• The Transactions of The Korean Institute of Electrical Engineers
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• v.59 no.6
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• pp.1059-1063
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• 2010

Shimming is an important technique in development of nuclear magnetic resonance (NMR) magnets where image resolution is highly dependent on magnetic field homogeneity. Classically, shimming may be categorized into two types: 1) active shimming that incorporates with extra coils and precise tuning of their currents; and 2)passive shimming that incorporates with pieces of steel placed in a bore of a main magnet and their uniform magnetization under homogeneous external fields. Additional magnetic fields, produced by the coils and/or the steel sheets, compensate original magnetic field from the main magnet in such a way that the total field becomes more homogeneous. In this paper, we developed a passive shimming method based on linear programming optimization. Linear programming is well known to be highly efficient to find a global minimum in various linear problems. We firstly confirmed the linearity of magnetization of ferromagnetic pieces under a presence of external magnetic fields. Then, we adopted the linear programming to find optimized allocation of the steel pieces in the inner bore of a main magnet to improve field homogeneity.

• Journal of Electrical Engineering and Technology
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• v.10 no.6
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• pp.2326-2333
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• 2015

Stator turn faults in permanent magnet synchronous motors (PMSMs) are more dangerous than those in induction motors (IMs) because of the presence of spinning rotor magnets that can be turned off at will. Condition monitoring and fault detection and diagnosis of the PMSM have been receiving a growing amount of attention among scientists and engineers in the past few years. The aim of this study is to propose a new detection technique of stator winding faults in a three-phase PMSM. This technique is based on the image analysis and recognition of the stator current Concordia patterns, and will allow the identification of turn faults in the stator winding as well as its correspondent fault index severity. A test bench of a vector controlled PMSM motor behaviors under short circuited turn in two phases stator windings has been built. Some experimental results of the phase to phase short circuits have been performed for diagnosis purpose.

• The Transactions of The Korean Institute of Electrical Engineers
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• v.56 no.1
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• pp.57-65
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• 2007

Along with the development of power electronics and magnetic materials, permanent magnet (PM) brushless direct current (BLDC) motors are now widely used in many fields of modern industry BLDC motors have many advantages such as high efficiency, large peak torque, easy control of speed, and reliable working characteristics. However, Compared with the other electric motors without a PM, BLDC motors with a PM have inherent cogging torque. It is often a principle source of vibration, noise and difficulty of control in BLDC motors. Cogging torque which is produced by the interaction of the rotor magnetic flux and angular variation in the stator magnetic reluctance can be reduced by sinusoidal air-gap flux density waveform due to reduction of variation of magnetic reluctance. Therefore, this paper will present a design method of magnetizing system for reduction of cogging torque and low manufacturing cost of BLDC motor with isotropic bonded neodynium-iron-boron (Nd-Fe-B) magnets in ring type by sinusoidal air-gap flux density distribution. An analytical technique of magnetization makes use of two-dimensional finite element method (2-D FEM) and Preisach model that expresses the hysteresis phenomenon of magnetic materials in order for accurate calculation. In addition, For optimum design of magnetizing fixture, Factorial design which is one of the design of experiments (DOE) is used.

• Journal of Aerospace System Engineering
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• v.6 no.1
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• pp.26-32
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• 2012

A shock absorber with magnetic effects is suggested for a lunar space-ship expected to launch in 2025. The device consists of a copper steel combined tube, two magnets, and a piston. The piston is designed to move a magnet through the tube when it is pushed by an external impact. While the magnet is moving in the tube, it generates the eddy current force with the copper part of the tube and it also makes the large friction force with the steel part of the tube. Beside, it gets resistive forces against its movement such as the magnetic force with a steel-ring at the first time of the movement and the repulsive force with a same pole opposed magnet at the end time of the movement. In this thesis, results of analyses and experiments of each force are represented and the expected performance of the electromagnetic shock absorber is drawn from the results.

• Progress in Superconductivity and Cryogenics
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• v.3 no.2
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• pp.15-20
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• 2001

This paper describes a model flux pump experiment recently performed at the MIT Francis Bitter Magnet Laboratory. The results of the model flux pump will be used in the development of a prototype flux pump that will be couple to a high-temperature superconductor (HTS) insert coil of a high-field NMR (Nuclear Magnetic Resonance) magnet, Such an HTS insert is unlikely to operate in persistent model because of the conductors low index(n) The flux pump can compensate fro field decay in the HTS insert coil and make the insert operate effectively in persistent mode . The flux pump, comprised essentially of a transformer an two switches. all made of superconductor, transfers into the insert coil a fraction of a magnetic energy that is first introduced in the secondary circuit of the transformer by a current supplied to the primary circuit. A model flux pump has been designed. fabricated, and operated to demonstrate that a flux pump can indeed supply a small metered current into a load superconducting magnet. A current increment in the range of microamperes has been measured in the magnet after each pumping action. The superconducting model flux pump is made of Nb$_3$ Sn tape, The pump is placed in a gaseous environment above the liquid helium level to keep its heat dissipation from directly discharged in the liquid: the effluent helium vapor maintains the thermal stability of the flux pump.

• Journal of the Korean Society for Precision Engineering
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• v.32 no.5
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• pp.483-490
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• 2015

This paper presents a proof-of-concept of a wheel-based magnetostrictive energy harvester (EH), which is a vibration-based EH. Coil-wound Galfenol cantilevers with two permanent magnets (PMs) act EH, while rotating wheels provide a forced vibration to EH. Four different cantilevers are designed and simulated for various end deflection. As expected from the simulation, the cantilever end deflection with triple cavity is the most. Three experiments are conducted to characterize the EH: the first with a magnetostrictive actuator, the second with a motor-driven wheel, and the third with the dummy weights. From the first experiment, the power reaches about 50 mV due to the relatively small displacement of the magnetostrictive actuator. From the second experiment, the power reaches about 120 mW. The power from the Galfenol cantilever is estimated to be about 60% of the total power from the wheel-based magnetostrictive EH.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.31 no.1 s.256
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• pp.43-49
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• 2007

A new electric coil design methodology using the notion of topology optimization is developed. The specific design problem in consideration is to find optimal coil configuration that maximizes the Lorentz force under given magnetic field. Topology optimization is usually formulated using the finite element method, but the novel feature of this method is that no such partial differential equation solver is employed during the whole optimization process. The proposed methodology allows the determination of not only coil shape but also the number of coil turns which is not possible to determine by any existing topology optimization concept and to perform single coil strand identification algorithm. The specific applications are made in the design of two-dimensional fine-pattern focusing coils of an optical pickup actuator. In this method, the concept of equivalent magnetizing current is utilized to calculate the Lorentz force, and the optimal coil configuration is obtained without any initial layout. The method is capable of generating the location and shape of turns of coil. To confirm the effectiveness of the proposed method in optical pickup applications, design problems involving multipolar permanent magnets are considered.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.44 no.5
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• pp.431-438
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• 2016

In this paper, a self-powered quadcopter is designed and manufactured. The quadcopter gains extra power via two types of generators which utilize the principle of electromagnetic induction. One of the types is to make use of a commercial BLDC motor which rotates together with a propeller and thus acts as a generator. The other type is to make use of a coil-wound ring around a propeller which generates electricity when small magnets embedded in a uniquely designed frame rotate together with the propeller. The proposed generators are expected to be of practical use when they are located under the propellers of a quadcopter.

• Journal of the Korean Society of Physical Medicine
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• v.15 no.4
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• pp.101-109
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• 2020

PURPOSE: This study investigates the therapeutic effect of a prototype of a hand rehabilitation device based on magnetic forces. METHODS: Using an electromagnet and permanent magnets, we developed an end effector type device that induces various movements of the finger in accordance with the magnetic field direction. A total of 26 subacute stroke patients were enrolled and assigned to two groups in this randomized controlled trial. The intervention group received 30 minutes hand rehabilitation therapy per day for 4 weeks, using the device developed by us. Conventional physical therapies were conducted equally twice a day, 30 minutes per session, during the same period in both groups. RESULTS: After 4 weeks, rate of the Wolf Motor Function Test as a primary outcome measure showed significant improvement in the intervention group as compared to control group(p = .036). Scores of the Manual Function Test and Fugl-Meyer Assessment of upper limb were also significantly increased in the intervention group as compared to control group(p = .038 and p = .042, respectively). Moreover, the Korean version of Modified Barthel Index tended to improve after subjecting to physical therapy in both groups. CONCLUSION: Our results indicate that the novel hand rehabilitation device developed using a magnetic force, improves the hand motor functions and activities of daily life in subacute stroke patients.

• Journal of the Korean Society for Precision Engineering
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• v.31 no.5
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• pp.417-422
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• 2014

A guitar pickup is a transducer that converts string vibration to an electrical signal. The magnetic and piezo pickups are the most commonly used for the respective electric and electroacoustic guitars. The magnetic pickups are prone to magnetic interference between the steel strings and permanent magnets, while the piezo ones are not free from signal inference between the strings. Thus, this paper presents the development of an optical pickup for the electroacoustic guitar. The proposed optical pickup has the top-to-bottom structure. It uses two of Infrared (IR) Light Emitting Diode (LED) and one photodetector. The developed optical pickup is subjected to the evaluation with commonly used piezoelectric pickup. It becomes obvious that SNR with the optical pickup is increased by 45 percent in average, compared with the piezoelectric pickup. It can be concluded that the developed optical pickup has a potential to be applied to the acoustic guitar.