Space-time representation of the main air gap flux of a three phase squirrel cage induction motor and its application to detect eccentricity

Abstract

The stator current and main air gap electromagnetic flux of an induction motor produces different types of harmonics even when they are run by balanced three-phase supply. A lot of these harmonics are created due to the spatial positions as well as slot structure of stator and rotor and thus known as space harmonics. If the supply is imbalanced or the motor is fed from an inverter, which is the most likely case in industrial environment, even more harmonics are created in time domain. As unwanted as these harmonics may be, they have a good use for condition monitoring of the motor. The harmonics show a certain behavior for a healthy motor and deviations from those behavior may lead to detect a fault as it happens. Most of the existing electrical techniques of condition monitoring are based on the time harmonics analysis of the stator current and/or leakage flux. We propose to analyze the space harmonics of the main air gap electromagnetic flux along with the time domain analysis to detect faults. To this aim, we present a model to analyze the space harmonics of the main air gap flux of a healthy as well as an eccentric induction motor in this paper. We also propose a method to detect and differentiate among different types of eccentricity, i.e. static, dynamic and mixed eccentricity by analyzing the main air gap flux with respect to both stator and rotor. Our presented results show that the space harmonics analysis can be used to detect and differentiate between different types of eccentricity unambiguously.