Local and global effect of a broken bar in induction machines using fundamental electromagnetic laws and finite element simulations

In this paper an induction machine with a broken rotor bar is studied. The present study shows that the theorem of superposition can lead to acceptable representation of a faulty machine in the case of saturable iron and this, even though local saturation is greatly affected by the presence of the broken bar. Several cases, representing healthy and faulty machines, saturable as well as faulty machines with permeability frozen at the healthy condition, are considered. The results obtained by finite element computations and interpreted graphically using Faradaypsilas and Amperepsilas laws, show that if we neglect the change of saturation due to the presence of the broken bar, the superimposed effect of the broken bar is not only the creation of a pulsating field but also to phase shift the stator currents. However, if local saturation is allowed to change due to the presence of the broken bar, the phase shift of the stator currents is annihilated and the only effect introduced by the broken bar is to create a pulsating field as in linear machines. Based on this conclusion, an analytical model of a faulty machine is obtained using the principle of superposition. This approach has the advantage to decompose the machine in healthy and faulty models and therefore reduce computation time. The model is used for precise detection and quantification of broken bars at early stage of the fault.