Mohamed Salah

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Master Technologist
Higher Institute of Technologi... · Electrical Engineering

Publications

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    ABSTRACT: In brushless excitation systems, the rotating diodes can experience open- or short-circuits. For a three-phase synchronous generator under no-load, we present theoretical development of effects of diode failures on machine output voltage. Thereby, we expect the spectral response faced with each fault condition, and we propose an original algorithm for state monitoring of rotating diodes. Moreover, given experimental observations of the spectral behavior of stray flux, we propose an alternative technique. Laboratory tests have proven the effectiveness of the proposed methods for detection of fault diodes, even when the generator has been fully loaded. However, their ability to distinguish between cases of diodes interrupted and short-circuited, has been limited to the no-load condition, and certain loads of specific natures.
    Energy Conversion, IEEE Transaction on 04/2014; pp(99).
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    ABSTRACT: 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.
    International Conference on Green Energy (ICGE'14), Sfax, Tunisia; 03/2014
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    ABSTRACT: 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.
    ISA transactions 02/2014; · 1.63 Impact Factor
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    ABSTRACT: 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 alternator’s frame.
    14th international conference on Sciences and Techniques of Automatic control & computer engineering (STA'13), Sousse, Tunisia; 12/2013
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    ABSTRACT: 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.
    ISA transactions 08/2013; · 1.63 Impact Factor
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    ABSTRACT: Because of initial manufacture and ageing phenomenon, rotating machines present an inherent mechanical disequilibrium. When a small mechanical unbalance fault affects the driving system part ensured by an induction machine, an additional undesirable torque will be superposed to the torque imposed by the load. This supplementary torque is a time varying signal according to rotor position. This time torque variation will be reflected on stator and rotor currents amplitudes by modulation behavior at rotational frequency. Stator current spectrum will contain sideband harmonics that are characteristic of mechanical unbalance defect. Moreover flux beaming around machine body, which results of rotor and stator currents variations, will be affected. In addition, the total inertia of the revolving part is also modified and creates a non uniform distribution of mass which, when the system rotates, generates a centrifugal force tending to move the rotor axis in radial direction. A suitable transducer allows the measurement of vibration signal that can be a useful amount to detect the apparition of the considered defect. This paper studies the effectiveness limits of stator current spectrum technique compared to classical vibration method for monitoring motor mechanical unbalance when it drives a real load presenting intrinsic mechanical unbalance. Experimental results show that axial stray flux analysis can be an alternative solution to cover effectiveness limitation of stator current analysis method.
    Codit'13, Hammamet, Tunisia; 05/2013
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    ABSTRACT: Mechanical unbalance fault is one of the most common reasons of excessive machine vibration. The control of such a type of defect makes it possible to avoid the damage and to guarantee machines functioning with the maximum effectiveness. This fault leads essentially to load torque oscillation around its average value at rotational frequency. Theoretical development based on magneto-motive forces approach shows the modulation effect of mechanical unbalance on rotor current expressions. Taking account rotor winding function distribution, supplementary induced currents in rotor meshes lead to an amplitude modulation of stator current around its supply frequency and its principal slot harmonic, at rotational frequency. Because a real load is not ideal and presents an intrinsic rotor asymmetry involving an additional time varying torque according to its rotor position, the stator motor current spectral harmonics produced by the load will overlap the harmonics caused by the main fault (rotor unbalance) since induction machine becomes sufficiently loaded. This paper examines detection of such machine defect by analysis of stator current spectrum for unloaded and loaded squirrel cage motor.
    2013 10th International Multi-Conference on Systems, Signals & Devices (SSD) Hammamet, Tunisia, March 18-21, 2013; 03/2013
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    ABSTRACT: Traditional faults diagnostic technique using motor current signature analysis is, generally, based on the assumption of a constant load torque. In fact, due to the design, construction, and ageing phenomenon of loads, torque imposed to the motor can not be strictly constant and, in most of cases, varies according to the machine rotor position. These variations of the load torque, which are not related to the health of the motor, involve amplitude modulation of machine stator current at rotational frequency. In this paper we propose a technique based on the introduction of a supplementary known mechanical unbalance on the motor rotor axis to estimate the unknown load unbalance level from time analysis of stator current signal. Simulation and experimental results show the effectiveness of the proposed method.
    Electrical Engineering and Software Applications (ICEESA), 2013 International Conference on; 01/2013
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    ABSTRACT: Rotating rectifier is a basic part of synchronous generators. Inappropriate operation of this component can prove costly for the machine’s owner. This paper presents theoretical analysis and experimental validation for detecting failure of brushless exciter rotating diodes that can fail either open circuit or short circuit. Harmonic analysis of the alternator output voltage waveform is performed when machine is unloaded as well as when it runs around its rated load. Apparition of characteristic frequencies can be useful to distinguish the rotating diodes state. By considering the relative amplitudes at specific harmonics, it is possible to discriminate short circuit diode failure case from open circuit diode breakdown.
    Electrical Engineering and Software Applications (ICEESA), 2013 International Conference on; 01/2013
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    ABSTRACT: The matter deposit on the blades of an induction machine fan is a frequent and potential cause of mechanical unbalance, especially in industries with dusty environment. This fault condition is one of the most common reasons of excessive machine vibration. The control of such a type of defect makes it possible to avoid the damage and to guarantee machines functioning with the maximum effectiveness. Several techniques have been proposed for the detection of mechanical unbalance. However, the analysis of this defect, generally, assumes that the load torque still constant. In fact, a real load is not ideal and presents an intrinsic rotor asymmetry involving an additional time varying tor-que according to its rotor position. Then, the load torque varies with the rota-tional speed and the stator motor current spectral harmonics produced by the load will overlap the harmonics caused by the main fault (matter deposit). This paper studies the limits of effectiveness of MCSA technique for the detection of mechanical unbalance when in the presence of a rotor position varying load.
    13th International Conference on Sciences and Techniques automatic Control and Computer Engineering STA 2012, Monastir, Tunisia; 12/2012
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    ABSTRACT: Context prediction is useful for energy saving and hence eco-efficient context-aware service by increasing the interval of context sensing. One way of predicting context is to recognize context patterns in an accurate manner. Traditionally, clustering method has been widely used in pattern recognition, as well as image processing and data analysis. Clustering aims to organize a collection of data items into a specific number of clusters, such that the data items within a cluster are more similar to each other than they are items in the other clusters. In this paper, a modified fuzzy possibilistic clustering algorithm is proposed based on the conventional Fuzzy Possibilistic C-means (FPCM) to obtain better quality clustering results. To show the feasibility and performance of the proposed method, numerical simulation is performed in an actual amusement park setting. The results of the numerical simulation shows that the proposed clustering algorithm gives more accurate clustering results than the FCM and FPCM methods.
    New Challenges for Intelligent Information and Database Systems [original works presented during a poster session organized within the 3rd Asian Conference on Intelligent Information and Database Systems, 20-22 April 2011, Daegu, Korea]; 01/2011
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    Mohamed Salah, Moncef Gossa
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    ABSTRACT: In order to maintain a low downtime of machinery and an acceptable quality of product, it is essential to develop a robust technique to detect incipient faults in electrical machines. Stray Flux Analysis Method remained primarily restricted with the detection of broken rotor bars, stator windings short-circuit, supply voltage unbalance and gearbox fault, which are related to induction machines. The aim of this paper is to apply this technique for detecting an outer raceway defect of a ball bearing which supports the rotor of a squirrel-cage motor. The experimental results show the effectiveness of an exploring coil, used as stray flux sensor, in the detection of the considered fault, but a sensitivity variation according to its axial or radial position.
    6th International Conference on Electrical Systems and Automatic Control JTEA 2010, Hammamet, Tunisia; 03/2010
  • Mohamed Salah, Moncef Gossa
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    ABSTRACT: Dans toute industrie, il y a toujours un risque de dysfonctionnement des équipements qui affecte la sûreté des installations même dans les conditions normales à cause du phénomène de vieillissement. Une stratégie de surveillance doit, alors, être mise en place et il est important de chercher à minimiser son coût. Dans ce papier nous effectuons une étude expérimentale comparative entre les signatures vibratoire et magnétique, à travers l’analyse du flux de dispersion, d’un défaut de roulement à billes d’un moteur à induction. Les résultats expérimentaux montrent l’efficacité de la détection d’un tel défaut moyennant une simple bobine exploratrice.
    5ème Conference Internationale JTEA 2008, Hammamet, Tunisia; 05/2008

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