THE EFFECT OF MAGNET GEOMETRY ON ELECTRIC MOTOR VIBRATION

A principal source of vibration in permanent magnet motors and generators is the travelling forces on the stator induced by the rotating permanent magnets. These forces are transmitted through the stator and to the surrounding system. The magnetic forces were calculated from the flux density by finite element methods. The dynamic reactions at the motor mounting points, which provide the forcing function to the base system, were also calculated by finite element methods. The vibration characteristics and the transmissibility of each frequency component were investigated using Fourier decomposition of the travelling magnetic force. The results showed that for a radially centered rotor, the frequency components of the magnetic force were integer multiples of the rotor speed multiplied by the number of magnetic poles. Higher harmonies were more difficult to transmit, except when stator structure resonance occurred. The edge shape of the permanent magnet determined the shape of the magnetic force and the magnitude of the frequency components. By proper shaping of the magnetic edges, the composition of the magnetic force spectrum can be assigned to higher frequencies, reducing the overall transmission to the base system.