Die Steigerung der elektromagnetischen Ausnutzung elektrischer Maschinen ist im Wesentlichen durch eine Erhöhung des Strombelages möglich. Die Belastungsgrenze resultiert u. a. aus der maximal zulässigen Temperatur der Wicklungsisolation. Aus der geringen spezifischen thermischen Leitfähigkeit herkömmlicher Isolierwerkstoffe resultieren große Temperaturgradienten im Wicklungsquerschnitt und es kommt zur Bildung von Hotspots z. B. in den Wicklungsköpfen. Im Beitrag werden die Entwicklung und die Charakterisierung von Polysiloxan-Kompositen mit keramischen Füllstoffen vorgestellt. Im Vergleich zu herkömmlichen Isoliermaterialen kann die spezifische thermische Leitfähigkeit damit etwa um den Faktor fünf gesteigert werden. Die Materialien werden für die Herstellung der Hauptisolation des Elektroblechpaketes in einem Tauchprozess sowie die Vakuum-Imprägnierung der Wicklung eingesetzt. Für einen elektrischen Radnabenantrieb werden die erwartete Wicklungstemperaturreduzierung und die daraus resultierende mögliche Leistungsdichtesteigerung diskutiert. / The increase of the electromagnetic utilisation of electrical machines is essentially achievable by increasing the electric load. The power limit results from the maximum permissible temperature of the winding insulation. The low specific thermal conductivity of conventional insulating materials results in large temperature gradients in the winding cross section and the appearance of hotspots in the end windings. The paper discusses the development and characterisation of polysiloxane composites with ceramic fillers. Compared to conventional insulating materials, therewith the specific thermal conductivity can be increased by a factor of five. The materials are used for the production of the main insulation of the laminated core in a dipping process and the vacuum impregnation of the winding. For an electric wheel hub motor, the expected winding temperature reduction and the possible power density increase are discussed.

Electrical machines have been widely used along the last decades with large life length under operating conditions. However, they will become more important in the upcoming years because of the emerging electric car industry. Thus, the maintenance cost of this technology can be reduced by extending the lifetime in the electrical machines. Despite the fact that existing numerous studies within the life length in these devices, only few study the effect of the thermomechanical stresses of insulation. The core of this master thesis is to study the influence of these stresses in the insulation material of a winding. The tested electrical machines were subjected to different test conditions, allowing to analyse multiple aging effects in the winding. To achieve these effects, power cycling tests were carried out on stators, where the windings were tested in cycles with different ΔT and two cooling methods: air cooling and oil cooling. The results showed large aging differences between the two cooling methods employed. The aging effect in the oil cooling method was higher than in the air cooling method for the same number of cycles. However, the aging effects regarding the same cooling process had not wide differences between the different test temperatures.

High voltage testing of the stator winding insulation is one of the most recognized methods used to determine the state of degradation in the insulation. HV tests performed at 0.1 Hz do have potential advantages compared to more traditional 50 Hz tests. This thesis therefore aims to perform and compare tan delta, capacitance and partial discharge measurements on stator windings when using a 0.1 Hz voltage source and a more traditional 50 Hz voltage supply. Several associated test parameters with considerable influence on the test results were varied during the tests. An associated data analysis followed that was focused on the differences and similarities of the analyzed parameters and the results due to the differences in frequency. The results show that there are substantial levels of noise present in the partial discharge measurements when utilizing the VLF voltage source. There are also more numerous partial discharges for VLF measurements than for regular power frequency measurements if the same amount of voltage cycles is considered. The generated patterns show similarities with those generated at 50 Hz, but a larger sample base is probably needed for more thorough conclusions. The tan delta/capacitance part of the test do indicate potential advantages compared to power frequency measurements regarding the sensitivity in the measurements.

It is common practice to perform accelerated aging with 60 Hz ac to determine the lifetime characteristics of insulation used in the machine. Comparable breakdown measurements are performed at different voltage levels and temperatures for the polyimide insulated machine winding under normal operating conditions of 60 Hz ac. The result shows that the time to failure can be represented by the inverse power law and the Arrhenius equation with respect to test voltage and temperature respectively. However, the use of power electronic devices causes harmonics, and spikes that electrically degrade the machine winding insulation. Applied high frequency pulse voltages can be used to study the impact of electrical degradation of the machine winding insulation that exists in electrical machines. Evaluation of change in dielectric strength, partial discharge and breakdown voltage is monitored versus aging caused by high frequency pulse voltage at 90% of operating temperature.

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The thesis Diagnostics of rotating electric machines deals with the theory of diagnostics of rotating machines, focusing especially on the diagnosis of asynchronous motors of high power. Due to the importance of maintaining the trouble-free condition of these machines with regard to serviceability, the importance of their diagnosis is growing. Due to inappropriate working conditions or for many other reasons, different types of malfunctions may occur in the machine. The list and the way of occurrence of possible failures are included and explained in this thesis. With the growing age of the machine, it is important to check the insulation status through insulation testing. These tests are performed in the practical part of the thesis and the results are presented. In the case of asynchronous machines, each anomaly leaves a signature in the form of an electrical, magnetic or vibrational signal. These signals are analyzed and evaluated using a specific measuring equipment with the intention of deciding on the current state of the machine. Motor current signature analysis (MCSA) is considered to be the main diagnostic method of asynchronous machines in this thesis. This method is described in detail in this paper and is being applied in electrical engineering.

High voltage rotating machines play a significant role in generation and use of electrical energy as the demand for power continues to increase. However, one of the main causes for down times in high voltage rotating machines is related to problems with the winding insulation. The utilities want to reduce costs through longer maintenance intervals and a higher lifetime of the machines. These demands create a challenge for the producers of winding insulations, the manufacturers of high voltage rotating machines and the utilities to develop new insulation materials which can improve the life of the equipment and reduce the maintenance cost. The advent of nanotechnology in recent times has heralded a new era in materials technology by creating opportunities to significantly enhance the properties of existing conventional materials. Polymer nanocomposites belong to one such class of materials that exhibit unique combinations of physical, mechanical and thermal properties which are advantageous as compared to the traditional polymers or their composites. Even though they show tremendous promise for dielectric/electrical insulation applications, there are no studies relating to the long term performance as well as life estimation of the nanocomposites. Considering this, an attempt is made to generate an understanding on the feasibility of these nanocomposites for electrical insulation applications. An epoxy based nanocomposite system is chosen for this study along with alumina (Al2O3) and silica (SiO2) as the nanofillers. The first and the foremost requirement for studies on polymer nanocomposites is to achieve a uniform dispersion of nanoparticles in the polymer matrix, as nanoparticles are known to agglomerate and form large particle sizes. A laboratory based direct dispersion method is used to process epoxy nanocomposites in order to get well dispersed samples. A detailed microscopy analysis of the filler dispersion using Scanning Electron Microscope (SEM) has been carried out to check the dispersion of the nanofiller in the polymer. An attempt is made to characterize and analyze the interaction dynamics at the interface regions in the epoxy nanocomposite by glass transition temperature (Tg) measurements and Fourier transform infrared (FTIR) spectroscopy studies. The values of Tg for the nanocomposites studied decreases at 0.1 wt% filler loading and then starts to increase gradually with increase in filler loading. This Tg variation suggests that there is certainly an interaction between the epoxy chains and the nanoparticles. Also no new chemical bonds were observed in the spectra of epoxy nanocomposite as compared to unfilled epoxy. But changes were observed in the peak intensity and width of the –OH band in the spectrum of epoxy nanocomposite. This change was due to the formation of the hydrogen bonding between the epoxy and the nanofiller. The thermal conductivity of the epoxy alumina and the epoxy silica nanocomposites increased even with the addition of 0.1 wt% of the filler. This increase in thermal conductivity is one of the factors that make these nanocomposites a better option for electrical insulation applications. The dielectric properties of epoxy nanocomposites obtained in this investigation also reveal few interesting behaviors which are found to be unique and advantageous as compared to similar properties of unfilled materials. It is observed that the addition of fillers of certain loadings of nanoparticles to epoxy results in the nanocomposite permittivity value to be lower than that of the unfilled epoxy over the entire range of frequencies [10-2-106 Hz] considered in this study. This reduction has been attributed to the inhibition of polymer chain mobility caused by the addition of the nanoparticles. The tan values are almost the same or lower as compared to the unfilled epoxy for the different filler loadings considered. This behavior is probably due to the influence of the interface as the strong bonding at the interface will make the interface very stable with fewer defects apart from acting as charge trapping centres. From a practical application point of view, the surface discharge resistant characteristics of the materials are very important and this property has also been evaluated. The resistance to surface discharge is measured in the form of roughness on the surface of the material caused by the discharges. A significant enhancement in the discharge resistance has been observed for nanocomposites as compared to unfilled epoxy/ microcomposites, especially at longer exposure durations. The partial discharge (PD) measurements were carried out at regular intervals of time and it is observed that the PD magnitude reduced with discharge duration in the case of epoxy alumina nanocomposites. An attempt was made to understand the chemical changes on the surface by conducting the FTIR studies on the aged surface. For all electrical insulation applications, materials having higher values of dielectric strengths are always desired and necessary. So AC breakdown studies have also been conducted. The AC breakdown strength shows a decreasing trend up to a certain filler loading and then an increase at 5 wt% filler loading for epoxy alumina nanocomposites. It has been also observed that the type of filler as well as the thickness of the filler influences the breakdown strength. The AC dielectric strength of microcomposites are observed to be lower than the nanocomposites. Extensive research by long term aging studies and life estimation are needed before these new nanocomposites can be put into useful service. So long term aging studies under combined electrical and thermal stresses have been carried out on unfilled epoxy and epoxy alumina nanocomposite samples of filler loading 5 wt%. The important dielectric parameters like pemittivity, tan  and volume resistivity were measured before and after aging to understand the performance of the material under study. The leakage current was measured at regular intervals and tan  values were calculated with duration of aging. It was observed that the tan  values increased drastically for unfilled epoxy for the aging duration considered as compared to epoxy alumina nanocomposites. The life estimation of unfilled epoxy as well as epoxy nanocomposites were also performed by subjecting the samples to different stress levels of 6 kV/mm, 7 kV/mm and 8 kV/mm at 60 oC. It is observed that the epoxy alumina nanocomposite has an enhanced life which is nine times the life of the unfilled epoxy. These results obtained for the nanocomposites enable us to design a better material with improved dielectric strength, dielectric properties, thermal conductivity, resistance to surface discharge degradation and enhanced life without sacrificing the flexibility in the end product and the ease of processing. Dry type transformers and stator winding insulation need to be cast with the above material developed and tested before practically implementing these in the actual application.

This thesis describes properties of windings of electric rotating machines and their insulation systems. There are winding and insulation low voltage machines tests listed with their procedures and criteria. Further it deals with coordination methodology and the last part contains execution and results assessment of tests conducted on stator samples.