The electromagnetic force due to slot combination has a strong influence on acoustic noise from induction motors. When both the vibration mode and natural frequency of the stator correspond to those of the electromagnetic force, the stator causes a resonance which results in a loud noise. Therefore, it is necessary to estimate accurately the electromagnetic force before acoustic noise can be reduced. The force is calculated numerically by a combination of transient magnetic field FEM analysis and harmonic analysis by using a Fourier series expansion. The moving of the rotor and eddy currents are also taken into account in the FEM method. The acoustic noise from three types of motors is measured to verify the developed method. The simulated frequency and space harmonic order of the electromagnetic force are in good agreement with the measured frequency and vibration mode.
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Kobayashi et al. (1997) studied this question.
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