Spark-Plasma Sintering and Related Field-Assisted Powder Consolidation Technologies

Electromagnetic field-assisted sintering techniques have increasingly attracted attention of scientists and technologists. Spark-plasma sintering (SPS) and other field-assisted powder consolidation approaches provide remarkable capabilities to the processing of materials into configurations previously unattainable. Of particular significance is the possibility of using very fast heating rates, which, coupled with the field-assisted mass transport, stand behind the purported ability to achieve high densities during consolidation and to maintain the nanostructure of consolidated materials via these techniques. Potentially, SPS and related technologies have many significant advantages over the conventional powder processing methods, including the lower process temperature, the shorter holding time, dramatically improved properties of sintered products, low manufacturing costs, and environmental friendliness

Feasability demonstration of the Vienna type excitation system for synchronous generators

The operation of the Vienna rectifier has been analyzed and relevant literature on the rectifier has been reviewed. Furthermore, the Vienna rectifier in scope of rotating power electronics in brushless excitation has been discussed and found to be an viable option. The Vienna Rectifier have been simulated with the parameters from the Svante testrig in Matlab Simulink with satisfactorily results. A re-connection of the main windings is necessary. Also demagnetizationstrategies and necessary modification to the Vienna Rectifier has been discussed and simulated. A demonstration of the Vienna Rectifier were build, but experimental results are remain qualitative due to practical issues with the demonstration rig. The control were designed in LabView Instruments and the workings of the control-scripts were confirmed. The main challenge is comes from dealing with the large inductance from the mains winding on the rotating exciter, but this challenge may be overcome. Additional circuitry consisting of a chopper on the output and demagnetization resistor and switch should be added to ensure better controllability and demagnetization-capability.M-M

High power density dc-to-dc converters for aerospace applications

Three dc-to-dc converter topologies aimed at high-power high-frequency applications are introduced. Major system parasitics, namely, the leakage inductance of the transformer and the device output capacitance are efficiently utilized. Of the three circuits, the single-phase and three-phase versions of the dual active bridge topology demonstrate minimal stresses, better utilization of the transformer, bidirectional, and buck-boost modes of operation. All circuits operate at a constant switching frequency, thus simplifying design of the reactive elements. The power transfer characteristics and soft-switching regions on the Vout-Iout plane are identified. Two coaxial transformers with different cross-sections were built for a rating of 50 kVA. Based on the single-phase dual active bridge topology, a 50 kW, 50 kHz converter operating at an input voltage of 200 Vdc and an output voltage of 1600 Vdc was fabricated. Characteristics of current-fed output make the dual active bridge topologies amenable to paralleling and hence extension to megawatt power levels. Projections to a 1 MW system operating from a 500 Vdc input, at an output voltage of 10 kVdc and a switching frequency of 50 kHz, using MOS-controlled thyristors, coaxially wound transformers operating at three times the present current density with cooling, and multilayer ceramic capacitors, suggests an overall power density of 0.075 to 0.08 kg/kW and an overall efficiency of 96 percent

Current source inverters for PM machine control

Brushless permanent magnet (PM) drive systems offer a high efficiency over a wide power/torque-speed operating envelope, however, there are a number of problems that may limit, or complicate, their operation particularly in automotive and aerospace vehicular applications, i.e. the loss of control of the power silicon gate drive circuitry during fluxweakening operation, control of high-speed low-inductance machines and the presence of large electrolytic capacitors on the inverter DC link. Current Source Inverters (CSIs) could potentially address some or all of the above issues. However, they have found little application to date due to the wide use of the Voltage Source Inverter (VSI) circuit topology.This thesis investigates feasibility of utilising Current Source Inverters (CSIs) to control permanent magnet synchronous machines in automotive and aerospace actuation systems. CSIs, switching at the fundamental frequency, were used in some of the first semiconductor based, electronic variable speed drive systems that utilised the simple, low maintenance AC induction motor. However, the rapid progress of semiconductors and discovery of Pulse Width Modulation (PWM) techniques soon resulted in the Voltage Source Inverter (VSI) replacing the CSI in all but the highest power applications. Modern power electronics and (micro-processor based) control systems mean that the advantages of VSI systems may no longer be significant and combined with the unique environmental conditions that automotive and aerospace applications present, could allow the CSI to offer advantages over VSIs in these applications.The thesis presents the switching and control logic for CSIs and mapping to the more conventional VSI logic. Analysis is made of the various loss mechanisms in VSI and CSI power circuitary. Simulation models of the VSI and CSI structures are presented and representative drive systems designed, built and tested to validate the model developed. Comparisons are made of the two inverter topologies based on power conversions and loss audits of the test validation hardware.EThOS - Electronic Theses Online ServiceThe University of Manchester Alumni FundGBUnited Kingdo

A novel test method for minimising energy costs in IGBT power cycling studies

Insulated Gate Bipolar Transistors (IGBTs) are popular power electronic switching devices with several advantages. However, they have been known to fail in the field when subjected to significant variations in power dissipation – known as power cycling. In the work presented here, a novel alternating-current (AC) power cycling test method for IGBTs together with their free-wheeling diodes is proposed and verified. A review of previous work revealed that the parameter that most affects IGBT lifetime under power cycling conditions is the variation in its junction-case temperature difference. Through simulation, the behaviour of a conventional single phase inverter (H-bridge) using simple pulse width modulation (PWM) control was quantified, and the effect of switching frequency and load power factor was studied. Results of the simulations and literature review were used to develop design criteria for a new AC test circuit. The new AC test circuit (a modified version of the conventional H-bridge) was then designed and its performance compared to the criteria and to the simulation results of the conventional circuit. The circuit was then built and its performance was validated. The circuit complied with the performance criteria, in particular the desired variation in 7jc, to an adequate degree of accuracy. The proposed test circuit is novel for several reasons. The stresses on devices used in a conventional H-bridge using a high power factor inductive load are reproduced using a low power factor inductive load, considerably reducing the energy cost of running such a test. IGBT switching losses are not actively reduced, as is normal practice, but instead are actively increased to generate the required losses. Free-wheeling diodes are also tested, but do not have significant switching losses, as the nature of the test circuit dictates that these be transferred to the IGBTs. The main drawback of the proposed test circuit is that a larger number of devices are needed; however, this tradeoff is necessary to obtain the energy cost savings provided by this circuit