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Thread: Power Distribution At Residential Level Understanding

  1. #1
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    Power Distribution At Residential Level Understanding

    I was debating some of the theory with a coworker and realized I lack some understanding on a piece of how the power grid functions:

    At a simple/brief level, power is generated with 3 phases, then stepped up and transmitted at high voltages, and then stepped down as it distributes and, with the circumstance my question lies, is split into single phase runs for sets of houses. I understand that these single phase runs are stepped down once more to become the 120/240V before being wired into houses, and I understand how 120/240V systems power devices (whether it be LN or LL). 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?

    Thanks for the help.

  2. #2
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    180511-1346 EDT

    Ranes:

    Yes it inherently happens, if both halves of the secondary are wound on the same core, the secondary coil is one winding all wound in the same direction, and the center tap is in the center of the secondary winding.

    If the secondary is made of two separate coils, then they must be phased correctly to get the result you describe. There are only two phasing possibilities.

    .

  3. #3
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    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

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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
    I spent most of my degree on Controls, actually. I wound up in a semi-power job, so sometimes I have to ask questions to learn. Choosing to not ask questions when you don't understand something entirely is a bad life philosophy, in my opinion . Thanks for your input though.

  5. #5
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    Quote Originally Posted by gar View Post
    180511-1346 EDT

    Ranes:

    Yes it inherently happens, if both halves of the secondary are wound on the same core, the secondary coil is one winding all wound in the same direction, and the center tap is in the center of the secondary winding.

    If the secondary is made of two separate coils, then they must be phased correctly to get the result you describe. There are only two phasing possibilities.

    .
    That makes sense, thanks for the info!

  6. #6
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    180511-1429 EDT

    Ranes:

    If you look in the Handbook of Chemistry and Physics, or or Handbook of Tables and Formulas, there are lists of many trig equations.

    An important concept is that the sum of two sine waves of identical frequency and that are phase related create a sine wave of the same identical frequency and with some possibly different phase relative to one of the original waves.

    There is one degenerate case where the resulting output amplitude is zero. This is the case where the secondary consists of two separate coils of identical output voltage but are connected in parallel. Here, if you connect one end of each winding together, and then measure the voltage difference between the other two ends that difference voltage will be zero or near zero for the paralleling case. If the wrong phase for paralleling, then the voltage will be double that of one winding.

    Get some small transformers, a scope, and AC voltmeter and play.

    .

  7. #7
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    Quote Originally Posted by Ranes View Post
    I spent most of my degree on Controls, actually. I wound up in a semi-power job, so sometimes I have to ask questions to learn. Choosing to not ask questions when you don't understand something entirely is a bad life philosophy, in my opinion . Thanks for your input though.
    Really, I'm not blaming you. I'm just trying to understand today's educational system that hands out what they like to call an EE degree without the recipient knowing fundamental concepts. Maybe it's because I'm an old grey haired guy who has worked with many EEs over the years. None would ever say "I don't understand analog because I spent most of my degree studying digital and computers".

    Guess it's just another symptom of our pathetic educational system.

    But I digress...

    -Hal

  8. #8
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    In a nutshell, when you Center tap (i.e., the neutral ) transformer, you provide a 120/240V secondary.

    If you put the leads on a voltmeter on phase A and C, which is typically the leg that is Center tapped in a high leg Delta with B being 208 volts, you will read 240 volts. from A to neutral or C to neutral, you will get 120 volts.

    if you think about where are you split that sine wave, say at the zero Crossing Point, one wave form will be going up while the other wave forum is going down. they are 180 degrees out of phase with each other in regards to the neutral, thus giving 240 volts from A to C.
    Electricians do it until it Hertz!

  9. #9
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    Quote Originally Posted by hbiss View Post
    Really, I'm not blaming you. I'm just trying to understand today's educational system that hands out what they like to call an EE degree without the recipient knowing fundamental concepts. Maybe it's because I'm an old grey haired guy who has worked with many EEs over the years. None would ever say "I don't understand analog because I spent most of my degree studying digital and computers".

    Guess it's just another symptom of our pathetic educational system.

    But I digress...

    -Hal
    You have not offered an answer to OP's question. . . instead, you spouted a sermon from your pulpit. . . which I think wasn't necessary.

  10. #10
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    think of it as a 3 winding xfmr
    1 prim
    2 sec, opposite signs (or hand, dot convention, etc) and a common center point
    this makes each sec v 180 deg out of phase

    sometimes it is better to grab a text and teach yourself
    I thought all abet bsee programs required basic power and machine courses?

    I don't believe hbiss meant insult, but like me was surprised that a bsee program would not cover it or at least give you the tools to develop the model or at least look it up?
    a xfmr is the most fundemental 'machine' and is the basis for understanding the application of fields, then on to gen/motors

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