Analysis and application of particular current signatures (symptoms) for cage monitoring in nonsinusoidally fed motors with high rejection to drive load, inertia, and frequency variations

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.