Permanent magnets can be divided into three main groups depending on the technology of production: cast, sintered or bonded magnets.
Bonded permanent magnets are very often referred to as dielectromagnets or ferroplast. The powder metallurgy methods have many advantages such as e.g., low losses of materials, and low consumption of labour and energy.
Another division of permanent magnets depends on the type of magnetic materials used.In this group are Alnico magnets, ferrite magnets, Sm-Co or Nd-Fe-B magnets. Figure 6: SmCo2 type permanent magnet used for levitation force test1.2.2 Ferrite magnetsThe permanent magnet used inside the levitator design is SmCo magnet.
The main raw materials for SmCo-magnets are samarium (~25% – 35%), small quantities of chrome and copper, and the remainder being cobalt. SmCo-magnets are manufactured primarily using presses and sinters. SmCo-magnets are characterized by excellent resistance to corrosion and can be used at temperatures of up to +250°C. Depending on the alloy, the remanence of SmCo-magnets lies between ca. 0.86 – 1.
16 Tesla. This remanence is comparable to NdFeB-magnets. The magnets are arranged in a circular shape close to each other. 14Ferrite magnets are manufactured by powder metallurgy technology. The most widespread application of ferrite magnets is in strontium and barium ferrite magnets.
Magnetic properties of ferrite magnets are not very high, but – their coercivity being higher than the coercivity of Alnico magnets – they are less sensitive to demagnetization. High resistivity is the reason ferrite magnets to work in a varying magnetic field with high frequency. Ferrite magnets have Curie temperature of 450°C. The behaviour of ferrite magnets in different temperatures is diverse than that of other permanent magnets. With elevated temperature coercivity increases but remanence decreases. The temperature coefficient of coercivity has a positive value.
In temperatures below 0°C, the remanence of ferrite magnets increases while coercivity decreases. Ferrite magnets can work in up to 250°C. The disadvantages of ferrite magnets are their brittleness and high shrinkage after sintering. 1.
2.3 ApplicationsPermanent magnets are used in many devices. The main recipient of permanent magnets is the electric machines and instruments industry.
They are used not only in electric motors but also in several types of electromagnetic transducers, analogue measurement devices with magnetic circuits. Nowadays the development of various sensors with permanent magnets can be observed. Magnetic sensors find a wide range of application in industrial automatics – in electric drives designed for specific purposes, in automotive technology, in the increasingly developing technology of the so-called “intelligent buildings”, in medical technology, etc. 15.
Some examples of magnetic sensors: ? proximity magnetic sensors? magnetic encoders? linear position sensors? precise position sensors? read relay