一种考虑电流过零的直线电机分段供电策略

为了解决长初级直线电机采用分段供电的驱动方式带来的电流波动、系统性能下降等缺点,以双边长初级直线感应电机为研究对象,提出了一种根据次级所在位置和电流过零点两个条件进行切换的新型分段供电策略,推导了考虑电流过零切换时分段供电暂态特性的直线电机方程,在Matlab/Simulink环境下建立了系统动态仿真模型,并在小型样机上进行了试验验证。仿真和试验结果表明:考虑电流过零切换的分段供电策略可显著减小电流的波动,满足系统性能和实际工程的需求。     

三相绕组切换电路分析与设计

针对在装甲车辆中体积、功率受限条件下,永磁电动机提升低速转矩与拓宽弱磁工作范围难以兼顾的问题,提出了采用电动机绕组换接的方法。提出了一种切换电路的设计方案,采用电路仿真软件Muhisim分析了切换电路的动态工作过程。在此基础上,构建了一套适用于三相绕组切换的零电流切换控制系统,可通过上位机观测电路中的电流波形,并实现对绕组的串、并联结构进行切换控制。试验结果表明：该电路可快速实现绕组的串／并联切换,而且基本上不会产生电压冲击。     

幅压直流电动机-中频发电机控制技术

幅压直流供电的电动-交流发电机组广泛应用于潜艇等由蓄电池供电的直流独立电源系统.发电机组输出交流电的供电质量受蓄电池的浮动电压和交流负载等因素影响,同时,还存在着输出电压漂移、频率不稳等现象.通过研究机组的工作特性以及独立供电的小容量系统负载冲击对机组输出电压稳定性的影响,针对影响输出电源品质的因素,采用有效的磁场补偿技术和PI控制技术相结合的方法,维持机组输出电压幅值、频率的稳定,试验验证了该方法的有效性.     

Norton集成运放及其应用(下)

4典型应用电路 LM3900可用于交直流放大、滤放、开关控制、波形产生等多种场合。下面选择一些典型电路予以介绍。4.1放大器 (1)高输入阻抗反相放大电路 如图1所示,R_1=R_3=R_4=1MΩ,R_2=2MΩ,R_5=10kΩ,C_1、C_2的容抗应尽量小。设直流参考电压为V_R,则可以求出输出端直流电压;输出端的交流电压     

过零比较器实验中相位失真问题的研究

本文探讨了过零比较器的频率特性,得出输出与输入电压之间的相位差随频率变化的原因,进而提出测量过零比较器截止频率的一种简易办法。     

新型模拟乘法器AD633JN的几种应用(编译)

新型模拟乘法器ＡＤ６３３是一种功能完整的、四－象限模拟乘法器,其输出为电压。本文以型号为ＡＤ６３３ＪＮ的ＡＤ６３３产品为例,首先介绍了其功能框图及其传输函数。然后介绍了它的几种应用电路,即：在一个输入端口接入控制电压,将有关管脚适当连接,并接入电阻或阻、容元件而组成的实现压控增益放大及压控低通、高通滤波的电路;将有关管脚适当连接,并接入阻容元件而组成的无直流分量输出的频率二倍器电路;将输入端口及有     

利用RLC电路合成高压方波脉冲

基于过阻尼RLC电路分时放电,提出了一种新型高压方波脉冲产生方法。理论分析表明过阻尼RLC电路产生的双指数电压波与PFL产生的方形电压波具有类似的上升沿和平顶。电路模拟表明通过人工过零技术可以对双指数电压波进行截尾,从而形成完整的高压方波脉冲。建立了原理验证性样机,由两组RLC电路构成,每组电路包含一台400 nF脉冲电容器和一只三电极场畸变气体开关,两组电路共用一个上升沿调节电感。实验证明样机可以在250 Ω电阻负载上输出幅值17 kV,平顶宽度330 ns~5.8 μs,上升沿100~ 350 ns 的单极性高压方波脉冲。该方法适应性强,对负载变化不敏感,同时具有良好的可调节性,方波上升沿、平顶宽度连续独立可调。     

带有高感性直流负载的桥式整流电路的计算

本文从带有高感性直流负载的单相及三相桥式整流电路稳态工作的物理情况出发,首先考虑交流侧电感的影响,导出了交流侧和直流侧之间的电压关系式和电流关系式。进而考虑交流侧电阻对电压关系的影响,将直流电压平均值用马克劳林级数表示,取前两项作为近似值,从而得出了足够准确而又便于实用的计算公式。三相桥式整流电路有三种可能的工作情况,主要取决于交流侧感抗对直流侧电阻比值的大小。本文确定了三种整流工作情况所分别对应的参数比值范围。     

新型SRD软开关功率电路分析与仿真

针对开关磁阻电机调速系统(SRD)电磁干扰及开关损耗大的问题,提出了一种新型并联谐振软开关变换器的电路拓扑。通过在传统公共逆变桥的各相开关上并联一个缓冲电容,并在直流母线上加入一个由两个电感与一个电容为主要组成元件的谐振环节,实现对所有功率开关和谐振开关的软动作。此电路没有采用电感与分压电容中性点连接方式,故不会因中性点电位变化而影响系统可靠性。理论分析及MATLAB仿真结果表明:该电路具有控制简单,母线电压过渡速度快,谐振元件参数小,功率损耗小等特点。     

基于潮流计算的环形直流微电网母线电压协调控制策略研究

针对现有的基于直流母线电压信号的传统控制方法在不同的工作模式运行时存在电压控制偏差的问题,首先对现有的分层控制策略进行了改进,通过潮流计算确定了本地控制器参数,并以负荷端电压为控制目标,实现了将不同模式下的负荷端电压均控制在相同的参考值上;然后,利用仿真软件搭建了四端环形直流微电网及其控制算法的模型,并对不同模式间的切换进行了仿真分析,仿真分析结果验证了该策略的可行性和有效性。     

Reviews

Douglas S. Derrer, We Are All the Target: A Handbook of Terrorism Avoidance and Hostage Survival. Annapolis, MD: US Naval Institute Press, 1992. Pp. x+135, notes, index. $14.95. ISBN 01–55750–150–5

Ian Knight, Zulu: Isandlwana and Rorke's Drift, 22–23 January 1879. London: Windrow & Greene, 1992. Pp.136, 150 illus., incl 8 colour plates, maps, biblio. £35. ISBN 1–872004–23–7

Ian Knight, By the Orders of the Great White Queen: Campaigning in Zululand through the Eyes of the British Soldiers, 1879. London: Greenhill Books and Novato: Presidio Press, 1992. Pp. 272, 17 illus., 1 map. £18.95. ISBN 1–85367–122–3

Manfried Rauchensteiner and Erwin A. Schmidl (eds.), Formen des Krieges: vom Mittelalter zum ‘Low‐intensity’ Conflict’. Graz : Verlag Styria, 1991. Pp.208. DM35. ISBN 3–22–12139–7

Harold J. Kearsley, Maritime Power and the Twenty‐First Century. Dartmouth: Dartmouth Publishing Company, Limited, 1992. Pp.xv + 203, 13 diagrams, index. £32.50. ISBN 1–85521–288–9     

Panjwai: a tale of two COINs in Afghanistan

The US Army has two approaches to counterinsurgency operations in Afghanistan. One is hard, or combat-focused, and the other is soft, or development-focused. This study examines two US Army task forces deployed to Panjwai District, Afghanistan from 2012 to 2013. CTF 4-9 and 1-38 offer a meaningful comparison because they pursued these contrasting approaches among the same population and against the same enemy at the same time and place. The study compares each unit’s approach and finds that neither approach was successful absent the other. The article concludes by recommending further research into combining the approaches at the operational level.     

线性核时频信号表示及其仿真

根据ｃｈｉｒｐ信号在模糊域的特点,本文提出了线性核时频表示方法。这种方法在提高分辨率、消除交叉项以及抑制噪声等方面都具有较高的性能。理论分析和实验结果都证实了这种方法的有效性     

Forecasting a Hurricane: Israeli and American Estimations of the Khomeini Revolution

The surprising ‘Arab Spring’ raises the question as to what would enable national intelligence to provide high quality warnings prior to the eruption of popular revolutions. This article uses new sources of evidence to trace and explain Israel's success in comparison to US failure at correctly estimating the course of the Iranian Revolution in 1977–79. In explaining this variance, the article shows that it was mainly the result of the intimate acquaintance of Israel's representatives in Iran with the local language, history and culture, as well as the ability to communicate with locals – tools which the Americans completely lacked.     

关于教师继续教育培训模式的思考

继续教育作为终身教育的重要组成部分和基本形式,对教师的发展成长意义重大。继续教育工作开展十多年来,取得了一定的成绩,但还存在不少问题,如学习动机较为被动,管理认识不到位,教学内容陈旧空泛,教学方法呆板机械等。随着课程改革的不断深入,继续教育必须采取相应的改进措施,树立以人为本,服务至上的观点,教学内容的前瞻性与实用性并重,教学人员进行整合,教学方法灵活多样,评价考核规范科学等,以此激活受训教师内在的学习动机,更大程度地提高继续教育培训的实际效果。     

用DirectShow实现网络视频

目前市场上的网络监控技术一般用昂贵的硬件设备来支撑,投资较大。在深入研究微软的DirectX9.0的基础上,提出如何用DirectShow软件技术和普通的数字摄像头来实现网络视频的监控和实现技术,内容涉及通信协议、视频的编码与解码技术、视频的编码格式和COM组件编程技术等,最终的实验结果令人满意,且费用低廉,技术可靠,为在高速局域网内构建视频监控系统提供有效的技术支持。     

Interpreting the Ribchester Helmet

In 1796 a 13-year-old boy playing on wasteland behind his father’s house in Ribchester, Lancashire discovered an assemblage of over 30 Roman artefacts, including a decorated brass helmet. Although partially corroded the Ribchester helmet is a fine example of a first-century AD cavalry sports helmet. This article considers how the interpretation of the helmet has evolved since its discovery. The multi-faceted iconography of the helmet with its mural crown diadem is that of an elite soldier and is paralleled in other examples of cavalry sports helmets of the Ribchester type. The long-neglected imagery on the helmet bowl is reinterpreted and seen not as a haphazard collection of randomly placed combatants but as a highly structured, symmetrical composition specifically designed to be viewed from different directions.     

Putin – the masked nemesis of the strategy of ambiguity

Recent events demonstrate the complex and adaptive approach employed by Russia to reassert influence in Europe. The changing face of Russia’s strategy commenced in 2007 when it launched a crippling cyber-attack against Estonia. This was followed by a large Russian conventional attack against Georgia in 2008, occupying two large areas of the nation. 2014 witnessed the Russian annexation of Crimea where in just a week, Russia seized control of Crimea “without firing a shot.” The annexation of Crimea was rapidly followed by a Russian inspired and led subversive war in eastern Ukraine. The common thread among these diverse Russian operations is its use of ambiguity to confound and confuse decision makers in the West.     

Desert insurgency: lessons from the third Tuareg rebellion

This article provides an in-depth examination and analysis of the 2006–2009 Tuareg rebellion in Mali and Niger. It identifies the underlying reasons behind the rebellion, explores contrasting counter-insurgency (COIN) strategies employed by the two governments, and presents some lessons learned. While both COIN approaches ultimately produced similar peace settlements, the article argues that the Malian strategy of reconciliation combined with the selective use of force was far more effective than the Nigerien iron fist approach at limiting the size and scope of the insurgency and producing a more sustainable peace. It concludes by looking at the role of external actors, particularly the United States, and how the failure to internationalize the conflict was actually more beneficial to the local COIN effort, as well as to the longer strategic interests of the United States in the region.