• Journal of Mechanical Science and Technology
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• 제17권11호
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• pp.1650-1658
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• 2003

This paper deals with a novel shunt circuit, which is capable of suppressing multimode vibration amplitudes by using a pair of piezoceramic patches. In order to describe the characteristic behaviors of a piezoelectric damper connected with a series and a parallel resistor-negative capacitor branch circuit, the stiffness ratio and loss factor with respect to the non-dimensional frequency are considered. The mechanism of the shunt damper is also described by considering a shunt voltage constrained by shunt impedance. To obtain a guideline model of the piezo/beam system with a negative capacitive shunting, the governing equations of motion are derived through the Hamilton's principle and a piezo sensor equation as well as a shunt-damping matrix is developed. The theoretical analysis shows that the piezo/beam system combined with a series and a parallel resistor-negative capacitor branch circuit developed in this study can significantly reduce the multiple-mode vibration amplitudes over the whole structural frequency range.

• Journal of Power Electronics
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• 제18권5호
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• pp.1555-1566
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• 2018

Proportional capacitor-current-feedback active damping (AD) is a common damping method for the resonance of LCL-type grid-connected inverters. Proportional capacitor-current-feedback AD behaves as a virtual resistor in parallel with the capacitor. However, the existence of delay in the actual control system causes impedance in the virtual resistor. Impedance is manifested as negative resistance when the resonance frequency exceeds one-sixth of the sampling frequency ($f_s/6$). As a result, the damping effect disappears. To extend the system damping region, this study proposes a virtual resistor-inductor-capacitor (RLC) AD method. The method is implemented by feeding the filter capacitor current passing through a band-pass filter, which functions as a virtual RLC in parallel with the filter capacitor to achieve positive resistance in a wide resonance frequency range. A combination of Nyquist theory and system close-loop pole-zero diagrams is used for damping parameter design to obtain optimal damping parameters. An experiment is performed with a 10 kW grid-connected inverter. The effectiveness of the proposed AD method and the system's robustness against grid impedance variation are demonstrated.

• Journal of Electrical Engineering and Technology
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• 제1권1호
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• pp.52-57
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• 2006

In this paper, a new 'impedance sensing' method is described. This method overcomes the shortcomings of the impedance sensing method. According to the new method, sensing voltage pulse is applied to the idle phase in the minimum inductance region and the beginning of the increasing inductance region to detect rotor position. The negative torque produced by the sensing voltage pulse can be neglected in the minimum inductance region and the efficiency of SRM is improved. In the minimum inductance region the back electromotive force (EMF) can be neglected. And in the increasing inductance region the EMF opposes the rise of current in the phase, so the position estimation scheme is reliable. Therefore the new 'impedance sensing' method is sufficiently precise even under the high back EMF effect. The adjustment of turn-on angle and turn-off angle is also easy to be realized. The technique is very useful in applications where cost or size is primary concerns, such as electric bicycle drives. Experimental results are presented to verify the proposed method.

• Advances in materials Research
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• 제2권2호
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• pp.119-131
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• 2013

Impedance and electrical conduction studies of $Ba(Nd_{1/2}Nb_{1/2})O_3$ ceramic prepared using conventional high temperature solid-state reaction technique are presented. The crystal symmetry, space group and unit cell dimensions were estimated using Rietveld analysis. X-ray diffraction analysis indicated the formation of a single-phase cubic structure with space group $Pm\bar{3}m$. Energy dispersive X-ray analysis and scanning electron microscopy studies were carried to study the quality and purity of compound. The circuit model fittings were carried out using the impedance data to find the correlation between the response of real system and idealized model electrical circuit. Complex impedance analyses suggested the dielectric relaxation to be of non-Debye type and negative temperature coefficient of resistance character. The correlated barrier hopping model was employed to successfully explain the mechanism of charge transport in $Ba(Nd_{1/2}Nb_{1/2})O_3$. The ac conductivity data were used to evaluate the density of states at Fermi level, minimum hopping length and apparent activation energy.

• 한국음향학회:학술대회논문집
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• 한국음향학회 2000년도 학술발표대회 논문집 제19권 2호
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• pp.251-254
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• 2000

• Journal of Power Electronics
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• 제19권1호
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• pp.234-243
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• 2019

Studying the control strategy of a microgrid under the load unbalanced state helps to improve the stability of the system. The magnitude of the power fluctuation, which occurs between the power supply and the load, is generated in a microgrid under the load unbalanced state is called negative sequence reactive power $Q^-$. Traditional power distribution methods such as P-f, Q-E droop control can only distribute power with positive sequence current information. However, they have no effect on $Q^-$ with negative sequence current information. In this paper, a stationary-frame control method for power sharing and voltage unbalance compensation in islanded microgrids is proposed. This method is based on the proper output impedance control of distributed generation unit (DG unit) interface converters. The control system of a DG unit mainly consists of an active-power-frequency and reactive-power-voltage droop controller, an output impedance controller, and voltage and current controllers. The proposed method allows for the sharing of imbalance current among the DG unit and it can compensate voltage unbalance at the same time. The design approach of the control system is discussed in detail. Simulation and experimental results are presented. These results demonstrate that the proposed method is effective in the compensation of voltage unbalance and the power distribution.

• 대한의용생체공학회:의공학회지
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• 제23권6호
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• pp.437-449
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• 2002

관상동맥은 체순환 동맥과 비교하여 매우 다른 유동조건을 갖는다. 본 연구에서는 이러한 관상동맥의 유동조건이 다른 혈관과 비교하여 동맥경화의 생성과 발전에 어떤 영향을 미치는지를 협착된 혈관 모델의 혈류역학적 전산유체해석을 통해 분석하였다. 연구 결과, 협착된 관상동맥 모델의 유동장과 벽면전단응력의 분포는 복부 대동맥 및 고동맥과 비교하여 큰 차이점을 보였다. 관상동맥은 혈관 지름이 혈류량에 비해 상대적으로 작으므로 큰 벽면전단응력값을 보이는 반면에, 혈관 협착부 목 후방부에 하나의 보텍스만 보이는 단순한 유동장 분포를 보였다. 한편, 복부대동맥과 고동맥 모델은 협착부 목 전반부와 후방부에 여러 개의 보텍스를 나타내는 복잡한 유동장을 형성하였다. 관상동맥 모델에서는 임피던스 페이즈앵글이 음의 큰 값을 가질 때에 벽면전단응력의 크기가 증가하고 재순환영역이 증가함이 관찰되었다. 그러므로 심장의 수축운동과 -110。에 이르는 관상동맥의 큰 음의 임피던스 페이즈앵글 값은 동맥경화를 촉진시키는 유동환경을 조성함을 알 수 있었다.

• 전력전자학회논문지
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• 제17권6호
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• pp.546-552
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• 2012

This paper presents an active anti-islanding detection method using negative sequence voltage injection to the grid through a three-phase photovoltaic inverters. Because islanding operation mode can cause a variety of problems, the islanding detection of grid-connected photovoltaic inverter is the mandatory feature. The islanding mode is detected by measuring the magnitude of negative sequence impedance calculated by the negative sequence voltage and current at the point of common coupling. Simulation and experimental test are performed to verify the effectiveness of the proposed method which can detect the islanding mode in the specified time. The test has been done in accordance with the condition on IEEE Std 929-2000.

• 전기화학회지
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• 제11권1호
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• pp.33-36
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• 2008

본 연구에서는 겔식 VRLA (valve regulated lead acid번지의 충전상태(SoC) 판단을 위해 임피던스 기법을 이용하여 조사하였다. 임피던스는 VRLA전지 (2V/1.2Ah)의 다양한 충전상태에서 진폭 10mV로 100kHz에서${\sim}$10mHz까지 측정하였다. 측정된 임피던스 데이터로부터 등가회로를 유도하고, CNLS (Complex Non-linear Least Squares) 법을 사용하여 분석하였다. 양극 쪽의 전하전이 저항과 전기이중층 커패시턴스가 음극보다 높았다. 겔 저항은 충전상태가 감소함에 따라 증가하며 이는 VRLA 전지의 충전상태를 판단하는데 중요한 파라미터임을 확인하였다.

• 대한전자공학회논문지
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• 제6권2호
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• pp.6-20
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• 1969

부저항소자인 턴넬다이오드와 수동선형부하로 부저항발진기를 구성하고 수동부하를 가변하여 주파수최대화에 관해서 연구했다. 이론적인 해석과 복소평면상의 임피이던스 궤적을 도시하고 병렬-RL부하와 저항부하의 두 경우의 최대발진주파수를 측정하여 두 경우의 실험결과를 보면 병렬-RL부하의 폭이 훨씬 높은 주파수를 발진하는 것을 알았다.