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As far as industrial applications of induction drives are concerned, one of the most critical issues is the statement of acceptable mixed eccentricity degree and of other mechanical abnormal conditions. These troubles cause characteristic spectral components in several drive quantities, but for the sake of economy, monitoring the electrical quantities involved in drive control is preferable. Among them, current signals (i.e., motor current signature analysis) are the signals of choice for assessing the machine conditions, although no analytical link between the current signature and the eccentricity degree through machine parameters has ever been presented. Each trouble produces spectral lines at typical frequencies in the stator current: monitoring them is a usual procedure, but the threshold to discern acceptable intrinsic defects from alarming situations is hardly defined. This paper presents the formulation of a mixed eccentricity index developed through the interaction between static and dynamic eccentricities on stator windings and rotor cage. The index can be computed by a no- or mild-load test without spectral resolution problems, and it does not require any preliminary knowledge of machine health conditions.