• Journal of Power Electronics
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• v.11 no.3
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• pp.294-303
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• 2011

In this paper an effective five-phase induction motor (IM) and its drive methods are proposed. Due to the additional degrees of freedom, the five-phase IM drive presents unique characteristics for enhancing the torque producing capability of the motor. Also the five-phase motor drives possess many other advantages when compared to traditional three-phase motor drives. Some of these advantages include, reducing the amplitude and increasing the frequency of the torque pulsation, reducing the amplitude of the current without increasing the voltage per phase and increasing the reliability. In order to maximize the torque per ampere, the proposed motor has concentrated winding, the produced back electromotive force (EMF) is almost trapezoidal, and the motor is supplied with the combined sinusoidal plus the third harmonic of the currents. For demonstrating the superior performance of the proposed five-phase IM, the motors are also analyzed on the synchronously rotating reference frame. To supply trapezoidal current waveform and to exclude the effect of the $3^{rd}$ harmonic current, a new control stratagem is proposed. The proposed control method is based on direct torque control (DTC) and rotor flux oriented control (RFOC) of the five-phase IM drives. It is able to reduce the acoustical noise, the torque, the flux, the current, and the speed pulsations during the steady state. The DTC transient merits are preserved, while a better quality steady-state performance is produced in the five phase motor drive for a wide speed range. Experimental results clearly demonstrated a more dynamic steady state performance with the proposed control system.

• The Transactions of the Korean Institute of Power Electronics
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• v.15 no.2
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• pp.134-141
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• 2010

Small PEM (Proton Exchange Membrane) fuel cell systems do not require humidification and have great commercialization possibilities. However, methods for controlling small PEM fuel cell stacks have not been clearly established. In this paper, a control method for small PEM fuel cell systems using a dual closed loop with a static feedforward structure is defined and realized using a DSP (Digital Signal Processor). The fundamental elements that need to be controlled in fuel cell systems include the supply of air and hydrogen, water management inside the stack, and heat management of the stack. For small PEM fuel cell stacks operated without a separate humidifier, fans are essential for air supply, heat management, and water management of the stack. A purge valve discharges surplus water from the stack. The proposed method controls the fan using double control loops to quicken transient response of the fan thereby improving the supply rate of air. Feedback control to compensate for the voltage change in fuel cell stack improves the response characteristics in fuel cell to load variations. The feasibility of proposed method was proved by the experiments with a 60W small PEM fuel cell system and operation of a notebook computer using this system.

• Nuclear Engineering and Technology
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• v.50 no.2
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• pp.246-252
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• 2018

A series of simulated reactivity-initiated accident (RIA) tests on irradiated fully recrystallized boiling water reactor Zircaloy-2 cladding has been performed by means of the expansion-due-to-compression (EDC) test method. The EDC method reproduces fuel pellet-clad mechanical interaction (PCMI) conditions for the cladding during RIA transients with respect to temperature and loading rates by out-of-pile mechanical testing. The tested materials had a large variation in burnup and hydrogen content (up to 907 wppm). The results of the EDC tests showed variation in the PCMI resistance of claddings with similar burnup and hydrogen content, making it difficult to clearly identify ductile-to-brittle transition temperatures. The EDC-tested samples of the present and previous work were investigated by light optical and scanning electron microscopy to study the influence of factors such as azimuthal variation of the Zr-hydrides and the presence of hydride rims and radially oriented hydrides. Two main characteristics were identified in samples with low ductility with respect to hydrogen content and test temperature: hydride rims and radial hydrides at the cladding outer surface. Crack propagation and failure modes were also studied, showing two general modes of crack propagation depending on distribution and amount of radially oriented hydrides. It was concluded that the PCMI resistance of irradiated cladding under normal conditions with homogenously distributed circumferential hydrides is high, with good margin to the RIA failure limits. To further improve safety, focus should be on conditions causing nonfavorable hydride distribution, such as hydride reorientation and formation of hydride blisters at the cladding outer surface.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.38 no.2
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• pp.193-203
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• 2014

Temperature-dependent stress analysis and heat transfer analysis of a rotating gas turbine blade made of functionally graded materials (FGMs) are presented considering turbine operating temperature and ceramic particle size. The material properties of functionally graded materials are assumed to vary continuously and smoothly across the thickness of the thin-walled blade. For obtaining system stiffness reflecting these characteristics, the one-dimensional heat transfer equation is applied along the thickness of the thin-walled blade for determining the temperature distribution. Using the results of the temperature analysis, the equations of motion of a rotating blade are derived with hybrid deformation variable modeling method along with the Rayleigh-Ritz assumed mode methods. The validity of the derived rotating blade model is evaluated by comparing its transient responses and temperature distribution with the results obtained using a commercial finite element code. The maximum tensile stress with operating speed and gradient index are obtained. Furthermore, the gradient index that minimizes blade temperature was investigated.

• Fire Science and Engineering
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• v.31 no.5
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• pp.12-18
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• 2017

The present study has been conducted to predict the mass burning flux of methanol pool fire using liquid vaporization model in FDS and examine the effect of thermal properties of liquid fuel such as radiative fraction and mean absorption coefficient. A series of calculation for the pool diameter of 5 cm to 200 cm were performed and the size of computational domain was determined by the scale of the pool diameter. The reference grid size was determined by the grid sensitivity analysis and the computational grids consisted of approximately 750,000 cells. For the methanol pool fire, the mass burning flux predicted by liquid vaporization model of FDS followed the trend of transient characteristics as a function of pool diameter and showed good agreement within measurement uncertainty range of previous studies. The mass burning flux increased with increasing the radiative fraction and the mean absorption coefficient greatly affected on relatively small pool diameter.

• Journal of Yeungnam Medical Science
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• v.31 no.2
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• pp.89-93
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• 2014

Background: This study was conducted to investigate the epidemiological features, clinical courses, and diagnoses of neonates who are transferred to neonatal intensive care unit of Yeungnam University Hospital due to tachypnea soon after birth. Methods: Based on medical records, we performed a retrospective study of neonatal intensive care unit admissions due to tachypnea from January 2010 to December 2013. Results: A total of 311 neonates were included in this study. The patient characteristics showed male predominance at 2.65:1. Among the 311 neonates with tachypnea, 127 (40.8%) neonates needed oxygen supply, and 54 (17.4%) neonates needed assisted mechanical ventilation. Transient tachypnea of the newborns (TTN) (158, 50.8%) showed the highest incidence, followed by pneumonia (63, 20.3%), extrapulmonary infection (37, 11.9%), respiratory distress syndrome (21, 6.8%), air leak (16, 5.1%), meconium aspiration syndrome (12, 3.9%), congenital heart disease (5, 1.6%), metabolic acidosis (3, 1%), primary pulmonary hypertension of newborns (2, 0.6%) and anemia (2, 0.6%). Conclusion: Although the neonates with tachypnea showed no other respiratory distress symptom, clinicians should be aware of the possibility of other pulmonary diseases as well as TTN and their extra-pulmonary causes. If tachypnea does not improve within a few hours, the clinician should consider further evaluation and management as soon as possible.

• Proceedings of the Korea Water Resources Association Conference
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• 2017.05a
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• pp.260-260
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• 2017

This study aims to compare the performance of TRIGRS (Transient Rainfall Infiltration and Grid-based Regional Slope-stability model) and TiVaSS (Time-variant Slope Stability model) in the prediction of rainfall-induced shallow landslides. TRIGRS employs one-dimensional (1-D) subsurface flow to simulate the infiltration rate, whereas a three-dimensional (3-D) model is utilized in TiVaSS. The former has been widely used in landslide modeling, while the latter was developed only recently. Both programs are used for the spatiotemporal prediction of shallow landslides caused by rainfall. The present study uses the July 2011 landslide event that occurred in Mt. Umyeon, Seoul, Korea, for validation. The performance of the two programs is evaluated by comparison with data of the actual landslides in both location and timing by using a landslide ratio for each factor of safety class ( index), which was developed for addressing point-like landslide locations. In addition, the influence of surface flow on landslide initiation is assessed. The results show that the shallow landslides predicted by the two models have characteristics that are highly consistent with those of the observed sliding sites, although the performance of TiVaSS is slightly better. Overland flow affects the buildup of the pressure head and reduces the slope stability, although this influence was not significant in this case. A slight increase in the predicted unstable area from 19.30% to 19.93% was recorded when the overland flow was considered. It is concluded that both models are suitable for application in the study area. However, although it is a well-established model requiring less input data and shorter run times, TRIGRS produces less accurate results.

• Journal of the Korean Society for Railway
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• v.20 no.3
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• pp.320-328
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• 2017

In order to analyze the influence on derailment safety according to the aging of primary rubber springs widely applied to railway vehicles, characteristic tests were carried out on aged primary rubber spring samples. To analyze the effect of primary rubber spring aging on derailment safety, a vehicle dynamic analysis was carried out. The results of the vertical characteristics test for the rubber spring specimens with 17 years of service life revealed that the displacement restoration function was degraded due to rubber aging and the spring stiffness significantly increased. The results of the running dynamic analysis simulating the twist track running in accordance with the EN14363 standard, compared with the normal vehicle model (Case 1), showed that the derailment coefficient and the wheel unloading of the vehicle model (Case 2) using the aging primary spring characteristic increased, and the derailment safety was degraded. IN particular, it was found that the derailment safety due to the reduction of the wheel load is weak in the transient section where a steep slope change occurs.

• Perinatology
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• v.29 no.4
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• pp.159-164
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• 2018

Objective: To compare neonatal respiratory morbidity of twins according to birth order related to gestational age and mode of delivery. Methods: We performed the retrospective research of the medical records of 3,224 neonates (1,612 twin pairs) born in a single center from January 2011 to December 2015. Subjects were classified into four gestational age groups: very (<32 weeks), moderate (32-33 weeks), late (34-36 weeks) preterm, and term (${\geq}37weeks$) groups. We investigated clinical characteristics and respiratory morbidity according to birth order related to gestational age group and mode of delivery. Results: We found increased risk of respiratory morbidity in second-born twin than first-born twin (P=0.039). Second-born twin was associated with increased risk of respiratory distress syndrome (RDS) in late preterm group (odds ratio [OR] 2.8, 95% confidence interval [CI] 1.30-5.95), and transient tachypnea of newborn in term group (OR 1.4, 95% CI 1.03-1.81). In vaginal delivery mode, there was no difference of respiratory morbidity between first and second-born twin in each group, but in cases of Cesarean delivery, second-born twin was related with a greater risk of RDS in late preterm group (OR 2.3, 95% CI 1.07-5.09). Birth order and Cesarean section independently increased the risk of RDS (adjusted OR [aOR] 1.69, 95% CI 1.12-2.54; aOR 2.14, 95% CI 1.25-3.66, respectively). Conclusion: Second-born twin and Cesarean delivery are associated with increased risk of RDS, especially in late preterm twins.

• Explosives and Blasting
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• v.37 no.1
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• pp.14-23
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• 2019

It has been an always issue for the blasting or the impact analysis to consider the strength characteristics of the rock materials associate with loading rate dependency. Due to the nature of transient loading, the dynamic rock test requires a careful technique to achieve the stress equilibrium state of the specimen. In this study, to investigate the relationship between the rock dynamic strength and the stress equilibrium state, a series of dynamic uniaxial compression tests for Pocheon granite were performed. As a result, the unbalanced stress state on the specimen can lead to the premature failure on the specimen and the less estimation of dynamic strength characteristic as well as the overestimation of strain rate. Consequently, a careful consideration of rock fracture process to achieve the dynamic force balance on the specimen should be required to make an reasonable evaluation of rock dynamic strength.