With the continuous development of materials technology, magnetic materials have been widely used in various industrial equipment. Among industrial equipment, motors are undoubtedly the most widely used, and magnetic materials play a decisive role in the performance of motors.
An electric motor can be described as the result of the interaction between the rotor and stator magnetic fields. One magnetic field needs to generate rotating magnets, so it must be the armature winding. The other magnetic field can take many forms; it can be generated by magnetic materials (magnets), generated by the armature, or generated by induction. The type that requires magnetic materials (magnets) to provide the magnetic field is the permanent magnet motor we are familiar with.
Magnetic materials that determine the performance of permanent magnet motors
Permanent magnet motors use permanent magnets to generate the motor's magnetic field, eliminating the need for excitation coils and excitation current. They are highly efficient and have a simple structure. Moreover, with the development of high-performance permanent magnet materials in recent years, the application of permanent magnet motors has become more widespread.
Based on their properties in a magnetic field, magnetic materials are generally classified into paramagnetic, diamagnetic, antiferromagnetic, ferromagnetic, and ferrimagnetic. For permanent magnet motors, this refers to ferromagnetic and ferrimagnetic materials, as these two types of magnetic materials exhibit strong responses to external magnetic fields and are classified as strongly magnetic materials.
Strong magnetic materials, all exhibiting spontaneous magnetization below the Curie temperature, include permanent magnets, soft magnetic materials, and magnetic recording materials, all of which have significant practical applications. Magnetic recording materials refer to magnetic materials that utilize magnetic properties and effects to input, record, store, and output information such as images and digital data, and are not closely related to motor applications. Permanent magnets and soft magnetic materials are more closely related to motor applications.
Permanent magnet materials are the core materials of permanent magnet motors. A key characteristic of permanent magnets is that once magnetized, they are very difficult to demagnetize. After being sintered, permanent magnet poles possess the ability to retain their magnetism. By magnetizing these poles, a constant magnetic field can be maintained. In terms of specifications, permanent magnet materials exhibit three characteristics: a wide hysteresis loop, high coercivity, and high remanence. The higher the coercivity, the less likely the material is to demagnetize. Therefore, permanent magnet materials, as the magnetic source of a motor, provide a stable magnetomotive force, much like the electromotive force in a circuit.
Soft magnetic materials are also easily magnetized, but they are prone to demagnetization after magnetization. From a parameter perspective, these magnetic materials possess low coercivity and high permeability, thus they are used as magnetic conductors to transmit magnetic field lines. Therefore, the two have different focuses: permanent magnet materials aim for higher coercivity to maximize demagnetization resistance, while soft magnets pursue higher permeability for smoother magnetic conduction.
Permanent magnet materials and high-performance motors
The choice of permanent magnet materials is closely related to the performance of motors, and the application of rare earth permanent magnet motors is now widely recognized. The extremely high magnetic energy product and coercivity of rare earth permanent magnet materials give rare earth permanent magnet motors advantages in size and weight, as well as lower losses, higher efficiency, and sufficient reliability.
Rare earth permanent magnet materials have undergone many generations of development, and the main rare earth materials have now evolved from samarium cobalt permanent magnets to sintered neodymium iron boron (NdFeB). Sintered NdFeB is currently the most important rare earth permanent magnet material used in permanent magnet motors, and it has extensive applications in automotive motors and industrial motors.
While the remanence of sintered NdFeB magnets is not particularly outstanding among permanent magnet materials, falling short of first-generation alloy permanent magnets, its coercivity and energy product far surpass other permanent magnet materials. This significant lead in energy product means that NdFeB magnets are smaller in volume per unit magnetic field strength, making miniaturization and weight reduction of high-performance motors possible.
Rare-earth permanent magnet motors inevitably utilize rare-earth materials. While second-generation permanent magnets, primarily made from iron oxide, don't boast the best performance, their raw materials are abundant, the manufacturing process is simple, and the cost is low. Iron oxide permanent magnets, emphasizing high cost-effectiveness, are widely used in various types of motors. However, this comes at the cost of reduced motor performance and size, given their lower coercivity and energy product.
Considering the performance of several strong magnetic materials currently available, rare-earth magnets are the optimal solution for achieving higher motor performance. In particular, high-performance neodymium iron boron (NdFeB) magnets can reduce motor size and weight while also minimizing energy loss, thus improving the overall efficiency of the motor system. With further development in magnetic materials, it's not impossible that one day high-performance motors will no longer require rare-earth materials.
summary
The choice of magnetic material has a decisive impact on the overall performance and cost of permanent magnet motors, and motors with appropriate magnetic materials need to be selected according to different application requirements.
