This is a question that everyone is concerned about. Some calculation formulas can be found on the Internet, and many websites of magnetic material manufacturers also have corresponding calculation programs to facilitate calculation. However, those who have used them can see that these formulas or tools can only calculate the surface magnetism of cylindrical and rectangular magnets, and they calculate the center surface magnetism. Is it difficult to calculate other shapes? Is it also difficult to calculate the highest surface magnetism? Yes, accurate calculation of surface magnetism is indeed very difficult and complicated. For cylindrical and rectangular magnets, we assume that the magnetic field distribution is ideal and symmetrical, and the center surface magnetism is assumed to be ideal and perpendicular to the magnetic pole surface. The magnetic permeability in the air gap is equal to the magnetic permeability of the magnet and both are equal to 1.0. Based on the above conditions, there will be a relatively simple calculation method. You can refer to the calculation formula of TDK:
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Based on the above formula, you can make an Excel calculator to facilitate calculations. This is also the source of formulas for calculation programs on many websites. However, in actual calculations, you will find that the accuracy of the calculation results does not seem to be enough. The calculated values and measured values of some products are very different. Take the N50 D10*10mm magnet as an example. Use a Gauss meter to measure the center position of the magnetic pole surface. We believe that the probe is close to the magnet and there is no air gap. Excluding the error of the measuring instrument, the Br calculated by the formula is 14kGs, then the center surface magnetism should be 6261Gs, but, no matter how you measure, it is impossible to reach this value, even if you use N54, it is impossible to reach this value. What is the problem? Is TDK's formula inaccurate?
1. First of all, the working point must be above the inflection point of the B-H demagnetization curve and retain an appropriate margin. In this way, special attention should be paid to N materials with a magnetic energy product of more than 45 and thin sheets with Pc less than 0.6. If it is below the inflection point, it means that the magnetism is unstable. Even if Bdi is calculated from the curve to calculate the center surface magnetic fluctuation, the fluctuation will be large. Hcj must be increased to maintain the working point below the inflection point or the BH line does not have an inflection point.
2. Under the premise that the first point can be met, Bdi=Br∙(Pc+1)/(μrec+Pc), μrec is determined according to the actual value, generally 1.08-1.1 below N40, 1.06-1.08 above N40, 1.05-1.06 for M gear, 1.04 for H gear, and 1.03 for others.
The final calculation formulas for the center surface magnetic of the cylindrical (or nearly cylindrical) and rectangular (or nearly rectangular) are:
