The principle of magnetic field shielding is introduced
The magnetic field is generated by the magnetosphere, which is pointed to the S pole by the N pole of the magnet, which forms a closed form outside the magnet, the closer the source of the source, the stronger the strength of the field. Magnetic field shielding principle is the use of magnetic shielding materials, change the direction of the magnetic field, due to the magnetic field through the channel has been bypassed by low magnetic resistance, so can ensure that blocked the magnetic flap liquid level gauge is not affected by the interference of magnetic field. Magnetic field shielding is divided into magnetostatic and electromagnetic shielding. Or steady current static magnetic field is produced by permanent magnet magnetic field, static magnetic shielding is the use of ferromagnetic materials of high permeability M shield to shield the external magnetic field, the principle of static magnetic shielding can be used to illustrate the concept of magnetic circuit, such as loop of ferromagnetic materials section as shown in figure 2, the outer magnetic field, the vast majority of the magnetic field is focused on the ferromagnetic loop. This can be used as a parallel magnetic circuit for the ferromagnetic material and the air in the cavity. Because the permeability of ferromagnetic materials is bigger than the air permeability of thousands of times, so the cavity of the magnetic resistance is much bigger than the reluctance of ferromagnetic materials, most of the external magnetic field of magnetic induction line along the wall in ferromagnetic materials through, into the cavity of the magnetic flux is few. In this way, the cavity of the magnetoplate liquid-gauge is shielded by ferromagnetic materials, and there is basically no external magnetic field, thus achieving the objective of magnetostatic shielding.
When the electromagnetic field is propagated in the conductive medium, the amplitude of the field volume (E and H) is attenuated by the increase of distance. From the point of view of energy, the electromagnetic wave propagation in the conductive medium has the energy loss, therefore, the performance is the decrease of the amplitude of the field. The surface of the surface of the magnetic flip plate is the largest, the more inside the conductor, the smaller the field. This phenomenon is also known as the skin effect. The use of the skin effect can prevent high-frequency electromagnetic waves from penetrating a good conductor and make an electromagnetic shielding device. Electromagnetic shielding is an effective means to restrain interference, enhance the reliability of the liquid level of the magnetic plate and improve the quality of the products. Reasonable use of electromagnetic shielding can inhibit the interference of foreign high-frequency electromagnetic wave, and can also avoid being a disturbance source to affect other equipments.
The selection of magnetic field shielding material
The magnetic permeability of magnetic shielding materials related to the intensity of the external magnetic field, the external magnetic field intensity, magnetic flap have different level of permeability of magnetic material, saturation flux density is different also, therefore, in the role of the shielding design of the magnetic field is different also. When shielded low frequency magnetic field, magnetic steel, permalloy, high magnetic permeability of materials such as iron, while high frequency magnetic field shield should choose of high conductivity materials such as copper and aluminum.
4.3 processing measures
From the above analysis, the hydroelectric generating set the oil bearing basin magnetic interference over suet potentiometer is belong to the low frequency magnetic field interference, shielding should choose high permeability material, because of the constraints, we choose the thickness of 5 mm plate as shielding material, and in the design, as far as possible little holes and cracks, avoid increasing the magnetic resistance of magnetic shield, thus reduce shielding effect, boot up the wound after verification, the guide bearing oil tank oil level anomaly signal did not quote us again.
Conclusion 5.
The interference of low frequency magnetic field is the important factor that causes the flu-bearing tank of the hydraulic generator in a hydropower station.
After many experiments and analysis, the following conclusion is obtained:
1) materials that shield low-frequency magnetic fields from high permeability materials;
When the strength of the magnetic field is greater, the thickness of the magnetic shielding material should be added.
3) minimize the sealing and opening of the shield, avoid the magnetic resistance of the magnetic shield and reduce the shielding effect. Cracks and holes are distributed in the direction of the magnetic field.
