• Journal of Power Electronics
• /
• v.15 no.5
• /
• pp.1305-1317
• /
• 2015

Distributed generation systems (DGSs) have been getting more and more attention in terms of renewable energy use and new generation technologies in the past decades. The self-excited induction generator (SEIG) occupies an important role in the area of energy conversion due to its low cost, robustness and simple control. Unlike synchronous generators, the SEIG has to absorb capacitive reactive power from the outer device aiming to stabilize the terminal voltage at load changes. This paper presents a novel static VAR compensator (SVC) called a magnetic energy recovery switch (MERS) to serve as a voltage controller in SEIG powered DGSs. In addition, many small scale SEIGs, instead of a single large one, are applied and devoted to promote the generation efficiency. To begin with, an expandable mathematic model based on a d-q equivalent circuit is created for parallel SEIGs. The control method of the MERS is further improved with the objective of broadening its operating range and restraining current harmonics by parameter optimization. A hybrid control strategy is developed by taking both of the stand-alone and grid-connected modes into consideration. Then simulation and experiments are carried out in the case of single and double SEIG(s) generation. Finally, the measurement results verify that the proposed DGS with SVC-MERS achieves a better stability and higher feasibility. The major advantages of the mentioned variable reactive power supplier, when compared to the STATCOM, include the adoption of a small DC capacitor, line frequency switching, simple control and less loss.

• Journal of Applied Reliability
• /
• v.14 no.4
• /
• pp.213-219
• /
• 2014

Wind turbine, which is attracting part of the renewable energy and is researching continuously, is going to be large size for high efficiency. There is a yaw system rotating the nacelle, weighted about 600 tons, to be perpendicular with the wind direction blowing in the large wind turbine. The wind turbine is focusing on the reliability improvement because working environment effect is bigger than any other points and specially, the reliability improvement of the yaw drive is required by the customer because it is the key component of the wind turbine. Because of this, the establishment of criteria for yaw drive is required because yaw drive system is the part of the wind turbine closely related with ensuring the reliability. So, this study did the failure analysis of the yaw drive system, which is consisted with 10 sets of yaw drives through researching and analyzing the site conditions. Also this study designed the life test method based on the failure analysis and working condition of the yaw drive. To design the accelerated life test of the yaw drive, this study reviewed the torque, lubrication condition, and frequency of use and etc. Finally, this selected the torque as the acceleration factor which is affected mainly to the system and also, the test equipment was developed based on the requirement of the life and performance test.

• Korean Journal of Human Ecology
• /
• v.19 no.1
• /
• pp.103-117
• /
• 2010

The purpose of this study was broken down into four sections. First, this study identified both single workers' lifestyles and their motives for going to restaurants. The study classified the consumers and then confirmed the attributes of selection that the consumers considered important when eating out at family restaurants. Second, the study analyzed the consumers' lifestyles, motives for going to restaurants (user motive), and the attributes of selection for the restaurant. Third, the study analyzed how the attributes of selection toward the family restaurant affect the consumer's satisfaction and the consumer's intention of revisiting the restaurant. Fourth, the study analyzed how consumer satisfaction affects the intention to revisit the family restaurant. After the execution of the aforementioned four steps, the study was successful in meeting its purpose: to provide information to enhance the satisfaction of the consumer and to provide foundation material for consumer policy. In regards to the analytical method, this study performed a Frequency Analysis, Factor Analysis, Reliability Analysis, Cluster Analysis, ANOVA Analysis, and Multiple Regression Analysis. The analytical results from the study are as follows. Lifestyle was categorized into 4 factors and 4 clusters. User motive was grouped into 3 factors and 4 clusters. The attributes of selection for family restaurants were categorized into 4 factors. Based on the lifestyle cluster, the attributes of selection for family restaurants showed a statistically significant difference. In addition, based on the cluster of user motive, the attributes of selection for family restaurants showed a statistically significant difference. Lastly, the attributes of selection for family restaurants were found to affect both the intention of revisiting the restaurant and consumer satisfaction after the consumers used the restaurants.

• Journal of the Korean Society for Aeronautical & Space Sciences
• /
• v.32 no.10
• /
• pp.93-101
• /
• 2004

A method to calculate stability limits is investigated to predict the characteristics of high-frequency combustion instability in liquid-propellant rocket engine. It is based on the theory of linear stability analysis proposed in previous works and useful to predict combustion stability at the beginning stage of engine development. The system of equations governing reactive flow in combustor has the simplified and linearized forms. The overall equation expressing stability limits is adopted. The procedures to evaluate quantitatively each term included in the equation are proposed. The thermo-chemical properties and flow variables required in the evaluation can be obtained from calculation of thermodynamic equilibrium, CFD results, and experimental test data. Based on the existent data, stability limits are calculated with actual rocket engine (KSR-III rocket engine). The present calculations show the reasonable stability limits in a quantitative manner and the stability characteristics of the engine are discussed. The prediction from linear stability analysis could be serve as the first approximation to the true prediction.

• The Mathematical Education
• /
• v.56 no.3
• /
• pp.281-300
• /
• 2017

This study analyzed the difficulties of middle school students in computer simulation of the law of large numbers through eye movement analysis. Some students did not attend to the simulation results and could not make meaningful inferences. It is observed that students keep the existing concept even though they observe the simulation results which are inconsistent with the misconceptions they have. Since probabilistic intuition influence student's thinking very strongly, it is necessary to design a task that allows students to clearly recognize the difference between their erroneous intuitions and simulation results. In addition, we could confirm through eye movements analysis that students could not make meaningful observations and inferences if too much reasoning was needed even though the simulation included a rich context. It is necessary to use visual representations such as graphs to provide immediate feedback to students, to encourage students to attend to the results in a certain intentional way to discover the underlying mathematical structure rather than simply presenting experimental data. Some students focused their attention on the visually salient feature of the experimental results and have made incorrect conclusion. The simulation should be designed so that the patterns of the experimental results that the student must discover are not visually distorted and allow the students to perform a sufficient number of simulations. Based on the results of this study, we suggested that cumulative relative frequency graph showing multiple results at the same time, and the term 'generally tends to get closer' should be used in learning of the law of large numbers. In addition, it was confirmed that eye-tracking method is a useful tool for analyzing interaction in technology-based probabilistic learning.

• Journal of Korea Multimedia Society
• /
• v.8 no.3
• /
• pp.362-371
• /
• 2005

This paper presents a digital video broadcasting - terrestrial (DVB-T) receiver system based on the orthogonal frequency division multiplexing (OFDM) modulation method, which has exhibited a good reception performance even with obstacles and a mobile reception. As such, an improved OFDM receiver is developed for a DVB-T system that also considers function expansion for further development. After manufacturing the DVB-T receiver system, the performance of the proposed system is compared with three other hardware systems, all of which are end products. The experimental results confirm the performance using the measured minimum required carrier-to-noise ratio and threshold of visibility signal for each system. In addition, a graphic user interface (GUI) and electronic program guide (EPG) are developed for the digital television user.

• Journal of Information Technology Services
• /
• v.19 no.1
• /
• pp.71-87
• /
• 2020

As diverse online information services are introduced and the use of them are increasing, the importance of the quality control over the online information services is also increasing. Therefore, studies are being attempted to derive quality evaluation models for online information services, but these showed many limitations. In order to overcome the limitations of the previous studies and create more appropriate quality evaluation model, this study analyzed 408 data collected through the interviews with the users of Korea national online information services (NDSL, NTIS) using content analysis and the coding method of ground theory. Through the analysis, we identified the quality evaluation model (13 elements, 6 dimensions) of online information services and the description and insights of each elements. And the priorities of the most significant element(amount of information) and other key elements(such as the usefulness of the information, the diversity of the field, etc.) were figured out through the frequency analysis. Through these inductive methods based on the responses of users, the quality control model could be derived, which is different from the previous researches. This study is the first study which derived the quality evaluation model of online information service through the inductive way based on 408 interview responses from users. Therefore, the results of this study will be helpful to the post researchers who conduct studies on the quality of online information service, etc. Moreover, the results of this study are expected to provide references of frame for decision-making of working-level officials who have to manage the quality of online information services.

• Korean Journal of Cognitive Science
• /
• v.11 no.2
• /
• pp.19-35
• /
• 2000

Learning by examples has proven to be an efficient method in mastering various subjects including programming languages. This study hypothesizes that the number of examples and the type of examples are two significant dimensions that influence the performance of learning programming languages by examples. A set of experiments was conducted to investigate the impacts of the two dimensions in the domain of JAVA programming. The results showed that providing two examples is more effective than providing only one example even though significantly more explanations are attached to the single example. Among the 'two-example' groups, the group that was given functionally similar examples performed better than those with functionally dissimilar examples. Explanations for these results are provided in this paper based on the behavioral patterns of individual subjects in terms of time and frequency. This paper concludes with the implications of the study results for the development of effective tutoring systems for programming languages.

• Journal of the Korea Institute of Information and Communication Engineering
• /
• v.15 no.9
• /
• pp.1971-1978
• /
• 2011

In RFID-based cold-storage systems, product locations are recognized as the logical locations of RFID readers which are attached to the entrance of the cold rooms. However, through these methods, product locations can be incorrect in many different situations of a storage. Cold storage rooms have a huge rage in temperature, and the product in the cold-storage spoils easily. Therefore, highly precise product locations is very important for the product quality In this paper, a new approach is suggested to manage the storage location inaccuracy by adding two readers to existing forklift and attaching RFID tags to the ceiling. One reader recognizes ceiling tags to acquire location data, and the other recognizes product tags to acquire product detail data loaded on the forklift. The product can be seamlessly tracked and traced in realtime by monitoring and analyzing data gathered from these readers. Through the proposed location management method, the effectiveness of the product management in cold-storage can be improved consequently.

• The KIPS Transactions:PartB
• /
• v.15B no.2
• /
• pp.87-94
• /
• 2008

In this paper, we propose a new vision-based fire detection method for a real-life application. Most previous vision-based methods using color information and temporal variation of pixel produce frequent false alarms because they used a lot of heuristic features. Furthermore there is also computation delay for accurate fire detection. To overcome these problems, we first detected candidated fire regions by using background modeling and color model of fire. Then we made probabilistic models of fire by using a fact that fire pixel values of consecutive frames are changed constantly and applied them to a Bayesian Network. In this paper we used two level Bayesian network, which contains the intermediate nodes and uses four skewnesses for evidence at each node. Skewness of R normalized with intensity and skewnesses of three high frequency components obtained through wavelet transform. The proposed system has been successfully applied to many fire detection tasks in real world environment and distinguishes fire from moving objects having fire color.