Leading & Lagging Phasor Diagrams

Discussion in 'Homework Help' started by myristate, Apr 2, 2017.

  1. myristate

    Thread Starter New Member

    Oct 21, 2016
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    0
    when drawing a voltage current phasor to show lagging leading in my example I have a voltage of 230V ∠60° and a current of 87A ∠32° Normally when drawing phasors you have a reference along the X plane but in my example neither voltage or current is at 0° So do you draw both phasors from a horizontal angle of 0° in which case I don't know what to call the reference or do I use the voltage even though its at 60° as the reference. I'm not sure what the accepted convention is when drawing these. Every example I find the voltage is always at 0° and is used as a reference along the horizontal.
     
  2. crutschow

    Expert

    Mar 14, 2008
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    6,270
    Just use 0° on phasor diagram as the reference and plot the voltage and current with respect to that.
     
  3. WBahn

    Moderator

    Mar 31, 2012
    23,980
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    Use whichever makes sense for what you are trying to do with THAT diagram. The reference is totally arbitrary -- in a very real sense it merely marks the point during the cycle at which you started a stopwatch. Nothing more.

    As long as ALL of the phasors on the diagram use the SAME reference, you can rotate all of them by the same amount by adding or subtracting the same angle from all of them.

    In most cases you are probably better off plotting the angles as given as that makes it less likely that you will make a silly arithmetic error while adjusting them to a nice reference.

    Also, remember that you always have the option to rotate them at any time, including after you have the final answer, if you think that aligning the voltage phasor with the reference will communicate your answer more clearly. But, of course, you need to be sure that all of the work that refers to that diagram is adjusted so that it is consistent with that diagram. This is often more hassle than it is worth.
     
  4. shamekhpower

    New Member

    Apr 14, 2012
    1
    0
    I am not in alignment with some comments above. However, here is my answer; draw each quantity as given with respect to the positive X-axis as a reference. The key is that positive angels rotate counter clockwise and negative angles rotate clockwise. Now you draw your phasors; look at the small angel between your phasors and express your answer in term of the current.

    Example: voltage of 230V ∠60° and a current of 87A ∠32. The phasor of these quantities tells us that the two vectors form a small angle between them that is (60°-32°= 28°). Finally, express this angle as leading or lagging with respect to the current. Therefore the current is lagging the voltage by 28°
    -Sayed
     
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