Eddy Current Loss of Rare Earth Permanent Magnet
Both samarium cobalt and neodymium iron boron are metal materials. Due to the good electrical conductivity of metal materials, the resistivity is very low. This is not a good thing for rotating machinery such as motors, because it will bring eddy currents to rotating machinery Loss, causing rotating machinery including magnets to heat up. Therefore, the eddy current losses of magnets that occur in rotating machinery must be considered by magnet and motor designers. Today, if you understand Magneto, I will take you to understand what is the eddy current loss of a magnet, and how to avoid eddy current loss in the production of magnetic materials.
To understand and reduce eddy current loss, we should first know how eddy current loss is generated. Here we need to introduce a concept – skin effect (skin effect)
When the alternating current passes through the wire, the distribution of the current density on the cross-section of the wire is uneven. With the increase of the frequency of the current change, the current is more and more concentrated on the surface of the wire, and the current inside the wire is getting smaller and smaller. The phenomenon is called the skin effect.
The cause of the skin effect is eddy currents. From the law of electromagnetic induction, it can be seen that an alternating magnetic field will be generated around an alternating electric field. When an alternating current passes through a conductor, an alternating magnetic field will be generated inside and around the conductor, causing a vortex-shaped induced current to be generated inside the conductor. The eddy current is referred to as eddy current. .
The closer to the center of the conductor, the higher the induced electromotive force generated by the alternating magnetic field in the conductor, the stronger the eddy current, the stronger the resistance to the original current, resulting in a small current density approaching the center of the conductor, while approaching the surface higher current density.
Since the induced electromotive force increases with the increase of the frequency, the skin effect becomes more significant with the increase of the frequency. When a current with a high frequency passes through the wire, it can be considered that the current flows only in a very thin layer on the surface of the wire, which is equivalent to a reduction in the cross-section of the wire, which greatly reduces the effective utilization of the conductor material.
Eddy current loss
Since the resistance of samarium cobalt and neodymium iron boron permanent magnets is relatively small, the eddy current in the alternating electric field is generally relatively large. Due to the heating effect of the current, the eddy current will heat the magnet, and thermal demagnetization will occur when the temperature is too high.
The size of the eddy current loss is related to factors such as the change mode of the magnetic field, the motion of the magnet, the shape of the magnet, the permeability and the resistivity. The higher the rotational speed (equivalent to the frequency) and the magnetic permeability of the rotating machinery, the lower the resistivity, the smaller the skin depth, and the greater the loss caused. In the fields of electric vehicles, elevators, etc., in order to control the speed, the permanent magnet motor is usually controlled by the inverter power source. Due to the presence of high-order harmonics of the carrier frequency, the eddy current loss in the magnet will also increase and cause thermal demagnetization.
Reducing Eddy Current Loss of Sintered NdFeB by Increasing Resistivity
From the perspective of motor design, in order to reduce the eddy current loss of permanent magnets in rotating machinery, several technical methods have been proposed, such as shielding columns surrounding the magnets, methods of dividing magnets and side isolating magnets, etc.
From the perspective of magnets, one of the most effective ways to reduce the eddy current loss of motors is to use bonded magnets. Because of the presence of binders and a sufficiently high volume fraction, the resistivity of bonded magnets is 102~104 of that of sintered magnets. times, but the power and maximum operating temperature of the motor are greatly limited, so the most direct way is to increase the resistivity of the sintered magnet itself.
There are many ways to increase the resistivity of sintered magnets, such as adding high resistivity powder (Al2O3, etc.), coating SiO2 coating, etc., but these methods will affect the magnetic properties of sintered magnets to a certain extent, so in the research and development of magnets A balance needs to be made between resistivity and magnetic properties in the process.