Jo May 21, 2022
Ho Tong Chol, a researcher at the Faculty of Electrical Engineering, has developed DC-DC chopper experimental equipment, a part of a power electronics integrated laboratory table. It is designed to enable experiments on typical DC-DC converters ― Buck, Boost and Buck-Boost.
It can easily get an operational waveform at any point thanks to the terminals placed on the front-plate, it is supported by AC 220V as main power source, and the circuit diagrams of the front-plate are intuitive and plain enough to provide users with convenient environment for experiments on a certain converter.
This equipment, composed of power supply, control block, driving circuit, main circuit, load and filter, is useful for raising or dropping static DC 12V and for measuring some parameters such as duty cycle, voltage and current ripple rate, instantaneous value, mean value, etc.
The control block using current typed PWM integrated circuit SG3525 compares the carrier in 32.5kHz with DC voltage(0~3.3V) before it generates a PWM pulse at that comparison position. This process is observed at the terminals.
Additionally, since it includes a soft-starting circuit, you can increase the duty cycle smoothly so that no shock current flows into the main switch even when it starts on the maximum duty cycle.
The control range of duty cycle is 0.2 to 0.8.
The control block is also available for the control of a transformer isolated DC-DC converter since it has two ground separated power supplies, which is useful for the experiment on a half-bridge DC-DC converter.
The driving circuit guarantees a reliable and safe switch of the main device. The driving circuit is fed from unipolar power source.
The main circuit consists of a main switching device, a reactor, FRD, a capacitor and a current sensing resistor. The load is a resistor in 200Ω and it can also include a filter when necessary.
The newly-developed experimental equipment is useful to students, researchers and teachers who major in power electronics.
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Jo May 18, 2022
Sim Jae Rim, a section head at the Faculty of Materials Science and Technology, has established a way of making WC-Co-B4C system electrode material for electric spark alloying and an alloying process by this material so that the service life of tools and machine parts could get more than 2.5 times as long.
B4C in the material has a good effect on the formation of alloying layers since it plays the role as flux which decreases the amount of oxide scale made during electric spark alloying. It also decreases the amount of oxides of low hardness and helps the formation of carbides of high hardness.
The reasonable electric conditions for electric spark alloying by WC-Co-B4C system electrode material are short circuit current I=9A, U=24V, C=420㎌ and t=2min/cm2. The thickness and hardness of an electric spark alloying layer by the new material are higher, that is, 90㎛ and 23.5GPa respectively, compared to 70㎛ and 19.5GPa by previous WC-Co system electrode material.
Electric spark alloying by WC-Co-B4C system electrode material forms alloying layers of high hardness on the surfaces of tools and machine parts, which leads to the lengthening of their working life.
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Jo May 16, 2022
The research team led by Ri Ju Hyok, a section head at the Faculty of Heat Engineering, has designed and built a cryogenic liquid tank for storage of oxygen liquid.
The tank is manufactured to store oxygen liquid produced by cryogenic distillation process for a certain period and to supply it in either liquid or gas form.
It is composed of an inner body for storage of oxygen liquid, an outer body, a pipeline for injection and exhaustion, safety factors including checker valves, safety valves, pressure control valves, a level meter and manometers and a self-evaporator for increasing the pressure inside the tank.
The space between the inner and outer bodies is filled with thermal insulation material expanded perlite and made vacuous to reduce evaporation loss and to keep cool.
The problem of preserving the cold to minimize cold loss by a heat bridge was solved, which decreased daily loss to only 0.3~0.8%.
The degree of vacuum in the insulated space is 1.3Pa, and the total heat conductivity of the expanded perlite is 0.017W/(m•K).
With all these advantages it can also be used for storage of other cryogenic liquids like nitrogen, argon, etc. as well as oxygen.
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Jo May 13, 2022
Kim Myong Il, a vice dean at the Faculty of Communications, has invented an indoor antenna for mobile communication.
Compared with previous antennas, this antenna has several advantages ― it ensures enough matching, gain and horizontal omni pattern in a wide band of frequencies; it is easy to feed, produce and assemble; it is light, good-looking and easy to install.
Consisting of a cone monopole, a double cone reflection panel, a curved plastic protection lid and coaxial feeding structure, the antenna assumes horizontal omni pattern in a wide range of frequency such as GSM, WCDMA, LTE, etc. With the gain of 3dBi, it can provide indoor mobile communication service in areas such as the underground of a building where mobile communication service cannot reach.
As a wide-range radiator, a cone monopole has top and bottom panels made of metal (Oblique angle is 25.5, radii are 12mm and 21mm respectively.), with an impedance of 50Ω and a wide radiation area. So it is of nearly omni directional pattern on the horizontal plane.
Vertical radiation can be controlled by a double cone reflection panel consisting of a primary cone reflection panel, a plain reflection panel, an upper cone reflection panel for coaxial feeding and it can bring a gain of 3dBi.
The inner conductor of a feeding coaxial cable is connected to the bottom panel and the outer conductor is linked to the reflection panel. The impedance of the antenna can be controlled by inserting an inclined line between the cone monopole and the reflection panel. Therefore, the impedance of the antenna remains constant in a wide frequency range. What is more, it is easy to install on the wall and it is covered tightly with a radome made of glass fiber plastic in order to prolong the life span and give a good-looking appearance.
In a wider frequency range(756~1 063/1 670~2 777MHz) than in the frequency range of former wire monopoles, the gain is 3.2dBi, which is 1.1dBi higher than former antennas (2.1dBi), so the service bound gets 1.4 times wider than before. The antenna is light, easy to install and meets aesthetic requirements.
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Jo May 11, 2022
Kim Ryul, a researcher at the Faculty of Electronics, has developed a new insulation bushing for a 10kV electric boiler that conforms to the operating conditions of the boiler and guarantees high reliability.
Unlike prior bushings of a cylindrical shape for a 3-phase 3kV electric boiler, a newly-developed one is of a conical shape, satisfies the operating conditions for a 10kV electric boiler and is more reliable and lighter. As its head has been changed into a ring type, they can increase yield, which means it can help reduce the level of environmental pollution and save raw and other materials
The new bushing is lighter by 0.7 times each, which means it has now become cheaper to make.
What is more, it is 2.4 times higher in dielectric strength, 3.3 times in working voltage, 1.25 times in output power, 2.7 times in 5-minute upkeep water pressure and 0.65 times lower in maximum current, which offers a scientific guarantee of the operating conditions and reliability of a 10kV electric boiler.
The yield has increased up to 93%.
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Jo May 6, 2022
Ssirum (Korean wrestling), part of the superior national heritage created and developed by the Korean people from ancient times, is a traditional sporting event where two people who grasp each other’s thigh bands compete to bring down the opponents onto the ground using their hands, legs and bodies.
Korean ssirum, registered as part of the intangible heritage of the World Heritage Committee of UNESCO, has a long history and tradition.
Korean Ssirum is said to have made a start in Ancient Korea and have been perfected in Koguryo.
Although several nations and countries including Mongolia and Japan did have similar strength events called ssirum, Koguryo ssirum had some unique features ― wrestlers wearing heavy cloth bands around either thighs and no garments on their upper bodies competed in strength with their chins tucked on the opponents’ shoulders in bare feet; ssirum was well organized according to strict rules; wrestlers were trained to be professional.
Unlike Koguryo ssirum, however, Mongolian ssirum allowed wrestlers to wear shoes and upper garments similar to leather waistcoats.
Meanwhile, influenced by Korean ssirum, the Japanese did ssirum with thigh bands, bare upper body and bare feet. This explains Japanese ssirum originated from Korean ssirum.
This assertion is also supported by the figures discovered in Inbe Hachimanyama Tomb (around 6th century) situated on the basin of the Ginogawa River in Wakayama Prefecture, Japan.
Figure 10 of strong man erected in the east of the group of tombs bears a close resemblance to Koguryo ssirum wrestlers.
It proves that Japanese ssirum was not invented and developed in Japan itself, but introduced from Koguryo.
Such information is also found in “Japanese History” and other Japanese historical records.
In a word, Korean ssirum, which was developed in Ancient Korea and perfected in Koguryo, was disseminated through the neighbouring countries of the same blood Paekje, Silla and Kaya to the Japanese archipelago, exerting a significant influence upon the cultural development of the island country.
Indeed, the time-honoured and superior culture and tradition created by the Korean nation from olden times was introduced into other countries, thus leaving an indelible mark in their history of cultural development.
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