新型制冷剂研究及应用冰箱压缩机的设计

为了解决R12的主流替代工质R134a作为冰箱制冷剂时存在的问题,提出给R134a系统加入R600a组成混合工质R134a/R600a的方案,提供物性和热力性能比较结果,证明R134a/R600a作为冰箱制冷剂比R134a和R600a更具有优势,并提出了采用新型制冷剂的冰箱压缩机的改进方案.R134a/R600a的性能试验结果证明其与矿物油的互溶性良好,成功的解决了R134a系统由于更换润滑油而产生的一系列问题.     

Computational finance models

The author discusses his involvement in developing computational finance software. These computational finance models attempt to model the randomness of a stock's price. At a fixed future time, a stock's price is modeled as a random variable with a normal distribution centered about the current price adjusted with a simple growth multiplier. The standard deviation of this normal distribution depends on the length of time into the future one peers and the volatility of the market. As the market becomes more volatile and we look further ahead, the less likely the stock will have a price near the adjusted current price. Implementing these ideas requires a tool borrowed from physics called the Brownian motion. In a sense, a stock's price is modeled as a point fluctuating about in "dollar space". Hence a financial modeler can no more predict what price a stock will have at a given instance in time than a physicist can predict where a particular air molecule might be.     

Steady-state corona charging behavior of a corotron over a movingdielectric substrate

Charging up a dielectric surface through corona discharge from a thin wire has been a common practice in electrophotographic processes. One of the widely used corona charging devices is called a corotron, which consists of a coronating wire enclosed in a rectangular shield with one constituent side being the surface to be charged. Uniform surface charge can be deposited on a dielectric substrate, such as a photoreceptor in the absence of light, by moving the substrate at a constant velocity through a stationary corotron that consistently emits corona current. To design an efficient corotron for charging dielectric substrates, a fundamental understanding of the electrostatic nature of the device is desired. In this paper, the steady-state behavior of corona charging with a corotron over a moving dielectric substrate is analyzed by computationally solving the nonlinearly coupled equation system with Galerkin finite-element method and Newton iterations. The predictions based on a first-principle model are shown to agree well with experimental measurements     

Negotiating expert status: who gets taken seriously

A lay person may listen puzzled as a specialist in a public hearing discusses a technical point. Similarly, a challenging question from a technically knowledgeable member of the audience may be difficult to fathom. Failure to understand what a speaker discusses-despite one's interest in a topic-can cause frustration, anger and despair. It makes a listener wonder why they are there trying to engage, and why technical specialists persist in such a style of presentation. Breakdowns in communication between technical and nontechnical people are linked to the concept of “expert”. This link involves the process of the “negotiation of expert status”; this refers to the social construction of the role of the “expert” by the participants in a conversation. The term “negotiation” refers to the sometimes subtle and sometimes overtly contentious nature of a debate about who deserves to be listened to as a voice of authority on a particular topic     

Teleoperation via computer network

In a teleoperation system, providing force information to a human operator can improve task performance. When a communication block between a master and a slave has a transmission delay, the system is easily destabilized. Anderson and Spong guaranteed passivity in the communication block by using scattering transformation and overcame this instability caused by the time delay. But this method can be applied to the communication block with a constant time delay. In a traditional teleoperation system, its communication block has a constant time delay. But time delay irregularly changes in a computer network because many users share telecommunication lines. This paper shows that the variable time delay destabilizes bilateral master-slave manipulator with scattering transformation and a new compensation method which keeps the time delay constant. This new compensation method has been implemented in a single-axis master-slave manipulator.     

A study of electric stress enhancement. Part 1. Implication in power cable design

The increase in electric stress due to a surface protrusion at the interface between a semiconductive shield and a polymer insulation layer in power cables can lead to localized electron injection into the polymer insulation and result in undesired material degradation. This paper reports the analyses of electric stress enhancement of surface protrusions in several commonly encountered medium- and high-voltage power cable configurations. First, a brief review of the electric stress enhancement theories is presented. Then evaluations using these theories for various power cable configurations are made, and it is shown that theories considering a hyperboloidal protrusion will exhibit more realistic stress enhancement results than the cases based on a spheroidal protrusion. Further examination reveals that, besides the sharpness of a protrusion, thickness of the insulation medium also plays a governing role in the stress enhancement at the interfaces between different dielectric media. Contrary to the conventional wisdom, it is a surprise to observe that a thicker insulation can actually cause higher stress enhancement at a protrusion tip, under a given applied voltage. Because of this, a flatter surface protrusion at a higher voltage setting, which usually has thicker insulation, can result in earlier degradation than a sharper protrusion at a lower voltage, even though sharper extrusions are expected to have higher stress enhancement. As a result, thicker insulation may not always be advantageous in the power cable design for higher voltage applications.     

Methods for controlling a linear asynchronous motor

This article describes the case of one of possible method of controlling a linear asynchronous motor (LAM) with a controlled normal component of the force of interaction between an inductor and a reactive bus. It is shown that the normal force in a unilateral linear asynchronous motor can affect significantly the operation of devices and systems on which a motor is installed. This normal force can attract an inductor to a reactive bus or repel it from a reactive bus. For the model of a motor, which is represented by four zones, namely, an inductor, an air gap, a conductive bus, and a steel core of a reactive bus, the analytical expressions for the calculation of the components of a magnetic field in an air gap were obtained. The effect of the longitudinal and transverse edge effects in the considered plane model of a motor is not considered. The expressions for the determination of the specific tangential and normal forces per area unit were found. It is shown that, under a change in the slide from negative to positive values, the tangential force changes similarly to the electromagnetic torque of a conventional asynchronous motor with a rotating rotor. Under a change in the slide, the normal force can attract an inductor to a reactive bus or repel it from a reactive bus. The force of attraction reaches the highest values under a zero slide. The slide under which the normal force is equal to zero is determined only by the pole pitch and the conductivity of material of a reactive bus and provides the operation of a motor at the highest power factor. It is proposed to use the measured normal force value as a parameter for the construction of a traction linear asynchronous motor-control system.     

Modeling and performance of a static frequency converter starting a 300 MVA synchronous machine

This paper presents a study of the dynamic behavior of a static frequency converter driving a 300 MVA synchronous generator which is used in a pumped storage power plant of Taiwan Power Company. The static frequency converter, which consists of a three-phase AC/DC phase converter, a filtering inductance, and a six-step current-source inverter, is used to start a 300 MVA synchronous machine. In this paper, first the mathematical model of the whole system which consists of a synchronous machine, a static frequency converter, a microprocessor, and sensors is established. Then, the control method of the static frequency converter based on field oriented control is presented. Next, a filter circuit has been designed to reduce the harmonics of the AC source. By using this filter, the harmonics can be effectively reduced. Finally, some computer simulated and experimental results are shown to verify the theoretical analysis. This paper presents a new direction in the use of advanced power electronic technology to drive a very large synchronous machine.     

A practical approach to harmonic compensation in powersystems-series connection of passive and active filters

The authors present a combined system with a passive filter and a small-rated active filter, both connected in series with each other. The passive filter removes load produced harmonics just as a conventional filter does. The active filter plays a role in improving the filtering characteristics of the passive filter. This results in a great reduction of the required rating of the active filter and in eliminating all the limitations faced by using only the passive filter, leading to a practical and economical system. The active filter has a much smaller rating than a conventional active filter. Experimental results obtained from a prototype model are shown to verify the theory developed     

Unit commitment using Lagrangian relaxation and particle swarm optimization

Unit commitment (UC) is a NP-hard nonlinear mixed-integer optimization problem. This paper proposes ELRPSO, an algorithm to solve the UC problem using Lagrangian relaxation (LR) and particle swarm optimization (PSO). ELRPSO employs a state-of-the-art powerful PSO variant called comprehensive learning PSO to find a feasible near-optimal UC schedule. Each particle represents Lagrangian multipliers. The PSO uses a low level LR procedure, a reserve repairing heuristic, a unit decommitment heuristic, and an economic dispatch heuristic to obtain a feasible UC schedule for each particle. The reserve repairing heuristic addresses the spinning reserve and minimum up/down time constraints simultaneously. Moreover, the reserve repairing and unit decommitment heuristics consider committing/decommitting a unit for a consecutive period of hours at a time in order to reduce the total startup cost. Each particle is initialized using the Lagrangian multipliers obtained from a LR that iteratively updates the multipliers through an adaptive subgradient heuristic, because the multipliers obtained from the LR tend to be close to the optimal multipliers and have a high potential to lead to a feasible near-optimal UC schedule. Numerical results on test thermal power systems of 10, 20, 40, 60, 80, and 100 units demonstrate that ELRPSO is able to find a low-cost UC schedule in a short time and is robust in performance.     

Microendoscopic Confocal Imaging Probe Based on an LVD Microlens Scanner

This paper reports a microendoscopic confocal imaging probe based on a large-vertical-displacement (LVD) microlens scanner. Photoresist reflow technique is used to form a microlens on a lens holder that is integrated with a LVD microactuator. With a footprint of 3 mmtimes2 mm, the LVD microlens scanner can vertically displace the integrated polymer microlens by 0.7 mm at a dc voltage of 23 V. The LVD microlens chip is directly packaged into a 5-mm-diameter imaging probe. The architecture of the imaging probe is very simple and is just composed of this microlens, a GRIN rod lens (Oslash1.8 mm), a metal mold, and a cover     

Power Supplies for Electronic High-Voltage Measuring-Current Transformers

A power supply for electronic high-voltage measuring-current transformers with extraction of energy from the measured current that flows through a high-voltage line has been considered. A magnetic core of the supply transformer from the windings of which the necessary power is extracted is mounted on the line for this purpose. When the line current changes over a wide range, the supply transformer should have a quickly saturated magnetic core. It has been shown that, to reduce the losses and decrease the loading of the elements of a power-supply circuit in a wide current range, a ballast load should be introduced in the circuit of a secondary winding. Three variants of the ballast load have been compared: resistive, capacitive, and in the form of a transistor-current limiter. The results of simulations in the VisSim software have been considered for all three variants, as well as for the case of absence of a ballast load in the power-supply circuit. The simulations were carried out for the lowest and highest measured currents, as well as for the line short-circuit mode. The results of simulations have shown the advantages of a power supply based on a saturated transformer with a transistor-current limiter. A variant of a circuit of a transistor-current limiter has been suggested.     

换热器管壁圆形泄漏口空间辐射声场分布特征数值研究

根据可分离辐射源的声辐射，利用叠加原理和无穷级数迭代求和，建立了换热器管道壁面圆形口非分离泄漏声辐射源的空间声场分布数学模型。计算了圆形口均匀幅度非分离声辐射源的空间声场远场分布，给出了其远场相对声压级和位相的空间分布图案。与方形口均匀可分离辐射源的空间声场分布特征相比，发现两种情况具有相同的空间指向性特征，在较高频率时辐射声场空间分布出现盲区，在x—y平面内声场分布与泄漏口形状无关，但与辐射频率有密切关系。     

有载调压变压器对系统中长期电压稳定性影响的仿真研究

应用电力系统中长期动态仿真软件Eurostag仿真研究有载调压变压器在系统不同的负荷特性下对中长期电压稳定性的影响。仿真分析表明, 有载调压变压器表现出“正调压效应”还是“负调压效应”主要取决于系统低压侧的负荷特性, 当系统重负荷且电压已经接近极限时, 对电压灵敏度高的负荷特性, 典型的是恒阻抗负荷, 有载调压变压器容易表现出“负调压效应”, 对系统电压稳定很不利; 受有载调压变压器调压影响不大的负荷特性, 典型的是恒功率负荷, 有载调压变压器对系统中长期电压稳定相当有利, 表现出很好的“正调压效应”。系统中有动态负荷时, 有载调压变压器容易表现出“负调压效应”, 但表现的程度受有载调压变压器的调压档距和容量的影响, 并且表现的方式也和静态负荷特性有所不同。     

Development of a web-based laboratory for control experiments on acoupled tank apparatus

The Internet provides a new environment for developing a variety of applications for educational and research purposes. This paper presents the implementation of a web-based laboratory experiment on a coupled tank apparatus, a multi-input-multi-output (MIMO) system. The web-based laboratory has been developed to serve students and staff in the Department of Electrical Engineering at the National University of Singapore (NUS). The laboratory is an educational tool for teaching students the basic principles and methodology in performing a series of experiments on a coupled tank apparatus at any time and from any location through the Internet. With the capability to implement strategies for manual, proportional integral derivative (PID), general state-space, and fuzzy logic control, the laboratory also provides a platform for research staff to test control algorithms. Video conferencing has been used to provide audio and video feedback, with a camera mounted on a movable platform so that the user can control both the zooming and viewing angle     

High speed transmission system directional protection using anElman network

Detection of the direction of a fault on a transmission line is essential to the proper performance of a power system. It would be desirable to develop a high speed and accurate approach to determine the fault direction for different power system conditions. To classify forward and backward faults on a given line, a neural network's abilities in pattern recognition and classification could be considered as a solution. To demonstrate the applicability of this solution, neural network technique is employed and a novel Elman recurrent network is designed and trained. Details of the design procedure and the results of performance studies with the proposed network are given and analysed in the paper. System simulation studies show that the proposed approach is able to detect the direction of a fault on a transmission line rapidly and correctly. It is suitable to realize a very fast transmission line directional comparison protection scheme     

Time and the eyes to see [waveform analysis]

In engineering we frequently make use of an oscilloscope to see the various forms of waves with which we work. If it is a sound wave, we would use a microphone to capture the wave. The microphone is known as a transducer, converting the acoustic energy into electrical energy. A photo-sensor acts as a transducer for a light wave and an antenna is able to retrieve a radio wave. To interpret the waves, of whichever kind, the waveform can be recorded and this is not a difficult task if the wave is displayable on the oscilloscope. Joseph Fourier (1768-1830) produced a theory that allowed us to analyze the waves that we record. His method can be likened to many of the tools that have been discussed. We have seen physical transformations to provide insights and this is a transformation of a mathematical kind.     

A capstone design course [electrical engineering]

A capstone electrical engineering design course has been developed at the University of San Diego with the goal of providing students with the opportunity to: study design alternatives and select a design responsive to a request for proposal; sell the design concept to a panel of independent evaluators with a written proposal and oral presentations; obtain experience in design within a cooperating group; and completely fabricate and document a working electronic system. While innovation is not overtly discouraged, emphasis is placed upon satisfactory completion of a working product with the time constraints of a two semester course which spans one summer. The course is intended not only to provide a meaningful design experience but also to accomplish a confidence-building transition to the role of practicing engineer