<p>Although a superconductor has zero resistivity when carrying a direct current, losses do occur when it is exposed to an alternating magnetic field and/or is carrying an alternating current. The magnitude of these so-called AC losses depends on the operating temperature, the amplitude and the direction of the magnetic field, the transport current, and the frequency. Therefore, the use of high-temperature superconductors, HTSs, in electric power components such as cables, transformers or reactors, requires knowledge of the AC losses.</p><p>This thesis deals with the development of AC loss models for HTSs, mainly for Bi-2223 tapes. In particular, the orientation of the applied magnetic field is taken into account in the modelling. The basis for the models is the results of experimental investigations.</p><p>The basic concepts of HTSs with special emphasis on the modelling of AC losses are presented. These can be broken down into several components. Their sources and natures are described. One of the components is the hysteretic loss and it is the dominating loss in AC applications at power frequencies. Therefore, the other loss components are neglected in the modelling.</p><p>Models are presented and the associated parameters are investigated with respect to their dependence of the magnetic field as well as the temperature. The AC losses for parallel and perpendicular magnetic field with respect to the wide side of the tape are calculated numerically. Moreover, a semi-empirical model for intermediate angels of the applied magnetic field is proposed. The comparisons show good agreement with experimental results.</p><p><b>Keywords:</b> High-temperature superconductors, AC loss modelling, hysteresis, E-J characteristic.</p>

<p>Vehicles with internal combustion (IC) engines fueled by hydrocarbon compounds have been used for more than 100 years for ground transportation. During the years and in particular in the last decade, the environmental aspects of IC engines have become a major political and research topic. Following this interest, the emissions of pollutants such as NO_x, CO₂ and unburned hydrocarbons (UHC) from IC engines have been reduced considerably. Yet, there is still a clear need and possibility to improve engine efficiency while further reducing emissions of pollutants. The maximum efficiency of IC engines used in passenger cars is no more than $40\%$ and considerably less than that under part load conditions. One way to improve engine efficiency is to utilize the energy of the exhaust gases to turbocharge the engine. While turbocharging is by no means a new concept, its design and integration into the gas exchange system has been of low priority in the power train design process. One expects that the rapidly increasing interest in efficient passenger car engines would mean that the use of turbo technology will become more widespread. The flow in the IC-engine intake manifold determines the flow in the cylinder prior and during the combustion. Similarly, the flow in the exhaust manifold determines the flow into the turbine, and thereby the efficiency of the turbocharging system. In order to reduce NO_x emissions, exhaust gas recirculation (EGR) is used. As this process transport exhaust gases into the cylinder, its efficiency is dependent on the gas exchange system in general. The losses in the gas exchange system are also an issue related to engine efficiency. These aspects have been addressed up to now rather superficially. One has been interested in global aspects (e.g. pressure drop, turbine efficiency) under steady state conditions.In this thesis, we focus on the flow in the exhaust port and close to the valve. Since the flow in the port can be transonic, we study first the numerical modeling of such a flow in a more simple geometry, namely a bump placed in a wind tunnel. Large-Eddy Simulations of internal transonic flow have been carried out. The results show that transonic flow in general is very sensitive to small disturbances in the boundary conditions. Flow in the wind tunnel case is always highly unsteady in the transonic flow regime with self excited shock oscillations and associated with that also unsteady boundary-layer separation. To investigate sensitivity to periodic disturbances the outlet pressure in the wind tunnel case  was varied periodically at rather low amplitude. These low amplitude oscillations caused hysteretic behavior in the mean shock position and appearance of shocks of widely different patterns. The study of a model exhaust port shows that at realistic pressure ratios, the flow is transonic in the exhaust port. Furthermore, two pairs of vortex structures are created downstream of the valve plate by the wake behind the valve stem and by inertial forces and the pressure gradient in the port. These structures dissipate rather quickly. The impact of these structures and the choking effect caused by the shock on realistic IC engine performance remains to be studied in the future.</p> / CICERO

Thesis (M.S. in Electrical Engineering)--Naval Postgraduate School, June 2004. / Thesis advisor(s): Robert Ashton. Includes bibliographical references (p. 73-74). Also available online.

Thesis (Ph. D.)--University of Washington, 2001. / Vita. Includes bibliographical references (p. 224-236).

The objective of this research is to explore circuit techniques and architectures suitable for implementation in digital technologies, that can be used to enhance the efficiency of power stages. Specifically, the use of switching power stages with pulse-width modulation techniques is considered. Switching power stages, such as Class D amplifiers, are inherently well-suited for implementation in deep-submicron CMOS. Pulse-width modulation (PWM) employs discrete amplitude levels and encodes signal information in local time-based averages, and as such can also benefit from such technologies. Additionally PWM does not suffer from quantization noise, and is well-suited for low noise applications. PWM designs, that can be applied for a range of signal bandwidth requirements, spanning several tens to hundreds of kHz are proposed. Applications for these architectures include audio systems, powerline communications and wireless communications. Design challenges and requirements that can arise in different application contexts are considered in the specification of the architectures. A common goal in the definition of the architectures is to minimize complexity of the designs. In the first part of the dissertation, a third-order self-oscillating PWM class-D amplifier for audio applications, that utilizes a hysteretic comparator is described. The design is analyzed and its THD is theoretically determined by employing an equivalent model, that relates the approach to natural sampling pulse-width modulation. The architecture eliminates the requirement for a high-quality carrier generator. A low-cost hysteresis compensation technique is utilized to enhance distortion performance at high output power levels. An implementation is presented in a 0.7um CMOS process. The design achieves a dynamic range (DR) of 116.5 dB, and a THD+N of 0.0012%, while delivering a power of 125 mW into an 8[Omega] load at 1 kHz. The THD+N is under 0.006% up to 90% of the maximum output power. The amplifier can deliver 1.45 W into the load with a THD of 5% with a 5 V power supply. The efficiency is greater than 84% for output power larger than 1 W. The area of the amplifier is 6 mm². The achieved performance indicates that the design is well-suited for high-performance audio applications. A class D line driver that utilizes a phase-locked loop (PLL) based PWM generation technique is presented next. The principle of operation, and implementation details relating to loop stability, linearity and noise performance are analyzed. An implementation is presented in a 130nm CMOS process. The amplifier can deliver 1.2 W into an 6.8[Omega] load with a 4.8 V power supply. The architecture eliminates the requirement for a high-quality carrier generator and a fast, continuous voltage comparator that are often required in PWM implementations. The design can achieve a THD of -65 dB, with a switching frequency that can be as high as 20 MHz. The peak efficiency is 83% for output power larger than 1 W, for a switching frequency of 10 MHz. The area of the amplifier is 2.25 mm². This architecture is potentially suitable for powerline applications. Finally, a phase-locked loop based PWM Cartesian transmitter with the capability to drive switched power amplifiers, such as a Class D power amplifier, is proposed. A phase-locked loop based technique is employed to generate a high-frequency PWM pulse stream centered at 1.28 GHz. The prototype is simulated in a 130 nm CMOS process, and achieves 35% peak efficiency for 17 dBm output power with a carrier frequency of 900 MHz. Operation of the architecture with non-constant envelope modulation, such as that employed in the WCDMA standard, is verified in simulation. / text

<p>Diss. (sammanfattning) Umeå : Universitet, 1992, Härtill 4 uppsatser</p> / digitalisering@umu

No description available.

Multifrequency averaging is one of the widely used modeling and simulation techniques today for the analysis and design of power electronic systems. This technique is capable of providing the average behavior as well as the ripple behavior of power electronic systems. Hysteresis current control has fast response and internal current stability through controlling switches to maintain the current within a given hysteresis band of a given current command. However the state space variables in a hysteresis controlled system cannot be directly approached by multifrequency averaging method because of time varing switching frequency. In this thesis, a method of applying multifrequency averaging to hysteresis current controlled dc-dc converters is proposed. A dc-dc converter model with the application of this method has been successfully developed and validated both in simulation and experiment.

Sudden cardiac death (SCD) is a leading cause of fatalities. Several methods have been developed to predict instability in myocytes which could lead to SCD. The focus of this study was on altering memory in myocytes, i.e. hysteresis in restitution of action potential duration (APD), by differing levels of calcium. Determination of alteration was implemented by using a diastolic interval (DI) control program that implements a sinusoidal change in DI. Plotting APD versus previous DI, i.e. restitution, produces a hysteresis loop. From these hysteresis loops, five parameters were used to determine measures of memory: area, thickness, overall tilt, max delay and min delay. Calcium levels were then altered with either verapamil or BAPTA-AM. Statistically significant effects were found for the verapamil study, but not for the BAPTA-AM study. Simulations were used to explain significant results. The verapamil findings support clinical studies that have shown verapamil to not have anti-arrhythmic effects. Theory predicts that a decrease in memory would decrease the stability of a system, and perhaps verapamil may not increase stability as hypothesized previously. The results of the BAPTA-AM study were inconclusive, and further investigation is needed before it can be determined that BAPTA-AM has no significant effect on memory.

We perform micromagnetic simulations in MuMax3 on various magneticnanostructures to study their magnetic state and response to external fields. Theinteraction and ordering of nanomagnetic arrays is investigated by calculating themagnetostatic energies for various configurations. These energies are then used inMonte Carlo simulation to study the thermal behaviour of systems of nanomagneticarrays. We find that the magnetic state of the nanostructures are related to theirshape and size and furthermore affect the emergent properties of the system, givingrise to temperature dependent ordering among the individual structures. Results fromboth micromagnetic and statistical mechanic simulations agree well with availableexperimental data, although the Monte Carlo algorithm encounter problems at lowsimulation temperatures.