• Journal of Electrical Engineering and Technology
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• 제9권5호
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• pp.1746-1752
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• 2014

This paper presents an application of proportional-resonant (PR) current controllers in modular multilevel converter-high voltage direct current (MMC-HVDC) system under unbalanced voltage conditions. The ac currents are transformed and controlled in the stationary reference frame (${\alpha}{\beta}$-frame). Thus, the complex analysis of the positive and negative sequence components in the synchronous rotating reference frame (dq-frame) is not necessary. With this control method, the ac currents are kept balanced and the dc-link voltage is constant under the unbalanced voltage fault conditions. The simulation results based on a detailed PSCAD/EMTDC model confirm the effectiveness of the proposed control method.

• 대한수학회지
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• 제56권4호
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• pp.1017-1029
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• 2019

In this paper, blows-up and global solutions for a class of nonlinear divergence form parabolic equations with the abstract form of $({\varrho}(u))_t$ and time dependent coefficients are considered. The conditions are established for the existence of a solution globally and also the conditions are established for the blow up of the solution at some finite time. Moreover, the lower bound and upper bound of the blow-up time are derived if blow-up occurs.

• Advances in nano research
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• 제16권6호
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• pp.601-613
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• 2024

A new analytical buckling solution of a thermo-electro-magneto-elastic (TEME) cylindrical nano-shell made of BiTiO₃-CoFe₂O₄ materials is obtained based on Hamiltonian approach. The Winkler and Pasternak elastic foundations as well as thermo-electro-magneto-mechanical loadings are applied, and two different types of edge conditions are taken into the investigation. According to nonlocal strain gradient theory (NSGT) and surface elasticity theory in conjunction with the Kirchhoff-Love theory, governing equations of the nano-shell are acquired, and the buckling bifurcation condition is obtained by adopting the Navier's method. The detailed parameter study is conducted to investigate the effects of axial and circumferential wave numbers, scale parameters, elastic foundations, edge conditions and thermo-electro-magnetic loadings on the buckling behavior of the nano-shell. The proposed model can be applied in design and analysis of TEME nano components with multi-field coupled behavior, multiple edge conditions and scale effect.

• Environmental Engineering Research
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• 제20권3호
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• pp.223-229
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• 2015

The application of coagulation for feed water pretreatment prior to microfiltration (MF) process has been widely adopted to alleviate fouling due to particles and organic matters in feed water. However, the efficiency of coagulation pretreatment for MF is sensitive to its operation conditions such as pH and coagulant dose. Moreover, the optimum coagulation condition for MF process is different from that for rapid sand filtration in conventional drinking water treatment. In this study, the use of response surface methodology (RSM) was attempted to determine coagulation conditions optimized for pretreatment of MF. The center-united experimental design was used to quantify the effects of coagulant dose and pH on the control of fouling control as well as the removal organic matters. A MF membrane (SDI Samsung, Korea) made of polyvinylidene fluoride (PVDF) was used for the filtration experiments. Poly aluminum chloride (PAC) was used as the coagulant and a series of jar tests were conducted under various conditions. The flux was $90L/m^2-h$ and the fouling rate were calculated in each condition. As a result of this study, an empirical model was derived to explore the optimized conditions for coagulant dose and pH for minimization of the fouling rate. This model also allowed the prediction of the efficiency of the coagulation efficiency. The experimental results were in good agreement with the predictions, suggesting that RSM has potential as a practical method for modeling the coagulation pretreatment for MF.

• 한국과학교육학회지
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• 제22권2호
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• pp.286-298
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• 2002

본 연구는 증발과 증발 조건에 관련된 활동으로부터 유치원부터 중학교 2학년까지 학생들의 개념 유형과 학년별 경향성을 알아보고자 하였다. 임의로 선정한 25명의 학생들을 대상으로 활동적 면접을 실시한 뒤, 수집된 자료를 질적으로 분석하였다. 증발에 대한 학생의 개념 유형은 5가지로 구분되었다. 또한 증발의 조건에 대한 학생의 개념 유형도 5가지로 구분되었다. 그리고 고학년으로 갈수록 과학적 개념으로 변화하지만 과학자의 개념과는 다른 개념이 지속되는 경향을 보였다.

• Journal of Power Electronics
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• 제13권2호
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• pp.232-242
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• 2013

To obtain phase currents information in AC drives, shunt sensing technology is known to show great performance in cost-effectiveness and therefore it is widely used in low cost applications. However, shunt sensing methods are unable to acquire phase currents in certain operation conditions. This paper deals with the derivation of the boundary conditions for phase current reconstruction in three-shunt sensing inverters and proposes a voltage injection method to expand the measurable areas. As the boundary conditions are deeply dependent on the switching patterns, they are typically analyzed on the voltage vector plane for space vector pulse width modulation (SVPWM) and discontinuous pulse width modulation (DPWM). In the proposed method, the voltage injection and its compensation are conducted within one sampling period. This guarantees fast current reconstruction and the injected voltage is decided so as to minimize the current ripple. In addition to the voltage injection method, a sampling point shifting method is also introduced to improve the boundary conditions. Simulation and experimental results are presented to verify the boundary condition derivation and the effectiveness of the proposed voltage injection method.

• 한국정밀공학회지
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• 제31권1호
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• pp.35-42
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• 2014

Recently, in industry field, many researchers are looking for ways to reduce the use of lubricant because of environmental and economical reasons. MQL lubrication is one of many lubrication technologies. The aim of this study is to evaluate the machinability considering lubrication methods and taper angles of workpieces for turning of SCM440. Workpieces of two shapes such as workpiece with and without taper angle are used. And two lubrication methods such as MQL and Wet have been considered. And cutting force and surface roughness are used as characteristic values. Cutting speed, feed rate, injection angle and distance are used as design parameters. The characteristic values were statistically analyzed by Taguchi method. From the results, main effects plot and importance of each parameter according to conditions are analyzed. Finally, this study has been suggested the optimum machining conditions according to the lubrication methods, machining conditions and shape of workpiece.

• 보건교육건강증진학회지
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• 제33권2호
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• pp.27-37
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• 2016

Objectives: This study investigated the relationship between adolescents' suicidal ideation and their physical health conditions to contribute to lower suicide rates among adolescents. Methods: Secondary analysis of 2108 high school students of $10^{th}$ grade was conducted using the Korean Child and Youth Panel survey in 2013. The association between suicidal ideation and socio-demographic and health characteristics were analyzed using Chi-square tests and regression models. Results: Adolescents' physical health conditions were significantly related to suicidal ideation. In particular, rhinitis and various symptoms such as feeling feverish, low appetite, feeling of nausea, stomachache while studying, and sleeping problem showed statistically significant relationships with suicidal ideation (P < .05). Furthermore, the higher frequency of diseases and body symptoms increases the rate of suicidal ideation. Considering socio-economic characteristics, family structure was also important. Conclusions: Physical health condition is a major contributor to adolescent's suicidal ideation. results suggest that improving social support system for adolescents' physical health conditions across family and school may be important in suicide prevention efforts.

• 아동학회지
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• 제34권4호
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• pp.105-123
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• 2013

The purpose of this study was to examine the effects of a child's demographic variables and maternal perceptions of the conditions of happiness on their happiness for mothers. The participants of this study comprised 916 mothers of young children, school-aged children, and adolescents. The results revealed that a child's age and the numbers of children influenced maternal happiness when mothers' demographic variables were controlled. In addition, mothers who exhibited high degree of need in terms of the conditions of happiness regarding existence, relatedness, and growth were more likely to be happy than mothers who exhibited low degree of needs. Finally, the expectations regarding growth, which refers to the pursuit of self-accomplishment and meaning, was the most important predictor of maternal happiness. These findings have implications for parent education programs for mothers of young children, school-aged children, and adolescents.

• 한국해양공학회지
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• 제23권1호
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• pp.81-88
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• 2009

An evaluation of the structural integrity of an oceanographic buoy subjected to extreme loads was carried out in this study. Load components, such as the current, waves, and wind load, which were required for the sea's environmental conditions, were calculated precisely. A non linear finite element analysis was conducted to elucidate the structural response of the buoy under extreme environmental conditions. Based on the surface drift velocity scheme, a dynamic impact analysis was also carried out for the case of collision accidents. The proposed numerical technique would be a useful and cost effective tool for design scheme evaluation in the field of oceanographic buoys.