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Describe the principles of operation of brushless DC motors, stepping motors, switched reluctance motors, universal motors, and single-phase motors Select an appropriate motor and determine the required performance characteristics for a given application Describe the characteristics of the electronic drives required for each of the important motors discussed in the chapter
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Figure 181a Two-pole brushless DC motor with three-phase stator winding
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In spite of its name, the brushless DC motor is actually not a DC motor, but (typically) a permanent-magnet synchronous machine; the name is actually due not to the construction of the machine, but to the fact that its operating characteristics resemble those of a shunt DC motor with constant eld current This characteristic can be obtained by providing the motor with a power supply whose electrical frequency is always identical to the mechanical frequency of rotation of the rotor To generate a source of variable frequency, use is made of DC-to-AC converters (inverters), consisting of banks of transistors that are switched on and off at a frequency corresponding to the rotor speed; thus, the inverter converts a DC source to an AC source of variable frequency As far as the user is concerned, then, the source of excitation of a brushless DC motor is DC, although the current that actually ows through the motor windings is AC (The operation of the inverters will be explained shortly) In effect, the brushless DC motor is a synchronous motor in which the torque angle, , is kept constant by an appropriate excitation current Brushless DC motors also require measurement of the position of the rotor to determine its speed of rotation, and to generate a supply current at the same frequency This function is accomplished by means of a position-sensing arrangement that usually consists either of a magnetic Hall-effect position sensor, which senses the passage of each pole in the rotor, or of an optical encoder similar to the encoders discussed in 13 Figure 181(a) depicts the appearance of a brushless DC motor Note how the multiphase winding is similar to that of the synchronous motor of 17 Figure 181(b) depicts the construction of a typical brushless DC servomotor The brushless motor consists of a stator with a multiphase winding, usually three-phase; a permanent-magnet rotor; and a rotor position sensor It is interesting to observe that since the commutation is performed electronically by switching the current to the motor rather than by brushes, as in DC motors the brushless motor can be produced in many different con gurations, including, for example, very at ( pancake ) motors Figure 181 shows the classical con guration of inside rotor, outside stator For simple machines, it is also possible to resort to an outside rotor, with greater ease of magnet attachment and inherently smoother rotation, but with inferior thermal characteristics, since a stator encased within the rotor structure cannot be cooled ef ciently In conventional DC motors, the supply voltage is limited by brush wear and sparking that can occur at the commutator, often resulting in the need for transformers to step down the supply voltage In brushless DC motors, on the other hand, such a concern does not exist, because the commutation is performed electronically without the need for brushes Further, since, in general, the armature
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Primary feedback device Rugged TENV, 1P65 waterproof construction O ring prevents rotation of outer bearing race for longer bearing life
Steel Optional shaft configurations Steel bearing insert for long bearing life Low-inertia samariumcobalt rotor
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Figure 181b A typical brushless DC servomotor
(load-carrying winding) is on the stator and thus the losses are concentrated in the stator, liquid cooling (if required) is feasible and does not involve excessive complexity You will recall that in a conventional DC motor the armature is on the rotor, and therefore auxiliary liquid cooling is very dif cult to implement Another important advantage of brushless DC motors is that by sealing the stator, submersible units can be built In addition to these operational advantages, it should be noted that these motors are also characterized by easier construction: the construction of the stator in a brushless DC motor is similar to that in traditional induction motors, and is therefore suitable for automated production The windings may also be tted with temperature sensors, providing the possibility of additional thermal protection The permanent-magnet rotor is typically made either of rare earth magnets (Sm-Co) or of ceramic magnets (ferrites) Rare earth magnets have outstanding magnetic properties, but they are expensive and in limited supply, and therefore the more commonly employed materials are ceramic magnets Rare earth magnet motors can be a cost-effective solution since they allow much greater uxes to be generated by a given supply current in applications where high speed, high ef ciency, and small size are important Brushless DC motors can be rated up to 250 kW at 50,000 rev/min The rotor position sensor must be designed for operation inside the motor, and must withstand the backlash, vibrations, and temperature range typical of motor operation Brushless DC motors do require a position-sensing device, though, to permit proper switching of the supply current Recall that the brushless DC motor replaces the cumbersome mechanical commutation arrangement with electronic switching of the supply current The most commonly used position-sensing devices are position encoders and resolvers The resolver, shown in Figure 182, is a rotating machine that is mechanically coupled to the rotor of the brushless motor and consists of two stator and two rotor windings; the stator windings are excited by an AC signal, and the resulting rotor voltages are proportional to the sine and cosine of the angle of rotation of the rotor, thus providing a signal that can be directly related to the instantaneous position of the rotor The resolver has two major disadvantages: First, it requires a separate AC supply; second, the resolver output must be appropriately decoded to obtain a usable position signal For these reasons, angular position encoders (see 13) are often used You will
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