How to calculate the surface magnetism of a magnet?

Readers often leave messages asking how to calculate the magnetism of the magnet? The calculation results of the calculation models found on the Internet are very different, which one is more accurate? Is there a corresponding relationship between surface magnetism and remanence? This issue of yueci will take you to understand the relationship between surface magnetism and magnet performance.

The concept and measurement of table magnetism

Surface magnetism is also called surface field and surface magnetic field, which refers to the magnetic induction intensity at a certain point on the surface of the magnet, and the unit is Gauss Gs or Tesla T (1T=10000Gs).
Surface magnetism is the easiest parameter to be directly measured in daily life. When the size of the magnet is fixed, people often judge and compare the performance of the magnet by comparing the surface magnetism. For some very large or very small magnets with special shapes, they are not suitable for routine measurement. At this time, the meter magnetism becomes very important. Regarding the magnetometer, the following two points must be known:
The surface magnetism is the value measured when the Gauss meter is in contact with a certain point on the surface of the magnet. It is a data reflection of the magnet itself to the measuring tool and does not represent the overall performance of the magnet.
The surface magnetism of different positions on the surface of the magnet is different. For a non-multipole magnetized magnet with regular shape, the center surface magnetism is generally measured. The surface magnetism is easily affected by the external environment. The same magnet, using Gauss meters from different manufacturers, may have different center surface magnetism; the same magnet, measured in different environments, may have different surface magnetism. . (The N-pole and S-pole center surface magnetism of the same magnet is also different~)
From the above two points, it can be seen that the surface magnetic measurement is not objective, it is not a parameter that can fully reflect the performance of the magnet, and it is not recommended as an evaluation index for product transactions.

The relationship between surface magnetism and magnetic properties

What is the relationship between table magnetism and magnet performance parameters (such as remanence Br, coercivity Hc and maximum magnetic energy product (BH) max, etc.)? Can you find the mathematical formula? These two questions are frequently asked by readers.
The answer to the first question is yes, but in the past, some people just made empirical statistics. For example, for a sintered neodymium iron boron cylinder with an aspect ratio of 1, the remanence Br is 2 to 3 times the surface field, but this statement The quantitative relationship cannot be established strictly.

The relationship between surface magnetism and remanence

Residual magnetization refers to the magnetic induction retained in the ferromagnetic body when the external magnetic field is gradually reduced to zero after the external magnetic field is used to magnetize the ferromagnetic body to a saturated state. Its full name is the residual magnetic induction intensity (Br represents). The remanence is determined by the characteristics of the magnet itself, and the remanence of the same magnet under certain conditions is constant and has a single value.
The remanence determines the surface magnetism of the magnet to a certain extent, but the surface magnetism of the same magnet is not the same. The surface magnetism is also affected by the shape, size, and magnetization of the magnet.
Two magnets with exactly the same shape, performance and size, the one with the higher remanence has the stronger magnetism. Two magnets with different shapes, performances and sizes cannot be judged simply by the level of surface magnetism.

The surface magnetism of the magnet is lower than the remanence

The remanence is tested in a closed circuit state, while the surface magnetism is tested with a Gauss meter in an open circuit condition. At the same time, the magnet itself has a demagnetizing field, so the maximum surface magnetism of a single magnet is much smaller than its remanence. At present, the maximum remanence of sintered NdFeB magnet is about 14000Gs, so we can say with certainty that the maximum surface magnetism of a single NdFeB magnet cannot exceed 14000Gs. (Note that it is a “single magnet”. In some magnetic components and magnet arrays, special magnetic circuit design can be used to improve the magnet surface magnetism)

Table magnetism calculation

Since the sintered NdFeB magnet has a very high magnetic anisotropy field, the magnetization vector is arranged in the easy magnetization direction, so we can regard it as a uniform magnetization, so that the current shell model can be used to calculate the magnetization in space magnetic field. At present, many surface magnetic calculation models on the Internet are based on this principle to derive the calculation formula, but there are two assumptions. One is that the magnet is a completely uniform magnetized body, and the other is that the demagnetization curve is completely straight. The actual situation is not completely true. , So the calculated result is different from the actual measurement result. In addition to remanence, the surface magnetism of a magnet is greatly affected by its shape and size, and the applicable surface magnetism calculation formulas for magnets of different shapes are different.

Source: China Permanent Magnet Manufacturer – www.rizinia.com