• Proceedings of the KOSOMBE Conference
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• v.1997 no.05
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• pp.227-229
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• 1997

A new phased-array quadrature RF coil for one or two RF acquisition channels is developed for spine MR imaging. Quadrature RF coils for MRI have been useful to improve the signal-to-noise ratio (SNR) by $"\sqrt{2}"$ using two orthogonal RF coils in combination [1]. More recently, the phased-array RF coil has been proposed for more improvement of SNR by using an array of RF coil elements with a reduced size and coverage for each element. Two new schemes are proposed for the new phased-array quadrature RF coil as follows : (1) Proper overlapping of two quadrature RF coils thus removing the mutual inductance and (2) Attaching preamplifiers right after the coil section and combining the signal with proper phase delays. The coil has been implemented for receive- only mode. It has been tested through phantom and volunteer imaging. The experimental results show the utility of the proposed RF coil.

• Journal of Power Electronics
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• v.15 no.6
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• pp.1429-1437
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• 2015

A new parallel hybrid soft switching converter with low circulating current losses during the freewheeling state and a low output current ripple is presented in this paper. Two circuit modules are connected in parallel using the interleaved pulse-width modulation scheme to provide more power to the output load and to reduce the output current ripple. Each circuit module includes a three-level converter and a half-bridge converter sharing the same lagging-leg switches. A resonant capacitor is adopted on the primary side of the three-level converter to reduce the circulating current to zero in the freewheeling state. Thus, the high circulating current loss in conventional three-level converters is alleviated. A half-bridge converter is adopted to extend the ZVS range. Therefore, the lagging-leg switches can be turned on under zero voltage switching from light load to full load conditions. The secondary windings of the two converters are connected in series so that the rectified voltage is positive instead of zero during the freewheeling interval. Hence, the output inductance of the three-level converter can be reduced. The circuit configuration, operation principles and circuit characteristics are presented in detail. Experiments based on a 1920W prototype are provided to verify the effectiveness of the proposed converter.

• Proceedings of the Materials Research Society of Korea Conference
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• 2011.05a
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• pp.31.2-31.2
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• 2011

We have investigated frequency dependant conductivity (or permittivity) of low dimensional oxide structures represented by [($Sr_{0.75}$, $La_{0.25}$)$TiO_3$]$_1$/1$[SrTiO_3]_n$ superlattices. The low dimensional oxide superlattice was made by cumulative stacking of one unit cell thick La doped $SrTiO_3$ and $SrTiO_3$ with variable thickness from 1 to 6 unit cell, i,e, [($Sr_{0.75}$, $La_{0.25}$)$TiO_3$]$_1$/$[SrTiO_3]_n$ (n=1, 2, 3, 4, 5, 6). We found two kinds of relaxation when n is 3 and 4, while, inductance component was observed at n=1. This behavior can be explained by electron modulation in ($Sr_{0.75}$, $La_{0.25}$)$TiO_3/SrTiO_3$ superlattices. When n is 1, electrons by La doping well extend to un-doped layer. Therefore, the transport of superlattices follows bulk-like behavior. On the other hand, as n increased, the doped electrons became two types of carrier: one localized and the other extended. These results in two kinds of transport phase. At further increase of n, most of doped electrons are localized at the doped layer. This result shows that dimensionality of the oxide structure significantly affect the transport of oxide nanostructures.

• Proceedings of the KIEE Conference
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• 1989.07a
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• pp.16-20
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• 1989

In this paper, we describe the optimal driving method and magnetic flux distribution of permanent magnet which enhance torque characteristics in small-sized 3-phase brushless DC motors. The disadvantages of conventional $120^{\circ}$ constant current drive method are torque ripple, switching noise and spike voltage due to the inductance of stator coil. This shortcommings can be avoided by the switching slew-rate of driving current which is called linear voltage driving method. The aim of this study is to analyze linear voltage driving method quantatively and to determine optimal drive current waveform through computer simulation. The selection of commutation angle and slew rate of a new driving current at switching instants makes torque ripple index minimize and average torque maximize. And the validity of this new driving method was assured by Fourier analysis. Considering two dimensional nonlinear magnetic flux distribution on the permanent magnet, we suggest optimal flux distribution according to the presented driving method which improves torque characteristics.

• The Transactions of The Korean Institute of Electrical Engineers
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• v.59 no.5
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• pp.860-865
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• 2010

When a circuit breaker is opened, a large capacitance around the buses, the circuit breaker and the potential transformer (PT) might cause PT ferroresonance. During PT ferroresonance, the iron core repeats saturation and unsaturation even though the supplied voltage is a rated voltage. This paper describes an analytical analysis of PT ferroresonance in the transient-state. To analyze ferroresonance analytically, the iron core is modelled by a simplified two-segment core model in this paper. Thus, a nonlinear ordinary differential equation (ODE) for the flux linkage is changed into a linear ODE with constant coefficients, which enables an analytical analysis. In this simplified model, each state, which is either saturated or unsaturated state, corresponds to one of the three modes, i.e. overdamping, critical damping and underdamping. The flux linkage and the voltage in each state are obtained analytically by solving the linear ODE with constant coefficients. The proposed transient analysis is effective in the more understanding of ferroresonance and thus can be used to design a ferroresonance prevention or suppression circuit of a PT.

• Journal of the Institute of Electronics Engineers of Korea SC
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• v.43 no.4 s.310
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• pp.45-52
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• 2006

In his paper, we have developed the SMD(surface mounted device) type PECL(positive emitter-coupled logic) VCXO of the $5{\times}7mm$ size for gratifying the requested specifications and the multilayer ceramic SMD(surface mounted device) package technology. The VCXO wired with the PECL(positive emitter coupled logic) package take place a stray inductance and a parasitic capacitance by the length and the inner pattern of the VCXO and the amplitude attenuation and signal loss due to the reflection of power source and the noise component. We have developed the Zig system to analyze the precise spectrum and evaluate the performance. The basic operating voltage is the 3.3 V and have the frequency range of 120MHz-180MHz. The Q factor is over 5K and it has the low jitter characteristics of 3.5 ps and low phase noise.

• Proceedings of the KIEE Conference
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• 2015.07a
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• pp.682-684
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• 2015

A 12/14 bearingless switched reluctance motor (BLSRM) with hybrid stator poles has been proposed due to the outstanding decoupling characteristics between the torque and suspending force. However, the motor is a two-phase motor. The output torque of the motor has torque dead zone and high torque ripple. Hence, the motor cannot self-start at some rotor positions. To solve the self-starting problems and reduce the torque ripple, a stepped rotor is proposed in this paper. Then, the motor with the stepped rotor is optimally designed. In the new designed motor, the majority parameters are kept the same with those of original motor; only the torque pole arc and rotor pole shape are optimally designed. The characteristics of the redesigned motor, such as inductance, torque and suspending force, are analyzed and compared with those in the original motor. Finally, the effectiveness of the proposed method is verified by the simulation results.

• Journal of Advanced Navigation Technology
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• v.14 no.2
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• pp.176-182
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• 2010

This paper proposes that an inverse class-E amplifier is used a tunable parallel resonator at output port in order to maintain a high power-added efficiency(PAE) and output power with wide frequency ranges. A tunable circuit has a constant Q factor at operating frequency ranges and because of using varactor diode, the inductor and capacitor values of resonator can be changed. Also, the inductance value for zero-current switching (ZCS) is implemented a lumped element and the capacitance value is made a distributed element for phase compensation. The inverse class E amplifier using tunable parallel resonator is obtained to deliver 25dBm output power and achieve maximum power added efficiency(PAE) of 75% at 65-120MHz frequency ranges.

• Proceedings of the KIPE Conference
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• 2003.07b
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• pp.677-680
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• 2003

The control of the SRM(Switched Reluctance Motor) is usually based on the non-linear inductance profiles with positions. So determination of optimal switching angle is very different. we present self-tuning control of SRM for maximum torque and efficiency with phase current and shaft position sensor During the sample time, micro-controller checks the number of pre-checked pulse. After micro-controller calculates between two data, it move forward or backward turn-off angle. When the turn-off angle is fixed optimal turn-off angle, turn-on angle moves forward or backward by a step using self-tuning control method. And then, optimal turn-off angle is searched once again. As such a repeating process, turn-on/off angle is moves automatically to obtain the maximum torque and efficiency. The experimental results are presented to validate the self-tuning algorithm.

• Tribology and Lubricants
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• v.11 no.5
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• pp.99-107
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• 1995

This paper reported research results to develop an algorithm of on-lin evaluation of surface profiles and roughness generated by turning. The developed module consisted of computer simulation of surface profiles using mechanism of cutting mark formation and cutting vibrations, and online measurement of cutting vibrations. The relative cutting vibrations between tool and worpkiece were measured through an inductance pickup at the rate of one sample per rotation of the workpiece. The sampling process was monitored using an encoder to avoid conceling out the phase lag between waves. The digital cutting signals from the Analog-to-Digital converter were transferred to the simulation module of surface profile where the surface profiles were generated. The developed algorithm or surface generation in a hard turning was analyzed through computer simulations to consider the stochastic and dynamic nature of cutting process. Cutting tests were performed using AISI 304 Stainless Steel and carbide inserts in practical range of cutting conditions. Experimental results showed good correlation between the surface profiles and roughness obtained using the developed algorithm and the surface texture measured using a surface profilemeter. The research provided the feasibility to monitor surface characteristics during tribelogical tests considering wear effect on surface texture in machining.