IMPULSE RESISTANCE OF ENAMELED WIRES

Yu. I. Linin, V. M. Anikeenko

(Presented by the scientific seminar of the Department of Electrical Insulation and Cable Engineering)

The wide use of enameled wires with high-strength enamel insulation in the windings of electric machines has led to the emergence of a number of requirements concerning their electrical-insulation and physicomechanical properties. These requirements take into account the influence of a complex of factors acting on the insulation of wires during machine operation. These include, mainly, mechanical effects: vibration, dynamic compressive forces, displacement relative to one another, the thermal effect of electrical voltage, the magnitude of which is determined by the continuously acting phase or turn-to-turn voltage. A more complete study of the operation of electric-machine insulation shows that the insulation is subjected to the action of increased impulse voltage arising at the moment of starting and stopping, or reversing, machines. The magnitude of overvoltages can reach significant values.

Thus, the need naturally arose to study the resistance of enameled wires to the action of voltage impulses.

We carried out a study of the insulation resistance of enameled wires of the PETV-TS and PETV-F-35 grades to the action of impulse voltage of various amplitudes, as well as the change in resistance depending on the number of coatings and on the eccentricity of the insulation. Samples of enameled wires in twists 125 mm long were subjected to impulse voltage; the number of twists was determined from the relation

\[ n=\frac{7.4}{d_{\text{ins}}}, \]

where

\(n\) — number of twists;
\(d_{\text{ins}}\) — diameter of the enameled wire over the insulation, in mm.

As the source of impulse voltage, a GIN was used, making it possible to obtain up to 50 impulses per second, with a nonstandard wave shape. Wire samples were subjected to impulses with amplitudes of 8, 7, 6, 5, and 4 kV.

Depending on the number of passes during enameling, the resistance to impulse voltage is presented in Table 1.

The results given in the table show a significant increase in the resistance of enameled wires to the action of impulses with an increase in the number of passes, i.e., in the layering of the insulation. Moreover, the influence of the number

passes becomes especially pronounced as the amplitude of the repeatedly recurring pulses is reduced.

Table 1

The presence of such defects in the insulation as eccentricity of the insulating layer leads to a considerable reduction in the pulse withstand strength of the wires. This is clearly shown in Table 2.

Table 2

When comparing the results given in Table 2, it is evident that the reduction in the average number of pulses withstood by the insulation

of enameled wires with 15% eccentricity, compared with the original wires, is 46.7% at a pulse amplitude of 8 kV, and 86.9% at 25%.

This comparison shows that the presence in the insulation of such defects as eccentricity, point damage, and air inclusions leads to a reduction in the resistance of the insulation to repeatedly recurring overvoltages.

When comparing the resistance of enamel insulation to the action of voltages with a frequency of 50 Hz and to a pulse voltage equivalent in amplitude on PETV-F-35 wires (Table 3), it is evident—

Table 3

—that the pulse voltage causes damage in the insulation 2.05 times more intensively than a voltage with a frequency of 50 Hz. This phenomenon must be taken into account when predicting the service life of electrical machines.