Design and Operation of Interior Permanent-Magnet Motors With Two Axial Segments and High Rotor Saliency

Skewing the magnets in a brushless interior permanent-magnet motor can be difficult. One method to overcome this problem is to use axial segments that are rotated (“twisted”) with respect to each other. Compared to other methods of rotor skewing, this method may reduce manufacturing cost and the complexity of the rotor. This paper addresses the use of two axial segments and the associated effects on the back-electromotive force (EMF) waveform and motor performance. The back-EMF waveforms of an interior permanent-magnet motor are deeply influenced by the tooth-slot and winding harmonics. They should be sinusoidal to reduce torque ripple for ac motor servo drives and other applications where smooth operation is required. In the paper, we present the two-segment rotor structure together with a simple technique for reducing high-order back-EMF harmonics, and we derive the optimal twisted angle of the proposed two-segment rotor. This minimizes the total harmonic distortion of the back-EMF waveform due to tooth-slot effects. We examine cogging torque and the reduction in cogging torque. We apply the twisted angle rotor to two different compressor motors. In addition to the back-EMF, we address the torque ripple under load and the effect of twist on back-EMF constant. We examine the results using finite-element analysis and validate them by experimental measurement.