Permanent magnetic materials are the core components of permanent magnet motors, and their characteristics directly determine the motor’s efficiency, size, durability, and overall equipment cost.
Definition of Permanent Magnetic Materials
Permanent magnetic materials, also known as hard magnetic materials, are those that can maintain a constant magnetic state once magnetized. These materials exhibit a wide hysteresis loop, high coercivity, and high residual magnetism, allowing them to retain magnetism for a long period after the external magnetic field is removed and withstand certain strengths of external magnetic interference. Permanent magnetic materials can convert, transmit, process, and store information and energy.
Indicators of Permanent Magnetic Materials Affecting Motor Performance
| Magnetic Permeability (μr): The magnetic permeability of permanent magnetic materials is a key parameter for evaluating their magnetic performance, typically expressed as relative permeability (μr), which is the ratio of the material’s magnetic permeability to that of vacuum magnetic permeability (μ0). Permanent magnetic materials usually have low permeabilities, approaching that of a vacuum, though specific values may vary among different material types and compositions.
| Remanence (Br): This refers to the magnetic induction strength that remains in the magnetic material after the external magnetic field has been reduced to zero. The unit is Tesla (T) or Gauss (G). Higher remanence signifies a stronger magnetic field that the magnet can provide externally, resulting in greater stability.
| Temperature Coefficient: Within the allowable working range of a permanent magnet, the percentage change in residual magnetism for every 1°C change in temperature is referred to as the residual magnetism temperature coefficient (αBr). Similarly, the percentage change in intrinsic coercive force is known as the intrinsic coercive force temperature coefficient (αHcj). The temperature coefficient indicates the temperature stability of the permanent magnet material; a positive coefficient means performance increases with temperature, while a negative coefficient shows performance decreases with rising temperature.
| Coercive Force (Hc): On the demagnetization curve of permanent magnetic materials, when the magnetic induction strength is zero, the magnetic field strength (H) is not zero but equals Hc, which is termed coercive force. It is generally measured in amperes per meter (A/m) or oersteds (Oe). A higher coercive force indicates a stronger ability of the magnet to resist demagnetization, meaning it is more difficult to magnetize or demagnetize.
Specific Effects of Permanent Magnetic Material Performance on Motor Performance
| Permeability and Iron Loss: High-permeability magnetic materials can reduce magnetic field losses, while low iron loss materials can minimize heat generated by hysteresis and eddy currents during motor operation. Motors with cores made from high-permeability and low-iron-loss cold-rolled silicon steel can achieve energy savings of 10%-30% compared to standard steel under the same power conditions, generating less heat and alleviating the burden on cooling systems. For example, in small precision motors, the use of high-permeability materials enables size reduction while meeting stringent spatial requirements for the equipment.
| Remanence and Coercive Force: Residual magnetism indicates the strength of the retained magnetic intensity after magnetization, while coercive force reflects the material’s resistance to demagnetization. Higher values for both metrics result in a stronger magnetic field from the material. For motors, a strong magnetic field enables “small size high power”: high-performance rare earth permanent magnet motors (such as neodymium-iron-boron) can be over 30% smaller than standard ferrite permanent magnet motors while achieving more than 50% higher power density. In motors requiring frequent starts and stops or significant load variations, high coercive force materials help the magnetic field quickly return to a stable state, enhancing dynamic response performance.
| Temperature Stability: Motor operation generates heat, and the temperature stability of magnetic materials directly determines the long-term reliability of the motor. Materials with poor temperature stability are prone to demagnetization at high temperatures, leading to reduced power, increased noise, and even failure. For example, industrial motors operating in high-temperature conditions require high-temperature stable samarium-cobalt permanent magnets (capable of withstanding temperatures above 200°C), while standard neodymium-iron-boron materials, particularly those below the UH grade, can demagnetize above 150°C and are unsuitable.
| Material Types: Rare earth permanent magnets (neodymium-iron-boron) exhibit excellent performance, while ferrite permanent magnets offer average performance, are cost-effective, and use readily available raw materials, suitable for common household appliances like washing machines and fans that do not demand high size or efficiency. Among silicon steel, oriented silicon steel outperforms non-oriented silicon steel and is more suitable for high-end motors. In design, a balance between performance requirements and cost constraints is necessary to select the appropriate materials.
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