Investigation of Worst-Scenario Rotor Tolerances in Halbach Machines With Different Magnet Segments

Tolerances are inevitable in machine manufacturing and additional low-frequency cogging torque harmonics with high-amplitude will be introduced. In this paper, the worst-scenario rotor tolerances, including PM thickness, angular width, position, and radial movement, in Halbach machines with different permanent magnet (PM) segment number per pole are investigated. Based on the tolerance sensitivity analysis, it is found that tolerances on the PM with high radial magnetization component and the PM thickness tolerance in the Halbach machine with more segments are more crucial to the cogging torque due to their large influence on the airgap flux density. Besides, a genetic algorithm combined with phasor diagram method is proposed to identify the worst-scenario cogging torque caused by rotor tolerances in Halbach machines. Compared to the peak values of the worst-scenario cogging torque due to a single tolerance, the peak values of the worst-scenario cogging torque are significantly aggravated due to the combined tolerances. Overall, PM thickness tolerance is more crucial in the Halbach machine with more segments and needs to be controlled strictly during manufacturing, especially when this tolerance acts on the PM with high radial magnetization component. The experiments have verified the cogging torque and back EMF predicted by the finite element analysis for the 2-segment Halbach machine with the worst-scenario rotor tolerances.