INFLUENCE OF THE LEVEL OF TURN-TO-TURN INSULATION CONTROL ON THE RELIABILITY OF RANDOM-WOUND WINDINGS OF INDUCTION MOTORS

Yu. P. Pokholkov, E. K. Strelbitskii

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

The share of failures due to breakdowns of the turn-to-turn insulation of random-wound windings of induction motors during the running-in period amounts to 60–80% of the total number of failures [1]. Breakdown of insulation that has not aged is caused by the presence of hidden defects in the windings that are not detected during tests.

Existing methods for checking the turn-to-turn insulation of windings of induction motors (SM-1, SM-2 apparatus) involve testing the insulation at an extremely low voltage—36–55 volts per phase. Such a voltage makes it possible to detect only metallic short circuits and does not reveal other weak points. At a phase voltage of 220 V, the probability of a 2.5 kV overvoltage occurring in every 1000 hours is equal to 1 [2, 3].

In the present work, a method is proposed for testing the turn-to-turn insulation of random-wound stator windings of induction motors with a high voltage of increased frequency.

The method is based on the principle of inducing a high voltage in the stator winding.

A low voltage of increased frequency (1000 Hz) is generated by a high-frequency generator and is supplied to the three-phase winding of a rotor inserted into the stator. In this case the rotor winding is connected in star, while the stator winding is open. Thus, the circuit constitutes a transformer with three secondary windings. The transformation ratio is selected so as to obtain a voltage of 2.5 kV on a phase of the stator winding. If there are no defects in the insulation, the load in all three phases of the rotor winding is symmetrical. The appearance of a turn-to-turn short circuit in one of the phases of the stator winding is equivalent to the connection of an additional secondary winding of the transformer operating in short-circuit mode. This leads to asymmetry of currents in the rotor circuit and makes it possible to detect the turn-to-turn short circuit.

It should be noted that the number of poles of the rotor winding must correspond to the number of poles of the stator winding. The air gap between the rotor and the stator must be minimal in order to reduce the no-load current. To prevent rapid overheating of the rotor iron by high-frequency currents, the core is assembled from E-44 steel.

An installation of this type was manufactured and tested at the Sibelektromotor plant. Tests of the turn-to-turn insulation of unimpregnated

windings of AO-32/4 and AO-32/4A machines were carried out after they had passed shop inspection.

One of the tasks in raising the level of turn-insulation inspection is to establish the optimum test voltage. To this end, we determined the dependence of turn-insulation rejects on the voltage in one phase of the winding.

Table 1 gives the test results. At each voltage, 60 stators were checked.

Table 1.

The reject percentage practically begins to increase only at a voltage of 1700–1800 V. This is explained by the fact that the breakdown voltage of the air gap between two defective conductors has a certain minimum; at normal pressure it cannot be less than 400–450 V [4]. In our case, when the winding has 4 sections, the maximum possible voltage acting between two conductors in a section is equal to \(0.25 U_{\phi}\).

Thus, defects in a section began to be detected at voltages

\[ U_{\text{sec}} = 0.25\,U_{\phi} = 0.25 \cdot 1700 = 425 \text{ volts}, \]

which agrees with theoretical concepts. At a voltage below 1700 V per phase, only metallic short circuits can be detected in the winding.

In general, increasing the test voltage leads to an increase in the reject fraction; however, the relative increase in rejects at voltages above 2500 V per phase decreases substantially. This is associated with the completion of detection of the bulk of the defects. Thus, the experimental data, as well as consideration of service conditions, make it possible to assume that for random-wound windings the level of the test voltage (per phase) should be no less than \((0.4\text{–}0.45)n\) kV, where \(n\) is the number of individual sections in a phase.

In determining the reliability of the turn insulation of windings during the running-in period, for one batch of machines one may use the expression [3]

\[ R = 1 - \frac{Q_{\text{pot}} - Q_{\text{zav}}}{1 - Q_{\text{zav}}}, \]

where \(Q_{\text{pot}}\) is the potential failure rate of the turn insulation during the running-in period (calculated from the mathematical reliability model [3] or taken from average statistical data),

\(Q_{\text{zav}}\) is the reject fraction during tests at the manufacturing plant.

The reliability of windings after testing them with a voltage of 2500 V per phase is 0.986 (data for a winding impregnated with varnish 321); at the same time, the reliability of windings after testing them with the SM-1 apparatus is only 0.937. Consequently, raising the level of turn-insulation inspection makes it possible to reduce the failure rate of machines during the running-in period by a factor of 4.5.

To verify the effectiveness of this method of turn-insulation inspection, accelerated tests of machines for service life and reliability were carried out. The test results are given in Table 2.

Table 2

These results show that the detection of hidden defects during testing reduces the intensity of failures not only during the running-in period, but also during the period of physical wear of the winding insulation.

Consequently, even minor defects not detected during testing gradually develop in operation and lead to failure more rapidly than defects that arise from aging of the insulation during the operation of the machine.

Conclusions

1. The existing level of testing of turn insulation is insufficient and does not reveal the majority of winding defects.

2. It has been established that the magnitude of the test voltage for the turn insulation of random-wound windings must be no less than 400 V per section.

3. The proposed method for testing the turn insulation of random-wound windings makes it possible to reduce the failure rate of windings during the running-in period by no less than a factor of 4.5 and to increase the service life of windings by more than a factor of two.

References

1. Development of recommendations for improving the reliability and efficiency of electrical machines. VNIIEM report OAB 143.834, 1965.

2. Z. G. Kaganov. Internal overvoltages and other effects on the turn insulation of electric motors. Testing of the turn insulation of electrical machines. Collection of articles, Gosenergoizdat, 1959.

3. Yu. P. Pokholkov. The influence of winding-and-insulating operations and the quality of insulation on the reliability of windings of electrical machines. Dissertation, Tomsk, 1966.

4. G. I. Skanavi. Physics of Dielectrics (the Region of Strong Fields). Fizmatgiz, 1958.