Analysis of the influence of turns on magnetic permeability test

Starting from the u~H curve of soft magnetic materials, the influence of the number of turns N of the test coil on the test results in the measurement of magnetic permeability u is studied. The reason why the magnetic permeability u value is the highest when N takes a certain value is explained. The influence of the radial non-uniformity of the test magnetic field H on the test sample ring is solved by the integration method. The difference between the ring magnetic permeability and the real magnetic permeability of the material is pointed out. The method of measuring the initial magnetic permeability ui of the material with a simple instrument is introduced.

As we all know, $magnetic permeability$μ is a key technical indicator of soft magnetic materials. The measurement of magnetic permeability μ is basically measured by winding on a standard sample ring. The inductance L of the winding coil is measured, and then the magnetic permeability of the material is calculated using L. However, for the same sample ring, the measured material magnetic permeability μ may vary greatly if different instruments or the same instrument are used for different test turns. Sometimes this will cause contradictions or disputes between the supply and demand parties. In particular, the different test μ values ​​caused by different test coil turns, sometimes the μ value measured with more turns is lower, but sometimes the μ value measured with more turns becomes higher. This will confuse some testers. This article attempts to explain the impact of different test instruments, test coil turns and calculation formulas on magnetic permeability testing based on the test principle.

Related concepts of magnetic permeability

For a uniform magnetic medium, if it is placed in a uniform magnetic field H and magnetized, the magnetic medium itself will generate an additional magnetic field H', and the direction of H' is the same as that of H. The total magnetic field strength of H' and H is called the magnetic flux density B of this magnetic material [1]. It can be seen that the magnetic flux density B and the magnetic field strength H are essentially physical quantities that characterize the strength of the magnetic field. However, the names and sizes of the units they use may be different. In the Gaussian unit system commonly used, the unit of H is Oersted (Oe) and the unit of B is Gauss (Gs). The two units of Oersted and Gauss have different names but are exactly the same in size. In the International System of Units that is now emphasized, the unit of B is Tesla (T) and the unit of H is Ampere per meter (A/m). The units of B and H are no longer equal, 1A/m = 4×10-7T. The ratio of magnetic flux density to magnetic field strength is called the magnetic permeability of the material. The ratio of their numerical values ​​is called absolute magnetic permeability μabsolute, and the ratio of the sizes of these two physical quantities is called relative magnetic permeability μ. Obviously, in the Gaussian system of units, since the units of B and H are equal, the ratio of their numerical values ​​is equal to the ratio of their magnitudes. Therefore, in the Gaussian system of units, the relative magnetic permeability of the material is equal to the absolute magnetic permeability, and there is no need to distinguish between absolute magnetic permeability and relative magnetic permeability. For a vacuum, it will not produce an additional magnetic field, and B is equal to H, so the magnetic permeability of the vacuum is equal to 1. In the International System of Units, since the unit sizes of B and H are no longer equal, their numerical ratio μ can never represent the ratio of their physical quantities μ, and the relative magnetic permeability of the material is no longer equal to its absolute magnetic permeability. For a vacuum, since no additional magnetic field can be produced, the two physical quantities B and H are equal, so the equal magnetic permeability μ of the vacuum is equal to 1. If the magnetic field intensity H at a point in the vacuum is yA/m, then the magnetic flux density B at that point should be 4πy×10-7T, so the numerical ratio of B and H at that point is the absolute magnetic permeability of the vacuum in the International System of Units μ0=4π×10-7H/m. Generally, the absolute magnetic permeability of a soft magnetic material is divided by μ0 to obtain its relative magnetic permeability μ=B/μ0H. Unless otherwise specified, the reference to magnetic permeability refers to the relative magnetic permeability.