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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Jo Aug 15, 2025
With the recent increase in gold production, low-grade, fine-grained gold ores difficult to cyanide and refractory gold ores containing harmful impurities such as sulfur, arsenic, copper, etc. have become the major resources for gold production.
In order to effectively extract gold from the refractory gold ore, oxidative pretreatment must be conducted prior to leaching to oxidize the sulfide bearing gold particles and convert their physical and chemical properties.
There are several pretreatment methods, each with their own advantages and disadvantages. Hydrothermal oxidation pretreatment of refractory gold ores is an effective one, which has been widely used in recent years, due to its high rate of desulfurization, little environmental pollution and high reaction rate.
Sin Hyok, a researcher at the Faculty of Chemical Engineering, has investigated the thermodynamic process of nitric acid oxidation and the factors affecting gold extraction when pyrite-and-arsenopyrite-based gold concentrates are pretreated by mechanochemical activation and nitric acid oxidation.
He has found the following.
The factors affecting gold extraction are nitric acid concentration, mechanical activation time and liquid-solid ratio. The decomposition of pyrite and arsenopyrite in nitric acid medium proceeds from above 1mol/L of nitric acid concentration at room temperature, and the mechanical activation starts decomposition from above 0.5mol/L.
After pretreatment using mechanical activation and nitric acid oxidation, the cyanidation leaching rate for gold concentrates is over 87%.
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Jo Aug 14, 2025
Semiconducting BaTiO3 ceramics are widely used in electronic information, automatic control, biological technology, energy management and traffic control as an important control component. The barium titanate (BaTiO3) shows high intrinsic resistivity of more than 1010Ω·cm when prepared in the oxidizing atmosphere. Therefore, the chemically unmodified BaTiO3, because of its large energy gap, is an electrical insulator. However, when polycrystalline BaTiO3 ceramics are doped with higher valence cations such as Y3+, La3+, Nb5+ or Ta5+, they show semiconducting properties and positive temperature coefficient of resistance (PTCR) effect.
SiO2 and Mn additions assist homogenization of the microstructure during sintering, improving the performance of PTCR under transient loads. In terms of electrical properties, the variation of permittivity with temperature is significantly affected by SiO2 additions: with increasing SiO2, the permittivity increases below Tc.
SiO2 has long been added to BaTiO3 as a sintering aid, but it has not been reported to significantly affect the electrical properties of PTCR thermistors.
Kong Myong Il, a researcher at the Semiconductor Institute, has investigated the effects of SiO2 addition on PTCR characteristics of Y-doped BaTiO3 ceramics prepared by solid-state reaction method.
He has found through the experiments that the optimum SiO2 content was 2mol.
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Jo Aug 13, 2025
At present, most injection molding designers depend on their old experiences in mold design. Due to the diversity and complexity of plastic workpieces and the limited experience of designers, it is difficult to precisely design good-quality products at low cost and in a short period.
Computer Aided Engineering (CAE) analysis techniques are useful for the manufacturing and design of injection molds. The operational tests of injection molds by CAE techniques makes it possible to predict the quality status of plastic products in the forming process of injection molding dies before injection molding. The simulation and analysis using Moldflow instead of operational tests of injection molds at the time of injection mold design help predict the potential defects of injection molds to modify and improve their structure.
Choe Tong Ju, a researcher at the Robotics Institute, has applied CAE simulation analysis to the injection molding of a container into a Hermetia illucens box, and proposed an improved design scheme of die structure.
First, he simulated the flow of plastic melt in the forming chamber. Then, he predicted the best gate position, packing pressure, temperature change, air bubble and weld line position by using the Moldflow software.
Applying the Moldflow to die design helps reduce the cost and the cycle of new product updating by predicting the defects that may occur during the injection process.
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Jo Aug 12, 2025
Generally, the film produced by the chemical film process is not as corrosion-resistant as that produced by the anodizing process, and that is why the chemical film process can only be used for the surface treatment of aircraft skins and non-structural parts.
Unlike the chemical film process, the anodizing process can produce a dense and thick porous oxide layer with excellent corrosion resistance performance. For a thicker and denser oxide layer on the aluminum sheet surface, an aluminum sheet should be immersed in a treatment tank with suitable electrolyte and a direct current or an alternating current should be applied to it. The porous oxide layer provides a large wetting area for adhering, thus enhancing the bonding strength of the aluminum sheet.
Chromic anodizing, sulfuric anodizing, hard anodizing and hot etching are the most commonly used surface treatments of aluminum sheets.
Kim Nam Il, a researcher at the Faculty of Materials Science and Technology, has found the most suitable surface treatment by studying the effect of the four different surface treatments on AlMgSi aluminum sheets when they are bonded with an epoxy-based film adhesive.
He performed four different surface treatments on aluminum sheets that were then bonded with an epoxy-based film adhesive in an autoclave to achieve good adhesion. He examined the microstructure of the porous oxide layer on the aluminum sheet surface and its effect on the bonding performance.
The analysis of the failure modes of the bonded surfaces, the peeling strength and the microstructure of the porous oxide layer showed that chromic anodizing of aluminum sheets have the best bonding performance, whereas sulfuric anodizing and hard anodizing exhibit the worst bonding performance.
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Jo Aug 11, 2025
A jottae (a Korean bamboo flute) is a national woodwind instrument from the period of Koguryo. It was also called transverse flute as it is played transversely. It was first made in Koguryo. In Paekje and Silla, they introduced the Koguryo jottae to make several kinds of bamboo instruments. Later, around the period of Koryo and feudal Joson dynasty, it became a major tune instrument. Its mouthpiece is oval and very big, which requires high technical skills to play.
You play the jottae with its head on your left shoulder and with your lips at the mouthpiece. You make vibrato by means of the up-and-down movements of your left elbow connected with the jottae. It is characterized by the gentle, soft, sweet and elegant timbre.
The Korean bamboo flute widely played by the Korean people was improved into three kinds: ordinary jottae, high-pitched jottae and low-pitched jottae, and it developed into the one capable of playing modern music skillfully with its peculiar features unchanged.
The improved ones are made of wood from paktal trees and red sandalwood instead of bamboo, which guarantees scientific accuracy and unity in their making.
Today, the Korean bamboo flute is mainly used for solos and ensembles, and, in particular, it is used as a major instrument for elegant and plaintive tones in the mixed orchestral music of a Juche type.
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