This dissertation investigates the control of the electrical energy input to a three-phase electric arc furnace (EAF). Graphite electrodes are used to convert electrical energy into heat via three-phase electric arcs. Constant arc length is desirable as it implies steady energy transfer from the graphite electrodes to the metallic charge in the furnace bath. With the charge level constantly changing, the electrodes must be able to adjust for the arc length to remain constant. In this dissertation electric arc current is used as the control variable. This is the most often used control variable in the electric arc furnace industry and implies fast adjustments of short circuits between the electrode tips and the metallic charge. The motivation behind the modelling of the electrode system for a three-phase electric arc furnace is to extend an existing EAF model developed at the University of Pretoria. The existing model investigates the control of the electric arc furnace process itself and it is assumed that the applied electrical energy input is constant. Proportional-Integral-Derivative (PID) control as well as Model-Predictive-Control (MPC) is applied to the electrode system. Time delays on the outputs of the hydraulic actuators makes it necessary to include approximations of time delays on the outputs of the linear model, which is needed for controller design. A well known general control problem is followed in this dissertation. All models are derived from first principles, and complete controller design is carried out. Most available literature lack in at least one of these fields. / Dissertation (MEng (Electronic Engineering))--University of Pretoria, 2007. / Electrical, Electronic and Computer Engineering / unrestricted

The objective of this study is to investigate the interaction between a lightning strike and carbon/fiber composites. The first approach is to characterize the damage development in a composite structure subjected to simulated lightning strikes. Several existing studies have acknowledged that the lightning induced damaged can be categorized into two separate domains of damage; a primary domain of damage that occurs at the attachment point, and a secondary domain of damage that is typically formed around the attachment point. Quantitative studies of the causes of the primary damage domain are not satisfactory for explaining the secondary damage domain and thus, these two domains are produced by presumably different mechanisms. There have been many reports and studies focused on the inspection of the primary damaged area. However, the secondary domain of damage has not yet been fully explained and understood. An experimental setup was configured with a recommissioned lightning current simulator to generate artificial lightning strikes consistent with the existing standard for lightning protection testing used in the aerospace industry. Carbon/epoxy composite laminates in various layups and Pultruded Rod Stitched Efficient Unitized Structure (PRSEUS) panels were subjected to high impulse currents of different magnitudes. The lightning induced damage to the protected and non-protected composite laminates and PRSEUS panels were evaluated, and the influence of different variables such as current magnitude, strike location, and laminate layup were studied. An interesting observation was the secondary damage area that expanded laterally beyond the intense damage area. The structure of a composite panel is such that it forces the current to flow along the carbon fibers directions, as opposed to metals where the relatively isotropic conductivity of the metal allows current to distribute radially. It is argued in this work that the secondary domain of damage may be related to the anisotropic electrical conductivity property of the composite panels. A comprehensive theory based on multidimensional electromagnetic field simulation was proposed to reveal the root cause mechanisms of the unique patterns of secondary damage in the carbon composite structural materials tested with simulated lightning current impulses.

Sediment deposited in back-arc basins preserves a record of the extensional, volcanic and tectonic history of the arc-backarc systems. Back-arc sedimentation is of particular interest as seafloor massive sulfide deposits may be preserved in back-arc basin sediments. The study of back-arc sedimentation using acoustic data, such as high-resolution sub-bottom profiling data (Parasound) and seismic reflection data, can be a much more cost effective approach than analysis of sediments recovered from drill cores. In this study, we use these two acoustic datasets to build a facies model of sedimentation in the northeast Lau Basin, an actively opening back-arc basin in the southwest Pacific Ocean. Using 830 km of Parasound and 730 km of seismic lines along 4 transects of the Lau Basin, we constructed one of the most detailed models of sedimentation in a back-arc basin to date. Parasound data show distinct echoes with sub-bottom reflections indicative of a high proportion of hemipelagic sediment, whereas the indistinct echoes with few to no sub-bottom reflections indicate a higher proportion of coarse, bedded, volcaniclastic turbidites. Hyperbolic echoes are associated with regions of rugged or uneven terrain characterized by exposed, rough basement or deposits formed by contour currents, turbidity currents, slumps or slides. These relationships form the basis of an echo-facies legend developed for typical back-arc basin sediments. The echo-facies observed in the Parasound, and confirmed by deeper-penetrating seismic reflection data, provide important insights into the sedimentary processes involved in back-arc sedimentation. We observed mass transport deposits (MTDs) in all of the sub-basins and slope deposits within and on the flanks of active rifts (e.g., the Fonualei Rift and Spreading Centre, FRSC), suggesting a direct correlation between MTDs and zones of active rifting. We observed an overall increase in sediment thickness toward the Tofua Arc which suggests it is the main sediment source, but local variations in sediment thickness suggest significant input from local intrabasinal seamounts. The uppermost echo-facies in over 60% of the sub-basins in the study area is dominated by hemipelagic material, which suggests an abrupt transition in the dominant sediment source from volcaniclastic to hemipelagic at around 0.3 Ma, when a period of volcanic quiescence from the Tofua Arc began. The study shows that a near complete record of basin evolution can be constructed using geophysical and acoustic methods and that this work may help to locate future drill sites where in situ data can be collected.

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No description available.

Around the world, the eutrophication of freshwater lakes and streams by the excess loading of phosphorus (P) has become one of the most important water quality issues. In Vermont, P pollution from urban and agricultural non-point sources has led to severe blooms of toxic cyanobacteria in Lake Champlain, and the degradation of the lake’s value as a drinking water source and its recreation potential. Electric arc furnace (EAF) steel slag has been identified as an effective material for use as a filter media for the removal of P from both point and non-point sources of pollution. In order to further assess the feasibility of this technology for use in Vermont, several investigations were carried out starting in the winter of 2006. Three objectives for research were identified: 1) to construct 2 EAF steel slag filters in-series at the Constructed Wetlands Research Center (CWRC) and investigate their efficiency in P, TSS and metals reduction from dairy waste water in a cold climate; 2) investigate the potential for reuse of P saturated EAF steel slag as a soil amendment and plant fertilizer by testing bioavailability of sorbed P and quantities of P released to surface runoff; 3) To elucidate the principal mechanisms responsible for the removal of P in EAF slag filters when used for the treatment of dairy effluent. The results indicated that 2 EAF steel slag filters constructed in-series are an effective method to increase the treatment efficiency and longevity of a filter system. Additionally, parameters for the concentrations of total suspended solids (TSS) and biological oxygen demand (BOD) were developed, both important factors for design of filter systems. In greenhouse trials, Medicago sativa plants achieved greater above ground biomass growth with P fertilization by triple super phosphate (TSP) than EAF steel slag at the shorter growth period (5 weeks). However, by the end of the longer growth period (10 weeks) except at the highest amendment rate, the plants treated with EAF steel slag had a higher growth rate than the TSP, suggesting that EAF steel slag is an effective slow release P source. Using a rain simulator, the amount of P lost to surface runoff from both a saturated and a semi-saturated EAF steel slag was found to be negligible, and except for total P in the saturated slag, to be below 1 mg L-1. Voltammetric analysis and geochemical modeling were used to identify possible mechanisms for the removal of P from waste effluent. The Ca mineral hydroxyapaptite and the Fe(II) mineral vivianite were both shown to be likely mechanisms given the chemical conditions in EAF steel slag filters. This research represents the first investigation of cold weather performance of EAF steel slag filters for the treatment of dairy parlor and milk house waste effluent. Additionally, it was also the first research on the bioavailability of P sorbed to EAF steel slag, and of the possibility of its reuse as a soil amendment, and of the mechanisms involved in P removal from dairy waste effluent.

Abstract In this study, a static electric contact tester with a vacuum system and gas supply system is used to investigate the effects of experimental parameters, such as inert gas kind, supply voltage, and the gap between the contacts, on the electric-arc characteristics and the erosion type for the electric contacts of silver. The experimental results show that chargeable threshold voltage in helium is higher than that in argon or air. In addition, the erosion surface of anode is observed, and the erosion area and volume are calculated and analyzed. Results show that the erosion area and volume in helium are smaller than that in argon or air. According to the observation of SEM photomicrographs, metal sputtering of the erosion hole in helium is smaller than that in argon or air. It is expected to provide the domestic electrical contact manufacturer in improving the research and development technique of the electrical contacts, and the design reference related to ultra-precision machining using a clean room.

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Whether by accretion, magmatic addition, or refinement of more mafic lithologies, continental arcs are likely zones for the creation of "average" continental crust with intermediate silica content. This dissertation contains the results of broadband seismic studies carried out in two field areas, an active subduction zone and the remnants of an extinct arc, with the aim of understanding lithospheric evolution at convergent margins. The analytical techniques of receiver function calculation and surface wave tomography are applied to data sets collected above the Andean subduction zone in Chile and western Argentina and in the Coast Mountains Batholith of central British Columbia. We present the first in-depth comparison of receiver functions calculated using the high frequencies available in records of intermediate-depth local earthquakes with those calculated from the lower frequency data in records of larger teleseismic events. The comparison reveals that the lower crust beneath the Western Sierras Pampeanas contains a gradational velocity increase over ~20km above a small velocity step at the Moho. Surface wave tomography confirms the existence of an unusually high velocity anomaly in the mantle above the slab and yields estimates of slab thickness on the order of 50 km. To the south of the flat slab region, we see evidence of active mantle wedge convection above the steep slab, but no evidence of the lithosphere-asthenosphere boundary beneath the subducting Nazca plate. In the Coast Mountains Batholith (CMB), receiver functions image a bright, continuous Moho throughout the study region. Combined with petrologic modeling, the receiver function data point toward convective removal of any ultramafic root that formed beneath the CMB. Low absolute shear wave velocities in the upper mantle resolved via surface wave analysis strengthen the case for root removal beneath the eastern section of the CMB. On the far western edge of the CMB, we find evidence of a partially reformed lithosphere outboard of a major tectonic boundary. These observations shed light on the distillation of felsic to intermediate continental crust from more mafic primary magmas in active subduction zones and the eventual return of the complementary ultramafic residuals to the convecting mantle.

No description available.