Analysis of Electromagnetic Noise Mechanism in Internal/External Rotor Axial-Flux Motors Considering Magnetic Field Modulation Effect

The mechanism of the electromagnetic noise in internal and external-rotor axial-flux motors is analyzed in this study, considering the magnetic field modulation effect. First, the spatiotemporal property of axial electromagnetic force (EMF) exerted on the stator teeth and permanent magnets (PMs) with the magnetic field modulation effect is derived analytically. Then, a numerical model is introduced to calculate the electromagnetic noise of the axial-flux motors. The magnetic field modulation effect on the mechanism of the electromagnetic noise in internal and external rotor axial-flux motors is revealed through comparison with different pole/slot combinations. Finally, the results are verified by the noise test. The results show that the high-order and 0-order EMF are the prime sources of acoustic noise in axial-flux motors, and their source becomes more abundant due to the modulation. In some conditions, the influence of the high-order EMF on acoustic noise is greater than the 0-order EMF. The primary source of noise, whether high-order or 0-order EMF, is determined by the specific type of motor, the pole/slot combinations, and operating conditions. This article provides guidelines for the design of different types of low-noise axial-flux motors.