Acoustic noise in induction motors: causes and solutions

This paper deals with the causes of acoustic noise in induction motors and presents methods to reduce the noise before and after the motor is manufactured. The acoustic noise in induction motors has three main sources: the first one is the ventilating system which is responsible for the noise level of 2 and 4 pole-motors due to the fan speed. The second is the magnetic noise, generated by vibration of the core due to the electromagnetic forces in the airgap. The third is the noise generated by the bearings. The influence of the PWM frequency inverters on the magnetic noise is also considered and results of tests performed on motors are also presented. It is shown that a reduction in the fan diameter decreases the noise emitted by the ventilating system. Related to the magnetic noise, solutions have been adopted during design stage by stator/rotor slot design. Finally, the paper presents advantages, disadvantages and general comments about each solution and comparing noise level with parameters described in standards.