What are the commonly used magnet materials in motors?

The commonly used permanent magnet materials in motors include sintered magnets and bonded magnets, the main types are AlNiCo, ferrite, SmCo, NdFeB, etc.

Alnico: Alnico permanent magnet material is the earliest widely used permanent magnet material, and its preparation process and technology are relatively mature. At present, there are factories producing it in Japan, the United States, Europe, Russia, and China.

Permanent ferrite material: In the 1950s, ferrite began to flourish, especially in the 1970s, when strontium ferrite with good performance in coercivity and magnetic energy product was put into production in large quantities, which rapidly expanded the use of permanent ferrite. As a non-metallic magnetic material, ferrite does not have the disadvantages of easy oxidation, low Curie temperature, and high cost of metal permanent magnet materials, so it is very popular.

Samarium cobalt material: A permanent magnet material with excellent magnetic properties that emerged in the mid-1960s, and its performance is very stable. Samarium cobalt is particularly suitable for manufacturing motors in terms of magnetic properties, but due to its high price, it is mainly used in the research and development of military motors such as aviation, aerospace, and weapons, and motors in high-performance high-tech fields.

NdFeB material: NdFeB magnetic material is an alloy of neodymium, iron oxide, etc., also known as magnetic steel. It has extremely high magnetic energy product and coercive force. At the same time, the advantages of high energy density make NdFeB permanent magnet material widely used in modern industry and electronic technology, making it possible to miniaturize, lighten and thin equipment such as instruments, electroacoustic motors, magnetic separation and magnetization. Because it contains a large amount of neodymium and iron, it is easy to rust.

The influence of magnetic materials on motor performance

One of the structural characteristics of the motor is that the stator pole is composed of permanent magnetic materials. The quality of the magnetic material directly affects the size of the motor magnetic circuit, the volume of the motor, as well as the functional indicators and motion characteristics. Permanent magnetic materials are also called hard magnetic materials. The main characteristics are large coercive force (Coercive Force) and high residual flux density. After saturation magnetization, the permanent magnetic material can still maintain stable magnetism for a long time after removing the external magnetic field, excite the permanent magnet motor, and establish a constant magnetic field in the air gap.

Remanence Br, coercive force Hcb

After the permanent magnet is magnetized to saturation, the magnetic field intensity (Magnetic Field Intensity) H of the external magnetic field is gradually reduced to zero, and the magnetic flux density (Magnet Flux Density) B of the permanent magnet is reduced from Bs to Br, and Br is called remanence. The reverse magnetic field is applied to reduce Br to zero. The absolute value of the reverse magnetic field intensity at this time is called the magnetic induction coercive force, or coercive force (Coercive Force) Hcb for short, as shown in the figure below. The B-H closed magnetization curve formed by continuously and slowly changing the magnetic field intensity of the external magnetic field for a cycle is called the hysteresis loop (Magnetic hysteresis loop). The hysteresis loop in the second quadrant is the demagnetization curve (Demagnetization Curve), which is the basic characteristic curve of permanent magnetic materials and an important basis for characterizing the quality of permanent magnetic materials.

Recoil permeability r

After the permanent magnet is magnetized, the external magnetic field is removed, and the magnetic density is Br. Under the action of the demagnetization field, the magnetic density drops to a certain point along the demagnetization curve, such as point K in the figure above, and then the demagnetization effect is reduced until the field strength H=0, but the magnetic density does not return to Br along the demagnetization curve, but to a lower point, such as point M. Later, when the demagnetization field strength is increased to Hk, the magnetic density will follow the new curve to point K, forming a local small loop. Since the area of the local loop is very small, it can be approximately represented by a straight line KM, which is called the recoil line. The slope of the recoil line is called the recoil permeability r, which is approximately equal to the slope of the demagnetization curve at Br, that is, the recoil line is parallel to the tangent line at Br on the demagnetization curve. r is an important parameter for the dynamic operation of permanent magnetic materials. When r is small, the permanent magnetic material has better dynamic performance.

Isotropy and Anisotropy

Due to different manufacturing processes, permanent magnet materials are divided into isotropy (Iotropy) and anisotropy (Anisotropy). The easy magnetization axes of different grains in isotropic permanent magnets are disorderly oriented, so the remanence Br is low, only about half of the saturation magnetic induction intensity Bs, and the corresponding maximum magnetic energy product (BH)max is also small. Anisotropic permanent magnets are formed by magnetic field and then sintered (or rolled). The easy magnetization axes of their grains are arranged in the same direction along the forming magnetic field, and Br is close to Bs, so that Br is about twice as high as isotropy, and for ferrite (BH)max. It is nearly four times higher. Therefore, motor magnets usually use anisotropic materials.