Fatigue aging of insulation of traction motors of diesel locomotives

. This paper analyzes the problem of occurrence and development of fatigue processes in the insulation of traction motors of autonomous locomotives. Factors causing destruction (cracking, delamination, occurrence of voids) of insulation in operation have been studied in the course of the research. Three characteristic stress cycles (reversible, repetitive, random) were described to make fatigue failure predictions. Outcomes of investigations of temperature conditions of windings of electric machines of diesel locomotives show that during operation the insulation experiences variable thermomechanical stresses. Received data can be recommended to take into account during the creation of new electric machines for diesel locomotives, as well as during the operation and


Introduction
Extension of service life of locomotive traction electric machines in operation is one of the priority tasks of locomotive engineering. The problem of increasing the reliability of transport electric machines, and in particular the study of fatigue strength processes of electrical materials (namely, insulation), is also given much attention in various research works. [1][2][3][4][5][6][7][8][9][10][11]. Such a concept as fatigue failure of insulation material refers to a type of mechanical failure. It is defined as a process in which progressive, local and permanent microstructural changes in the material structure occur when forces are applied. This outcomes in cyclic or sign-variable stresses and strains at some point or several points simultaneously.
It is known that the concept of fatigue in practice is usually associated with metals or their alloys, but it can occur in all materials capable of plastic deformation. Such materials include polymers and multilayer composite materials. Plastically non-deformable materials (e.g., glass or ceramics), in which deformations are only elastic, are not subject to fatigue failure due to cyclic repeated stresses.
Insulation of electric machines of locomotives in operation experiences not only mechanical but also thermal fatigue. When investigating thermal fatigue processes, it is necessary to take into account the climatic and operational peculiarities of locomotive circulation areas on the railroads of the Russian Federation. But not only the temperature range is crucial in assessing the serviceability of insulation materials of locomotive traction motors, but the thermal cycle in real time also has a great impact.
It should be noted that the phenomenon of fatigue of the insulation material of locomotive traction electric machines is a rather complex process, in which different types of damage (fracture, cracking, delamination of material, delamination, etc.) can occur either gradually or interact with each other, generating different growth rates.

The fatigue life of insulation materials
Based on the data studied, it is known that mainly analytical models are created for specific types of layered materials with different boundary conditions, as well as a large range of cyclic loads. [12][13][14]. Their extrapolation to existing structures is a rather difficult problem since the service life of such materials, namely the insulation of traction electric machines, under cyclic loading is determined by a set of related factors.
If the stresses in the insulation material are well below the elastic limit, microscopic damage begins to occur under continuous cyclic loading. Such micro damage can accumulate throughout the winding material of an electrical machine and lead to cracks. The cracks then grow to a size that can destroy the insulation material. This destruction occurs after a certain number of stress or strain cycles and causes the loss of serviceability of the traction electrical machine in operation. The external manifestation of fatigue failure of traction motor insulation material can occur suddenly without any pre-detectable signs. But at the same time processes of destruction of traction electric motor insulation material and creation of favorable conditions for loss of this electric machine serviceability could be going on from the beginning of the operation period. Processes of accumulation of indicators, leading to insulation failures, occur most often in certain local areas of traction motor insulation material and are not homogeneous throughout the whole volume. Such vulnerable zones are capable of having high local concentrations of deformations or stresses caused by abrupt changes in traction motor winding geometry, internal and external defects of electrical insulation material, or specific operating conditions of locomotive electric machines.
The fatigue life of insulating material can be defined as the total number of stress cycles that can cause damage to that material. The evolution of fatigue processes leading to the formation of damage to the insulation material of traction electrical machines of rolling stock can be considered as separate stages, which is demonstrated in Figure 1. In the initial period of operation, there are foci in the insulation material of traction motors, in which the process of micro-crack initiation begins. Further, if the operating mode remains unchanged, this process causes the formation of micro-cracks. Minor damage can also occur (second stage). Such damage leads to the consolidation and development of microcracks in the insulation material (third stage). Such changes in the structure of the insulation material of a traction electric machine do not affect its operability in operation. But at this stage the probability of cracking increases, which is mainly determined by the intensity of arising stresses. Unfortunately, the problem of the influence of such damages on the serviceability and reliability of traction electric machine insulation material is practically not considered in existing literature sources.
The determining causes of subsequent crack growth or delamination of the insulation material are the presence of voids, the presence of foreign inclusions, and manufacturing defects. Since these factors are closely related to the intensity and density of stress concentrations in local zones. The next stage of fatigue failure is associated with crack growth, which proceeds steadily until it reaches a critical size. When the crack reaches a critical size, it begins to propagate through the insulation material of the electrical machine, mainly affecting the impregnation composition. However, the crack itself in the insulation material does not lead to a significant reduction in the overall performance of the traction motor. But the accumulation of atmospheric moisture, dust, or wear products of electric machine elements in the crack space leads to the emergence of conductive paths, which in turn causes complete or partial failure of the traction electric motor.
In materials such as metals and metal alloys, the stage of gradual and hidden from observation reduction of material quality takes almost the entire life cycle of the part. In the first stage, the fatigue process proceeds without a significant decrease in stiffness. The beginning of the second stage is already associated with the emergence of small cracks, which are the only macroscopically observable sign of the occurrence of damage processes. The subsequent growth and union of such cracks relatively quickly become the cause of a single crack and lead to the final failure of the part. Consequently, in the first stage, as long as the stiffness characteristics of the metal are almost unreduced, the relationship between stresses and strains can be assumed to be linear. In such a case, the simulation of the fatigue process can be considered in general based on linear elastic analysis and linear fracture mechanics.
The incubation period for the nucleation of an insulating material defect is rather short. However, the area of the damage propagation zone constantly increases in the process of uninterrupted loading. During the operation of an electrical machine, not only the size but also the type of defect in these areas can change. As a consequence, small size cracks in the insulating material with increasing loads, with increasing duration of their action, or with an increasing number of load cycles lead to destruction or delamination at the interfaces of glass fabric material and impregnation composition. It is worth noting that taking into account the actual state and predicting the final state (time and zone of destruction) of the insulation structure requires modeling a rather complex process of stage-by-stage development of the defect. At the same time, the weakest areas of insulation of electric machines of traction rolling stock should be taken into account.
Usually, in the insulation material, fatigue failure can take the form of any fault in the traction motor (cracking, delamination, growth of voids inside the insulation material, delamination of the interfacial surface, etc.). Such manifestations of damage can interact with each other, which leads to the transition of one type of damage to another. Figure 2 shows the process of insulation material defect development during the whole period of fatigue life.
The study of the fatigue accumulation process of the material of insulating structures does not always rely on stress reactions. It may also be based on the number of load cycles that outcome in micro-or macro-cracking. Consequently, a distinction can be made between lowcycle and high-cycle fatigue processes.
Low-cycle fatigue must be considered in light of loading conditions since stresses can be quite high. Such operating conditions of an electrical machine can lead to irreversible damage to the insulation material. The transition boundary from low-cycle to high-cycle fatigue is determined by the properties of the insulation material. In most cases, it is within 10 2 -10 4 load cycles. The essence of such physical processes is that during multi-cycle fatigue, the stresses are small enough, so the insulation material is in the range of its elastic strain limit. When low-cycle fatigue is considered, the range of stresses in the insulation material can exceed its elastic strain limit. A consequence of such operating conditions of traction electric machine windings is cracking and delamination of insulation material, which ultimately leads to reduction of residual service life of locomotive traction motor.

Fatigue fracture modeling
Typically, periodic loading cycles are used to solve fatigue strength problems of metals and their alloys. It becomes necessary to operate with millions of cycles. When analyzing the thermal fatigue failure of insulating material of electrical machines, modes with a relatively small number of load cycles are used. Relatively small stresses are also considered. In such studies it is necessary to interact with tens and hundreds, sometimes thousands of cycles. Therefore, the problem arises, which is that to assess the performance of insulation material under cyclic loading, the necessary amount of research outcomes is currently missing.
Even though fatigue failure of electric machine insulation material of locomotives is difficult to predict with high accuracy, the creation of a methodology for prediction and prevention of fatigue failure development is necessary for creating modern traction electric motors.
Several models can be relied upon to predict the fatigue failure processes of the insulation material. For example, three characteristic stress cycles can be analyzed based on which loads can be applied to the object in question.
The reversible loading cycle is the simplest of them all (Fig. 3). It is a sinusoidal cycle where the maximum and minimum stresses are equal and differ only in sign. An example of such a stress cycle is the axis on which the stress is reversed after each half-period. The second type of cycle, which is called a repetitive loading cycle, is used more often in studies. In this case, the maximum and minimum stresses are asymmetrical, but not equal or opposite to each other (Fig. 4). The curve describing such a cycle is a sinusoid Fig. 4. Repetitive loading cycle.
These two types of cyclic loading represent an idealization of the alternating loads applied to the object under study.
The closest to the existing operating conditions of locomotive traction electric machines is a random loading cycle. In this case, the voltage, frequency, and duration of the cycle change during the operation of traction rolling stock electrical equipment randomly (Fig.5). But the such course of the process is associated with the emergence of significant difficulties in the necessity of mathematical description of the stress-strain state, as well as in the creation of fatigue damage models. To predict the service life of the insulation material of traction motors, cyclic parameters of temperature change should be taken into account. An example of such a case can be the movement of a locomotive with a train, where thermal cycles are created whenever the traction motors supply voltage is switched on or off (Fig.6).   Fig. 6. Dependence between winding temperature (T) and phase current ( I f ).
If the electrical load is switched on and off, the locomotive traction motor components are affected by thermo-mechanical effects. This effect leads to an increase in the temperature of the conductors made of copper. As a consequence, the copper will expand mainly in the axial direction. (It is accepted that one cycle will correspond to one heating process followed by one cooling process.) Also, due to the different coefficients of thermal expansion of the copper and the insulation material, one or both components will experience additional stress, and it is the insulation that suffers the most damage. The cycle will end when one of the winding components of the electrical machine fails. Failure of the traction motor insulation occurs because the electrical parameters of the material change so much that their values do not match those necessary to fully perform the basic functions. Consequently, the insulation of the traction motor groove should have the necessary compressive and shear strength to ensure that it will not be damaged in service.
In the case of continuous locomotive motion in idling mode, the temperature of traction electric machine windings often becomes almost equal to the ambient temperature. In this case, the operating temperature of traction motor windings under load can reach 130-150 ºC and higher. Consequently, when predicting the lifetime of the traction motor insulation material, cyclic ranges of temperature changes should be taken into account. Especially, such processes should be taken into account when operating locomotives in conditions of low temperatures (for example, in the Far North and Siberia), where the ambient temperature is-45 ºC in winter and reaches+35 ºC in summer. [15].
To demonstrate random loading cycle during locomotive equipment operation, the outcomes of operational tests of traction electric motor type ED118A of diesel locomotive series 2TE116-1186A, equipped with DC traction electric motors, when moving with freight trains on the sections of the October railroad (Fig. 7). The presented outcomes allow us to conclude that the values of the weighted average heating of the active part of the armature surface are in the range of 65-75 ºC. The maximum temperature was 102 ºC and the minimum temperature corresponded to-16 ºC. At the same time the ambient temperature during the study was-22 ºC. Figure 8 shows an example of transcription of ASC data of motion parameters of diesel locomotive 2TE25A-049A with asynchronous traction motors during 3 hours and 25 minutes of operation in the BAM conditions. The ambient temperature was-32 ºC. Based on the data obtained, the phase current of the DAT-350 traction motor varied in the range from 0 to 350 A, and rectifier unit voltage-in the range from 0 to 1200 V. At the same time the temperature value of the stator winding of the investigated electric machine varied in the range from 45 to 135 ºC. Based on the consideration of the dependencies shown in Figures 7 and 8, the number of cycles of temperature change is not too great. Cycle duration is from tens of minutes to several hours when there is heating and cooling of traction motor windings with monotonous speed. But at the same time, complete testing for thermal fatigue of locomotive electric machine insulation material in real-time is practically unfeasible in practice during the operation of autonomous traction rolling stock.
The data obtained during operational tests of the thermal condition of DC and AC traction motor windings indicate that regardless of the design features, throughout the entire service life the insulation is exposed to alternating thermomechanical stresses. The value of these stresses is determined by the modes of operation and the intensity of operation of traction rolling stock.

Conclusions
At the moment, the thermal fatigue of layered materials is studied in a simplified way. The fact that the formation of plastic deformations inside the insulation material during a temperature cycle proceeds not in micro, but in macro volumes, allows us to approach this question from the perspective of a continuous medium scheme. Thus, the following conclusions should be made: if after several temperature cycles the cyclic regime of purely elastic deformations is established, the temperature fatigue failure at a small number of cycles will not occur. If plastic deformations of the material are continuously formed in the steady-state cycle conditions, after a certain number of cycles a significant expansion of the crack dimensions occurs. Further it can be penetrated by atmospheric moisture, or by wear products of rubbing parts of the traction electric machine, which will lead to the occurrence of currentcarrying paths and further to the breakdown of insulation.
In practice, the calculation of fatigue characteristics of insulating material plays an important role in modeling its damage and predicting its residual life. Even though a significant amount of experimental research is required, and, therefore, high economic costs, the study of damage mechanisms involved during fatigue loading is basic in creating a model for predicting the degree of aging of locomotive traction motor insulation. Regardless of the model created, it is necessary to apply the outcomes of experiments to predict fatigue life using failure criteria, continuous damage mechanics, or statistical functions.
The outcomeing thermomechanical fatigue of the insulation material also includes multiaxial stress states, which are characterized by thermal expansion coefficients, on which one can base an understanding of the failure mechanisms of electrical machine insulation The obtained outcomes of studies of heat flow distribution processes in asynchronous electric machines and DC electric machines can be used when installing non-contact temperature sensors required for diagnosing the thermal state of traction motors.