Vibration reduction in electric machine by magnet interlacing

A principal source of vibration in permanent magnet motors and generators is the traveling attractive forces from the rotating permanent magnets acting on the stator. This magnetic force is calculated from the flux density by finite-element methods and the Maxwell stress tensor, and the characteristics of the frequency components are investigated by means of a Fourier decomposition of the traveling magnetic force. Various parameters are investigated for their effect on this forcing function. It is concluded that the amplitudes of the driving frequencies increase as the pole separation increases. Radial magnetization produces a more uniform radial flux density along the permanent magnet, which reduces not only the torque ripple but also the amplitudes of the driving frequencies. This effect can be greatly enhanced by an interlacing of higher energy magnets and a changing of the magnetic orientation at the pole transitions