Asymmetric DC Research Articles

Multilevel voltage space vector generation with hexagonal and dodecagonal voltage space vectors for induction motor drives

The proposed power circuit that can perform multilevel inverter operation consists of three cascaded two level inverters with asymmetric DC link voltages and results in hexagonal voltage space vector structure in lower modulation region and a 12-sided polygonal space vector structure in the higher modulation region. The proposed topology reduces torque pulsations and losses due to the complete elimination of the 5th and 7th harmonic voltages. In the lower modulation region, inner space vectors may be switched, and in higher modulation region the 12-sided polygonal space vectors and hexagonal voltage space vectors are used for the PWM generation. FFT analysis is proposed to verify the reduction in harmonics in the output wave form. A detailed simulation study using Simulink is presented in this paper. The first part of the study uses a dodecagonal space vector structure for the PWM generation. The computer simulation result is experimentally verified using resistive load.

Fractal-based low computation space phasor generation scheme for a four-level inverter using an open-end winding induction motor

In this study, a space vector-based pulse width modulation scheme for an inverter with even number of levels is proposed. An approach based on fractal theory which utilises the self-similarity of the triangular sectors is employed in the present work, to find the sector and the inverter switching vectors. The space vector locations and associated computations are performed in a 60° coordinate system. Representation in a 60° framework avoids fractional arithmetic, thereby reducing computational complexity compared with the methods based on a Cartesian coordinate system. The duration of the switching vectors is determined after mapping the instantaneous reference space vector to an equivalent two-level inverter. The scheme is experimentally verified by using a four-level inverter configuration. four-level inversion is achieved with a dual inverter fed induction motor in the open-end winding configuration. The individual inverters are supplied with asymmetric DC link voltages. The switching vectors for the two inverters are automatically generated without using look-up tables. The scheme is implemented and tested with a 2-HP open-end winding induction motor drive.

Artificial Neural Network for monitoring the asymmetric half bridge DC–DC converter

This paper investigates the effectiveness of an Artificial Neural Network (ANN) while training as an asymmetric half bridge DC–DC converter circuit used in sensitive applications. The ANN is trained to form a mapping between the inputs and outputs of converter circuit using back propagation algorithm. After training the Artificial Neural Network monitor the asymmetric half bridge converter in order to detect the performance deviations. The ultimate objective of the designed Neural network is to perform like the trained converter circuit when the circuit is changed from its original conditions. Such information can be valuable for many sensitive power electronic applications. Typical simulation results are provided that indicates the effectiveness of the proposed scheme.

Dynamic response analysis and output voltage control of asymmetric half bridge DC–DC converter for low voltage applications

Asymmetric control scheme is an approach to achieve Zero Voltage Switching (ZVS) for half bridge isolated DC–DC converters. Due to the switching property included in their structure, DC–DC converters have a nonlinear behavior and their controller design is accompanied with complexities. But by employing the average method it is possible to approximate the system into a linear system and then linear control methods can be used. Dynamic performance of half bridge converters output voltage can be controlled by Pole placement and PID controllers. In this paper, Genetic Algorithm is used to optimize the system to achieve the optimum dynamic response. Matrix coefficients and dominant poles of closed loop transfer function is selected based on Genetic Algorithm. The results show an improvement in voltage control response in a short time.

The influence of boron contents on the microstructure and mechanical properties of Cr–B–N thin films

Seven Cr–B–N films with various boron contents ranging from 3.7 to 20.6 at.% were deposited by a bipolar asymmetric pulsed DC reactive magnetron sputtering system. The structures of thin films were characterized by X-ray diffraction (XRD). The surface and cross-sectional morphologies of thin films were examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The surface roughness of thin films was explored by atomic force microscopy (AFM). A nanoindenter was used to evaluate the hardness and properties of thin films, respectively. The phases of each Cr–B–N coating were mainly CrN and amorphous BN regardless the boron content. It was found that the grain size decreased with increasing boron content. The hardness and elastic modulus of Cr–B–N thin films decreased with increasing boron concentration, which were mainly attributed to the nanostructure configuration of small CrN nanograins and large intergranular amorphous BN phase.

Effects of Boron and Nitrogen Contents on the Microstructures and Mechanical Properties of Cr-B-N Nanocomposite Thin Films

Two series of Cr–B–N nanocomposite thin films with various nitrogen and boron contents were deposited by a co-sputtering process using a bipolar asymmetric pulsed DC reactive magnetron sputtering system, respectively. The crystalline structures and BN bonding nature of thin films were characterized by X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR), respectively. The cross sectional morphologies of thin films were examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The nanoindentation technique was used to evaluate the hardness and elastic modulus of thin films. The hardness, elastic modulus and H/E ratio increased with increasing nitrogen content, and decreased with increasing boron content. The optimal mechanical properties were achieved on the Cr-3.7 at.% B-54.6 at.% N thin film with a very fine and dense columnar structure.

Speed control of grid-connected switched reluctance generator driven by variable speed wind turbine using adaptive neural network controller

In wind energy conversion system, variable speed operation is becoming popular nowadays, where conventional synchronous generators, permanent magnet synchronous generators, and doubly fed induction generators are commercially used as wind generators. Along with the existing and classical solutions of the aforementioned machines used in wind power applications, the switched reluctance generator (SRG) can also be considered as a wind generator due to its inherent characteristics such as simple construction, robustness, low manufacturing cost, etc. This paper presents a novel speed control of switched reluctance generator by using adaptive neural network (ANN) controller. The SRG is driven by variable speed wind turbine and it is connected to the grid through an asymmetric half bridge converter, DC-link, and DC–AC inverter system. Speed control is very important for variable speed operation of SRG to ensure maximum power delivery to the grid for any particular wind speed. Detailed modeling and control strategies of SRG as well as other individual components including wind turbine, converter, and inverter systems are presented. The effectiveness of the proposed system is verified with simulation results using the real wind speed data measured at Hokkaido Island, Japan. The dynamic simulation study is carried out using PSCAD/EMTDC.

DC 스퍼터법과 비대칭 바이폴라 펄스 DC 스퍼터법으로 증착된 TiN 코팅막의 물성 비교연구

This work investigated the effect of duty cycle and pulse frequency on the microstructures and properties of titanium nitride thin films deposited by asymmetric bipolar pulsed DC sputtering system. Oscilloscope traces of the I-V waveforms indicate high power and high current density outputs during the asymmetric bipolar pulsed mode. The grain size decreases with decreasing duty cycle. The duty cycle has a strong influence not only on the microstructural properties but also on the mechanical properties of titanium nitride films. Comparing with the continuous DC sputtering, the titanium nitride films prepared by pulsed DC asymmetric bipolar process exhibit better properties.

Ion flux-film structure relationship during magnetron sputtering of WO3

In this work, we have investigated the influence of the magnetic field configuration during magnetron sputtering of WO 3 in order to establish the ion flux-film structure relationship. An asymmetric bipolar pulsed DC magnetron sputtering (PDMS) was used with two magnetic field configurations: balanced (BM) and unbalanced (UMB) magnetic configurations. Ion Energy Distribution Functions (IEDFs) of the main ion populations (Ar + , O + ) were recorded. The IEDFs are broad with, in addition of the thermalized distribution around 2 eV, two peaks with available ion kinetic energy up to 40–100 eV associated with the positive part of the pulse. Comparing the BM and UBM data, we calculated an increase by a factor of 5 of the ionic current while the average energy per ion was kept constant (~44 eV). X-ray diffraction demonstrates the influence of the magnetic configuration on the coating phase constitution. The films are crystallized in the WO 3 monoclinic phase with preferential orientations along the c axis using the BM configuration and along the a axis using the UBM one’s. On the other hand, it has been demonstrated that the grain size increases with the thickness using the BM configuration (up to 18 nm) while it remains constant using the UBM one’s (~7 nm).

Microstructure and mechanical property evaluation of pulsed DC magnetron sputtered Cr–B and Cr–B–N films

CrB2 and four Cr–B–N films with high Cr/B ratio and various nitrogen contents were deposited by a co-sputtering process using a bipolar asymmetric pulsed DC reactive magnetron sputtering system. The structures and BN bonding nature of the thin films were characterized by X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR), respectively. The surface and cross sectional morphologies of the thin films were examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The surface roughness of the thin films was explored by atomic force microscopy (AFM). Nanoindentation, microscratch and ball-on-disk wear techniques were used to evaluate the hardness, and tribological properties of the thin films, respectively.The microstructure of the Cr–B–N thin films changed from a coarse columnar structure to a glassy and featureless morphology as the nitrogen content increased from 15.2at.% to 54.5at.%, whereas the corresponding structure developed from an amorphous state to a nanocomposite structure consisting of CrN nanograins and amorphous BN phases. It was found that high hardness, good tribological and brittle properties were obtained for the CrB2 coating. The hardness and elastic modulus of the Cr–B–N thin films decreased with increasing nitrogen content until the nanocomposite structure of nanocrystalline CrN grains and an amorphous BN matrix was formed. However, the hardening effect induced by the nanocomposite structure was limited due to the fact that the small CrN nanograins were surrounded by a thick intergranular soft amorphous BN layer. On the other hand, the fracture toughness and resistance against elastic strain to failure of the Cr–B–N coatings were effectively enhanced by the addition of nitrogen.

비대칭 펄스 DC 반응성 마그네트론 스퍼터링으로 증착된 나노결정질 TiN 박막의 성장거동

Nanocrystalline TiN films were deposited on Si(100) substrate using asymmetric pulsed DC reactive magnetron sputtering. We investigated the growing behavior and the structural properties of TiN films with change of duty cycle and pulsed frequency. Grain size of TiN films were decreased from 87.2 nm to 9.8 nm with decrease of duty cycle. The 2θ values for (111) and (200) crystallographic planes of the TiN films were also decreased with decrease of duty cycle. This shift in 2θ could be attributed to compressive stress in the TiN coatings. Thus, the change of plasma parameter has a strong influence not only on the microstructure but also on the residual stresses of TiN films.

Influence of bilayer period and thickness ratio on the mechanical and tribological properties of CrSiN/TiAlN multilayer coatings

Nanostructured CrSiN/TiAlN multilayer coatings were deposited by a bipolar asymmetric reactive pulsed DC magnetron sputtering system. The thickness ratio of CrSiN to TiAlN layers was fixed at 1:1. The bilayer periods of the coatings were controlled to be from 6 to 40nm. Furthermore, two CrSiN/TiAlN multilayer coatings with the same bilayer period (20nm) but different CrSiN/TiAlN thickness ratios (2:8 and 8:2) were also deposited to explore the influence of thickness ratio on the mechanical properties of the multilayer coatings. The crystalline structures of the coatings were determined by a glancing angle X-ray diffractometer. The microstructures of thin films were examined by a scanning electron microscopy and a transmission electron microscopy, respectively. A nanoindenter, a micro Vickers hardness tester, and a pin-on-disk wear tester were used to evaluate the hardness, the toughness and the tribological properties of the thin films, respectively. The maximum hardness of the multilayers was obtained when the bilayer period was at 10nm for the coating with the same thickness ratio of CrSiN to TiAlN layers (1:1). Meanwhile, the thickness ratio of CrSiN to TiAlN layer had great influence on the hardness and the toughness properties of the multilayer coatings. The hardness and the toughness of the CrSiN/TiAlN multilayer coatings increased as the individual TiAlN layer thickness increased.

Dry etching of GaAs in asymmetric bipolar pulsed dc BCl3 plasmas

Dry etching of GaAs in asymmetric bipolar pulsed dc BCl3 plasmas using only mechanical pumping is reported. The mean pulsed dc voltage on the cathode was used as a process variable in the experiment. When the mean pulsed dc voltage changed from −350 to −550 V at the cathode electrode, the dependent mean current and power were increased from 0.22 to 0.33 A and from 100 to 220 W, respectively. The etch rate of GaAs increased from 0.01 to 0.55 μm/min, and etch selectivity over photoresist was also strongly increased from 0.2:1 to 2.2:1 with this voltage change. The threshold mean voltage for plasma ignition pulsed dc plasma was −350 V. Scanning electron microscopy micrographs showed that dry etching at −500 V provided excellent surface smoothness (rms roughness of ∼1 nm) and sidewall passivation. At a lower mean voltage of −400 V, there was a rough surface and some undercutting of GaAs after etching at −400 V. Degradation of the photoresist surface was negligible in both cases.

Nano-scratching and nano-machining in different environments on Cr 2N/Cu multilayer thin films

Five nanostructured Cr 2N/Cu multilayer coatings were deposited by a bipolar asymmetric reactive pulsed DC magnetron sputtering system, and various bilayer periods (Λ) were achieved by controlling the holding time of Si substrates in the plasma of Cr or Cu. The hardness and elastic modulus of multilayer coatings were investigated by means of a nanoindenter. Nano-scratch and nano machining experiments on multilayered coatings were conducted using atomic force microscopy (AFM) in air and DI-water, respectively. According to the groove depth, width , and coefficient of friction (COF) obtained from nano-scratch tests, influences of scratch cycle numbers and bilayer periods on the scratchability of Cr 2N/Cu multilayered thin films were examined. It was observed that after nano-scratch experiments in air and water, the COF values and the amount of removed material increased with increasing bilayer period. After nano-machining tests in air and water, different types of the cutting chip pile-ups were observed. In this work, the surface tribological properties and machinability of Cr 2N/Cu multilayered thin films when using an AFM are discussed.

Corrections to “Impedance-Transforming Symmetric and Asymmetric DC Blocks” [Sep 10 2463-2474

In the above titled paper (ibid., vol. 58, no. 9, pp. 2463-2474, Sep. 10), there is a publication error [Fig. 8]. The correct Fig. 8 is presented here.

A Hybrid Cascade Converter Topology With Series-Connected Symmetrical and Asymmetrical Diode-Clamped H-Bridge Cells

A novel H-bridge multilevel pulsewidth modulation converter topology based on a series connection of a high-voltage diode-clamped inverter and a low-voltage conventional inverter is proposed in this paper. A dc link voltage arrangement for the new hybrid and asymmetric solution is presented to have a maximum number of output voltage levels by preserving the adjacent switching vectors between voltage levels. Hence, a 15-level hybrid converter can be attained with a minimum number of power components. A comparative study has been carried out to present high performance of the proposed configuration to approach a very low total harmonic distortion of voltage and current, which leads to the possible elimination of the output filter. Regarding the proposed configuration, a new cascade inverter is verified by cascading an asymmetrical diode-clamped inverter, in which 19 levels can be synthesized in output voltage with the same number of components. To balance the dc link capacitor voltages for the maximum output voltage resolution as well as synthesize asymmetrical dc link combination, a new multi-output boost converter is utilized at the dc link voltage of a seven-level H-bridge diode-clamped inverter. Simulation and hardware results based on different modulations are presented to confirm the validity of the proposed approach to achieve a high-quality output voltage.

The influences of the second harmonics together with the asymmetries of junctions on the voltage–current characteristics of high T C DC SQUIDs

We consider a superconducting quantum interference device having two arbitrary different over damped junctions transporting different currents. By replacing the governed two-dimensional Fokker–Planck equation with two one-dimensional equations, two density probability currents are appeared which determine the statistical average of the time-averaged total voltage across the device. To obtain the density probability currents, two coupled integral equations are introduced. These equations together with two other equations coming from normalizing conditions, found one generalized formulation for the voltage–current characteristics of the device. Based on that, the voltage–current characteristics of large inductance asymmetric DC SQUIDs having first and second harmonics in their current-phase relations are obtained and some predictions are illustrated.

Circulating Current Ratio for DC SQUIDs

The circulating current in one Josephson junction of a DC SQUID is in the direction of translation current while in the other Josephson junction is in the opposite direction. Therefore, one interesting function is the circulating current ratio (CCR) which we define as: CCR = (dissipative translation current1 − circulating current)/(dissipative translation current2 + circulating current). We discuss the circulating current ratio in the loop of DC SQUIDs by considering the 20 on-chip high TC DC SQUIDs that Novikov et al. measured their transfer functions and flux shifts and reported in I.L. Novikov et al. (Physica C 469:30, 2009). A comparison between the CCR characteristics of those SQUIDs may be helpful in understanding the voltage-current and voltage-flux characteristics of asymmetric high TC DC SQUIDs.

Microstructure, mechanical and electrochemical properties evaluation of pulsed DC reactive magnetron sputtered nanostructured Cr–Zr–N and Cr–Zr–Si–N thin films

Cr–Zr–Si–N thin films with various Zr contents were deposited by a bipolar asymmetric pulsed DC reactive magnetron sputtering system. In addition, a Cr–Zr–N film without Si addition was fabricated as a reference. The influence of Zr on the constitution, microstructure, mechanical, tribological and electrochemical properties of Cr–Zr–Si–N films was investigated. The microstructure of thin films was determined by a glancing angle X-ray diffractometer (GA-XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), respectively. A nanoindenter, a Vickers micro hardness tester and pin-on-disk wear tests were adopted to evaluate the hardness, toughness and tribological properties of thin films, respectively. The electrochemical properties of thin films were also evaluated in 3.5 wt.% NaCl aqueous solution. In case of the Cr–Zr–Si–N films, the Si content was fixed around 6–8 at.% and various Zr contents ranging from 0.5 to 13.6 at.% were achieved by changing the Zr target power density. In comparison to the Cr–Zr–N reference film, the addition of ~ 7.0 at.% Si in Cr–Zr–Si–N films resulted in a refined columnar structure and enhanced mechanical and anti-corrosion properties. A lattice constant expansion of these films was observed with increasing Zr content. A nanostructured thin film with around 5–10 nm grain size was obtained in case of a Cr–13.6 at.% Zr–6.8 at.% Si–N film. In general, the hardness, plastic deformation resistance and corrosion resistance increased also with increasing Zr content in the Cr–Zr–Si–N films. The Cr–Zr–Si–N film containing 13.6 at.% Zr exhibited a combination of high hardness, good mechanical properties, adequate tribological performance and excellent corrosion resistance in this study.

Impedance-Transforming Symmetric and Asymmetric DC Blocks

Design formulas of symmetric and asymmetric dc blocks are presented for achieving Chebyshev and Butterworth frequency responses. They can also be used for impedance transformers. Design formulas of symmetric dc blocks are first derived and those of asymmetric dc blocks are also computed based on the symmetric ones. For easy derivation of design formulas of the symmetric dc blocks, coupling coefficient, being proportional to bandwidth, is defined. It is then clarified that symmetric Chebyshev dc blocks are not possible for the impedance transforming. For the symmetric Butterworth dc blocks, three design methods are exploited. To verify the symmetric Butterworth dc blocks, a microstrip dc block with 50- and 25-Ω termination impedances designed at a center frequency of 2 GHz was fabricated. The measured <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</sub> = <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">21</sub> and <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sub> = <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">22</sub> are - 0.19 and - 57 dB around 2 GHz, showing quite good agreement with simulation results. Based on synthesized symmetric dc blocks, the coefficients of transfer function | <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</sub> | <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> are calculated and design formulas of the asymmetric dc blocks are derived based on the symmetric ones. Asymmetric Chebyshev dc blocks are verified by the measurements provided in a previously published paper and asymmetric Butterworth dc blocks by the symmetric Butterworth dc block measured in this paper.