118型传真机自动对相电路原理

用于接收气象卫星云图的118型传真机,如要实现自动接收,关键是将传真机由手工对相改为自动对相,即需在传真机里增加一个自动对相电路。本文简要介绍了此电路的设计思想及其工作原理。

相差高频保护两侧电流回路的对相方法

相差高频保护的基本原理是直接比较两侧电流的相位,根据相位差值的大小来决定保护是否动作.因此,要求两侧操作元件中通入的电流相别应一一对应.这样,两侧发出的高频信号才能对线路故障进行如实地反应.所以在新装保护投运前,对两侧电流同路进行相别核对(简称电流对相)是十分必要的.现场常反映电流对相没有一个统一的方法及判断标准,以至有时对结果无法分析.本文拟以操作电压正比于量为典型,对电流对相方法进行阐述与分析.1 电流对相方法在图1所示系统中,MN为被保护线路,并假定由M侧向N侧送有功及无功.则电流对相方法按下列步骤进行(负荷电流二次值宜大于lA,此要求值为参考值,实际只要二次电流能可靠操作发信就行。

用V—V型电压互感器对相原理及实用对照表

本文介绍了用不完全星形接法的电压互感器(V-V型)校对同一电压系统的两条回路相序、相位的基本原理及方法,并总结出了一种实用对照表,给实际工作带来了很大的方便。

基于CPLD三相全控桥整流电源

三相晶闸管可控整流电源广泛应用于大功率工业控制领域,其中的核心控制环节触发器和调节器的设计是电力电子领域的热点。可控硅整流电源系统具有复杂性、非线性、时变性等特点,合理设计各个控制环节以达到最优的控制效果是本文研究的重点。本文提出的数字同步触发电路,产生的触发脉冲稳定性好,可靠性高,并且不需同步变压器、具有相序自适应功能,它可以在线系统编程,调试方便,在数字移相技术中将会得到广泛使用。

评《观测与相对论》

《北京工业大学学报》本期"学术争鸣"栏目中的《观测与相对论:光速在爱因斯坦狭义相对论中为什么不变?》(以下简称《观测与相对论》),试图否定光速不变性原理,挑战爱因斯坦相对论!爱因斯坦是人类历史上最伟大的科学家。在物理学界,爱因斯犹如上帝一般地存在,而爱因斯坦相对论则犹如圣经一般。

试谈《新编对相四言》的来龙去脉

为了考查古代教育,特别是语文教育的情况,我做过一点收集,查索、探讨古代蒙学用书的工作。其间,曾在明初《文渊阁书目》和《录竹堂书目》中查到一本《对相识字》。这是见于著录的最早的看图识字课本("对相"即图、文对照之意)。

基于实测响应轨迹稳定边界的电力系统暂态不稳定识别

为充分利用广域测量系统WAMS(wide area measurement system)信息实现电力系统暂态稳定性快速在线识别,提出一种基于实测响应轨迹稳定边界的暂态不稳定识别方法.根据单机"位能脊"推导了单机-无穷大系统在相平面上的暂态稳定边界;证明单机无穷大系统任意比例剖分点处,由扰动能与电压相角构成的平面上的轨迹与相平面轨迹具有相似的几何特征,为间接利用发电机端口外网络测量信息识别电力系统暂态不稳定性提供了依据;证明了临界机组对的相轨迹上二阶导数等于零的点构成了系统的不返回边界,提出用临界机组对的相轨迹几何特征来识别系统暂态稳定性.为避免判据在线应用时受参数及不确定性干扰可能造成误判,对判据进行了实用性改进.利用PSASP 6.28 WEPRI 36节点仿真算例验证了所提判据的有效性.

带CPLD新型通用数字触发器的设计

针对模拟触发器和基于微机控制的数字触发器存在的不足。提出了一种以复杂可编程逻辑器件(CPLD)和锁相环技术为核心的新型通用数字触发器,对其硬件电路和软件设计进行了详细分析。最后通过实验验证了该触发器的有效性和可行性。另外,由于CPLD具有在线多次编程功能,因此该触发器不必改变外围电路,只需在线修改软件程序,适用于各种晶闸管触发装置中。该触发器不仅控制精度高,调式方便,而且应用灵活,成本低廉,可以满足不同类型触发电路的要求。

一种基于CPLD的新型通用数字触发器

研究一种基于CPLD的新型通用晶闸管数字触发器 ,详细介绍了CPLD的软件及其外围硬件电路 ,对锁相环的动静态特性进行了分析 。

Effects of magnesium chloride-based multicomponent salts on atmospheric corrosion of aluminum alloy 2024

Atmospheric corrosion of aluminum alloy 2024 （AA2024） with salt lake water was simulated through a laboratory- accelerated test of cyclic wet-dry and electrochemical techniques. Effects of the soluble magnesium salt contained in the salt water were investigated by scanning electron microscope （SEM）, transmission electron microscope （TEM）, energy dispersive spectrometer （EDS）, electron probe micro analyzer （EPMA）, X-ray diffraction （XRD）, infrared transmission spectroscope （IR）, and atmospheric corrosion monitor （ACM）. The results showed that, with the deposition, atmospheric corrosion of AA2024 could occur when the relative humidity （RH） was lower than 30%. A main crystalline component of corrosion products, layered double hydroxides （LDH）, [Mg1-xAlx（OH）2]^x＋ Clx-·mH2O （LDH-C1）, was determined, which meant that magnesium ion played an important role in the corrosion process. It not only facilitated the corrosion as a result of deliquescence, but also was involved in the corrosion process as a reactant.

Properties and microstructure of Cu/diamond composites prepared by spark plasma sintering method

Cu/diamond composites have been considered as the next generation of thermal management material for electronic packages and heat sinks applications. Cu/diamond composites with different volume fractions of diamond were successfully prepared by spark plasma sintering（SPS） method. The sintering temperatures and volume fractions（50%, 60% and 70%） of diamond were changed to investigate their effects on the relative density, homogeneity of the microstructure and thermal conductivity of the composites. The results show that the relative density, homogeneity of the microstructure and thermal conductivity of the composites increase with decreasing the diamond volume fraction; the relative density and thermal conductivity of the composites increase with increasing the sintering temperature. The thermal conductivity of the composites is a result of the combined effect of the volume fraction of diamond, the homogeneity and relative density of the composites.

Study on Physiological Characteristics of Three Different Cold-resistant Tomato Strains

[Objective]The aim was to study cold-resistant indexes and physiological characters of three different cold-resisitant tomato strains selected under natural conditions. [Method]With 3-4 weeks tomato seedlings as tested materials,through simulating external environment conditions by the cold treatment in lab,the survival ratio and growth status of seedlings were identified; by studying the morphological change of in vitro leaves after cold treatment and determining the relative conductivity,the cold-resistant degree of different tomato strains was identified and the correlation between conductivity and survival rate was analyzed; the variation of soluble sugar and praline content in plant of tomato was determined under the condition of cold stress. [Result]The results showed that leaf morphological changes,size of conductivity and growth status of different tomato strains after cold treatment have significant differences; the variation of soluble sugar contents in plant has significant differences,while the variation of proline content has no significant differences. [Conclusion]There is a certain correlation between conductivity and survival rate under cold treatment conditions,and plant cold tolerance is related to accumulation of soluble sugar content.

NOVEL GENERALIZED GREY INCIDENCE MODEL BASED ON INTERVAL GREY NUMBERS

To extend the traditional generalized grey incidence model, a novel grey incidence model based on inter- val grey numbers is constructed. Considering the numerical information of indexes cannot be accurately obtained and can be defined as interval grey numbers, the interval grey numbers are defined as standard interval grey num- bers which are split in white part and grey part. The absolute degree of incidence and relative degree of incidence based on the interval grey numbers are constructed and their arithmetic are given. Finally, an example about commercial aircraft index selection illuminates the effectiveness of the model. The results show that the model can sort indexes better and can extend the grey incidence models significantly.

Effects of Sand Burial on the Survival, Growth, and Biomass Allocation in Semi-shrub Hedysarum laeve Seedlings

Hedysarum laeve Maxim. (Leguminosae) is one of the major species used frequently in revegetation of dune_field in the sandlands of the northern part of China by means of aerial sowing. Seedlings of the species after emergence above the sand surface may be buried in sand to various depths during its establishment in late spring and early summer. A study was made to examine the effects of sand burial at different levels of 0 (control), 33%, 67%, 100% and 133% of their shoot height, on the survivorship, growth, and biomass allocation pattern of H. laeve seedlings (one and two weeks old after emergence). When burial depth was up to 100% of their shoot height, about 70% seedlings died; and the burial at depth of 133% of their shoot height led to death of all seedlings. When seedlings was buried at depth of 33% and 67% of their shoot height, respectively, after six_week growth, their biomass of whole plant, blade, and root and relative growth rate were higher than the unburied counterparts. The seedlings in both 33% and 67% sand burial treatments did not significantly change their biomass allocation pattern comparing with the unburied ones. Furthermore, the number of leaves and shoot height of the seedlings in both 33% and 67% sand burial treatments were not significantly different from those of unburied individuals, respectively. The newly born leaves of the surviving seedlings, in 33%, 67%, and 100% burial treatments, during the period of experiment, were significantly more than those in control.

In situ investigation of atmospheric corrosion behavior of PCB-ENIG under adsorbed thin electrolyte layer

The effects of relative humidity (RH) on a printed circuit board finished with electroless nickel immersion gold (PCB-ENIG) under an adsorbed thin electrolyte layer (ATEL) were investigated in situ via the measurement of cathodic polarization curves, electrochemical impedance spectroscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy to clearly elaborate the corrosion behavior of PCB-ENIG in the atmospheric environment. Results indicated that the cathodic process of PCB-ENIG under ATEL was dominated by the reduction of dissolved oxygen, corrosion products, and H2O. The cathodic current density of PCB-ENIG increased progressively with increasing RH. Moreover, its cathodic current density in the solution was greater than that under ATEL. This result demonstrated that the diffusion process was not the controlling step during the limiting reduction of cathodic oxygen. When the polarization potentials were located in a more negative region, the cathodic polarization current density gradually decreased under 75% and 85% RH. Notably, the anodic process became the controlling step in the extremely thin liquid film during the remainder of the experiment.

DUAL QUATERNION CURVE INTERPOLATION ALGORITHM FOR FORMATION SATELLITES

The traditional algorithms for formation flying satellites treat the satellite position and attitude sepa- rately. A novel algorithm combining satellite attitude with position is proposed. The principal satellite trajectory is obtained by dual quaternion interpolation, then the relative position and attitude of the deputy satellite are ob- tained by dual quaternion modeling on the principal satellite. Through above process, relative position and atti- tude are unified. Compared with the orbital parameter and the quaternion methods, the simulation result proves that the algorithm can unify position and attitude, and satisfy the precision requirement of formation flying satel- lites.

An Evaluation of Crown Structure and Site Index to Determine Crown Closure in Eucalyptus urophylla × E. grandis Stands in Guangxi,China

Eucalyptus forests are grown in many parts of the world for their commercial value and use in construction projects. Density management becomes im- portant as a means to attain the management goals in these forests. Changes in canopy and tree crown structure were quantified for Eucalyptus urophylla x E. grandis forests at different ages to determine when canopy closure occurs and the onset of competition begins. Site index was developed for these forests to determine whether site quality affects the canopy structure. Site index had little effect in the forests sampled, with the forest canopy on the better sites becoming slightly more elongated. Based upon crown projection ratio and crown diameter： dbh （diameter at breast height） ratio, it appears that crown closure occurs by age 4 years in these forests. The age at which this occurs was also checked and verified with the evaluation of relative spacing, RS （the ratio of the mean distance between trees to the average dominant height of the stand）. The RS value displayed a rapid decline until age 4 years, and then became relatively flat through age 21 years. The rapid height growth during the first 3 years with no change in density accounted for this rapid decrease. By age 4 years, reductions in the number of trees due to mortality began to have more of an influence on this value, resulting in a more gradual de- cline. The implications for management are discussed.

基于CPLD新型通用数字触发器的实验研究

针对模拟触发器和基于微机控制的数字触发器存在的不足。提出了一种以复杂可编程逻辑器件(CPLD)和锁相环技术为核心的新型通用数字触发器,对其硬件电路和软件设计进行了详细分析。最后通过实验验证了该触发器的有效性和可行性。另外,由于CPLD 具有在线多次编程功能。因此该触发器不必改变外围电路,只需在线修改软件程序,适用于各种晶闸管触发装置中。该触发器不仅控制精度高,调式方便,而且应用灵活,成本低廉,可以满足不同类型触发电路的要求。

Winter Wheat Salt Tolerance Appraisal Indices with Varied Mineral Content of Saline Water

[Objective] The aim was to study wheat salt resistance appraisal indicators under the condition of saline water irrigation.[Method] A trial was conducted with five varieties irrigated with saline water at 1,2,4,6,and 8 g/L during 2009-2010 and 2010-2011 in the research station of Dry-land Farming Institute of Hebei Academy of Agricultural and Forestry Sciences.With standardized indicators for measuring treatment,the response relations among different salt stress levels and winter wheat growth index were analyzed in this study.[Result] Under the condition of different salinity of irrigation,relative plant height after jointing stage,relative leaf area index,relative dry matter weight,relative ear number per unit area showed significant differences among treatments and yield showed significant correlation,which can be taken as salt-resistance examination index of wheat.[Conclusion] The results showed that the relative height after jointing stage was recommended as the most practical salt-tolerance appraisal indices,because it was easy to be observed and sensitive to salt stress.