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Power calculation

  1. May 24, 2015 #1
    in the calculation of complex power S=VI we sometimes use S=VI* and sometimes as S=V*I.what is the physical significance of these two notations and in which case these two expression has different meaning?
     
  2. jcsd
  3. May 24, 2015 #2

    davenn

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    what is S ?
     
  4. May 24, 2015 #3

    Simon Bridge

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    The difference is in the phase of the reactive power.
    To understand that - you need to understand the significance of the reactive power.
    http://www.allaboutcircuits.com/vol_2/chpt_11/2.html

    The two relations get you the same magnitudes for the components and you normally only need the power delivered to the load anyway.
    If you derive the equation for S from the usual phasors, then you get ##S=VI^\star##.
     
  5. May 24, 2015 #4

    davenn

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    Thanks Simon

    was wondering what the heck he was talking about
     
  6. May 24, 2015 #5
    I agree with Simon, of course.In my opinion we use the conjugate current in formula S=VxI* since in an inductive circuit the actual current lags the voltage but the reactive power is considered positive[conventional].So if S[apparent power]= VxI=(Vre)x(Ire-jIim)=(VrexIre)+(-VrexIim)j then Q=-Vre*Iim.
    If S[apparent power]= VxI*=(Vre)x(Ire+jIim)=(VrexIre)+(VrexIim)j and Q=+Vre*Iim.
    If the current leads the voltage [capacitive circuit] then the reactive power is negative[conventionally].
     
  7. May 25, 2015 #6

    Simon Bridge

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    ... erk: here lets tidy that up a bit:

    ##S = VI^\star = V_{re}\big(I_{re}+jI_{im}\big) = V_{re}I_{re} + j V_{re}I_{im}## and ##Q=+V_{re}I_{im}##.

    ... better? But did you take the conjugate properly? - I decided not to wade through all those letters to check.

    LaTeX: worth the learning curve.
     
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