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Power Electronic Handbook P10

Brushless DC (BLDC) motors are synchronous motors with permanent magnets on the rotor and armature windings on the stator. Hence, from a construction point of view, they are the inside-out version of DC motors, which have permanent magnets or field windings on the stator and armature windings on the rotor. A typical BLDC motor with 12 stator slots and four poles on the rotor is shown in Fig. 10.1. The most obvious advantage of the | 10 AC Machines Controlled as DC Machines Brushless DC Machines Electronics Hamid A. Toliyat 10.1 Introduction Texas A M University 10.2 Machine Construction Permanent Magnets Stator Windings Tilak Gopalarathnam 10.3 Motor Characteristics Texas A M University Mathematical Model 10.4 Power Electronic Converter Unipolar Excitation Fault-Tolerant Configuration Current Source Inverter 10.5 Position Sensing Position Sensorless Control 10.6 Pulsating Torque Components 10.7 Torque-Speed Characteristics 10.8 Applications 10.1 Introduction Brushless DC BLDC motors are synchronous motors with permanent magnets on the rotor and armature windings on the stator. Hence from a construction point of view they are the inside-out version of DC motors which have permanent magnets or field windings on the stator and armature windings on the rotor. A typical BLDC motor with 12 stator slots and four poles on the rotor is shown in Fig. 10.1. The most obvious advantage of the brushless configuration is the removal of the brushes which eliminates brush maintenance and the sparking associated with them. Having the armature windings on the stator helps the conduction of heat from the windings. Because there are no windings on the rotor electrical losses in the rotor are minimal. The BLDC motor compares favorably with induction motors in the fractional horsepower range. The former will have better efficiency and better power factor and therefore a greater output power for the same frame because the field excitation is contributed by the permanent magnets and does not have to be supplied by the armature current. These advantages of the BLDC motor come at the expense of increased complexity in the electronic controller and the need for shaft position sensing. Permanent magnet PM excitation is more viable in smaller motors usually below 20 kW. In larger motors the cost and weight of the magnets become 2002 by CRC Press LLC Rotor FIGURE 10.1 Three-phase BLDC motor with four poles on the rotor and