Influence of the number of pole pairs on the audible noise of inverter-fed induction motors: Radial force waves and mechanical resonances

This paper examines the influence of the motor pole pair number on the audible noise of electric motors for inverter operation. The number of pole pairs affects different parameters related to electromagnetic noise generation: the mode shape of the related force waves, the motor eigenfrequencies, the speed range where flux weakening is activated, as well as the frequencies of the radial force waves for a certain motor speed (and also modulation index). The radiated noise of induction motors with the same shaft height but different number of pole pairs has been measured in the laboratory and significant differences have been identified. A theoretical analysis of the excitation forces and calculations with the help of finite element analysis was performed in order to investigate the reasons for these differences.