A rotor and a stator of an outer rotor motor often resonate with electromagnetic force working between them. Sometimes resonating rotor and stator cause irritating magnetic noise. When they make noises with their resonance, noise from the stator shows its resonance frequency. However, noise from the rotor shows different frequency from its resonance frequency. To clarify the mechanism of the phenomenon, surface vibration of the rotor during rotation was measured by using cylindrical microphone array that covered the entire rotor and was calculated by using advanced non-stationary conformal mapping technology by Equivalent Source Method (ESM) approach. As a result, it was found that the rotor is resonating with rotation and the amplitude of air vibration is modulated on the surface of the rotor.
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