**3.1 Magnetic steel**

There exist various electric steel materials used in servomotors. Material type and grade depends mainly on the application and cost. High quality materials with high saturation and low loss levels are used in high performance and high speed applications while thick and high loss materials are used in low speed and cost effective applications. Nonoriented electrical steels are usually used in electric motor applications. Low magnetic loss and high permeability characteristics are valuable for applications where energy efficient, low loss, low noise and small size are important. One of the most frequently used magnetic steel lamination material is M270-35A (similar to M19 in the US). This material or similar grade is used in most PM servomotor applications. If high saturation levels and low losses at high speeds are required, materials such as Vacoflux50 would be a good option. The BH curve of these materials in addition to materials with high loss and thin high saturation level are all displayed in Fig. 8. Moreover, Table 3 shows the electrical and mechanical properties of various non-oriented electrical steel materials used in different motor applications.

**Phase A-B** 

**Phase B-C** 

**Phase C-A** 

(a) (b)

There are two types of PM servomotor alternatives: Sinusoidal and trapezoidal motors. This is made on the basis of back-EMF waveforms. Trapezoidal servomotors have a back-EMF in trapezoidal manner and sinusoidal servomotors have a sinusoidal back-EMF as illustrated in Fig. 7. In addition to back-EMF, the supply current is trapezoidal and sinusoidal in each

There exist various electric steel materials used in servomotors. Material type and grade depends mainly on the application and cost. High quality materials with high saturation and low loss levels are used in high performance and high speed applications while thick and high loss materials are used in low speed and cost effective applications. Nonoriented electrical steels are usually used in electric motor applications. Low magnetic loss and high permeability characteristics are valuable for applications where energy efficient, low loss, low noise and small size are important. One of the most frequently used magnetic steel lamination material is M270-35A (similar to M19 in the US). This material or similar grade is used in most PM servomotor applications. If high saturation levels and low losses at high speeds are required, materials such as Vacoflux50 would be a good option. The BH curve of these materials in addition to materials with high loss and thin high saturation level are all displayed in Fig. 8. Moreover, Table 3 shows the electrical and mechanical properties of various non-oriented electrical steel materials used

Fig. 7. Trapeziodal (a) and sinusoidal back-EMF (b) waveforms of a PM servomotor

individual type of motors.

**3. Magnetic materials** 

in different motor applications.

**3.1 Magnetic steel** 

Fig. 8. Examples of steel materials with magnetic and structural properties


Table 3. Non-oriented electric steel material properties (Source: Cogent)

#### **3.2 Permanent magnets**

Permanent magnet materials have been used in electric motors for decades. One important property of permanent magnets is the maximum energy product (*MEP*) which is the multiplication of residual flux density (*Br*) and coercive force (*Hr*). In other words, *MEP* represents the maximum energy available per unit volume (kJ/m3). *MEP* is also an indication of magnet force. Furthermore, the larger the *MEP*, the smaller the magnet material needed for the same force. Permeability is another important property of the

Brushless Permanent Magnet Servomotors 283

NdFeB magnets are more common rare earth magnets than SmCo, cheaper but more brittle. SmCo magnets are widely used in applications in which higher operating temperature and higher corrosion and oxidation resistance are crucial. A basic comparison of the four major

types of permanent magnet materials used in motors today is illustrated in Fig. 9.

Fig. 9. Flux density versus magnetizing field for the important magnets classes

be finalized after a design passes all of the main steps (Aydin et al. 2006).

Fig. 10. PM servomotor design process

Design procedure for any PM servomotor is shown in Fig. 10. This process comprises three main steps: Electromagnetic, structural and thermal designs. Electromagnetic design starts with magnetic circuit modeling and parameter optimization with a given set of design specifications. A series of optimizations such as pole number, loading, current density, dimensional limits etc. have to be performed to find the optimum parameters of the motor before proceeding further. When a design is obtained that meets the technical spec, a quick motor simulation and the influence of parameter variation must be carried out using simulation software such as SPEED (PC-BDC Manual, 2002). A detailed electromagnetic finite element analysis (FEA) either in 2D or 3D is the next step to verify that the design meets the specified torque-speed characteristics and performance. After an electromagnetic design is finalized, structural and thermal analyses (MotorCAD Manual 2004) have to be completed. It should be pointed out that structural analysis is not a necessity at low speed servomotor designs. If the motor does not meet the structural or thermal tests, then the electromagnetic design study should be repeated for a better design. A motor design has to

**4. Basic permanent magnet motor design process** 

magnets. It is the slope of the demagnetization curve in the linear region. Small permeability means high flux levels before the magnet is irreversibly demagnetized.

Alnico magnets which are Aluminum, nickel, iron and later addition of cobalt based materials was one of the important discoveries in permanent magnet technology and is still widely used today. These magnets can be magnetized in any direction by simply heating the magnet and cooling them in a magnetic field to give a preferred magnetic direction. Traditionally, Alnico magnets were largely used in PM motors. One advantage of Alnico magnets is that they have a high residual flux density (*Br*). They have excellent temperature stability and strong corrosion resistance level. Their working temperatures can go up to 500 degrees. However, they can be demagnetized very easily. In addition, the maximum energy product of these magnets is not very high.

Ferrite magnets, also called ceramic magnets, are one of the cheapest magnets manufactured in industry. They have very high intrinsic coercive force (*Hci*) and therefore, they are very difficult to demagnetize. They can easily be magnetized in a variety of formats. The raw material is so abundant that it is found in numerous applications. This kind of magnet material has a good resistance to corrosion and can operate at high temperatures up to 300 degrees. These materials are used even today for applications where space and cost are not important requirements.

Rare-earth magnets are strong permanent magnets made from the alloys elements such as Neodymium and Samarium. Discovery of these strong magnets have changed the future of permanent magnet motor technology as well as servomotors and the magnetic field can be increased to 1.5T levels. There are two types of rare-earth magnets available: Neodymium magnets and Samarium cobalt magnets.

The first generation rare earth magnets use Samarium and Cobalt (SmCo). One of the biggest advantages of such magnets is that they provide very high MEP compared to Alnicos and Ferrites. This big improvement in high MEP is made possible by the high coercive force. Nonetheless, they are very brittle and both the raw material cost and the production cost are quite high compared to other types of magnets. The revolution of rare earth magnets accelerated with the discovery of Neodymium Iron-Boron (NdFeB) magnets with even higher MEP in 1982. NdFeB magnets are produced by pressing powders in a magnetic field and their energy products can go up to 420 kJ/m3. This material is much stronger than SmCo and the cost is much lower simply because they are composed of mostly iron which is much cheaper than cobalt. However, they have to be protected against corrosion and their working temperature is also lower compared to SmCo magnets.

A brief comparison of different magnets used in PM motors is illustrated in Table 4. The rare earth magnets are the most common magnet materials used in PM servomotors and the table clearly shows significant benefits of such magnets. NdFeB magnets have higher flux density levels up to 1.5T and higher MEPs but their working temperature is lower (up to 200 oC).


Table 4. Typical permanent magnet material magnetic properties

magnets. It is the slope of the demagnetization curve in the linear region. Small permeability

Alnico magnets which are Aluminum, nickel, iron and later addition of cobalt based materials was one of the important discoveries in permanent magnet technology and is still widely used today. These magnets can be magnetized in any direction by simply heating the magnet and cooling them in a magnetic field to give a preferred magnetic direction. Traditionally, Alnico magnets were largely used in PM motors. One advantage of Alnico magnets is that they have a high residual flux density (*Br*). They have excellent temperature stability and strong corrosion resistance level. Their working temperatures can go up to 500 degrees. However, they can be demagnetized very easily. In addition, the maximum energy

Ferrite magnets, also called ceramic magnets, are one of the cheapest magnets manufactured in industry. They have very high intrinsic coercive force (*Hci*) and therefore, they are very difficult to demagnetize. They can easily be magnetized in a variety of formats. The raw material is so abundant that it is found in numerous applications. This kind of magnet material has a good resistance to corrosion and can operate at high temperatures up to 300 degrees. These materials are used even today for applications where space and cost are not

Rare-earth magnets are strong permanent magnets made from the alloys elements such as Neodymium and Samarium. Discovery of these strong magnets have changed the future of permanent magnet motor technology as well as servomotors and the magnetic field can be increased to 1.5T levels. There are two types of rare-earth magnets available: Neodymium

The first generation rare earth magnets use Samarium and Cobalt (SmCo). One of the biggest advantages of such magnets is that they provide very high MEP compared to Alnicos and Ferrites. This big improvement in high MEP is made possible by the high coercive force. Nonetheless, they are very brittle and both the raw material cost and the production cost are quite high compared to other types of magnets. The revolution of rare earth magnets accelerated with the discovery of Neodymium Iron-Boron (NdFeB) magnets with even higher MEP in 1982. NdFeB magnets are produced by pressing powders in a magnetic field and their energy products can go up to 420 kJ/m3. This material is much stronger than SmCo and the cost is much lower simply because they are composed of mostly iron which is much cheaper than cobalt. However, they have to be protected against

corrosion and their working temperature is also lower compared to SmCo magnets.

Alnico 1.2 10 6 500 500 Ferrite 0.43 10 5 300 300

Table 4. Typical permanent magnet material magnetic properties

SmCo Up to 1.1 Up to 820 Up to 240 Up to 820 Up to 350 NdFeB Up to 1.5 Up to 1033 Up to 422 Up to 380 Up to 200

A brief comparison of different magnets used in PM motors is illustrated in Table 4. The rare earth magnets are the most common magnet materials used in PM servomotors and the table clearly shows significant benefits of such magnets. NdFeB magnets have higher flux density levels up to 1.5T and higher MEPs but their working temperature is lower (up to 200 oC).

**Materials Br [T] Hc [kA/m] BHmax [kJ/m3] TC [oC] Tw-max [oC]** 

means high flux levels before the magnet is irreversibly demagnetized.

product of these magnets is not very high.

magnets and Samarium cobalt magnets.

important requirements.

NdFeB magnets are more common rare earth magnets than SmCo, cheaper but more brittle. SmCo magnets are widely used in applications in which higher operating temperature and higher corrosion and oxidation resistance are crucial. A basic comparison of the four major types of permanent magnet materials used in motors today is illustrated in Fig. 9.

Fig. 9. Flux density versus magnetizing field for the important magnets classes
