Improved micro-contact resistance model that considers material deformation, electron transport and thin film characteristics

This paper reports on an improved analytic model forpredicting micro-contact resistance needed for designing microelectro-mechanical systems (MEMS) switches. The originalmodel had two primary considerations: 1) contact materialdeformation (i.e. elastic, plastic, or elastic-plastic) and 2) effectivecontact area radius. The model also assumed that individual aspotswere close together and that their interactions weredependent on each other which led to using the single effective aspotcontact area model. This single effective area model wasused to determine specific electron transport regions (i.e. ballistic,quasi-ballistic, or diffusive) by comparing the effective radius andthe mean free path of an electron. Using this model required thatmicro-switch contact materials be deposited, during devicefabrication, with processes ensuring low surface roughness values(i.e. sputtered films). Sputtered thin film electric contacts,however, do not behave like bulk materials and the effects of thinfilm contacts and spreading resistance must be considered. Theimproved micro-contact resistance model accounts for the twoprimary considerations above, as well as, using thin film,sputtered, electric contact

Future of electromechanical switchgear

The current development of the electrical networks towards a DC system imposes certain changes in the design and functionality of several network components. This paper aims to offer an overview of the challenges and the opportunities that are raised by the DC system over the electromechanical switchgear in order to identify its future path in the electrical network. The paper starts with a review process of the previous statements regarding the future of the low-voltage (LV) electromechanical switchgear. In the second part, the existing developments in the LV technology towards a mixed (centralized and decentralized) DC grid are presented. The third part presents the main influencing factors and the developments in the classic electromechanical switchgear as well as in the relatively new switchgear technologies represented by the hybrid and power electronic switches. Following, several questions concerning the standardization and the DC ageing behaviour of the insulating materials will be presented and discussed. The last part will present the conclusions of the current overview

Modelization and analysis of the electric arc in low voltage circuit breakers

246 p.Tesis doctoral que presenta un nuevo modelo de arco eléctrico para interruptores de baja tensión mediante simulación FV y validación experimental

ELECTRICAL CONTACT BOUNCE AND THE CONTROL DYNAMICS OF SNAP-ACTION SWITCHES

Experimental and theoretical studies are made of a typical snap-action rocker switch, to establish the wear mechanisms in the pivoting contact. The rocker switch, used extensively in consumer goods, operates in the medium duty current range, (1 - 30 Amps). Highspeed photographic studies have shown that the main cause of wear is arcing, occurring during separation and bounce at the pivot contacts. To reduce the bounce a computer-based mathematical model of the system dynamics is developed and optimised; this results in recommended design changes. These changes are tested under full current endurance conditions, and show significant improvements in wear. The model of the switch dynamics relates the mathematics of motion to the bounce occuring at the pivot contact, without the influence of current. To show the effect of current and arcing, an automatic test system is developed for the controlled testing of electrical contacts. The system has the ability to evaluate arc energy, bounce times, and contact resistance. The results presented detail the influence of d.c current on contact bounce time, and identify the importance of the subsequent bounce time; which is defined for a single make operation, as the total duration of the bounces occurring after the first bounce. To compare the erosion profiles of the switch and test system, the system is operated under full load current endurance conditions, to evaluate wear. This comparison shows that the wear in the real switch contacts is greater, as result of the additional contact movement of slip and rolling.Arrow-Hart (Europe) Ltd, Plymouth, Devo

Development of carbon nanotube-reinforced nickel matrix composites: processing, microstructure and physical properties

The present thesis is focused on the design of a processing route which would deliver a microstructurally tailored metal matrix composite. By controlling the final microstructure, the resulting physical properties can be predicted and therefore optimized for a certain application. Process parameters were optimized considering the CNT defect state after dispersion. Additionally, the microstructural evolution is analysed during sintering, considering the potential chemical interactions. It was observed that the CNTs act as microstructural controller due to boundary pinning, resulting in finer final microstructures for higher CNT concentrations up to 3.0 wt.% CNTs, concentration beyond which no further refinement is detected. This stagnation is a consequence of the CNT agglomeration due to the mass transport during sintering. The mechanical, thermomechanical, tribological and electrical characterization of the composites was performed with complementary techniques. The improvement of the mechanical properties is associated to a Hall-Petch effect. Thermomechanical behaviour shows a decrease in the coefficient of thermal expansion. The anchoring effect of the CNTs is the responsible for this reduction and three model mechanisms for the behaviour are proposed. Tribological behaviour showed reduced friction and wear loss in the composites. Finally, the influence of the CNT concentration and distribution are correlated to the improvement in the electrical conductivity.Die vorliegende Arbeit beschäftigt sich mit der Entwicklung einer Herstellungsroute, welche die Synthese mikrostrukturell maßgeschneiderter Metallmatrixkomposite erlaubt. Dies ermöglicht eine Vorhersage und Optimierung der sich einstellenden physikalischen Eigenschaften für bestimmte Anwendungsfälle. So wurden die Prozessparameter hinsichtlich des Defektzustandes von CNTs optimiert und die während des Sintervorganges potentiell möglichen chemischen Wechselwirkungen analysiert. CNTs fungieren durch Behinderung der Korngrenzbewegung als Steuerungselement für die Mikrostruktur. Mit steigender CNT-Konzentration (bis zu 3 Gew.-%) nimmt die Korngröße ab. Höhere CNT-Konzentrationen führen zu Agglomerationen aufgrund des Massentransportes während des Sintervorganges und tragen nicht mehr zum Kornfeinungseffekt bei. Eine mechanische, thermomechanische, tribologische und elektrische Charakterisierung der Komposite wurde mit komplementären Methoden durchgeführt. Die Verbesserung der mechanischen Eigenschaften kann mit dem Hall-Petch Effekt korreliert werden. Gleichzeitig wird eine Abnahme des Wärmeausdehnungskoeffizienten festgestellt, welche mit dem Verankerungseffekt der CNTs zusammenhängt und durch drei Modellansätze beschrieben wird. Die tribologischen Untersuchungen zeigen eine Verringerung des Reibkoeffizienten und des Verschleißes. Abschließend wird eine Verbesserung der elektrischen Leitfähigkeit auf den Einfluss von CNT-Konzentration und -Verteilung zurückgeführt

Enhancement of domestic solar photovoltaic unit productivity through the use of a cost effective tracking system

The majority of new and existing small photovoltaic (PV) installations in South Africa are fixed-panel systems, largely due to the cost of photovoltaic panel components having reduced steadily in recent years where an increased requirement is met with a larger number of panels, whilst tracking system costs remain prohibitively expensive. Fixed installations realise only a part of their energy potential as they are truly effective for only short periods of the day. The aim of this study is to investigate and build on the current technology of PV tracking systems with the aim of specifying a simple control and actuation system which performs the tracking function. The eventual purpose of this thesis is to reliably produce more energy from solar photovoltaic installations than similar installations using fixed panels. This would be achieved by the use of an effective and affordable tracking system which yields acceptable accuracy and reliability and opens the potential for the system to be further developed for other purposes. These alternative uses could be the control of sunlight into green buildings, control of dampers for building ventilation and cooling and Trombe wall air control. This study has investigated the potential of several passive and active methods to actuate a sun tracking system. A useful closed loop system, which uses low pressure hydraulics, was developed and tested. The prototype is detailed in the drawings, Appendix D.Thesis (PhD) -- Faculty of Engineering, the Built Environment, and Technology, 202

Enhancement of domestic solar photovoltaic unit productivity through the use of a cost effective tracking system

The majority of new and existing small photovoltaic (PV) installations in South Africa are fixed-panel systems, largely due to the cost of photovoltaic panel components having reduced steadily in recent years where an increased requirement is met with a larger number of panels, whilst tracking system costs remain prohibitively expensive. Fixed installations realise only a part of their energy potential as they are truly effective for only short periods of the day. The aim of this study is to investigate and build on the current technology of PV tracking systems with the aim of specifying a simple control and actuation system which performs the tracking function. The eventual purpose of this thesis is to reliably produce more energy from solar photovoltaic installations than similar installations using fixed panels. This would be achieved by the use of an effective and affordable tracking system which yields acceptable accuracy and reliability and opens the potential for the system to be further developed for other purposes. These alternative uses could be the control of sunlight into green buildings, control of dampers for building ventilation and cooling and Trombe wall air control. This study has investigated the potential of several passive and active methods to actuate a sun tracking system. A useful closed loop system, which uses low pressure hydraulics, was developed and tested. The prototype is detailed in the drawings, Appendix D.Thesis (PhD) -- Faculty of Engineering, the Built Environment, and Technology, 202

Technology for large space systems: A bibliography with indexes (supplement 22)

This bibliography lists 1077 reports, articles, and other documents introduced into the NASA Scientific and Technical Information System between July 1, 1989 and December 31, 1989. Its purpose is to provide helpful information to the researcher or manager engaged in the development of technologies related to large space systems. Subject areas include mission and program definition, design techniques, structural and thermal analysis, structural dynamics and control systems, electronics, advanced materials, assembly concepts, and propulsion