In this paper the diagnosis of induction machine rotor electrical faults is considered. Two approaches are compared: the current spectrum analysis and the apparent rotor resistance estimation. For the first approach the authors have developed several procedures based on different fault models of the machine. Their experience is used to approach the parameter estimation method from a theoretical point of view: the resistance variation of the balanced per-phase model is computed using the faulted machine model. It is possible to obtain, by the simulation, the expected resistance variation when some bars break in a specific machine. Moreover the numerical results can be generalized. Using a simplified model of a faulted machine a relationship is obtained, which correlates the apparent resistance variation with the number of broken bars. This relationship needs several assumptions and therefore it is an approximate one, but can be used to define the threshold level for the apparent resistance variation expected in the case of one broken bar. By this relationship it is possible to have an indication on the sensitivity of the parameter estimation approach. The superiority of the current spectrum approach over the parameter estimation approach is shown.
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