Investigation of the magnetic excitation forces due to simultaneous existence of eccentricities and uneven magnetization in a permanent magnet motor

Abstract

The noise and vibration are important performance indices to evaluate permanent magnet (PM) motors, especially in PM motors with neodymium or other rare earth materials. Because they have greater retentivity, coercivity, and maximum energy product, and the attractive magnetic force in an air gap increases approximately with the square of the magnetic flux density, the magnetic forces arising from designs using rare earth magnets are significantly greater than from those without PM. These magnetic excitation forces are originated from the distorted flux density distribution due to the manufacturing errors inevitable in mass productions. The major magnetic excitation forces causing magnetically induced vibration and noise are torque ripple and unbalanced magnetic pull (UMP). This dissertation investigates the torque ripple and UMP due to manufacturing errors such as uneven magnetization and eccentricities. The effects of simultaneous uneven magnetization and eccentricities are also evaluated. Firstly, the background of the dissertation is presented, which covers major manufacturing errors in mass production and excitation forces causing magnetically induced vibration and noise in a PM motor. The previous researches on characteristics of magnetic excitation forces due to uneven magnetization and eccentricities are presented. Secondly, the characteristics of torque ripple and UMP due to uneven magnetization are investigated. The uneven magnetization is classified into two categories, uneven magnetization due to non-pole pair harmonics and even pole pair harmonics in magnetization. The non-pole pair harmonics in magnetization generate slot harmonics and slot harmonics±1 of torque ripple and UMP, respectively. The even pole pair harmonics in magnetization lower main harmonic of cogging torque from the least common multiple (LCM) of pole and slot numbers to LCM of pole pair and slot numbers, and also increase the cogging torque, and torque ripple significantly. The even pole pair harmonics in magnetization do not generate the UMP. The characteristics of magnetic excitation forces without and with armature current are verified through finite element (FE) analysis. The characteristics of cogging torque due to uneven magnetization originated from the even pole pair harmonics in magnetization are also verified experimentally. Thirdly, the characteristic of torque ripple and UMP due to eccentricities are investigated. The eccentricity is classified into static, dynamic, and mixed eccentricity, which is the state that static and dynamic eccentricities exist simultaneously. The static eccentricity generates the pole harmonics in torque ripple and UMP. The dynamic eccentricity generates the slot harmonics and slot harmonics±1 of torque ripple and UMP, respectively. The mixed eccentricity generates the pole harmonic±l of torque ripple and UMP at operating condition. The relative magnitudes of the harmonics of magnetic excitation are evaluated by order of eccentricities. The characteristics of magnetic excitations are verified by FE analysis. The characteristics of UMP due to mixed eccentricity are also verified experimentally. Finally, the characteristics of torque ripple and UMP due to simultaneous existence of uneven magnetization and eccentricities are investigated. The uneven magnetization originated from non-pole pole pair harmonics in magnetization modulates the harmonic of magnetic excitation forces by -q when it simultaneously exist static and mixed eccentricities. The harmonics of excitation forces are not varied when it exists with dynamic eccentricities. The uneven magnetization due to even pole pair harmonics in magnetization generates the pole pair harmonics of torque ripple, and UMP with static eccentricity. It also generates slot harmonics and slot harmonic±1 of torque ripple and UMP with dynamic eccentricity, and pole pair harmonics±l of magnetic excitation forces with mixed eccentricity. The uneven magnetization due to even pole pair harmonics in magnetization increases the pole pair harmonics of magnetic excitation forces significantly.