Extraction of frequency information for the reliable screening of rotor electrical faults via torque monitoring in induction motors

Conventional diagnostic methods applied to industrial induction motors, such as the Motor Current Signature Analysis, may lead to false-negative diagnostic outcomes in several cases. Such a case consists of the non-adjacent rotor breakages occurrence. Various alternatives with advanced digital signal processing algorithms have been proposed that concern the monitoring and analysis of the stator current, or the stray magnetic flux, of the motor during the transient start-up. Those methods work efficiently in most cases; however, the real issue is that most large industrial motors have very few start-ups during their long operating life time. In that sense, it is not feasible to implement the transient analysis-based methods. This paper addresses an alternative methodology that solves this issue for induction motors at steady state. The method relies on a two-stage signal processing technique for frequency information tracking and extraction, where the higher harmonic index of the motors torque around the sixth harmonic is evaluated during each stage. By the results of the method, it is evident that the fault and its severity level can be reliably detected at the steady state. The methods efficacy is proven valid even for challenging cases of large industrial motors, where the likelihood of a false diagnostic decision is increased during the signature screening.