Electromagnetically Generated Acoustic Noise in DC Machines

Acoustic noise of generated in DC motors is difficult to predict, and its exact mechanism is unclear. It is a common observation that brushless DC motors with rare earth magnets are often acoustically inferior to motors of equivalent output built with conventional magnets. In this paper, the acoustic noise of electromagnetic origin is investigated using a magnetic frame which emulates a DC motor. The driving electromagnetic force is calculated using finite element analysis and the resulting vibration and acoustic noise is measured. Acoustic noise of purely electromagnetic origin was also measured from a DC brushless motor to confirm the results of the magnetic frame. The results of the study show that the mechanism of noise generation can be a quasi-static response of a stator not only at the fundamental frequency but also at higher harmonic frequencies of alternating switched DC excitation of motor phases. Noise generation is significantly aggravated when some of those harmonics match the resonant frequencies of the stator. Eddy current flow within the magnets due to the time varying electromagnetic field act as a shorted transformer secondary winding, and results in a reduction of the electrical phase impedance. This reduced impedance results in a faster rise time with a sharper current shape during transients, and increased current magnitude during steady state, thus making the motor noisier.