**2.2 Permanent magnet servomotors**

There exist various permanent magnet (PM) servomotors in the literature. They can be classified into two main categories, which are surface mounted PM motors where magnets are glued on the rotor surface and buried PM motors where magnets are buried into the rotor.

The use of surface mounted PM motors increases the amount of PM material per pole used in the motor. Using more magnet material usually increases the torque production of the motor while it also increases the motor volume and thus the cost. Buried PM motor and interior PM motor use the flux concentration principles where the magnet flux is concentrated in the rotor core before it gets into the airgap. These motors usually have considerable reluctance torque which arises from the fact that the use of flux concentration in the iron core introduces a position dependent inductance and hence reluctance torque that can be beneficial in certain cases.

PM motors are also classified based on the flux density distribution and the shape of the current excitation. They are listed into two categories, one of which is PM synchronous motors (PMSM) and the other is PM brushless motors (BLDC). PMSM, also called permanent magnet AC (PMAC) motors, has sinusoidal flux density, current and back EMF variation while the BLDC has rectangular shaped flux density, current variation and back EMF. Classification of these two motor types is explained in Table 1.




Table 2. Basic comparison of surface magnet and buried magnet motors

Brushless Permanent Magnet Servomotors 279

Fig. 6 shows typical torque-speed characteristics of a brushless PM servomotor. There are two main torque parameters to describe a PM servomotor: Rated torque (*TR*) and maximum torque (*Tmax*). In addition, there are two major speed points: Rated speed and maximum speed. The region up to rated speed is called constant torque region and the region between the max speed (ω*max*) and rated speed (ω*R*) is called constant power region. During constant torque region, the motor can be loaded up to rated torque usually without any thermal problem. On the other hand, during constant power region, the motor torque starts to drop but the power stays almost constant. Another important characteristic of a PM motor is maximum load point which shows the overload capability of the motor. During this period, the motor can deliver higher torque for a short time to handle cases such as motor overload,

*Speed* [rpm]

Max speed ω*max*

Constant Power Region

Max torque *Tmax*

Rated torque *TR*

Fig. 5. Stator stack showing the motor laminations

Fig. 6. Torque-speed characteristics of a PM servomotor

Rated speed ω*R*

Constant Torque Region

start-up etc.

*Torque*

[Nm]

**2.4 PM servomotor torque-speed and back-EMF characteristics** 

Fig. 3. Typical servomotor (Courtesy of FEMSAN Motor Co.)

Each PM motor type explained has some advantages over another. For instance, surface magnet motor has very simple rotor structure with fairly small speed limits. Buried or interior PM motors have wide speed ranges but their rotor is more complex than both surface magnet and inset PM rotors. In addition, buried or interior PM motors can go up to very high speeds unlike surface magnet motors although their control is more complex than surface magnet type motors. This comparison is also tabulated in Table 2.

## **2.3 Permanent magnet servomotor structure**

A conventional surface mounted PM servomotor structure is illustrated in Fig. 1 (a). The motor has a stator and a PM rotor. The stator structure is slotted and formed by the laminated magnetic steel. A close picture of a laminated stator is shown in Fig. 4. Polyphase windings are placed into the stator slots although a slotless versions of servomotors are also available. The rotor structure is formed by the permanent magnets mounted on the rotor surface, rotor core and shaft. The rotor core is usually laminated. Fig. 5 shows both the stator and the rotor of a typical permanent magnet servomotor with high energy NdFeB magnets.

Fig. 4. Stator stack showing the servomotor laminations

Each PM motor type explained has some advantages over another. For instance, surface magnet motor has very simple rotor structure with fairly small speed limits. Buried or interior PM motors have wide speed ranges but their rotor is more complex than both surface magnet and inset PM rotors. In addition, buried or interior PM motors can go up to very high speeds unlike surface magnet motors although their control is more complex than

A conventional surface mounted PM servomotor structure is illustrated in Fig. 1 (a). The motor has a stator and a PM rotor. The stator structure is slotted and formed by the laminated magnetic steel. A close picture of a laminated stator is shown in Fig. 4. Polyphase windings are placed into the stator slots although a slotless versions of servomotors are also available. The rotor structure is formed by the permanent magnets mounted on the rotor surface, rotor core and shaft. The rotor core is usually laminated. Fig. 5 shows both the stator and the rotor of a typical permanent magnet servomotor with

Fig. 3. Typical servomotor (Courtesy of FEMSAN Motor Co.)

**2.3 Permanent magnet servomotor structure** 

Fig. 4. Stator stack showing the servomotor laminations

high energy NdFeB magnets.

surface magnet type motors. This comparison is also tabulated in Table 2.

Fig. 5. Stator stack showing the motor laminations
