VIBRATIONS AND NOISES OF MECHANICAL ORIGIN IN ASYNCHRONOUS SQUIRREL-CAGE ELECTRIC MOTORS

E. A. Knippenberg

(Presented by the scientific seminar of the Departments of Electrical Machines and General Electrical Engineering)

Vibrations and noises of mechanical origin in asynchronous squirrel-cage electric motors, considered apart from the installation of the electric motor and the mechanisms coupled to it, are produced mainly by imbalance of the rotating parts and by the bearings.

It is especially important to consider these questions when it is necessary to obtain motors with low levels of noise and vibration (low-noise electric motors, high-precision electric motors). Careful balancing, and especially balancing of electric motors as an assembly, makes it possible to obtain a sufficiently small imbalance (when balancing as an assembly it can be reduced practically to zero). In this case, the components of noise and vibration of bearing origin come to the fore. With very high requirements on the level of vibration (on the order of one or several microns), the degree of influence of disturbances of bearing origin is so great that they make balancing electric motors as an assembly extremely difficult. In this case, and especially for high-speed electric motors, bearing components are one of the most intense sources of noise and vibration, the reduction of which is the most difficult.

Bearing components are caused by purely mechanical reasons. Noise and vibrations arise because of defects in the shape of the bearing races and rolling elements, which may be aggravated by errors in manufacture and assembly.

But even bearings of a very high class may produce considerable vibration and noise, since the periodic displacement of the rolling elements through the lines of bearing loading causes periodic deformation transmitted to the housing.

Even very carefully made bearings, mounted in an electric motor with misalignment or interference, cause considerable vibrations and noises (this can be explained by a change in the elastic characteristic—it may become nonlinear).

The use of sliding bearings makes it possible substantially to reduce the component of bearing noise and vibration; however, in this case (as is evident from the experimental results), vibrations often arise in the low-frequency range.

Recommendations for reducing the noise and vibration components caused by mechanical factors

Taking into account experience in designing and refining especially high-precision electric motors, special low-noise electric motors, the results of studies of high-speed electric motors for the woodworking industry and AO2 electric motors of frames 2, 3, and 4, as well as data from the literature, the following measures may be recommended for obtaining low levels of vibration and noise of mechanical origin:

1. Dynamic balancing of rotors, and, when necessary, of other rotating parts of the machine, makes it possible to obtain a sufficiently small unbalance at the fundamental rotational frequency. For special high-precision electric motors, very good results are obtained by balancing the machine in the assembled state. By using a special test stand, this operation can be performed fairly quickly.

2. When plain bearings are used, in order to reduce vibration and noise at low frequencies, special attention must be paid to ensuring strict perpendicularity of the end surfaces of parts that are in contact with one another during rotation relative to the axis of rotation of the electric motor; otherwise, considerable vibrations and noises at low frequencies, unstable in time, may appear (Fig. 1).

Fig. 1. Spectrograms of airborne noise of the AO2 32-2 electric motor at \(U=380\) V, phase connection—Y, \(f=50\) Hz. 1—with rolling bearings, 2—with plain bearings

To reduce vibration at twice the rotational frequency, it is necessary to ensure the minimum possible ellipticity of the inner surface of the bearing and of the shaft journal. Plain bearings make it possible to reduce considerably the vibrations and noises in the frequency range from 200 Hz and above.

1. When rolling bearings are used, the best results are obtained with rolling bearings of higher accuracy classes; low-noise bearings have the smallest distortions of the rolling elements. The design of the bearings and bearing assemblies must provide for the constant presence of lubricant and prevent dirt from entering it.

2. The manufacture of electric motors with high vibroacoustic characteristics is impossible when only precision bearings are used. The bearing seats must be made with maximum accuracy. Measures should be provided that exclude ellipticity and taper of the bearing seats, ne...

coaxiality of the boring axes and end runout of the supporting surfaces; failure to meet these requirements leads to a considerable increase in vibration and noise over a broad frequency range.

1. A tight fit of the outer bearing race in the bearing shield is inadmissible, since in this case, because of skews, nonuniform compression of the outer bearing race, the appearance of internal nonuniform stresses in the bearing shield and bearings, and, as a consequence, nonlinearities of the elasticity coefficients, nonuniform periodic noise and vibration of a resonant character arise.

The most favorable fit of the outer bearing ring in the bearing shield is a sliding fit, allowing the bearing to self-align.

1. The vibroacoustic characteristics of electric motors, with the obligatory presence of a sliding fit of the outer bearing race, can in many cases be improved by creating a preliminary load on the outer ring (Figs. 2, 3). However, it must be taken into account that too small a preliminary load leads to the occurrence of increased noise and vibration of a high-frequency character, while too large a preliminary load causes addit—

Fig. 2. Spectrogram of vibration accelerations of electric motor AO2 32-2SV at \(U=200\) V, connection scheme — Y, \(f=50\) Hz. 1 — with axial load, 2 — without axial load

Fig. 3. Spectrograms of vibration accelerations of electric motor AO2 32-2SV at \(U=400\) V, connection scheme — Y, \(f=100\) Hz. 1 — with axial load, 2 — without axial load

…excessive heating of the bearing and low-frequency noises and vibrations. The optimum value of the preliminary load should be selected experimentally. With an optimally selected load, a positive effect is achieved in most cases only for the frequency range from 50 Hz to 700–1000 Hz; in the range of frequencies above those indicated, the vibroacoustic characteristics deteriorate (Figs. 2, 3).

It should be noted that in many cases (Figs. 2, 3), when the vibration acceleration is reduced at certain low frequencies, the vibration accelerations at other frequencies increase so much that the overall displacement level, important for high-precision electric motors, does not decrease and may even increase.

It is well known that vibrations are undesirable especially in the low-frequency range for high-precision electric motors, whereas for low-noise electric motors, on the contrary, frequencies around 800–1000 Hz and above are undesirable. Therefore it is obvious that simultaneous improvement of the noise and vibration characteristics with one and the same preliminary load is impossible. In designing special low-noise electric motors for phonographs (with a maximum sound level of 50 dB), numerous studies have shown that the sound level in most cases is only stabilized, but not reduced.

Taking all the above into account, preliminary loading of the bearing can be recommended in some cases, but in doing so the optimum pressing force on the outer ring of the bearing must be selected experimentally.

1. Also worthy of attention are the structural materials of electric motors. The use of welded structures in electric motors is undesirable, since the internal damping of steel is 50–1000 times less than the damping of cast iron. Weak damping can cause a considerable amplification of resonance phenomena. For the manufacture of low-noise and low-vibration electric motors, the use of gray cast iron is most advisable. Owing to graphite inclusions, gray cast iron possesses exceptional damping properties. According to the studies cited in [2], the ratio of the damping properties of gray cast iron with graphite inclusions, to gray cast iron with inclusions in the form of granules, and to steel is characterized as 25:6:2.

2. A significant influence on the magnitude of vibration and noise is exerted by the lack of flatness of the feet and flange. Misalignments under rigid fastening of the feet and flange cause a large increase in the level of vibration and sound over a wide frequency range. This can be explained by the appearance of misalignments and internal stresses in the feet, frame, and bearing assemblies, as a result of which the elasticity characteristics of the structural elements become nonlinear, which can cause resonance phenomena. Therefore it is very important to ensure the greatest possible flatness of the feet and flange.

REFERENCES

1. Proceedings of VNIIEM. Vol. 20, “Energiya,” 1965.
2. Fol'kman Kh. Design and technological prerequisites for the production of low-noise three-phase asynchronous electric motors with low vibration in the power range up to 10 kW. Elektrie, No. 11, 1968.