European Transactions on Electrical Power Engineering vol:5 issue:1 pages:5-13
The audible noise produced by three-phase, squirrel-cage induction motors, is caused by the space harmonics of the flux-density distribution in the air gap of the machine. When the motor is supplied by a frequency inverter in order to control the speed, time harmonics in both current and voltage are present, yielding new components in the audible noise spectrum. In the paper; a number of motor-inverter combinations are compared for medium-sized induction motors (90 kW). As a reference case, the motors are supplied with a generator, delivering a variable frequency without harmonics. Theoretical explanations for the increase of the audible noise level and the additional frequency components are given.