Chapter 27.pdf

Curve is known as the normal curve while the j versus

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curve is known as the normal curve, while the J versus H curve is called the intrinsic curve (polarisation J=μ o M ). These curves, also called hysteresis loops, are shown in figure 27.16b. The loops show the properties of the magnetic material as it is cycled between magnetization (saturated) and demagnetization (under the influence of an external magnetic field). The second (and third) quadrant of the loops display the magnetic properties as the magnet performs work. When the current is reversed from zero to i d , the B-H characteristic operating point traverses along a hysteresis loop with a trajectory from point c to point d . If the current is maintained at i d , the magnet operating point is maintained at point d , where H d , B d generally denotes the operating point on the second quadrant demagnetising curve. This same operating point would be attained if the material were to start at point c , and with zero current excitation, from equation (27.8), an air gap of length g = m (A g / A m )( μ o H d / B d ) were introduced into the core. If the current becomes more negative, the trajectory continues to trace out the hysteresis loop toward point e . If instead of increasing the current magnitude, it is reduced from i d to zero, the trajectory does not retrace the hysteresis loop toward point c . Rather it inscribes a minor hysteresis loop, reaching point f at zero current. If the current is then varied between zero and i d , the B-H characteristic operating point traces out the minor loop shown in figure 27.16c. Unlike soft magnetic material, the absence/removal of an external magnetic field does not lead to demagnetization. The B-H trajectory between points d and f can be represented by a straight line, termed the recoil line , with a slope μ o μ rc , where μ rc is defined as the relative recoil permeability. At the vertex of the minor hysteresis loop, point d, the material has been partially demagnetized, with the effective remanent magnetization of the magnetic material having been reduced to that represented by point f , which is less than the remanent magnetization B r , point c . If the demagnetization is increased past point d , to point e of figure 27.16c, a new minor loop will be created, with a new recoil line but with a similar recoil permeability (slope μ o μ rc ). If the demagnetizing field is increased beyond point g , H c , the operating point of the magnet now moves into the third quadrant of the normal curve. Recoil still results in a positive remanence flux density. Before the intrinsic coercivity H ci is reached, which is show in figure 27.16b, the magnet becomes completely demagnetized, since recoil is back to the origin, point a . When H ci is reached and exceeded, the recoil flux density is negative, below point a . Intrinsic coercivity H ci is therefore a measure of a magnet’s ability to resist demagnetization.
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  • Fall '19
  • Magnetism, Magnetic Field, Ferro, Ferromagnetism

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