Electrical eng, my vectors for KVL and KCL aren't adding up - no errors in math

In summary, according to the circuit diagram attached, the voltage across R2 and C2 in parallel is not equal to the voltage across R1 and C1. The voltage across R2 and C2 should be added once, and the voltage across R1 and C1 should be added twice.
  • #1
Yarnzorrr
11
0
I've been asked to prove KVL and KCL using this circuit, but my vectors don't add up.
ei Vr1+ Vr2 + Vc1+Vc2 ≠ Vtotal, same goes for Current, but I at least got the correct angle for the current
Tried using vector addition, using addition of the vectors in complex form and even tried using this http://www.1728.org/vectors.htm vector addition calculator

I know my calculation are correct, I got my lab supervisor to check them.

Noting works, I don't know what I'm doing wrong, please help!

These are the values that I've got for my series parallel RC circuit (attached a photo of the circuit diagram)

Voltage (V)
R1 1.60< 49.97°
C1 2.78 < - 40.03°
R1 1.68 <19.4°
C2 1.68 <19.4°
Total 4.75< 0°

Currant (mA)
R1 1.956 < 49.97°
C1 1.956 < 49.97°
R2 1.68<19.4°
C2 0.995< 109.4°
Total 1.956 < 49.9°

Impedance (Ω)
R1 820 < 0°
C1 1421< -90°
R2 1000 < 0°
C2 1693< -90°
Total 2427.6 < - 49.9°
 

Attachments

  • Screen Shot 2012-09-27 at 4.38.07 PM.png
    Screen Shot 2012-09-27 at 4.38.07 PM.png
    3.4 KB · Views: 426
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  • #2
Yarnzorrr said:
I've been asked to prove KVL and KCL using this circuit, but my vectors don't add up.
ei Vr1+ Vr2 + Vc1+Vc2 ≠ Vtotal, same goes for Current, but I at least got the correct angle for the current
Tried using vector addition, using addition of the vectors in complex form and even tried using this http://www.1728.org/vectors.htm vector addition calculator

I know my calculation are correct, I got my lab supervisor to check them.

Noting works, I don't know what I'm doing wrong, please help!

These are the values that I've got for my series parallel RC circuit (attached a photo of the circuit diagram)

Voltage (V)
R1 1.60< 49.97°
C1 2.78 < - 40.03°
R1 1.68 <19.4°
C2 1.68 <19.4°
Total 4.75< 0°

Currant (mA)
R1 1.956 < 49.97°
C1 1.956 < 49.97°
R2 1.68<19.4°
C2 0.995< 109.4°
Total 1.956 < 49.9°

Impedance (Ω)
R1 820 < 0°
C1 1421< -90°
R2 1000 < 0°
C2 1693< -90°
Total 2427.6 < - 49.9°

Can you show your analysis steps? I can see issues with your result values, but without seeing how you got there it's difficult to know how to help you.
 
  • #3
Forgot to mention that the AC voltage input value on the diagram is wrong, its actually 4.75 < 0° sorry!

first found Z with:

Ztotal = (R2*Xc2)/(√(R2^2+Xc2^2) + R1 + Xc1

Where
R2 = 1000
Xc2 = -j 1693
R1 = 820
Xc2 = -j 1421

break it down to polar form to get

861 < 30.57° + 820 <0° + 1421 <-90° = 2427.6 < - 49.97°

Then made a table and using Ohm's Law got the following (attached)
 

Attachments

  • Screen Shot 2012-09-27 at 10.05.34 PM.png
    Screen Shot 2012-09-27 at 10.05.34 PM.png
    6.4 KB · Views: 396
  • #4
and to get the current through R2 and Xc2:

Itotal * (R2/(-jxc2+R2) = 1.956 < 49.97° * (1000 /-j1693+1000)

then to get IXc swapped the value for R2 on the denominator for Xc

got the following answers:
IR2 = 0.999 <109.4°
IXc2 = 1.68 < 19.4°
 
  • #5
Yarnzorrr said:
Forgot to mention that the AC voltage input value on the diagram is wrong, its actually 4.75 < 0° sorry!
In that case, your individual calculated values look fine.

Perhaps your issue lies with using the calculated values with KVL and KCL. For example, in your original post you wrote: "Vr1+ Vr2 + Vc1+Vc2 ≠ Vtotal". The question is what closed KVL path does this expression correspond to in the circuit? After all, R2 and C2 are in parallel, so why would you add the voltage across them twice?
 

Related to Electrical eng, my vectors for KVL and KCL aren't adding up - no errors in math

1. Why are my vectors for KVL and KCL not adding up?

There could be several reasons for this. First, check your math to make sure there are no errors. Double check all your calculations and make sure you are using the correct values. Also, make sure you are using the correct direction for your vectors.

2. What is KVL and KCL?

KVL stands for Kirchoff's Voltage Law, which states that the sum of all voltages in a closed loop must equal zero. KCL stands for Kirchoff's Current Law, which states that the sum of all currents entering a node must equal the sum of all currents leaving the node.

3. How do I know if I am using the correct values for KVL and KCL?

You can use Ohm's Law (V=IR) and the equations for voltage and current division to calculate the correct values for KVL and KCL. You can also use circuit analysis techniques such as nodal analysis or mesh analysis to determine the correct values.

4. Are there any shortcuts for solving KVL and KCL problems?

Yes, there are some techniques that can make solving KVL and KCL problems easier. One technique is to redraw the circuit and simplify it before applying KVL and KCL equations. Another technique is to use Thevenin's or Norton's theorem to convert the circuit into an equivalent circuit with a single voltage or current source.

5. Can I use KVL and KCL for any circuit?

Yes, KVL and KCL can be applied to any circuit, as long as the circuit is in steady state and follows the principles of Ohm's Law. However, for more complex circuits, other techniques such as superposition, Thevenin's theorem, or nodal analysis may be more efficient to use.

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