614 Pm Brushless Dc Motors

Permanent magnet AC machines with trapezoidal back-emf waveforms are known as the PM brushless DC machines. The concentrated windings of the machine instead of the sinusoidally distributed windings on the stator are the reason for the trapezoidal-shaped back-emf waveforms. PM brushless DC motors are widely used in a range of applications, from computer drives to sophisticated medical equipment. The reason behind the popularity of these machines is the simplicity of control. Only six discrete rotor positions per electrical revolution are needed in a three-phase machine to synchronize the phase currents with the phase back-emfs for effective torque production. A set of three Hall sensors mounted on the stator facing a magnet wheel fixed to the rotor and placed 120° apart can easily give this position information. This eliminates the need for the high-resolution encoder or position sensor required in PM synchronous machines, but the penalty paid for position sensor simplification is in performance. For EV/HEV applications, a high-resolution encoder/resolver may be necessary for phase advancing at high speeds. Vector control is not possible in PM brushless DC machines because of the trapezoidal shape of back-emfs.

The three-phase back-emf waveforms and the ideal phase currents of a PM brushless DC motor are shown in Figure 6.4. The back-emf waveforms are fixed with respect to rotor position. Square-wave phase currents are supplied such that they are synchronized with the back-emf peak of the respective phase. The controller achieves this obj ective using rotor position feedback information. The motor basically operates like a DC motor, with such a controller configuration, from a control point of view; hence, the motor is designated as a brushless DC motor.

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