Study on rotor structure with different magnet assembly in high-speed sensorless brushless DC motors

High-speed permanent magnet (PM) brushless DC motors have gained more and more interests for many applications. The technique of detecting the electromotive force (EMF) zero-crossings is a common method in sensorless operation of PM brushless DC motors. However, the zero-crossings of the phase EMF will be obscured and the sensorless control will be malfunctioned, since the free-wheeling diode conduction can last more than 30 elec-degs under high-speed condition. Therefore it is supposed that the utilisation of third-harmonic EMF is more preferable, as it is not influenced by the free-wheeling diode conduction. Some design features for the high-speed sensorless operation are presented, primarily focusing on the rotor structure with different magnet assembly. The influence of the pole-arc to pole-pitch ratio on the third-harmonic airgap field will be analysed with both finite-element method and analytical model. Based on these analyses, another method which can improve both the third harmonic and fundamental components in the airgap field, by segmenting the parallel-magnetised PMs, is employed and studied. Motor prototypes are then designed and fabricated, and the experimental results confirm the validity of the proposed design method.