Aspirations for miniaturization and increase in device efficiency have lead to a search for magnets with greater and greater energy densities.
The figure below shows the development of magnets
Fig. 7. Change in energy density (B.H)max of magnets within the space of recent hundred years (source: Rare - Earth Permanent Magnets VACODYM - VACOMAX, Catalogue PD-002, Vacuumschmelze, 1997 and 2000)
The choice of a proper magnet depends not only on the energy density but also on other magnetic parameter.
Table 4. Typical properties of commonly used rare-earth magnets and hard ferromagnetics (source: Soinski M.: Magnetic materials in engineering, p. 163; in Polish)
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Neodymium (NdFeB) and samarium (SmCo) magnets have increasingly wider applications superseding conventional ferrite and AlNiCo magnets.
Neodymium and samarium magnets are inter alia used in:
- motors with rotating armature windings
- motors with armature windings in the form of hollow cylinder
- brushless motors
- multi-phase synchronous motors with permanent magnets
- stepper motors
- torque motors
- magnetic separators
- magnetic catchers
- magnetic pillars
- magnetic clutches and bearings
- devices of microwave and space engineering
- acoustic transducers
Except for required magnetic parameters (mainly energy density) rare-earth magnets should meet the requirements as to shape, magnetization method and additional anticorrosive and mechanical protection
The most often used shapes of these magnets are:
- rectangular
- disc
- annular
- arc
Magnetization methods are presented in Fig. 8
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Fig. 8. Magnetization methods (after Vacuumschmelze)
Nickel and tin-plating protective layers for rare-earth magnets are mainly used.
The following rare-earth magnets are in the offer of Vacuumschmelze GmbH & Co. KG:
- neodymium magnets called VACODYM
- samarium magnets called VACOMAX
