Comparison of thermal stresses developed during transients on a damaged rotor cage

Abstract

This is the accepted version of the original article published by IEEE.Abstract-Structural defects in the rotor cage of large electrical machines significantly impact their expected operational lifetime. This work simulates the stresses developed in a damaged rotor cage during different transient profiles and for different sizes of the imperfection. A combined model featuring electrical, thermal and mechanical stages as well as three different meshes reflecting a progressing narrowing of one of the bars in its junction to the end ring are used for this purpose. The results qualitatively agree with a previously presented fatigue test and show that under severe conditions the capability of aluminum to absorb stress is limited, and a state of plastic deformation is rapidly reached. The effect of a possible mitigation strategy is also studied.

Study of thermal stresses developed during a fatigue test on an electrical motor rotor cage

“…Besides that, given that the values of the convection coefficients in the model are linked to the mechanical speed of the machine, stall transients, considered the most damaging ones since there is no end space and air gap ventilation [13], are also easily simulated for comparison purposes by establishing in the movement equations of the rotor a load inertia high enough to avoid its turning. The progressing widening of the drills next to the bar executed during the last stage of the experimental test is reproduced here by using three different meshes in the simulations, featuring a bar's remaining section reduced to 12%, 8% and 4% of its original value, complementing the results of [9] in which just the latter was employed, and completing the ones presented in [14]. In this manner the full development of the fault can be depicted and the simulated results compared to the experimental ones published in [12], showing a good agreement despite all the assumptions made.…”

“…Contrary to [10] and complementing [14] with further simulations, in this work the study of three different meshes showing the development of the fault allows checking the resemblance of the evolution of electromagnetic quantities as the fault progresses (thermal ones don't differ from the case shown in [10]). Transient current analysis is employed on the stator currents to obtain an accurate quantification of the rotor cage asymmetry for the startup cases, both from the current waveforms yielded by the model in the simulations and from the recorded experimental results, by tracking an integrating the energy in the LSH-50 harmonic during the transient and normalizing it with the one from the main current component.…”

“…A sensibility analysis regarding the interbar resistance parameter, difficult to measure or estimate, is also carried out in this work. Based on the analysis of this complete simulated results, a novel mitigation strategy, not included in [14], is developed.…”