Acoustic models are useful tools to increase understanding of cochlear implant perception. Two particular issues in modelling cochlear implant perception were considered in the present study, which aimed at improving acoustic models. The first included an electricallayer in the model, while the second manipulated synthesis signal parameters. Two parts of the study explored the effects of current decay, compression function and simultaneous stimulation, by including the electrical layer. The SPREAD model, which incorporated this layer, yielded the asymptote in speech intelligibility at seven channels observed in CI listeners. It was shown that the intensity of border channels was deemphasized in relation to more central channels. This was caused by the one-sided effects of current spread from neighbouring channels for the border channels, as opposed to the two-sided effects for the more central channels. It was theorised that more compressive mapping functions would affect spectral cues and consequently speech intelligibility, but speech intelligibility experiments did not confirm this theory. A simultaneous analogue stimulation (SAS) model, which modelled simultaneous stimulation, yielded intelligibility results that were lower than those of the SPREAD model at 16 channels. The SAS model also appeared to introduce more temporal distortion than the SPREAD model. A third part of the study endeavoured to improve correspondence of acoustic model results with cochlear implant listener results by using nine different synthesis signals. The best synthesis signal was noise-band based. The widths of these increased linearly from 0.4 mm at the apical to 8 mm at the basal end. Good correspondence between speech recognition outcomes using this synthesis signal with those of CI listeners was found. AFRIKAANS : Akoesties modelle word algemeen gebruik om die persepsie van inplantingsgebruikers beter te verstaan. Twee benaderings tot die modellering van kogleêre inplantingsgebruikerpersepsies is voorgestel om akoestiese modelle te verbeter. In die eerste benadering is die generiese model verbeter deur die byvoeging van 'n elektriese laag en in die tweede benadering is sinteseseinparameters gemanipuleer om die ooreenkoms met inplantingsgebruikersuitkomste te verbeter. Twee dele van die studie het die effek van stroomverspreiding, samedrukkings-funksie en gelyktydige stimulasie ondersoek deur die insluiting van die elektriese laag. Die SPREAD-model het die asimptoot in spraakherkenning by sewe kanale getoon. Die intensiteit van grenskanale is onderbeklemtoon in verhouding met meer sentrale kanale. Dit is veroorsaak deur die eensydige effekte van stroomverspreiding vir die grenskanale, teenoor die tweesydige effekte wat meer sentrale kanale tipies beïnvloed. Die model het gesuggereer dat meer samedrukkende funksies spektrale inligting sou affekteer, maar spraakherkenningsdata het nie hierdie teorie bevestig nie. Die gelyktydige- analoogstimulasiemodel, wat gelyktydige stimulasie gemodelleer het, het soortgelyke tendense getoon, maar met meer temporale effekte as die SPREAD-model. Die gelyktydige- analoogstimulasiemodel-model se resultate was ook swakker by 16 kanale as die SPREAD-modelresultate. Die derde deel van die studie het gepoog om beter ooreenkoms tussen modeluitkomste en inplantingsgebruikeruitkomste te verkry deur nege verskillende sinteseseine te gebruik. Die beste sintesesein was die ruisband met veranderende wydte; hierdie wydte het verbreed vanaf 0.4 mm by die apeks tot by 8 mm by die basis. 'n Goeie ooreenkoms is verkry tussen modeluitkomste en inplantingsgebruikeruitkomste deur hierdie sintesesein te gebruik. / Thesis (PhD)--University of Pretoria, 2010. / Electrical, Electronic and Computer Engineering / Unrestricted

No / In this paper, a model for urea prilling tower with co-current flow of cooling air and urea prills (particles) is presented. The process is modelled by simultaneous solution of the differential equations for hydrodynamics, heat and mass transfer between the air and prills. The process variables such as temperature, absolute and relative humidity of air along the height of the tower were obtained from this model. Temperature and moisture distribution of urea prills and their radial and vertical velocities were also calculated. The results of the present model were compared with the counter-current operation model available in the literature. The simulation results show that heat transfer performance for co-current operation is significantly less than that of the counter-current scenario. This is more pronounced for small prills, i.e. 1.0 mm than that of the large prills. The advantage of the model is that it can be used to investigate influence of operating parameters on efficiency of the co-current process. This also helps us to set the process control strategies for design and quality control purposes of the process.

The integrated high-quality rectifier-regulators [1] are not practical for universal input voltage and wide load range applications because of high bulk-capacitor voltage stress at lighter loads. This load-dependent characteristic of the bulk-capacitor voltage is due to the integration of a discontinuous conduction mode boost converter and a continuous conduction mode dc-dc converter. In addition, this power factor correction technique suffers from a high-voltage spike on the switch at turn-off. In this thesis, variable frequency control, swinging choke, and low loss LC snubber techniques are proposed to alleviate these problems. Finally, several experimental converters with different specifications are evaluated with respect to efficiency and ability to meet the IEC555-2 standards. / Master of Science

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Approved for public release; distribution is unlimited / The main focus in the design of the next generation combatant, DD(X), is the US Navy's proposed Integrated Power System (IPS) which includes an all-electric propulsion drive system. The reduction of current waveform harmonics is critical in combatant propulsion systems such as the IPS. One method of reducing the current harmonics is to utilize a multilevel converter topology. The multi-level converter, as compared to a standard converter, features low dv/dt losses and low switching losses. This thesis examines the design, construction and testing of two multi-level converters operated in tandem, called a Cascaded Multi-Level Converter (CMLC). A digital logic switching circuit is designed and constructed to control the CMLC during the operational testing phase. The CMLC is demonstrated in a three-phase high-voltage configuration with 178.5 V zero-to-peak voltage, 2.10 A zero-to-peak current achieved using an R-L load. / Lieutenant Commander, United States Navy

M.Phil. (Electrical and Electronic Engineering) / There are two methods that can be used when power is converted from one frequency to another. One way uses a DC link as an intermediate stage and the other uses no intermediate stage. When no DC link exists, the conversion process is referred to as AC-AC conversion, (Cha, Enjeti & Ratanapanachote, 2004:2237). This method essentially chops the AC signal in a specific manner which yields an AC signal with a different frequency directly. Depending on how the AC signal is chopped, it would result in a varied number of wave shapes are constructed. The construction of these various wave shapes results in devices that can accomplish different tasks. One such device is called a unity displacement factor frequency converter (UDFFC). This research attempts to develop a model, to simulate and design a unity displacement factor frequency converter. The primary idea about this device is that the power factor or displacement factor is unity regardless of the load. There are three parts in the development of the UDFFC. The power circuit is an array or matrix of switches that link the three phase supply to the three phase load. The power circuit is interfaced to the control circuit which generates pulses that control the matrix of switches. The control circuit utilizes embedded control software that performs the control logic in a microcontroller. The development of the UDFFC has a potential to solve real power engineering problems, such as eliminating the DC link in a converter. This achieves the conversion of AC directly to AC in the absence of a DC link. The other major advantage is the variation of the output frequency to a desired value. The most important is that the displacement factor can be controlled as well.

Nos processos industriais envolvendo eletrólise, como na produção de alumínio, cobre, zinco, níquel, manganês, carbeto de silício, cloro e seus derivados, as correntes envolvidas em geral atingem patamares que excedem o valor nominal de um único semicondutor, exigindo a associação de componentes em paralelo com uma distribuição de corrente equilibrada. Este trabalho apresenta uma análise comparativa da distribuição de corrente em associações de semicondutores em paralelo em vinte retificadores de alta potência com diferentes tipos de arquitetura física de barramentos. A particularidade deste estudo é a medição simultânea das correntes em todos os semicondutores de todos os ramos do retificador obtendo-se uma imagem da distribuição real de correntes do conversor. / The industrial electrolytic processes to produce, aluminum, cooper, zinc, nickel, manganese, silicon carbide, chlorine and its derivative, require current values which greatly exceed the capability of single rectifying devices and paralleled combinations of semiconductors or equipments are necessary. This study evaluated through a comparative analyzes twenty rectifiers, with different busbar geometries, in order to verify the current distribution in paralleled devices. The differential of this study is the simultaneous measurement of currents in all semiconductors of each branch in order to obtain a real representation of the rectifiers current distribution.