Electromagnetic and mechanical finite element analysis of end region of large-sized three-phase squirrel-cage induction machines

This piece of research is related to the design of large-sized radial flux electric machines. It aims at studying the end-region magnetic field under the steady-state operation in order for an analysis of some important electromagnetic and mechanical phenomena that require careful thought during the design of the end region. The research work is focused on the end region of two large-sized three-phase squirrel-cage induction machines. In addition to the end-region magnetic field, the stator end-winding leakage inductance, the eddy currents inside the end shield and the end frame, and the end-winding forces and vibrations are covered. The 3-D time-harmonic finite element analysis of the above aspects, carried out with a commercial software package, forms the main part of the research work. The corresponding measurement, as a complement to the finite element analysis, is mainly for validating the models and the numerical calculations. A 3-D time-harmonic finite element analysis of the end-region magnetic field with suitable boundary conditions can give an accurate distribution of the magnetic field. The stator end-winding leakage only affects the core ends; hence, the end-winding leakage inductance can be calculated from the corresponding magnetic energy separated from the total magnetic energy. The eddy currents inside the end shield and the end frame do not affect the end-region magnetic field seriously, and it is feasible to consider them as surface currents and to model them with the standard impedance boundary condition. The end-winding forces cause vibrations and deformation, but the amplitude of vibration and the degree of deformation are small because of the end-winding bracing system. With the bracing system, the natural frequency of the most excitable mode of the end winding and its bracing system is also greatly raised, so resonances may be avoided. Under the steady-state operation of large-sized induction machines, the end-region magnetic field does not bring about severely disadvantageous electromagnetic and mechanical phenomena in the end region. However, from the viewpoint of optimisation, the end frame can be shifted farther from the end winding for eliminating potential hot spots. In addition, fewer bracing parts can be fixed to the knuckle portion and the lower part of the involute portion of the end connections for improving the cooling of the end region.