Academic literature on the topic 'Electric current'

This work was on the effect of electric current on the processing of ceramics. The focus was on electromigration/electrochemistry and plasma effects. While there is no solid evidence that there is plasma in Spark Plasma Sintering, (SPS), newer techniques e.g. flash, use different conditions so there is an interest in understanding the conditions under which a plasma forms. The minimum arcing voltage was found from literature to be from 10-15V for materials of interest. This is above that found in SPS (10V). However, due to the many contact points in a powder compact much higher voltages (50V) were required in practical experiments. Optical spectroscopy was used to verify the formation of a plasma, and emission peaks from the powder compact material were visible implying they were vaporised and formed the plasma. Electromigration was exploited to alter the oxidation of zirconium diboride, by passing current through the oxide layer (120μm zirconia base grown at 1200°C) oxygen could be pumped either away or toward the diboride bulk. Small cubes (3mm) of diboride had platinum foil electrodes applied on both sides and oxidation was performed at 1400°C for 5hr. Without a field the oxide grew to 360μm, by applying 10V and 100mA the oxide grew to 150μm under the +ve electrode but 1400μm under the -ve electrode. Electrochemical reduction was believed to have occurred due to the electrical properties of the material changing during oxidation and visible blackening of the oxide. Combining the techniques from both earlier works, a contactless flash sintering setup was developed. This used two plasma arcs as electrodes to heat and pass current through the sample. Various materials, currents and times were used, but the best result was with SiC:B4C which was sintered in 3s with 6A, the microstructure showed sharp grains, no segregation and limited grain growth ( initially 0.7μm SiC and 0.5μm B4C, this grew to 1.1μm and 1.4μm). This was the first recorded case of contactless flash sintering and the technique has the potential to sinter ceramics in a continuous manner.

There is ever-increasing demand in telecommunication system, data server and computer equipment for low voltage, high current power supply. LLC resonant converter is a good topology on primary side of the converter because it has soft switching and resonant conversion. However, the passive rectifier in the secondary side has high power dissipation. Synchronous rectifier is a popular method to reduce this rectification loss. Although there are many types of synchronous rectifier for PWM converter, most of them do not function well in LLC resonant converters. It is because the wave form of LLC resonant converter is different from PWM. The objective of this research is to reduce the power dissipation and physical size at the same time. In this thesis, a novel current driven synchronous rectifier with saturable current transformer and dynamic gate voltage control for LLC resonant Converter is presented. This novel circuit reduces the rectification loss and size of the current transformer in the synchronous rectifier. This synchronous rectifier has several outstanding characteristics compared with generic voltage driven and current driven synchronous rectifier. The saturable feature reduces the current transformer turns. Inherent dynamic gate voltage controlled by saturable current transformer reduces gate loss in the MOSFET. A novel driving circuit is proposed for accurate turn off time. It reduces loss significantly. This synchronous rectifier is completely self-contained which can replace the rectifier diode as a drop in replacement. It is insensitive to parasitic inductance. In order to explain the current transformer saturable, a model of saturable current transformer is proposed. A prototype demonstrates the advantages of the proposed current driven synchronous rectifier. Furthermore, a novel integrated synchronous rectifier is presented which provides a more compact system. The synchronous rectifier current transformer is integrated with the main transformer which reduces the number of circuit joints in power path. Each soldering joint generates significance loss in power converter. A pair of 0.5mΩ soldering joint in 25A current path produces 0.62W loss. The placement of the integrated current transformer is important. A criterion for the placement of the current transformer within the main transformer is to avoid interference to the current transformer from the magnetic flux of the main transformer. Thus, a placement method to integrate the current transformer into the main transformer is proposed. An integrated current transformer model is suggested to explain the operation of the integrated synchronous rectifier. A prototype demonstrates the advantages of the integrated synchronous rectifier.<br>published_or_final_version<br>Electrical and Electronic Engineering<br>Master<br>Master of Philosophy

The University of 'Canterbury Mark II' electric vehicle has been out of service for several years due to the lack of a suitable inverter to provide a variable frequency AC supply to its traction motors. This thesis describes the design and construction of such an inverter, using high current bipolar junction transistors as the switching elements, so that the car might be returned to service in the near future. The inverter is based on an existing commercial AC motor speed controller. Modifications to this AC motor speed controller were made to suit the low voltage, high current rating of the traction motors. These modifications are described and it is shown that these modifications permitted the inverter to deliver the required increase in current. The inverter differs from most conventional AC motor speed controllers in that it acts to shape load current rather than potential, and uses an asynchronous switching technique to do this. The Thesis describes this technique and the control hardware constructed to implement it. Test results, showing the performance of the combined inverter/motor system are then presented graphically and discussed with reference to standard AC motor theory, giving consideration to the harmonic content of the AC waveforms. Consideration is also given to a suitable closed-loop control system which could be expected to ensure that the inverter's output frequency is controlled in such a manner as to give a safe and predictable response to the brake and accelerator controls.

Electrical systems have become ubiquitous, and with them come the need to accurately monitor electric current. The aerospace industry is no exception. Modern aircraft may contain more than one hundred current sensors, each one critical to a properly functioning vehicle. While these sensors function acceptably, several areas have been identified for improvement: size, weight, and cost. Each sensor is bulky, taking up valuable space. They are also costly to manufacture. The existing design is based on the Hall effect, and has remained fundamentally unchanged for decades. With the recent progress in manufacturing techniques and materials, it would be beneficial to reexamine these sensors and determine if improvements can be made using the accomplishments of recent years. Of particular interest are microelectromechanical systems, also known as MEMS. Using a sensor based on MEMS technologies in which design, function, and fabrication are closely intertwined would automatically meet two of the three goals: reducing size and weight. MEMS additionally have the potential to allow existing systems to be miniaturized. Also of interest are advanced materials, some of which can behave as transducers, linking different physical phenomenon. The goal of this dissertation is to use advances in manufacturing techniques and materials, specifically those discussed above, to design a better current sensor. As part of this goal, several potential solutions were studied and optimized. Finally, proof-of-concept prototypes were fabricated and tested to validate the feasibility of the designs and offer insight into continued sensor development.

The oxyfuel cutting method is still widely used nowadays, even though it is not a fully autonomous process. Thisthesis presents a computational model to study ion and electron transport and current-voltage characteristics inside a methane-oxygen flame. By finding the relationship between current-voltage characteristics and critical parameters,such as standoff, fuel oxygen ratio, and flow rate, a control algorithm could be implemented into the system and make it autonomous. Star CCM+ software is used to develop preheat phase computational models by splitting the simulations into the combustion and electrochemical transport parts. Both the laminar and turbulent flows are considered. Several laboratory experiments are used to compare test data with the numerical results generated using this model. The initial and boundary conditions used in the simulation were to the extent possible similar to the experimental conditions in the laboratory experiment. In the combustion part, the general GRI3.0 mechanism plus three additional ionization reactions are applied, and the combustion part results are then used as input into the electrochemical transport part. A particular inspection line inside the domain is created to analyze the results of the electrochemical transport part. Ions, electrons number density, and current density are studied in the interval from -40V to 40V electric potential. The ions are heavier and more challenging to move than electrons. The results show that at both the torch and work surfaces, charged sheaths are formed, which cause three different regions of current-voltage relations to form in a similar manner as observed in the tests.<br>Master of Science<br>Oxyfuel cutting is essential to numerous industries, such as shipbuilding, rail, earth moving equipment, commercial building construction, etc. Tuning the process parameters and diagnosing problems with the oxyfuel process still relies on experienced operators. The main obstacles to the automation of the oxyfuel process come from the limitations of the sensing suites currently in use. Since typical sensors are highly unreliable in the harsh environment near the high-temperature flame, an alternate method is proposed to find the co-dependence between the flame's electrical characteristics and critical parameters of the oxyfuel cutting system (standoff, flow rate, F/O ratio, etc.). The relevant electrical characteristics are the electrical potential and distribution of ions and electrons. Two-dimensional models are created to analyze the combustion of methane-oxygen flame and transport of ions and electrons. The models allow the derivation of the current-voltage characteristic between the torch and work surface. Also, the way sheath phenomena of ions and electrons on the surface affect the current-voltage relationship can be analyzed from ions and electrons distribution. The electric field is added to the model by applying a constant voltage to the torch tip surface. To validate the models, a laboratory experiment with a similar geometry arrangement is used as a comparision. The models' results reveal three different regimes in the current-voltage relationship.