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In this paper the authors present some original and effective fault indicators for broken bars detection in power squirrel cage induction motors. Motor Phase Current Signature Analysis (MPCSA) can be performed by evaluating the typical ratios I(7-2s)f/I5f and l(5+2s)f/I7f, I(13-2s)f/I11f and I(11+2s)f/I13f, etc., which appear in the phase current spectrum of faulted motors fed by non-sinusoidal low-switching-frequency voltage sources. Main advantages of the new indicators are: a) accentuate insensitivity to disturbs like load torque, drive inertia, and frequency variations, b) independence with respect to machine parameters and c) linear dependence on fault gravity. They can be directly applied on motors fed by GTO/Thyristor converters. Railway traction motor diagnostics is a possible target. The mathematical formulation has been based on the Multi- Phase Symmetrical Components Theory; parameter identification was obtained by 2D-3D FEM analyses; experimental work was performed by using a motor with an appositely prepared cage.