May 2012

Ferrite magnets for micro motors

Slim, powerful and economical

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Using a new production technology, TDK has developed ultra-thin anisotropic ferrite magnets whose size and performance approach that of premium rare-earth magnets. The innovative TDK FB13B and FB14H magnets enable the economical design of miniaturized high-performance motors and actuators.

Demand is growing for small and lightweight motors that are both energy-saving and powerful, especially for automotive applications, where the number of electric motors is continually increasing. The selection of the magnet is a determining factor for these parameters. The highest-performance magnet currently available is the sintered neodymium magnet with dysprosium (Dy) added to improve the intrinsic coercive force (HCJ). As rare earth metals, however, the supplies of neodymium and especially dysprosium are very limited.

This has led TDK to intensify its efforts to find alternatives to rare earth materials. In addition to participating in research projects with Japan’s largest public R&D organization, the New Energy and Industrial Technology Development Organization (NEDO), for the development of rare earth-saving motors, TDK has focused on the development of high-powered magnets that use much more abundant ferrite materials. The target: enable motors that combine high performance with a competitive cost for use in high volume applications, such as compressor motors for air-conditioners and the growing number of micro motors in electric vehicles.

Leader in advanced ferrite magnets
TDK is a pioneer in high-performance ferrite magnets, for which only small amounts of rare earth metals are necessary. For example, already in the 1990s TDK introduced innovative composite materials with additional trace elements lanthanum (La) and cobalt (Co) that significantly improved the intrinsic coercive force (HCJ) and now offers a strong portfolio of anisotropic magnets fabricated using dry and wet molding process technologies (Figure 1).

Figure 1: Improving performance of sintered TDK ferrite magnets

Portfolio of anisotropic TDK magnets fabricated using dry and wet molding process technologies.

Each technique has its advantages. Dry molding enables smaller and more complex shapes than are possible with wet molding. By contrast, wet-molded magnets, as exemplified by the FB12 material, offer higher performance. For example, the temperature coefficient of FB12 is one third that of its precursor material FB6, thus significantly improving the material’s demagnetization resistance at low temperatures. With an HCJ of 402 kA/m at −40 °C its intrinsic coercive force is about 1.9 times that of FB6N. As a result, the permeance coefficient (PC) can be made small, or the magnet extremely thin.

Despite the superior magnetic performance of wet-molded magnets, the current technology has its limitations: the minimum thickness possible when molded is 3 mm, and it requires a considerable allowance for grinding after calcination. Thus, extra energy is required for grinding and there is the risk of breaking during grinding and/or degradation of the product strength. Based on the conventional dry and wet molding technologies, design engineers have a choice of either a thinner form factor or higher magnetic performance.

New solution for ultra-thin ferrite magnets
In order to solve the size/performance dilemma, TDK developed a new fabrication technology called the NS1 method. NS1 is a new near net shape molding technique for thin-shape anisotropy, which ensures that the size and shape of the manufactured product is very close to the final (net) shape, thus reducing the need for surface finishing. In addition, NS1 also provides even better magnetic characteristics than the wet molding technology. The major feature of this method is that it delivers molded products with uniform density and optimized orientation. The result is the new FB13B and FB14H anisotropic ferrite magnets that deliver magnetic performance very near to that of rare earth magnets with a very slim form factor of as thin as 1 mm, thus opening the door to further miniaturized motors in a multipolar design (Table 1).

Table 1: Motor design with conventional and TDK NS1 ferrite magnets

MaterialFB6BFB9BFB13B
Number of poles224
Motor diameter [mm]403733.4
Motor volume ratio [%]1008670
Magnet wall thickness [mm]5.03.51.9
Total magnet weight ratio [%]1007240
Compared to the conventional anisotropic ferrite magnet materials, FB6B and FB9B,
the new ultra-thin TDK FB13B and FB14H magnets enable multipolar brush micro
motors that are 30 percent smaller and 60 percent lighter.

Improved design freedom and power
The demand for smaller and lighter motors in automotive applications is on the rise due to the mandatory improvement of fuel efficiency and reduction of CO2 emissions. Thin FB13B and FB14H magnets are an economical alternative to neodymium magnets for the design of miniaturized multipolar motors. Moreover, they enable the design of micro motors that are 30 percent smaller and 60 percent lighter. The new ferrite magnets also have clear advantages in terms of their heat resistance, corrosion resistance, and magnetizability.

Figure 2: Torque of a micro motor with TDK FB13B material

Micro motors that use TDK FB13B material can deliver up to 14 percent more torque than motors using conventional materials (evaluation based on Ø 14 mm motor).

Until now, dry-molded magnets had to be used out of necessity because wet-molded magnets are not thin enough. But with the development of the NS1 method, FB13B and FB14H magnets can now replace conventional thin dry-molded magnets in brush micro motors (Ø 14 mm) that among others are used in automotive applications, significantly improving motor performance (Figure 2). Furthermore, the shapability of the NS1 method has allowed the manufacture of C-shaped magnets with sharp arcs (both edges being sharp), as shown in Figure 3; they have 14 percent more torque compared to FB5D with the same shape. In this case, results very comparable to a motor with a neodymium magnet can be achieved.

Figure 3: Configuration freedom enables improved performance

TDK’s new NS1 manufacturing method offers greater design freedom and enables wide-arc shapes with substantially greater torque.

Excellent cost-performance ratio
Because of their excellent mechanical and magnetic properties combined with their thin shape capability, the TDK FB13B and FB14H ferrite magnets are well suited for use in motors for power windows, power seats and fuel pumps, as well as for other small motors and actuators. Table 2 shows the key magnetic properties of the two materials.

 

Table 2: Magnetic properties of TDK FB13B and FB14H ferrite magnets

MaterialFB13BFB14H
Residual flux density BR [mT]475 ±10470 ±10
Coercive force HCB [kA/m]340 ±20355 ±20
Intrinsic coercive force HCJ [kA/m]380 ±20430 ±20
Maximum energy product BH [kJ/m³]44.0 ±1.643.1 ±1.6
Temperature coefficient of BR [%/K]-0.18-0.18

Against a background of growing concerns regarding the supplies of rare earth elements, thin, high-performance ferrite magnets certainly meet today’s requirements. The new FB13B and FB14H ferrite magnets represent great advances in cost-performance ratio as a result of optimized manufacturing and grinding processes and a reduction in the volume of raw materials used.

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