• Journal of Welding and Joining
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• 제22권3호
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• pp.69-76
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• 2004

Effects of friction stir welding parameters such as tool rotation speed and welding speed on the joints properties of 5052 Al alloys were studied in this study. A wide range of friction stir welding conditions could be applied to join 5052 AA alloy without defects in the weld zone except for certain welding conditions with a lower heat input. Microstructures near the weld zone showed general weld structures such as stir zone (SZ), thermo-mechanically affected zone (TMAZ) and heat affected zone (HAZ). Each zone showed the dynamically recrystallized grain, transient grain and structure similar to base metal's, respectively. Hardness distribution near the weld zone represented a similar value of the base metal under wide welding conditions. However, in case of 800 rpm of tool rotation speed, hardness of the stir zone had a higher value due to the fine grain with lots of dislocation tangle, a higher angle grain boundary and some of Al3Fe particles. Except joints with weld defects, tensile strength and elongation of the joints had values similar to the base metal values and fracture always occurred in the regions approximately 5mm away from the weld center.

• 한국전산유체공학회지
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• 제20권3호
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• pp.27-34
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• 2015

In this study, preconditioned adjoint equations for the quasi one-dimensional Euler equations are developed, and their computational benefit at all speed is assessed numerically. The preconditioned adjoint equations are derived without any assumptions on the preconditioning matrix. The dissipation for Roe type numerical flux is also suggested to scale the dissipation term properly at low Mach numbers as well as at high Mach numbers. The new preconditioned method is validated against analytical solutions. The convergence characteristics over wide range of Mach numbers is evaluated. Finally, several inverse designs for the nozzle are conducted and the applicability of the method is demonstrated.

• Journal of Advanced Marine Engineering and Technology
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• 제30권2호
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• pp.284-290
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• 2006

For the case of high head and critical low flow rate range of micro hydropower resources, it requires very low specific speed turbines which are lower than conventional impulse turbine's specific speed. In order to satisfy the request for very low specific speed turbine with high efficiency, a new positive displacement turbine is developed. The performance characteristics of the new turbine is tested and compared with a conventional impulse turbine, which is used for automatic water faucet system. The purpose of present study is to develop an high performance turbine that can be used to extract micro hydropower potential of a water supply system. The test results show that the positive displacement turbine is much more efficient than the conventional turbine and it can sustain high efficiency under the wide range of operating conditions. The pressure pulsations at the inlet and outlet of the positive displacement turbine can be considerably minimized by using simple pressure damper.

• 전력전자학회:학술대회논문집
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• 전력전자학회 2001년도 전력전자학술대회 논문집
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• pp.276-279
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• 2001

Direct torque control(DTC) is known to produce quick response in ac drives. Some drawbacks of the classical DTC are the relatively large torque ripple in a low speed range and notable current pulsation during steady state. They are reflected speed response and increased acoustical noise. In this paper, The DTC quick response are preserved at transient state, while better qualify steady state performance is produced by Space Vector Modulation(SVM). It is able to reduce the acoustical noise, torque ripple and current pulsation during steady state. The system presented are closed loop stator flux and torque observer for wide speed range that inputs are currents and voltages sensing of motor terminal. Simulation results for the 2.2kw general purposed induction motor are presented and discussed.

• 대한전기학회논문지
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• 제43권5호
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• pp.747-752
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• 1994

The switched reluctance motor has high efficiency over a wide range of speed and torque. However the switched reluctance motor exhibit comparatively low efficiency in the chopped excitation which produces additional inverter losses. This paper examines the optimal efficiency operation using choppingless excitation under the changed supply voltage. The excitation is designed to produce the required torque and speed satisfying the optimal efficiency in a nonlinear switched reluctance motor. The optimal algorithm is applied to 119 operation points which are chosen over the full range of driving torque and speed. The result in this paper indicate the improved efficiency comparing to the efficiency obtained by both switched and chopped excitataions under a fixed supply voltage.

• 제어로봇시스템학회논문지
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• 제7권4호
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• pp.288-294
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• 2001

In this study, a new PWM method considering the actuation delay of high speed solenoid valves is proposed to improve the response characteristics of electro hydraulic servo systems controlled by high speed solenoid valves. In addition, the decision method for the system gain, the basic period of PWM, and the sampling time is proposed, Since the conventional system controlled by high speed solenoid valves is too slow to apply this method, a high speed driving circuit(Quick-Drive) which enables rapid switching of the high speed solenoid valve at a high speed sampling mode is applied to realize this method. The experimental result shows that it is possible to achieve precision and quiet control without occurrence of limit cycle and wide range dead band.

• 대한전기학회논문지:전기기기및에너지변환시스템부문B
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• 제48권11호
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• pp.632-639
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• 1999

SRM(Switched Reluctance Motor) drives require the accurate position and speed information of the rotor. These informations are generally provided by a shaft encoder or resolver. High temperature, EMI, and dust may make detection performance deteriorate. Therefore, the elimination of the position and speed sensor is desirable. In this paper, a nonlinear adaptive observer using the MRAS(Model Reference Adaptive System) is proposed. The rotor speed and position are estimated by the adaptation law using the real and estimated currents. The stability of the adaptive observer is proved by Lyapunov stability theory. The proposed methods are implemented with TMS320C31 DSP. Experimental results prove that the observer has a good estimation performance of the rotor speed and position despite of the parameter variations and loads, and the speed control can be accomplished in the wide speed range.

• 한국자기학회:학술대회 개요집
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• 한국자기학회 2008년도 Asian Magnetics Conference
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• pp.21.2-21.2
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• 2008

• 대한전자공학회:학술대회논문집
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• 대한전자공학회 2003년도 하계종합학술대회 논문집 II
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• pp.1201-1204
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• 2003

This paper described a replica-delay adaptive lock-acquisition scheme for high-speed DLLs. The proposed scheme provides the fast and correct locking cycle that is variable according to the magnitude of the arbitrary replica delay (fixed delay). The scheme guarantees the wide operation range and the fast lock-aquisition time. It has been confirmed by HSPICE simulations in a 0.35${\mu}{\textrm}{m}$ CMOS process.

• 대한전기학회:학술대회논문집
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• 대한전기학회 2008년도 제39회 하계학술대회
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• pp.659-660
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• 2008

This paper presents a hybrid winding connection method for torque characteristics improving of a traction SRM. In order to get a high torque in wide speed range and torque ripple reduction, series and parallel winding connection are changed according to operating speed. From the analysis of torque character operation mode and efficiency, the proposed control scheme is verified.