Thesis (MScEng)--University of Stellenbosch, 2002. / ENGLISH ABSTRACT: This thesis describes. the design and evaluation of a constant current angle controller for a variable speed reluctance synchronous machine (RSM) drive, as an energy efficient high performance drive. An accurate model of the RSM, with the use of finite element analysis, is derived and implemented in simulation software. The current- and speed controllers are designed and evaluated using a complete simulation model of the whole drive. The controller is implemented on a TMS320F240 DSPbased digital controller, which was developed. The dynamic performance of the constant-current-angle control is compared with that of the conventional constant-daxis- current control method. The results obtained from the RSM drive confirm the simulation results. In the comparison of the two control methods it is shown that the constant-current-angle controlled RSM drive is an energy-efficient drive with good dynamic performance. / AFRIKAANSE OPSOMMING: Hierdie tesis beskryf die ontwerp en evaluering van 'n konstante stroomhoek beheerder vir 'n reluktansie sinchroon masjien (RSM) aandryfstelsel vir optimum effektiwiteit en dinamika. 'n Akkurate model van die RSM, met behulp van eindige element analise, is opgestel en geimplimenteer in 'n simulasie pakket. Die stroom- en spoedbeheerders is ontwerp en geëvalueer deur middel van 'n simulasiemodel vat die volledige aandryfstelsel. Die beheerder is geimplimenteer deur gebruik te maak van 'n TMS320F240 DSP-gebaseerde digitale beheerder, wat ontwikkel is. Die konstantestroornhoek beheer is vergelyk met die konvensionele konstante-d-as-stroom beheer metode. Die resultate van die praktiese stelsel korreleer baie goed met die resultate van die simulasie. Deur die vergelyking van die twee metodes is dit bevind dat die konstante-stroomhoek beheerde RSM aandryfstelsel 'n energie effektiewe aandryfstelsel met baie goeie dinamiese vermoë is.

Thesis (MEng)--Stellenbosch University, 2015. / ENGLISH ABSTRACT: The growing contribution of wind energy to utility grids has sparked interest in small-scale wind turbines and thus a growing global cumulative installed capacity. Small-scale wind turbines find use in the saving of cost of electricity or for the carbon footprint reduction of small farms and small-holdings, as well as the electrification of rural communities. A goal of any wind turbine is to produce power at as low of a cost per unit energy as possible. Thus, a generator with a high power density and high efficiency is essential. The reluctance synchronous machine (RSM) is a strong competitor in this regard. Additionally, the RSM is a robust brushless topology that has good properties of manufacturability. However, studies published on the use of RSMs as generators in wind turbines is limited. This study serves to explore the performance and controllability of an RSM as a generator in a small-scale 9:2 kW wind turbine. For maximum power capture, it is desirable to have a wind turbine vary its rotor speed. However, there is a limit to the power that the generator may produce and so techniques are employed to reduce the captured power when operating above the rated wind speed. A turbine controller is developed that employs a speed-controlled maximum power point tracking (MPPT) technique for maximum power capture and soft-stalling of the blades to reduce power capture at excessive wind speeds. The RSM is modelled along with a turbine simulation model, complete with a wind source generator, to evaluate the performance of the system. Speed-controlled MPPT is known to sacrifice torque smoothness for fast tracking performance. To mitigate these harsh effects on the drivetrain, the speed reference of the generator is filtered to provide an average response to the optimal speed reference. This is shown to reduce the frequent and excessive speed, torque, and electrical power variations though optimal performance is not possible. However, any reduction on drivetrain fatigue that will maximise operation time of the turbine is considered an important gain. The RSM proves to have qualities that are applicable to wind turbine applications with its high efficiency, good manufacturability properties, low cost, and high robustness. Its higher power density over induction machines is also favourable though power electronics are required for optimal operation of the machine. / AFRIKAANSE OPSOMMING: Die groeiende bydrae van wind energie te nut roosters het aanleiding gegee tot belangstelling in kleinskaalse wind turbines en dus 'n groeiende w^ereldwye kumulatiewe geïnstalleerde kapasiteit. Kleinskaalse wind turbines vind ook gebruik in die besparing van koste van elektrisiteit, of vir die koolstofvoetspoor vermindering van klein plase en klein-hoewes, sowel as die elektrifisering van landelike gemeenskappe. Een van die doelwitte van enige wind turbine is om krag te produseer teen so laag van 'n koste per eenheid energie as moontlik. Dus, 'n kragopwekker met 'n hoë krag digtheid en hoë doeltreffendheid is noodsaaklik. Die reluktansie sinchroonmajien (RSM) is 'n sterk mededinger in hierdie verband. Daarbenewens is die RSM 'n robuuste borsellose topologie wat goeie eienskappe van vervaardigbaarheid het. Maar studies oor die gebruik van RSMs as kragopwekkers gepubliseer in die wind turbines is beperk. Hierdie studie dien om die prestasie te ondersoek en die beheerbaarheid van 'n RSM as 'n a kragopwekker in 'n klein-skaal 9:2 kW wind turbine te verken. Vir maksimum krag vang is dit wenslik dat die wind turbine sy rotor spoed wissel. Maar daar is 'n beperking op die krag wat die kragopwekker kan produseer en daarom work tegnieke gebruik om die gevange krag te verminder wanneer daar bo die gegradeerde wind spoed gewerk word. 'n Turbine beheerder word ontwikkel wat werk om 'n spoedbeheer maksimum kragpunt dop tegniek vir maksimum krag vang en die sagtestaking van die lemme krag vang deur oormatige wind spoed te verminder. Die RSM is gemodeleer saam met 'n turbine simulasie model kompleet met 'n wind bron kragopwekker om die prestasie van die stelsel te evalueer. Spoedbeheerde maksimum kragpunt dop is bekend om wringkrag gladheid vir 'n vinnige dop prestasie te offer. Om hierdie harde gevolge op die kragoorbringstelsel te versag is die spoed verwysing van die kragopwekker gefiltreer om 'n gemiddelde reaksie op die optimale spoed verwysing te verskaf. Dit word getoon om gereelde en hoë spoed, wringkrag en elektriese krag variases te verminder al is optimale prestasie nie moontlik nie. Enige afname van aandrystelsel moegheid wat operasie tyd van die turbine maksimeer word beskou as 'n belangrike gewin. Die RSM bewys eienskappe wat van toepassing is op die turbine aansoeke na aanleiding met sy hoë doeltreffendheid, goeie vervaardigbaarheid eienskappe, lae koste end ' hoë robuustheid. Sy hoër krag digtheid oor induksiemasjien is ook gunstig al is drywingselektronika nodig vir optimale werking van die masjien.

With the increasing environmental impact from the automotive industry, electric vehicles become more and more popular. This combined with the great breakthroughs in fast electronics the switched reluctance motor (SRM) has again gained popularity in recent years. Due to its cheap and rugged construction it is a good alternative to the permanent magnet motors and to the induction motor. The ́two main problems holding the SRM back are torque ripple and the acoustic noise generated from it. A lot of research is currently being performed in order to find a solution to these issues. This thesis has investigated different materials for the rotor in a high speed SRM. Different materials have been evaluated based on both mechanical and magnetic properties. This is done through simulations of the forces acting on the rotor combined with simulations of the magnetic field. The forces are simulate in the DASSULT SYSTEMS ABAQUS program and the magnetic field is simulate using AVL FIRE. Three different kinds of alloys are investigated, two different cobalt alloys are simulated as well as a silicon alloy with pure iron as a reference. The results show that the material needs to have a yield strength of at least 349 MPa to withstand the forces affecting the rotor. And that by using the high purity cobalt-iron alloy the generated torque could be increased with up to 20.9%, but with a cost increase of 3151.9% compared to the silicon alloy.

MPH / Department of Public Health / Prostate cancer (PC) screening is a strategy to identify cancer before it causes symptoms. However, men’s participation in prostate cancer screening seems inadequate and remains a public health concern worldwide. This leads most men to be diagnosed with an advanced prostate cancer where cancer cells spread to other parts of the body. The aim of this study was to assess the knowledge and attitudes of males towards prostate cancer screening at a selected village in Thulamela Municipality, Limpopo province. The study adopted a quantitative approach using a descriptive cross-sectional survey. A well-structured questionnaire was used to collect data from 245 men who are 40 years and above. The sample was selected using the simple random sampling technique. The Statistical Package for Social Scientists (SPSS) version 25.0 was used to analyse the collected data; and the results were presented in percentages, frequencies and tables. Cross tabulation, Chi square and Phi and Cramer’s V test were also utilised to test for association and effects size respectively at .05 level of significance. Respondent’s knowledge as an explanatory variable, screening practices as response variable was assessed. The findings of this study showed that 64.1% of respondents had inadequate knowledge about prostate cancer. About 62.4% respondents had no prior knowledge regarding prostate cancer and 69% of respondents didn’t know the age at risk for the development of PC, while 81.9% of respondents had never heard about PC, and 35.9% didn’t know that PC can be treated. 84.9% of respondents had positive attitudes towards PC screening, however, 96.7% had never undergone screening for prostate cancer and 46.9% indicated that they will never undergo PSA test. Furthermore, the study found a significant association between men’s knowledge of PC and their willingness to undergo PC screening, X2 (3, N=245) = 48.44, p = .001; men’s knowledge of PC was significantly related to their attitudes towards PC, X2 (1, N = 245) = 17.63, p = .001. The effect size was moderate, ɸ = .27. Knowledge was significantly associated with all the demographic variables. Therefore, this study recommends widespread public health campaigns focusing on educating men about prostate cancer risk factors, symptoms, treatment and ways to prevent and manage it through healthy lifestyles. / NRF

No description available.

This thesis presents advanced torque control methods for torque ripple reduction and performance improvement in switched reluctance motor (SRM) drives. A new offline torque sharing function (TSF) method is proposed for torque ripple reduction in SRMs. The proposed TSF achieves lower current tracking error by establishing a new current reference generation strategy. The phase current reference is first derived from the torque command using offline calculations and also from the phase current response that is obtained from the dynamic model of the SRM. Then, an optimization problem is formulated to shape the current reference for the objective of minimizing the torque ripple and copper losses, while maintaining the required average output torque at the given operating speed. The dynamic simulation of the SRM model is also utilized in the optimization problem. A new online TSF method is proposed for torque ripple reduction in SRMs. The proposed TSF takes the current dynamics and induced electromotive force into account by establishing a new online current profile generation technique. First, a primary phase current reference derived from the torque reference is applied to the SRM. Then, the decaying phase current after the turn-off angle is sampled, and it is used to update the current reference. A new online optimization strategy is performed to shape the current reference during the operation of the machine. Owing to the proposed current profile generation technique, the optimization process is decoupled to independently minimize the torque ripple by optimizing the turn-on angle and minimizing copper losses by optimizing the turn-off angle. Compared to the conventional TSFs and existing optimization-based TSFs, the proposed two TSFs achieve accurate torque control, improved torque-speed capability, reduced torque ripple, and better current tracking performance. All the proposed TSF methods are validated by both simulations and experiments on a 3-phase, 12/8 SRM. / Dissertation / Doctor of Philosophy (PhD)

This thesis presents a torque control mechanism for switched reluctance machine (SRM) drives. The proposed mechanism is capable of maintaining ripple free torque control while minimizing the copper loss or mode-0 radial force or both at a fixed switching frequency. In the proposed approach, the torque control problem is addressed by splitting it into two parts. The first part consists of identification of optimum phase current references while the second part incorporates the design of an efficient current controller. For the identification of optimum phase current references, three algorithms are presented in the form of a developmental process. The nature of the online optimization problem is demonstrated using a simple 2-dimensional gradient descent method. Subsequently, a parametric form gradient descent algorithm is presented which transforms the original optimization problem into two 1-dimensional problems, viz. torque error minimization and identification of optimum search direction. This method yields improved computational efficiency and accuracy. The third algorithm incorporates projection using equality constraint on the phase torque contributions to achieve a 1-dimensional solution process. Although this algorithm takes more iteration as compared to the parametric form gradient descent algorithm, it demonstrates greater accuracy and computational efficiency. A comparative analysis of these algorithms is performed in at different operating conditions in terms of the torque ripple magnitude and computational effort. The thesis also presents a comprehensive analysis of well known control techniques for application in SRM current control in discrete-time domain. This analysis also presents a comparative evaluation of these control techniques under different operating conditions. On account of this analysis, several recommendations pertaining to the performance improvement are presented. Finally, a digital sliding-mode based model-free current controller suitable for fixed switching frequency operation is presented. The proposed controller is capable of providing a consistent dynamic response over wide operating range without utilizing any model information. The reference current tracking performance of this controller is verified through simulation studies in MATLAB/Simulink® environment and over a 1.2kW, 100V, 2500RPM, 12/8 experimental SRM drive. / Thesis / Doctor of Philosophy (PhD)

No description available.

Switched reluctance machines (SRMs) are gaining increasing interest in industrial applications due to their low manufacturing cost, simple and robust structure, excellent fault-tolerant capability, and reliable operation in high-temperature operating environments. However, the inherent pulsative torque and radial force lead to the well-known torque ripples and acoustic noise issues. Although there are numerous advanced methods to address the above two issues, the high nonlinearity inevitably brings difficulties in controlling the SRMs. Since the linear SRM voltage and toque equations are simple, it would be beneficial to explore the control algorithm by using such simple linear model. The application of the linear torque model is firstly explored. To utilize such simple model, the connections between the linear toque model and the nonlinear torque model are built through the mapping. The features of these mapping curves are studied in detail. Applying the linear torque equation to generate the reference currents in the optimization-based torque sharing function method shows a significant reduction of the time consumption in solving the bi-optimization problem. Later, the complete equivalent linear SRM model is constructed by introducing the linear voltage equation and corresponding mapping. Since the linear model is easy to predict the behaviour of SRMs, it is beneficial to apply such model in the model predictive torque control (MPTC) methods. The application of the equivalent linear model in the finite control set (FCS) MPTC method shows a low computational burden and occupies less storage space. Besides, the improved switching table in the proposed FCS MPTC method also enhances the torque control performance in high-speed operation conditions. To further reduce the torque ripples, the continuous control set (CCS) MPTC method is developed based on the constructed equivalent linear SRM model. The impossibility in analytically solving the optimization problem in the CCS MPTC method if using the original nonlinear SRM model is innovatively addressed by using the equivalent linear SRM model and properly modifying the cost function. Extensive simulation and experimental results prove the low-ripple feature of the proposed CCS MPTC method in a wide speed range. The high nonlinearity also makes the current control of SRM drives difficult. An intersection-method-based current controller is presented to ensure good current tracking performance for SRMs. The employed adaptive flux-linkage observer makes this current controller show robust performance when there is a deviation on the employed flux-linkage characteristics. Finally, the key but unmeasurable radial force information for the advanced acoustic reduction method is reconstructed based on the measured flux-linkage curves and some core relationship. This core relationship, which is between the square root of the radial force and the flux-linkage, is explored in detail. Simulation results reveal that the proposed method shows good radial force estimation accuracy when there is even 50% airgap length variation. / Thesis / Doctor of Philosophy (PhD)

Rotor position estimation of switched reluctance machines (SRMs) is the main focus of this work. Rotor position sensors are a crucial component of optimal motor controls. Fail-safe operation and system cost reduction have been extensively researched and implemented in industry and academia. Position sensorless control on switched reluctance machines introduces a new challenge due to high nonlinearity under different operating conditions. A comprehensive review of SRM analytical modeling is presented, detailing each technique's main advantages and drawbacks. A least square-based analytical model (LSA) is proposed, which provides a simpler implementation and improved performance when compared to the methods commonly used in the literature. A literature review of rotor position sensor technology, position sensor fail modes, and position sensorless control is presented, providing a good roadmap of potential development and current limitations of the current technology. A wide speed range sensorless control is usually required when considering fail-safe techniques, fail detection methods, and low-cost applications. A unified nonlinear optimization-based sensorless control is proposed in this thesis, where a single method is used for startup, low and high speeds, with reduced memory allocation where a look-up table is not required, optimal transient response due to the elimination of a phase-locked-loop (PLL), and robustness against parameter variation. The method is validated at a wide speed range and torque conditions, thus showing the performance against conventional methods. / Thesis / Doctor of Philosophy (PhD)