Design of permanent magnet machines for field-weakening operation

Abstract

This research focuses on the electromagnetic design of permanent magnet (PM) machines in terms of the iron loss, torque pulsations and field-weakening performance. It covers the investigation of the effect of stator-slot and rotor-pole number combinations for surface-mounted PM (SPM) machines, and the stator-slot and rotor-effective-slot number combinations for interior permanent magnet (IPM) machines. The effect of changing the number of slots and poles on the performance of a particular SPM machine design is studied in detail using finite element analysis. This includes examining the back-EMF, the open-circuit/full-load power losses, the cogging/ripple torque, and the field-weakening performance. The simulation results are compared with the expected relationships to provide electric machine designers useful insights on the effect of the number of slots and poles on the performance of SPM machines. Operation at high speed in traction drives corresponds to deep field-weakening conditions. Due to the high electrical frequencies, the iron loss of IPM machines at high speeds can significantly affect the overall efficiency. This thesis investigates the rotor- cavity positioning and the combination of stator-slot and rotor-effective-slot number on the eddy-current loss for IPM/reluctance machines operating under deep field-weakening conditions. A new closed-form expression for the stator and rotor eddy-current loss is developed. The optimal barrier-positioning for the minimum total loss and the effect on the eddy-current loss of varying the stator-slot and rotor-effective-slot number are investigated for 1-, 2-, 3- and full-layered rotors. FEM optimisation and experimental verification of an example IPM machine design are presented. An optimized 30 slot, 4 pole (slot/pole/phase = 2.5) three-layered IPM machine with a significantly reduced iron loss under field-weakening operation is proposed and compared to the baseline 36-slot 4-pole (slot/pole/phase = 3) three-layered IPM machine. The detailed comparison of the optimized and baseline designs using a combination of the analytical, FEM and experimental tests are presented.