Power calculations on an AC circuit

Thread Starter

mifster

Joined Dec 12, 2017
1
I have a resistor and an inductor in parallel, picture included and I want to calculate the reactive and true power given a voltage U. This is my solution but its not correct.

\(Z = \frac{Z_4Z_5}{Z_4+Z_5} = \frac{R_2\omega ^2L_2^2}{R_2^2+\omega ^2L_2^2} + j\frac{R_2^2\omega L_2}{R_2^2+\omega ^2L_2^2} = R + jX\)

The complex power is equal to.

\(S = \frac{UI^*}{2}\)

\(I = \frac{U}{Z} = \frac{UZ^*}{ZZ^*}\)

\(\Rightarrow S = \frac{UU^* Z^*}{ZZ^*} = \frac{|U|^2(R-jX)}{R^2+X^2}\)

This now gives the true and reactive power, P and Q respectivley

\(P = real(S)\)

\(Q = imag(S)\)

Thanks for any help
 

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WBahn

Joined Mar 31, 2012
32,823
Where did the 2 come from in your first equation for S and why did it then disappear?

What do you get when you write your R and X back in terms of the original resistance and inductance?
 
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