PROCEEDINGS
OF THE TOMSK POLYTECHNIC INSTITUTE
AWARDED THE ORDER OF THE OCTOBER REVOLUTION
AND THE ORDER OF THE RED BANNER OF LABOR
named after S. M. Kirov

Volume 284　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　1974

STUDY OF THE RELATIONSHIP BETWEEN THE PARAMETERS OF SPARK-ARC DISCHARGES ON A COMMUTATOR AND THE INTENSITY OF SPARKING IN POINTS

A. Ya. TSIRULIK, V. S. STUKACH, Z. F. IDRISOV

(Submitted by the scientific seminar of the departments of electrical machines and apparatus
and general electrical engineering)

According to GOST 183-66, the intensity of sparking on a commutator is a characteristic determined by the intensity of the light radiation of spark-arc discharges. The intensity of the light radiation is evaluated visually, which is the reason for the non-objectivity of this method of measuring sparking. Attempts are being made to develop objective methods for measuring sparking by means of instruments that record and measure the parameters of certain objective signals generated by the sparking process.

The best-known instruments are those of the II type and their latest modification, IE-1, created under the direction of M. F. Karasev and based on measuring the voltage of spark-arc discharges at the trailing edge of the brushes. In all these instruments one principle is used—the averaging of the parameters of signals from discharges on all lamellae and the obtaining of a certain integral index in arbitrary units, which is considered proportional to a definite level of sparking expressed in points on the GOST scale. From our point of view, such a principle does not correspond to the physics of the phenomenon for the following reasons:

1. The creators of instruments for measuring sparking seek, with the aid of the instrument, to measure such a signal parameter whose values could subsequently be correlated with the scale of sparking in points. This is a natural aspiration, since the sparking scale in points is the only scale provided for by the standard.

At the same time, it is assumed that the visual impression of the intensity of sparking on the commutator, when sparking is assessed visually, is obtained by a person as the result of the total light effect from all spark discharges on all lamellae of the commutator. Obviously, therefore, in instrumental methods of measuring sparking they also strive to obtain some integral characteristic of the signal averaged over the entire commutator.

In reality, as is clear from physical considerations and as the investigations we have carried out have shown, the visual assessment of sparking is determined not by the combined action of all discharges on the commutator, but only by the light effect produced by the most intense discharges occurring on certain lamellae of the commutator. If this were not so, then the visual assessment of sparking would depend on the rotational speed of the commutator. At different rotational speeds, with the same intensity of discharges, the human eye detects per unit time ...

a different number of discharges; however, the sparking intensity in points does not appear to differ. When the rotational speed is varied over a wide range, one can observe a change in the frequency of pulsations (flashes) of the spark discharges, but the level of sparking seems to remain the same.

The fact that, with very weak sparking on all bars but strong sparking on one or two bars, a person without any doubt rates the sparking intensity at the commutator with a high score is also inconsistent with the concept of the cumulative effect of all discharges. In this case the instrument will show a low sparking intensity, since one or two outstanding discharges will change the average characteristic of the signal only to a very small extent.

Investigations carried out by us on mock-ups and on machines have shown that, in the visual assessment of sparking, each class of sparking in points corresponds to spark-arc discharges of a definite intensity; moreover, the sparking class is determined by several of the most intense discharges among all the discharges occurring on different bars of the commutator.

1. The electroerosive wear of commutator bars depends on the intensity of the discharges on the bars. Each bar wears at a rate proportional to the energy of the discharges on that bar. From this point of view it is important to know not the average assessment of sparking for the whole commutator, but the sparking on individual bars, since the commutator is considered to have failed if even one of the bars has been worn by sparking to inadmissible limits. The operating life of the commutator to failure is determined by the operating life to failure of that bar which wears faster than the others and on which sparking is most intense. Thus, the service life of the commutator is determined not by the average sparking index at the commutator, but by the parameters of the most intense spark-arc discharges.

It is known that, according to GOST 183-66, each sparking class corresponds to a definite intensity of the processes of commutator destruction. This confirms the conclusion that sparking is visually assessed according to the most intense discharges. If, for example, on one commutator the discharges on all bars are equally intense, while on another such discharges occur only on two bars and on the remaining bars the sparking is weak, then, nevertheless, as follows from the foregoing, the service life of these commutators will be the same. At the same time, the integral sparking characteristic for the first commutator will be many times greater than for the second.

What has been set forth in paragraphs 1 and 2 shows that, when creating objective methods for measuring sparking with instruments, one should measure the parameters of individual spark-arc discharges and assess sparking according to the discharges of maximum intensity. This corresponds to the physico-biological nature of the process of visual assessment of sparking at the commutator and, consequently, to the scale of GOST 183-66, and at the same time corresponds to the nature of the processes of electric-spark wear of the bars. Methods will be developed for calculating the durability indicators of commutators operating with sparking. It will be necessary to measure objectively the parameters of the most intense discharges at the commutator in order to obtain the initial data for the calculation. Instruments must measure precisely these parameters, and not the average over all discharges at the commutator.

Of course, the integral indicator of the signal is related to the sparking intensity, but the relationship is correlational, not functional. The characteristics of the relationship depend on the statistical law governing the distribution of the discharge parameters on individual commutator bars. The laws of distribution on different commutators are different, and the characteristics—

connection. It is for this reason that instruments determining the integral indicator have to be calibrated visually for each machine. Instruments determining the characteristics of individual discharges will have a universal scale.

The voltage pulses at the trailing edge of the brush have a rectangular form with duration \(\tau_i\) and voltage \(U_i\).

Studies by A. E. Lifshits and others [1] have shown that “for the practically possible combinations encountered of electrode materials, medium, and polarity, the volt–time characteristic of an arc discharge remains (with an accuracy of \(\pm 10\text{--}15\%\)) parallel to the time axis, independent of the pulse duration, the voltage of the power source, and the resistance in the circuit...” Then the current in the arc varies linearly from \(i_0\) to zero, and the discharge energy is equal to

\[ W_i=\frac{L\cdot i_0^2}{2}=\frac{U_i^2\cdot \tau_i^2}{2L}, \tag{1} \]

where \(L\) is the inductance of the opened section of the armature winding.

For constant \(L\), the parameters \(U_i\) and \(\tau_i\) of the discharge-voltage pulse uniquely determine the discharge energy. The voltage magnitude \(U_i\) does not depend on the inductance of the circuit, the breaking current, the rate of opening, or the brush grade, and may be regarded as constant and approximately equal to \(12.5\ \mathrm{V}\). Then the parameter \(\tau_i\) can serve as the sole characteristic of spark discharges, which, for constant \(L\), is uniquely related to the discharge energy. The discharge durations \(\tau_i\) can be determined rather simply by oscillographing the discharge-voltage pulses.

We have determined the relation between the duration of spark-arc discharges and the intensity of sparking, expressed in points according to GOST 183-66. This was done by constructing a training sequence of values of discharge durations and the sparking levels in points assigned to them, i.e., by solving the problem of classifying discharge durations by sparking levels.

The experiment was conducted as follows. By varying the excitation current, sparking on the machine commutator was brought to the boundary between adjacent sparking classes according to visual assessment, because the boundary sparking level can be recorded visually more clearly and definitely than the sparking level within a single class. Within one class there exists a fairly broad range of possible discharge intensities, and the eye distinguishes this, but only one sparking point can be assigned.

The boundary sparking level can be recorded much more accurately when, by visual assessment, sparking is maximally intense for the lower sparking class and at the same time minimally intense for the next higher sparking class, and when such a sparking level can with equal probability be assigned to either class. The sparking assessment was made by a group of experts consisting of five people. Boundary sparking levels were established repeatedly on one machine, and the experiment was repeated on machines of different frame sizes of series P, in order to ensure reliable statistics. The boundary sparking levels established were \(1\text{--}1\frac{1}{4}\), \(1\frac{1}{4}\text{--}1\frac{1}{2}\), \(1\frac{1}{2}\text{--}2\). In what follows, for convenience we shall express sparking levels as decimal fractions. At each sparking level in each individual experiment, the voltage pulses of the spark discharges on all commutator bars were oscillographed and the discharge durations were determined.

Thus, for each boundary level of sparking, a set of values of discharge durations has been determined, distributed according to a certain statistical law.

The investigations were carried out for brushes EG4, EG63, EG74, and EG14, used in machines of series P. The sparking boundary between classes 1 and 1.25 is observed for pulse durations \(\tau_i = 0 \div 0.3\ \mu\text{sec}\). The distributions of \(\tau_i\) for other boundary levels of sparking are given in Table 1, and the distribution polygons are shown in Fig. 1.

Table 1

In Fig. 1, for brushes EG-4, EG-63, and EG-74, at sparking 1.25–1.5, all pulses with a duration of less than \(3\ \mu\text{sec}\) belong to class 1.25, while pulses with a duration of \(3\ \mu\text{sec}\) correspond to the lower boundary of class 1.5. At sparking 1.5–2, all pulses with a duration of less than \(3\ \mu\text{sec}\) belong to class 1.25; pulses with a duration of less than \(6\ \mu\text{sec}\) and more than \(3\ \mu\text{sec}\) belong to class 1.5; and pulses with a duration of \(6\ \mu\text{sec}\) correspond to the lower boundary of class 2. Class 1.25 corresponds to discharge durations of \(3\)–\(6\ \mu\text{sec}\). Class 1.5 corresponds to discharge durations of \(3\)–\(6\ \mu\text{sec}\). Class 2 corresponds to discharge durations of more than \(6\ \mu\text{sec}\). It is difficult to establish the upper boundary for class 2, since it is difficult to determine the boundary between sparking classes 2 and 3. However, this is of no essential significance, since both of these sparking classes are not permitted during prolonged operation of machines. For brush EG-14, class 1.25 corresponds to discharge durations of \(1\)–\(4\ \mu\text{sec}\), class 1.5 to durations of \(4\)–\(7\ \mu\text{sec}\), and class 2 to durations exceeding \(7\ \mu\text{sec}\). Combining the data for brushes EG-4, EG-63, and EG-74, we obtain the following classification of discharge durations (Table 2).

Table 2

In Fig. 2 are shown the boundaries of possible discharge durations (dashed lines) and the distribution polygons of the durations for each sparking class. When the sparking of machines of a certain series is evaluated visually, at a given sparking class of the machines there will be different values of discharge durations. The average value of the discharge duration for the series will be equal to the mathematical expectation of the discharge duration for the corresponding sparking class.

therefore the graph of the dependence of the sparking intensity in points on the duration of discharges is naturally drawn through the points corresponding to the mathematical expectation of the discharge duration for each sparking class, namely, 1.2 μsec for class 1.25 and 4.05 μsec for class 1.5. The sparking scale in points becomes continuous, and it becomes possible also to give a physical interpretation to intermediate numbers on the sparking scale \(I\): these numbers correspond to discharges at the commutator whose duration is not equal to 1.2 or 4.05. Since previously intermediate sparking classes were not provided for, then some arbitrariness is possible in constructing the function relating sparking \(I\)

Fig. 1. Distribution polygons of discharge durations for limiting sparking levels.

Fig. 2. Relation of discharge durations to the sparking scale in points.

to the duration of discharges \(\tau_i\). In Fig. 2 this function is specified by a straight solid line passing through the points with coordinates \(I = 1.25\), \(\tau_i = 1.2\) and \(I = 1.5\), \(\tau_i = 4.05\). It is described by the expression

\[ I = 1.14 + 0.089 \cdot \tau_i,\ \text{points}. \tag{2} \]

Here \(\tau_i\) is in microseconds. The number 1.14 corresponds to the boundary between dark commutation and the appearance of the first signs of sparking. The fact that this boundary did not turn out to be at point 1 on the \(I\) scale is not something unexpected or unnatural. At one time the numbers \(1.1\frac{1}{4}\), \(1\frac{1}{2}\), 2 and 3 as indices of sparking levels were taken quite arbitrarily.

Function (2) makes it possible to evaluate sparking objectively if the discharge parameters \(\tau_i\) at the commutator are measured by instruments. It also makes it possible to determine, approximately, the discharge parameters \(\tau_i^n\) for a known sparking level of a series of machines or of an individual machine by the formula

\[ \tau_i = \frac{I - 1.14}{0.089},\ \mu\text{sec}. \tag{3} \]

Conclusions

1. The intensity of sparking on the collector, as judged by the voltage on the collecting edge of the brushes, should be assessed not by an integral indicator determined from all discharges on all collector segments, but by the parameters of the most intense discharges on the collector.

2. The dependence of the intensity of sparking on the collector, in points, on the duration of spark-arc discharges on the collector has been determined.

References

1. A. E. Livshits, A. T. Kravets, I. S. Rogachev, A. B. Sosenko. Electropulse Machining of Metals. Moscow, “Mashinostroenie,” 1967.