Electromagnetic noise radiated by brushless permanent magnet DC drives

The paper describes an investigation into the electromagnetic noise radiated by a brushless permanent magnet DC drive, based on analytical techniques for predicting the electromagnetic, vibrational, and acoustic behaviour. Theoretical and experimental investigations are carried out on a 4-pole, 12-slot, 3-phase, surface-mounted magnet brushless DC motor, for which instantaneous open circuit, armature reaction and resultant airgap field distributions are computed and compared with corresponding finite element predictions. The effect of frame overhang as well as the end shields on the natural frequencies of the stator are investigated, and predicted dominant components of acoustic power radiated by the motor are validated against measurements made in an anechoic chamber. Finally, the effects of asymmetry in the magnetisation of the rotor magnets and/or rotor eccentricity on the noise radiation are investigated.