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Hard RLC Series Circuit

  • Engineering
  • Thread starter ragamuffin_8
  • Start date
  • #1

Homework Statement



An RLC circuits consists of R1 a 10-ohm resistor, R2 a resistor that takes 50 W, C1 a capacitor with 5-ohm reactance, and L1 an inductor that takes 100 var. Find the value of R1, R2, and Xl (inductive reactance).


Homework Equations



P = (I^2)R
Xc = 1/(2∏fC)
Xl = 2∏fL
Z = √(R^2 + XeqL^2)

The Attempt at a Solution



I tried equating the currents but I don't know what to do next. I tried to solve for the equivalent impedance but without the frequency of the source, my efforts were futile. Perhaps I could assume a frequency of 60 Hz?
 
Last edited:

Answers and Replies

  • #2
1,097
2

Homework Statement



An RLC circuits consists of R1 a 10-ohm resistor, R2 a resistor that takes 50 W, C1 a capacitor with 5-ohm reactance, and L1 an inductor that takes 100 var. Find the value of R1, R2, and Xl (inductive reactance).


Homework Equations



P = (I^2)R
Xc = 1/(2∏fC)
Xl = 2∏fL
Z = √(R^2 + XeqL^2)

The Attempt at a Solution



I tried equating the currents but I don't know what to do next. I tried to solve for the equivalent impedance but without the frequency of the source, my efforts were futile. Perhaps I could assume a frequency of 60 Hz?
Is this parallel or series RLC?
 
  • #3
Is this parallel or series RLC?
I forgot to mention, this is connected in SERIES. I'm sorry
 
  • #4
NascentOxygen
Staff Emeritus
Science Advisor
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I think we need another clue, such as the line voltage, or that the load has unity power factor.

By assuming a current, I, in all the elements I can find the voltage across each in terms of that I, but that's as far as I can get without more information.
 
  • #5
I'm sorry I forgot to mention the line voltage. 100 Vac. But the frequency was not given, how do I start attacking this problem?
 
  • #6
NascentOxygen
Staff Emeritus
Science Advisor
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I'm sorry I forgot to mention the line voltage. 100 Vac.
Forgot?! :frown: :mad:

how do I start attacking this problem?
Start by drawing a large schematic, and mark on the quantities you are given for each element.

Assume a branch current, I, and using what you are told about each element, determine the voltage across that particular element in terms of I. The only unknown on the right-hand side of each equation will be I, any other terms on the right-hand side will be known numbers that you can work out from the information provided.

You do not need to know the line frequency.

Good luck!
 
  • #7
Forgot?! :frown: :mad:



Start by drawing a large schematic, and mark on the quantities you are given for each element.

Assume a branch current, I, and using what you are told about each element, determine the voltage across that particular element in terms of I. The only unknown on the right-hand side of each equation will be I, any other terms on the right-hand side will be known numbers that you can work out from the information provided.

You do not need to know the line frequency.

Good luck!

Thanks Sir NascentOxygen!

I was not thinking of KVL that's why I'm having a hard time in this problem. :shy: I'm sorry

My attempt:

100 V = IR1 + IXc + IR2 + IXL

but:
P = VR2I
VR2 = 50/I

P = I2XL
XL = 100/I2

so:

100 = 10I + 5I + 50/I + 100/I

Solving for I, I got two values I = 4.39 and I = 2.28, which value should I choose?
 
  • #8
Forgot?! :frown: :mad:



Start by drawing a large schematic, and mark on the quantities you are given for each element.

Assume a branch current, I, and using what you are told about each element, determine the voltage across that particular element in terms of I. The only unknown on the right-hand side of each equation will be I, any other terms on the right-hand side will be known numbers that you can work out from the information provided.

You do not need to know the line frequency.

Good luck!

Thanks Sir NascentOxygen!

I was not thinking of KVL that's why I'm having a hard time in this problem. :shy: I'm sorry

My attempt:

100 = IR1 + IXc + IR2 + IXL

but:
P = VR2I
VR2 = 50/I

P = I2XL
XL = 100/I2

so:

100 = 10I + 5I + 50/I + 100/I

Solving for I, I got two values I = 4.39 and I = 2.28, which value should I choose?
 
  • #9
NascentOxygen
Staff Emeritus
Science Advisor
9,244
1,072
My attempt:

100 = IR1 + IXc + IR2 + IXL
That's a good start, but we distinguish resistance from reactance by associating an angle with reactance. So the equation above needs to be fixed to include this. There are a couple of ways to represent angle, use whichever you like to correct the above equation.


but:
P = VR2I
VR2 = 50/I
yes

P = I2XL
XL = 100/I2
What law did you rely on here?
 
  • #10
That's a good start, but we distinguish resistance from reactance by associating an angle with reactance. So the equation above needs to be fixed to include this. There are a couple of ways to represent angle, use whichever you like to correct the above equation.
I don't quite understand sir. Would you please elaborate?
 
  • #11
NascentOxygen
Staff Emeritus
Science Advisor
9,244
1,072
We write VL for an inductor as [itex]{\color{Blue} {j I X_{L}}} \text{ or as } {\color{Blue}{IX_{L} \angle 90^{\circ}}}\text{ where } X_{L} [/itex] is the magnitude of the inductive reactance.

And something similar for the voltage across a capacitor. Then addition of voltages takes the form of addition of vectors.
 

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