Complex Power = VI* or VI?

Discussion in 'Homework Help' started by HBenzing, Feb 6, 2011.

  1. HBenzing

    Thread Starter New Member

    Feb 6, 2011
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    This is more of an understanding problem then a homework problem.

    Can anyone tell me why complex power is calculated using the conjugate of the current? Everywhere ive looked it just states S=VI* without really giving an explanation, why isn't it just S=VI

    Anything that would help explain this would be much appreciated.
     
  2. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    5,448
    782
    Suppose Vrms=|Vrms|/_θ1 and Irms=|Irms|/_θ2

    [The /_ 'thingy' is meant to signify the phase angle]

    To construct the complex power triangle one might assume (as you did) that this could be found from the product Vrms x Irms

    However, this would give the incorrect relationship

    S=|Vrms|x|Irms|/_(θ1+θ2)

    whereas the apparent power should be given by

    S=|Vrms|x|Irms|/_(θ1-θ2)

    The complex conjugate form Vrms x Irms* in fact gives the correct relationship.
     
  3. HBenzing

    Thread Starter New Member

    Feb 6, 2011
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    Ok that makes sense. Thanks for explaining it!
     
  4. Ankush Sharma

    New Member

    Oct 29, 2015
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  5. Ankush Sharma

    New Member

    Oct 29, 2015
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    what about leading phase angle..wouldn't it get subtracted?
     
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