Multipulse rectifiers are used in large industrial drives, aircraft, electric ships, and other systems, where limits on harmonic currents prohibit the use of conventional three-phase, six-pulse ...rectifiers. The large power rating of these rectifiers and the dynamic nature of the loads often necessitate the study of their dynamic interactions with the source, which can lead to system stability and power quality problems if not properly designed. This paper presents input impedance models for multipulse rectifiers that can be used for such system stability and dynamic power quality analysis. The modeling method is based on the concept of harmonic linearization and makes use of analytical mapping functions that describe voltage and current transfer through multiphase diode rectifiers. Input impedance models are developed first for general N -phase rectifiers, and then combined with a model of the phase-shifting transformer to define the three-phase input impedance. Magnetizing inductance and line commutation due to transformer leakage inductance are also considered. Details of the modeling method are presented using an 18-pulse rectifier as example, and experimental measurement results are also presented to validate the models.
•Effect of laser remelting on mechanical properties under cyclic loading was studied.•Effect of laser remelting on wear volume and friction coefficient was studied.•Effect of laser remelting and heat ...treatment on Charpy impact energy was studied.•Anisotropy in hardness, residual stress, Charpy impact energy and wear was studied.•Reciprocating sliding tests with ball above and below configuration were conducted.
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The reliability and quality of additively manufactured parts are questionable. The present study aimed to improve the surface quality and mechanical and tribological properties of a Ti6Al4V alloy manufactured using selective laser melting (SLM). The effects of the laser remelting on its surface topology, mechanical properties, and sliding wear in a ball-on-plate configuration with a ball either above or below the plate were investigated systematically. The influence of the laser remelting approach on the anisotropy of the mechanical and tribological properties was compared with that of the heat treatment approach. The surface quality and high cycle fatigue strength improved with an increase in the number of melting steps. The compressive and impact strengths also increased with an increase in the number of melting steps. The wear resistance in both configurations was higher in the remelted samples than in the samples after SLM. The results confirmed that laser remelting, as an affordable approach, could significantly improve the reliability of parts fabricated by the SLM process.
The effects of silicon oxide (SiO₂) nanoparticles at concentrations of 50, 100, 200, 400, and 800 mg/L on Triticum aestivum L. seedlings were investigated. We showed that SiO₂ nanoparticles, at ...concentrations higher 200 mg/L, had negative impacts on wheat seedlings. At these concentrations, SiO₂ nanoparticles significantly decreased roots and shoots fresh weight, decreased roots and shoots dry weight, decreased amounts of chlorophyll a and b in leaves, decreased amount of carotenoids in leaves, increased proline content in leaves, increased lipid peroxidation in leaves, and increased catalase activity in leaves. Results of this study indicate that at lower concentrations (such as 50 and 100 mg/L), SiO₂ nanoparticles not only have negative effects on wheat seedlings, but can have even some positive effects.
This study aimed to investigate the microstructure and tribological properties of the equiatomic AlCoCrFeNi high-entropy alloys (HEAs) fabricated using selective laser melting (SLM) and spark plasma ...sintering (SPS) processes. Sliding wear behavior in a ball-on-disk configuration with a ball either above or below the disk of the SLM and SPS samples was investigated. A detailed study of the effects of wear debris on the sliding wear behavior was undertaken to clarify the effect of wear debris entrapment in a wear scar. The wear volumes in the as-built SLM samples were slightly higher than in the SPS counterparts in both configurations. Wear volume and coefficient of friction values in the ball-above configuration were significantly lower than in the ball-below configuration. The present results indicated that the wear debris significantly influenced the mechanical contact properties of the HEA, providing new insight into the performance of the new materials in various conditions that could be considered to widen classical approaches by including the fate of the detached particles in the wear process. The impacts of wear debris on the design and application of these novel alloys should be considered.
In this work, influences of external electric and magnetic fields on the optical rectification coefficient, the linear and the third-order nonlinear optical absorption coefficients as well as ...refractive index changes of finite semi-parabolic quantum dots are investigated. In this regard, energy eigenvalues and eigenfunctions of the system are calculated numerically, and optical properties are obtained using the compact density matrix approach. The results show that external electric and magnetic fields have a great influence on these optical quantities.
The powder particle properties in additive manufacturing processes such as selective laser melting (SLM) influence the material properties. The microstructure, hardness, and chemical composition of ...gas-atomized AlCoCrFeNi powder particles were investigated, which showed a biphasic structure, consisting of FCC and BCC with a significant deviation in hardness. SLMed samples, consisting of BCC phase, indicated poor printability, inhomogeneous microstructure, and cracks. Microstructural inhomogeneity of SLM AlCoCrFeNi parts must reflect the inhomogeneity inherited from the atomized powder.
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Based on the effective mass and parabolic one band approximations, simultaneous effects of the geometrical size, hydrogenic impurity, hydrostatic pressure, and temperature on the intersubband optical ...absorption coefficients and refractive index changes in multilayered spherical quantum dots are studied. Energy eigenvalues and eigenvectors are calculated using the fourth-order Runge–Kutta method and optical properties are obtained using the compact density matrix approach. The results indicate that the hydrogenic impurity, hydrostatic pressure, temperature and geometrical parameters such as the well and barrier widths have a great influence on the linear, the third-order nonlinear and the total optical absorption coefficients and refractive index changes.
•Hydrogenic impurity effects on the optical properties of a MSQD are investigated.•Hydrostatic pressure and temperature effects are also studied.•Hydrogenic impurity has a great influence on the linear and nonlinear ACs and RICs.•Hydrostatic pressure and temperature change the linear and nonlinear ACs and RICs.
Motion planning is a key factor in enhancing the autonomy level of unmanned flying vehicles. A new dynamic hybrid algorithm is developed to solve the motion planning problem in real-time using a ...heuristic optimization approach. The proposed algorithm effectively combines desired features such as rapid convergence to an optimal path with reduced computational effort. In addition to the terrain obstacles, the proposed algorithm is able to avoid random threats that may arise sporadically in the terrain. Using the maneuver automaton concept, nonlinear dynamic model and performance constraints are also considered in the process of motion planning to further ensure feasible trajectories. Evaluation of the proposed algorithm against several simulated scenarios has effectively demonstrated its potential for generating optimal contour-matching trajectories that succeed in avoiding stochastic obstacles.
This paper presents a systematic evaluation approach of three-phase pulsewidth-modulated (PWM) AC-AC converter topologies for high-density applications. All major components and subsystems in a ...converter are considered and the interdependence of all the constraints and design parameters is systematically studied. The key design parameters, including switching frequency, modulation scheme, and passive values, are selected by considering their impacts on loss, harmonics, electromagnetic interference (EMI), control dynamics and stability, and protection. The component selection criteria as well as the physical design procedures are developed from the high-density standpoint. The concept of using the same inductor for harmonic suppression and EMI filtering is introduced in the design. With the proposed methodology, four converter topologies, a back-to-back voltage source converter (BTB-VSC), a nonregenerative three-level boost (Vienna-type) rectifier plus voltage source inverter (NTR-VSI), a back-to-back current source converter (BTB-CSC), and a 12-switch matrix converter, are analyzed and compared for high specific power using SiC devices. The evaluation results show that with the conditions specified in this paper, BTB-VSC and NTR-VSI have considerably lower loss, resulting in higher specific power than BTB-CSC and the matrix converter. The proposed methodology can be applied to other topologies with different comparison metrics and can be a useful tool for high-density topology selection.
In this work, the linear, the third-order nonlinear and the total optical absorption coefficients and refractive index changes of a strained InGaN/GaN quantum well are investigated numerically. In ...the effective-mass approximation, the electronic energy levels and the corresponding wave functions are calculated by taking into account the effects of spontaneous and piezoelectric polarization fields on the conduction band edge. Effects of intense laser field, In composition and the well width on the optical properties of the strained quantum well are studied. Results indicate that the laser field as well as the strain induced piezoelectric field considerably affects the confining potential of the quantum well. Results also show that the resonant peaks experience a red-shift (blue-shift) with the increase in the laser field intensity and well width (In composition).
