BH Magnetization Curve


The BH Magnetization Curve shows in the image.1 how much flux density (B) results from increasing the flux intensity (H). The curves in image.1 are for two types of soft iron cores plotted for typical values. The curve for soft iron 1 shows that flux density B increases rapidly with an increase in flux intensity H, before the core saturates, or develops a "knee." Thereafter, an increase in flux intensity H has little or no effect on flux density B. Soft iron 2 needs a much larger increase in flux intensity H before it reaches its saturation level at H = 5000 At/m, B = 0.3 T.

Air, which is nonmagnetic, has a very low BH profile, as shown in image.1.

Image.1

The permeability (μ) of a magnetic material is the ratio of B to H. Equation ..1 is the mathematical representation for magnetic material permeability.


μ -  B/H                                                                                                ..1

The average value of permeability is measured where the saturation point, or knee, is first established. Figure 27 shows the normal or average permeability for the two irons as follows.

In SI units, the permeability of a vacuum is μo = 4 p x 10-7 H/m or 1.26 x 10-6 or T-m/At. In order to calculate permeability, the value of relative permeability μr must be multiplied by μo. Equation (1-18) is the mathematical representation for permeability.

                        Î¼ = μr x μo

Example: Find the permeability of a material that has a relative permeability of 100.


Hysteresis

Hysteresis means “remaining” in Greek, an effect remains after its cause has disappeared. Hysteresis, a term coined by Sir James Alfred Ewing in 1881, a Scottish physicist and engineer (1855-1935), defined it as When there are two physical quantities M and N such that cyclic variations of N cause cyclic variations of M, then if the changes of M lag behind those of N, we may say that there is hysteresis in the relation of M to N". The most notable example of hysteresis in physics is magnetism. Iron maintains some magnetization after it has been exposed to and removed from a magnetic field. When the current in a coil reverses direction thousands of times per second, hysteresis can cause a considerable loss of energy. Hysteresis is defined as "a lagging behind." The magnetic flux in an iron core lags behind the magnetizing force.

Consider a magnetic material being subjected to a cycle of magnetization. The graph intensity of magnetization (M) vs. the magnetizing field (H) gives a closed curve called the M-H loop.   Consider the portion AB of the curve given below. The intensity of magnetization M does not become zero when the magnetizing field H is reduced to zero. Thus the intensity of magnetization M at every stage lags behind the applied field H. This property is called magnetic hysteresis. The M-H loop is called the hysteresis loop. The shape and area of the loop are different for different materials.

Hysteresis loop

The hysteresis loop is a series of curves that shows the characteristics of magnetic material in image.2. Opposite directions of current will result in opposite directions of flux intensity shown as +H and -H. Opposite polarities are also shown for flux density as +B or -B. Current starts at the center (zero) when unmagnetized. Positive H values increase B to the saturation point, or +Bmax, as shown by the dashed line. Then H decreases to zero, but B drops to the value of Br due to hysteresis. By reversing the original current, H now becomes negative. B drops to zero and continues on to -Bmax. As the -H values decrease (less negative), B is reduced to -Br when H is zero. With a positive swing of current, H once again becomes positive, producing saturation at +Bmax. The hysteresis loop is completed. The loop does not return to zero because of hysteresis.

Image.2

The value of +Br or -Br, which is the flux density remaining after the magnetizing force is zero, is called the retentivity of that magnetic material. The value of -Hc, which is the force that must be applied in the reverse direction to reduce flux density to zero, is called the coercive force of the material.

The greater the area inside the hysteresis loop, the larger the hysteresis losses.

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