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Star Delta Line/Phase Voltage Confusion

  1. Apr 20, 2015 #1
    As I am new to this forum, I must apologise if I have began this thread in the wrong category but I have a bit of a problem.

    I don't necessary study physics, I study Electrical Installation at college. As a result we overlap on some physics topics

    We have recently covered Star Delta. Now, I understand that in Star the Line and Phase current are equal and line voltage is *sqrt 3 times the phase voltage. And vice versa for delta.

    I am somewhat experiencing some doubt as to what line current actually means. It is always between the phases of a load? Or can it also be between the line and neutral?

    Any help would be appreciated
  2. jcsd
  3. Apr 20, 2015 #2

    jim hardy

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    Sometimes our thinker gets its gears confused.

    You said current but it seems you're asking about voltage.
    Voltage is measured between two points
    Current is measured at one point.

    [PLAIN]http://upload.wikimedia.org/wikipedia/commons/thumb/4/48/3-phase_flow.gif/357px-3-phase_flow.gif[/SIZE][/COLOR] [Broken]


    I would say: "Neither. Line current is in the lines between source and load."
    Last edited by a moderator: May 7, 2017
  4. Apr 20, 2015 #3

    So just to clarify the is the line VOLTAGE across all three phases

    ....but the PHASE voltage is between line and neutral.?

    I maybe over thinking this haha
    Last edited by a moderator: May 7, 2017
  5. Apr 20, 2015 #4

    jim hardy

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    In three phase power the term "Line Voltage" is taken to mean between two lines not line to neutral.

    It is good practice to name your voltage so as to define the two points between which you intend to describe or measure it

    VL-L for line to line volts
    VL-N for line to neutral volts

    Rigor in your naming conventions will keep your thinking straight
    and will make you aware of how many folks aren't rigorous in their naming, calling lines "phases", and when naming voltages omitting the second measurement point .
  6. Apr 20, 2015 #5

    I'm sure you are right in what you say but my course is at a fairly basic level and the exam format I have will most likely Label them as Line Voltage (VL) and Phase Voltage (VP) which can cause confusion.

    Would it be possible to send you an email of a practice exam question? We marked it in class but I'm not so sure about it now
  7. Apr 20, 2015 #6

    jim hardy

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    not overthinking at all.

    Again, voltage is between two points
    Line voltage is between any two lines, and usually all 3 line to line voltages are the same.

    Phase voltage is a term i reserve for the internals of a device, like one generator winding.
    That's because the windings can be connected star or delta.
    In star machine, phase voltage is going to be line to neutral
    in delta machine phase voltage is going to be line to line.

    This is an example of words leading the mind to the correct (or incorrect) physical concept.
    Most folks look up at a transmission line and call those big wires "phases". I call them "Lines" for a reason.
    When you look at a transformer
    it's pretty apparent that "phase" refers to a winding and its branch of the core.

    This dual use of the term "phase" , once for a winding and once for the external wires, leads the mind to also mix up the physical concepts of phase and line currents and voltage.

    So my advice is -
    depart from convention and reserve the term "phase" for an individual winding or load element
    use "line" for the three wires connecting devices together,

    and after the concepts are firmly planted you'll raise your eyebrows when you encounter less than rigorous naming
    but you'll be able to figure out what the author meant.

    Laviosier: "In correcting our language we reason better"
  8. Apr 20, 2015 #7
    When you say 'line voltage is between any two lines' does this therefore include between Line and Neutral in, say, a star connected load?

    Sorry if these questions seems a little stupid but I'm determined to get this locked in my brain :)
  9. Apr 20, 2015 #8

    jim hardy

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    sorry, you posted while i was typing....

    indeed it causes confusion. My AC circuits instructor cautioned us boys about this very point and taught us rigor first thing.

    I'd say post the practice question here. Who knows, we might have somebody else interested.....

    This picture makes it clear to me why one should use "phase voltage" inside the device and "line voltage" outside devices.
    If you cover up both devices , the term "phase" voltage is irrelevant because you have only three "lines".
    Were there a neutral, i'd use VL-N for voltage measurements to there.
    On left device Vphase = VL-N . On right device Vphase = VL-L .
    Once you have that concept nailed you can write it on the exam using whatever nomenclature your class prefers.
    I sometimes used exams to pass to teachers advice on how to avoid such confusion : "Professor : Please See Reverse for suggestion to improve pedagogical methodology, yours truly....."
    Last edited: Apr 20, 2015
  10. Apr 20, 2015 #9

    jim hardy

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    good for you. i hope post immediately prior answered the question.

    Line to neutral should be VL-N
    again, voltage is between two points which should be (but too seldom are) unambiguously defined.
    Voltage is defined as potential difference and it takes two points to have a difference.
    Too bad Mr Volta didn't have a compound last name, if he had then the name of the unit Volt would be two words not one, leading the mind to the concept of two points.

    What's in a name, eh ?
  11. Apr 20, 2015 #10

    jim hardy

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    I see irony here.

    Ampere is distinctly two syllables
    yet an amp is the charge flowing past a single point per unit time.

    Volt is distinctly one syllable yet is the potential difference between two points.
    Try for a few days calling them "volt-ahs"
  12. Apr 20, 2015 #11
    https://twitter.com/3Collector/status/590162401670070272 [Broken]

    Here you can find the question, any help is appreciated. I seem to grasp the concept but this question is playing tricks with my mind! Is the answer actually A? Or is it C?
    Last edited by a moderator: May 7, 2017
  13. Apr 20, 2015 #12

    jim hardy

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    https://pbs.twimg.com/media/CDCtk4gWgAEY1cT.jpg [Broken]

    Aha ! "Line voltage" ?

    Poorly worded question.
    Line to line
    line to neutral ?

    By convention, "Line voltage" is taken to mean "line to line", did teacher not cover that ?

    So which meter is connected between two "Lines" ?

    What's in a name, eh ?

    Observe even Wikipedia is weak on defining their naming convention up front
    until they get to their "definitions" paragraph
    Now that's clear.
    So i'd write immediately underneath that question:
    "Professor - Author has failed to distinguish between voltage 'line to line' or 'line to neutral'. C measures the former and A the latter. Your humble student, my name, "
    Last edited by a moderator: May 7, 2017
  14. Apr 20, 2015 #13
    Perfect! I will run this by him tomorrow - hopefully he will be able to make some sense of it!

    Suppose a question like that came up again, would I be right in assuming its referring as Line to Line? It seems pretty lazy on the exam boards part.
  15. Apr 20, 2015 #14

    jim hardy

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    I want to say "yes"
    It was fifty years ago i took the course.

    Mention to teacher you have it on good authority that
    "When slide rules roamed the earth, term 'line voltage' was taken to mean line-to-line not line-to neutral . Is this today's convention ?"

    Reason i say that is, some months ago i got a surprise in a thread on "Power Factor".
    That term is used by some authors nowadays to mean 'a factor to calculate RMS power of arbitrary waveforms' , as opposed to VIcos(theta) . New math ?
    A bit used to mean 1/8dollar, too .

    back on topic - I agree with you, it's sloppy notation.

    old jim
  16. Apr 20, 2015 #15

    jim hardy

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    I hope you take away from this an appreciation for the value of rigorous upfront definition of terms. It's one thing i look for in textbooks.
  17. Apr 24, 2015 #16
    I had a question similar to this a while back; Jim Hardy helped me on that one so thanks again Jim from all of us beginners.

    I just wanted to chime in with an addendum to the issue of clearly defining your question. First of all, it's asking for voltage so B and D are irrelevant since they're not between two points. But more important is that the question fails to define line-line or line-neutral. Which brings me to the point I'm trying to make: always qualify your work! This is something that I deal with all the time. People simply give numbers with no labels attached to them. I know it seems like a pretty elementary thing to say but as you get out working in the field you can become so into a routine you start making the mistake of just listing results without qualifying them. Heck, the guy who wrote that question got it wrong because he wasn't specific enough. My advice is that while you're in school label literally everything, even things that don't necessarily have to be.

    You're never getting "about 20 volts from the stator". You're getting "20.3 ACV from line 1 to line 2".
  18. Apr 26, 2015 #17


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    Line current is the current in the line. :wink: Trite but true!

    Whether the load is star or delta is immaterial to how you measure line current.

    If everything is balanced (and in most exam questions it always is :smile: ) then all line currents will have the same magnitude. As I believe Jim said, you measure line current by imagining you have an ammeter in series with the line. You can take your pick whether you choose red, blue, or yellow when the load is balanced. It is always a good idea to mark on the circuit exactly how and where you will connect any meter, before you perform the task.

    3ɸ outlets can provide fearsome sparks. i0om5.gif
  19. Apr 27, 2015 #18

    jim mcnamara

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    i0om5.gif +++ for Sparky the gif-ted electrician. I was equally careless one time. Never again.
  20. Apr 27, 2015 #19


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    This Smilie would come in handy at times here in the EE Forum. Maybe Greg would like to add it to the "Old School" Smiles; if there's no copyright problems.

    This one would be good for the Chemistry Forum. :devil:

  21. Jul 25, 2015 #20
    I think this is really about convention

    In the power industry in the UK (for example), the convention for three-phase power is:

    Line voltage is the voltage across two phases
    Phase voltage is the voltage across a single phase (referenced to earth neutral)

    The relationship between this is sqrt3 which is most easily shown geometrically

    In a delta system,
    Line and phase voltage are the same
    Line current is sqrt3 * phase current

    In a star system
    Line voltage is sqrt3 * phase voltage
    Line current = phase current
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