Method for extracting current envelope for broken rotor bar fault detection of induction motors at time-varying loads

When the broken rotor bar (BRB) fault occurs in induction motors, the amplitude of stator fundamental current is modulated by fault components, forming the current envelope with a particular frequency, which can be considered as a powerful criterion for BRB detection. However, it is relatively difficult to extract especially at time-varying loads due to the non-stationary characteristics. Thus, a novel envelope extraction method (EEM) combining fast Fourier transform and sliding overlapping window is proposed and the overall performance is tested by simulation. The results show that the method is capable to effectively extract the current envelope, but the extraction accuracy is unsatisfied due to the spectrum leakage in the case of non-integer periodic truncation. The technique of discrete spectrum correction is then introduced into EEM to improve its extraction accuracy, and then, an improved EEM (IEEM) is proposed and then tested by simulation. The results show that the IEEM is able to eliminate the impact of non-integer periodic truncation, effectively and accurately extract the current envelope. Finally, the two methods are applied in BRB fault online detection of 1.1 and 2.2 kW induction motors at time-varying loads. The corresponding experimental results demonstrate its validity and universality again.