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

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, start-up etc.

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

Brushless Permanent Magnet Servomotors 281

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

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

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

**3.2 Permanent magnets** 

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

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 individual type of motors.
