1. Sep 18, 2012 #1

   Yarnzorrr

   

   here is a screenshot from one of my lectures talking about how to find active power (P) and reactive power (Q)
   
   He calculates P using 100*10 Cos (30-(-30), which I understand, though the answer he gets is 200W, but whenever I calculate it I get 500 because cos60 = 0.5 and 0.5*1000 is 500.
   
   Just wanting to know what I'm doing wrong.
   Thanks.

    

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     Screen Shot 2012-09-18 at 5.18.48 PM.png

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   Yarnzorrr, Sep 18, 2012
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3. Sep 18, 2012 #2

   NascentOxygen

   

   Staff: Mentor

   It does look like it should be 500.

    

   NascentOxygen, Sep 18, 2012
4. Sep 18, 2012 #3

   I_am_learning

   

   Yes, its obviously 500.
   Also, VA = Sqrt(Watts^2+Var^2)
   1000 ~= sqrt(500^2+866^2)

    

   I_am_learning, Sep 18, 2012
5. Sep 18, 2012 #4

   yungman

   

   I think it's 500
   
   I use complex number. From the picture of the board:
   
   V=100∠+30 deg = 100(0.866+j0.5)= 86.6+50j
   I=10∠-30 deg = 10(0.866-j0.5)= 8.66-j5.
   
   Power =Re[IV]= Re[(86.6+j50)(8.66-j5)]= 749.956-250=500.

    

   yungman, Sep 18, 2012

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