Experimental-Analytical Determination of the Durability of Collectors of Series "P" Electric Machines
Abstract Generated abstract
This paper estimates the durability of commutators in P-series electric machines of insulation heat-resistance classes B and H, using operating-time data from plant reliability tests. The authors combine statistical evaluation of machine service life with design-based wear allowances, measured commutator wear, and the expected number of possible undercuttings before complete wear. The resulting calculated commutator lives are 225,000 hours for class B machines with plastic-housed commutators and 122,000 hours for class H machines with steel-housed commutators, under proper operation of the commutator and brush assembly. The study concludes that these commutators, made of M-1 copper and used with EG-74 and EG-74K brushes, do not appear to limit the service life of the machines considered, although aging of insulating materials was not included in the calculation.
Full Text
DETERMINATION BY AN EXPERIMENTAL-ANALYTICAL METHOD OF THE DURABILITY OF COMMUTATORS OF P-SERIES ELECTRICAL MACHINES
A. N. Nimirovskii, I. A. Volkomirskii
(Presented by the scientific seminar of the Department of Electrical Machines)
The durability of commutators was determined for machines of the “P” series with insulation heat-resistance class B and commutators in a plastic housing, and class H with commutators in a steel housing.
In determining durability, statistical data on the operating time of the above-mentioned machines, obtained in the plant’s reliability laboratory, were used.
In the course of testing and calculation, the following quantities were determined:
a) the average service life of the machine for each heat-resistance class;
b) the total wear of the commutator;
c) the amount of commutator undercutting;
d) the number of undercuttings before complete wear;
e) the service life of the commutator until complete wear.
The average service life of the machines was determined from the results of testing \(n\) machines and the subsequent calculation of its root-mean-square value. In this case \(n = 14\) machines. The average service life of the machine was: for class B, 17,910 hours; for class H, 7,570 hours.
The amount of complete wear of the commutator was determined from its design data and was, respectively, for class B 3 mm on the radius, and for class H 4 mm on the radius.
The amount of commutator undercutting was determined as the sum of two terms, of which the first is the value of commutator wear along the radius over the testing time (from the test data it ranges from 0.03 to 0.13 mm), and the second is the value of the technological allowance necessary to obtain the specified surface finish of the working surface of the commutator (in practice it is taken to be of the order of 0.15–0.25 mm).
The number of undercuttings was accordingly determined as the quotient obtained by dividing the amount of commutator wear by the amount of commutator undercutting. Thus, for class B the average number of undercuttings was 13, and for class H—16.
Knowing the number of undercuttings and the average service life of the machine over the testing period, we calculate the total service life of the commutators of both heat-resistance classes.
The service life of the commutators until complete wear was determined as the product of the number \(N\) of commutator undercuttings by the average service life \(L_{\mathrm{sr}}\) of the machine:
\[ L = N \cdot L_{\mathrm{sr}}, \]
where
\[ L_{\text{av}}=\overline{X}-\frac{t_p}{\sqrt{n-1}}\cdot S, \]
\[ S=\sqrt{\frac{1}{n}\cdot\sum_{i=1}^{n}(X_i-\overline{X})^2}, \]
\(X_i\) is the operating time, in hours, of the \(i\)-th commutator before turning,
\(\overline{X}\) is the mean value of the operating time of \(n\) commutators.
The values of \(\dfrac{t_p}{\sqrt{n-1}}\) are determined from Table 1 for a confidence probability \(P=0.99\);
Table 1
| \(n\) | \(\dfrac{t_p}{\sqrt{n-1}}\) |
|---|---|
| 3 | 4.93 |
| 4 | 2.62 |
| 5 | 1.87 |
| 6 | 1.51 |
| 7 | 1.29 |
| 8 | 1.14 |
The service life of the commutators amounted to 225,000 hours for class B and 122,000 hours for class H.
Conclusions
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Provided that the operating rules for the commutator-and-brush assembly are observed, the theoretical service life of commutators to complete wear is 225,000 hours for heat-resistance class B and 122,000 hours for heat-resistance class H.
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The assumption made in the statistical evaluation of the average service life of electrical machines of both heat-resistance classes yielded somewhat understated results for the service life of commutators, since the machines continue to operate.
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The calculation was carried out without taking into account the aging of insulating materials and, consequently, without taking into account its effect on the mechanical strength of the commutator as a whole. In view of the current absence of a method for calculating the mechanical strength of a commutator with allowance for the influence of insulation aging, this can be refined only after lengthy experimental checks.
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The data obtained give grounds to assert that commutators made of M-1 grade copper, in combination with EG-74 and EG-74K brushes, can ensure—by a considerable margin above that required by the technical specifications for these machines—the resource and service life, provided that maintenance personnel observe the operating rules for the commutator-and-brush assembly.
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The view that has existed up to now, namely that the commutator assembly limits the service life of electrical machines of the type under study, is not confirmed by the results of bench tests and inspections of products at the consumer’s site.