Distributed energy resources like solar power (PV Panels) are usually connected to the AC grid through a single phase voltage source inverter (VSI). The major drawback associated with single phase grid tie inverters is the double frequency component of the grid that appears on the DC bus link. Large electrolytic capacitors are generally employed in the inverters to eliminate the ripple component. However, their bulkiness and relatively short lifetime are motivational factors to replace them with small film capacitors. This paper presents a synchronous boost/buck based active power decoupling circuit in parallel with the dc-bus link capacitor and discusses the different types of control strategies implemented. Simulation results are presented for each control technique and it is shown that the ripple on the DC bus link is largely reduced due to inclusion of this circuit along with an expected extension of the lifetime due to the reduction in the amount of dc-bus capacitance used. / text

In recent decades there has been a rapidly growing demand for high quality, uninterrupted power. In light of this fact, this study has addressed some of the causes of poor power quality and control strategies to ensure a high performance level in inverter-fed power systems. In particular, specific loading conditions present interesting challenges to inverter-fed, high power systems. No-load, unbalanced loading, and non-linear loading each have unique characteristics that negatively influence the performance of the Voltage Source Inverter (VSI). Ideal, infinitely stiff power systems are uninfluenced by loading conditions; however, realistic systems, with finite output impedances, encounter stability issues, unbalanced phase voltage, and harmonic distortion. Each of the loading conditions is presented in detail with a proposed control strategy in order to ensure superior inverter performance. Simulation results are presented for a 90 kVA, 400 Hz VSI under challenging loading conditions to demonstrate the merits of the proposed control strategies. Unloaded or lightly loaded conditions can cause instabilities in inverter-fed power systems, because of the lightly damped characteristic of the output filter. An inner current loop is demonstrated to damp the filter poles at light load and therefore enable an increase in the control bandwidth by an order of magnitude. Unbalanced loading causes unequal phase currents, and consequently negative sequence and zero sequence (in four-wire systems) distortion. A proposed control strategy based on synchronous and stationary frame controllers is shown to reduce the phase voltage unbalance from 4.2% to 0.23% for a 100%-100%-85% load imbalance over fundamental positive sequence control alone. Non-linear loads draw harmonic currents, and likewise cause harmonic distortion in power systems. A proposed harmonic control scheme is demonstrated to achieve near steady state errors for the low order harmonics due to non-linear loads. In particular, the THD is reduced from 22.3% to 5.2% for full three-phase diode rectifier loading, and from 11.3% to 1.5% for full balanced single-phase diode rectifier loading, over fundamental control alone. / Master of Science

Submitted by Maria Suzana Diniz (msuzanad@hotmail.com) on 2015-11-06T15:48:55Z No. of bitstreams: 1 arquivototal.pdf: 3791340 bytes, checksum: 58dd4dde93f8552a891a0760151a7c10 (MD5) / Made available in DSpace on 2015-11-06T15:48:55Z (GMT). No. of bitstreams: 1 arquivototal.pdf: 3791340 bytes, checksum: 58dd4dde93f8552a891a0760151a7c10 (MD5) Previous issue date: 2015-02-27 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES / The interest for the study of multiphase (more than three phases) machines and variable speed drives has substantially increased in the last two decades. The advantages presented by the multiphase systems compared to their three-phase counterparts have being the the main effort to increase researches all over the world. Reduction in torque oscillation, lower current ratings, high drive reliability, better fault tolerance and harmonic content reduction in the DC bus are some of these advantages. The additional degrees of freedom due to extra phases make multiphase systems very flexible when it comes to control systems and modulation strategies. Although three-phase systems are predominant in industrial applications, the use of multiphase machines and drives has increased in very specific areas such as locomotive traction, electric ship propulsion, aerospace industry (more electric aircraft), electric and hybrid vehicles and industrial high power systems. Due to high coupling degree between electric variables in multiphase systems, modeling of multiphase machines has been and still is a challenge task in research centers. In this present work, analitic modeling of symmetric and asymmetric nine-phase machines using natural variables and space vector decomposition are presented. The principles used in the study are the same used for three-phase systems. However, multiphase systems are analised in multiple d-q planes. PWM modulation strategies based on space vectors theory (SVPWM) for a voltage source inverter (VSI) are presented. The multiphase drive system presented works in the linear operation region with sinusoidal voltage generation. Results for both machines and drive modeled are verified by simulation programs developed in C programming language an Matlab. / O interesse pelo estudo de máquinas de indução multifases (mais de três fases) e dos seus sistemas de acionamento cresceu substancialmente nas últimas duas décadas. As muitas vantagens apresentadas pelos sistemas multifases, em relação aos sistemas trifásicos convencionais, têm sido fatores motivadores para o aumento de pesquisas em todo o mundo. Redução das oscilações de torque, redução da corrente por fase, maior confiabilidade do acionamento, grande tolerância à faltas e redução no conteúdo harmônico da corrente no barramento CC são algumas destas vantagens. O maior grau de liberdade proporcionado pelas fases extras torna os sistemas multifases bastante flexíveis quanto às estratégias de modulação e de controle. Apesar da atual predominância da utilização das máquinas e acionamentos trifásicos na indústria, as máquinas multifases estão sendo cada vez mais utilizadas em áreas de aplicações específicas tais como tração de locomotivas, propulsão de navios elétricos de grande porte, indústria aeroespacial, tração de veículos híbridos e elétricos e sistemas industriais de alta potência. Devido ao alto grau de acoplamento entre as variáveis elétricas de um sistema multifases, a modelagem e análise desses sistemas tem representado uma tarefa desafiadora nos centros de pesquisa. No presente trabalho são apresentadas as modelagens analíticas de uma máquina de indução de nove fases simétrica e de uma máquina de nove fases assimétrica pelo método de variáveis naturais e pelo método de decomposição vetorial. Os princípios utilizados na modelagem são os mesmos utilizados nos sistemas trifásicos. No entanto, sistemas multifases são analisados em múltiplos planos d-q. Estratégias de modulação PWM baseadas na teoria de vetores espaciais (SVPWM) para um inversor de nove fases tipo VSI (Inversor Fonte de Tensão) são apresentadas para acionamento das máquinas. O sistema de acionamento apresentado trabalha na região linear de operação e gera tensão de saída senoidal. Dados de simulação obtidos a partir de programas desenvolvidos em linguagem C e Matlab são apresentados para ambas as máquinas de nove fases modeladas.

Within the past decade, talk has arisen of shifting the utility grid from centralized, radial sources to a distributed network of sources, also known as distributed generation (DG); in the wake of deregulation, the California energy crisis, and northeastern blackouts. Existing control techniques for DG systems are designed to operate a system either in the connected or disconnected (islanding) mode to the utility; thus not allowing for both modes to be implemented and transitioned between. Existing detection and re-closure algorithms can also be improved upon. Dependent upon the method implemented, detection algorithms can either cause distortions in the output or completely miss a disturbance. The present re-closure process to reconnect to the utility is to completely shutdown and wait five minutes. The proposed methods of this study improve upon existing methods, via simulation and hardware experimentation, for DG systems that can intentionally islanding themselves. The proposed, "switched-mode", control allows for continuous operation of the system during disturbances by transitioning the mode of control to reflect the change in the system mode (grid-connected or islanding). This allows for zero downtimes without detrimental transients. The proposed detection method can sense disturbances that other methods cannot; and within 25 ms (approximately 1.5 line-cycles at 60 Hz). This method is an improvement over other methods because it eliminates the need to purposely distort the outputs to sense a disturbance. The proposed re-closure method is an improvement over the existing method due to the fact that it does not require the system to de-energize before re-synchronizing and reconnecting to the utility. This allows for DGs to continuously supply power to the system without having to shut down. Results show that the system is generally ready to reconnect after 2 to 5 line cycles. / Master of Science

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.

Voltage source inverter (VSI) is used to control the speed of an induction motor by applying AC voltage of variable amplitude and frequency. The semiconductor switches in a VSI are turned on and off in an appropriate fashion to vary the output voltage of the VSI. Various pulse width modulation (PWM) methods are available to generate the gating signals for the switches. The process of PWM ensures proper fundamental voltage, but introduces harmonics at the output of the VSI. Ripple in the developed torque of the induction motor, also known as pulsating torque, is a prominent consequence of the harmonic content. The harmonic voltages, impressed by the VSI on the motor, differ from one PWM method to another. Space-vector-based approach to PWM facilitates a large number of switching patterns or switching sequences to operate the switches in a VSI. The switching sequences can be classified as conventional, bus-clamping and advanced bus-clamping sequences. The conventional sequence switches each phase once in a half-carrier cycle or sub-cycle, as in case of sine-triangle PWM, third harmonic injection PWM and conventional space vector PWM (CSVPWM). The bus-clamping sequences clamp a phase to one of the DC terminals of the VSI in certain regions of the fundamental cycle; these are employed by discontinuous PWM (DPWM) methods. Popular DPWM methods include 30 degree clamp PWM, wherein a phase is clamped during the middle 30 degree duration of each quarter cycle, and 60 degree clamp PWM which clamps a phase in the middle 60 degree duration of each half cycle. Advanced bus-clamping PWM (ABCPWM) involves switching sequences that switch a phase twice in a sub-cycle besides clamping another phase. Unlike CSVPWM and BCPWM, the PWM waveforms corresponding to ABCPWM methods cannot be generated by comparison of three modulating signals against a common carrier. The process of modulation in ABCPWM is analyzed from a per-phase perspective, and a computationally efficient methodology to realize the sequences is derived. This methodology simplifies simulation and digital implementation of ABCPWM techniques. Further, a quick-simulation tool is developed to simulate motor drives, operated with a wide range of PWM methods. This tool is used for validation of various analytical results before experimental investigations. The switching sequences differ in terms of the harmonic voltages applied on the machine. The harmonic currents and, in turn, the torque ripple are different for different switching sequences. Analytical expression for the instantaneous torque ripple is derived for the various switching sequences. These analytical expressions are used to predict the torque ripple, corresponding to different switching sequences, at various operating conditions. These are verified through numerical simulations and experiments. Further, the spectral properties are studied for the torque ripple waveforms, pertaining to conventional space vector PWM (CSVPWM), 30 degree clamp PWM, 60 degree clamp PWM and ABCPWM methods. Based on analytical, simulation and experimental results, the magnitude of the dominant torque harmonic with an ABCPWM method is shown to be significantly lower than that with CSVPWM. Also, this ABCPWM method results in lower RMS torque ripple than the BCPWM methods at any speed and CSVPWM at high speeds of the motor. Design of hybrid PWM methods to reduce the RMS torque ripple is described. A hybrid PWM method to reduce the RMS torque ripple is proposed. The proposed method results in a dominant torque harmonic of magnitude lower than those due to CSVPWM and ABCPWM. The peak-to-peak torque in each sub-cycle is analyzed for different switching sequences. Another hybrid PWM is proposed to reduce the peak-to-peak torque ripple in each sub-cycle. Both the proposed hybrid PWM methods reduce the torque ripple, without increasing the total harmonic distortion (THD) in line current, compared to CSVPWM. CSVPWM divides the zero vector time equally between the two zero states of a VSI. The zero vector time can optimally be divided to minimize the RMS torque ripple in each sub-cycle. It is shown that such an optimal division of zero vector time is the same as addition of third harmonic of magnitude 0.25 times the fundamental magnitude to the three-phase sinusoidal modulating signals. ABCPWM applies an active state twice in a sub-cycle, with the active vector time divided equally. Optimal division of active vector time in ABCPWM to minimize the RMS torque ripple is evaluated, both theoretically and experimentally. Compared to CSVPWM, this optimal PWM is shown to reduce the RMS torque ripple significantly over a wide range of speed. The various PWM schemes are implemented on ALTERA CycloneII field programmable gate array (FPGA)-based digital control platform along with sensorless vector control and torque estimation algorithms. The controller generates the gating signals for a 10kVA IGBT-based two-level VSI connected to a 5hp, 400V, 4-pole, 50Hz squirrel-cage induction motor. The induction motor is coupled to a 230V, 3kW separately-excited DC generator.

A voltage source inverter is commonly used to supply a variable frequency variable voltage to a three phase induction motor in a variable speed application. A suitable pulse width modulation (PWM) technique is employed to obtain the required output voltage in the line side of the inverter. Real-time methods for PWM generation can be broadly classified into triangle comparison based PWM (TCPWM) and space vector based PWM (SVPWM). In TCPWM methods such as sine-triangle PWM, three phase reference modulating signals are compared against a common triangular carrier to generate the PWM signals for the three phases. In SVPWM methods, a revolving reference voltage vector is provided as voltage reference instead of three phase modulating waves. The magnitude and frequency of the fundamental component in the line side are controlled by the magnitude and frequency, respectively, of the reference vector. The fundamental line side voltage is proportional to the reference magnitude during linear modulation. With sine-triangle PWM, the highest possible peak phase fundamental voltage is 0.5Vdc, where Vdc is the DC bus voltage, in the linear modulation zone. With techniques such as third harmonic injection PWM and space vector based PWM, the peak phase fundamental voltage can be as high as (formula) (i.e., 0:577Vdc)during linear modulation. To increase the line side voltage further, the operation of the VSI must be extended into the overmodulation region. The overmodulation region extends upto the six-step mode, which gives the highest possible ac voltage for a given (formula). In TCPWM based methods, increasing the reference magnitude beyond a certain level leads to pulse dropping, and gradually leads to six-step operation. However, in SVPWM methods, an overmodulation algorithm is required for controlling the line-side voltage during overmodulation and to achieve a smooth transition from PWM to six-step mode. Numerous overmodulation algorithms have been proposed in the literature for space vector modulated inverter. A well known algorithm among these divides the overmodulation zone into two zones, namely zone-I and zone-II. This is termed as the 'existing overmodulation algorithm' here. This algorithm is modified in the present work to reduce computational burden without much increase in the line current distortion. During overmodulation, the fundamental line side voltage and the reference magnitude are not proportional, which is undesirable from the control point of view. The present work ensures a linear relationship between the two. Apart from the fundamental component, the inverter output voltage mainly consists of harmonic components at high frequencies (around switching frequency and the integral multiples) during linear modulation. However, during overmodulation, low order harmonic components such as 5th, 7th, 11th, 13th etc., are also present in the output voltage. These low order harmonic voltages lead to low order harmonic currents in the motor. The sum of the lower order harmonic currents is termed as 'lower order current ripple'. The present thesis proposes a method for estimation of lower order current ripple in real-time. In closed loop current control, the motor current is fed back to the current controller. During overmodulation, the motor current contains low order harmonics, which appear in the current error fed to the controller. These harmonic currents are amplified by the current error amplifier deteriorating the performance of the drive. It is possible to filter the lower order harmonic currents before being fed back. However, filtering introduces delay in the current loop, and reduces the bandwidth even during linear modulation. In the present work, the estimated lower order current ripple is subtracted from the measured current before the latter is fed back to the controller. The estimation of lower order current ripple and the proposed current control are verified through simulation using MATLAB/SIMULINK and also experimentally on a laboratory prototype. The experimental setup comprises of a field programmable gate arrays (FPGA) based digital controller, an IGBT based inverter and a four-pole squirrel cage induction motor. (Pl refer the original document for formula)

Στόχος της παρούσας διπλωματικής εργασίας είναι η εργαστηριακή εξομοίωση της μηχανικής ροπής που εμφανίζεται στο δρομέα μιας πραγματικής ανεμογεννήτριας με τη μέθοδο της ταχείας προτυποποίησης. Η μέθοδος αυτή παρέχει τη δυνατότητα εξομοίωσης του προς μελέτη αντικειμένου, στην προκειμένη περίπτωση της ανεμογεννήτριας, χωρίς να είναι απαραίτητες οι δοκιμές σε πραγματικό εξοπλισμό, διευκολύνει τη μελέτη της ανεμογεννήτριας σε διάφορες συνθήκες και με διαφορετικές κάθε φορά παραμέτρους, χωρίς να είναι απαραίτητη η αναμονή προκειμένου ο άνεμος να είναι κατάλληλος ώστε να κάνουμε δοκιμές στο πραγματικό σύστημα και τέλος δίνει το πλεονέκτημα της αποφυγής βλαβών του πραγματικού εξοπλισμού. Απαραίτητα στοιχεία για να επιτευχθεί ο στόχος της εργασίας είναι μια πειραματική διάταξη, στην οποία θα πραγματοποιηθούν οι δοκιμές και οι μετρήσεις, καθώς επίσης κάποια ανεμολογικά δεδομένα σε συνδυασμό με τα χαρακτηριστικά της πραγματικής ανεμογεννήτριας της οποίας τη ροπή θα εξομοιώσουμε υπό κλίμακα. Η πειραματική διάταξη μπορεί να περιγραφεί συνοπτικά από το σχήμα της Εικ.1. Χρησιμοποιώντας τα ανεμολογικά δεδομένα υπολογίστηκε η ροπή στον δρομέα της πραγματικής ανεμογεννήτριας συναρτήσει της ταχύτητας του ανέμου. Αυτή την ροπή την παρήγαμε υπό κλίμακα στο εργαστήριο, στον άξονα ενός ασύγχρονου τριφασικού κινητήρα, εφαρμόζοντας έλεγχο ροπής. Η πειραματική διάταξη που χρησιμοποιήθηκε περιλαμβάνει μια ασύγχρονη μηχανή, μια μηχανή συνεχούς ρεύματος, ένα μονοφασικό ωμικό φορτίο, έναν τριφασικό αντιστροφέα πηγής τάσης και έναν μικροεπεξεργαστή με τον οποίο υλοποιείται η διαδικασία του ελέγχου. Ο έλεγχος της ασύγχρονης μηχανής γίνεται ρυθμίζοντας την τάση εξόδου του αντιστροφέα κατά πλάτος και συχνότητα, παλμοδοτώντας κατάλληλα τα διακοπτικά στοιχεία του. Το κύκλωμα παλμοδότησης υλοποιείται μέσω του μικροεπεξεργαστή, χρησιμοποιώντας τη μέθοδο ημιτονοειδούς διαμόρφωσης εύρους παλμών (Sinusoidal Pulse Width Modulation, SPWM). Ανάλογα με την τιμή του σφάλματος της ροπής ρυθμίζονται κατάλληλα οι παράμετροι της παλμοδότησης χρησιμοποιώντας τη μέθοδο του ασαφούς ελέγχου (Fuzzy Control). Για τη μέτρηση της ροπής στον άξονα του ασύγχρονου κινητήρα, που αποτελεί το σήμα ανάδρασης του ελέγχου, χρησιμοποιήθηκε ένα ροπόμετρο. Ιδιαίτερα ενδιαφέρουσα είναι η παραγωγή του κώδικα με τον οποίο γίνεται η εξομοίωση και ο έλεγχος. Αρχικά γίνεται μοντελοποίηση του κυκλώματος στο περιβάλλον Simulink και στη συνέχεια, χρησιμοποιώντας τα κατάλληλα εργαλεία, ακολουθείται μια αυτόματη διαδικασία παραγωγής του κώδικα και εκτέλεσή του από τον μικροεπεξεργαστή (DSP). Η χρήση του μικροεπεξεργαστή προσέφερε επίσης αρκετά πλεονεκτήματα και διευκόλυνε την πειραματική διαδικασία. Χρησιμοποιώντας τον μικροεπεξεργαστή για τη διεξαγωγή του ελέγχου, αποφεύχθηκε η χρήση επιπλέον διατάξεων ελέγχου. Επίσης, οι περιφερειακές μονάδες του ήταν ιδιαίτερα χρήσιμες κατά τη δειγματοληψία των μεταβλητών ανάδρασης, κατά την παραγωγή των παλμών της SPWM αλλά και κατά την καταγραφή των δεδομένων. / The main objective of this thesis is the implementation of the mechanical torque that appears on the rotor of a real wind turbine, using the method of rapid prototyping. That method has many advantages. The main advantage is that the use of a real wind turbine was avoided and that minimized the cost of research. A second advantage is that it was not necessary to wait for convenient weather conditions in order to carry out the experiments. In addition, damages of equipment were avoided using the method of rapid prototyping In order to accomplice this objective it is necessary to have an experimental construction, which will be used for tests and measurements, as well as the use of wind speed data and the characteristics of a real wind turbine, which will be used to calculate in scale the real torque that appears on the rotor. The experimental construction is shown in Pic.2. The torque on the rotor of the real wind turbine was calculated as a function of wind speed. That torque was implemented in scale using torque control of an induction motor in the laboratory. The experimental construction includes an induction machine, a constant current machine, a single-phase resistive load, a three-phase voltage source inverter and a digital signal processor, which is used to accomplice the control procedure. The torque control of the induction machine is achieved by regulating the amplitude and the frequency of the output voltage of the inverter, using the appropriate pulses to drive the IGBTs. The microprocessor produces the pulses using the method of Sinusoidal Pulse Width Modulation (SPWM). The parameters of the pulses are proportional to the torque error and are appropriately calculated using the method of Fuzzy Control. A torque meter was used in order to measure the torque on the shaft of the induction motor, which was the feedback signal for the control procedure. The code generation is achieved using a microprocessor (DSP). Initially, a simulation model is made using the program Simulink and then, using the right tools, the code is generated and run using the microprocessor. Using the microprocessor had many advantages and made the experiment procedure easier. Initially, additional control devices were not necessary during the experiments. Also, the microprocessor peripherals were useful during the sampling of feedback signals, during the calculation of SPWM pulses and during data recording.

Voltage source inverter (VSI) fed induction motors are increasingly used in industrial and transportation applications as variable speed drives. However, VSIs generate non-sinusoidal voltages and hence result in harmonic distortion in motor current, motor heating, torque pulsations and increased acoustic noise. Most of these undesirable effects can be reduced by increasing the switching frequency of the inverter. This is not necessarily true for acoustic noise. Acoustic noise does not decrease monotonically with increase in switching frequency since the noise emitted depends on the proximity of harmonic frequencies to the motor resonant frequencies. Also there are practical limitations on the inverter switching frequency on account of device rating and losses. The switching frequency of many inverters often falls in the range 2 kHz - 6 kHz where the human ear is highly sensitive. Hence, the acoustic noise emission from the motor drive is of utmost important. Further, the acoustic noise emitted by the motor drive is known to depend on the waveform quality of the voltage applied. Hence, the acoustic performance varies with the pulse width modulation (PWM) technique used to modulate the inverter, even at the same modulation index. Therefore a comprehensive study on the acoustic noise aspects of induction motor drive is required. The acoustic noise study of the motor drive poses multifaceted challenges. A simple motor model is sufficient for calculation of total harmonic distortion (THD). A more detailed model is required for torque pulsation studies. But the motor acoustic noise is affected by many other factors such as stator winding distribution, space harmonics, geometry of stator and rotor slots, motor irregularities, structural issues controlling the resonant frequency and environmental factors. Hence an accurate model for acoustic noise would have to be very detailed and would span different domains such as electromagnetic fields, structural engineering, vibration and acoustics. Motor designers employ such detailed models along with details of the materials used and geometry to predict the acoustic noise that would be emitted by a motor and also to design a low-noise motor. However such detailed motor model for acoustic noise purposes and the necessary material and constructional details of the motor are usually not available to the user. Also, certain factors influencing the acoustic noise change due to wear and tear during the operational life of the motor. Hence this thesis takes up an experimental approach to study the acoustic noise performance of an inverter-fed induction motor at any stage of its operating life. A 10 kVA insulated gate bipolar transistor (IGBT) based inverter is built to feed the induction motor; a 6 kW and 2.3 kW induction motors are used as experimental motors. A low-cost acoustic noise measurement system is also developed as per relevant standards for measurement and spectral analysis of the acoustic noise emitted. For each PWM scheme, the current and acoustic noise measurements are carried out extensively at different carrier frequencies over a range of fundamental frequencies. The main cause of acoustic noise of electromagnetic origin is the stator core vibration, which is caused by the interaction of air-gap fluxes produced by fundamental current and harmonic currents. In this thesis, an experimental procedure is suggested for the acoustic noise characterization of an induction motor inclusive of determination of resonant frequencies. Further, based on current and acoustic noise measurements, a vibration model is proposed for the stator structure. This model is used to predict the acoustic noise pertaining to time harmonic currents with reasonable accuracy. Literature on motor acoustic noise mainly focuses on sinusoidal PWM (SPWM), conventional space vector PWM (CSVPWM) and random PWM (RPWM). In this thesis, acoustic noise pertaining to two bus-clamping PWM (BCPWM) schemes and an advanced bus-clamping PWM (ABCPWM) scheme is investigated. BCPWM schemes are mainly used to reduce the switching loss of the inverter by clamping any of the three phases to DC rail for 120◦ duration of the fundamental cycle. Experimental results show that these BCPWM schemes reduce the amplitude of the tonal component of noise at the carrier frequency, compared to CSVPWM. Experimental results with ABCPWM show that the overall acoustic noise produced by the motor drive is reduced at low and medium speeds if the switching frequency is above 3 kHz. Certain spread in the frequency spectrum of noise is also seen with both BCPWM and ABCPWM. To spread the acoustic noise spectrum further, many variable-frequency PWM schemes have been suggested by researchers. But these schemes, by and large, increase the current total harmonic distortion (THD) compared to CSVPWM. Thus, a novel variable-frequency PWM (VFPWM) method is proposed, which offers reduced current THD in addition to uniformly spread noise spectrum. Experimental results also show spread in the acoustic noise spectrum and reduction in the dominant noise components with the proposed VFPWM. Also, the current THD is reduced at high speeds of the motor drive with the proposed method.

This thesis proposes control schemes and converter configurations for high power wound field synchronous motor (WFSM) drives. The model for a salient pole WFSM in any general rotating reference frame is developed which can be used to derive models along known rotor (dq) and stator flux (MT) reference frames. Based on these models, the principle of sensor-less stator flux oriented field-oriented control (FOC) for salient pole WFSM is developed. So far in the literature, control of cylindrical rotor machine only has been addressed and the effects of saliency have generally been neglected. The performance of the proposed sensor-less FOC has been demonstrated by experimentally operating a 15.8 HP salient pole WFSM using a three-level IGBT based voltage source inverter (VSI). The principle of FOC has been later extended to the control of current source load commutated inverter (LCI) fed salient pole WFSM drives, where the drawbacks present in conventional self-control method such as rigorous off-line calculation for generation of look up tables, coupling between flux and torque control etc. are eliminated. This thesis also proposes the combination of a VSI with the LCI power circuit to overcome the different disadvantages that are present in the existing LCI topology. Firstly, a novel starting scheme is proposed, where the LCI fed WFSM is started with the aid of a low power auxiliary VSI converter in a smooth manner with sinusoidal motor currents and voltages. This overcomes the difficulties of the present complex dc link current pulsing technique that has drawbacks such as pulsating torque, long starting time etc. In a second mode of operation, it is shown that the VSI can be connected to the existing LCI fed WFSM drive as a harmonic compensator in On-The-Fly mode; this will make the terminal stator current and voltage sinusoidal apart from cancellation of torque pulsations thus improving the drive performance. The above two schemes have potential as retrofit for existing drives. It is possible to combine both the advantages, mentioned above, by permanently connecting the VSI with the LCI power circuit to feed the WFSM. This proposed hybrid LCI/VSI drive can be regarded as a universal solution for high power synchronous motor drives at all power and speed ranges.