STUDY OF THE PULSE RESISTANCE OF INSULATION OF ENAMELED WIRES

V. M. Anikeenko, V. V. Leskov

(Presented by the scientific seminar of the Department of EIKT)

The insulation of electrical machines and apparatus during operation is subjected to the action of electrical overvoltages that have a pulse character and arise at the moments of switching on, reversing, and switching off. The amplitude, duration, and shape of the overvoltage waves that arise will depend not only on the parameters of the motor windings, but also on the type of machine, the supply network, and also on the cause of the change in operating mode (that is, switching on, reversing, or switching off).

The magnitude of the overvoltage amplitudes may reach values up to \((4 \div 6) U_{\text{n}}\) [1], and in some cases will correspond to the breakdown voltage of the turn insulation. Repeated action of overvoltage pulses will cause electrical aging of the insulation, with all the ensuing consequences [2].

In this connection, the study of the resistance of turn insulation, in particular the enamel insulation of wires, to repeated action of pulse voltage is of definite interest. To study the repeated action of pulse voltage on enamel insulation, a wire of the PETV-\(F\)-35 grade with a diameter of \(0.83\) mm was used. The study was carried out on samples of enameled wire made in the form of a twist \(125\) mm long. To reduce ionization processes in the air gaps directly between the wires, the samples (twists) were additionally coated with enamel varnish, followed by drying in a thermostat.

The source of pulse voltage was a pulse-voltage generator with a pulse repetition frequency of up to 50 per second. To compare the degree of action of the amplitude and duration of the pulse voltage, the samples were subjected to pulses with amplitudes of 2, 4, 6, and 8 kV, with front durations of 3 and 2 μsec and a total wave length of 12 μsec.

The dependence of the pulse resistance of the insulation, that is, the number of pulses withstood by the insulation before breakdown, on the amplitude value of the voltage \(U_a\), is shown in Fig. 1. As the amplitude of the pulses increases, the resistance of the insulation continuously decreases, and the rate of decrease increases as the amplitude grows. The results were processed using methods of mathematical statistics. In Fig. 1 the dependence \(\ln n_{\text{imp}}\) as a function of \(U_a\) is presented for 50% and 95% probability of failure of the samples.

Fig. 1. Dependence of \(\lg n_{\text{imp}}\) on the magnitude of the amplitude of the impulse and alternating voltage. 50 and 95% probability of specimen failure: 1 — impulse front duration \(3\,\mu\text{s}\); 2 — alternating voltage, \(f = 50\ \text{Hz}\).

Axes: \(U_a\) (kV); \(\lg n_{\text{imp}}\).

The resistance of enamel insulation to the action of an alternating voltage of frequency 50 Hz is shown in Fig. 1. The dependence presented, \(\lg n_{\text{imp}} = f(U_a)\), shows a greater effect of impulse voltage in comparison with alternating voltage, by approximately a factor of 1.05–1.08. The influence of the wave-front duration is presented in Fig. 2. Reducing the wave front

Fig. 2. Dependence of \(\lg n_{\text{imp}}\) on the front duration of the impulse voltage for 50 and 95% probability of sample failure: 1 — front duration 3 μsec, 2 — 2 μsec

from 3 μsec to 2 μsec leads to a decrease in impulse resistance over the entire range of acting voltage magnitudes. Consequently, the service life of the windings of electrical machines and apparatus will to a significant extent be determined not only by the amplitude of the overvoltages that arise,

Fig. 3. Influence of temperature on the impulse resistance of enamel insulation: 1 — temperature 20°C, 2 — temperature 50°C, 3 — temperature 100°C

but also by the spectrum of the forms and, probably, the repetition frequencies of the impulses. This circumstance should be taken into account when calculating the reliability of electrical machines.

During the operation of electrical machines, the temperature of the windings is in many cases lower than the long-term permissible temperature for the given insulation class. In this connection, to study the effect of temperature on pulse resistance, tests were carried out at 20, 50, and 100°C. The test results are presented in Fig. 3. Thus, increasing the temperature from 20 to 100°C causes a decrease in the pulse resistance of the insulation.

Mathematical processing of the test results made it possible to establish that the character of the distribution of sample failures under multi-pulse exposure is described by the Weibull law. Agreement between the experimental distribution and the theoretical one was checked using probability paper and the Kolmogorov criterion [3, 4].

Conclusions

1. As the pulse amplitude increases, the pulse resistance of enamel insulation decreases.

2. An increase in temperature, as well as a decrease in the duration of the wave front, is accompanied by a decrease in pulse resistance.

3. The character of the distribution of sample failures under multi-pulse exposure obeys the Weibull law.

References

1. I. A. Kozyrev. Insulation of Electrical Machines and Methods of Testing It. “Energiya,” 1962.

2. S. N. Koikov, A. N. Tsikin. Electrical Aging of Solid Dielectrics. “Energiya,” 1968.

3. A. K. Mitropolsky. Techniques of Statistical Computation. “Nauka,” 1971.

4. Ya. B. Shor. Statistical Methods for the Analysis and Quality Control of Reliability. “Sovetskoe Radio,” 1962.