Harmonic Theory of Noise in Induction Motors

This paper presents a theory of noise generation particularly adapted to use with small motors. Since single-phase motors predominate in the small-motor field, a single-phase point of view has been employed. The underlying philosophy of the paper is to reduce the theory to simplest terms and to concentrate attention upon the most probable sources of noise. While the results are shown as a large collection of numbers representing force waves the development steps of the theory are really simple and easily understood. The resultant air-gap flux wave in an induction motor is the difference between the fundamental stator flux plus its harmonics and the fundamental rotor flux plus its harmonics. The difference between the two fundamental fluxes is usually less than the larger one and often less than either one but the harmonics, with few exceptions, are neither of the same velocity nor space order and, therefore, do not add or subtract from each other It follows that the resultant flux wave has more harmonics of much greater per-unit amplitude than does either the stator or rotor flux wave alone. It is the interaction of two large harmonic flux waves of slightly different space order and traveling at high frequency with respect to each other that causes high-frequency magnetic-force waves of long space pitch which are effective in producing noise. The theory has been used for some time in noise investigations and has been found to bear out test results on a large number of motors.