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

  1. Sep 27, 2012 #1
    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°
     

    Attached Files:

  2. jcsd
  3. Sep 27, 2012 #2

    gneill

    User Avatar

    Staff: Mentor

    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.
     
  4. Sep 27, 2012 #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)
     

    Attached Files:

  5. Sep 27, 2012 #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°
     
  6. Sep 27, 2012 #5

    gneill

    User Avatar

    Staff: Mentor

    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?
     
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