**1. Introduction**

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Electrical motors drive a variety of loads in today's world. Almost every industrial process relies on some kind of electrical motors and generators. There exist billions electric motors used in different applications all over the world. Majority of them are small fractional HP motors use in household appliances. However, they used about 5% of the electricity used by the motors. Three phase motors are used in heavier applications and consume substantial amount of electricity. These electric motors operate long hours and consume more than half of the electricity used by motors.

The oldest type of electric motor, wound field DC motor, was the most popular motor for years and easiest for speed control. Although they are replaced by adjustable AC drives in many applications, they are still used in some low power and cost effective applications. The main reason why DC drives faded away over the last decade is that they require converters and maintenance, not to mention their lower torque densities compared to AC motors. Induction motors are also one of the most widely used motors in AC drive applications. They are reliable and don't require maintenance due to the absence of brushes and slip rings. The availability of single phase power is another big plus for these motors. The fact that the rotor windings are present makes the induction motors less efficient and creates cooling problems of the rotor. One crucial drawback of the induction motors is the parameter variation due to the heat caused by the rotor winding.

Variable reluctance motors are also frequently used in the industry and robotics. It's simple and robust stator and rotor structures reduce the cost dramatically compared to other types of motors. The converter requirement is also not very severe. A simple half bridge converter can easily be used to drive the motor. On the other hand, variation of reluctance does also create significant cogging, vibration and audible noise.

As for the synchronous motors, they have benefits and drawbacks of both DC and induction motors. The synchronous motors with field winding can be more efficient than a DC or induction motors and are used in relatively large loads such as generating electricity in power plants. If the rotor winding in synchronous motors is replaced by permanent magnets, another variation of synchronous motors is obtained. These motors are called permanent magnet motors which can be supplied by sinusoidal or trapezoidal currents. These motors have three major types based on their magnet structures as displayed in Fig. 1.

The lack of slip rings and rotor windings as well as high power density, high efficiency and small size make these motors very attractive in the industrial and servo applications. In

Brushless Permanent Magnet Servomotors 277

Electric motors are also classified by their slots. They are called slotted motors if they do have slots and called non-slotted or slotless motors if they do not have any slot structures. Furthermore, one major classification method is identified by the main flux direction. If the motor has a main flux component which is radial to the shaft, they are called radial flux motors and if the flux component is axial to the motor shaft, then the motors are called axial flux motors where they find various applications because of their structural flexibility.

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

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

 **PMSM BLDC** 

**Surface PM motor Buried/Interior PM motor** 

Table 1. Classification of permanent magnet motors based on their excitation and back EMF

Convenience BLDC PMSM

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

Flux distribution Square or Sinusodial Usually Sinusoidal Complexity of rotor Simple Complex Speed limit ~1.2 x ω<sup>R</sup> ~3 x ωR or higher High speed capability Difficult Possible Control Relatively easy More complex

EMF. Classification of these two motor types is explained in Table 1.

Phase current excitation Sinusoidal Trapezoidal Flux density Sinusoidal Square Phase back EMF Sinusoidal Trapezoidal Power and Torque Constant Constant

**2.2 Permanent magnet servomotors** 

that can be beneficial in certain cases.

waveforms

Fig. 1. Surface mounted PM (a), buried PM (b) and spoke type PM (c) motor types

addition, PM servomotors have better torque-speed characteristics and high dynamic response than other motors. Their long operating lives, noise-free operations and high speed ranges are some of the advantages of brushless servomotors.
