Power in AC circuits.

Thread Starter

Kml565

Joined Oct 11, 2017
3
For the circuit given in the power factor is 0.72 lagging and the power dissipated is 375 W. Determine the: (1) apparent power (2) reactive power (3) the magnitude of the current flowing in the circuit (4) the value of the impedance Z and state whether circuit is inductive or capacitive. (1) Apparent power = True power / Power Factor (S=P/pf) = 375 / 0.72 = 520.8333 VA (2) Reactive Power = SQRT (apparent power^2) - (True Power^2) = SQRT (520.8333^2)-(375^2) = SQRT (130642.3264) = 361.4448 VAR (3) Magnitude of Current Power= Voltage*Current *Power factor (P=V*I*pf) 375 = 120*I*0.72 I = P/(V*pf) I = 375/(120*0.72) I = 4.34 A (4) Total ohms = Voltage/Current = 120/4.34 =27.6498 ohms (minus 10) z = 17.65 ohms The power factor is lagging within the circuit therefore the the virvuit is inductive Am i on the right lines here? or is there a better way of calculating the above results

Also if I calculate the current using I*2 R for the resistive part of the circuit I get a different result!

Please help.

Kml565
 

WBahn

Joined Mar 31, 2012
29,979
For the circuit given in
For the circuit given in.... what?

Some of the questions can be answered independent of the circuit, but others can't, at least not without having to guess at details of the circuit that you've chosen not to provide. Since engineering isn't about guessing (most of the time), I'll wait for more information.
 

WBahn

Joined Mar 31, 2012
29,979
"(3) Magnitude of Current Power=" That's new to me. What is Current Power?
I stumbled on the same thing. The key is to look at the original problem and note that part (3) is to find "the magnitude of the current flowing in the circuit". So he should probably have a colon or a period after "Magnitude of Current" to indicate that it is just a label for what he is trying to find and "Power =" is the start of the work.
 

MrAl

Joined Jun 17, 2014
11,389
For the circuit given in the power factor is 0.72 lagging and the power dissipated is 375 W. Determine the: (1) apparent power (2) reactive power (3) the magnitude of the current flowing in the circuit (4) the value of the impedance Z and state whether circuit is inductive or capacitive. (1) Apparent power = True power / Power Factor (S=P/pf) = 375 / 0.72 = 520.8333 VA (2) Reactive Power = SQRT (apparent power^2) - (True Power^2) = SQRT (520.8333^2)-(375^2) = SQRT (130642.3264) = 361.4448 VAR (3) Magnitude of Current Power= Voltage*Current *Power factor (P=V*I*pf) 375 = 120*I*0.72 I = P/(V*pf) I = 375/(120*0.72) I = 4.34 A (4) Total ohms = Voltage/Current = 120/4.34 =27.6498 ohms (minus 10) z = 17.65 ohms The power factor is lagging within the circuit therefore the the virvuit is inductive Am i on the right lines here? or is there a better way of calculating the above results

Also if I calculate the current using I*2 R for the resistive part of the circuit I get a different result!

Please help.

Kml565
Hi,

Looks like you are on the right track but why did you subtract 10 from the 27.6498 ohms?
 

Thread Starter

Kml565

Joined Oct 11, 2017
3
For the circuit given in.... what?

Some of the questions can be answered independent of the circuit, but others can't, at least not without having to guess at details of the circuit that you've chosen not to provide. Since engineering isn't about guessing (most of the time), I'll wait for more information.
Sorry it is a simple series circuit having a 120V RMS supply in series with a 10 ohm resistor and an inductor of unknown value. No frequency of the AC supply is given.
Thanks.
 
Last edited by a moderator:

MrAl

Joined Jun 17, 2014
11,389
Sorry it is a simple series circuit having a 120V RMS supply in series with a 10 ohm resistor and an inductor of unknown value. No frequency of the AC supply is given.
Thanks.
Hi,

That makes sense now.
No need for AC frequency if we can choose whatever inductor we want, and also interesting is we did not even need to know if it was series or parallel until you specified the resistor value, and even then we could deduce it from the resistor value. Just a side note. I am assuming the voltage is 120vac too from your work.
 

Thread Starter

Kml565

Joined Oct 11, 2017
3
Hi,

That makes sense now.
No need for AC frequency if we can choose whatever inductor we want, and also interesting is we did not even need to know if it was series or parallel until you specified the resistor value, and even then we could deduce it from the resistor value. Just a side note. I am assuming the voltage is 120vac too from your work.
Yes I would be interested to know you're answers. Thanks.

Karl
 

MrAl

Joined Jun 17, 2014
11,389
Yes I would be interested to know you're answers. Thanks.

Karl
Hi,

Well maybe we should take a look at your calculation first.

You seem to have come up with the value of 17.65 for 'z', and is that for the reactive part?
If so, then is that the inductor value you would choose if we have angular frequency w=1 ?
If that is also true, then do you get the right angle between current and voltage when using that value along with the known resistor value of 10 ohms?

Please feel free to correct any of these assumptions, but if so, try to calculate the angle with whatever you actually got for the inductor value or however you want to do it. I would be interested to see what you get for the angle after you find your component.value or whatever way you want to do it other than using the original problem specification of 0.72 power factor. In other words, a check to see that you get the same angle as the original specification.

LATER:
Oh yeah BTW, is it possible that there is no solution for this, or is that resistor of 10 ohms certain?
 
Last edited:
Top