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Urgent - Need help checking my thevinins theroem calculations

  1. Aug 23, 2013 #1
    I have to find current on part of a circuit using thevinins theroem with polar and rectangular coordinates. Circuit is attached

    Sorry - this is a large amount of working and I cant really type it all out. Nor can i type out a circuit diagram. Find attached the question as well as my solution. there is an imgur album as well as attached files. apologies for the circuit diagram not being very clear on the question sheet. It is drawn out on my first page of solutions.


    http://imgur.com/N8eDDJd&692gycQ&TvgOkMv&qIyzsKh&tRKoeFu#0

    This is the imgur album. Has high res images. Sorry the pdf is so blurry. Had to use a lower resolution so i could attach it.

    Any help would be greatly appreciated.
     

    Attached Files:

  2. jcsd
  3. Aug 23, 2013 #2

    gneill

    User Avatar

    Staff: Mentor

    Whew! That's a lot of numerics to wade through. While your basic approach is okay (using mesh analysis to find the currents, then the resulting open circuit potential), plugging in the complex numbers right away and then carrying them through all the manipulations is error prone and very frustrating to follow.

    If I may suggest, why not break down the problem into smaller morsels that you can attack symbolically first, then plug in the numbers. At least then the starting expressions will be verifiable and calculation errors can be tracked to manageable bites.

    To that end, first note that the circuit consists of four parallel branches. Since the order of the branches will not affect the result, move the "load" branch to one end and consider the rest of the circuit for which you want to find the Thevenin equivalent.

    One approach to compartmentalize the solution is to build up your Thevenin equivalent one step at a time. Note the red arrows dividing up the circuit in this diagram:

    attachment.php?attachmentid=61091&stc=1&d=1377272029.gif

    The circuit segment to the left of arrow A comprises a simple voltage divider, so you can determine the expressions for its Thevenin equivalent easily and work the math. Then move to the next "slice" at arrow B. Add the new components to the previous Thevenin equivalent and determine the new, combined equivalent. Again, the expressions should be manageable symbolically and then numerically.

    Alternatively, you might want to look at using nodal analysis to find a single symbolic expression for the open-circuit potential (that of the top node). The single expression will be a bit large, but can be attacked in pieces.

    A third alternative is to convert both source branches to Norton equivalents. Combine the parallel stuff into a single Norton equivalent (easy!), then convert the result to its Thevenin equivalent. Again it becomes stepwise-manageable.
     

    Attached Files:

  4. Aug 23, 2013 #3
    Thanks for the reply!

    I definately like the idea of seperating out the sections of the circuit and working out the thevinin equivalent for each component and collecting them, definately easier for finding problems! When I checked through I realised I had made a fairly large mistake early on and it was a pain to change throughout the calculation (had to rewrite 3 pages!). The reason I used the method I did was because my lecturer had a similar example and I could compare the steps making sure I had missed nothing out.

    I think the biggest problem - as you said - was working in the cartesian and polar coordinates. They added a level of complexity that was unnecassary and would have been far easier to follow if I had a symbolic equation to follow instead.

    Thanks for the advice! The work is submitted now but I think I will definately take note (it's going next to where I file my solution) of your advice.
     
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