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What is hysteresis? Draw a hysteresis loop for ferromagnetic material and explain the various important points on it. What is the technical significance of the area enclosed by the loop?

For a transformer, what kind of material will you choose- the one with small hysteresis area or the one with the big one?
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  • Hysteresis: -

    • Hysteresis is the dependence of the output of a system not only on its current input, but also on its history of past inputs.

    • The dependence arises because the history affects the value of an internal state

    • To predict its future outputs, either its internal state or its history must be known.

  • Hysteresis Loop: -

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  • The above figure shows the behavior of a ferromagnetic core subjected to cyclically varying magnetic field.

    • Initially, the core is demagnetized and the strength of magnetic field “H” is set to zero. Thus point “O” marks the starting point of the curve.

    • On increasing magnetic strength the magnetic induction (B) increases non-linearly, as shown, up to point P, after which it remains constant.

    • Thus, point P is known as the saturation point.

    • On reducing “H” to zero, the core still remains magnetized. The residual value of “B” at point Q is called remnant induction.

    • On further reduction of “H” (i.e. increasing “H” in reverse direction), at point R, the core gets completely demagnetized.

    • The value of magnetic strength at that point is called coercive field.

    • As the magnetic field increases in the negative direction, again the magnetic induction appears to remain constant after a certain point S.

    • This is the saturation point in the negative direction.

    • Similarly, points T and U corresponds to points Q and R respectively as the value of “H” is increased reaching point P again.

    • The full cycle of magnetization and demagnetization of core is termed as Hysteresis loop.

(Note: If the time is less and the weightage for the question is not much, just mention the points and write their significance. No need to explain the process)

  • Significance of the Area Enclosed: -

The area enclosed by the Hysteresis loop represents the energy loss per unit volume of the material per unit cycle.

  • Material for transformer: -

For transformer cores, soft magnetic materials are used which has a smaller Hysteresis area.

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