• Journal of Electrical Engineering and Technology
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• 제12권6호
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• pp.2317-2323
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• 2017

Recently, linear compressors of cooling systems such as refrigerators, which have a free piston driven by a linear motor, have attracted much attention because of their high efficiency. For structural reasons, linear compressors applied in refrigerators should use an AC capacitor to ensure stable control. However, AC capacitors are expensive and bulky. In this paper, we propose a new method to realize stable control without a real AC capacitor by implementing a virtual AC capacitor with software. To realize a virtual AC capacitor, a pure integral is calculated. Nonetheless, if an offset current exists, the calculation may diverge to infinity. To solve this problem, a high-pass filter is applied and the compensation for the phase angle and magnitude are realized with a new method. Finally, a virtual AC capacitor enables variable frequency operations. Hence, in case of a lack of voltage, we can compensate by running the linear compressor in high-frequency operations. To improve efficiency, we may optimize the operation frequency. The validity of a virtual AC capacitor has been verified through simulations and experimental results.

• 전력전자학회:학술대회논문집
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• 전력전자학회 1998년도 연구회 합동 학술발표회 논문집
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• pp.60-63
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• 1998

The widely used power supply (Switched Mode Power Supply : SMPS) as a source in order to stabilize direct current for electronics or communication systems has merits, when it is compared to the existing source for stability, such as high efficiency, small size, light weight by means of switching process of the semiconductor device which controls the flow of power. However, due to existence of inductors and capacitors used for charging energy, the source part in electronic or communication systems hasn't reached the speed, that is supposed to get, for achieving smaller size and lighter weight. In order to get smallness in size, it is necessary to increase switching frequency. And that makes devices for measuring energy smaller. Nevertheless, the rise switching frequency brings increases in switching loss, inductor loss, and power loss. Also, the occurrence of surge and noise caused by high frequency switching is getting higher. The resonant converter has been considered as one of methods that give solutions for the problems of SMPS and that method have been paid attention as a source technology in electronics and communication.

• Journal of Power Electronics
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• 제9권2호
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• pp.180-187
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• 2009

When converters operate in megahertz range, the passive components and magnetic devices generate high losses. However, the eddy current issues and choices of magnetic cores significantly affect on the design stage. Apart from that, the components' reduction, miniaturization technique and frequency scaling are required as well as improvement in thermal capability, integration technique, circuit topologies and PCB layout optimization. In transformer design, the winding and core losses give great attention to the design stage. From simulation work, it is found that E-25066 material manufactured by AVX could be the most suitable core for high frequency transformer design. By employing planar geometry topology, the material can generate significant power loss savings of more than 67% compared to other materials studied in this work. Furthermore, young researchers can use this information to develop new approaches based on concepts, issues and methodology in the design of magnetic components for high frequency applications.

• Journal of Electrical Engineering and Technology
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• 제9권4호
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• pp.1369-1374
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• 2014

Most accidents of medium-voltage cables installed in nuclear power plants result from the initial defect of internal insulators or the initial failure due to poor construction. However, as the service years of plants increase, the possibility of cable accidents is also rapidly increases. This is primarily caused by electric, mechanical, thermal, and radiation stresses. Recently, much attention is paid to the study of cable diagnoses. To date, partial discharge and Tan${\delta}$ measurements are known as reliable methods to diagnose the aging of medium-voltage cables. High frequency partial discharge measurement techniques have been widely used to diagnose cables in transmission and distribution systems. However, the on-line high frequency partial discharge technique has not been used in the nuclear power plants because of the plant shutdown risk, degraded measurement sensitivity, and application problems. In this paper, the partial discharge measurement with a portable device was tried to evaluate the integrity of the 4.16kV and 13.8kV cable lines. The test results show that the high detection sensitivity can be achieved by the high frequency partial discharge technique. The present technique is highly attractive to diagnose medium voltage cables in nuclear power plants.

• 한국정밀공학회:학술대회논문집
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• 한국정밀공학회 2006년도 춘계학술대회 논문집
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• pp.151-152
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• 2006

Nowadays, great attention has been shown to the question of ultrasonic linear motor for accomplishing the high positioning accuracy and high driving force in the semiconductor and optical industry. Ultrasonic linear motors have many advantages such as simple structure, quick response, ability to maintain position without energy consumption, and electromagnetic effect. And BLT has attracted attention to accomplish large vibration amplitude and large mechanical force. Authors designed and developed the new type of ultra sonic linear motor with multi support mechanism, achieved 75N of output force and 0.45m/s of velocity.

• 말소리와 음성과학
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• 제15권1호
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• pp.1-7
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• 2023

This study investigated the influences of probabilistic accessibility of the word being produced - as determined by its usage frequency and neighborhood density - on native and high-proficiency L2 speakers' realization of six English monophthong vowels. The native group hyperarticulated the vowels over an expanded acoustic space when the vowel occurred in words with low frequency and high density, supporting the claim that vowel forms are modified in accordance with the probabilistic accessibility of words. However, temporal expansion occurred in words with greater accessibility (i.e., with high frequency and low density) as an effect of low phonotactic probability in low-density words, particularly in attended speech. This suggests that temporal modification in the opposite direction may be part of the phonetic characteristics that are enhanced in communicatively driven focus realization. Conversely, none of these spectral and temporal patterns were found in the L2 group, thereby indicating that even the high-proficiency L2 speakers may not have developed experience-based sensitivity to the modulation of sub-categorical phonetic details indexed with word-level probabilistic information. The results are discussed with respect to how phonological representations are shaped in a word-specific manner for the sake of communicatively driven lexical intelligibility, and what factors may contribute to the lack of native-like sensitivity in L2 speech.

• 한국의류학회지
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• 제25권6호
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• pp.1025-1036
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• 2001

This paper explores the effects of price discount frequency on consumer evaluation of clothing brand equity as well as the impact on consumers who exhibit different consumer characteristics. Following a preliminary investigation, two clothing brand groups were identified according to the frequency of their price discounts, one with a high frequency of price discounts, the other with a low frequency. Each brand group consisted of three women's clothing brands. A questionnaire was developed and administered to 351 females between the ages of nineteen and thirty-four in October 2000. The questionnaire included questions on evaluations on clothing brand equity (i.e., perceived quality, brand image, perceived value, reliability and brand knowledge), on consumer characteristics (i.e., clothing knowledge, clothing evaluative criteria, price perception and demographic characteristics), as well as on price discount frequency. Statistical tests, such as factor analysis. t-test and pearson's correlation, were used to analyze the data. Results of this research showed that price discount frequency negatively impacted consumer perception of clothing brand equity. The negative influence was particularly strong on the perceived quality and brand image dimensions of brand equity. The findings also suggested the effect of price discount frequency on consumer assessment of clothing brand equity was related to consumer characteristics as well. The equity of the clothing brand group with a low frequency of price discounts was positively related to clothing knowledge, price-quality inference, price-prestige inference, sale prone-ness. price mavenism, valuing the fashionable and symbolic uses of clothing and family income. On the other hand, it was negatively related to low price consciousness. The equity of the clothing brand group with a high frequency of price discounts was positively related to utilitarian economic aspects of clothing values as well as to low price consciousness. To establish and maintain high brand equity, marketers must pay attention to the frequency of price discount as it may have a negative impact on clothing brand equity.

• 전기학회논문지P
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• 제60권2호
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• pp.76-81
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• 2011

Power system frequency is the main index of power quality indicating an abnormal state and disturbances of systems. The nominal frequency is deviated by sudden change in generation and load or faults. Power system is used as frequency relay to detection for off-nominal frequency operation and connecting a generator to an electrical system, and V/F relay to detection for an over-excitation condition. Under these circumstances, power system should maintain the nominal frequency. And frequency and frequency deviation should accurately measure and quickly estimate by frequency measurement device. The well-known classical method, frequency estimation technique based on the DFT, could be produce the gain error in accuracy. To meet the requirements for high accuracy, recently Wavelet transforms and analysis are receiving new attention. The Wavelet analysis is possible to calculate the time-frequency analysis which is easy to obtain frequency information of signals. However, it is difficult to apply in real-time implementation because of heavy computation burdens. Nowadays, the computational methods using the Wavelet function and transformation techniques have been searched on these fields. In this paper, we apply the Recursive Discrete Wavelet Transform (RDWT) for the frequency estimation. In order to evaluate performance of the proposed technique, the user-defined arbitrary waveforms are used.

• 한국전기전자재료학회:학술대회논문집
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• 한국전기전자재료학회 2003년도 추계학술대회 논문집 Vol.16
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• pp.587-590
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• 2003

The first attention in designing a transformer for low temperature rise should be to reduce losses. Leakage inductance and temperature rise are two of the more impotent problems facing the magnetic core technology of today's high frequency transformers. Excessive leakage inductance increases the stress on the switching transistors and limits the duty-cycle, and excessive temperature rise can lead the design limitation of high frequency transformer with high current. The flat transformer technology provides a very good solution to the problems of leakage inductance and thermal management for high frequency power. The critical magnetic components and windings are optimized and packaged within a completely assembled module. The turns ratio in a flat transformer is determined as the product of the number of elements or modules times the number of primary turns. The leakage inductance increase proportionately to the number of elements, but since it is reduced as the square of the turns, the net reduction can be very significant. The flat transformer modules use cores which have no gap. This eliminates fringing fluxes and stray flux outside of the core. The secondary windings are formed of flat metal and are bonded to the inside surface of the core. The secondary winding thus surrounds the primary winding, so nearly all of the flux is captured.

• 한국진공학회:학술대회논문집
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• 한국진공학회 2014년도 제46회 동계 정기학술대회 초록집
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• pp.292-292
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• 2014

Wide band gap semiconductors, such as III-nitrides (GaN, AlN, InN, and their alloys), SiC, and diamond are expected to play an important role in the next-generation electronic devices. Specifically, GaN-based high electron mobility transistors (HEMTs) have been targeted for high power, high frequency, and high temperature operation electronic devices for mobile communication systems, radars, and power electronics because of their high critical breakdown fields, high saturation velocities, and high thermal conductivities. For the stable operation, high power, high frequency and high breakdown voltage and high current density, the fabrication methods have to be optimized with considerable attention. In this study, low ohmic contact resistance and smooth surface morphology to AlGaN/GaN on 2 inch c-plane sapphire substrate has been obtained with stepwise annealing at three different temperatures. The metallization was performed under deposition of a composite metal layer of Ti/Al/Ni/Au with thickness. After multi-layer metal stacking, rapid thermal annealing (RTA) process was applied with stepwise annealing temperature program profile. As results, we obtained a minimum specific contact resistance of $1.6{\times}10^{-7}{\Omega}cm2$.