Phase current NOT EQUAL to line current?

Discussion in 'General Electronics Chat' started by powersys2005, Jan 12, 2010.

  1. powersys2005

    Thread Starter New Member

    Jan 3, 2010
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    0
    I'm a bit confused with the figure below (extracted from here: http://www.allaboutcircuits.com/vol_2/chpt_10/7.html). If I'm not mistaken, the phase current and line current of a star-connected system should be the same. However, as shown in figure below, they look different because the line current contains (w,5w,7w) whilst the phase current contains (w,3w,5w,7w,9w). Is it correct? Thanks.



    [​IMG]
     
  2. rvh002@gmail.com

    Active Member

    May 15, 2009
    118
    2
    Hi, your question does not seem to agree with your drawing? The blue line ( your Problem) starts off as a full system and then looses some component along the way, on a solid wire?? Your original statement is correct.
     
  3. powersys2005

    Thread Starter New Member

    Jan 3, 2010
    29
    0
    Hi rvh002,

    Thanks for your reply. However, I don't quite understand your point. Let's look at the load side. Clearly, all phase currents contain (w,3w,5w,7w,9w). However, if we look at the line currents, they only contain (w,5w,7w). Therefore, in this star-connected case, the phase current and the line current are not equal. Kindly note that I extract the figure from "All About Circuits" ebook (http://www.allaboutcircuits.com/vol_2/chpt_10/7.html). Kindly advise if it's correct. Thanks.
     
  4. 3ldon

    Active Member

    Jan 9, 2010
    82
    3
    That's because third, 6th and 9th harmonics cancel, in theory the line currents will not have any third harmonic current in them, and the neutral will have a third and 9th harmonic voltage present, which may be enough to cause a shock hazard.
     
  5. powersys2005

    Thread Starter New Member

    Jan 3, 2010
    29
    0
    Hi 3ldon, thanks. So, do you agree that the line currents should have no 3w and 9w, right? So, the diagram provided in All About Electrics is wrong, right?
     
  6. 3ldon

    Active Member

    Jan 9, 2010
    82
    3
    The diagram is correct, there are third and ninth harmonics across line to neutral, but line to line they cancel.
     
  7. powersys2005

    Thread Starter New Member

    Jan 3, 2010
    29
    0
    Hi 3ldon, thanks for your reply again. Let's say we place two current probes i.e. ILine and Iph, as shown in figure below. If the diagram is correct, that means the current measured by Iph probe will contain w,3w,5w,7w,9w... whilst the current measured by ILine probe will contain w,5w,7w... But in theory, the line and phase currents are in series and therefore the currents measured by Iph and ILine probes must be the same, right? I'm really confused...


    [​IMG]
     
  8. 3ldon

    Active Member

    Jan 9, 2010
    82
    3
    I see why you are getting confused.
    The diagram is showing the harmonic voltages present across the load, and the harmonic current on the line, without differentiating between the two
     
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