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(7-2s)f /I5f and I(5+2s)fI7f, I(13-2s)f /I11f and I(11+2s)f /I13f , 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.