• Journal of international Conference on Electrical Machines and Systems
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• v.1 no.1
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• pp.54-60
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• 2012

In this paper, a new permanent-magnet (PM) linear motor is proposed, in which both the magnets and armature windings are placed in the short mover, while the long stator consists of iron core only. Hence, this new PM linear motor can be called a primary permanent-magnet linear motor. It exhibits the advantages of robustness, low cost, high efficiency, high power factor, and high thrust force density. It is especially suitable for long stator applications such as urban rail transit. In this paper, the topology and operation principle of this motor are discussed in detail. The steady-state characteristics including field distributions, flux-linkage, back-EMF, phase inductance and thrust force are investigated. In addition, the technique of skewing stator teeth is adopted to improve the electromagnetic performance. Results from finite element method (FEM) verified the theoretical analysis results.

• Membrane and Water Treatment
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• v.6 no.6
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• pp.451-476
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• 2015

Membrane distillation (MD), which can utilize low-grade thermal energy, has been extensively studied for desalination. By incorporating solar thermal energy, the solar membrane distillation desalination system (SMDDS) is a potential technology for resolving the energy and water resource problems. Small-scale SMDDS (s-SMDDS) is an attractive and viable option for the production of fresh water for small communities in remote arid areas. The minimum-cost design and operation of s-SMDDS are determined by a systematic method, which involves a pseudo steady state approach for equipment sizing and the dynamic optimization using overall system mathematical models. The s-SMDDS employing three MD configurations, including the air gap (AGMD), direct contact (DCMD) and vacuum (VMD) types, are optimized. The membrane area of each system is $11.5m^2$. The AGMD system operated for 500 kg/day water production rate gives the lowest unit cost of $5.92/m^3$. The performance ratio and recovery ratio are 0.85 and 4.07%, respectively. For the commercial membrane employed in this study, the increase of membrane mass transfer coefficient up to two times is beneficial for cost reduction and the reduction of membrane heat transfer coefficient only affects the cost of the DCMD system.

• International Journal of Air-Conditioning and Refrigeration
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• v.12 no.2
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• pp.79-86
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• 2004

An analytical and experimental study has been conducted to determine the optimal charging mass of working fluid for the maximum heat transport capacity of heat pipe with axially grooved wick. When the heat pipe is operated in a steady state, the liquid-vapor meniscus recession of working fluid to the bottom of groove is occurred in the evaporator region. In this work, the optimal charging mass of working fluid was obtained by considering the meniscus recession from the axial variation of capillary pressure, the radius of curvature and wetting angle of meniscus of liquid-vapor interface. Experimental results were also obtained by varying the charging mass of working fluid within a heat pipe, and presented for the trend of maximum heat transport capacity corresponding to the operating temperature and the elevation of heat pipe. Finally, the analytical results of the optimal charging mass of working fluid were compared with those from the experiment, both of which were in good agreement with each other.

• Journal of Korean Society of Industrial and Systems Engineering
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• v.20 no.43
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• pp.153-162
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• 1997

M/G/1 queueing system is one of the most widely used one to model the real system. When operating a real systems, since it often takes cost, some control policies that change the operation scheme are adopted. In particular, the N-policy is the most popular among many control policies. Almost all researches on queueing system are based on the assumption that the arrival rates of customers into the queueing system is constant, In this paper, we consider the M/G/1 queueing system whose arrival rate varies according to the servers status : idle, set-up and busy states. For this study, we construct the steady state equations of queue lengths by means of the supplementary variable method, and derive the PGF(probability generating function) of them. The L-S-T(Laplace Stieltjes transform) of waiting time and average waiting time are also presented. We also develop an algorithm to find the optimal N-value from which the server starts his set-up. An analysis on the performance measures to minimize total operation cost of queueing system is included. We finally investigate the behavior of system operation cost as the optimal N and arrival rate change by a numerical study.

• Journal of the Korean Solar Energy Society
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• v.24 no.3
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• pp.27-38
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• 2004

A numerical investigation has been carried out for a solar system, which consists of all glass solar vacuum tubes Water is heated as it flows through the coaxial fluid conduit inserted in each tube. The space between the exterior of the fluid conduit and the glass tube is filled with antifreeze solution. This is to facilitate heat transfer from the solar heated absorber surface to water and to prevent the functional problems due to freezing in frigid weather conditions. A one-dimensional steady state model is fully described which will be used to develop three-dimensional model using STAR-CD. These models could be used efficiently in designing all-glass solar collector tubes with different geometrical parameters other than those considered in the present analysis. Results show good agreement when compared with other experimental data demonstrating the reliability of the present model.

• International Journal of Naval Architecture and Ocean Engineering
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• v.10 no.6
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• pp.782-793
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• 2018

A small-scale horizontal axis hydrokinetic turbine is designed, manufactured and studied both experimentally and numerically in this study. The turbine is expected to work in most of China's sea areas where the ocean current velocity is low and to supply electricity for remote islands. To improve the efficiency of the turbine at low flow velocities, a magnetic coupling is used for the non-contacting transmission of the rotor torque. A prototype is manufactured and tested in a towing tank. The experimental results show that the turbine is characterized by a cut-in velocity of 0.25 m/s and a maximum power coefficient of 0.33, proving the feasibility of using magnetic couplings to reduce the resistive torque in the transmission parts. Three dimensional Computational Fluid Dynamics (CFD) simulations, which are based on the Reynolds Averaged Navier-Stokes (RANS) equations, are then performed to evaluate the performance of the rotor both at transient and steady state.

• The Transactions of The Korean Institute of Electrical Engineers
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• v.61 no.9
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• pp.1232-1241
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• 2012

The ac side current of an high voltage direct current (HVDC) converter is characterized by highly non-sinusoidal waveform. If the harmonic current is allowed to flow in the connected ac system, it may cause unacceptable levels of distortion. Therefore, ac side filters are required as part of the total HVDC converter station, in order to reduce the harmonic distortion of the ac side current and voltage to acceptably low levels. The ac side filters are also employed to compensate network requested reactive power because HVDC converters also consume substantial reactive power. Among different types of filters, double-tuned filters have been widely utilized for HVDC system. This paper presents two design methods of double-tuned filter; equivalent method and parametric method. Using a parametric method, in particular the paper proposes a new design algorithm for a realistic system. Finally, the performance of the design algorithm is evaluated for a 80kV HVDC system in Jeju island with PSCAD/EMTDC program. The results cleary demonstrate the effectiveness of proposed design method in harmonics elimination and steady-state stability.

• Journal of Electrical Engineering and Technology
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• v.12 no.3
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• pp.1187-1194
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• 2017

In this paper, a high step-up DC-DC PWM converter with continuous input current and low voltage stress is presented for renewable energy application. The proposed converter is composed of a boost converter integrated with an auxiliary step-up circuit. The auxiliary circuit uses an additional coupled inductor and a balancing capacitor with voltage doubler and switching capacitor technique to achieve high step-up voltage gain with an appropriate switch duty cycle. The switched capacitors are charged in parallel and discharged in series by the coupled inductor, stacking on the output capacitor. In the proposed converter, the voltage stress on the main switch is clamped, so a low voltage switch with low ON resistance can be used to reduce the conduction loss which results in the efficiency improvement. A detailed discussion on the operating principle and steady-state analyses are presented in the paper. To justify the theoretical analysis, experimental results of a 200W 40/400V prototype is presented. In addition, the conducted electromagnetic emissions are measured which shows a good EMC performance.

• Journal of Power Electronics
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• v.4 no.4
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• pp.205-216
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• 2004

In this paper, a three-phase induction machine-based wind power generation scheme is proposed. This scheme uses a low-cost diode bridge rectifier circuit connected to an induction machine via an ac load voltage regulator (AC-LVR) to regulate dc power transfer. The AC-LVR is used to regulate the DC load voltage of the diode bridge rectifier circuit which is connected to the three-phase self-excited induction generator (SEIG). The excitation of the three-phase SEIG is supplied by the static VAR compensator (SVC). This simple method for obtaining a full variable-speed wind turbine system by applying a back-to-back power converter to a wound rotor induction generator is useful for wind power generation at widely varying speeds. The dynamic performance responses and the experimental results of connecting a 5kW 220V three-phase SEIG directly to a diode bridge rectifier are presented for various loads. Moreover, the steady-state simulated and experimental results of the PI closed-loop feedback voltage regulation scheme prove the practical effectiveness of these simple methods for use with a wind turbine system.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.28 no.12B
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• pp.1106-1114
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• 2003

Several analytic models describe transmission control protocol (TCP) performance such as steady-state throughput as an averaged ratio of number of transmissions to latency. For more detailed analysis of TCP latency, the latency during packet losses are recovered should be considered. In this paper, we derive the expected duration of loss recovery latency considering the number of packet losses recovered by retransmissions. Based on the numerical results verified by simulations, TCP using selective acknowledgement (SACK) option is more effective than TCP NewReno from the aspect of loss recovery latency.