Fault diagnosis of induction motors by space harmonics analysis of the main air gap flux

Abstract

A new fault diagnosis scheme based on the monitoring of main air gap flux of the three phase squirrel cage induction motor is proposed. Most of the existing electrical techniques of condition monitoring are based on the stator current and/or leakage flux measurement outside the motor. Besides, most of these methods only concentrate on the time harmonics analysis of the acquired signal. We propose to analyse the space harmonics of the main air gap electromagnetic flux along with the time domain analysis. To this aim, we propose to monitor the main air gap flux using Hall Effect Flux Sensors (HEFS) at all the stator slots of an induction motor, which can be used to address the stator and rotor slot effects not only through frequency analysis of the time varying magnetic flux, but also by magnitude and phase shift comparison of sensors located at different geometric positions around the stator. Moreover, by using HEFS in all the stator slots, the flux density along the circumference of the stator can be acquired and the space harmonics of the flux can be analysed, which shown promising results in detecting and locating different faults. In this paper, we concentrate on detecting different types of rotor faults such as broken rotor bar fault and eccentricity.

A New Approach for Broken Rotor Bar Detection in Induction Motors Using Frequency Extraction in Stray Flux Signals

“…The same rotor faults are studied in [36]- [38] using internal Hall-effect flux sensors, providing an assiduous analytical description of the air-gap flux space harmonics. Also, stray flux signature analysis is applied for bar breakages in [39] & [40], and in [41]- [43] for bearing fault detection using also a statistical based approach.…”

“…When a bar breakage occurs, a backward rotating magnetic field is generated due to the open-circuited bar and the lack of inter-bar and eddy currents in the point of breakage creating in the rotor cage an asymmetry that is clearly reflected in the motor's harmonic content [6], [11], [35]- [38]. This fault asymmetry is known to cause additional frequency sidebands, distanced by multiples of the motor's slip s from the fundamental frequency and its multiples [21], [36]. These appear in the spectrum of stator-related quantities (stator current or stray flux), modulated by the component (1 − ) because of the continuous induction from the rotor to the stator and vice-versa.…”

“…These appear in the spectrum of stator-related quantities (stator current or stray flux), modulated by the component (1 − ) because of the continuous induction from the rotor to the stator and vice-versa. The equation for these fault related sideband signatures is the following [36], [37]:…”

Analysis of Stray Flux Spectral Components in Induction Machines under Rotor Bar Breakages at Various Locations

“…Similar approach, focusing on the low frequency components is introduced in [35] for examination of eccentricity and broken bar faults. The same rotor faults are studied in [36]- [38] using internal Hall-effect flux sensors, providing an assiduous analytical description of the air-gap flux space harmonics. Also, stray flux signature analysis is applied for bar breakages in [39] & [40].…”

“…When a bar breakage occurs, a backward rotating magnetic field is generated due to the open-circuited bar and the lack of inter-bar and eddy currents in the point of breakage, creating in the rotor cage an asymmetry that is clearly reflected in the motor's harmonic content [6], [11], [35]- [38]. This fault asymmetry is known to cause additional frequency sidebands, distanced by multiples of the motor's slip s from the fundamental frequency fs and its multiples [21], [36]. These appear in the spectrum of stator-related quantities (stator current or stray flux), modulated by the component (1-s)fs because of induction from the rotor to the stator and viceversa.…”