Undriven vibrations in brushless DC motors

This paper investigates motor vibrations due to reluctance forces. Forcing frequencies caused by motor design, part geometries, and errors in assembly are tabulated for several popular pole/slot ratios. Motor torque and translational force fluctuations are found to exhibit a rich set of shapes and frequencies, especially when multiple defects occur simultaneously. Two analysis methods are employed: the FEA approach and the convolution method. Both techniques predict similar frequency spectra. The convolution method is faster, easier to implement, and more intuitive, but less accurate