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In this paper, some original and effective fault indicators for broken-bar detection in power squirrel-cage induction motors are presented. A motor phase-current signature analysis can be performed by evaluating the typical ratios [I.sub.(7-2s)f]/[I.sub.5f] and [I.sub.(5+2s)f]/[I.sub.7f], [I.sub.(13-2s)f]/[I.sub.11f] and [I.sub.(11+2s)f]/[I.sub.13f], etc., which appear in the phase-current spectrum of faulted motors fed by nonsinusoidal voltage sources. The main advantages of the new indicators are the following: 1) accentuate insensitivity to disturbs such as load torque, drive inertia, and frequency variations; 2) low dependence with respect to machine parameters (except the pole number); and 3) linear dependence on fault gravity. They can be directly applied on motors fed by open-loop low-switching frequency gate turn-off/thyristor converters. Railway traction drives are possible targets. Application to mains-fed motors can be tried too, if suitable harmonics are present in the plant supply. A detailed analytical formulation for fault indicators is furnished, based on the multiphase symmetrical component theory; theoretical results have been supported by experimental work, performed by using a motor with an appositely prepared cage, and successively, method validation was achieved on three other industrial motors. Index Terms--Broken bars, fault indicators, induction motor, sideband calculation, spectral analysis, symmetrical components.