Finite element study of rotor slot designs with respect to current monitoring for detecting static airgap eccentricity in squirrel-cage induction motors

Abstract

Previous research using the MMF and permeance wave approach and experimental investigations has established that there are specific frequency components in the input current signal to an induction motor which are a function of static eccentricity, rotor slotting and saturation. The magnitudes of these components increase as the level of static eccentricity increases. The work reported in this paper applies finite element analysis to model a motor with static eccentricity to predict the severity of the fault from the magnitudes of these current components. The finite element results are compared with measured results from the test-rig motor being modelled and the agreement between them is found to be consistently closer than was achieved on previous attempts using the MMF and permeance wave approach. The finite element analysis is also used to determine what effect rotor slotting has on the magnitude of these components in relation to the overall changes in magnitude due to increasing static eccentricity levels. Different rotor slot designs were modelled (open, closed, semi-closed) and it was found that in terms of online current monitoring of induction motors the effect of different rotor slots on the magnitudes of the components was significant. This study has provided valuable information in terms of monitoring different motors in industry with the severity of the fault at different stages.