Because of the orderly arrangement of magnetic domains inside the core, the magnetic field generated by the current is greatly enhanced. The greater the current, the more orderly arranged magnetic domains, the greater the magnetic field generated, and the greater the magnetic flux passing through the coil, which is basically proportional to the current. Inductance is defined as the self-inductance coefficient of the coil, which is equal to the ratio of magnetic flux to current, so under normal circumstances, the inductance L is constant. When the current reaches a certain level, all the magnetic domains in the magnetic core have been arranged in an orderly manner at this time. Even if the current is increased again, there is no surplus magnetic domain that can be arranged in an orderly manner to increase the magnetic field, so the magnetic field strength basically does not increase. At this time, we say that the inductance is saturated, the current increases, and the magnetic flux no longer increases. The inductance value is equal to the magnetic flux divided by the current, so the inductance value decreases. In general, the saturation current of the inductor we actually use is generally defined as the current value corresponding to when the inductor value drops by 30% relative to the initial value.
The abscissa is the magnetic field strength H, and the ordinate is the magnetic induction strength B. It is difficult to understand the difference between them literally. The magnetic field strength H is usually generated by the energized current, so it can be understood as the magnetic field strength generated by the energized coil itself without any dielectric material. And B, that is, filled with magnetic material is the total magnetic induction intensity. H is mainly related to the magnitude of the current, while B is related to the magnetic material. The B- H hysteresis loop describes the characteristics of repeated magnetization of magnetic materials.
Let's look at the OS section first. If the magnetic material has not been magnetized before, then the initial magnetism is 0, which is at point O. At this time, if magnetization is carried out and H is increased, it will reach point S along the curve, and point S will be completely magnetically saturated. At this time, if H is reduced to 0, it will not return to point O, but will only return to point Br, which is remanence, because some magnetic domains are rigidly deflected. Because of the existence of remanence, in theory, the magnetic material can no longer return to the O point unless heated to the Curie temperature. This magnetization curve does not coincide with the demagnetization curve, and the phenomenon that the change of B lags H is called hysteresis.
After reaching Br, continue to apply the reverse magnetic field to reach-Hc, then B can be 0. Hc is also called coercivity. The meaning is that due to hysteresis, to make B in the magnetic medium 0, a certain reverse magnetic field strength is required.
Continue to increase the reverse magnetic field and reach-Hs, at which time reverse magnetic saturation occurs. If the reverse magnetic field is reduced to 0 at this time, it will reach-Br point, and if the forward magnetic field is increased again, it will reach Hc point, and if it continues to increase, it will reach S point. This is a complete hysteresis loop.
What's the use of understanding this hysteresis loop?
It is easy to think that the permanent magnet is the kind of remanence Br is relatively large, belongs to the hard magnetic material.
And we use the inductor, the core should be soft magnetic material, remanence is relatively small. It can be understood that our ideal inductor is an energy storage element, which stores energy when there is current, and releases energy when there is no current, and does not consume energy itself, and this energy is magnetic field energy. However, in the actual magnetic core, when the current flows through, a magnetic field is generated. With the magnetic field energy, the current becomes 0. Because of hysteresis, the magnetic core will have remanence, that is to say, the magnetic core has not returned all the magnetic field energy and has left a part of itself. This part is actually the hysteresis loss of the magnetic core. Therefore, the greater the hysteresis, the greater the loss. In order to reduce the loss, the inductor core naturally chooses the soft magnet material.
In addition, we can infer that after the current reaches a certain value, the inductance will decrease with the increase of the current. Since B = uH, the permeability u is the slope of this curve. It can be seen that the whole curve is similar to the S-type. When the current is relatively small, H and B are basically linear, and the magnetic permeability u is basically unchanged, so the inductance is also unchanged. When the current is relatively large, H and B are nonlinear, and the slope gradually becomes lower, that is to say, U gradually becomes smaller, so the inductance of the inductance also gradually becomes smaller. Believe here, you can understand why the inductance and current curve is like that in the inductance specification manual.