Analytical method for modal analysis of submerged electric motor stator

Abstract

Modal analysis of the stator is important for reducing motor noise and diagnosing motor faults. An accurate analytical model is beneficial for the establishment of the relationship between the motor stator parameters and the natural frequencies, which will further aid in optimizing machine designs. Up to now, the existing modal analysis models are mainly aimed at onshore motors. When the motor is submerged in dense fluid, such as those used in the rim-driven thrusters, the vibration behaviors are quite different due to the high characteristic impedance of the fluid. In a bid to analyze the wet natural frequencies of motor stator, this paper proposes analytical models with one, three and five degrees of freedom for different application scenarios based on Donnell, Love and Soedel’s theories. The coupling between the structure and fluid is realized by the acoustic pressure load derived from the acoustic wave function. The resulting non-linear dispersion equations are then solved by Newton’s iterative method. The proposed models are applied to analyze the natural frequencies of motor stators of various sizes and with a wide range of thickness-to-radius and length-to-radius ratios. The results show that the analytical models are insensitive to the stator radius but sensitive to the thickness-to-radius and length-to-radius ratios. For thick and short motor stators, it is necessary to consider the shear deformation and rotatory inertia in the analytical model to compute the natural frequency accurately, especially when the fluid loading is taken into account.