Abstract :
In order to predict the noise level of a synchronous machine the paper develops equations to represent the permeance and m.m.f. variation at the stator bore. From these two equations expressions are derived which give the force variation causing vibrations in the core. The method for calculating the amplitude of, and number of nodes associated with, any particular harmonic is given. From this the loudness of the noise in decibels is obtained. Six examples are given of the application of the method to actual machines, on five of which the comparison is made between the results predicted by calculation and those obtained in service. Good agreement is obtained between calculation and observation, and it is thus possible to draw certain conclusions concerning the factors which influence the generation of open-circuit noise, in particular the objectionable noise at about slot frequency in synchronous machines. The paper shows that there are six such factors, namely (a) The stator permeance harmonics caused by the presence of the stator slots. (b) The rotor m.m.f. harmonics caused mainly by the shape and configuration of the rotor poles. (c) The pole pitch of force harmonics. (d) The frequency of the noise which depends on the number of poles and the number of slots. (e) Peripheral velocity of the noise harmonic. (f) Moment of inertia and natural frequency of the stator core. For noise to be generated, factors (a) and (b) must both be present in combination with one or more of the remaining factors. If required the method can also be used to calculate the noise spectrum over a range of frequencies. It is shown that, theoretically, skewing of the stator or rotor may reduce or eliminate open-circuit noise in a machine, but that practical difficulties restrict its field of application.
