Engineering How Do You Calculate Apparent and Reactive Power in AC Circuits?

[Electest]

Sorry this is how the learning material shows how to write it. It's not so much an L but a symbol that denotes an angle or am I missing the point.

Ztotal = 27.65 +44 degrees Ohms

Z = 21.52 +62.58 degrees Ohms

Regards

 

[gneill]

Sorry this is how the learning material shows how to write it. It's not so much an L but a symbol that denotes an angle or am I missing the point.

Ztotal = 27.65 +44 degrees Ohms

Z = 21.52 +62.58 degrees Ohms

Regards

Ah! Okay, understood. If you go to the Advanced editing panel to compose your post then you will find an actual angle symbol ∠ available in the Quick Symbols menu.

 

[Electest]

Ok brilliant.

So is my answer now correct for the question (4) or is there anything else to do?

Thanks

 

[gneill]

Ok brilliant.

So is my answer now correct for the question (4) or is there anything else to do?

Thanks

You're good to go with either representation of the impedance (they are after all just two representations of the same complex value).

 

[Electest]

Thanks again

So my answer should show the Z values calculated not the Z totals. I've got it now :-)

 

[Lightning9]

Hi, sorry to resurrect this thread but I'm confused by your working out. I understand your reason for using complex numbers but when I try and confirm the voltage drops they don't add up to the supply voltage 120V?

or am I missing something?
Thanks

 

[gneill]

Hi, sorry to resurrect this thread but I'm confused by your working out. I understand your reason for using complex numbers but when I try and confirm the voltage drops they don't add up to the supply voltage 120V?

or am I missing something?
Thanks

There's no way to tell if you're missing something if you don't show us what you have...

Show your work!

 

[Lightning9]

I was just using the equation Vs = √ V_R² + V_L²

But I now see you can't because of the complex number.

 

[woodyzzz]

For the circuit given in the power factor is 0.72 lagging and
the power dissipated is 375 W.

View attachment 42861

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

Hello Charger and Oneill

I am currently going through the same coursework and just wanted to share my answers and thoughts- hoping for some feedback so that I can be sure that what my understanding is, is correct

Now...there are numerous equations that involve P, R and I the first one that jumps out is the P=VI or P=I^2R P = dissipated power V= supply voltage I = current

now my thinking is you can't use the P=VI as this would only give you the true power value, which in turn would not give you the correct I value

I then ventured onto P= I^2R - but then i thought this would only apply to a purely resistive circuit, which this obviously isn't as the pf is lagging 0.72

I then decided on (s) = Vs*I s= apparent power (which includes the total circuit voltage) Vs= voltage supply

I = 520.83/ 120 = 4.34 A

is this the correct way of thinking ?

 

[gneill]

I then decided on (s) = Vs*I s= apparent power (which includes the total circuit voltage) Vs= voltage supply

I = 520.83/ 120 = 4.34 A

Yes, that will work.

 

[woodyzzz]

Yes, that will work.

Is it correct to assume that the current value is the same throughout the circuit ? or would you obtain different value i.e. across the resistor

 

[gneill]

Is it correct to assume that the current value is the same throughout the circuit ? or would you obtain different value i.e. across the resistor

It is a series circuit, so there's no option: The current must be identical throughout.