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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
41

OPTIMISATION DE LA COMMANDE D'UN ONDULEUR MLI A TROIS NIVEAUX DE TENSION POUR MACHINE ASYNCHRONE

Depernet, Daniel 18 December 1995 (has links) (PDF)
Les progrès réalisés ces dernières années dans la commande des onduleurs MLI à deux niveaux de tension pilotant en vitesse variable des machines asynchrones de moyenne et grande puissance ont permis d'améliorer sensiblement les performances. L'utilisation récente des onduleurs MLI à trois niveaux de tension est très prometteuse en raison de la capacité de cette nouvelle structure d'onduleur à améliorer la qualité des courants dans la machine et à étendre sa commande à des domaines de puissance supérieurs à 2 MW. Nous définissons le principe de calcul et les limites de validité des principales techniques de modulation sinusoïdales à trois niveaux. Certaines d'entre elles se révèlent être tout à fait adaptées et performantes pour commander la machine dans les zones de faible vitesse. Nous proposons ensuite une méthode systématique de construction d'une commande optimisée. Les signaux sont calculés pour minimiser les harmoniques de courant et les pulsations de couple. Cette méthode est appliquée à un exemple concret, et la commande est implantée sur le microcontrôleur 80C196KC. La simulation des régimes établis permet d'affiner les études qualitatives des signaux. La bonne concordance entre les résultats expérimentaux et les résultats théoriques permet de confirmer la conservation des caractéristiques des signaux par la méthode de génération en temps réel. L'étude de l'association de la commande optimisée et d'une loi de contrôle vectoriel permet enfin de prouver la faisabilité d'un tel contrôle. Elle permet d'envisager l'amélioration des performances des entraînements de grande puissance en régimes permanent et transitoire simultanément.
42

Load Commutated SCR Current Source Inverter Fed Induction Motor Drive With Sinusoidal Motor Voltage And Current

Banerjee, Debmalya 01 July 2008 (has links)
This thesis deals with modeling, simulation and implementation of Load Commutated SCR based current source Inverter (LCI) fed squirrel cage induction motor drive with sinusoidal voltage and sinusoidal current. In the proposed system, the induction motor is fed by an LCI. A three level diode clamped voltage source inverter (VSI) is connected at the motor terminal with ac chokes connected in series with it. The VSI currents are controlled in such a manner that it injects the reactive current demanded by the induction motor and the LCI for successful commutation of the SCRs in the LCI. Additionally, it absorbs the harmonic frequency currents to ensure that the induction motor draws sinusoidal current. As a result, the nature of the motor terminal voltage is also sinusoidal. The concept of load commutation of the SCRs in the LCI feeding an induction motor load is explained with necessary waveforms and phasor diagrams. The necessity of reactive compensation by the active filter connected at the motor terminal for the load commutation of the thyristors, is elaborated with the help of analytical equations and phasor diagrams. The requirement of harmonic compensation by the same active filter to achieve sinusoidal motor current and motor voltage, is also described. Finally, to achieve the aforementioned induction motor drive, the VA ratings of the active filter (VSI) and the CSI with respect to VA rating of the motor, are determined theoretically. The proposed drive scheme is simulated under idealized condition. Simulation results show good steady state and dynamic response of the drive system. Load commutation of the SCRs in the LCI and the sinusoidal profile of motor current and voltage, have been demonstrated. As in LCI fed synchronous motor drives, a special mode of operation is required to run up the induction motor from standstill. As the SCRs of the LCI are load commutated, they need motor terminal voltages for commutation. At standstill these voltages are zero. So, a starting strategy has been proposed and adopted to start the motor with the aid of the current controlled VSI to accelerate until the motor terminal voltages are high enough for the commutation of the SCRs in the LCI. The proposed drive is implemented on an experimental setup in the laboratory. The IGBT based three level diode clamped VSI has been fabricated following the design of the standard module in the laboratory. A generalized digital control platform is also developed using a TMS320F2407A DSP. Two, three phase thyristor bridges with necessary firing pulse circuits have been used as the phase controlled rectifier and the LCI respectively. Appropriate protection scheme for such a drive is developed and adopted to operate the drive. Relevant experimental results are presented. They are observed to be in good agreement with the simulation results. The effect of capacitors connected at the output of the LCI in the commutation process of the SCRs in the LCI is studied and analyzed. From the analysis, it is understood that the capacitors form a parallel resonating pair with filter inductor and the motor leakage inductance, which results in an undesired oscillation in the terminal voltage during each of the commutation intervals leading to commutation failure. So, in the final system, the capacitors are removed to eliminate any chance of commutation failure of the SCRs in the LCI. It is shown by experiment that the commutation of the SCRs takes place reliably in the absence of the capacitors also. The commutation process is studied and analyzed without the capacitors to understand the motor terminal voltage waveform of the experimental results.

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