Noise and Vibration Analysis due to Electromagnetic Excitation in PM Motors

Abstract

The noise and vibration of permanent magnet (PM) motor systems are important specifications. Noise and vibration arising from an electromagnetic excitation have the characteristics of discrete frequencies which are proportional to operating speed of PM motors. Prominent discrete harmonics of vibration and noise are increasingly regulated as an important indicator in evaluating the vibration and noise in PM motor systems. Electromagnetic excitations of PM motors are ripples of torque and unbalanced magnetic force (UMF). This dissertation investigates the effects of four major electromagnetic sources on noise and vibration of a PM motor system: Rotor eccentricity, axial electromagnetic asymmetry, and the additional harmonics of the driving current and connecting wires between the slot windings. Firstly, the background of the dissertation is presented, which covers the characteristics of noise and vibration in PM motor systems and the electromagnetic excitations of PM motors. Secondly, this dissertation investigates the characteristics of torque and UMF generated by the rotor eccentricity of PM motors, and their effects on the noise and vibration of PM motor systems. Rotor eccentricity generates slot number harmonics of cogging torque and the harmonics in slot number±1 of the UMF in non-operating conditions. In the operation of a PM motor, the harmonics of the commutation frequency of the torque ripples (obtained through multiplying the pole and phase) are increased by the interaction of the driving current and the rotor eccentricity. The major excitation frequencies of the UMF during operation are also the harmonics of the slot number±1, and the harmonics of the commutation frequency±1. It is verified through experimentation that the source of the harmonics of slot number±1 and the harmonics of the commutation frequency±1 in the noise and vibration are the UMF caused by the rotor eccentricity. Thirdly, this dissertation investigates the characteristics of axial UMF generated by the axial electromagnetic asymmetry of PM motors, and their effects on the noise and vibration of PM motor systems. The axial electromagnetic asymmetry of PM motors generates the axial UMF ripples with the least common multiple (LCM) harmonics for the pole and slot numbers. The axial UMF can be eliminated by the optimal positioning of a permanent magnet with respect to the stator core. It is experimentally verified that the LCM harmonics of the pole and slot in the noise and vibration mostly originate from the axial UMF ripple, and the proposed design can effectively minimize the LCM harmonics in the noise and vibration of PM motor systems. Fourthly, this dissertation investigates the characteristics of the torque and UMF generated by the driving current of the PM motors, and investigates their effects on the noise and vibration of PM motor systems. The additional harmonics of the driving current generates electromagnetic excitations in PM motor systems. UMF is not generated in an ideal PM motor, even when the driving current has additional harmonics. However, the additional harmonics of the driving current generate the additional UMF harmonics once a PM motor develops stator or rotor eccentricity. It is experimentally verified that the electromagnetic excitations originating from the additional harmonics of the driving current affect the noise and vibration of a PM motor system. Finally, this dissertation investigates the characteristics of torque and UMF generated by the connecting wire between slot windings of PM motors, and investigates their effects on the noise and vibration of PM motor systems. The connecting wires between slot windings generate pole number harmonics in the torque ripple and UMF. Thus, a novel pattern for the connecting wire, where all the coils are located at rotationally symmetrical positions, is proposed to eliminate the pole number harmonics of the torque ripple and UMF induced by connecting wires. The experimental findings show that the proposed pattern can effectively reduce the magnitude of the pole number harmonics for the noise and vibration of a PM motor.