STUDY OF PULSATIONS AND VIBROACOUSTIC CHARACTERISTICS OF PRODUCTION TRANSVERSE-FIELD EMAs AND EMAs WITH A SMOOTH ARMATURE

A. I. SKOROSPEKSHKIN, G. G. KONSTANTINOV, Yu. V. GORST

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

The development of modern automation equipment and highly sensitive measuring apparatus imposes ever higher requirements on the magnitude of pulsations and on the vibroacoustic characteristics of electrical machines. At present these characteristics are among the principal indicators characterizing the technical perfection of electrical machines.

The voltage of direct-current generators consists of constant and alternating components. The magnitude of the alternating component in most generators lies within the range of 1–2 percent, and in some generators up to 3 percent and higher. Vibrations and noise also reach significant values.

In the present work, the sources and causes of pulsations and of elevated levels of vibration and noise in transverse-field electric-machine amplifiers are investigated. Methods are proposed for reducing the principal types of pulsations and improving vibroacoustic characteristics by means of design changes in the EMA.

The investigations were carried out on a number of production amplifiers of types EMU-12 and EMU-5, both with a straight slot and with a slot skewed by one tooth pitch, and also on electric-machine amplifiers with a smooth armature developed by us [1, 2].

The principal types of pulsations that make up the alternating component of the voltage of a direct-current generator are: phase pulsation, tooth pulsation, pulsation due to anisotropy of the magnetic properties of the steel, pulsations caused by production and operating tolerances, commutation pulsations, and others [3].

The known sources of vibration and noise in electrical machines are mechanical, aerodynamic, and magnetic.

As a result of comparing the calculated frequencies of the principal exciting forces with experimentally obtained frequency spectra of pulsations, vibrations, and noise, it was established that the dominant components in the frequency spectrum of EMA pulsations are tooth and phase components, while in the frequency spectrum of vibrations and noise they are magnetic components of vibration and noise.

In this connection, the main attention in the work was devoted to finding the most rational way of reducing the level of tooth and phase pulsations, as well as the magnetic components of vibration and noise of electric-machine amplifiers.

Tooth and Phase Pulsations in a Transverse-Field EMA

Tooth pulsation is caused by transverse and longitudinal oscillations of the magnetic flux arising during rotation of a toothed armature as a result of periodic variation of the magnetic permeance of the tooth zone.

The magnetic permeance of the tooth zone depends on the mutual arrangement, shape, ratio of geometrical dimensions, and structural elements of the magnetic circuit that form the air-gap profile, such as: tooth pitch, pole-shoe width, slot-skew coefficient, slot-opening width, and the magnitude of the air gap.

The frequency of tooth pulsation is calculated by the formula

\[ f_z = z \cdot n \quad [\text{Hz}], \tag{1} \]

where

\[ \begin{aligned} z &\text{ — number of actual armature slots,}\\ n &\text{ — armature rotational speed (rpm).} \end{aligned} \]

To reduce transverse pulsations of the magnetic flux, it is recommended to skew the armature slot by one tooth pitch along the axis of the pole shoe. To reduce longitudinal pulsations of the magnetic flux, it is advisable to take the number of slots per pair of poles as odd, since in this case the change in magnetic reluctance under one pole is noticeably compensated by the change in this reluctance under the other.

All these design changes make it possible only to reduce tooth pulsations somewhat. The use of a smooth armature, however, makes it possible to eliminate completely both transverse and longitudinal pulsations of the magnetic field.

Phase pulsation arises as a consequence of the finite number of sections of the armature winding and their arrangement relative to the pole axis.

The frequency and magnitude of this pulsation are determined by the parameters of the d.c. polygon. Pulsations occur within large limits if the d.c. polygon consists of an even number of sides [4]:

\[ \frac{\Delta E}{E_{\text{av}}}\cdot 100 = \tg^2 \frac{\alpha}{4} \quad [\%]; \tag{2} \]

and within smaller limits for an odd number of sides

\[ \frac{\Delta E}{E_{\text{av}}}\cdot 100 = \tg^2 \frac{\alpha}{6}, \tag{3} \]

where \(\alpha\) is the phase angle and is determined by the formula

\[ \alpha = \frac{2\pi}{m} = \frac{2\pi t}{z}, \tag{3} \]

here \(t\) is the greatest common divisor of \(z\) and \(p\),

\(p\) is the number of pole pairs.

The frequency of phase pulsation is determined by the formulas, according to [3]:

for an even number of phases

\[ f_{\phi.\text{e}} = p \cdot m \cdot n \quad [\text{Hz}] \tag{4} \]

and

for an odd number of phases

\[ f_{\phi.\text{o}} = 2p \cdot m \cdot n \quad [\text{Hz}]. \tag{5} \]

The magnitude and frequency of phase pulsation in a transverse-field EMA with a smooth armature are determined by formulas (2, 4) and (3, 5), taking into account here that the number of phases in this case is equal to the number of sections of the armature winding.

In all amplifiers with a smooth armature, a simple loop winding with diametral pitch is used. Table 1 gives some design data for the production amplifier and the amplifier with a smooth armature, needed for calculating the frequencies and magnitudes of tooth and phase pulsations.

Table 2 gives, in relative units, the results of an experimental study of the pulsations of these amplifiers.

Figures 1 and 2 show the amplitude values of the frequency components of the spectrum of pulsations of the output voltage of the production amplifier and of the amplifier with a smooth armature at rated load and at no load.

Table 1

Table 2

The oscillogram (Fig. 3) illustrates the character and magnitudes of these pulsations at rated load. Oscillography was carried out on a two-beam oscillograph SI-18 simultaneously on both channels, with identical positions of the control knobs of amplifiers U-I and U-II.

As can be seen from Table 2 and Figs. 1, 2, and 3, in amplifiers with a smooth armature there are no tooth pulsations ($f_z = 1400$ Hz); the magnitude sharply decreases and the frequency of the phase pulsations increases ($f_\phi = 7830$ Hz). The overall level of pulsations of the output voltage, in comparison with production amplifiers with a skewed slot, decreases by almost a factor of 2, and, compared with a straight slot, by more than a factor of 5.

Magnetic vibrations and noise in transverse-field EMAs

The physical nature of vibrations and noise of magnetic origin in transverse-field EMAs is the same as in induction motors and dc machines. The cause of magnetic vibrations and noise is the variability of magnetic forces due to changes in the magnetic conductance of the air gap during rotation of the toothed armature. In addition, vibrations and noise may be caused by unilateral magnetic forces due to nonuniformity of the air gap, asymmet—

Fig. 1. Spectra of pulsations of the output voltage of the EMA at rated load

Fig. 2. Spectra of pulsations of the output voltage of the EMA at no load

…regularity in the placement of poles, brushes, windings, incorrectly selected ratios of the numbers of slots, as well as various technological factors.

Not all known recommendations [6—9] for reducing or eliminating magnetic vibrations and noise of electrical machines give positive results as applied to EMAs, and some of them, in connection

Fig. 3. Oscillogram of pulsations of the output voltage of an EMA.
1 — production EMA, 2 — EMA with a smooth armature

with the design features of a transverse-field EMA are completely unacceptable.

For example, the optimal ratios of the numbers of stator and rotor slots recommended for asynchronous electric motors are unacceptable in an EMA. The same can be said of recommendations for increasing the air gap and of a number of others. To reduce magnetic noise and pulsation losses, in [10] it is recommended to satisfy the condition under which the tooth divisions of the compensating slots and of the armature-winding slots differ from one another by \(\pm 5\) percent. But by satisfying only one condition it is impossible to achieve a significant reduction in magnetic vibrations and noise. Because of the small air gap of a transverse-field EMA, even relatively small deviations of individual parts and assemblies from their nominal dimensions and geometrical shape can substantially worsen its vibroacoustic characteristics and, in particular, lead to a sharp increase in the levels of vibration and noise of magnetic origin. But excessive tightening of tolerances in the manufacture of parts and assemblies is associated with well-known production difficulties. Therefore, it is most expedient to achieve a reduction in the magnetic components of vibration and noise of an EMA by means of design changes, which may include: increasing the air gap, shortening the winding pitch, closing and selecting the correct slot shape, skewing the slot, etc. The influence of skewing the armature slot in a transverse-field EMA on the level of magnetic vibrations and noise was considered in [11]. But it does not appear possible to eliminate these components of vibration and noise completely by introducing a slot skew by one slot pitch, since the radial magnetic forces are not eliminated; rather, they are redistributed along the length of the armature. Being in antiphase along the length of the armature, the radial magnetic forces excite torsional vibrations of the frame, the frequency of which is also equal to or a multiple of the tooth frequency.

Consequently, an indispensable condition for eliminating the magnetic components of vibration and noise of an electric machine is the elimination of their source, i.e., the toothed structure of the armature. One of the solutions

![Figure 4 graph: vibration spectrograms of an electric machine amplifier; vertical axis \(L\), dB, horizontal axis \(f\), Hz. Curves: 1—production EMA, 2—EMA with a smooth armature.]

Figure 4. Vibration spectrograms of an EMA: 1 — production EMA,
2 — EMA with a smooth armature

![Figure 5 graph: noise spectrograms of an electric machine amplifier; vertical axis \(L\), dB, horizontal axis \(f\), Hz. Curves: 1—production EMA, 2—EMA with a smooth armature.]

Figure 5. Noise spectrograms of an EMA: 1 — production EMA,
2 — EMA with a smooth armature

the question may be the manufacture of an electric machine with a smooth armature.

The results of the investigation of the frequency composition of the vibration and noise of a transverse-field EMA made with a smooth armature are presented in Figs. 4 and 5. Vibration measurements were carried out with an IVPSh-type vibrometer, and noise measurements with an Sh-63-type sound-level meter. Frequency analysis of the noise and vibrations was carried out with a PF-1-type half-octave filter and an S4-7-type spectrum analyzer. From a comparison of the vibration spectrogram (Fig. 4) and noise spectrogram (Fig. 5) of the EMA with a smooth armature and the production EMA, it is evident that the level of magnetic vibrations and noise in the EMA with a smooth armature is lower by 11 and 12 dB, respectively. It should be noted that the investigations

were carried out on an EMA of single-frame design. This leaves its imprint on the frequency composition of the vibrations and noise, since the influence of the built-in motor is felt. Measurements of vibration and noise carried out with the EMA generator disconnected showed that their spectral composition practically does not differ from the spectrograms of an EMA with a smooth armature (curves 2, Figs. 4 and 5). From the above it follows that in an EMA with a smooth armature it is possible to achieve complete elimination of magnetic vibrations and noise at the slot frequency.

Conclusions

1. In the frequency spectra of vibrations, noise, and pulsations, the dominant components are magnetic vibrations and noise, as well as slot and phase pulsations.

2. In an EMA with a smooth armature, the overall level of output-voltage pulsations, in comparison with a production EMA with a skewed slot, is reduced almost twofold, and in comparison with a straight slot—by more than five times.

3. In the frequency spectrum of pulsations of an EMA with a smooth armature, the slot component disappears, while the amplitude of the phase component sharply decreases and its frequency increases.

4. In an EMA with a smooth armature, vibrations and noise of magnetic origin are completely eliminated.

REFERENCES

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2. A. I. Skorospeshkin, G. G. Konstantinov. Some results of a study of the commutation characteristics of a transverse-field EMA with a smooth armature. Izvestiya TPI, Vol. 160, 1967.

3. I. M. Vodyakho. Phase pulsation. Elektrichestvo, No. 10, 1966.

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