Design of Adjustable Cogging Torque Permanent Magnet Machine With High Contrast Ratio

This article presents a new adjustable cogging torque permanent magnet (ACT-PM) machine, which can provide two distinct operating states: the low cogging torque (LCT) state and the high cogging torque (HCT) state. The contrast ratio is defined as the ratio of the cogging torques between HCT and LCT states. The LCT state of this machine is designed to have similar performance as a conventional permanent magnet machine, with high average torque and low torque ripple that offer smoothness and control precision. For the HCT state, on the other hand, the machine generates HCT that can provide passive self-lock ability under power-OFF condition, serving as a parking brake that prevents the driveshaft from rolling. Consequently, a conventional mechanical parking brake that has corrosion and wearing-out concerns can be removed. The potential applications of this machine include auto-guided vehicles and other types of electric vehicles. This article illustrates the design flowchart and the proposed candidate structure, followed by the investigation and comparison of electromagnetic performances between different stator/rotor combinations. Then, multiobjective optimization and single parameter optimization are applied to the candidate structure to further improve the contrast ratio for cogging torque. A prototype machine with the optimal design parameters is fabricated and experimentally verified, where a mechanical switch is designed to adjust the machine between LCT and HCT states. The test result shows a good agreement with the finite-element analysis, and this confirms the feasibility of the proposed ACT-PM machine.