Predetermination of Sound Pressure Levels of Magnetic Noise of Polyphase Induction Motors [includes discussion]

This paper gives a general procedure for calculating in decibels the slot frequency sound pressure level produced by a polyphase induction motor operating at any arbitrary speed and load. To do this, it is necessary, first, to calculate the frequencies, pole numbers, and magnitudes of the important air-gap magnetic fields; second, to calculate from these the frequencies and magnitudes of those air-gap force waves with relatively few nodes that are most likely to produce noise; third, to calculate the stiffness and resonant frequencies of the core and frame structure for vibrations of low numbers of nodes; fourth, from all these, to calculate the radial deflections of the stator core and frame that are due to the significant slot frequency force waves; and, fifth, to calculate the decibel level of sound pressure at any point in space that is caused by the calculated deflection of the frame. After outlining the procedure required for these five steps in some detail, the paper gives the numerical results of calculations and the corresponding test sound levels for a particular 4-pole, 30-horsepower polyphase induction motor of open-frame construction. Calculations and tests were made for six operating conditions with different loads, voltages, and line frequencies. The calculated values for the six conditions were 86, 88, 91, 75, 62, and 82 decibels respectively, and the corresponding test values were 88, 90, 94, 74, 64, and 85, showing excellent agreement.