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Micro Zero Padding for the Reduction of Spectral Leakage in the Diagnosis of Rotor Asymmetries Faults in Large Induction

Large induction machines play a key role in modern industry, both as large motors (e.g. in mining industries) or large generators (e.g. in wind turbines). Condition monitoring of such machines assures their continuity of service, and may avoid costly breakdowns. Among the methods for condition monitoring, motor current signature analysis is being increasingly used, because it is non-invasive, and can identify a wide variety of faults. A challenging problem is the detection of the fault signature of rotor asymmetries in large induction machines, which have a very low rated slip. In this case, the fault harmonics are so close to the fundamental component that its spectral leakage may hide them, even with significant fault severity. Using a filter window, such as the Hanning one, reduces the spectral leakage, but distorts the amplitude of the fault components, thus decreasing the reliability of the fault diagnosis. This problem is addressed in the technical literature by removing the fundamental component, with notch filters, or using high-resolution spectral techniques; but these techniques are costly in terms of computing power. This paper proposes a new, efficient method to solve this problem, using a small amount of zero padding that eliminates the leakage of the fundamental component, without distorting the amplitude of the fault harmonics. The method is theoretically explained and experimentally validated with the diagnosis of a broken bar fault in a 3.15 MW induction motor.