The Role of the Mechanical Speed Frequency on the Induction Motor Fault Detection via the Stray Flux

Lately, the monitoring and analysis of the induction motor stray flux has been a modern trend and significant research work has been accomplished. Most papers have focused on the monitoring of rotor electrical faults around the fundamental stray flux signature, imitating in this way the traditional Motor Current Signature Analysis (MCSA). However, more significant fault related harmonics exist at other frequencies and most significantly around the mechanical frequency. The existence of the mechanical frequency in the stator current is still the best signature for detection of the mixed rotor eccentricity fault. Even healthy motors present this harmonic due to some low level inherent eccentricity. Despite that, it will be shown for the first time in this paper, with extensive Finite Element Analysis (FEA) and experimental testing, that the mechanical frequency associated harmonics in the stray flux can be purely rotor electrical fault related and completely independent from any rotor eccentricity and rotor imbalance. This makes this specific harmonic unreliable for any rotor fault diagnosis although can be a good indicator of rotor electrical faults at low slip operation. Finally, the sidebands of the mechanical frequency harmonics appear to be very sensitive to the broken rotor bar fault while quite immune to the number of the rotor bars.