Natural frequencies of stators of small electric machines

In designing electric machines for low vibration and noise levels, the accurate calculation of the natural frequencies of the stator is of great importance, since it is essential to avoid coincidence between these frequencies and those of the exciting forces. Existing methods give unsatisfactory results for small and medium-sized 2-pole machines, owing to their high ratio of core thickness to mean radius of the stator core. As an extension of earlier work, the paper presents a method of calculating the natural frequencies of a stator having a thick core loaded with teeth and windings coupled solidly through ribs (key bars) to an outer thin frame, taking into account bending, shear, extension and rotary inertia. Analytical results are compared with experimental results, and good agreement is obtained.