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Phasor Mesh Current-what am I doing wrong?

  1. Nov 30, 2011 #1
    Phasor Mesh Current--what am I doing wrong??

    1. The problem statement, all variables and given/known data
    Circuit diagram:
    meshphasor.jpg
    Calculate the phasor mesh currents, I1 and I2.

    2. Relevant equations
    KVL/Mesh Current analysis with phasors/complex numbers.

    3. The attempt at a solution

    Mesh 1: (-550@0deg) + j75(I1) + 30(I1 - I2) - j40(I1) - j24(I2) = 0

    Mesh 2: (600@-30deg) + 30(I2 - I1) + j50(I2) + 80(I2) - j24(I1) = 0

    (30 + j35)I1 - (30 + j24)I2 = 550
    -(30 + j24)I1 + (110 + j50)I2 = (-600@-30deg)

    I got:

    I1 = (9.45503221553 @ -38.8333374187 degrees) A

    I2 = (2.79176352576 @ 93.1719060416 degrees) A

    This problem was even demonstrated in class. Yet the homework site keeps telling me I'm wrong. I don't see where I've made a mistake...?
     
  2. jcsd
  3. Dec 1, 2011 #2

    rude man

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    Re: Phasor Mesh Current--what am I doing wrong??

    How did you incorporate the 600V at -30 degrees in your equations? You didn't show us that, so we can't find your mistake for you, if there is one.
     
  4. Dec 1, 2011 #3

    gneill

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    Staff: Mentor

    Re: Phasor Mesh Current--what am I doing wrong??

    Maybe check that the signs of the mutual inductance terms respect the dot notation and choice of loop current directions?
     
  5. Dec 1, 2011 #4
    Re: Phasor Mesh Current--what am I doing wrong??

    Mesh 2: (600@-30deg) + 30(I2 - I1) + j50(I2) + 80(I2) - j24(I1) = 0

    How else can I demonstrate? It's there.

    If you mean how did I mix polar notation with rectangular notation, I punched it all into the TI-89, in which the modes were set to degree, polar, and approximate.

    Otherwise, I don't know what you're getting at.
     
    Last edited: Dec 1, 2011
  6. Dec 1, 2011 #5
    Re: Phasor Mesh Current--what am I doing wrong??

    I don't entirely understand how to deal with the polarity marks, but I've tried switching plus and minus for the j24Ω term. It's my understanding that they should both be -j24Ω, multiplied by I1 or I2 respectively, but I've tried both minus, both plus, one minus, one plus, the other way around, and none of the answers are correct.
     
  7. Dec 1, 2011 #6

    gneill

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    Re: Phasor Mesh Current--what am I doing wrong??

    I've taken a closer look at your circuit and I believe that you have them correctly oriented. Sorry for the bother.

    The idea is that a positively increasing current flowing into one coil at its dot produces a positive polarity of induced voltage at the dot of the second coil. So you have to be attentive as to whether a positive increase in the mesh current will create a positive increase in the current flowing into the coil dot, or whether it will in fact cause a decrease.

    attachment.php?attachmentid=41401&stc=1&d=1322725464.jpg
     

    Attached Files:

  8. Dec 1, 2011 #7

    rude man

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    Re: Phasor Mesh Current--what am I doing wrong??

    That is not a viable equation. You must transform the "600V@-30deg" the same way you transform components. Example: you didn't write "wL@+90deg" for the inductive impedance, did you? You wrorte jwL. jwL is the transform of wL@+90deg. Do the same for the 600V@-30 deg. I don't know what your TI-59 is doing & I can't guess.

    I checked & your mutual inductance polarities are correct and in fact your equations are correct, except the second equation is not an equation. :frown:
     
    Last edited: Dec 1, 2011
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