Browsing by Author "Prather, Carl L."
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- Analysis and Design of a Novel E-Core Common-Pole Switched Reluctance MachineLee, Cheewoo (Virginia Tech, 2010-03-03)In this dissertation, a novel two-phase switched reluctance machine (SRM) with a stator comprised of E-core structure having minimum stator core iron is presented for low-cost high-performance applications. In addition, three new magnetic structures for the E-core SRM comprising two segmented stator cores or a monolithic stator core are proposed for good manufacturability, mechanically robustness, ease of assembly, and electromagnetic performance improvement. Each E-core stator in the segmented structure has three poles with two small poles at the ends having windings and a large center pole containing no copper windings. The common stator pole at the centers in the segmented E-core is shared by both phases during operation. Other benefits of the common poles contributing to performance enhancement are short flux paths, mostly flux-reversal-free-stator, constant minimum reluctance around air gap, and wide pole arc equal to one rotor pole pitch. Therefore, two additional common poles in the monolithic E-core configuration are able to significantly improve efficiency due to more positive torque and less core loss by the unique design. Using a full MEC analysis, the effect of the common-pole structure on torque enhancement is analytically verified. Efficiency estimated from the dynamic simulation is higher by 7% and 12% at 2000 rpm and by 3% and 7 % at 3000 rpm for the segmented and single-body SRMs, respectively, compared to a conventional SRM with four stator poles and two rotor poles. The new E-core SRMs are suitable for low-cost high-performance applications which are strongly cost competitive since all the new E-core SRMs have 20% cost savings on copper and the segmented E-core SRMs have 20% steel savings as well. Strong correlation between simulated and experimentally measured results validates the feasibility of the E-core common-pole structure and its performance. A simple step-by-step analytical design procedure suited for iterative optimization with small computational effort is developed with the information of the monolithic E-core SRM, and the proposed design approach can be applied for other SRM configurations as well. For investigating thermal characteristics in the two-phase single-body E-core SRM, the machine is modeled by a simplified lumped-parameter thermal network in which there are nine major parts of the motor assembly.
- Design and Control of A Ropeless Elevator with Linear Switched Reluctance Motor Drive Actuation SystemsLim, Hong Sun (Virginia Tech, 2007-03-23)Linear switched reluctance motor (LSRM) drives are investigated and proved as an alternative actuator for vertical linear transportation applications such as a linear elevator. A one-tenth scaled prototype elevator focused on a home elevator with LSRMs is designed and extensive experimental correlation is presented for the first time. The proposed LSRM has twin stators and a set of translator poles without back-iron. The translators are placed between the two stators. The design procedures and features of the LSRM and the prototype elevator are described. The designed LSRM is validated through a finite element analysis (FEA) and experimental measurements. Furthermore, a control strategy for the prototype elevator is introduced consisting of four control loops, viz., current, force, velocity, and position feedback control loops. For force control, a novel force distribution function (FDF) is proposed and compared with conventional FDFs. A trapezoidal velocity profile is introduced to control vertical travel position smoothly during the elevator's ascent, descent, and halt operations. Conventional proportional plus integral (PI) controller is used for the current and velocity control loops and their designs are described. The proposed control strategy is dynamically simulated and experimentally correlated. Analytical and experimental results of this research prove that LSRMs are one of the strong candidates for ropeless linear elevator applications. However, the proposed FDF is assuming that the feedback current signals are ideal currents indicating actual phase currents without any measurement disturbances mainly arising from sensor noise, DC-link voltage ripple, measurement offset, and variations in the plant model. Meanwhile, real control systems in industry have measurement disturbance problems. Phase current corrupted by measurement disturbances increases torque or force ripple, acoustic noise and EMI. Therefore, this dissertation also presents a novel current control method to suppress measurement disturbances without extra hardware. The controller is based on an extended state observer (ESO) and a nonlinear P controller (NLP). The proposed method does not require an accurate mathematical model of system and can be implemented on a low-cost DSP controller. The proposed ESO is exploited to estimate the measurement disturbances on measured phase currents, and the proposed NLP compensates for the measurement disturbances estimated by the ESO. The performance of the proposed current control is validated through extensive dynamic simulations and experiments. Moreover, this rejection of measurement disturbances results in a reduction of force ripple and acoustic noise. Due to superior and robust current control performance, it is believed that the proposed method can be successfully applied into other motor drive systems to suppress measurement disturbances with the same promising results without extra hardware.
- Doubly-Salient Permanent Magnet Flux-Reversal-Free-Stator Switched Reluctance MachinesLobo, Nimal (Virginia Tech, 2011-01-21)A new hybrid machine having variable reluctance and permanent magnets (PMs) is presented. The machine makes use of the features of a PM machine and variable reluctance machine. The resulting machine is doubly salient and has a structure free of flux reversals. Unlike conventional doubly salient permanent magnet machines (DSPMs), the one proposed in this report is driven by unipolar currents and uses an asymmetric converter which is used to drive switched reluctance machines. The reason to have a new hybrid machine without the drawbacks of conventional flux-reversal-free-stator SRMs and conventional DSPMs is also described. Conventional doubly salient permanent magnet machines which are driven by alternating currents, do not use reluctance torque and have flux reversals in the stator iron. Homopolar flux at the peak flux density lowers hysteresis and eddy-current loss, since the machine's core operates in only one magnetizing quadrant. Due to unbalanced forces in conventional stator-flux-reversal-free machines, their deployment in industrial and end-user applications has been hindered. The presented hybrid machine has balanced radial forces. Therefore, it maintains the advantages of conventional stator-flux-reversal-free machines while shedding its disadvantages. The proposed machine has significantly increased power density and is more electromechanically efficient than its predecessor. A experimental prototype motor has been designed and built. Its static torque characteristics correlated well with predicted data. Experimental operation of the drive under open loop speed control shows the efficiency to be 90.8% under non-ideal driving conditions. In the current energy conscious environment and market, this motor because of its high efficiency has a significant role in reducing the energy consumption in household, industrial and automotive applications requiring electric motors.
- Exponential Stability for a Diffusion Equation in Polymer Kinetic TheoryMulzet, Alfred Kenric (Virginia Tech, 1997-04-22)In this paper we present an exponential stability result for a diffusion equation arising from dumbbell models for polymer flow. Using the methods of semigroup theory, we show that the semigroup U(t) associated with the diffusion equation is well defined and that all solutions converge exponentially to an equilibrium solution. Both finitely and infinitely extensible dumbbell models are considered. The main tool in establishing stability is the proof of compactness of the semigroup.
- Mechanical Properties and Electrochemical Durability of Solid Oxide Fuel CellsAn, Ke (Virginia Tech, 2003-12-19)The mechanical properties of unaged and aged constituent materials for solid oxide fuel cells were evaluated using microindentation, plate tensile, four-point bend, ball on ring and pressure on ring tests. The Vickers hardness of the anode, interconnect and electrolyte was determined before and after 1000 hours aging at 1000 oC in air. The fracture toughness KIC was found for the electrolyte materials. Finite element analysis (FEA) was validated and used to calculate the stress distribution and peak stress for the biaxial strength test. A Weibull analysis was carried out on the test/FEA-predicted peak stresses, and Weibull strength, modulus and material scale parameters were found for each test methodology. The methodologies were evaluated based on the results of the Weibull analysis and the pressure on ring test is preferred one for brittle thin film fracture strength testing. Half cell SOFCs with composite cathode (Pr0.7Sr0.3)MnO3±Î´ /8YSZ on the 8YSZ electrolyte were aged 1000 hours at 1000 oC in air with/without polarization and investigated using Electrochemical Impedance Spectroscopy (EIS), Scanning Electron Microscopy (SEM), Brunauer-Emmett-Teller (B.E.T.) method and X-ray Diffraction (XRD). The performance of the half cell SOFCs degraded after aging with/without polarization compared to the initial state, which was ascribed to the decrease of the electrolyte conductivity. The current load was shown to have impact on the performance by slowing down the decreasing rate of the polarization resistance of the SOFCs. After aging, the microstructural properties - pore size and pore volume changed, and growth of grains was found on the (Pr0.7Sr0.3)MnO3 phases, which may have contributed to the decrease of the activation polarization by decreasing the capacitance and increasing the number of active sites. After aging the high frequency EIS arcs/peaks shifted to a lower frequency range, and the low frequency arcs/peaks became unapparent compared to before aging. A 3-D multiphysics finite element model was used to simulate the performance of the half cell SOFC. The effective exchange current density and the effective ionic conductivity of the cathodes showed much influence on the performance of the SOFC. Predicted and observed performance was compared. Suggestions were given for the further experiments on the composite cathode.
- Modal Analysis Techniques in Wide-Area Frequency Monitoring SystemsBaldwin, Mark W. (Virginia Tech, 2008-02-29)The advent of synchronized wide-area frequency measurements obtained from frequency disturbance recorders and phasor measurement units has presented the power industry with special opportunities to study power system dynamics. I propose the use of wide-area frequency measurements in identifying system disturbances based on power system post-event modal properties. In this work, power system dynamics are examined from an internal system energy viewpoint. Since an electric power system is composed of coupled rotating machines (large generators) which have air gap magnetic fields that are essentially static, or quasi-static, the power system may be modeled as a system with energy stored in quasi-static magnetic fields. The magnetic fields in the machines do change with time but may be modeled as static as far as wave propagation is concerned. The dynamic model that I develop treats this magnetic energy specifically as potential energy. Each rotating machine also contains an inertia due to the mass and motion of its rotor train and so each machine contains a rotational kinetic energy. Thus the internal system energy for a power system dynamic model may be considered to be contained in potential (magnetic) and kinetic (rotating mass) energies. This notion of internal energy lends itself to the use of a state-space model where each system state is associated with either a kinetic energy or a potential energy. An n-machine system would have a total of 2n states and would thus be a 2n-th order system. For many power system disturbances, I postulate that a linearized version of this model may be used to examine system natural response in terms of frequency and phasor measurements. The disturbances that I will investigate include generator and line outages. For any particular outage, the power system exhibits a very specific natural response in terms of its kinetic and potential energies. Kinetic energy in the system is directly related to each specific machine's rotational speed. I propose that the kinetic energy corresponds directly with bus frequencies through a linear transformation. Likewise magnetic field energy in each machine corresponds directly with a torque angle. The potential energy in the system thus corresponds directly with bus angles through a linear transformation. The primary focus of this work is on frequency deviation modal characteristics – specifically damped oscillation frequencies, mode shapes, and damping ratios. This work presents how specific disturbances on a power system will lead to specific oscillation frequencies in the deviation quantities and that these oscillation frequencies may be used to identify the disturbance. The idea of disturbance identification stems out of previous work done in locating disturbances by using a distributed parameter (DP) model of an electric power system. This DP model, which assumes a wave-like motion of frequency and phase quantities, was used to locate disturbances via a triangulation method. This present work, instead of using a DP model of the power system, assumes lumped parameters and focuses on disturbance identification strictly via modal characteristics – particularly oscillation frequency in the frequency deviations. This model is not concerned with geographic location but focuses on system topology, loading, and machine mass as lumped parameters. Advantages of disturbance identification include mainly reliability enhancements but can also be used in marketing applications. The state-space model used to realize this theory is verified via simulation using small, "academic" systems which should prove useful in classroom settings. Additionally the model is verified on a larger test system in order prove its validity and potential usefulness on large power systems.
- Note on a role for entire functions of the classes andPrather, Carl L. (Hindawi, 1981-01-01)We use the B and B* operators of Levin on the Classes P and P* and a comparison principle to prove a Gauss-Lucas Theorem for differential operators. The connection with the determination of final sets for differential operators is then clarified.
- Numerical Simulation of Adiabatic Shear Bands and Crack Propagation in Thermoviscoplastic MaterialsLear, Matthew Houck (Virginia Tech, 2003-04-24)Plane strain deformations of an elastoplastic material are studied using numerical methods. In the first chapter, a meshless formulation of the static small strain elastic-plastic problem is formulated using the meshless local Petrov-Galerkin method. The code is validated against the small strain plasticity routines in the commercial finite element code ABAQUS for two basic configurations with loading, unloading, and reloading. The results are found to agree within 5%. The validated code is then used to analyze the stress intensity factor (SIF) in a double edge-cracked plate. Deformations of the plate are studied both with and without exploiting the symmetry conditions. The penalty method is used to enforce the essential boundary condition in the former case. When analyzing the deformations of the entire plate, the diffraction method is employed in order to introduce the discontinuity in the displacement field across the crack faces. The log-log and a higher order extrapolation technique due to Dally and Berger (1996) are used to calculate the SIF. It is found that the penalty method was inadequate to enforce the essential boundary conditions in the vicinity of the crack tip and that in this region the deformations were oscillatory. Consequently, the SIF calculation using the higher order technique was not accurate. It is also found that for a small plastic zone (3% of the cracked length) the SIFs do not differ significantly from their values for the corresponding linear elastic problem. In the second chapter, a finite element formulation of the dynamic deformations of a micro-porous thermoviscoplastic solid is formulated. The heat conduction in a material is assumed to be governed by a hyperbolic heat equation; thus thermal and mechanical waves propagate with finite speeds. The formation and propagation of an adiabatic shear band (ASB) inplane strain tensile deformations is studied for eleven materials. The ASB is assumed to form when the maximum shear stress has been reduced to 80% of its peak value at a point and it is deforming plastically. The materials are ranked according their susceptibility to the formation of an ASB. A parametric study of the effect of the initial defect strength where the defect is assumed through an initially inhomogeneous distribution of porosity, the thermal conductivity, the thermal wave speed, and the applied strain-rate upon the ASB initiation and propagation is conducted. It is found that the susceptibility ranking for this configuration differs somewhat from that previously found for simple shear and torsion of thin-walled tubes. It is also found that thermal conductivity influences ASB initiation and propagation only for materials with large values of · and that for such materials an adiabatic model may not be adequate. The effects of initial defect strength and the nominal strain-rates are both found to be consistent with simple shearing studies except that the ASB propagation speed was found to decrease with increasing nominal strain-rate. It is found that the criterion employed for ASB initiation accurately predicts the onset of the collapse of the total axial load applied to the body. In the final chapter, the formulation from the previous chapter is modified to permit the formation and propagation of brittle and ductile fracture. Deformations of the impact loaded double edge-crack specimen of Kalthoff and Winkler (1987) are studied. The brittle to ductile failure mode transition with increasing impact speed was found. Previous studies have focused on identifying the transition speed and did not allow for crack propagation. In this study, crack propagation is achieved through a nodal release algorithm and interpenetration of the crack surfaces is prevented using stiff-spring contact elements. Brittle fracture is assumed to occur when the maximum tensile principal stress achieves a critical value and the ductile fracture is assumed to occur when the effective plastic strain reaches a critical value. It is found that the transition speed for 4340 steel is approximately 54 m/s. For the brittle failure, the stress field is found to be significantly modified by the propagating crack and in the vicinity of the propagating crack the field is mode-I dominant. The crack formed through brittle fracture is found to completely propagate through the plate. For the ductile failure, the distribution of effective plastic strain about the crack tip is not significantly altered by the formation of the crack. The temperature rise in the vicinity of the ductile crack is found to be approximately 45% of the melting temperature of the material.
- On the number of zeros of iterated operators on analytic Legendre expansionsPrather, Carl L. (Hindawi, 2001-01-01)Let L=(1−z2)D2−2zD, D=d/dz and f(z)=∑n=0∞cnpn(z), with Pn being the nth Legendre polynomialand f analytic in an ellipse with foci ±1. Set Lk=L(Lk−1), k≥2. Then the number of zeros of Lkf(z) in this ellipse is O(klnk).
- Power Systems Analysis in the Power-Angle DomainArana, Andrew Jex (Virginia Tech, 2009-12-04)The idea of performing power systems dynamic analysis in the power-angle domain has been hinted at by previous researchers, but this may be the first published document to develop detailed techniques by which entire power systems can be represented and solved in the power-angle domain. With the widespread deployment of phasor measurement units and frequency data recorders the industry is looking for more real-time analytical tools to turn real-time wide-area measurements into useful information. Applications based on power-angle domain analysis are simple enough that they may be used online. Power-angle domain analysis is similar to DC load-flow techniques in that a flat voltage profile is used and it is assumed that real power and voltage angle are completely decoupled from reactive power and voltage magnitude. The linearized equations for the dynamics of generators and loads are included in the model, which allows the electromechanical response to be solved using conventional circuit analysis techniques. The effect of generation trips, load switching, and line switching can be quickly approximated with nodal analysis or mesh analysis in the power-angle domain. The analysis techniques developed here are not intended to be as accurate as time-domain simulation, but they are simpler and fast enough to be put online, and they also provide a better analytical insight into the system. Power-angle domain analysis enables applications that are not readily available with conventional techniques, such as the estimation of electromechanical propagation delays based on system parameters, the formulation of electromechanical equivalents, modal analysis, stability analysis, and event location and identification based on a small number of angle or frequency measurements. Fault studies and contingency analysis are typically performed with detailed time-domain simulations, where the electromechanical response of the system is a function of every machine in the interconnection and the lines connecting them. All of this information is rarely known for the entire system for each operating condition; as a result, for many applications it may be more suitable to compute an approximation of the system response based on the current operating state of only the major lines and generators. Power-angle domain analysis is adept at performing such approximations.
- Real-Time Image Processing Using Acousto-Optic Bragg DiffractionDunn, Derrek Butler (Virginia Tech, 1998-01-07)Optical image processing systems using an acousto-optic cell have been studied previously. However, these previous studies have been limited to two diffracted order in the Bragg regime and two spatial dimensions. Some comparisons between experimental data and theoretical predictions have been made. This dissertation studies image processing by acousto-optic Bragg diffraction to perform image enhanment. Theoretical results involving two diffracted orders in three spatial dimensions is presented. Experimental data is presented that confirms the validity of the theoretical results. Detailed analysis of several optical image processing system using acousto-optic modulators is presented. Also, the methodology use to derive an analytically expression in three spatial dimension for the interaction of an arbitrary light profile and a rectangular sound column in an acousto-optic cell is presented. Lastly, the ability to change the characteristics of the derived transfer function that mathematically represents the interaction of light and sound inside the acousto-optic cell is discussed and future research topics is given.
- Synchrophasor-Only Dynamic State Estimation & Data ConditioningJones, Kevin David (Virginia Tech, 2013-08-30)A phasor-only estimator carries with it intrinsic improvements over its SCADA analogue with respect to performance and reliability. However, insuring the quality of the data stream which leaves the linear estimator is crucial to establishing it as the front end of an EMS system and network applications which employ synchrophasor data. This can be accomplished using a two-fold solution: the pre-processing of phasor data before it arrives at the linear estimator and the by developing a synchrophasor-only dynamic state estimator as a mechanism for bad data detection and identification. In order to realize these algorithms, this dissertation develops a computationally simple model of the dynamics of the power system which fits neatly into the existing linear state estimation formulation. The algorithms are then tested on field data from PMUs installed on the Dominion Virginia Power EHV network.