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Dielectric properties of pole insulation of traction motors under constant thermal ageing

Glykos Dimitrios

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Summary

This diploma thesis is a study of the behavior of the dielectric properties of the pole insulation of a permanent magnet synchronous axial flux motor having the yokeless and segmented armature topology when these poles are subjected to constant thermal aging. As the need to reduce the burning of fossil fuels leads to the gradual widespread adoption of electric vehicles, this type of motor is increasingly used due to its reliability, high torque density, compact structure and high efficiency. The windings of this motor are insulated to protect the turns of the coils from stator faults. Among them, the inter-turn short circuits pose a significant threat to the condition of insulation systems due to their rapid evolution into catastrophic breakdowns. However, various stresses may arise in the environment of the motor during its operation, which are associated with insulation degradation and failure. These are known as TEAM stresses, an abbreviation for Thermal, Electrical, Ambient and Mechanical stresses. This thesis aims to provide significant insights into the behavior of the windings insulation after the application of fixed thermal stress. First, the characteristics of the electric motors used in electric vehicles (EVs) are outlined, emphasizing on the permanent magnet synchronous motors (PMSM). Next, the potential faults that can occur are analyzed, including electrical, mechanical and magnetic faults. Additionally, the widely employed diagnostic methods are described that aim at early fault detection. Subsequently, the pole insulation systems are discussed, insulation degradation with the increase of partial discharge activity is analyzed and the diagnostic tests to assess the insulation condition are described. Moreover, the thesis delves into fixed thermal stress testing as an accelerated aging mechanism along with relevant literature on constant thermal aging testing with their respective experimental outcomes. The experimental procedure encompasses the development of quality testing procedures, modeling the equivalent circuit of the poles, conducting experimental tests and the analysis of the applied fixed thermal stresses on the poles. Impedance spectroscopy, Nyquist diagrams and breakdown voltage measurements are the primary methods for monitoring and evaluating the insulation condition of the poles. A frequency response (FRAX) analyzer and an Insulation Resistance Tester (Megohmmeter) are utilized as the main measuring equipment. Particularly, the performed work on the development of the complex equivalent circuit for the poles is described. Then the impact of different temperatures acting as different fixed thermal stresses on the pole insulation is explored, elevating gradually the temperature between the cycles. Their results are recorded using the aforementioned diagnostic tools after each cycle to assess the insulation degradation. The thermal cycling test is also investigated to highlight the impact of the thermomechanical effects observed during this test on the insulation. Furthermore, multi- stress mechanisms are examined using a specially designed mechanical accelerator to conduct electromechanical stress testing, following the thermal aging procedures.

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