In DC-micromotors, permanent magnets are today used almost exclusively for producing the magnetic excitation flux. Four different magnetic materials (hard ferrite, AlNiCo, SmCo and NdFeB) are available. These differ from one another primarily with respect to the following parameters: temperature behavior, corrosion behavior, costs and above all by the remanent flux density and coercive field strength.
The properties of the four materials are compared in the following.
Hard ferrite
These are oxidic materials with barium or strontium that are quantitatively of the highest importance in the area of micromotors. The necessary raw materials are inexpensive and easily available. The disadvantage is the low remanence of less than 0.5 T (T is the unit symbol for the unit tesla) and the strong dependence of the magnetic properties on the temperature.
AlNiCo material
This type of permanent magnet is a metal cast or sintered material made of aluminum, nickel and cobalt with a remanence of just less than 1 T. Magnets made of AlNiCo material have only a very small coercive field strength and are, therefore, easy to demagnetize. This applies, in particular, to T-armature motors due to their pronounced armature reaction. Large currents, such as those that "flow" in the event of a malfunction, can lead to demagnetization or partial demagnetization of the magnet and thereby render the motor unusable. Bell-type armature motors are less susceptible to this due to their construction. The advantage of AlNiCo magnets is their usability at high temperatures, their very flat temperature response and their very good resistance to corrosion.
SmCo materials
Rare-earth cobalt materials are sintered materials made of samarium and cobalt (SmCo5, Sm2Co7) with very high coercive field strength and a remanence of approx. 1.1 T. Like AlNiCo materials, they also have a very flat temperature response. SmCo magnets are very expensive, which is why they are increasingly being replaced by NdFeB magnets. They are used above all in applications in which a very high-performance motor is required for operation at high ambient temperatures and there are special requirements on corrosion resistance.
NdFeB material
This group of permanent magnets consists of anisotropic composites made of neodymium, iron and boron. They have an approximately 20% higher remanence than SmCo magnetic materials. Compared to SmCo, their disadvantage is a higher temperature response and poorer corrosion behavior; their advantage is the lower price. To protect against corrosion, special coatings may be necessary. Exact knowledge of the usage conditions is therefore essential.
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