Radial Force Density Analysis of Switched Reluctance Machines: The Source of Acoustic Noise

Closer attention has been given to the acoustic noise performance of electric motors as electrified powertrains penetrate into our transportation system. In particular, switched reluctance machine introduces a new challenge to acoustic noise aspect given that the temporal harmonics of high amplitudes on the radial direction can excite the natural frequencies of the main circumferential modes. A practical understanding of the radial force density decomposition is crucial in identifying the primary source of acoustic noise at different operating points, and it is the main contribution of this paper. An analytical expression is introduced to identify the temporal harmonic orders that excite different spatial mode shapes. The combination of the circumferential and temporal harmonic orders results in a force density surface wave that travels in the clockwise (CW) or counter CW direction. The mode excitation is investigated along with the sound pressure level produced by the primary vibrating mode shapes. Acoustic noise characteristics for each mode and the corresponding natural frequency at different speeds have been analyzed by using a waterfall plot.