Rozprawy doktorskie na temat „FED INDUCTION MOTOR”

Induction motors are widely used in industrial processes. The malfunction of a motor may not only lead to high repair costs, but also cause immense financial losses due to unexpected process downtime. Since thermal overload is one of the major root causes of stator winding insulation failure, an accurate and reliable monitoring of the stator winding temperature is crucial to increase the mean time to catastrophic motor breakdown, and to reduce the extraordinary financial losses due to unexpected process downtime. To provide a reliable thermal protection for induction motors fed by motor control devices, a dc signal-injection method is proposed for in-service induction motors fed by soft-starter and variable-frequency drives. The stator winding temperature can be monitored based on the estimated stator winding resistance using the dc model of induction motors. In addition, a cooling capability monitoring technique is proposed to monitor the cooling capability of induction motors and to warn the user for proactive inspection and maintenance in the case of cooling capability deterioration. The proposed cooling capability monitoring technique, combined with the proposed stator winding temperature monitoring technique, can provide a complete thermal protection for in-service induction motors fed by motor control devices. Aside from online thermal protection during a motor's normal operation, the thermal protection of de-energized motors is also essential to prolong a motor's lifetime. Moisture condensation is one of the major causes to motor degradation especially in high-humidity environments. To prevent moisture condensation, a non-intrusive motor heating technique is proposed by injecting currents into the motor stator winding using soft-starters. A motor's temperature can be kept above the ambient temperature due to the heat dissipation, so that the moisture condensation can be avoided. To sum up, active stator winding temperature estimation techniques for induction motors under both operating and de-energization conditions are proposed in this dissertation for both thermal protection and optimizing the operation of a motor system. The importance of these proposed techniques lies in their non-intrusive nature: only the existing hardware in a motor control device is required for implementation; a motor's normal operation is not interrupted.

The Power Electronics Research group at Aberdeen University is working towards providing a complete time domain model of an electrical drive for marine applications. Effects such as vibration, shaft and propeller induced oscillations and noise in ships due to the use of cycloconverters will be studied in this way. This thesis therefore reports the investigation of machine vibrations resulting from electromagnetic forces. The machines of primary interest are ac induction machines, but much of the work reported in this thesis has broader significance. To assess the vibration problems of propulsion motors, the exciting force and the vibration behaviour of the motor must be modelled. The investigation has therefore been carried out in five principal areas. The development and use of a new time domain magnetic force model. The development of a method to calculate machine stator natural frequencies. A laboratory investigation of system parameters of both a stator and also an entire motor, which draws important conclusions not previously reported. A machine vibration prediction method based on analysis of interactions between the magnetic forces and the motor system parameters. Vibration measurements on a machine to provide data for correlation with predicted results. The Saber simulator has been used throughout to develop the force calculation model. The force model is used to calculate the time and space dependent magnetic force wave, in particular, when the motor is loaded and fed from a nonsinusoidal converter, which makes this model particularly useful for machine vibration calculations. The computer model can be readily used with previously developed cycloconverter models to achieve a complete cycloconverter propulsion simulation.

This thesis analyzes the power losses in induction machines and how the losses depend on the harmonic content of the applied voltages. Two cases are compared, one case where a machine is fed with a sinusoidial voltage and one case with a modular multilevel converter (M2C). The sine is representing an ideal grid while the M2C represents a case with harmonic content. The usage of converters for electrical drive systems is increasing due to advantages when the rotor speed could be variable by changing the frequency of the voltage. This is usually increasing the efficiency of the overall system, but is also adding harmonics fed to the machine and switching losses in the converter. Low switching losses in the inverter usually create higher harmonic content that instead increases the losses of the machine. The M2C is then proposed as a converter topology that can keep the harmonic content low while keeping the switching losses relatively low. This study focuses on the iron losses, the part of the total losses that is most hard to predict or measure. Today’s methods used to calculate the iron losses are often rough approximations that do not take the impact of the harmonic content of voltage into consideration, even though the iron losses are dependent on the harmonics. Experimental results in the study show that the losses of a M2C-fed case do not differ much from a sine-fed case. The difference could be explained by low increase of iron losses caused by the small harmonic content from the M2C. The increase of iron losses was linked to the harmonic content of the voltage.
Detta examensarbete analyserar effektförluster i induktionsmaskiner och hur förlusterna beror på övertonsinnehållet i den matande spänningen. Två fall kommer att jämföras, ett fall där en maskin är matad från en sinus spänning och ett fall med en modulär multinivå omvandlare (M2C). Sinusen representerar ett idealt nät medan M2C representerar ett fall med övertonsinnehåll. Användning av omvandlare för elekriska drivsystem ökar på grund av fördelarna när rotorhastighet kan varieras genom att ändra frekvensen från den matande växelriktaren. Detta ökar vanligtvis verkningsgraden på det sammanlagda systemet, men detta bidrar även med övertonsinnehåll matat till maskinen och switchförluster i omvandlaren. Låga switchförluster i omvandlaren medför oftast ett högt övertonsinnehåll som istället ökar förlusterna i maskinen. M2C är därför föreslaget som en teknik som håller övertonsinnehållet lågt medan switchförlusterna är relativt låga. Denna studie fokuserar på järnförluster, den del av de totala förlusterna som är som svårast att förutse eller mäta. De metoder som finns för att beräkna järnförlusterna är vanligtvis grova skattningar som inte tar hänsyn till inverkan från spänningens övertoninnehåll, även om järnförluster beror på övertonerna i stor utsträckning. Experimentella resultat i studien visar att förlusterna i ett M2C-matat fall inte avviker i stor utsträkning jämte ett sinusmatat fall. Skillnanen kan förklaras utifrån den lilla ökningen av järnförluster från det låga övertonsinnehållet från M2C:n. Järnförlusterna ses vara kopplade till övertonsinnehållet i spänningen.

Induction motors are usually fed by voltage source inverters. In some cases, it is necessary to increase the speed of the motor beyond the rated speed. This could be done by increasing the supply voltage (which is not always possible) or by field weakening. Field weakening algorithms are desirable since the current and the voltage of the motor cannot exceed the rated values. However, the range of the controlled speed is limited because of the limitation of the input voltage of the inverter. In this project, considering the limitation of current and voltage, the fundamentals of the field weakening in induction motor are reviewed. Th motor performance is evaluated under a variable range of the input DC voltage. A three-phase boost converter is used to regulate the input DC voltage of the inverter. In fact, when the requested voltage of the control algorithm is greater than the producible voltage of the inverter, the input DC voltage of the inverter is increased by the boost converter. The performance of this drive is evaluated in terms of current, torque and speed of induction motor. Besides, the efficiency of the drive is discussed.

The use of variable frequency drives in centrifugal pump applications has raised the question of how to select a drive. Clogging obstacles in waste water applications create unknown transient loads for the pump system. A sudden load increase occurrence can clog the pump if the drive cannot supply enough current to reach the motor’s torque demand. In order to select a suitable drive, an empirical approach has been implemented, investigating three different drives. Results have shown that selecting a drive with the highest possible overload capabilities, even if for a short time is most suitable. Operation in vector speed control gives the most reliable operation if an automatic parameter tuning is performed by the drive.
Användningen av frekvensomriktare i centrifugalpumpar har väckt fr ågan om hur en omriktare skall väljas. Igensättande objekt i avloppsvatten kan ge upphov till transienta laster i pumpsystemen. En oförutsedd lastökning kan sätta igen pumpen om frekvensomriktaren inte kan förse motorn tillräckligt med ström för att möta momentbehovet. För att välja en lämplig omriktare har ett empiriskt tillvägag ångssätt valts i en undersökning av tre olika omriktare. Resultat har visat att det är lämpligast att välja en omriktare med högst överbelastningskapacitet, även om under en kort tid. Vektor hastighetskontroll är metoden som ger stabil körning om omriktaren f ått automatiskt ställa in motorparametrarna.

The development of an online method for the early detection of a stator turn insulation deterioration is the objective of the research at hand. A high percentage of motor breakdowns is related to the failure of the stator insulation system. Since most of the stator insulation failures originate in the breakdown of the turn-to-turn insulation, the research in this realm is of great significance. Despite the progress that has been made in the field of stator turn fault detection methods, the most popular and the best known ones are still limited to the detection of solid turn faults. The time span between a solid turn fault and the breakdown of the primary insulation system can be as short as a few seconds. Therefore, it is desirable to develop a method capable of detecting the deterioration of the turn insulation as early as possible and prior to the development of a solid turn fault. The different stresses that cause the aging of the insulation and eventually lead to failure are described as well as the various patterns of an insulation failure. A comprehensive literature survey shows the methods presently used for the monitoring of the turn insulation. Up to now no well-tested and reliable online method that can find the deterioration of the turn insulation is available. The most commonly used turn insulation test is the surge test, which, however, is performed only when the motor is out of service and disconnected from the supply. So far no research at all has been conducted on the application of an online surge test. The research at hand examines the applicability of the surge test to an operating machine. Various topologies of online surge testing are examined with regard to their practicability and their limitations. The most practical configuration is chosen for further analysis, implementation and development. Moreover, practical challenges are presented by the non-idealities of the induction machine like the eccentricity of the rotor and the rotor slotting, and have to be taken into account. Two solutions to eliminate the influence of the rotor position on the surge waveform are presented. Even though the basic concepts of online surge testing can be validated experimentally by a machine with a solid turn fault, it is preferable to use a machine with a deteriorated turn insulation. Therefore, a method, which does not require complex and expensive hardware, to experimentally emulate the turn insulation breakdown is implemented. The concepts at any stage of the work are supported by simulations and experimental results. In addition, the theory of surge testing is further developed by giving new definitions of the test's sensitivity, i.e., the frequency sensitivity and the error area ratio (EAR) sensitivity.

Due to the ruggedness, reliability and low maintenance required, small inverter-fed induction motors (1.5 kW to 25 kW) have earned a reputation and are utilized more and more frequently in industry for variable speed applications. The applied voltage and current waveforms are generally quite non-sinusoidal and polluted with higher time harmonic components. The higher time harmonics affect the electrical and mechanical performance significantly, and adverse effects are investigated. Literature on the effect of higher time harmonics is reviewed. Recommendations referring to various aspects under non-sinusoidal excitation are made when installing inverter-fed induction motors. Different inverter supplies (6-Pulse Square Wave voltage source inverter (VSI), PWM VSI and sinusoidal excitation) are used with a small 3-Phase, 3-kW squirrel cage induction motor, configured under synchronous characteristic Vᵣ/50 and subsynchronous characteristic Vᵣ/87. The respective voltage and current frequency spectrums of the excitation source are analyzed and the harmonic content determined. Performance executed under identical operating respectively and results are compared with pure sinusoidal (fundamental) excitation. tests are conditions respect to To determine the electrical performance the efficiency, power factor, temperature rise of the stator and induced shaft voltages were analyzed. The mechanical performance under inverter operation was analyzed by measuring the torque, torque pulsations, critical speeds and resonance frequencies of frame vibrations and torque oscillations as well as vibrations in x-, y-, and z-direction of the inverter-fed induction machine. Consequences of time harmonics on the induction motor and system are discussed. Conclusions on the effect of time harmonics and performance of different inverter types compared to sinusoidal excitation are drawn and are theoretically summarized and justified. Summary in English and German. Bibliography: pages 148-154.

Induction machine drives with various configurations are getting a lot of attention in several industrial applications. Due to this increasing demand in industrial applications, the significance of developing effective control approaches for obtaining a high dynamic performance from the induction machine drives became essential. Up to the present time, the control of induction machine drives using power converters has been based on the principle of mean value, using pulse width modulation with linear controllers in a cascaded structure. Recent research works have demonstrated that it is possible to use Predictive Control to control induction machine drives with the use of power converters, without using modulators and linear controllers. This new approach will have a strong impact on control in power electronics in coming decades. The advantages of Predictive Control are noticed through the ability to consider a multi-objective case within the model, easy inclusion of non-linearities within the model, simple treatment of system constraints, easy of digital implementation, and flexibility of including modifications and extension of control horizons according to the required applications. Upon this, the research presented in this thesis concerns with developing different control topologies for various configurations of induction machine drives based on finite control set model predictive control (FCS-MPC) principle, which actuates directly the switch states of the voltage source inverter (VSI). In addition, for enhancing the robustness of the induction machine drives, different sensorless approaches are utilized and tested for validations. The first topology of induction machine drives that has been studied is the induction motor (IM) drive. An effective model predictive direct torque control (MP DTC) approach is used to control the torque and stator flux of the motor through the utilization of an effective cost function, through which the understanding and comparing implementation variants and studying convergence and stability issues can be easily investigated. The speed sample effect on the control variants and overall performance of the proposed MP DTC is analyzed, which enables the understanding of the real base principle of DTC, as well as why and when it works well. Two different sensorless procedures for estimating the speed and rotor position are used by the proposed MP DTC approach; the first utilizes a model reference adaptive system (MRAS) observer, while the other exploits the prediction step during the implementation of proposed MP DTC to get the speed information through performing a linear extrapolation of the speed values starting from the last two estimated samples. Extensive simulation and experimental tests have been carried out to validate the effectiveness of both sensorless approaches in achieving precise tracking of speed commands for a wide range of variations. For enhancing the robustness of proposed MP DTC, the stator flux as a control variable is replaced with controlling the flow of the reactive power through the induction motor drive. As the reactive power is a measured quantity compared with the estimated value of stator flux, thus, the sensitivity of the control against parameters variation is limited, and this confirmed through the obtained results from both simulation and experimental tests. In addition, an effective alternative approach to the MP DTC is presented, which based on controlling the instantaneous values of the active and reactive powers of the IM drive based on model predictive principle, instead of controlling the torque and flux as in MP DTC. This technique has the advantage that all controlled variables are became measured quantities (active and reactive powers), thus the estimation problems that commonly present in classic DTC schemes are effectively limited. For the last two control approaches (MP DTC reactive power control, and MP IPCactive and reactive power control), the sensorless that utilizes the predictive feature is also adopted. Obtained results via simulation and experiments confirm the feasibility of the two alternatives control procedures in obtaining a robust dynamic response of IM drive. To limit the accompanied ripple contents in the controlled values of electromagnetic torque and stator flux of induction motor, an effective ripple reduction technique has been presented. The technique is based on the derivation of the optimal value for the weighting factor (w_f) used in the cost function. A detailed mathematical derivation of the optimal value of w_f is introduced based on the analysis of torque and flux ripples behaviors. The proposed ripple reduction technique has been validated via simulation utilizing Matlab/Simulink software, and experimentally tested using a fast control prototyping dSpace 1104 board. In addition, the prediction step based sensorless approach is adopted during implementation. The performance of the IM drive using the proposed approach is compared with the results obtained from MP DTC approach that uses an arbitrary value of w_f. The comparison confirms the validity of the proposed ripple reduction procedure in reducing the ripple contents in the controlled variables while preserving the permissible computation burdens during the implementation. The FCS-MPC principle is also utilized to control the current of induction motor as an alternative to classic field oriented control (FOC), the proposed model predictive current control (MPCC) approach belongs to the class of the hysteresis predictive control (for limiting the switching frequency) as the MPCC is triggered by the exceeding of the error of a given threshold. In addition, a sensorless drive is achieved by including an effective Luenberger observer (LO) for precise estimation of rotor flux vector together with stator current, speed and load torque. The stator currents are estimated to eliminate the accompanied noise in their values when they are directly measured, thus the currents noise during prediction is limited. An effective pole placement procedure for the selection of observer gains has been adopted. The procedure is based on shifting the poles of the observer to the left of the motor poles in the complex (s-plane) with low imaginary part, so that the stability of the observer is enhanced for wide speed range. The feasibility of the sensorless MPCC for IM drive is confirmed through the obtained simulation and experimental results. The second topology of induction machine drives that has been studied is the doubly fed induction motor (DFIM) drive. An effective model predictive direct torque control (MP DTC) algorithm is developed for controlling the torque and rotor flux of DFIM drive. In addition, an effective sensorless approach is presented, which estimates the speed and rotor position in an explicit way without the need for involving the flux in the estimation process, thus the effect of parameters variation on the overall performance of the sensorless observer is effectively limited, this has been approved through the obtained results that are performed for a wide speed range from sub-synchronous to super-synchronous speed operation. During the operation, the stator resistance and magnetizing inductance values are changed from their original values to study the variation effect on the observer performance. Matlab/Simulink software and a prototyping dSpace 1104 control board are used to validate the effectiveness of proposed sensorless MP DTC approach through simulation and experiments, respectively. The results proof the robustness of the proposed sensorless approach and its ability to achieve precise estimation of the speed and rotor position. The third topology of induction machine drives that has been studied is the doubly fed induction generator (DFIG). A detailed analytical derivation for the proposed model predictive direct power control (MP DPC) approach for DFIG is presented, which as a sequence considered as a transposed control approach from the MP DTC used before for doubly fed induction motor (DFIM). A sensorless approach based on model reference adaptive system (MRAS) observer is adopted for estimating the speed and rotor position. Both simulation using Matlab/Simulink software and experimental test using a prototyping dSpace 1104 control board have tested the dynamic performance of the drive. Obtained results affirm the feasibility of the proposed MP DPC approach in achieving a decoupled control of active and reactive powers for DFIG. In summary, it can be said that the proposed model predictive control approaches have proved their ability in achieving high dynamic performance for different topologies of induction machine drives. In addition, the proposed sensorless techniques have confirmed their effectiveness for a wide range of speed variations. All of this are approved and validated through extensive simulation and experimental tests.
Gli azionamenti con machine ad induzione (macchine asincrone nelle loro varie configurazioni), stanno riacquistando molta attenzione in diverse applicazioni industriali. A causa di questo crescente interesse applicativo, è diventato di essenziale importanza lo sviluppo di efficaci tecniche di controllo per ottenere dagli azionamenti in questione elevate prestazioni dinamiche. Fino ad oggi, il controllo degli azionamenti con macchina a induzione alimentati da convertitori di potenza è basato sul “principio del valore medio” delle grandezze in commutazione, utilizzando la modulazione di larghezza di impulsi con controllori lineari in una struttura a cascata. Recenti ricerche hanno dimostrato che è possibile utilizzare il Controllo Predittivo per controllare gli azionamenti con macchina a induzione, con l'utilizzo di convertitori di potenza senza utilizzare modulatori e controllori lineari. Questo nuovo approccio avrà un forte impatto sul controllo dell'elettronica di potenza nei prossimi decenni. I vantaggi del Controllo Predittivo derivano dalla possibilità di perseguire problemi multi-obiettivo, di includere facile le non linearità all'interno del modello, di trattare in modo semplice i vincoli di sistema, nonché dalla facilità di implementazione digitale e dalla flessibilità di includere modifiche ed estensioni al controllo secondo le applicazioni richieste. Inlinea con tutto ciò, la ricerca presentata in questa tesi riguarda lo sviluppo di diverse topologie di controllo per varie configurazioni di azionamenti con macchine a induzione, basate sul principio di Controllo Predittivo a modello con insieme finito degli stati di controllo (Finite Control Set Model Predictive Control - FCS-MPC), che definisce direttamente l’assetto dell'inverter di tensione (VSI). Inoltre, per aumentare la robustezza degli azionamenti, vengono proposti e sperimentati diversi approcci senza sensori elettromeccanici (sensorless). La prima topologia studiata di azionamenti con macchina a induzione (IM) è l'azionamento con motore a gabbia. Il controllo diretto di coppia (DTC) è aggiornato in termini di controllo predittivo a modello (MP DTC) e usato per controllare la coppia e il flusso statorico attraverso l'utilizzo di una efficace funzione di costo attraverso la quale è anche possibile facilmente comprendere e confrontare le varianti di implementazione e studiare i problemi di convergenza e di stabilità. Viene analizzato l'effetto della velocità sulle diverse versioni di controllo e sulle prestazioni complessive del MP DTC proposto; ciò consente di comprendere appieno il principio del DTC, nonché perché e quando esso funzioni bene. Vengono utilizzate due diverse procedure di stima della posizione e della velocità del rotore nel MP DTC proposto; il primo utilizza uno stimatore adattivo con modello di riferimento (MRAS), mentre l'altro sfrutta la stessa fase di predizione del MP DTC proposto per ottenere le informazioni sulla velocità effettuando infine un'estrapolazione lineare dei valori di velocità a partire dagli ultimi due campioni stimati. Sono state eseguite numerose prove in simulazione e sperimentali per convalidare l'efficacia di entrambi gli approcci sensorless nell’ottenere un preciso inseguimento del comando di velocità per una vasta gamma di situazioni. Per migliorare la robustezza del MP DTC proposto rispetto alle variazioni parametriche, il controllo del flusso dello statore viene sostituito con quello della potenza reattiva assorbita dal motore ad induzione; di conseguenza la sensibilità del controllo alle variazioni dei parametri è limitata e ciò è confermato attraverso i risultati ottenuti sia dalla simulazione che dalle prove sperimentali. Inoltre, viene presentato un ulteriore efficace approccio alternativo per il MP DTC, basato sul principio del controllo predittivo a modello dei valori istantanei delle potenze attive e reattive dell'azionamento, invece di controllare la coppia e il flusso come nell’usuale MP DTC. Questa variante ha il vantaggio che tutte le variabili controllate sono divenute quantità misurate (potenze attive e reattive) e quindi i problemi di stima comunemente presenti nei classici schemi DTC sono efficacemente limitati. Per gli ultimi due approcci di controllo (controllo di coppia e di potenza reattiva e controllo di potenza attiva e reattiva) viene anche adottato la stima della velocità rotorica che sfrutta la funzione predittiva del controllo. I risultati ottenuti attraverso la simulazione e la sperimentazione confermano la fattibilità delle due procedure alternative di controllo per ottenere una risposta dinamica robusta dell’azionamento con IM. Per limitare il ripple che accompagna gli andamenti controllati della coppia e del flusso statorico del motore, è stata presentata una tecnica efficace di riduzione della sua ampiezza. La tecnica è basata sull’impiego di un valore ottimale per il fattore di ponderazione w_f utilizzato nella funzione di costo per sommare i due contributi che la definiscono. Viene introdotta una derivazione matematica dettagliata del valore ottimale di w_f attraverso l'analisi dei comportamenti dell’ondulazione di coppia e del flusso. La tecnica di riduzione del ripple proposta è stata verificata tramite la simulazione usando il software Matlab/Simulink e sperimentalmente utilizzando la scheda di rapida prototipazione del controllo dSpace 1104. Ancora, l'implementazione adotta l'approccio sensorless basato sulla fase di predizione. Le prestazioni dell’azionamento con IM utilizzando quest’ultimo approccio proposto sono confrontate con i risultati ottenuti con l'approccio MP DTC che utilizza invece un valore arbitrario di w_f. Il confronto conferma la validità della procedura di riduzione del ripple nelle variabili controllate mantenendo nel contempo gli oneri di calcolo entro i limiti consentiti per l'implementazione. Il principio FCS-MPC è anche utilizzato per controllare la corrente del motore di induzione come alternativa al controllo classico ad orientamento di campo (Field Oriented Control -FOC). L'approccio proposto di controllo di corrente di tipo predittivo (Model Predictive Current Control - MPCC) appartiene alla classe del controllo predittivo ad isteresi (per limitare il frequenza di commutazione) in quanto il MPCC viene attivato dal raggiungimento dell’errore di corrente di una determinata soglia. In questo caso, la caratteristica sensorless dell’azionamento è ottenuta includendo un efficace osservatore Luenberger (LO) per una precisa stima del vettore del flusso del rotore insieme alla coppia di carico e alla velocità. È stata adottata una efficace procedura di allocazione dei poli per la selezione dei guadagni dell'osservatore; la procedura si basa sul posizionamento dei poli dell'osservatore a sinistra di quelli del motore nel complesso (piano di s) con una ridotta parte immaginaria, in modo che la stabilità dell'osservatore sia migliorata in un'ampia gamma di velocità. La fattibilità dell'azionamento sensorless con MPCC è ancora confermata attraverso la simulazione e i risultati sperimentali. La seconda topologia degli azionamenti con macchina a induzione che è stata studiata è l'azionamento con motore ad anelli con rotore alimentato da invertitore e statore da rete (Doubly Fed Induction Motor DFIM). È stato sviluppato un efficace algoritmo predittivo a modello (MP DTC) per il controllo dinamico della coppia e del flusso di rotore dell'azionamento DFIM. Inoltre, viene presentato un approccio efficace di soluzione sensorless che valuta la velocità e la posizione del rotore in modo esplicito senza la necessità di coinvolgere la stima del flusso nel processo di predizione; di conseguenza l'effetto delle variazioni dei parametri sulle prestazioni complessive dell'osservatore di posizione e velocità è sensibilmente limitato. Questo è stato provato attraverso i risultati ottenuti con test eseguiti in un'ampia gamma di velocità, dal sub-sincronismo a velocità super-sincrona. Durante l'operazione, la resistenza dello statore e i valori di induttanza di magnetizzazione sono stati modificati rispetto ai valori reali per studiare l'effetto di variazioni parametriche sulle prestazioni dell'osservatore. Anche in questo caso, il software Matlab/Simulink e una scheda di controllo dSpace 1104 sono stati utilizzati per convalidare l'efficacia dell'approccio sensorless del MP DTC per l’azionamento. I risultati dimostrano la robustezza del controllo sensorless proposto e la sua capacità di ottenere una precisa stima della posizione e della velocità del rotore. La terza topologia di azionamenti con macchina a induzione che è stata studiata è quella del generatore ad induzione con rotore avvolto (DFIG) e invertitore sul rotore. Viene presentata una derivazione analitica dettagliata del controllo predittivo diretto di potenza (MP DPC) per DFIG, che trasferisce ed estende l’approccio di controllo del MP DTC citato prima per il motore a induzione a doppia alimentazione (DFIM). Una soluzione sensorless ancora basata sull'osservatore adattivo a modello di riferimento (MRAS) è adottato per stimare la velocità e la posizione del rotore. Sia le simulazioni usando il software Matlab/Simulink che i test sperimentali utilizzando la scheda dSpace 1104 hanno mostrato le elevate prestazioni dinamiche dell'azionamento. I risultati ottenuti confermano la fattibilità del metodo MP DPC proposto per ottenere un controllo disaccoppiato di potenze attive e reattive per DFIG. In sintesi, si può dire che l'utilizzo proposto del controllo predittivo a modello ha dimostrato la sua capacità di ottenere elevate prestazioni dinamiche per le diverse topologie degli azionamenti con macchina ad induzione considerati. Inoltre, le tecniche sensorless proposte hanno confermato la loro efficacia per una vasta gamma di velocità. Tutto questo è stato verificato e validato attraverso una vasta attività analisi simulativa e di sperimentazione in laboratorio.

Electric motor systems are the largest consumers of industrial electrical energy. As Variable Speed Drives continue to dominate various industrial processes, there is need for stakeholders to fully understand and quantify the converter-fed motor losses over a wide range of operating conditions. Such knowledge is crucial for both manufacturers and end-users in performing energy-efficiency optimizations for motor-drive applications. Although there is an increase in legislative activities, particularly in Europe, toward classification and improvement of energy efficiency of electric motor-drive systems, the available standards for quantifying the various losses are still in their early stages of development. None of these standards have yet passed through all the required phases for them to be considered full international standards, owing to a lack of consensus on many technical issues. Therefore, the need for researchers to provide feedback to the relevant standards committees cannot be over-emphasized. One of the most challenging issues in estimating the efficiency of converter-fed motors is the accurate determination of additional harmonic losses due to the PWM voltages and currents. Although the recently introduced IEC 60034-2-3 Technical Specification has proposed a method of determining these losses through experimental testing, the approach is still undergoing validation. Moreover, it only considers the rated motor frequency and voltage whereas induction motor drives are usually operated over a wide range of speed and torque. The main emphasis of the work presented in this dissertation was to develop a thorough understanding of various converter-fed induction motor losses, and hence efficiency, when fed from a 2-level Voltage Source Inverter. In particular, the dissertation provides a healthy questioning of some concepts in the proposed IEC method, with a view to providing useful feedback for improving the standard. Comparisons are also drawn between the related standards to identify areas for improvement. This study further attempts to explain some conflicting reports cited in literature regarding the nature of additional harmonic losses. The experimental results obtained by testing three induction motors demonstrate some of the technical issues associated with the determination of additional harmonic losses. To mitigate the adverse effect of varying technical skill and competence levels on efficiency test results, an automated testing procedure was developed and implemented on the 110kW test rig in the UCT Machines Lab. The test rig, which boasts of a Genesis 7i high-speed Data Acquisition System, also provides an energy-efficient platform for investigating the steadystate and dynamic characteristics of converter-fed motors. By utilizing the capability of the Data Acquisition System to segregate the fundamental and harmonic components of measured input electrical power, it was found that a PWM power supply can be used in place of a conventional Variac to estimate the sinusoidal supply efficiency of an induction motor. This is a welcome development for both laboratory and field efficiency testing applications.

This paper focuses on the Double Fed Induction Machine in the Pumped Storage Power Plants. It includes listing of this plants in the world, where such a technology is used; principle of Double Fed Induction Machine, its usage with variable speed, advantages and disadvantages and also experience of its usage.

With an increased demand on high-efficiency electrical machines in traction applications, better understanding in the losses induced by the converter is needed. The harmonic losses from the converter are non-negligible part of the total losses in an induction machine, and these harmonic losses have been studied here with a focus on iron, skin-effect and proximity losses. The main goal of the thesis is to advise how the modulation method can be changed to lower these time harmonic losses in an induction machine. A model of the induction machine was created in the FEM(Finite Element Method) software FLUX. The iron, the proximity and the skin-effect losses were simulated in the FLUX model with different modulation method to analyse the time harmonic losses due to respective phenomena. The modulation method at higher operation is square-wave modulation, this is due to that maximum fundamental voltage has been reached. At lower operation, pulse width modulation is used, and the switching frequency can be increased to lower the time harmonics losses in the motor. The iron losses simulated in FLUX show that the losses increase with frequency even if the motor enters flux weakening operation at higher frequencies. The proximity phenomena gives rise to a distinct increase of resistive losses in the conductor closest to the air gap in the stator winding. The skin-effect phenomena affects the losses at square-wave modulation of high frequencies mostly as the time harmonics are larger and of low order.
Med en ökad efterfrågan på högeffektiva elektriska maskiner i traktionsapplikationer, måste man ha en högre förståelse i de förluster som induceras av frekvensomvandlaren. Förlusterna från frekvensomvandlaren är en icke försumbar del av de totala förlusterna i induktionsmaskinen. Förlusterna har här analyserats med fokus på järn-, skineffekt- och proximityförluster. Huvudsyftet med examensarbetet är att ge råd i hur moduleringsmetoden kan ändras för att sänka övertonsförlusterna i en induktionsmaskin. En modell av induktionsmaskinen skapades i FEM(Finita Elementmetoden) mjukvaran FLUX. Järn-, proximity- och skineffektförlusterna simulerades i FLUX-modellen med olika moduleringsmetoder för att analysera övertonsförluster på grund av respektive fenomen. Moduleringsmetoden vid högre drift är fyrkantmodulering, det är på grund av att maximal fundamental spänning har uppnåtts. Vid lägre drift har pulsbreddsmodulering använts och switchfrekvensen kan ökas för att sänka övertonersförlusterna i motorn. Järnförlusterna som simulerades i FLUX-modellen visar att förlusterna ökar med frekvensen även när motorn utsatts för fältförsvagning vid högre frekvensker. Proximityfenomenet ger upphov till en tydlig ökning av resistiva förluster i ledaren närmast luftgapet i statorlindningen. Skineffektfenomenet påverkar förlusterna på fyrkantsmoduleringen av höga frekvenser, mest på grund av att övertonerna är större och av låg ordning.

Le développement de commandes en boucle fermée pour des entraînements électriques nécessite l'installation de capteurs pour avoir l'information de la rétroaction. Cependant, un éventuel défaut survenant sur l'un des capteurs installés (de courant, de vitesse/position,…) implique un disfonctionnement de la commande conduisant dans la plupart du temps à la mise hors service du système. Ces conséquences sont contraires aux exigences des industriels qui demandent des degrés de fiabilité du système de plus en plus élevés. Des statistiques montrent que le défaut capteur est fréquent. Il est donc impératif de trouver des solutions pour assurer la continuité de service des systèmes électriques dans le cas de présence de ce type de défaut. Tout d'abord, l'étude présentée dans ce manuscrit présente les technologies des différents capteurs installés et ce pour comprendre les raisons et le type de pannes qui pourraient survenir. Ensuite, le système sur lequel la validation des algorithmes développés est décrit. Il s'agit d'un entraînement électrique basé sur une machine à Double Alimentation (MADA) fonctionnant en mode moteur et connectée au réseau via deux convertisseurs. La commande associée est une Commande Directe de Couple (CDC). Elle est validée en mode sain aussi bien par simulation qu'expérimentalement. Après, les études réalisées prennent en considération les défauts capteurs de courants alternatifs et de vitesse/position. Les algorithmes développés, permettant une continuité de service, utilisent une redondance analytique et sont basés sur l'estimation et aussi sur la Détection et l'Isolation d'un éventuel Défaut (DID). Ils sont caractérisés par leur simplicité. Aussi, ils ne sont pas gourmands en termes de consommation en ressources matérielles et leur temps d'exécution est très court. Enfin, la validation expérimentale de ces algorithmes montre bien leur efficacité en cas de défaut, vu que le système s'avère insensible au défaut et continue à fonctionner sans interruption. La commande obtenue est alors tolérante aux défauts capteurs
The development of closed loop controls for electrical drives requires the sensor installations in order to get feed back information. Nevertheless, any occurred sensor fault (current sensor,speed/position sensor,…) shows an operation system deterioration which leads in most cases to its shut down. This consequence is in contrast to industrial expectations especially concerning the system high accuracy that they are asking for. Statistic studies point out the sensor faults as frequent. So, it is necessary to find out solutions ensuring the system service continuity in case of any sensor fault. Firstly, the study presented in this work shows the used sensor technologies in order to understand both of the reason and the kind of occurred faults. Secondly, the studied system is presented which is an electrical drive based on a Doubly Fed Induction Machine (DFIM) operating in motor mode and connected to the grid by two inverters. The control developed is a Direct Torque Control (DTC). The control validation, in healthy operating mode, is realised throw simulation and experimentally. After, a study considering alternative current sensor and speed/position sensor faults are achieved. The developed algorithms are based on signal estimation, on a Fault Detection Isolation (FDI) and reconfiguration algorithms. In fact, they are simple to carry out, they don't need much hardware resources for implementation and their execution time is short. Finally, the experimental validation of the developed algorithms shows their efficiency. The system continues working even in presence of a sensor fault. Thus, the obtained control becomes a fault tolerant control thanks to these algorithms

This master thesis presents a comparative analysis of the starting performance and losses at rated operation for a 15 kW, 4-pole industrial induction motor, mounted with standard skewed, unskewed and unskewed asymmetrical die-cast aluminium rotors through measurements and simulations. It is a well-known fact that rotor skewing suppresses the synchronous torques at low speeds and also reduces the audible noise of the machine. However, the casting process results in a low resistive path between the rotor bars and the iron laminations, for skewed rotors, this promotes the flow of inter-bar currents. These currents, flowing between the rotorbars, increase the harmonic torques during a start and create additional losses at rated operation. For standard unskewed rotors, these losses are ideally zero, but these rotors may produce high audible noise. Studies have shown that rotors with asymmetrical rotor slot pitch can reduce the audible noise level in unskewed machines. By removing the skew, the inter-bar current losses are suppressed to a negligible level; ultimately increased machine efficiency is obtained. In this work the electrical performance is verified through measurements on the built prototypes. Direct-on-line starts and rated performance for motors with different rotor slot arrangements is simulated using 2D FEM tool FCSmek. The three prototypes are tested in the laboratory according to IEC 60034-2-1 standard and the simulation results are in good agreement with the measured results. An additional test for the measurement of high frequency delta connected stator winding currents for each prototype machine is also performed, in order to study the losses induced in the stator winding. Results have shown that by introducing the proposed asymmetry in the rotor slots, the synchronous torques at low speeds are suppressed effectively, thus, improving the starting performance of the asymmetrical rotor compared to the standard unskewed rotor. Additionally, a higher pull-out torque is obtained for the unskewed rotor motor compared to the standard skewed rotor motor. However, the losses were more or less re-distributed in the unskewed rotor motor, resulting in similar efficiency as the standard skewed rotor motor. One important observation is that; to capture the inter-bar current losses which are estimated to be 5.5% of the total losses, requires more accurate methods of measurements than the existing. And sufficient repeatability must be achieved; alternatively one should rely on statistical data obtained from measurements on several number of motors.

This Master Thesis deals with the study of analytical and finite-element (FE) models for calculation of losses in traction induction motors. Motors are fed through inverters for this type of application. Therefore, both fundamental and harmonic losses are considered. The study is done with one particular motor and one initial analytical model. In order to validate the model and improve it, a FE model is developed with the tool FLUX 2D. Several chosen operating points with different modulation patterns are simulated both with FE and analytical models and results are compared. Stator and rotor Joule losses are studied first. A model to calculate stator Joule losses at strand level is proposed as an improvement to the current analytical model. Then iron losses, both in stator and rotor, are calculated. Two different computations methods with data extraction from FE are studied: the Bertotti model and a recently developed method called MVPRS in the report. It is based on a mathematical model for curve fitting of the core loss material data. Results with the two methods are compared with the ones from ana-lytical model. Finally total fundamental and harmonic losses are compared with measurements and conclusions are drawn on the quality and accuracy of the analytical model.
Detta examensarbete handlar om analytiska och Finita Element (FE) modeller för beräkning av förluster i asynkronmotorer för traktion. Motorer matas genom växelriktare för denna typ av applikation. Därför är det nödvändigt att både grundläggande och harmoniska förluster beaktas. Studien görs för en särskild motor och en redan existerande analytisk modell. För att validera modellen och förbättra den, utvecklats en FE modell med verktyget FLUX 2D. Flera arbetspunkter med olika moduleringsmönster simuleras både med FE och analytiska modeller och resultaten jämförs. Först studeras stator och rotor Joule förluster. En modell för att beräkna stator Joule förluster i varje ledare föreslås som en förbättring av den nuvarande analytiska modellen. Sedan beräknas järnförluster, både i stator och rotor, beräknas. Två olika metoder baserad på flödestäthet variationer i tid och rum från FE simuleringar studeras med Bertottis modell och en nyutvecklad metod som kallas MVPRS. Den är baserat på en matematisk modell för kurvanpassning av materialet förlust data. Resultaten med de två metoderna jämförs med de från analytiska modellen. Slutligen jämförs totala fundamentala och harmoniska förluster med mätningar och slutsatser dras om kvalitet och noggrannhet av analytiska modellen.

This diploma thesis is dedicated to design of highspeed induction motor with power 30 kW, 60 000 rpm. In the beginning of the thesis basic problematic of highspeed motors, used materials and motor types is discussed. The practical part of the thesis concerns design of the motor itself, creating a model in ANSYS Maxwell software, performing several simulations and optimalization of the designed machine. In the last parf of this thesis each simulation is evaluated and the final model of the machine is introduced.

The electrification era has been marked up by an increase in volume of electric vehicles which are directly or indirectly powered by electricity. Railways, roadways and airways are being electrified as we speak at their own respective rate. In addition to that upcoming concepts for transport solution such as hyperloop also described as the fifth mode of transportation will be electrified. The current thesis work is based on developing the model and control of the propulsion system of a scaled Hyperloop pod designed by student team KTH Hyperloop representing KTH. The team competes in Hyperloop competition organized by Spacex and the goal is to achieve the highest possible speed in a given distance and track designed by SpaceX. In order to achieve the goal of being the fastest, the scaled pod uses a Double Sided Linear Induction Motor (DSLIM) as mentioned in the subsequent chapter. The motor modelling is done on Simulink and is similar to a rotary induction motor (RIM). However the presence of end effect in DSLIM makes it different from RIM and has been discussed subsequently. The control strategy uses a synchronous frame PI control for the current control and sensor based speed control for controlling the speed of the pod.The speed control output is a reference current which is used as an input to the current controller which finally gives voltage as the control output. The corresponding bandwidth for the various loops have been calculated based on motor parameters as discussed in the method section. The validation of the motor model and the corresponding controller has been discussed in the result section, where the accuracy of the controller for the designed modelled is discussed.
Elektrifieringstiden har präglats av en ökning i volym av elfordon som direkt eller indirekt drivs med el. Järnvägar, vägar och luftvägar elektrifieras just nu med deras respektive takt. Utöver det kommer kommande koncept för transportlösning som hyperloop som också beskrivs som det femte transportsättet att elektrifieras. Detta examensarbete bygger på att utveckla modellen och regleringen av framdrivningssystemet för en nedskalad Hyperloop-pod utvecklad av studentteamet KTH Hyperloop som representerar KTH. Teamet tävlar i Hyperloop-tävlingen organiserad av SpaceX och målet är att uppnå högsta möjliga hastighet på ett visst avstånd och spår framtaget av SpaceX. För att uppnå målet om att vara snabbast använder den nedskalade podden en dubbelsidig elektrisk linjär induktionsmotor (DSLIM) som nämns i det följande kapitlet. Den elektriska motormodelleringen görs i Simulink och liknar en roterande induktionsmotor(RIM). Men närvaron av ’end effect’ i DSLIM gör den annorlunda än RIM och har diskuterats därefter. Styrstrategin använder en synkron ram-PI-styrning för strömstyrning och sensorbaserad hastighetsreglering för att styra hastigheten på podden. Varvtalsstyrningsutgången är en referensström som används som en ingång till den nuvarande styrenheten som slutligen ger spänning som slutling styrning. Motsvarande bandbredd för de olika slingorna har beräknats baserat på elektriska motorparametrar som diskuterats i metodavsnittet.Valideringen av elmotormodellen och motsvarande styrenhet har diskuterats i resultatsektionen, där noggrannheten hos styrenheten för den konstruerade modellerna diskuteras.

Tato práce řeší návrh komplexního modelu asynchronního stroje, pomocí něhož se dá stroj analyzovat. Model se skládá z dílčích modelů, které popisují asynchronní stroj z hlediska elektro-magnetického, elektrického a tepelného. Elektromagnetický model je řešen pomocí metody koneč-ných prvků v programu zvaném FEMM. Další výpočty jsou provedeny pomocí Matlabu. Tepelný model počítá s ustáleným stavem stroje v požadovaném pracovním bodě. Elektrický model je řešen pomocí náhradního schématu, ze kterého jsou odvozeny vztahy pro elektrické veličiny. Výsledný propojený model je naprogramován v Matlabu a slouží jako nástroj pro analyzování asynchronního stroje.

AC motors are largely used in a wide range of modern systems, from household appliances to automated industry applications such as: ventilations systems, fans, pumps, conveyors and machine tool drives. Inverters are widely used in industrial and commercial applications due to the growing need for speed control in ASD systems. Fast switching transients and the common mode voltage, in interaction with parasitic capacitive couplings, may cause many unwanted problems in the ASD applications. These include shaft voltage and leakage currents. One of the inherent characteristics of Pulse Width Modulation (PWM) techniques is the generation of the common mode voltage, which is defined as the voltage between the electrical neutral of the inverter output and the ground. Shaft voltage can cause bearing currents when it exceeds the amount of breakdown voltage level of the thin lubricant film between the inner and outer rings of the bearing. This phenomenon is the main reason for early bearing failures. A rapid development in power switches technology has lead to a drastic decrement of switching rise and fall times. Because there is considerable capacitance between the stator windings and the frame, there can be a significant capacitive current (ground current escaping to earth through stray capacitors inside a motor) if the common mode voltage has high frequency components. This current leads to noises and Electromagnetic Interferences (EMI) issues in motor drive systems. These problems have been dealt with using a variety of methods which have been reported in the literature. However, cost and maintenance issues have prevented these methods from being widely accepted. Extra cost or rating of the inverter switches is usually the price to pay for such approaches. Thus, the determination of cost-effective techniques for shaft and common mode voltage reduction in ASD systems, with the focus on the first step of the design process, is the targeted scope of this thesis. An introduction to this research – including a description of the research problem, the literature review and an account of the research progress linking the research papers – is presented in Chapter 1. Electrical power generation from renewable energy sources, such as wind energy systems, has become a crucial issue because of environmental problems and a predicted future shortage of traditional energy sources. Thus, Chapter 2 focuses on the shaft voltage analysis of stator-fed induction generators (IG) and Doubly Fed Induction Generators DFIGs in wind turbine applications. This shaft voltage analysis includes: topologies, high frequency modelling, calculation and mitigation techniques. A back-to-back AC-DC-AC converter is investigated in terms of shaft voltage generation in a DFIG. Different topologies of LC filter placement are analysed in an effort to eliminate the shaft voltage. Different capacitive couplings exist in the motor/generator structure and any change in design parameters affects the capacitive couplings. Thus, an appropriate design for AC motors should lead to the smallest possible shaft voltage. Calculation of the shaft voltage based on different capacitive couplings, and an investigation of the effects of different design parameters are discussed in Chapter 3. This is achieved through 2-D and 3-D finite element simulation and experimental analysis. End-winding parameters of the motor are also effective factors in the calculation of the shaft voltage and have not been taken into account in previous reported studies. Calculation of the end-winding capacitances is rather complex because of the diversity of end winding shapes and the complexity of their geometry. A comprehensive analysis of these capacitances has been carried out with 3-D finite element simulations and experimental studies to determine their effective design parameters. These are documented in Chapter 4. Results of this analysis show that, by choosing appropriate design parameters, it is possible to decrease the shaft voltage and resultant bearing current in the primary stage of generator/motor design without using any additional active and passive filter-based techniques. The common mode voltage is defined by a switching pattern and, by using the appropriate pattern; the common mode voltage level can be controlled. Therefore, any PWM pattern which eliminates or minimizes the common mode voltage will be an effective shaft voltage reduction technique. Thus, common mode voltage reduction of a three-phase AC motor supplied with a single-phase diode rectifier is the focus of Chapter 5. The proposed strategy is mainly based on proper utilization of the zero vectors. Multilevel inverters are also used in ASD systems which have more voltage levels and switching states, and can provide more possibilities to reduce common mode voltage. A description of common mode voltage of multilevel inverters is investigated in Chapter 6. Chapter 7 investigates the elimination techniques of the shaft voltage in a DFIG based on the methods presented in the literature by the use of simulation results. However, it could be shown that every solution to reduce the shaft voltage in DFIG systems has its own characteristics, and these have to be taken into account in determining the most effective strategy. Calculation of the capacitive coupling and electric fields between the outer and inner races and the balls at different motor speeds in symmetrical and asymmetrical shaft and balls positions is discussed in Chapter 8. The analysis is carried out using finite element simulations to determine the conditions which will increase the probability of high rates of bearing failure due to current discharges through the balls and races.

碩士
國立臺灣科技大學
機械工程系
105
With the advance of science and technology, motors are wildly used in industry, daily life and traffic. Nowadays, people are using motor controlled by digital signal rather than those using carbon brush to reduce the instability and cost. Sinusoidal Pulse Width Modulation(SPWM) and Space Vector Pulse Width Modulation(SVPWM) are usually used to control AC motors. For SPWM, we produce sinusoidal power sources whose frequency and magnitude can be tuned. For SVPWM, we consider the inverter and AC motor as a system. The model of SVPWM technique is simpler, and it is easier to take real-time control for MCU. In this thesis, we use SVPWM technique to produce inverter signals in order to control motors. We use control kit from Texas Instruments and AC motor from TECO. Taking no load and lock rotor experiments, given frequency command from 9.6 to 60 Hz, the errors of results are within 15%.

The need for studies of control strategies for induction motor drives is justified with the proliferation of application of these drives in industries and production plants where materials have to be both processed and transported and where a high productivity is one of the key factors. This work is focused on developing effective control strategies and configurations for control scheme of industrial induction motor drives. The work is extended to harmonics and reactive power control of the industrial induction motor drive with a PWM converter. The proposed control strategy is analyzed and confirmed by the simulation studies of the mathematical models used. The complete mathematical model of field orientation control (FOC) and direct torque control (DTC) of induction motor is described and simulated in MATLAB for studies of 200 hp cage type induction motor drives. The indirect vector controlled induction motor drives involve decoupling of the stator current in to torque and flux producing components. PI control is used for the estimation of the instantaneous magnitude of the rotor speed, current and torque. The direct torque control employs direct control of stator flux linkages and the electromagnetic torque by the selection of an optimum voltage vector. The pulse width modulated (PWM) converter designed as controllable switching pulses generator offers a flexible solution to the problem of current harmonics and reactive power requirement of the designed industrial drives. The dissertation successfully demonstrates the field oriented control and direct torque control fed with a PWM converter topology. Both motor control strategies are compared to decide the performance of the motor.

The inverter dead-time is integral to the safety of a voltage source inverter (VSI). Dead-time is introduced between the complementary gating signals of the top and bottom switches in each VSI leg to prevent shoot-through fault. This thesis reports and investigates dead-time induced sub-harmonic oscillations in open-loop induction motor drives of different power levels, under light-load conditions. The thesis develops mathematical models that help understand and predict the oscillatory behaviour of such motor drives due to dead-time act. Models are also developed to study the impact of under-compensation and over-compensation of dead-time act on stability. The various models are validated through extensive simulations and experimental results. The thesis also proposes and validates active damping schemes for mitigation of such sub-harmonic oscillations. The thesis reports high-amplitude sub-harmonic oscillations in the stator current, torque and speed of a 100-kW open-loop induction motor drive in the laboratory, operating under no-load. Experimental studies, carried out on 22-kW, 11-kW, 7.5-kW and 3.7-kW open-loop induction motor drives, establish the prevalence of dead-time induced sub-harmonic oscillations in open-loop motor drives of different power levels. An experimental procedure is established for systematic study of this phenomenon in industrial drives. This procedure yields the operating region, if any, where the motor drive is oscillatory. As a first step towards understanding the oscillatory behaviour of the motor drive, a mathematical model of the VSI is derived in a synchronously revolving reference frame (SRF), incorporating the of dead-time on the inverter output voltage. This leads to a modified dynamic model of the inverter-fed induction motor in the SRF, inclusive of the dead-time act. While the rotor dynamic equations are already non-linear, dead-time is found to introduce nonlinearities in the stator dynamic equations as well. The nonlinearities in the modified dynamic model make even the steady solution non-trivial. Under steady conditions, the dead-time can be modelled as the drop across an equivalent resistance (Req0) in the stator circuit. A precise method to evaluate the equivalent resistance Req0 and a simple method to arrive at the steady solution are proposed and validated. For the purpose of stability analysis, a small-signal model of the drive is then derived by linearizing the non-linear dynamic equations of the motor drive, about a steady-state operating point. The proposed small-signal model shows that dead-time contributes to different values of equivalent resistances along the q-axis and d-axis and also to equivalent cross-coupling reactance’s that appear in series with the stator windings. Stability analysis performed using the proposed model brings out the region of oscillatory behaviour (or region of small-signal instability) of the 100-kW motor drive on the voltage versus frequency (V- f) plane, considering no-load. The oscillatory region predicted by the small-signal analysis is in good agreement with simulations and practical observations for the 100-kW motor drive. The small-signal analysis is also able to predict the region of oscillatory behaviour of an 11-kW motor drive, which is con consumed by simulations and experiments. The analysis also predicts the frequencies of sub-harmonic oscillations at different operating points quite well for both the drives. Having the validity of the small-signal analysis at different power levels, this analytical procedure is used to predict the regions of oscillatory behaviour of 2-pole, 4-pole, 6-pole and 8-pole induction motors rated 55 kW and 110 kW. The impact of dead-time on inverter output voltage has been studied widely in literature. This thesis studies the influence of dead-time on the inverter input current as well. Based on this study, the dynamic model of the inverter fed induction motor is extended to include the dc-link dynamics as well. Simulation results based on this extended model tally well with the experimentally measured dc-link voltage and stator current waveforms in the 100-kW drive. Dead-time compensation may be employed to mitigate the dead-time and oscillatory behaviour of the drive. However, accurate dead-time compensation is challenging to achieve due to various factors such as delays in gate drivers, device switching characteristics, etc. Effects of under-compensation and over-compensation of dead time are investigated in this thesis. Under-compensation is shown to result in the same kind of oscillatory behaviour as observed with dead-time, but the fundamental frequency range over which such oscillations occur is reduced. On the other hand, over-compensation of dead-time effect is shown to result in a different kind of oscillatory behaviour. These two types of oscillatory behaviour due to under- and over-compensation, respectively, are distinguished and demonstrated by analyses, simulations and experiments on the 100-kW drive. To mitigate the oscillatory behaviour of the drive, an active damping scheme is proposed. This scheme emulates the effect of an external inductor in series with the stator winding. A small-signal model is proposed for an induction motor drive with the proposed active damping scheme. Simulations and experiments on the 100-kW drive demonstrate effective mitigation of light-load instability with this active damping scheme. In the above inductance emulation scheme, the emulated inductance is seen by the sub-harmonic components, fundamental component as well as low-order harmonic components of the motor current. Since the emulated inductance is also seen by the fundamental component, there is a fundamental voltage drop across the emulated inductance, leading to reduced co-operation of the induction motor. Hence, an improved active damping scheme is proposed wherein the emulated inductance is seen only by the sub-harmonic and low-order harmonic components. This is achieved through appropriate altering in the synchronously revolving domain. The proposed improved active damping scheme is shown to mitigate the sub-harmonic oscillation effectively without any reduction in flux.

The inverter dead-time is integral to the safety of a voltage source inverter (VSI). Dead-time is introduced between the complementary gating signals of the top and bottom switches in each VSI leg to prevent shoot-through fault. This thesis reports and investigates dead-time induced sub-harmonic oscillations in open-loop induction motor drives of different power levels, under light-load conditions. The thesis develops mathematical models that help understand and predict the oscillatory behaviour of such motor drives due to dead-time act. Models are also developed to study the impact of under-compensation and over-compensation of dead-time act on stability. The various models are validated through extensive simulations and experimental results. The thesis also proposes and validates active damping schemes for mitigation of such sub-harmonic oscillations. The thesis reports high-amplitude sub-harmonic oscillations in the stator current, torque and speed of a 100-kW open-loop induction motor drive in the laboratory, operating under no-load. Experimental studies, carried out on 22-kW, 11-kW, 7.5-kW and 3.7-kW open-loop induction motor drives, establish the prevalence of dead-time induced sub-harmonic oscillations in open-loop motor drives of different power levels. An experimental procedure is established for systematic study of this phenomenon in industrial drives. This procedure yields the operating region, if any, where the motor drive is oscillatory. As a first step towards understanding the oscillatory behaviour of the motor drive, a mathematical model of the VSI is derived in a synchronously revolving reference frame (SRF), incorporating the of dead-time on the inverter output voltage. This leads to a modified dynamic model of the inverter-fed induction motor in the SRF, inclusive of the dead-time act. While the rotor dynamic equations are already non-linear, dead-time is found to introduce nonlinearities in the stator dynamic equations as well. The nonlinearities in the modified dynamic model make even the steady solution non-trivial. Under steady conditions, the dead-time can be modelled as the drop across an equivalent resistance (Req0) in the stator circuit. A precise method to evaluate the equivalent resistance Req0 and a simple method to arrive at the steady solution are proposed and validated. For the purpose of stability analysis, a small-signal model of the drive is then derived by linearizing the non-linear dynamic equations of the motor drive, about a steady-state operating point. The proposed small-signal model shows that dead-time contributes to different values of equivalent resistances along the q-axis and d-axis and also to equivalent cross-coupling reactance’s that appear in series with the stator windings. Stability analysis performed using the proposed model brings out the region of oscillatory behaviour (or region of small-signal instability) of the 100-kW motor drive on the voltage versus frequency (V- f) plane, considering no-load. The oscillatory region predicted by the small-signal analysis is in good agreement with simulations and practical observations for the 100-kW motor drive. The small-signal analysis is also able to predict the region of oscillatory behaviour of an 11-kW motor drive, which is con consumed by simulations and experiments. The analysis also predicts the frequencies of sub-harmonic oscillations at different operating points quite well for both the drives. Having the validity of the small-signal analysis at different power levels, this analytical procedure is used to predict the regions of oscillatory behaviour of 2-pole, 4-pole, 6-pole and 8-pole induction motors rated 55 kW and 110 kW. The impact of dead-time on inverter output voltage has been studied widely in literature. This thesis studies the influence of dead-time on the inverter input current as well. Based on this study, the dynamic model of the inverter fed induction motor is extended to include the dc-link dynamics as well. Simulation results based on this extended model tally well with the experimentally measured dc-link voltage and stator current waveforms in the 100-kW drive. Dead-time compensation may be employed to mitigate the dead-time and oscillatory behaviour of the drive. However, accurate dead-time compensation is challenging to achieve due to various factors such as delays in gate drivers, device switching characteristics, etc. Effects of under-compensation and over-compensation of dead time are investigated in this thesis. Under-compensation is shown to result in the same kind of oscillatory behaviour as observed with dead-time, but the fundamental frequency range over which such oscillations occur is reduced. On the other hand, over-compensation of dead-time effect is shown to result in a different kind of oscillatory behaviour. These two types of oscillatory behaviour due to under- and over-compensation, respectively, are distinguished and demonstrated by analyses, simulations and experiments on the 100-kW drive. To mitigate the oscillatory behaviour of the drive, an active damping scheme is proposed. This scheme emulates the effect of an external inductor in series with the stator winding. A small-signal model is proposed for an induction motor drive with the proposed active damping scheme. Simulations and experiments on the 100-kW drive demonstrate effective mitigation of light-load instability with this active damping scheme. In the above inductance emulation scheme, the emulated inductance is seen by the sub-harmonic components, fundamental component as well as low-order harmonic components of the motor current. Since the emulated inductance is also seen by the fundamental component, there is a fundamental voltage drop across the emulated inductance, leading to reduced co-operation of the induction motor. Hence, an improved active damping scheme is proposed wherein the emulated inductance is seen only by the sub-harmonic and low-order harmonic components. This is achieved through appropriate altering in the synchronously revolving domain. The proposed improved active damping scheme is shown to mitigate the sub-harmonic oscillation effectively without any reduction in flux.

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.

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.

Electric Vehicles is call for today and tomorrow and induction motor drives are best chosen for the motional part in the system because of their enormous advantages. Three phase inverter fed induction motor drives are being used in the vehicles that have been made yet and research are still going to reduce the cost and complexity along with increased reliability to meet it at consumer ends without compromising its performance. In this dissertation an effort is made to reduce the cost of system by utilizing four-switch three phase inverter to feed induction motor drive. Also if conventional six switch inverter system incurred a fault in any one leg then shifting the motor load to four switch three phase alternatives would increase the reliability to great extent. Performance analysis for three induction motor drives with FS3P topology is presented along with the comparison between the conventional six-switch and proposed four switch inverter is made and discussed. Space Vector Pulse Width Modulation technique is used in proposed topology to produce three phase voltages at the output of four switch inverter, also SVPWM has advantages like better DC voltage utilization, Switching losses reduction and easier digital implementation. The validity of FS3P topology and performance of induction motor drives is verified by simulated results and MATLAB/SIMULINK platform has been utilized for simulation purpose.

碩士
國立東華大學
電機工程學系
100
The thesis aims at development of a DSP controller-based voltage-fed inverter for both induction motor (IM) drive and permanent magnet synchronous motor (PMSM) drive. In the inverter development, the hardware circuit setup contains that 1) an intelligent power electronic module(IPM) with six IGBT switches and three-phase full-bridge rectifier is used for pulse-width modulation(PWM) devices, 2) three hall-effect current sensors for detecting three-phase ac current of the motor drives are adopted, 3) three differential amplifiers for encoder signals are adopted, 4) optical isolation circuits are used for PWM triggering signals to the IPM, and 5) a TMS320F2812-chip DSP development kit is taken for the control algorithm implementation. In the control algorithm development, the main techniques are as follows: 1) a space-vector PWM and field orientation control are used, 2) a proportional-integral-derivative (PID) speed controller is designed, and 3) a simple start-up scheme for PMSM drive is adopted. To verify the validity of the development, experimental results from a prototype DSP controller-based voltage-fed inverter are illustrated in the thesis.

The PWM (pulse-width-modulated) type voltage source converters (VSC) allow a precise speed control of induction motors with maximum achievable energy efficiency. However, the rapid growth of this technology has created quite a concern, as the PWM waveform produces complex transients that stress the motor insulation, to much severe levels, compared to the normal sinusoidal voltage waveforms. As a result, the machine may fail prematurely due to increased dielectric heating; high turn-to-turn stress caused by non-linear potential distributions; increased partial discharge (PD) activities due to overshoots in pulse waveforms; and built-up space charge by high frequency signals. The present work therefore addresses the problems associated with enamelled wires and groundwall insulation in motor stator coil working under PWM-VSC.

Z-Source inverter is a one stage power converter and used for buck or boost the voltage without using additional dc-dc converter bridge. It uses a special network which connects main circuit and power supply, and hence providing special features which will not be achieved by using traditional current and voltage-source inverters, where capacitor and inductor arranged together in X-fashion. It provides voltage buck or boost properties and reduces voltage stress across capacitors and thereby increases efficiency of circuit. This topology finds many applications in renewable energy source where voltage varying from time to time. Actual speed of an induction motor can be sensed by connecting speed sensor which is compared with reference speed. During voltage changes and normal operating conditions it will provide effective speed control of drive. This paper presents performance characteristics of induction motor drives fed with ZSI. To make easy understanding of speed control of ZSI fed induction motor drive, V/F control with simple boost control PWM technique is used. A PI controller is used in this circuit which helps in running motor at reference speed. Simulation is done in MATLAB/ Simulink and Simulation results of induction motor drive with z-source inverter compared with traditional VSI fed induction motor drive which gives good performance at variable speed and torque characteristics.

This thesis deals with a new solution for medium voltage drives. Load Commutated Inverter (LCI) fed synchronous motor drive is a popular solution for high power drive applications. Though the induction machine is more rugged and cheaper compared to the synchronous machine, LCI fed induction motor drive solution is not available. The basic advantage of a synchronous machine over an induction machine is the fact that the synchronous machine can operate at leading power factor. Due to this property load commutation of SCR switches of the LCI is achievable for synchronous machine. On the contrary an induction machine always draws lagging power factor current; this makes it unsuitable as a drive motor for LCI technology. In this thesis a new LCI fed induction motor drive configuration is developed as an alternative for synchronous motor drives. A new variant of six phase induction motor is proposed in this context. The machine is named as Active Reactive Induction Machine (ARIM). The ARIM contains two sets of three-phase windings with isolated neutral. Both the windings have a common axis. One winding carries the active power and can be wound for higher voltage (say 11kV). The other winding supplies the total reactive power of the machine and can be wound for lower voltage (say 2.2 kV). The rotor is a standard squirrel cage. High power induction machines usually demand lesser magnitude of reactive power compared to the total power rating of the machine ( 20% ). Therefore excitation winding has a smaller fraction of the total machine rating compared to the power winding. A VSI with an LC filter supplies reactive power to the ARIM through the excitation winding and ensures leading power factor at the power winding. This is similar to the excitation control of the LCI fed synchronous machine. The direct VSI connection is possible due to the lower voltage rating for the excitation winding. In this way, the VSI voltage rating does not limit the highest motor voltage that can be handled. An LCI supplies the real power into the ARIM from the power winding. The LCI currents are quasi square wave in shape. Therefore they have rich low order harmonic content. They cause 6th and 12th harmonic torque pulsations in the machine. This is a problem for the LCI fed synchronous machine drive. In the proposed drive, the VSI can compensate these low frequency m.m.f. harmonics inside the machine air gap to remove torque pulsation and rotor harmonic losses. The advantage of the proposed topology is that no transformer is required to drive an 11kV machine. It is always desirable to feed sinusoidal voltage and current to both the power winding and the excitation winding. To address this problem, a second configuration is proposed. A low power three-level VSI is connected in shunt at the power winding with the proposed ARIM drive as discussed above. This VSI compensates the low frequency harmonic currents to achieve sinusoidal motor currents at the motor winding. This VSI acts as a shunt active filter and compensates for the lower order harmonics injected by the LCI. The proposed topologies have LC filters to maintain sinusoidal motor voltages and currents by absorbing the VSI switching frequency components. But the motor terminal voltage oscillates at system resonant frequency due to the presence of LC filters. These resonant components in the terminal voltages are required to be eliminated for smooth terminal voltages and safe load commutation of the thyristors. In this thesis a simple active damping method is proposed to mitigate these issues. The proposed topologies are experimentally verified with an ARIM with 415 V power winding and 220 V excitation winding. The control is carried out on a digital platform having a TMS 320LF 2407A DSP processor and an ALTERA CYCLONE FPGA processor. Results from the prototype experimental drive are presented to show the feasibility and performance of the proposed drive configurations.

This thesis deals with a new solution for medium voltage drives. Load Commutated Inverter (LCI) fed synchronous motor drive is a popular solution for high power drive applications. Though the induction machine is more rugged and cheaper compared to the synchronous machine, LCI fed induction motor drive solution is not available. The basic advantage of a synchronous machine over an induction machine is the fact that the synchronous machine can operate at leading power factor. Due to this property load commutation of SCR switches of the LCI is achievable for synchronous machine. On the contrary an induction machine always draws lagging power factor current; this makes it unsuitable as a drive motor for LCI technology. In this thesis a new LCI fed induction motor drive configuration is developed as an alternative for synchronous motor drives. A new variant of six phase induction motor is proposed in this context. The machine is named as Active Reactive Induction Machine (ARIM). The ARIM contains two sets of three-phase windings with isolated neutral. Both the windings have a common axis. One winding carries the active power and can be wound for higher voltage (say 11kV). The other winding supplies the total reactive power of the machine and can be wound for lower voltage (say 2.2 kV). The rotor is a standard squirrel cage. High power induction machines usually demand lesser magnitude of reactive power compared to the total power rating of the machine ( 20% ). Therefore excitation winding has a smaller fraction of the total machine rating compared to the power winding. A VSI with an LC filter supplies reactive power to the ARIM through the excitation winding and ensures leading power factor at the power winding. This is similar to the excitation control of the LCI fed synchronous machine. The direct VSI connection is possible due to the lower voltage rating for the excitation winding. In this way, the VSI voltage rating does not limit the highest motor voltage that can be handled. An LCI supplies the real power into the ARIM from the power winding. The LCI currents are quasi square wave in shape. Therefore they have rich low order harmonic content. They cause 6th and 12th harmonic torque pulsations in the machine. This is a problem for the LCI fed synchronous machine drive. In the proposed drive, the VSI can compensate these low frequency m.m.f. harmonics inside the machine air gap to remove torque pulsation and rotor harmonic losses. The advantage of the proposed topology is that no transformer is required to drive an 11kV machine. It is always desirable to feed sinusoidal voltage and current to both the power winding and the excitation winding. To address this problem, a second configuration is proposed. A low power three-level VSI is connected in shunt at the power winding with the proposed ARIM drive as discussed above. This VSI compensates the low frequency harmonic currents to achieve sinusoidal motor currents at the motor winding. This VSI acts as a shunt active filter and compensates for the lower order harmonics injected by the LCI. The proposed topologies have LC filters to maintain sinusoidal motor voltages and currents by absorbing the VSI switching frequency components. But the motor terminal voltage oscillates at system resonant frequency due to the presence of LC filters. These resonant components in the terminal voltages are required to be eliminated for smooth terminal voltages and safe load commutation of the thyristors. In this thesis a simple active damping method is proposed to mitigate these issues. The proposed topologies are experimentally verified with an ARIM with 415 V power winding and 220 V excitation winding. The control is carried out on a digital platform having a TMS 320LF 2407A DSP processor and an ALTERA CYCLONE FPGA processor. Results from the prototype experimental drive are presented to show the feasibility and performance of the proposed drive configurations.

Wind energy utilization has been growing at a rapid rate, fuelling research and development in wind turbine – generator systems. Doubly fed induction generator (DFIG) driven by a wind turbine is a commonly used wind energy conversion system. To advance research and education in wind energy conversion systems, a controlled test bed is necessary that does not depend on wind availability. Hence this thesis deals with emulation of wind turbine using a power electronic controlled squirrel cage induction motor (SCIM). The thesis also concerns improved control techniques for the DFIG to enhance the performance of the wind energy system. Wind turbine emulation involves controlling the SCIM drive such that the motor exhibits the characteristics of a wind turbine. The inertia of the motor being much lower than the inertia of the emulated turbine is an important challenge in such wind turbine emulation schemes. This thesis proposes one degree of freedom (1-DOF) and two degree of freedom (2-DOF) control structures for wind turbine emulation, overcoming this challenge. Regarding control of DFIG in wind energy applications, position sensorless operation of DFIG is desirable from considerations of cost, maintenance, cabling and reliability. This thesis proposes two stator flux based model reference adaptive observers (SF-MRAOs) for estimation of rotor speed and position in stand-alone DFIG. One of the proposed SF-MRAO is shown to work with good dynamic performance in vector control of stand-alone DFIG. The thesis also proposes a PLL based MRAO (PLL-MRAO) which does not require integration of sensed quantities, unlike other existing MRAOs. The linearized SF-MRAO is further utilized to propose a modified direct voltage control (DVC) of stand-alone DFIG with simplified design of controllers compared to existing DVC. Grid integration of DFIG system requires synchronization of stator induced voltages with grid voltages before grid connection, and active and reactive power (PQ) control after grid connection. This thesis also proposes a unified control algorithm for synchronization and power control, enabling a seamless transfer from synchronization mode to PQ control mode. Synchronization requires initial rotor position information, obtained through either a novel rotor parking scheme or PLL-MRAO, which are proposed in the thesis. It is shown that the unified control has negligible transients during grid integration of DFIG. All the proposed observers and control algorithms are validated through simulations and experiments, performed on a 7.5 kW doubly-fed induction generator coupled to a 5.5 kW squirrel cage induction motor, available in the laboratory.

Current-Controlled Pulse Width Modulated (CC-PWM) Voltage Source Inverters (VSIs) are extensively employed in high performance drives (HPD) because of the considerable advantages offered by them, such as, excellent dynamic response and inherent over-current protection, as compared to the voltage-controlled PWM (VC-PWM) VSIs. Amongst the different types of CC-PWM techniques, hysteresis current controllers offer significant simplicity in implementation. However, conventional type of hysteresis controllers (with independent comparators) suffers from some well-known drawbacks, such as, limit cycle oscillations (especially at lower speeds of operation of machine), overshoot in current error, generation of sub-harmonic components in the current, and random (non-optimum) switching of inverter voltage vectors. Common problems associated with the conventional, as well as current error space phasor based hysteresis controllers with fixed bands (boundary), are the wide variation of switching frequency in the fundamental output cycle and variation of switching frequency with the change in speed of the load motor. These problems cause increased switching losses in the inverter, non-optimum current ripple, excess harmonics in the load current and subsequent additional machine heating. A continuously varying parabolic boundary for the current error space phasor is proposed previously to get the switching frequency variation pattern of the output voltage of the hysteresis controller based PWM inverter similar to that of voltage controlled space vector PWM (VC SVPWM) based VSI. But the major problem associated with this technique is the requirement of two outer parabolas outside the current error space phasor boundary for the identification of sector change which gives rise to some switching frequency variations in one fundamental cycle and over the entire operating speed range. It also introduces 5th and 7th harmonic components in the voltage causing 5th and 7th harmonic currents in the induction motor. These harmonic currents causes 6th harmonic torque pulsations in the machine. This thesis proposes a new technique which replaces the outer parabolas and uses current errors along orthogonal axes for detecting the sector change, so that a fast and accurate detection of sector change is possible. This makes the voltage harmonic spectrum of the proposed hysteresis controller based inverter exactly matching with that of a constant switching frequency SVPWM based inverter. This technique uses the property that the current error along one of the orthogonal axis changes its direction during sector change. So the current error never goes outside the parabolic boundary as in the case of outer parabolas based sector change technique. So the proposed new technique for sector change eliminates the 5th and 7th harmonic components from the applied voltage and thus eliminates the 5th and 7th harmonic currents in the motor. So there will be no introduction of 6th harmonic torque pulsations in the motor. Using the proposed scheme for sector change and parabolic boundary for current error space phasor, simulation study was carried out using Matlab-Simulink. Simulation study showed that the switching frequency variations in a fundamental cycle and over the entire speed range of the machine upto six step mode operation is similar to that of a VC-SVPWM based VSI. The proposed hysteresis controller is experimentally verified on a 3.7 kW IM drive fed with a two-level VSI using vector control. The proposed current error space phasor based hysteresis controller providing constant switching frequency is completely implemented on the TI TMS320LF2812 DSP controller platform. The three-phase reference currents are generated depending on the frequency command and the controller is tested with drive for the entire operating speed range of the machine in forward and reverse directions. Steady state and transient results of the proposed drive are presented in this thesis. This thesis also proposes a new hysteresis controller which eliminates parabolic boundary and replaces it with a simple online computation of the boundary. In this proposed new hysteresis controller the boundary computed in the present sampling interval is used for identifying next vector to be switched. This thesis gives a detailed mathematical explanation of how the boundary is computed and how it is used for selecting vector to be switched in a sector. It also explains how the sector in which stator voltage vector is present is determined. The most important part of this proposed hysteresis controller is the estimation of stator voltages along alpha and beta axes during active and zero vector periods. Estimation of stator voltages are carried out using current errors along alpha and beta axes and steady state equivalent circuit of induction motor. Using this estimated stator voltages along alpha and beta axes, instantaneous phase voltages are computed and used for finding individual voltage vector switching times. These switching times are used for the computation of hysteresis boundary for individual vectors. So the hysteresis boundary for individual vectors are exactly calculated and used for vector change detection, making phase voltage harmonic spectrum exactly similar to that of constant switching frequency VC SVPWM inverter. Sector change detection is very simple, since we have the estimated stator voltages along alpha and beta axes to give exact position of stator voltage vector. Simulation study to verify the steady state as well as transient performance of the proposed controller based VSI fed IM drive is carried out using Simulink tool box of Matlab Simulation Software. The proposed hysteresis controller is experimentally verified on a 3.7 kW IM drive fed with a two-level VSI using vector control. The proposed current error space phasor based hysteresis controller providing constant switching frequency profile for phase voltage is implemented on the TI TMS320LF2812 DSP controller platform. The three-phase reference currents are generated depending on the frequency command and the proposed hysteresis controller is tested with drive for the entire operating speed range of the machine in forward and reverse directions. Steady state and transient results of the proposed drive are presented for different operating conditions.