• International Journal of Fluid Machinery and Systems
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• v.4 no.1
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• pp.179-190
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• 2011

The swirling flow developing in Francis turbine draft tube under part load operation leads to pressure fluctuations usually in the range of 0.2 to 0.4 times the runner rotational frequency resulting from the so-called vortex breakdown. For low cavitation number, the flow features a cavitation vortex rope animated with precession motion. Under given conditions, these pressure fluctuations may lead to undesirable pressure fluctuations in the entire hydraulic system and also produce active power oscillations. For the upper part load range, between 0.7 and 0.85 times the best efficiency discharge, pressure fluctuations may appear in a higher frequency range of 2 to 4 times the runner rotational speed and feature modulations with vortex rope precession. It has been pointed out that for this particular operating point, the vortex rope features elliptical cross section and is animated of a self-rotation. This paper presents an experimental investigation focusing on this peculiar phenomenon, defined as the upper part load vortex rope. The experimental investigation is carried out on a high specific speed Francis turbine scale model installed on a test rig of the EPFL Laboratory for Hydraulic Machines. The selected operating point corresponds to a discharge of 0.83 times the best efficiency discharge. Observations of the cavitation vortex carried out with high speed camera have been recorded and synchronized with pressure fluctuations measurements at the draft tube cone. First, the vortex rope self rotation frequency is evidenced and the related frequency is deduced. Then, the influence of the sigma cavitation number on vortex rope shape and pressure fluctuations is presented. The waterfall diagram of the pressure fluctuations evidences resonance effects with the hydraulic circuit. The influence of outlet bubble cavitation and air injection is also investigated for low cavitation number. The time evolution of the vortex rope volume is compared with pressure fluctuations time evolution using image processing. Finally, the influence of the Froude number on the vortex rope shape and the associated pressure fluctuations is analyzed by varying the rotational speed.

• Journal of Magnetics
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• v.21 no.3
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• pp.413-420
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• 2016

The permanent magnet synchronous motor has high efficiency driving performance and high power density output characteristics compared with other motors. In addition, it has good regenerative operation characteristics during braking and deceleration driving condition. For this reason, permanent magnet synchronous motor is generally applied as a power train motor for electrical vehicle. In permanent magnet synchronous motor, the most probable causes of fault are demagnetization of rotor's permanent magnet and short of stator winding turn. Therefore, the demagnetization fault of permanent magnet and turn fault of stator winding should be detected quickly to reduce the risk of accident and to prevent the progress of breakdown of power train system. In this paper, the fault diagnosis method using high frequency low voltage injection was suggested to diagnose the demagnetization fault of rotor permanent magnet and the turn fault of stator winding. The proposed fault diagnosis method can be used to check the faults of permanent magnet synchronous motor during system check-up process at vehicle starting and idling stop mode. The feasibility and usefulness of the proposed method were verified by the finite element analysis.

• Journal of the Korean Society of Combustion
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• v.4 no.1
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• pp.95-103
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• 1999

Experimental study on combustion characteristics of a regenerative combustor has performed. High-temperature air combustion in the regenerative combustor is obtained through heat recovery from exhaust gas flow by porous ceramic materials and through alternation of air flow direction through the combustor. Temperature field, CO and NOx emission with respect to the frequency of alternation are measured. It is found that at initial stage of the alternation, temperature of inlet section of main combustion chamber is increased sharply since both high temperature air preheated by the ceramics and prompt fuel injection results in rapid combustion. Following this initial stage, combustion temperature is reduced as the preheated air temperature is reduced. However peak temperature in the chamber and exhaust gas temperature are decreased as the alternation period is reduced, increased temperature of ceramic is observed. CO and NOx emission with respect to the alternation period is also examined. It is found that there exists a range of optimum alternating period for CO and NOx emission characteristics.

• Journal of the Korean Institute of Illuminating and Electrical Installation Engineers
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• v.25 no.12
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• pp.82-87
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• 2011

In this paper, a sensorless speed control system is designed for the next generation high speed railway at zero and low speed region. The applied vector control scheme is a maximum torque per ampere(MTPA) method to utilize reluctance torque of IPMSM. The designed sensorless control scheme is a rotating high frequency voltage signal injection method. To verify the designed system, a simulator for the vector controller and sensorless controller is implemented using Matlab/simulink.

• Electrical & Electronic Materials
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• v.11 no.10
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• pp.94-99
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• 1998

Suitable replacement materials for ultrathin SiO2 in deeply scaled MOSFETs such as lattice polarizable films, which have much higherpermittivities than SiO2, have bandgaps of only 3.0 to 4.0 eV. Due to these small bandgaps, the reliability of these films as a gate insulator is a serious concern. Ramped voltage, time dependent dielectric breakdown, and hot carrier effect measurements were done on 190 layers of TiO2 which were deposited through the metal-organic chemical vapor deposition of titanium tetrakis-isopropoxide (TTIP). Measurements of the high and low frequency capacitance indicate that virtually no interface state are created during constant current injection stress. The increase in leakage upon electrical stress suggests that uncharged, near-interface states may be created in the TiO2 film near the SiO2 interfacial layer that allow a tunneling current component at low bias.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.37 no.12
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• pp.1489-1495
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• 2013

High-pressure flows are ubiquitous in many industrial fields. A representative application is fuel injection using a common-rail control system in diesel engines, where the injection pressure in the injector exceeds 1000 bar. In high-speed injection, the fluid injected through the nozzle undergoes breakup owing to the interaction with the ambient gas. The breakup process influences mixture formation, which in turn influences combustion in diesel engines. Therefore, it is very important to analyze the breakup process of fuel spray. The Reitz and Diwakar model and cascade atomization and breakup (CAB) model were used in this study as sub-models for the numerical analysis of the breakup process of fuel spray. This study aims to precisely analyze the breakup process of spray and to investigate the breakup frequency of the injected fuel. Consequently, it proposes a suitable sub-model for analyzing the breakup process of a diesel spray by using CFX, a commercial CFD program.

• Journal of Acupuncture Research
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• v.24 no.2
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• pp.187-201
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• 2007

Objectives: This study was to investigate the effect of high frequency electro-acupuncture (EA) at $ST_{36}$ acupoint on the Freund's Complete Adjuvant (FCA)-induced arthritis in rats. Methods : Arthritis was induced by intradermal injection of FCA into base of tail. Experimental groups were divided into 4 groups; Normal, Control, $ST_{36}$ and Non-Acupoint (NA). $ST_{36}$ group was treated by 120Hz EA at $ST_{36}$ acupoint. Body weight, paw edema volume and ankle joint thickness were measured after treatment. And we investigated the effects of 120Hz EA via WBC count, segmen neurophil, lymphocyte, PGE2 assay and NADPH-d histochemistry. Results and Conclusions : The mean of body weight and ankle joint thickness of $ST_{36}$ group was increased compared with control group. The mean of paw edema volume, WBC count, segment neutrophil and lymphocyte of were not decreased with control group. The mean of PGE2 concentration ($1,851.00{\pm}160.11$) and NADPH-d positive neurons ($13.12{\pm}1.23$) were changed significantly (p

• The Transactions of the Korean Institute of Electrical Engineers B
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• v.50 no.9
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• pp.453-458
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• 2001

This paper presents a new vector control algorithm of the surface mounted permanent magnet motor (SMPMM) without any rotational tranceducer. Originally, SMPMM does not have any magnetic saliency in structure, but it has a little magnetic saliency due to the saturation by the flux of the permanent magnet. Moreover, it varies according to the load conditions and the control performance of schematics using the saliency can be easily degraded. To prevent it and to improve the performance of the proposed algorithm, the saliency of a SMPMM under various load conditions is analyzed. In the proposed algorithm, the saliency or the impedance difference related to the saliency is utilized in order to estimate the position and speed of the rotor. And the high frequency signal is injected into the motor to measure the impedance difference and also to enhance the control performance of the system. The experimental results verify the performance of the proposed sensorless algorithm.

• ETRI Journal
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• v.34 no.3
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• pp.361-368
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• 2012

A complete high-frequency small-signal circuit model of a 40 Gb/s butterfly electroabsorption modulator integrated laser module is presented for the first time to analyze and optimize its electro-optic (E/O) response and reflection characteristics. An agreement between measured and simulated results demonstrates the accuracy and validity of the procedures. By optimizing the bonding wire length and the impedance of the coplanar waveguide transmission lines, the E/O response increases approximately 5% to 15% from 20 GHz to 33 GHz, while the signal injection efficiency increases from approximately 15% to 25% over 18 GHz to 35 GHz.

• Journal of IKEEE
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• v.23 no.2
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• pp.622-627
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• 2019

This paper propose an initial rotor position detection method for sensorless operation of a single-phase permanent magnet synchronous motor(SP-PMSM) with asymmetric air-gap. In general, the sensorless control based on back-emf estimation is difficult to estimate the back-emf at the zero and low speed regions. For this reason, an open loop start-up technique is indispensable, and it is also necessary to detect the initial position of the rotor in order to rotate in a certain direction. In this paper, we propose a method to detect rotor polarity by adding offset voltage to high frequency voltage signal based on the magnetic characteristics of SP-PMSM. The validity and usefulness of the proposed algorithm are verified through several experimental results.