No-Load Performance Analysis of Brushless DC Machines With Axially Displaceable Rotor

Brushless dc (BLDC) machines with a surface mounted permanent magnet (SMPM) rotor meet the high-torque and high-efficiency requirements for automotive applications. However, their constant-power operation region is limited due to the low phase inductance. As an alternative to the electrical field-weakening methods, the speed range of radial-flux BLDC machines can be extended by mechanically reducing the axially overlapping length of the stator and the rotor. In this paper, the no-load performance of an SMPM-rotor BLDC machine with an axially displaceable permanent-magnet rotor is analyzed. The effectiveness of this mechanical field-weakening method is limited through the flux components due to the stator/rotor misalignment and the additional losses. The cause of the flux components due to the stator/rotor misalignment and the dependence of back-electromotive-force waveforms on the axial rotor position are investigated by using 3-D finite-element method (FEM) analysis, where the effects of the end-winding geometry and design are taken into account. Moreover, the additional loss mechanisms due to the stator/rotor misalignment are identified, and the no-load additional losses are determined by using the experimental and 3-D FEM analysis results. Finally, the numerical results are verified by using test-bench measurements.