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Homework Help: Magnetic circuits with air coil question

  1. Dec 5, 2015 #1
    1. The problem statement, all variables and given/known data
    a magnetic circuit with a coil of 500 turns and a core of sheet steel has an air gap of 1mm. the sheet steel has a mean path length Lss = 0.25 m and a cross section that measures 1cm x 1.6 cm. (assume: u air = u0(not) = 4pi x 10^-7 wb/(Am).

    determine how much coil current is required to establish a flux of phi = 2.24 x 10^-4 wb. neglect fringing

    2. Relevant equations
    b = phi/A
    F=Phi x R

    I also have a B-H curve for sheet steel.

    3. The attempt at a solution
    Ive tried solving using B=(Unot)NI / L. And solving rearranging phi =b/a.
    My textbook doesn't do a good job of explaining this material. There isn't any questions similar to this one in it or in my teachers examples. We covered the material in 1 class because "we are behind" and I write a final incorporating this material in 1.5 weeks.

    Anybody know any good resources for this material because you tube offered nothing.
    I'm very frustrated and being on a timeline isn't making this much easier.
    Thanks in advance.

    *most of the formulas I have are in relation to the mmf. I have 0 formulas in my text that relate current and the magnetic field being produced from the coil
  2. jcsd
  3. Dec 6, 2015 #2


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    You need to mention the relative permeability of steel ( I think its 1000).
    You can use mmf= NI=Φ*s
    You can calculate s(reluctance) as s=l/μa
    Also you need to keep in mind that this is a series magnetic circuit.
  4. Dec 9, 2015 #3

    rude man

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    In a practical world the reluctance of the sheet steel would be assumned infinite so your total reluctance would just be the reluctance of the air gap. But alas, textbook questions do not necessarily reflect real life so you have to come up with a series value for the reluctance of the steel. Pick some value between the two hysteresis curves for the slope μ.
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