Khmais Bacha

Comparative investigation of diagnosis media for induction machine mechanical unbalance fault

For an induction machine, we suggest a theoretical development of the mechanical unbalance effect on the analytical expressions of radial vibration and stator current. Related spectra are described and characteristic defect frequencies are determined. Moreover, the stray flux expressions are developed for both axial and radial sensor coil positions and a substitute diagnosis technique is proposed. In addition, the load torque effect on the detection efficiency of these diagnosis media is discussed and a comparative investigation is performed. The decisive factor of comparison is the fault sensitivity. Experimental results show that spectral analysis of the axial stray flux can be an alternative solution to cover effectiveness limitation of the traditional stator current technique and to substitute the classical vibration practice.

An improved spectral analysis of the stray flux component for the detection of air-gap irregularities in squirrel cage motors

For machines׳ monitoring purpose, the classical motor current signature analysis has shown its weakness in distinguishing the eccentricity occurrence in presence of others mechanical faults. Although Park׳s vector approach can cover this drawback, the high cost due to the requirement to use three current sensors associated with an advanced processing technique, makes it less desired by industrialists. In this paper, we suggest an alternative diagnosis method based on a suitable processing of the stray flux data. The experimental results have revealed the potential of a simple search coil for the detection and the distinction of the accurate eccentricity nature even in presence of similar mechanical faults.

A low-cost sensor for online detection of a transient abrupt drop in the drive speed of a three-phase synchronous generator

Foremost systems of electrical energy production are based on three-phases synchronous generators. In this work we are going to propose an efficient diagnosis technique, based on the use of a low-cost sensor, for the online detection of a transient abrupt drop in the drive speed system. Tests have been conducted on a four-pole brushless alternator driven by a DC shunt motor. Through conventional time-frequency analysis, four different techniques are suggested. Whereas the two first are based on the load currents analysis (one phase and three phases), the two other methods are based on the stray fluxes analysis (axial and transversal components). The experimental results have shown that, without the need of high-cost sensors (current probes), the investigated system failure can be effectively detected by means of simple search coil placed in a radial position near the alternators frame.

Experimental investigation of the eccentricity impact on the line current spectrum for induction motors fault diagnosis purposes

This paper deals with the use of the stator current signature analysis as a technique for the diagnostic of static, dynamic and mixed air-gap-eccentricity conditions in working three-phase induction motors. Associated spectra are analyzed and attribute fault harmonics are determined for an induction motor, under several types of air-gap eccentricity conditions. The effect of load levels in air-gap eccentricity fault detection is presented. The experimental results have revealed the potential of the spectral analysis of the stator current for the air-gap eccentricity fault detection.

A Time-Frequency-Based Approach for Monitoring Three-Phase Synchronous Generators Speed

In this study, we address the topic of monitoring the drive speed of synchronous generators by time-frequency analysis of electromagnetic quantities available at the machines outputs. In this regard, after recalling the commonly used techniques to deal with signals of scalable spectral content, we present and discuss the detection of an abrupt variation of the drive speed by suitable processing of load currents. Afterward, we propose an alternative technique based on the instantaneous frequency estimation of the stray flux. Through experimental trials conducted on a laboratory test-rig, the effectiveness of the suggested approach has been assessed under various load conditions, and its performance has been judged against those stemming from load currents processing. Promising results have been concluded in terms of generator speed monitoring by the use of a low-cost noninvasive sensor.

A contact-less sensor for detecting low-frequency ripples of the load driven by a squirrel cage motor

Despite the well-known efficiency of the conventional MCSA (Motor Current Signature Analysis) technique for detecting the occurrence of load torque ripples by monitoring of sidebands close to the supply frequency, many other rotor faults produce this same spectral signature leading the user to be faced with a confusing situation. The aim of this paper is to propose an alternative that could overcome this major drawback. The experimental results conducted on a laboratory test-rig have revealed that the spectral signature of the emf induced at the terminals of a search coil placed near the motor frame may provide relevant information of diagnosis purpose, when the load torque oscillates at low frequencies.

Stator current signature analysis to monitor shaft misalignment in induction motor speed-controlled

In this work we discuss the efficiency of various stator current signatures for monitoring shaft misalignment that could affect mechanical systems when driven by an induction machine speed-controlled. In that respect, we have considered the Motor Current Signature Analysis, the Current Space Vector, and the Current Park Loci, under no-load as well as under full load conditions. On the other hand, the effect of the machines input-frequency on the detection efficiency is also discussed. Thus, we have assessed these three techniques when the motor is operating under full speeds condition and under half speeds condition. The presented results are experimentally validated on a laboratorys test-rig simulating an angular shaft misalignment.

Bearing fault detection in wind turbines using dominant intrinsic mode function subtraction

This paper deals with a fault detection method based on an empirically data-driven approach combined to a statistical tool. This approach is an enhanced version of the empirical mode decomposition. The proposed fault detector application to bearing defects in wind turbine based on induction generator clearly shows that it is well suited for stationary and non-stationary behavior regardless the rank of the intrinsic mode function introduced by the fault.

A novel technique for detecting unbalance condition of the stator currents drawn by a squirrel cage motor

The aim of this work is to provide an alternative that could replace the conventional technique based on the spectral analysis of the line current to detect an unbalance state due to a weak amplitude drop affecting one of the three-phase currents drawn by a squirrel cage motor. From a theoretical investigation, it has been stated that the spectral analysis of the emf induced at the terminals of a simple search coil placed close to the motor frame may be of interest for diagnosis purpose. The experimental results have revealed that this suggested sensory signal provides an excellent fault indicator regardless of the load level.

Cepstral analysis of the stator current for monitoring mechanical unbalance in squirrel cage motors

Elderly loads cannot impose to the driving rotation process a strict unvarying torque. This truth can occur even with new loads because of inherent manufacturing imperfections. In the majority of circumstances, the load torque varies according to the machine rotor position and these variations, which are not related to the motor health condition, involve an amplitude modulation of the machine stator current at rotational frequency. Thus, harmonics caused by an eventual mechanical unbalance can be overlapped and the user can not distinguish a normal operating condition from a failure mode. Since an induction machine becomes sufficiently loaded, classical diagnosis technique based on the spectral analysis of the stator phase current becomes insufficient to monitor such fault incidence; in this paper we suggest an alternative diagnosis practice based on the analysis of the complex cepstrum. Simulation and experimental results show the effectiveness of the proposed method.