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The demand for quieter machines has led to a general procedure for calculation of sound pressure levels of polyphase induction motors. The origin of the magnetic driving forces which are responsible for the motor noise, and the method of calculating the mechanical response of the motor structure, are described in this paper. A method is developed to calculate the sound pressure levels of the magnetic noise at a specified point near the motor. The paper gives numerical results of calculations, and the corresponding sound pressure levels for a particular 4-pole, 30-horsepower polyphase induction motor of open frame construction under different operating conditions. The agreement is unexpectedly good.

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