Electromagnetic Vibration Analysis Of Permanent Magnet Synchronous Motor

In a permanent magnet synchronous motor(PMSM), a strong electromagnetic force exists between the rotor magnets and the stator teeth, having components in both the tangential and radial directions. The tangential force will result in a constant toque and a torque ripple. The radial force directly causes mechanical deformation and vibration of the stator. In small and middle PM motors, vibration is mainly from electromagnetic force of stator. In the dissertation, electromagnetic vibration induced by electromagnetic force will be addressed. The main contents are as follow:1. A weak-coupling electromagnetic and structure FE(finite element) model for PMSMs is built utilizing the APDL(Ansys Parametric Design Language). After the structure parameters and the load are set, the electromagnetic force of the motor can be automatically solved, and transferred to the structure FE model. The vibration of stator is obtained through the transient structure FE method. To validate the simulation model, the predicted and measured back-EMF and vibration spectrum are compared for a PMSM with the 12 slots and 8 poles. And the influence of electromagnetic force harmonics having the different mode number on the vibration is also investigated by the structure FE model. Natural frequency of the stator is the important parameter in the vibration analysis. A FE model to analyze the modal of motor stator is built. The effects of stator structure parameters on natural frequency are analyzed. The modal test for the motor is conducted to compare to the predicted results.2. The force harmonics with the low mode number can induce the high machine vibration, especially for PMSMs with the fractional slot combination. The general expression of the frequency and corresponding mode number of radial force harmonics for PMSM is formulated. The lowest mode number of radial force is given for PMSM with the different winding configuration. The electromagnetic vibration of ten motor types with different slot/pole number combinations, such as 9/6,18/6,36/6,12/8,18/8, 24/8,36/8,48/8,12/10,15/10, is investigated. The expressions of the mode number and frequency of radial force are validated. The predicted vibration for ten motors is compared. It can be obtained that the motor having the lower mode number force harmonic tends to the higher vibration level.3. The torque ripple and two radial force harmonics with the frequency 8f1(f1 is the rotational frequency of motor) and the mode number being 4 and 8 for the 12-slots 8-poles motor have a significant impact on vibration level. The effect of the structure parameters on the torque ripple and two radial force harmonics is studied. Those parameters include magnet pole-arc angle, magnet pole eccentric distance, slot opening width, slot opening depth and bridge width etc. It can be gotten that when magnet pole-arc angle is increased, torque ripple and the amplitude of two force harmonics under consideration can be decreased. Increasing magnet pole eccentric distance is a good method to eliminate torque ripple. But it leads to increase the amplitude of (8.8f1) force harmonic. Its influence on (4,8f1) force harmonic is dependant on pole-arc angle. If slot opening width is small, torque ripple and the amplitude of two force harmonics becomes small. When slot opening depth is increased, torque ripple and the amplitude of (8,8f1) force harmonic become also large, and the amplitude of (4,8f1) force harmonic decrease. Increasing bridge width is helpful to mitigate torque ripple and the amplitude of two force harmonics. However, it leads to increase leakage flux of motor and reduce output torque.4. In PMSMs with fractional slot combination, its electromagnetic vibration is generally larger than that of integer slot combination motors, because its force harmonics with the lowest mode number have a major impact on electromagnetic vibration. A method, eliminating the harmonic with the lowest mode number, is proposed to suppress electromagnetic vibration. For the vibration with the frequency being equal to 4f1, the electromagnetic force distribution and electromagnetic vibration of three motors with 12-slots 8-poles,24-slots 8-poles and 12-slots 10-poles respectively are analyzed. It is illustrated that the proposed method is suitable to the fractional slot motor while not to the integer slot motor. Furthermore, to mitigate the vibration with the frequency being 16f1, the method injecting the 5th current harmonic with the suitable phase angle is proposed. The validity of the proposed method is confirmed by both simulated and experimental results.