Acoustic characteristics concerning construction and drive of axial-flux motors for electric bicycles

Ride quality, including perceptible noise and tactile vibration, is one of key considerations for electric bicycles. Featured with high torque density and slim shape, axial-flux permanent magnet (AFPM) motors fulfill most of the integration requirements for electric bicycles. Such a pancake shape construction, however, is prone to structural vibration since large axial force exerts on the stator by the rotor magnets. In this study, two conventional bicycles were modified to equip with either inrunner or outrunner AFPM motors, and induced noise concerns during riding. Measured data of phase currents, vibration and noise were analyzed by time signature, spectrum or cepstrum for both motors. Additional modal testing was performed for the outrunner motor as structural resonances occurred. Through investigations both on the motor structure and the motor drive, the major vibration and noise peaks were correlated to their excitation sources. In this study, the torque ripple induced by current control scheme was the root cause of the inrunner motor noise. The outrunner motor noise was mainly caused by the stator slotting effect and the coincidence with structural resonance. Moreover, the perceptibility of switching noise was highly linked to pulse-width-modulation switching frequency. After the comprehensive cause-effect analysis and effective remedies to refine the drive scheme or the controller´s software, we obtained a satisfactory impression of motor noise with remarkable noise reductions, 16 dB and 6 dB for the inrunner motor and outrunner motor, respectively. As a result, the operating noise at rider´s ear location was below 60 dB and fulfilled the expectations of most cyclists.