Jo Aug 22, 2025
Induction heating generators are series resonant inverters (SRI) or parallel resonant inverters (PRI) in which the resonant tank consists of an induction coil and a capacitor. They are used to heat metals to be welded, melted, brazed or hardened.
In order to regulate the output power, SRI uses a diode bridge rectifier as a DC voltage source, and employs frequency (FC), phase-shift (PS) variation or pulse density modulation (PDM). These power control schemes, however, may result in an increase of switching losses and electromagnetic noise because it is impossible for switching devices to be always turned on and off under zero-current condition.
Therefore, only MOSFET inverters can be used in high-frequency induction heating applications. Nevertheless, isolated-gate bipolar transistors (IGBTs) are preferred in high-power industrial applications.
Ri Nam Jin, a researcher at the Faculty of Automation Engineering, has proposed an induction heating system of 15kW and 30kHz for industrial applications, which uses a novel control scheme based on a PS IGBT full-bridge SRI that allows us to significantly improve its reliability by increasing the lifetime of IGBTs.
In this power control mode, load adaptive variable frequency regulation and automated dead-time management are performed simultaneously in order to ensure that the inverter ensures zero voltage switching at all power levels and load conditions.
The comparative study between the proposed method and standard PS power control has demonstrated the effectiveness of the proposed one.
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Jo Aug 21, 2025
354 aluminum cast alloy is widely used for the manufacture of complex parts in various fields such as automotive industry due to its light weight and high strength. However, the tensile properties of this alloy, like other Al-Si alloys, decline rapidly at the temperatures above 300℃.
Up to date, many researchers have carried out numerous studies to improve the microstructure and high-temperature tensile properties of Al-Si-Cu-Mg alloys, by micro additions of alloying elements such as Zr, Ni and Ce and heat treatments. However, no studies have focused on determining the optimum addition amounts of Zr, Ni and Ce and heat treatment process parameters to maximize the tensile properties of 354 alloy at 300℃.
Ri Wi Song, a researcher at the Faculty of Materials Science and Technology, has conducted a study to determine the optimum addition amounts of Zr, Ni and Ce and heat treatment process parameters for the maximum tensile properties at 300℃ of 354 alloy, using TOPSIS method and Taguchi method.
As a result, he has found that the optimum addition amounts and heat treatment process parameters are as follows: Zr 0.3%, Ni 0.2%, Ce 0.1%, SHT (solution heat treatment) temperature 510℃, SHT time 10h, aging temperature 180℃ and aging time 6h.
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Jo Aug 20, 2025
Efficient block assembly scheduling is very important for building ships in the shortest possible time with efficient use of the facilities and resources of shipyards. Process planning is usually focused on determining the precedence constraints and weight factors of appropriate assembly units and tasks, but previous studies have mainly dealt with manufacturing processes such as welding.
The hull is a complex structure, so the deformation and stress generated during welding are also complex. The confinement of deformation during welding will increase residual stress, whereas the freedom of deformation will increase residual strain, making it difficult to assemble hull blocks. Therefore, the longitudinal strength of a hull must be taken into account when assembling it.
Ro Un Hyok, a researcher at the Faculty of Shipbuilding and Ocean Engineering, has proposed a process planning method based on the determination of precedence constraints and weighting factors for hull block assembly scheduling, taking into account the longitudinal strength of hulls.
According to the practical situation where precedence constraints and machine constraints exist, he proposed a mathematical model for hull block assembly scheduling, and a solution algorithm based on Lagrangian relaxation.
By addressing the precedence constraints based on assembly sequence and weight factor determination according to welding method in the hull block assembly process, he has laid a theoretical basis for process planning for block assembly scheduling, and he developed mathematical modeling and solution methods for these problems.
The results of the practical examples show that the proposed method provides rapid and effective solutions and it can be applied to the industry.
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Jo Aug 19, 2025
The separation of acid gases from gas mixtures and waste gases is very important for environmental protection. Acid gases which can form an acidic solution when mixed with water include carbon dioxide (CO2), hydrogen sulfide (H2S), and sulfur dioxide (SO2).
Sulfur dioxide (SO2) is generated mainly from combustion of sulfur-containing fossil fuels such as coal or oil. It is also produced from some production processes such as fertilizer production, aluminum smelting and steel production. Sulfur dioxide is a major component of acid rain because it reacts with water vapor in the atmosphere to produce sulfuric acid (H2SO4).
Therefore, there have been many attempts to remove SO2. Among them, membrane-based gas separation is of great importance in the chemical industry due to its simplicity, ease of operation, reduced energy consumption and compact structure.
Ri Chang Ryop, a researcher at the Institute of Nano Science and Technology, has conducted a mass transfer simulation to remove sulfur dioxide (SO2) using tubular membranes, and proposed a general mass transfer model to predict the absorption of SO2 into N,N-dimethylaniline solvent.
He solved the governing equations including continuity and momentum equations by computational fluid dynamics (CFD) methods. In order to obtain the concentration distribution of SO2 in the membrane, he investigated the effect of parameters on the performance of the membrane by simulating the behavior of the vapor and liquid phases.
The simulation results showed that the removal of SO2 increased with decreasing gas velocity at the membrane surface.
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Jo Aug 18, 2025
Non-destructive testing will estimate the size, shape and state of defects without damaging the structures. Non-destructive testing methods include eddy current inspection (ECT), ultrasonography, X-ray inspection, etc. Ultrasonography shows good accuracy and reproducibility, but it needs direct contact with specimens and special liquids, which means complicated procedure and high cost. X-ray inspection has the disadvantage that operators can be exposed to irradiation. Although eddy current technology has the advantages of relatively simple structure, no contact with test specimens, low cost and fast testing, it still has a disadvantage. If sine wave is used as an excitation current, the increase in power can cause the excitation coil to be severely heated and damaged, and increasing frequency limits the depth to detect defects.
Pulsed eddy current (PEC) technique can overcome these limitations. PEC technique is widely used for contactless inspection of large areas of pressure body or tube made of ferromagnetic materials.
What is important in detecting internal defects of nonmagnetic materials by PEC technology is to increase the excitation power and the sensitivity. However, some parameters such as pulse amplitude and width cannot be increased constantly. Differential signals of defective and non-defective parts must be obtained to identify defects. The key to designing PEC probes is to optimize the magnetic field near defects to the maximum.
Jo Kwang Myong, a researcher at the Faculty of Mechanical Science and Technology, has built a finite element model of a concentric sensor and obtained the type of a relatively sensitive sensor, and, on this basis, he developed a sensor. In addition, he has made a differential sensor in the same way and compared it with the concentric sensor.
From the internal defects located 3mm below the surface in a stainless steel plate of 4mm in thickness, he determined the structural dimensions of a concentric sensor for maximum signals and manufactured it for a test. The concentric sensor determined the presence or absence of defects, but not their exact size or depth. In the meanwhile, the same test by the differential sensor showed that its signal strength is very high with a significant change in the depth of defects and it can fully process them without amplifying the signals.
His study demonstrates that differential sensors are effective in detecting the internal defects of nonmagnetic materials.
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Jo Aug 17, 2025
The excitation field circuits of synchronous generators are typically isolated under normal operating conditions. The field winding is subjected to mechanical and thermal stress cycles due to the increase in rotation speed and temperature. In addition to the normal stress, the field winding can be exposed to abnormal mechanical or thermal stress due to overspeed, vibrations, excessive field currents, poor cooling or stator negative sequence currents. This may result in the breakdown of insulation between the field winding and the rotor core at the points where the stress has the highest value.
When the excitation system is isolated, a single ground fault in the field winding or its associated circuits, causes a negligible fault current, which does not lead to any immediate danger. However, if a second ground fault occurs, high fault currents and severe mechanical unbalances may quickly arise, leading to serious damage. In some cases, the field current, flowing through the rotor core, could generate enough heat to melt it.
It is essential, therefore, that the first insulation failure has to be detected, and the generator has to be removed from service to check the insulation health.
Jong Chol Min, a section head at the Faculty of Electrical Engineering, has proposed an on-line rotor ground fault location method for synchronous machines with static excitation, and conducted computer simulations to validate its effectiveness. In addition, he has proposed a new algorithm for estimating the ground fault resistance value in rotor windings in order to improve the accuracy of location.
This novel technique has two important advantages. First, it does not need any additional injection source to detect faults. Second, the new algorithm locates the fault position under normal operating conditions.
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