1. Main properties of commonly used permanent magnet materials in electric motors
1.1 Permanent magnet materials commonly used in electric motors
The permanent magnet materials commonly used in motors include sintered magnets and bonded magnets, with the main types being AlNiCo, ferrite, Samarium Cobalt, and Neodymium Iron Boron.
AlNiCo materials were widely used before the 1980s. They have advantages such as excellent temperature stability, time stability, and suitability for ultra-high temperature operating environments. They are used in special applications such as motors for military or instrument applications with high operating temperature requirements and excellent magnetic stability.
Ferrite materials are non-metallic permanent magnet materials, and are inexpensive. They are mainly used in economical micro-motors where performance and size requirements are not high and the demand is large. Examples include toy motors, household appliance motors, audio-visual motors, office equipment and general instrument motors, automotive and motorcycle motors, and small-power drive motors for industrial use.
Samarium cobalt (SMC) is a permanent magnet material with excellent magnetic properties that emerged in the mid-1960s, exhibiting very stable performance. SMC is particularly suitable for manufacturing electric motors due to its magnetic properties; however, its high price limits its application primarily to military motors used in aviation, aerospace, and weaponry, as well as high-performance motors in high-tech fields where price is not a primary factor. Neodymium iron boron (NdFeB) is a third-generation high-performance permanent magnet material that appeared in the 1980s. While its magnetic properties are superior to SMC, it has poorer thermal stability and is prone to corrosion, requiring surface protection treatment. However, its lower price led to its rapid adoption. With continuous advancements in NdFeB materials, its temperature performance has improved, especially since the 1990s. Low-temperature-coefficient, high-temperature-resistant NdFeB materials have been successfully developed, with high-performance heat-resistant NdFeB capable of operating at 200°C. Furthermore, its price has continued to decrease, leading to the widespread adoption of NdFeB in most industrial and civilian motors, and its eventual replacement of many ferrite materials in low-cost, economical motors.
Bonded permanent magnet materials are composite permanent magnet materials made by mixing binders and permanent magnet materials and then molding them through compression, injection, or extrusion. These include bonded ferrite, bonded AlNiCo, bonded Samarium Cobalt, and bonded Neodymium Iron Boron (NdFeB). Among them, bonded NdFeB is currently the best bonded permanent magnet material. Compared with sintered permanent magnet materials, it has advantages such as better machinability, easier molding, the ability to be made into various complex shapes, better uniformity of magnetic properties, and ease of multi-pole magnetization. However, the magnetic properties of bonded permanent magnet materials are lower than those of sintered magnets of the same type, with a magnetic energy product of approximately 40% to 70% of that of sintered magnets made of the same material. Among bonded permanent magnet materials, bonded NdFeB has the best prospects. If the processing issues and quality improvements are resolved, bonded NdFeB will become the permanent magnet material with the widest application potential. Currently, it is mainly used in small brushless DC motors and precision micro-motors such as stepper motors.
1.2 Main Properties of Permanent Magnet Materials
(1) Remanence. This refers to the magnetic induction intensity of a permanent magnet material after it has been magnetized to saturation in an external magnetic field, when the external magnetic field is zero. This value directly relates to the air gap magnetic flux density in the motor. A higher magnetic induction intensity value will result in a higher air gap magnetic flux density, leading to optimal values for key motor parameters such as torque constant and back EMF coefficient. This also ensures a more reasonable relationship between the motor's electrical and magnetic loads, ultimately achieving optimal efficiency.
(2) Coercivity Hc (Magnetic Induction Coercivity Hcb). The reverse magnetic field strength required when the remanent magnetic induction intensity Br drops to zero under saturation magnetization of a permanent magnet material. This indicator is related to the motor's demagnetization resistance, i.e., overload multiple and air gap magnetic flux density. The larger the Hc value, the stronger the motor's demagnetization resistance, the larger the overload multiple, and the stronger its adaptability to strong demagnetization dynamic working environments. Simultaneously, the motor's air gap magnetic flux density will also increase.
(3) Maximum magnetic energy product BHmax. The maximum magnetic field energy provided by the permanent magnet material to the external magnetic circuit. This indicator is directly related to the amount of permanent magnet material used in the motor. The larger BHmax is, the greater the magnetic field energy that the permanent magnet material can provide to the external magnetic circuit, that is, the less permanent magnet material is used in the motor under the same power conditions.
(4) Intrinsic coercivity Hci. This index refers to the magnetic field strength value when the residual magnetization M drops to zero. The Hcb value corresponding to B=0 on the demagnetization curve only indicates that the permanent magnet cannot provide energy to the external magnetic circuit at this time, and does not mean that the permanent magnet itself does not have energy. However, the Hci value corresponding to M=0 indicates that the permanent magnet has truly demagnetized at this time and has no magnetic field energy stored at all. Although Hci is not directly related to the motor operating point, it is the true coercivity of the permanent magnet material, representing that the permanent magnet material has magnetic field energy and the ability to resist demagnetization. The magnitude of intrinsic coercivity is closely related to the temperature stability of the permanent magnet material. The higher the intrinsic coercivity, the higher the operating temperature of the permanent magnet material can be.
(5) Temperature coefficient α. Temperature is one of the main factors affecting the magnetic properties of permanent magnet materials. The percentage change in magnetic properties that occurs reversibly for every 1°C change in temperature is called the temperature coefficient of the magnetic material. The temperature coefficient can be divided into the temperature coefficient of remanence and the temperature coefficient of coercivity. This index has a significant impact on the performance stability of the motor. The higher the temperature coefficient, the greater the change in the index when the motor operates from a cold state to a hot state, which directly limits the operating temperature range of the motor. It also indirectly affects the power-to-volume ratio of the motor.
2. Permanent magnet motors and their characteristics
The maximum power of permanent magnet motors has reached 1000kW, and the smallest diameter...
With a diameter of 0.8mm, a maximum speed of 300,000 r/min, and a minimum speed of 0.01 r/min, permanent magnet motors have the following characteristics compared to electrically excited motors.
2.1 Simple structure and high reliability
Using permanent magnet materials for excitation allows the pole shoes and excitation coils in the original electrically excited motor to be replaced by one or more permanent magnets, significantly reducing the number of parts and greatly simplifying the structure. Furthermore, eliminating the need for excitation slip rings and brushes not only improves the motor's manufacturability but also greatly enhances its mechanical reliability and extends its lifespan.
2.2 Excellent performance
Permanent magnet motors, especially those using rare-earth permanent magnet materials, can significantly improve air gap magnetic flux density, allowing for optimal motor performance design. The direct result is a reduction in motor size and weight. Furthermore, compared to other motors, permanent magnet motors exhibit superior control performance. This is because: Firstly, the high performance of rare-earth permanent magnet materials greatly improves the motor's torque constant, torque-to-inertia ratio, and power density. Through proper design, the moment of inertia, electrical and mechanical time constants can be significantly reduced, resulting in substantial improvements in key servo control performance indicators. Secondly, modern permanent magnet motors feature well-designed permanent magnet circuits, and the high coercivity of rare-earth permanent magnet materials greatly enhances the motor's resistance to armature reaction and other demagnetizing capabilities, significantly reducing the impact of external disturbances on motor control parameters. Thirdly, replacing electrical excitation with permanent magnet materials reduces the design of excitation windings and magnetic fields, thus reducing parameters such as excitation flux, excitation winding inductance, and excitation current, directly decreasing controllable variables or parameters. Considering all the factors mentioned above, it can be said that permanent magnet motors have excellent controllability.
For example, currently, fully digital permanent magnet AC servo motors exhibit excellent speed control performance, with sinusoidal AC servo motors achieving speed ratios up to 1:100,000. Stepper motors and low-speed synchronous motors, after adopting permanent magnet materials, show significant improvements in output torque and dynamic response characteristics. Therefore, compared to motors of the same specifications, permanent magnet motors offer substantial improvements in dynamic performance, steady-state performance, control performance, and reliability compared to ordinary motors.
2.3 High efficiency and energy saving
Permanent magnet motors not only reduce resistance losses but also effectively improve the power factor. For example, permanent magnet synchronous motors can maintain high efficiency and power factor within a load range of 25% to 120% of rated capacity. A micro permanent magnet DC motor with a capacity of 16 or less is 10% to 20% more efficient than an electrically excited motor of the same specifications. The widespread replacement of motors in fan and pump loads with permanent magnet motors has resulted in significant overall energy savings. The higher the power, the greater the proportion of excitation losses in the total losses, thus highlighting the high efficiency advantage of permanent magnet motors.
3. Issues that should be studied in depth in the design of permanent magnet motors
3.1 Utilization rate of permanent magnet materials
In permanent magnet motors, the cost of permanent magnet materials accounts for a large proportion of the total cost. Therefore, saving materials and improving material utilization are among the most pressing concerns for permanent magnet motor manufacturers. Theoretically, the maximum energy product point of a permanent magnet represents the maximum energy it can provide, and this maximum operating point can also be determined from the demagnetization curve. However, practical applications are far more complex. It is necessary to specifically study the motor's application environment, analyze its intended functions, identify its key performance indicators, and thus determine the optimal operating point. The shape and volume of the permanent magnet must also be rationally determined, while the influence of its manufacturing process must be considered. Only after comprehensively considering various factors can the optimal design of the motor be achieved in terms of function, performance, and cost.
3.2 Overload and Demagnetization
Demagnetization of magnetic materials includes temperature-induced demagnetization, time-induced demagnetization, and environmental demagnetization. It is further divided into reversible and irreversible demagnetization. In-depth research is needed on the relationship between the coercivity and intrinsic coercivity of permanent magnet materials and their stable operating temperature; the impact of the temperature coefficient on motor performance indicators and the demagnetization safety factor; the definition of the maximum operating temperature of a motor based on changes in magnetic properties; the proportion of reversible and irreversible demagnetization within the motor's operating temperature range and their impact on motor performance; and the remagnetization and reuse of demagnetized permanent magnet materials.
3.3 Analysis and Design
The theory and design of modern permanent magnet motors are relatively mature. Numerous design procedures and methods based on magnetic circuit analysis and calculation exist, and numerical analysis of permanent magnet magnetic fields is widely used in engineering practice. However, in permanent magnet motors, the permanent magnet serves as both the field excitation source and the magnetic source of the magnetic circuit, and is also a component of the magnetic field and the magnetic circuit. Furthermore, the manufacturing process, shape, size, magnetization tools, and magnetization methods of permanent magnet materials can all lead to non-ideal consistency and uniformity, sometimes resulting in significant dispersion. Even permanent magnet materials of the same grade and batch may exhibit substantial differences in performance data. Therefore, the dispersion of permanent magnets poses certain difficulties for the design analysis and numerical calculation of permanent magnet magnetic fields in permanent magnet motors, affecting the accuracy of the design. For example, issues such as the establishment and equivalence of permanent magnet models in theoretical and numerical field analysis, and the accurate calculation of leakage flux coefficient, local demagnetization, and armature reaction in engineering magnetic circuit calculations all present larger errors than those in the analysis and calculation of electrically excited motors.
3.4 Magnetization and Magnetism Measurement
The design of permanent magnet motors is based on the saturation magnetization of permanent magnet materials. Therefore, whether the magnets used in the motor are fully magnetized and saturated is a crucial question. If the permanent magnets are supplied magnetized and pre-magnetized by the magnetic material manufacturer, there is generally no problem. However, when the entire motor is magnetized, ensuring that the permanent magnets are fully magnetized, and maintaining the uniformity and consistency of magnetic properties while achieving saturation magnetization, are issues worthy of further investigation.
Similarly, there are still many issues worth studying regarding magnetic performance testing. For example, how can magnets supplied with magnetic components be effectively, easily, and relatively accurately inspected upon arrival at the motor manufacturing plant? Currently, most motor manufacturers are unable to effectively measure the magnetic properties of permanent magnet materials at the component stage, and problems with the magnetic materials are often only discovered when the overall machine fails performance testing.
3.5 Corrosion resistance
The corrosion susceptibility of neodymium iron boron (NdFeB) materials significantly impacts motor quality. Currently, the issue of surface protection for NdFeB remains unresolved in China, with methods such as electroplating often resulting in plating peeling and motor malfunctions. Furthermore, their limited tolerance to harsh environmental conditions (such as humidity, salt spray, and specific gases) hinders the motor's adaptability to such environments. Further improvements in the surface protection capabilities of permanent magnet materials are hoped for.
3.6 Magnetic property stability, uniformity, and consistency
To ensure that the performance of the motor does not change significantly during its life cycle, especially for some military motors with special requirements, whose reliable working life is required to be more than 15 years, it is desirable for the magnetic properties of the permanent magnet material to remain stable over a long period of time.
Motors, especially high-precision motors, have high requirements for the uniformity and consistency of the magnetic properties of permanent magnet materials. Non-uniform magnetic properties will lead to an uneven magnetic field in the motor, increased torque fluctuations, increased output voltage ripple in the generator, poor linearity, and reduced accuracy in motor control. Furthermore, inconsistencies in the magnetic properties of the same grade of permanent magnet material across different batches can sometimes result in batches of unqualified motors. Therefore, high-precision motors require the magnetic property consistency of permanent magnet materials to have an error within 5%, and the uniformity error to be within 3%.
3.7 Processing Technology
There are still many processing technology issues to be studied in the manufacturing of permanent magnet motors. For example, how to control the process parameters during bonding, and whether they will affect the magnetic properties; whether the impact, vibration and processing environment during machining will affect the magnetic properties; and how to take protective measures for magnetic components during process turnover and assembly.
In permanent magnet motors, permanent magnets are often supplied pre-magnetized. While this reduces the difficulty of magnetization for motor manufacturers, it significantly increases the complexity of the manufacturing process. For example, permanent magnet brushless DC motors and permanent magnet synchronous motors often employ surface mount structures where multi-pole, multi-block magnets are directly bonded to the rotor surface. High-performance sintered NdFeB magnets are commonly used, but their bonding process is complex and difficult to operate, and at high speeds, bonding reliability can be compromised. If sintered magnets could be made into radially crystalline, magnetized, multi-pole ring structures, the manufacturing process of the motor rotor would be greatly simplified. Currently, bonded magnets are achievable, but their magnetic properties are not high. If sintered magnets could be produced, mass-produced, and inexpensive (reports indicate that some manufacturers are already conducting research in this area), it would have a promising future.
Conclusion
In conclusion, the development of permanent magnet materials has driven the development of permanent magnet motors, while the widespread application of permanent magnet motors and the increasing demands on permanent magnet materials have further spurred improvements in these materials. With the widespread use of permanent magnet motors in various fields such as aviation, aerospace, national defense, industrial and agricultural production, and daily life, the application prospects of permanent magnet materials in motors will be quite extensive.
