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Homework Help: EE: Mutual Inductances in Magnetic Circuit

  1. Aug 24, 2013 #1
    1. The problem statement, all variables and given/known data

    The symmetric magnetic circuit has three windings. Winding A and B each have N turns and are wound on the two bottom legs of the core. The core dimensions are indicated in the figure.

    b.) Find the mutual inductances between the three pairs of windings.

    figure : http://i844.photobucket.com/albums/ab2/cpage86/133ece321hmwk1_zps0b1b4f42.png

    2. Relevant equations

    [itex]\varphi[/itex] = [itex]\Im[/itex]/[itex]\Re[/itex]

    Inductance = [itex]\lambda[/itex]/current

    [itex]\lambda[/itex] = N[itex]\varphi[/itex]

    3. The attempt at a solution

    In the problem I went through and found the self-inductances for part a.), but cannot get the mutual inductances. I have tried about 15 times, and according to the solution I am close, but still not 100% correct.

    I have tried finding the flux in each leg and determining the flux linkages of the coils, but I keep ending up with a huge mess of papers with never-ending equations.

    I have tried just utilizing the flux through the gap.

    I have tried to blindly guess which reluctances are necessary to solve this.

    This class just started this week, and I have read the book. I have gone back and forth through the chapter and the only example that is close to this one is of a square loop core with a single gap and two windings. In that particular example they disregarded the core permeability and completed the necessary work in a few steps; which is pretty misleading not helpful.

    Part of my work is here:


    According to the book I am pretty close, but the "la" term in the numerator is not supposed to be there. Other than that, my solution is right on.

    I mainly am hoping to gain some insights on how to go about setting this up and solving for the mutual inductances. If you have any advice or wisdom to drop on me I am all ears.
  2. jcsd
  3. Aug 26, 2013 #2

    rude man

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    Sorry, can't read your work so I'll start from scratch. Sorry if much or all of this is redundant:

    First, do you see that there are only two mutual inductances to be computed? I mean, there are 6 (M1a, Ma1, M2a, Ma2, M12, M21) but 4 are redundant by symmetry and mutual inductance theory.

    So let us choose to compute M1a and M12:

    Are you familiar with magnetomotive force (mmf) and reluctance R in addition to the usual H, B, flux ψ and current i? What equation relates mmf to reluctance R and flux ψ? where ψ = BA, A = cross-sectional area.

    Then, what is the defintion of mutual inductance between coil a and coil 1, assuming coil a is excited by a current ia?

    The method is to equate mmf to the sum of magnetic potential drops (mpd) around the closed magnetic path.
  4. Aug 26, 2013 #3
    Rude man, Thanks for the reply, I've got some good news. While dining at Arby's yesterday on my lunch break I had a breakthrough and finally solved that bad boy.

    The feeling of relief was huge.

  5. Aug 26, 2013 #4

    rude man

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    Good going!
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