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How to find True power dissipated and apparent power?

Electrical Engineering Asked by Michael scott on January 25, 2021

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I have to find the true power dissipated and apparent power for this RLC AC circuit. I’ve already found the impedance, the current from the supply and the current from each branch but I’ve forgotten how to find the true power dissipated and apparent power. Help would be appreciated. Sorry for the rough circuit.

2 Answers

I've forgotten how to find the true power dissipated and apparent power.

You've already calculated true power in comments but not so accurately. True power is (as transistor points out) $V^2/R$ = 847.06 watts and, the current in the resistor is 3.5294 amps.

For the apparent power I'd calculate individually the currents in the inductor and capacitor, these being: -

  • Inductor current = 15.279 amps
  • Capacitor current = 4.2977 amps

And, the trick here is to realize that that actual current that flows into the pair of them is the difference current. This is because L and C form a partial tuned circuit.

  • Net LC current is 10.981 amps

Total current is resistive plus net reactive current added as phasors (or using pythagoras): -

$$sqrt{3.5294^2+10.981^2}$$

And that equals 11.535 amps (this is the total RMS current from the 240 volt supply).

Hence apparent power is 240 x 11.535 = 2768.3 watts.

Correct answer by Andy aka on January 25, 2021

Well, the input impedance is given by:

$$underline{text{Z}}_{spacetext{in}}=frac{1}{frac{1}{text{R}}+frac{1}{text{j}omegatext{L}}+text{j}omegatext{C}}tag1$$

The true power is given by:

$$text{P}=text{V}_{text{in}}^*cdottext{I}_{text{in}}^*cdotcosleft(varphiright)tag2$$

Where $x^*$ is the RMS value.

In your case, $text{V}_{text{in}}^*=240$ is given. So, we get:

  • $$text{I}_{text{in}}^*=frac{1}{sqrt{2}}cdotfrac{240}{left|underline{text{Z}}_{spacetext{in}}right|}tag3$$
  • $$varphi=argleft(underline{text{Z}}_{spacetext{in}}right)tag4$$

It is not hard, to show that:

$$underline{text{Z}}_{spacetext{in}}=frac{1}{68 left(left(frac{1}{5 pi }-frac{57 pi }{10000}right)^2+frac{1}{4624}right)}+frac{2890000 pi left(57 pi ^2-2000right)}{59642000 pi ^2-938961 pi ^4-1156000000}cdottext{j}tag5$$

So:

  • $$text{I}_{text{in}}^*=frac{3 sqrt{578000000-29821000 pi ^2+frac{938961 pi ^4}{2}}}{2125 pi }tag6$$
  • $$varphi=-arctanleft(frac{68}{5pi}-frac{969pi}{2500}right)tag7$$

The answer, gives:

$$text{P}=frac{7200 sqrt{2}}{17}approx598.961spacetext{W}tag8$$

Answered by Jan on January 25, 2021

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