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Completely stuck on mesh analysis

  1. Dec 5, 2017 #1
    1. The problem statement, all variables and given/known data
    I have a circuit that I am to analyse and find I. I am told to use an excel matrix sheet given to me to solve 3 unknowns. I have 4 loops so this is a problem. I have made an attempt using three loops and referred to a previous thread on here for some help. It says I can do it with three loops.
    https://www.physicsforums.com/threads/ac-circuit-analysis-mesh-and-nodal.791744/



    2. Relevant equations I have attempted the question many times and am including my results. They do not seem to agree with the results for the agreed answer on here.
    Loop 1 --- 120-(2)I1-(I1-I2)(-j5)=0
    Loop2 --- (-j5)(I2-I1)+(j4)(I2-I3)-14.14+j14.14=0
    Loop3 --- (j4)(I3-I2)+(4)I3-j120=0

    Answers;
    I1=19.1624+j12.5642
    I2= 14.1367-j3.77088
    I3= 23.9538+j20.1829

    I cant seem to upload spread sheet

    3. The attempt at a solution
    I am incuding this and a spead sheet I am using. any pointers would be amazing as I am completely lost wheln I use the spreadsheet.
     

    Attached Files:

  2. jcsd
  3. Dec 5, 2017 #2
    here is the pic of the spread sheet
     

    Attached Files:

  4. Dec 5, 2017 #3

    gneill

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    Staff: Mentor

    Are you confident about the sign of the voltage terms in loops 2 and 3? Read through the related conversation with the OP in the thread you referenced: he had the same issue!
     
    Last edited: Dec 5, 2017
  5. Dec 5, 2017 #4
    hi gneil

    thanks again for help. I have changed my answers but they still wont work in spread sheet
     

    Attached Files:

  6. Dec 5, 2017 #5

    gneill

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    Staff: Mentor

    As far as I can tell with a quick glance, your recent attemp2.pdf shows correct loop equations. Good!

    Regarding the spreadsheet's inability to solve the equations, take a look at the conversation that occurred in the thread that you referenced previously, starting around post #98 :wink:
     
  7. Dec 5, 2017 #6
    thanks gneill will do tommorow. Thanks again so much
     
  8. Dec 6, 2017 #7

    scottdave

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    I'm trying to follow along. I put a 0.001 in the center cell of the Real matrix, as the warning suggested. This allows the spreadsheet to calculate values.
    I put the same matrix into Matlab and solved the system and arrive at the same answers as the spreadsheet.
    As far as I can tell, your loops and matrices look correct. Are the answers in the original post the "correct" answers that you are supposed to arrive at?
     
  9. Dec 6, 2017 #8
    I have done this question and can tell you that I couldn't get the thing to work at all with some zero values. As scottdave above says, you need to put in a very small number just to allow it to compute.

    Having just looked at my tutor feedback document, I did get the phases incorrect in my answer (+ instead of -), but the magnitudes were all correct, and mine don't match any of those in your original post.
     
  10. Dec 6, 2017 #9

    scottdave

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    Matlab is commercial software package. If you don't have access to that, Scilab and Octave are pretty good alternatives, which are free.
     
  11. Dec 6, 2017 #10
    Thanks scottdave and Numbskull.

    I have obtained the correct answers by entering 0.0001 in the spreadsheet. Thanks for help though. I always thought it would be me wrong and not the spreadsheet
     
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