Investigating and controlling noise of axial-flux motors for electric bicycles

Featured with high torque density and slim pancake shape, axial-flux permanent magnet (AFPM) motors fulfill most of the integration requirements for electrified vehicles. AFPM traction motors as the core of propelling power, however, may cause noise annoyance and degrade ride quality if improperly designed. A modified electric bicycle equipped with an AFPM motor was used as the research platform. The developing AFPM motor has high operating noise problem with irregular abnormal noise occurrences. This study investigates the effects of structural and motor drive designs on the noise generated by AFPM motors. The noise sources and their potential transfer paths were addressed. The relationships between the electromagnetic related frequencies and dominant noise frequencies were fully explored. In addition to examining electromagnetic frequencies, manipulating waveforms of phase currents was proposed as an effective countermeasure for noise control. Data on noise, vibration, motor structural design and motor drive are correlated and explored by using time signature, spectrum and cepstrum analyses. By establishing an index ?ph that is related to phase currents, this study showed excellent correlation between the index and the abnormal noise occurrence. Validation tests showed the motor drive scheme relating with current waveforms and PWM switching frequency was a crucial attribute to the motor vibration and noise. Furthermore, permeance variations because of the stator slotting effect caused high operating noise. With comprehensive cause-effect analysis and the effective remedies, this study reduced the operating noise from 65 dB to below 60 dB and eliminated the abnormal noise.