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Core

Most of the magnetic core materials are poor conductors of flux and have low permeability, while non-conductive materials such as air, copper, and paper have the same order of magnitude of permeability. Some materials, such as iron, nickel, cobalt and their alloys, have high permeability.

In order to improve the magnetic properties of the air-core coil, a magnetic core is introduced, as shown in Figure 1.2. The advantage of introducing a magnetic core is that in addition to its high permeability, its magnetic path length (MPL-magnetic path length) is clear at a glance. Except where Z is close to the coil, the magnetic flux is mainly confined to the core.

Before the magnetic core is filled and part of the coil returns to the hollow state, there is a cut-off point for how much magnetic flux can appear in the magnetic data.

Magnetomotive force, magnetic field strength and magnetoresistance

MMF and magnetic field strength H are two important concepts in magnetism. They have a causal relationship: MMF=NI, N is the number of turns of the coil, and I is the current.

Magnetic field intensity H, which is defined as the magnetic force per unit length: H= MMF /MPL

Magnetic flux density B, defined as the number of magnetic field lines per unit area: B = φ/Ae

The flux generated by MMF in a given data depends on the resistance of the data to flux. This resistance is called magnetoresistance Rm

The relationship between MMF, magnetic flux and magnetic resistance is similar to the relationship between electromotive force, current and resistance.

Air gap

When the magnetic path length MPL and the core cross-sectional area Ae are given, the magnetic core composed of high permeability data has low magnetic resistance. If the magnetic circuit contains an air gap, its magnetic resistance is different from that of a magnetic core made of low resistivity data (such as iron). Almost all the reluctance of this path will be in the air gap, because the reluctance of the air gap is much greater than that of the magnetic data. In practical applications, the magnetic resistance is controlled by controlling the size of the air gap.

Equivalent permeability

The air gap reluctance is Rg, the air gap length is LG, and the total core reluctance is Rmt.

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