The growth of embedded generation and portable electrical installations has led to an increased demand for low cost, flexible and reliable generator systems for military and commercial applications. An interior permanent magnet (IPM) machine has high power density due to its reluctance torque and magnetic torque components so it can produce a large constant power-speed range. However, an IPM machine needs demagnetizing current at high-speed during the flux-weakening region and thus develops an inverter shutdown problem in an uncontrolled generator mode operation. In order to overcome the disadvantages of the IPM machine, the permanent magnet assisted synchronous reluctance generator (PMa-SynRG) can be a good solution for low cost, high efficiency reliable generator systems. A PMa-SynRG can produce a high efficiency drive by utilizing the proper amount of magnet and reluctance torque. This work proposes a PMa-SynRG with two flux barriers and permanent magnets embedded in the second layer of the rotor. A neodymium magnet (NdFeB) was used as permanent magnets in the rotor to prevent demagnetization. Finding the minimum amount of magnet is one of the goals of the optimization process. The objectives of this work are to build an optimal design for the 3kW generator and an advanced power electronics converter for the PMa-SynRG drive system. In order to find the optimized 3kW machine, a Lumped Parameter Model (LPM) was used to achieve fast computation, and Differential Evolution Strategy (DES) was used to embed the LPM in an efficient numerical optimization routine to identify optimum designs. Finite Element Analysis (FEA) was used for test performance of optimum designs. On the basis of differences between LPM and FEA, model predictions were used to fine tune the LPM model. For new optimum design converges, numerical optimizations and iterations were performed to produce LPM and FEA predictions. For the drive system, the thyristor based, current-fed drive is much simpler and has lower power losses compared to the pulse width modulation (PWM) drive. Eliminating the requirement for self-controlled switches is a distinct advantage for lower cost. Another feature of the developed current-fed drive is its inherent capability to provide generating action by making the PMa-SynRG operates as a generator, rectifying the phase voltages by means of the three-phase rectifier and feeding the power into the load. These features make the current-fed drive a good candidate for driving any type of synchronous generators including the proposed PMa-SynRG.

The use of high power converters has increased tremendously. Increased demand for transportation, housing and industrial needs means that more number of power converters interact with the utility power grid. These converters are non-linear and they draw harmonic currents, significantly affecting power quality. To reduce harmonics, filters, power factor correction circuits and capacitor banks are required. And the development of hybrid technologies and renewable energy power stations trigger a demand for power converters with bi-directional capabilities. The objective of this thesis is to develop a high power quality, bi-directional AC-DC power converter that is a solution to the aforementioned problems. This thesis studies an existing topology for a high power AC-DC power conversion with transformer isolation. The topology consists of an uncontrolled rectifier followed by a DC-DC converter to produce a set voltage output. A design example of the topology is simulated using the PSIM software package (version 6). Critical performance characteristics such as power factor and total harmonic distortion are analyzed. Following that study a new topology is proposed, which is an improvement over the older design, with reduced power conversion stages. The new topology has a fully controlled current source Pulse Width Modulation (PWM) rectifier at the front end to replace the uncontrolled rectifier and DC-DC combination. This topology has multiquadrant operational capabilities and the controller employs Selective Harmonic Elimination techniques to produce the programmed PWM switching functions for the rectifier. A design example of the converter and the digital controller are simulated in PSIM environment. The converter input current THD (Total Harmonic Distortion) and input power factor are within IEEE 519 and DoE standards. The converter is simulated in both first and fourth quadrant operations. A side-by-side comparison of the two topologies is done with respect to design and performance features such as power factor, THD, filter size, etc. The new topology converter provides performance superior to that of the older topology. Finally the thesis explores possible applications for the converter in power supplies, renewable energy and hybrid technologies.

Significant advances in modern ac/ac power converter technologies and demands of industries have reached beyond standard ac/ac power converters with voltage-source inverters fed from diode rectifiers. Power electronics converters have been matured to stages toward compact realization, increased high-power handling capability, and improving utility interface. Modern ac/ac power converter topologies with various control strategies have been introduced for the further improvements, such as matrix converters, current-fed converters, PWM rectifiers, and active power filters. In this dissertation, several new converter topologies are proposed in conjunction with developed control schemes based on the modern ac/ac converters which enhance performance and solve the drawbacks of conventional converters. In this study, a new fault-tolerant PWM strategy is first proposed for matrix converters. The added fault-tolerant scheme would strengthen the matrix converter technology for aerospace and military applications. A modulation strategy is developed to reshape output currents for continuous operation, against fault occurrence in matrix converter drives. This study designs a hybrid, high-performance ac/ac power converter for high power applications, based on a high-power load commutated inverter and a mediumpower voltage source inverter. Natural commutation of the load commutated inverter is actively controlled by the voltage source inverter. In addition, the developed hybrid system ensures sinusoidal output current/voltage waveforms and fast dynamic response in high power areas. A new topology and control scheme for a six-step current source inverter is proposed. The proposed topology utilizes a small voltage source inverter, to turn off main thyristor switches, transfer reactive load energy, and limit peak voltages across loads. The proposed topology maximizes benefits of the constituent converters: highpower handling capability of large thyristor-based current source inverters as well as fast and easy control of small voltage source inverters. This study analyzes, compares, and evaluates two topologies for unity power factor and multiple ac/ac power conversions. Theoretical analyses and comparisons of the two topologies, grounded on mathematical approaches, are presented from the standpoint of converter kVA ratings, dc-link voltage requirements, switch ratings, semiconductor losses, and reactive component sizes. Analysis, simulation, and experimental results are detailed for each proposed topology.

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High-frequency and large amplitude current is a driving requirement for applications such as induction heating, wireless power transfer, power amplifier for magnetic resonant imaging, electronic ballasts, and ozone generators. Voltage-fed resonant inverters are normally employed, however, current-fed (CF) resonant inverters are a competitive alternative when the quality factor of the load is significantly high. The input current of a CF resonant inverter is considerably smaller than the output current, which benefits efficiency. A simple, parallel resonant tank is sufficient to create a high-power sinusoidal signal at the output. Additionally, input current is limited at the no-load condition, providing safe operation of the system. Drawbacks of the CF resonant inverter are associated with the implementation of the equivalent current source. A large input inductor is required to create an equivalent dc current source, to reduce power density and the bandwidth of the system. For safety, a switching stage is implemented using bidirectional voltage-blocking switches, which consist of a series connection of a diode and a transistor. The series diode experiences significant conduction loss because of large on-state voltage. The control of the output current amplitude for constant-frequency inverters requires a pre-regulation stage, typically implemented as a cascaded hard-switched dc/dc buck converter. The pre-regulation also reduces the efficiency. In this dissertation, a variety of CF resonant inverters with two input inductors and two grounded switches are investigated for an inductive-load driver with loaded quality factor larger than ten, constant and high-frequency (~500 kHz) operation, high reactive output power (~14 kVA), high bandwidth (~100 kHz), and high efficiency (over 95 %). The implementation of such system required to question the fundamental operation of the CF resonant inverter. The input inductance is reduced by around an order of magnitude, ensuring sufficient bandwidth, and allowing rich harmonic content in the input current. Of particular importance are fundamental and second harmonic components since they influence synchronization of the zero-crossing of the output voltage and the turn-on of the switches. The synchronization occurs at a particular frequency, termed synchronous frequency, and it allows for zero switching loss in the switches, which greatly boosts efficiency. The synchronous conditions were not know prior this work, and the dependence among circuit parameters, input current harmonics, and synchronous frequency are derived for the first time. The series diode of the bidirectional switch can reduce the efficiency of the system to below 90 %, and has to be removed from the system. The detrimental current-spikes can occur if the inverter is not operated in synchronous condition, such as in transients, or during parametric variations of the load coil. The resistance of the load coil has a wide variance, five times or more, while the inductance changes as well by a few percent. To accommodate for non-synchronous conditions, a low-loss current snubber is proposed as a safety measure to replace lossy diodes. The center-piece of the dissertation is the proposal of a two-phase zero-voltage switching buck pre-regulator, as it enables fixed frequency and synchronous operation of the inverter under wide parametric variations of the load. The synchronous operation is controlled by phase-shifting the switching functions of the pre-regulator and inverter. The pre-regulator reduces the dc current in the input inductors, which is a main contributor to current stress and conduction losses in the inverter switches. Total loss of the inverter switches is minimized since no switching loss is present and minimal conduction losses are allowed. The dc current in the input inductors, once seen as a means to transfer power to load, is now contradictory perceived as parasitic, and the power is transferred to the load using a fundamental frequency harmonic! The input current to the resonant tank, previously designed to be a square-wave, now resembles a sine-wave with very rich harmonic content. Additionally, the efficiency of the pre-regulator at heavy-load condition is improved by ensuring ZVS for with an additional inductive tank. The dissertation includes five chapters. The first chapter is an introduction to current-fed resonant inverters, applications, and state-of-the-art means to ensure constant frequency operation under load's parametric variations. The second chapter is dedicated to the optimization of the CF resonant inverter topology with a dc input voltage, two input inductors, and two MOSFETs. The topology is termed as a boost amplifier. If the amplifier operates away from the synchronous frequency, detrimental current spikes will flow though the switches since the series diodes are eliminated. Current spikes reduce the efficiency up to few percent and can create false functioning of the system. Operation at the synchronous frequency is achieved with large, bulky, input inductors, typically around 1-2 mH or higher, when the synchronous frequency follows the resonant frequency of the tank at 500 kHz. The input inductance cannot be reduced arbitrarily to meet the system bandwidth requirement, since the synchronous frequency is increased based on the inductance value. The relationship between the two (input inductance and the synchronous frequency) was unknown prior this work. The synchronous frequency is determined to be a complicated mathematical function of harmonic currents through the input inductors, and it is found using the harmonic decomposition method. As a safety feature, a current snubber is implemented in series with the resonant tank. Snubber utilizes a series inductance of cable connection between the tank and the switching stage, and it is more efficient than the previously employed series diodes. Topology optimization and detailed design procedure are provided with respect to efficiency and system dynamics. The mathematics is verified by a prototype rated at 14 kVA and 1.25 kW. The input inductance is reduced by around an order of magnitude, with the synchronous frequency increase of 2 %. The efficiency of the power amplifier reached 98.5 % and might be improved further with additional optimization. Silicon carbide MOSFETs are employed for their capability to operate efficiently at high frequency, and high temperature. The third chapter is dedicated to the development of the boost amplifier's large signal model using the Generalized State-space Averaging (GSSA) method. The model accurately predicts amplifier's transient and steady-state operation for any type of input voltage source (dc, dc with sinusoidal ripple, pulse-width modulated), and for either synchronous or non-synchronous operating frequency. It overcomes the limitation of the low-frequency model, which works well only for dc voltage-source input and at synchronous frequency. As the measure of accuracy, the zero-crossing of the resonant voltage is predicted with an error less than 2° over a period of synchronous operation, and for a range of interest for input inductance (25 μH – 1000 μH) and loaded-quality factor (10 – 50). The model is validated both in simulation and hardware for start-up transient and steady-state operation. It is then used in the synthesis of modulated output waveforms, including Hann-function and trapezoidal-function envelopes of the output voltage/current. In the fourth chapter, the GSSA model is employed in development of the PWM compensation method that ensures synchronous operation at constant frequency for the wide variation of the load. The boost amplifier is extended with a cascaded pre-regulator whose main purpose is to control the output resonant voltage. The pre-regulator is implemented as two switching half-bridges with same duty-cycle and phase-shift of 180°. The behavior of the cascaded structure is the same as of the buck converter, so the half-bridges are named buck pre-regulators. ZVS operation is ensured by putting an inductive tank between the half-bridges. Each output of half-bridges is connected to each of input inductors of the boost to provide the PWM excitation. Using the GSSA model, the synchronous condition and control laws are derived for the amplifier. Properties of the current harmonics in the input inductors are well examined. It is discovered that the dc harmonic, once used to transfer power, is unwanted (parasitic) since it increases conduction loss in switches of the boost. A better idea is to use the fundamental harmonic for power transfer, since it does not create loss in the switches. Complete elimination of the dc current is not feasible for constant frequency operation of the amplifier since the dc current depends on the load coil's resistance. However, significant mitigation of around 55 % is easily achievable. The proposed method improves significantly the efficiency of both the buck pre-regulator and the boost. Synchronous operation is demonstrated in hardware for fixed switching frequency of 480 kHz, power level up to 750 W, input voltage change from 300 V to 600 V, load coil's resistance change of three times, and load coil's inductance change of 3.5 %. Measured efficiency is around 95 %, with a great room for improvements. Chapter five summarizes key contributions and concludes the dissertation. / Ph. D.