first 4-5 courses are the same
    ckts 1 and 2
    basic machines/power
    basic ss devices (lol mine included tubes)
    fields
    Of course, but when I switched fields it had been over 20 years since those classes and I had lived in low voltage DC land since then. I remembered that ideally VI on one side equals VI on the other, but if I would have been asked how split phase was made I wouldn't have known the answer. At one time I could have solved Fourier and convolution problems and plotted root loci but certainly not now. Use it or lose it.

  8. #28
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    Quote Originally Posted by drcampbell View Post
    Could happen, if all your work was residential/light commercial (NM & MC) or industrial. (rigid conduit assembled with straight segments and pre-fab elbows)
    Yep, and I remember one threadabout a lifetime industrial electrician that had never handled Romex.

    I bet I put less than a stick of EMT a year in my pipe benders, but have run more Smurf pipe (ENT) than I care to admit.

    It takes all kinds... that's one thing that makes this forum great, is the wide variety of backgrounds and experience of the posters and members.
    Electricians do it until it Hertz!

  9. #29
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    Quote Originally Posted by Ranes View Post
    What I don't understand is how do we take a single phase with 60Hz frequency and break that into the two 120V phases that are 180° out of phase.

    Just to be clear, I understand how we can step down the higher single phase run (before transformer) from higher voltage primary to 240V and run a neutral to divide into 120V "pieces" on the secondary side, but what I can't find is some clarification as to why these are out of phase with each other. Is this something that just inherently happens when we tap a transformer with the neutral to get two 120V phases?
    It goes back to the basics of how we define a voltage. For any voltage we must pick a reference point. No reference point is pre-defined. It really is just that simple.

    If we pick the center-tap as the reference point, then the signals (L1-N and L2-N) are 180 degrees out of phase. If we pick L1 as a reference for one signal and N as the reference for the other signal (N-L1 and L2-N) then they are in phase. Fundamental physics and EE stuff that you can refresh yourself with.

    The defined set of voltages with a phase difference is not just a paper definition but is aligned with the physical world. Kind of the whole point in making definitions to follow what we discover in the physical world. We use these to harness the world around us and they really would not be useful if they did not help us interact with the physical world.

    For example, we can use this relationship to produce a third phase from only two phases using an open wye-wye transformer bank. The math says we should get the same set of three-phase voltages we would get if we had separate transformers with a 180 degree difference. With the open-wye bank, we use half of the winding in one polarity and half in the other polarity and the physics produce exactly what the math predicts.
    BB+/BB=?

  10. #30
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    Quote Originally Posted by Ingenieur View Post
    doesn't everyone take ckts 1 and 2?
    kvl, kcl, thevenin, etc, applied to ac/dc
    at most abet schools regardless of concentration the first 4-5 courses are the same
    ckts 1 and 2
    basic machines/power
    basic ss devices (lol mine included tubes)
    fields
    For EE yes. Doesn't mean they will remember it later if they haven't been playing with it since then. But they should be able to refresh that knowledge.
    BB+/BB=?

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