Recently, as technology develops towards high frequency and high speed, the eddy current loss of magnets has become a major problem. Especially the Neodymium Iron Boron (NdFeB) and the Samarium Cobalt (SmCo) magnets, are more easily affected by temperature. The eddy current loss has become a major problem.
These eddy currents always result in generation of heat, and then degradation of performance in motors, generators, and sensors. Anti-eddy current technology of magnets usually suppresses the generation of eddy current or suppresses the movement of induced current.
“Magnet Power” has been developed the Anti-eddy-current technology of NdFeB and SmCo magnets.
The Eddy Currents
Eddy currents are generated in conductive materials which are in an alternating electric field or alternating magnetic field. According to Faraday's law, alternating magnetic fields generate electricity, and vice versa. In industry, this principle is used in metallurgical melting. Through medium-frequency induction, conductive materials in the crucible, such as Fe and other metals, are induced to generate heat, and finally the solid materials are melted.
The resistivity of NdFeB magnets, SmCo magnets or Alnico magnets is always very low. Shown in table 1. Therefore, if these magnets work in electromagnetic devices, the interaction between the magnetic flux and conductive components generates eddy currents very easily.
Table1 The resistivity of NdFeB magnets, SmCo magnets or Alnico magnets
|
Magnets |
Resistivity(mΩ·cm) |
|
Alnico |
0.03-0.04 |
|
SmCo |
0.05-0.06 |
|
NdFeB |
0.09-0.10 |
According to Lenz’s Law, Eddy currents generated in NdFeB and SmCo magnets, leads to several undesirable effects:
● Energy Loss: Due to eddy currents, a part of the magnetic energy is converted into heat, reducing the efficiency of the device. For example, the iron loss and copper loss due to eddy current is the main factor of efficiency of motors. In the context of carbon emission reduction, improving the efficiency of motors is very important.
● Heat Generation and Demagnetization: Both the NdFeB and SmCo magnets have their maximum operating temperature, which is a critical parameter of permanent magnets. The heat generated by eddy current loss causes the temperature of the magnets rise. Once the maximum operating temperature is exceeded, demagnetization will occur, which will eventually lead to a decrease in the function of the device or serious performance problems.
Especially after the development of high-speed motors, such as magnetic bearing motors and air bearing motors, the demagnetization problem of rotors has become more prominent. Figure 1 shows the rotor of a air bearing motor with a speed of 30,000 RPM. The temperature eventually rose by about 500°C, resulting in demagnetization of the magnets.
Fig1. a and c is the magnetic field diagram and distribution of normal rotor, respectively.
b and d is the magnetic field diagram and distribution of demagnetized rotor, respectively.
Furthermore, NdFeB magnets have a low Curie temperature (~320°C), which makes them demagnetization. The curie temperatures of SmCo magnets, is ranging between 750-820°C. NdFeB is easier to be affected by eddy current than SmCo.
Anti-Eddy Current Technologies
Several methods have been developed to reduce the eddy currents in NdFeB and SmCo magnets. These first method is to change the composition and structure of magnets to enhance the resistivity. The second method which is always be used in engineering to disrupt the formation of large eddy current loops.
1.Enhance the resistivity of magnets
Gabay et.al have been added CaF2, B2O3 to SmCo magnets to improve the resistivity, which wan enhanced from 130 μΩ cm to 640 μΩ cm. However, the (BH)max and Br decreased significantly.
2. Lamination of Magnets
Laminating the magnets, is the most effective method in engineering.
The magnets were sliced into thin layers and then glued them together. The interface between two pieces of magnets is insulating glue. The electrical path for the eddy currents is disrupted. This technology is widely used in high-speed motors and generators. “Magnet Power” has been developed a lot of technologies to improve the resistivity of magnets. https://www.magnetpower-tech.com/high-electrical-impedance-eddy-current-series-product/
The first critical parameter is the resistivity. The resistivity of laminated NdFeB and SmCo magnets produced by “Magnet Power” is higher than 2 MΩ·cm. These magnets can significantly inhibit the conduction of current in the magnet and then suppress the heat generation.
The second parameter is thickness of the glue between pieces of magnets. If the thickness of the glue layer is too higher, it will cause the volume of the magnet to decrease, resulting in a decrease in overall magnetic flux. “Magnet Power” can produce laminated magnets with the thickness of glue layer of 0.05mm.
3. Coating with High-Resistivity Materials
Insulating coatings are always applied on the surface of magnets to enhance the resistivity of magnets. These coating act as barriers, to reduce the flow of eddy currents on the surface of the magnet. Such as epoxy or parylene, of ceramic coatings are always used.
Benefits of Anti-Eddy Current Technology
Anti-eddy current technology is essential applied in many applications with NdFeB and SmCo magnets. Including:
● High-speed motors: In high-speed motors, which means the speed is between 30,000-200,000RPM, to suppress the eddy current and to reduce heat is the key requirement. Figure 3 shows the comparation temperature of normal SmCo magnet and anti-eddy current SmCo in 2600Hz. When the temperature of normal SmCo magnets (left red one) exceeds 300℃, the temperature of anti-eddy current SmCo magnets (right bule one) does not exceed 150℃.
● MRI Machines: Reducing eddy currents is critical in MRI to maintain the stability of the systems.
Anti-eddy current technology is very important for improving the performance of NdFeB and SmCo magnets in many applications. By using lamination, segmentation, and coating technologies, the eddy currents can be significantly reduced in “Magnet Power”. The anti-eddy current NdFeB and SmCo magnets is possible to be applied in modern electromagnetic systems.
Post time: Sep-23-2024
