Classification and Characteristics of Rare Earth Permanent Magnets for Motors

Rare earth permanent magnets are a mature technology. The impact of the rare earth crisis in 2011 led to a re-evaluation of many ideas from the 1980s and 1990s regarding new hard magnets that might contain little or no rare earths (or heavy rare earths). Nd-Fe-B magnets have been carefully and skillfully optimized for a wide range of applications that require high performance at a reasonable price. When high-temperature stability is required, Sm–Co is the material of choice, while Sm–Fe–N magnets are entering certain special applications. The scope for improvement of these basic materials through substitution has been explored quite comprehensively, and the influence of processing techniques on the microstructure and hysteresis is widely understood. A big idea a generation ago was exchange spring hard/soft nanocomposite magnets with orientation potential, which had the real potential to greatly increase the record energy product. However, this has proven difficult to achieve.

Neodymium iron boron(NdFeB)

In 1983, Sumitomo Corporation of Japan and General Electric Company of the United States successfully developed the rare earth permanent magnet material neodymium iron boron (NdFeB) in different ways. Due to its high performance and relatively low price, it has been rapidly and widely applied. The most prominent feature of NdFeB is its high magnetic performance. Its remanence Br is 3 to 5 times that of ferrite, and its intrinsic coercivity Hcj is 5 to 15 times that of ferrite. The theoretical value of the maximum magnetic energy product (HB)max can reach 640 KJ/m3. The demagnetization curve of NdFeB permanent magnets is approximately a straight line. Under normal conditions, the recoil line basically coincides with the demagnetization curve. It has hard intrinsic properties and strong demagnetization resistance. It has a low density, high hardness and compressive strength, is not easy to break, and has good mechanical properties. The raw materials of NdFeB are abundant, and its price is relatively cheap compared with other rare earth permanent magnets, about 1/3 to 1/4 of that of rare earth cobalt.

Due to the high magnetic properties of neodymium iron boron (NdFeB), when applied to micro-motors, the volume of the magnet is much smaller than that of the corresponding ferrite. It is short and thin, enabling NdFeB permanent magnet motors to have a small volume, light weight, small inertia, high power, and high efficiency. Its torque can be 40% - 50% greater than that of ferrite motors of the same volume. At the same time, its intrinsic magnetic permeability is close to the vacuum magnetic permeability μ0, endowing the motor with not only good steady-state operating characteristics but also good dynamic performance. NdFeB has a strong demagnetization resistance, preventing the motor from demagnetizing due to external influences such as vibration and impact, and improving the magnetic stability of the permanent magnet motor.

The disadvantages of NdFeB are that its Curie point Tc is low, only about 300°C, its operating temperature is usually below 110°C, and its thermal stability is poor. The magnetic induction temperature coefficient ab is relatively large, about -0.126%/°C, only slightly smaller than that of ferrite (-0.19%/°C) (for permanent magnet motors, the smaller the ab, the better, preferably ab approaching zero). The intrinsic coercivity temperature coefficient ah of ferrite is positive, and the magnetic flux change is reversible at positive temperatures. However, NdFeB has a relatively large negative ah value, reaching -0.6%/°C. Therefore, irreversible demagnetization occurs at positive temperatures. When the temperature is 150°C, the irreversible demagnetization usually exceeds 5%. In addition, NdFeB has poor corrosion resistance and chemical stability, and is extremely prone to rusting in a high-temperature and humid air. The above defects limit the application of NdFeB in wet, hot, and high-temperature motors.

The chemical formula of neodymium iron boron is Nd2Fe14B. Similar to ferrite, neodymium iron boron powder can be polymerized with a polymer compound (binder) to produce bonded neodymium iron boron. Compared with sintered neodymium iron boron, the addition of the binder reduces the density of the material and somewhat decreases its magnetic properties.

Alnico(AlNiCo)

Alnico permanent magnet materials have a large remanence Br, and their maximum magnetic energy product (HB)max is at an upper-middle level among permanent magnet materials. They have a high Curie point Tc, and the maximum operating temperature can reach above 500°C. The temperature coefficient ab is the smallest among permanent magnet materials. Their organizational structure is stable, they have good oxidation resistance and strong corrosion resistance. They are one of the permanent magnet materials applied to motors relatively early.

The biggest disadvantage of Alnico is its low coercivity Hcb, non - linear demagnetization curve which almost coincides with the intrinsic curve, and poor resistance to demagnetization. Therefore, Alnico motors are extremely prone to losing magnetism, and special magnetic stability treatment is required after magnetization. In addition, Alnico materials have poor toughness, high brittleness, nickel (Ni) is a rare metal with low production volume, and cobalt (Co) is an even rarer strategic material with an expensive price. In the motor industry, Alnico is gradually being replaced by ferrite and neodymium iron boron.

Rare earth cobalt(Rare Earth Co)

Rare earth cobalt is an intermetallic compound formed by some rare earth metals such as samarium (Sm), praseodymium (Pr), cerium (Ce) and cobalt (Co). Represented by samarium cobalt permanent magnets SmCo5 and Sm2Co17, it was the most important rare earth permanent magnet material before the development of neodymium iron boron. Rare earth cobalt has excellent comprehensive magnetic properties. Its Br, Hcb and Tc are much higher than those of neodymium iron boron. The working temperature can reach 350°C. Moreover, it has a small volume, good thermal stability, strong thermal demagnetization ability and magnetic stability, and excellent oxidation resistance.

The disadvantages of rare earth cobalt are that the material is hard and brittle, prone to fragmentation, has low mechanical strength, poor impact toughness, and very poor machining performance. It is extremely prone to edge chipping and corner damage, and the processing technology is complex. However, its biggest drawback is that its components are the rare earth metal samarium (Sm) with a scarce reserve and the scarce and expensive strategic metal cobalt (Co), making it expensive. Rare earth cobalt permanent magnets are mostly used in micro - special motors in special fields such as military and aerospace.

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