Torque-Speed Characteristics of Interior-Magnet Machines in Brushless AC and DC Modes, with Particular Reference to Their Flux-Weakening Performance

The torque-speed characteristics of a 3-phase permanent magnet brushless machine, having an interior permanent magnet rotor and an essentially sinusoidal back-emf waveform, are determined experimentally when the motor is operated in brushless AC (BLAC) mode, with 3-phase, sinusoidal current waveforms, and in brushless DC (BLDC) mode with both 2-phase, 120deg conduction and 3-phase, 180deg conduction rectangular current waveforms. The performance is compared on the basis of (i) the same peak phase current, (ii) the same torque in the constant torque operating region, and (iii) the same RMS phase current. It is shown that while 2-phase, 120deg BLDC operation results in the highest torque capability for the same peak current and BLAC operation results in the highest specific torque per rms current when operating below the base-speed, 3-phase, 180deg BLDC operation generally results in the best performance in the flux-weakening region. Hence, although the IPM motor has an essentially sinusoidal back-emf waveform, it may be advantageous to operate it in a hybrid mode, i.e. BLAC in the constant torque region and 3-phase, 180deg BLDC in the flux-weakening region