INVESTIGATION OF SWITCHING OVERVOLTAGES IN THE WINDINGS OF ASYNCHRONOUS ELECTRIC MOTORS

Yu. P. POKHOLKOV, P. P. BESPERSTOV

(Presented by the scientific seminar of the departments of Electrical Machines and General Electrical Engineering)

Reliable operation of an electrical machine is determined mainly by the reliability of the winding insulation. Winding failures during the running-in period and during normal operation can be explained by the presence of defects in the insulation. One of the principal factors that reveal defects in winding insulation is switching overvoltages, which arise in an electrical machine during switching-on and reversing. The magnitudes of switching overvoltages in asynchronous motors have scarcely been studied, since it is assumed that switching overvoltages in low-voltage electrical machines do not have a substantial effect on turn-to-turn and ground insulation. Owing to the lack of data on the level of switching overvoltages, the selection of insulation systems presents considerable difficulties.

For the justified establishment of permissible overvoltages acting on the insulation and for the development of comprehensive measures aimed at limiting them, it is most expedient to accumulate substantial statistical material on overvoltages by carrying out measurements of switching overvoltages on a large number of different electric motors under various switching operations.

We investigated switching overvoltages arising when asynchronous motors of frame sizes 3, 6, 7, and 8, with 2, 4, 6, and 8 poles, were switched on.

The electrical circuit for investigating switching overvoltages is shown in Fig. 1. Switching overvoltages were recorded by means of a DÉSO-1 cathode-ray oscillograph. To reduce errors, the signal from the motor was fed directly to the plates of the oscillograph. For each motor, an average of 100 oscillograms of switching-overvoltage pulses was taken.

Statistical processing of the test results made it possible to determine the nature of the distribution of the magnitudes of the mean coefficients of switching overvoltages (the switching-overvoltage coefficient \(k_{\mathrm{p}}\) was taken to be the ratio of the pulse amplitude to the amplitude of the rated phase voltage).

On the basis of the test results, an approximation was made of the distributions of \(k_{\mathrm{p}}\) obtained when the motors were switched on. The distribution of \(k_{\mathrm{p}}\) is approximated by the superposition of two normal laws. The general form of the expression by means of which one can estimate the probability of the occurrence of \(k_{\mathrm{p}} > k_{\mathrm{pi}}\) is written as follows:

Fig. 1. Electrical circuit for the investigation of switching overvoltages.

\[ P(k_n > k_{ni}) = 1 - \left[ V_1 F\left(\frac{k_{ni}-\overline{k}_{n1}}{\sigma_1}\right) + V_2 F\left(\frac{k_{ni}-\overline{k}_{n2}}{\sigma_2}\right) \right], \]

where

\(V_1, V_2\) are the volume fractions of the I and II normal laws in the overall distribution of \(\overline{k}_n\) (Table 2).

\(\overline{k}_{n1};\ \sigma_1;\ \overline{k}_{n2};\ \sigma_2\) are the parameters of the I and II normal laws.

Table 1 gives the mean values of the switching-overvoltage coefficient for the motors investigated.

A decrease in the average level of switching overvoltages with an increase in the number of pole pairs for the motors studied is evidently associated with a change in the oscillatory circuits formed by the winding.

Table 1

The presence of two maxima in the distribution of \(k_n\) is due to the presence of two oscillatory circuits: one of them may be caused by the interturn capacitance and the inductance of the winding, the other by the inductance and capacitance of the winding relative to the grounded frame. As the size of the machine increases, the overvoltage ratio increases.

Approximation of the distribution of \(k_n\) makes it possible to calculate the probability of the occurrence of a dangerous overvoltage during \(\nu\)-switchings of the machine

\[ P_{\nu}(k_n > k_{n\mathrm{op}}) = 1\left[1 - P(k_{\mathrm{pop}} > k_{ni})\right]^{\nu}. \]

Table 2

The hazardous overvoltage coefficient was taken to be the coefficient calculated from the level of the switching overvoltage that produces, on one coil of a phase, a voltage equal to or exceeding the voltage \(U_v\)

\[ k_{\mathrm{haz}}=\frac{U_v \cdot n_k}{U_m}, \]

where

\(U_v\) is the average breakdown voltage of air with a thickness equal to twice the insulation thickness;

\(n_k\) is the number of coils in a phase;

\(U_m\) is the amplitude value of the phase voltage of the machine.

Conclusions

1. A method has been developed for the experimental investigation of switching overvoltages in low-voltage electric motors.

2. It has been established that, when a machine is switched on, overvoltages arise that are hazardous to the insulation of electric machines.

3. The distribution of \(k_p\) is approximated by a superposition of two normal laws. The parameters of the laws have been determined.

4. The values of \(k_p\) depend substantially on the number of poles and on the frame size of the machine. With an increase in the number of pole pairs from 2 to 8, \(k_p\) decreases by 24%. With an increase in the frame size of the machine from the 3rd to the 8th, it increases by 20%.