Permanent Magnet (PM) Brushless Motors The permanent magnet brushless dc motor has been in existence for about three decades and is finally being widely used for many applications1,5,6. Its being brushless has frequently been mentioned as justification for its cost, higher than most other motor types. However, the PM brushless motor has two other features that are arguably more important than the fact that it contains no mechanical brushes and commutator for commutation of the phase windings to the power source. First, it produces the highest continuous output power per unit volume of any motor yet invented. The other important virtue of a PM brushless motor is that it produces its output power with the least input power. For a given size and output performance envelope, it has the most efficient motor of any electric motor yet invented. With the emphasis on reducing power consumption, this type of motor will be used in many pump applications. In addition, and perhaps most importantly for the application of pumps, the PM brushless motor possesses one other very important feature or use of the permanent magnets contained within the rotor assembly. Although the magnets are there to supply a magnetic field from the rotor to pass through the air gap of the motor into the stator, these same magnets can serve as a magnetic coupling. If a sleeve-shaped liquid barrier made of a suitable material to be impervious to the liquid is fitted between the rotor and stator, the PM brushless can be used as the best hermetically sealed wet motor known. There are, of course, many possible configurations of this PM brushless hermetic motor concept. These PM brushless hermetic motors can be configured as axial gap2, 
FIGURE 9A through C Types of dc motors. (A) Shunt motor has field winding of many turns of fine wire connected in parallel with the armature circuit. The interpole winding aids commutation. (B) Series motor has field in series with the armature. Field has a few turns of heavy wire carrying full-motor current flowing in the armature. (C) Compound motor has both a shunt and a series field to combine characteristics of both shunt- and series-type motors in the same machine (from Powerspecial report, “Motors,” June 1969).
FIGURE 10 Speed, torque, and power characteristics of dc motors (from Powerspecial report, “Motors,” June 1969) radial gap inside rotor, or radial gap outside rotor motors (Figure 12). The choice depends upon the best way to design the pump integration. All PM brushless motors must be powered with an electronic inverter and controller, which can be used for controlling pump performance in controlled loop systems of all types. The electronic drive can be a simple square wave, a six-step trapezoid type, or a sinusoidal drive. The PM motor is a synchronous machine and the controller needs to know the shaft position; in some applications a resolver or encoder is used. However, in many pumping applications a ‘sensorless’ controller can be used. The choice of controller topology is application specific, but not difficult. Three permanent magnet choices can be used for these motors. The lowest priced magnet material is known as ferrite or ceramic with a magnetic flux output of about one-third 
FIGURE 11 The equivalent circuit of a dc motor with a permanent magnet stator 
FIGURE 12 Inner, outer, and axial rotor PM brushless motor choices for pump drives to one-half of the other two magnet choices. By careful design, this magnet material can be used for 50 hp (37 kW) and up in PM brushless machines to keep costs competitive with legacy motors. The two high energy or rare earth magnet materials, Samarium Cobalt and Neodynium Iron Boron, can be used from 1 to 50,000 hp (36 MW). The more expensive of the two, Samarium Cobalt, is used for high temperature and other hostile environment pump motor applications. The newer rare earth magnet material, Neodymium Iron Boron, is much lower in cost, with the highest useful magnetic flux at moderate operating temperatures. Permanent magnets can be assembled onto the surface of the rotor or they can be imbedded within the rotor structure. The imbedded magnet design can be used to direct drive high hp (kW) pumps to several thousand rpm. In either design, the rotor can be made to be quite robust and yield a very long useful operating life. In fact, the only failure modes for PM brushless motors have to do with bearings or winding insulation. Figure 13 shows examples of large PM brushless motors. The speed can be controlled to greater than 100 to 1 if required with constant power over the highest speed range of any other motor type. The efficiency remains very high over the entire speed range. There are many design possibilities including axial gap and outside rotor configurations. For example, because of the high magnetic strength of the permanent magnets, the rotor of a PM brushless motor can also serve as the impeller of a pump that is integrated with the motor in a hermetically sealed package with no shaft seals. Utilizing the axial-gap rotor configuration of Figure 12, the concept is illustrated in Figure 14. Commercially significant impeller torque levels can be generated by the PM brushless motor in this configuration3. This eliminates the need for a separate magnetic coupling or larger-size canned induction motor (see the discussion at the end of this section and in Section 2.2.3). The PM brushless motor can be driven with either square wave currents over a 120 degrees electrical commutation angle or with sinusoidal currents over 180° electrical commutation angle. The latter scheme generally yields lower torque fluctuations, which is sometimes important for pump applications. Either drive requires shaft angle feedback data to tell the phases when to be powered. This feedback data are also a requirement for vector driven ac induction motors. The sensor used for this feedback can be as simple as Hallswitches mounted in the motor that send out a pulse to the controller each time a rotor magnetic pole changes polarity as the motor rotates. An optical encoder or a resolvercan also be used for this purpose. Several sensorless or remote sensors have been developed that capture the rotor angle location from the stator phase windings so a shaft-mounted sensor is not required. 
FIGURE 13 Large horsepower (kW) high-performance permanent magnet inside rotor brushless motors (courtesy of Pacific Scientific) Switched Reluctance Brushless Motor The SR brushless dc motor is one of the oldest motors known, but it has not been used much until recently. Its main feature is that it is a true brushless motor with most of the virtues of its PM cousin, but it does not require permanent magnets4. Proponents of switched reluctance motors point out its inherent robustness. The rotor of the SR motor is so simple and rugged it can survive very at high speeds as well as in a variety of other unusual environmental conditions. 
FIGURE 14 Permanent magnet brushless motor integrated into a sealless pump. Permanent magnets are mounted on the impeller, which serves as the rotor of the motor. (Courtesy of Flowserve Corporation)3
FIGURE 15 Switched reluctance brushless dc motor, (6) rotor poles, (2) stator poles per phase Figure 15 shows the typical cross section of a switched reluctance motor, illustrating how simple this motor is with phase coils placed around the stator poles. The rotor consists of a set of magnetic steel gear-shaped laminations taken from the bore of the stator laminations. They are stacked and retained on the motor shaft. No magnets or windings of any kind are required to produce torque. The SR motor is said to be a doubly salient pole machine. The torque is produced by the magnetic attraction of the closest rotor poles to those stator poles which are magnetized by the phase coils. There is an abundance of technical information regarding the performance details of this technology. The important point is that the SR motor is an excellent choice for high-speed high-powered centrifugal pumps. There is a considerable difference in the inverter topology for the SR motor as compared to either the PM brushless or the ac induction motor. The SR machine must be driven as a unipolar machine rather than a bipolar motor like the other two. This means the standard ac inverter cannot be used for the SR motor. The phases are each connected to the dc power in parallel rather in the standard bridge fashion with all of the phases connected together at a center tap. Figure 16 shows the normal power circuit for the (4) phase SR motor shown in Figure 15. The same circuit is applicable for both two phase or three phase. It is essentially an independent half bridge circuit for each phase. Certain two-phase designs are very cost effective for high speed pump drives that are required to rotate in only one direction without the need for reversing. | 
