Surface magnetism 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). The surface magnetism at different positions on the surface of the magnet is not the same. Generally speaking, “surface magnetism” refers to the center surface magnetism of the magnet working surface.
Surface magnetism measurement
Surface magnetism is the easiest and most direct measurement of magnet performance parameters in daily life. It can be directly measured with a gauss meter, which is very convenient. When the shape and size of the magnet are certain, people often judge and compare the performance of the magnet by comparing the surface magnetism (in the case of non-multi-pole magnetization, etc.). However, for some magnets with particularly large or small sizes and special shapes, it is difficult to measure the magnetic flux. At this time, it is very important to measure the magnetism of the meter.
To measure the magnetism of a magnet, a Gauss meter, also called a Tesla meter, is generally used. The Hall sensing elements on the gauss meter of different manufacturers are different, and the surface magnetism measured by the same magnet is not exactly the same. In addition, it should be noted that the gauss meter measurement standards used in different countries are different.
The surface magnetism measurement is not objective, and it is not a parameter that can fully reflect the performance of the magnet, so it is not recommended as an evaluation index for product transactions.
The relationship between surface magnetism and magnet performance (remanent magnetism)
Remanence refers to the magnetic induction intensity retained in the ferromagnet when the external magnetic field is used to magnetize the ferromagnet to a saturated state and then gradually reduce the external magnetic field to zero. The full name is the residual magnetic induction intensity (indicated by Br), and the unit is Tesla or Gaussian. The remanence is determined by the characteristics of the magnet itself. The remanence of the same magnet is constant under certain conditions and has a single value.
The remanence determines the surface magnetism of the magnet to a certain extent, but it does not mean that the magnets with the same remanence have the same surface magnetism. The surface magnetism is also affected by the shape, size and magnetization method of the magnet.
For two magnets with the same shape and size, the surface magnetism with higher remanence is also higher.
For two magnets with different shapes and sizes, the residual magnetism cannot be judged simply by the height of the surface magnetism
The residual magnetism is tested in a closed-circuit state, while the surface magnetism is tested in an open-circuit condition with a Gauss meter. At the same time, the magnet itself has a demagnetization field, so the maximum surface magnetism of a single magnet is much smaller than its residual magnetism. At present, the maximum remanence of sintered NdFeB magnets is around 14000Gs, so we can say for sure that the maximum surface magnetism of a single NdFeB magnet cannot exceed 14000Gs. (Note that it is a “single magnet”. In some magnetic assemblies and magnet arrays, special magnetic circuit designs can be used to improve the surface magnetism of the magnet)
Calculation of surface magnetism of NdFeB magnet
Since the sintered NdFeB magnet has a very high magnetic anisotropy field, the magnetization vectors are all aligned in the direction of easy magnetization, so we can regard it as a uniform magnetization body, so that the current shell model can be used to calculate the magnetization generated by the magnet in space magnetic field. At present, many surface magnetism calculation models on the Internet are based on this principle to derive the calculation formula, but there are two assumptions in it, 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 established. , so there is a certain difference between the calculated result and the actual measurement result. In addition to remanence, the surface magnetism of a magnet is greatly affected by its shape and size, and the calculation formulas for surface magnetism applicable to magnets of different shapes are different.
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