• 소음진동
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• 제5권2호
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• pp.215-224
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• 1995

In this paper, the three-dimensional finite element model is established to investigate the structural dynamic characteristics of rotor blade using a finite element analysis. Six natural frequencies and mode shapes are calculated by computer simulation. The first three flapping modal frequencies, the first two lead-lag modal frequencies, and the first feathering modal frequency are validated through comparison with the modal test results of the fixed rotor blade. The computer simulation results are found in good agreement with experimentally measured natural frequencies. The important results are obtained as follows: (1) Natural frequencies are changed due to the variation of rotational speed and fiber angle of rotor blade, (2) Weak coupling between flapping mode shape and lead-lag mode shape are detected, (3) Centrifugal force has more effect on flapping modal frequency than lead-lag modal frequency.

• 한국항해항만학회지
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• 제45권2호
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• pp.69-74
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• 2021

This study aims to investigate the lead-lag relationship between the agricultural produce import price in Korea and the corresponding shipping freight rate. Since the Korean economy has pursued an export-driven growth strategy, mainly based on the manufacturing sector, the country has to depend on the vast majority of its agricultural produce consumption after import from foreign countries. Moreover, compared with other high-value products, transportation cost occupies a substantial share of the agricultural commodity price, resulting in changes in the shipping freight rate being a pivotal determinant of agricultural produce import. In this respect, this study explores the possible association between agricultural produce import in Korea and shipping freight rate and the lead-lag relationship. Using a monthly dataset of agricultural produce import prices and freight rates for Handysize and Panamax dry-bulkers for the period between January 2010 and November 2020, this study determines that the shipping freight rate, in general, leads the agricultural commodity price.

• 대한전기학회논문지:전기기기및에너지변환시스템부문B
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• 제51권7호
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• pp.387-398
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• 2002

In this paper, a new Three-Phase Lead-Lag Random Pulse Position PWM(LL-RPWM) scheme is proposed and implemented for decreasing audible acoustic noise of motor drives. In the proposed RPWM(Random PWM), each of three phase pulses is located randomly in each switching interval. Based on the space vector modulation technique, the duty ratio of the pulses is calculated. Along with the randomization of the PWM pulses, we can obtain the effects of spread spectra of voltage, current as in the case of randomly changed switching frequency. To verify the validity of the proposed LL-RPWM, the simulation and experimental study was tried. Along with the randomization PWM pulses, the space vector modulation is also executed in the C167 micro-controller. The simulation and experimental results show that the voltage and current harmonics are spread to a wide band area and that the audible acoustic noise is reduced by the proposed RPWM method.

• Nuclear Engineering and Technology
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• 제24권3호
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• pp.236-242
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• 1992

노심의 과온도 Delta-T보호식에 설정된 동적보정함수들의 시정수 변화에 따른 특성이 조사되었으며, 출력운전 중 제어봉집합체의 비통제된 인출사고의 경우에 있어서 위 동적보정함수들의 과온도 Delta-T보호식에 대한 영향을 시스템 코드인 NLOOP및 노심열수력 코드인 PUMA를 사용하여 연구하였다. 위 연구를 바탕으로, 과온도 Delta-T 보호식의 동적보정함수에 대한 최적화 절차가 제시되었으며, 고리 3&4 호기 친이노심의 경우에 대해 최적화된 동적보정함수를 구하였다. 그 결과, 시스템의 최소 DNBR에 가장 영향을 줄 수 있는 동적보정함수는 노심평균온도에 대한 lead-lag항으로 판명되었으며, 이때 최적화된 시정수값은 lead시간 21초, lag시간 4초로 나타났다. 이러한 동적보정함수의 최적화를 통하여 안전한계치를 변경하지 않고서도 노심의 운전영역을 개선할 수 있을 것으로 기대된다.

• 전력전자학회논문지
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• 제8권2호
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• pp.151-164
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• 2003

본 연구에서는 3상 변조된 RLL(Random Lead-Lag) PWM 보다 스위칭 소음의 스펙트럼 광역화의 관점에서 비교적 우수한 2상 변조된 RLL-PWM기법을 제안하고, 유도모터 속도제어 시스템을 제작하여 그 타당성을 확인하였다. 공간벡터 RPWM(Random PWM)의 발생은 TMS320C31 DSP에 의하고 있으며 균일한 확률 분포를 갖는 랜덤 발생 함수에 의하여 각 상의 펄스를 랜덤하게 배치하였다. 부하가 있는 상태에서 유도모터의 일정 v/f 속도 제어 성능을 3상 및 2상 중앙정렬 SVM(Space Vector Modulation), 3상 변조된 RLL-PWM 그리고 제안된 2상 변조된 RLL-PWM과 비교 검토한 결과, 일정 v/f 속도 제어의 성능은 모두 유사하였다. 그러나 제안된 방법은 전압, 전류 및 스위칭 소음 스펙트럼의 광대역화 효과의 측면에서 종전의 방법보다 우수함을 확인할 수 있었다.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2007년도 춘계학술대회A
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• pp.1149-1154
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• 2007

Fluctuation of control rod was experienced when plant was operating in normal operation mode in WH type NPPs. In order to cope with increased control rod fluctuation, the lead-lag controller setpoint for rod control system was optimized and resulted in increasing the margin of operation and minimizing unnecessary control rod movement. By optimization of the time constant, the margin of operation was increased by $1.5^{\circ}F$ and the control rod movement was not occurred due to mitigation of temperature fluctuation in loop. According to the mitigation of time constant, the margin of operation was increased but safety margin can be affected badly, so that the influences to FSAR design reference was evaluated. As the result of this evaluation, it satisfied the design reference of the existing safety analysis and was applied to NPP after obtaining the approval.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2003년도 추계학술대회
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• pp.1590-1595
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• 2003

Direct drive servovalve(DDV) is a kind of one-stage valve since the rotary motion of DC motor is directly transferred to the linear motion of valve spool through the link. Since the structure of DDV is simple, it is less expensive, more reliable and offers reduced internal leakage and reduced sensitivity to fluid contamination. However, the flow force effect on the spool motion is significant such that it induces large steady-state error in a step response. If the proportional control gain is increased to reduce the steady-state error, the system becomes unstable. In order to satisfy the system design requirements, the lead-lag controller is designed using the complex method that is one kind of constrained direct search method.

• 대한기계학회논문집A
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• 제28권11호
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• pp.1719-1726
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• 2004

Direct drive servovalve(DDV) is a kind of one-stage valve because the main spool valve is directly driven by the DC motor. Since the structure of DDV is simple, it is less expensive, more reliable and offers reduced internal leakage and reduced sensitivity to fluid contamination. However, the flow force effect on the spool motion is significant such that it induces large steady-state error in a step response. If the proportional control gain is increased to reduce the steady-state error, the system becomes unstable. In order to satisfy the system design requirements, the lead-lag controller is designed using the complex method that is one kind of constrained direct search method.

• Nuclear Engineering and Technology
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• 제49권1호
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• pp.141-147
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• 2017

Mechanical shim is an advanced technology for reactor power and axial offset control with control rod assemblies. To address the adverse accuracy impact on the ex-core power range neutron flux measurements-based axial offset control resulting from the variable positions of control rod assemblies, the lead-lag-compensated in-core self-powered vanadium detector signals are utilized. The prompt ${\gamma}$ current of self-powered detector is ignored normally due to its weakness compared with the delayed ${\beta}$ current, although it promptly reflects the flux change of the core. Based on the features of the prompt ${\gamma}$ current, a method for configuration of the lead-lag dynamic compensator is proposed. The simulations indicate that the method can improve dynamic response significantly with negligible adverse effects on the steady response. The robustness of the design implies that the method is of great value for engineering applications.

• Journal of Korea Trade
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• 제23권2호
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• pp.34-45
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• 2019

Purpose - This study investigates the lead-lag relations between the prices of major commodities imported into Korea and corresponding shipping freight rates. This paper aims to provide implications for cross-market causal relations between related economic segments. Design/Methodology - For economic long-run equilibrium between commodity prices and freights, a Johansen (1988) cointegration test is employed first. Then, Granger (1987) causality tests are performed under the vector error correction model (VECM) framework. Findings - The results indicate that the direction of causality varies by raw materials, which is attributable to different economic mechanisms in the corresponding shipping transportation sectors. In addition, the significance of causality becomes blurred during the post-2008 period. Practical Implication - Corporate managers in commodity trading, steelmaking, power generation, and oil refinery sectors can take advantage of the findings in this study as identifying leading economic indicators can be helpful for decision making in both short- and long-term strategies. Originality/value - This study is the first attempt to analyze the inter-relations between commodity prices and corresponding freight rates focusing on raw material imports of Korea.