On a Method for Reducing Magnetic Vibrations and Noise in Electrical Machines
Abstract Generated abstract
This paper examines excessive vibration and noise in single-frame electrical-machine amplifiers whose armatures remained problematic after conventional two-plane balancing. Analytical and resonance tests of bending natural frequencies showed that the operating speeds were about 0.6 to 0.7 of the first critical speed, allowing distributed unbalance to produce elastic armature deflection and a rotating nonuniform air gap. The authors argue that this deflection increases magnetic-origin vibration and noise, with experiments indicating increases of about 5 to 6 dB for the studied unbalance levels. Balancing the armatures at operating speed in three correction planes using special devices reduced vibration both at rotational frequency and in higher-frequency magnetic spectral components.
Full Text
ON A METHOD FOR REDUCING MAGNETIC VIBRATIONS AND NOISE IN ELECTRICAL MACHINES
Yu. V. GORST, P. T. MALYTSEV
(Presented by the scientific seminar of the Department of Applied Mechanics)
Balancing the armatures of certain electrical machines under production conditions on standard balancing equipment with an accuracy of up to 2–3 microns did not make it possible to reduce to permissible limits the vibration level at the frequency corresponding to the number of revolutions of the armature. The balancing weights were installed in two transverse planes near the armature supports. The spectral components of machine vibration at the rotational frequency, in machines assembled with such armatures, reached 70–80 dB. Calculations by the method described in the literature [1] showed that such a vibration level at the rotational frequency may be caused by a displacement of the armature’s center of gravity from the axis of rotation by 40–45 microns. In the vibration and noise spectra, components of magnetic origin also had very high levels.
In connection with the increased requirements for the vibroacoustic characteristics of certain electrical machines, the manufacturing plant was faced with the task of reducing the levels of their vibration and noise.
Two types of single-frame EMU were studied ($n = 5400$ rpm and $n = 6000$ rpm). By installing unbalancing weights in the transverse plane of the armature, well balanced at the working rotational speed, a vibration level was observed which, in the case of rigid rotors, could be caused by an unbalance twice as large as that actually produced by the installed weight. These experiments led to the assumption that, at the working rotational speed, the armatures of the machines under study have an elastic deflection caused by unbalance.
The natural frequencies of the bending vibrations of the armatures of the machines under study were determined analytically and experimentally. The calculation of the natural frequencies of bending vibrations was carried out by the Ritz method and by the Dunkerley method [2]. The experimental determination of the natural frequencies of the armatures was carried out by the resonance method. As a result, it was established that the working rotational speed of the armatures of the machines under study is 0.6–0.7 of the first critical speed.
Thus, at rotational speeds equal to 0.6–0.7 of the first critical speed, under the action of an unbalance distributed along the length, the armature acquires an elastic deflection. In the deflection, the component corresponding to a half-wave of a sinusoid predominates [3]. Such a deflection of the armature creates a rotating nonuniform air gap, which along the length
Fig. 1. Vibration spectra: a) EMU-12 ($n = 5400$ rpm),
b) EMU-5 ($n = 6000$ rpm).
1 — after balancing on standard balancing equipment;
2 — after balancing on B-1 and B-2.
the machine changes from a maximum in the middle transverse plane to a minimum in the end planes. Therefore, the nonuniformity of the air gap arising from the dynamic deflection of the armature for a single-housing EMU design will, in a first approximation, be analogous to the nonuniformity that is formed when the armature axis intersects the axis of the stator bore.
Calculations showed that the magnitude of the dynamic deflection of the armature of the EMUs under study at operating rotational speed may exceed half the value of the nominal gap. The resulting nonuniformity of the air gap may lead to an increase in vibration and noise levels of magnetic origin by 8–9 dB, determined by formula [4]
\[ \Delta L_{\varepsilon}=40\left(\lg \frac{1}{1-\frac{1}{2}\varepsilon^{2}}+\lg \frac{1}{1-\varepsilon^{2}}\right), \]
where $\varepsilon$ is the value of the relative eccentricity.
In practice, in the case under consideration, this increase should be smaller, since as one approaches the armature supports the nonuniformity of the air gap gradually decreases. As a result of experiments it was established that, with an armature unbalance on the order of 40–45 microns, the levels of vibration of magnetic origin increase by 5–6 dB. For balancing the armatures of the electrical-machine amplifiers considered, the manufacturer used special balancing devices B-1 and B-2 [5, 6]. On these balancing devices, the armatures are balanced at operating speeds by installing balancing weights in three correction planes. Such balancing makes it possible to eliminate the dynamic deflection of the armature shaft.
It can be seen from the graph that, after balancing the armatures on balancing devices B-1 and B-2, a reduction in vibration levels is achieved not only at the rotational frequency (in the range 85–120 Hz), but also in the spectral components of magnetic origin (in the range 1920–2720 Hz).
References
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Yu. V. Gorst, P. T. Mal’tsev. The influence of armature eccentricity on magnetic vibrations and noise of a transverse-field electrical-machine amplifier. In: Power-Supply and Low-Power Electric-Drive Devices, vol. 2, Low-Power Electrical Machines and Apparatus. Energiya Press, Moscow, 1969.
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P. T. Mal’tsev, Yu. V. Gorst. A device for balancing rotors of electrical machines at operating speeds. Proceedings of VNIIEM, vol. 19. Energiya Press, 1965.