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浙江大学本科毕业论文外文文献翻译

浙江大学本科毕业论文外文文献翻译

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未经允许,请勿外传!浙江大学本科毕业论文外文文献翻译The influence of political connections on the firm value of small and medium-sized enterprises in China政治关联在中国对中小型企业价值的影响1摘要中小型企业的价值受很多因素的影响,比如股东、现金流以及政治关联等.这篇文章调查的正是在中国政治关联对中小型企业价值的影响。

通过实验数据来分析政治关联对企业价值效益的影响.结果表明政府关联是关键的因素并且在中国对中小型企业的价值具有负面影响。

2重要内容翻译2。

1引言在商业界,有越来越多关于政治关联的影响的经济研究。

它们发现政治关联能够帮助企业确保有利的规章条件以及成功获得资源,比如能够最终提高企业价值或是提升绩效的银行贷款,这种政治关联的影响在不同的经济条件下呈现不同的效果。

在高腐败和法律制度薄弱的国家,政治关联对企业价值具有决定性因素1的作用.中国由高度集权的计划经济向市场经济转变,政府对市场具有较强的控制作用,而且有大量的上市企业具有政治关联。

中小型企业发展的很迅速,他们已经在全球经济环境中变得越来越重要。

从90年代起, 政治因素对中国的任何规模的企业来说都变得越来越重要,尤其是中小型企业的价值。

和其他的部门相比较,中小型企业只有较小的现金流,不稳定的现金流且高负债率.一方面,中小型企业改变更加灵活;另一方面,中小型企业在由于企业规模以及对银行来说没有可以抵押的资产,在筹资方面较为困难。

企业如何应对微观经济环境和政策去保证正常的企业活动,并且政治关联如何影响企业价值?这篇论文调查政治关联和企业价值之间的联系,并且试图去研究企业是否可以从政治关联中获利提升企业价值。

2.2定义这些中小型企业之所以叫中小型企业,是和管理规模有关。

对这些小企业来说,雇员很少,营业额较低,资金一般由较少的人提供,因此,通常由这些业主直接管理企业。

外文文献翻译(图片版)

外文文献翻译(图片版)

本科毕业论文外文参考文献译文及原文学院经济与贸易学院专业经济学(贸易方向)年级班别2007级 1 班学号3207004154学生姓名欧阳倩指导教师童雪晖2010 年 6 月 3 日目录1 外文文献译文(一)中国银行业的改革和盈利能力(第1、2、4部分) (1)2 外文文献原文(一)CHINA’S BANKING REFORM AND PROFITABILITY(Part 1、2、4) (9)1概述世界银行(1997年)曾声称,中国的金融业是其经济的软肋。

当一国的经济增长的可持续性岌岌可危的时候,金融业的改革一直被认为是提高资金使用效率和消费型经济增长重新走向平衡的必要(Lardy,1998年,Prasad,2007年)。

事实上,不久前,中国的国有银行被视为“技术上破产”,它们的生存需要依靠充裕的国家流动资金。

但是,在银行改革开展以来,最近,强劲的盈利能力已恢复到国有商业银行的水平。

但自从中国的国有银行在不久之前已经走上了改革的道路,它可能过早宣布银行业的改革尚未取得完全的胜利。

此外,其坚实的财务表现虽然强劲,但不可持续增长。

随着经济增长在2008年全球经济衰退得带动下已经开始软化,银行预计将在一个比以前更加困难的经济形势下探索。

本文的目的不是要评价银行业改革对银行业绩的影响,这在一个完整的信贷周期后更好解决。

相反,我们的目标是通过审查改革的进展和银行改革战略,并分析其近期改革后的强劲的财务表现,但是这不能完全从迄今所进行的改革努力分离。

本文有三个部分。

在第二节中,我们回顾了中国的大型国有银行改革的战略,以及其执行情况,这是中国银行业改革的主要目标。

第三节中分析了2007年的财务表现集中在那些在市场上拥有浮动股份的四大国有商业银行:中国工商银行(工商银行),中国建设银行(建行),对中国银行(中银)和交通银行(交通银行)。

引人注目的是中国农业银行,它仍然处于重组上市过程中得适当时候的后期。

第四节总结一个对银行绩效评估。

毕业设计论文外文文献翻译

毕业设计论文外文文献翻译

毕业设计(论文)外文文献翻译院系:财务与会计学院年级专业:201*级财务管理姓名:学号:132148***附件: 财务风险管理【Abstract】Although financial risk has increased significantly in recent years risk and risk management are not contemporary issues。

The result of increasingly global markets is that risk may originate with events thousands of miles away that have nothing to do with the domestic market。

Information is available instantaneously which means that change and subsequent market reactions occur very quickly。

The economic climate and markets can be affected very quickly by changes in exchange rates interest rates and commodity prices。

Counterparties can rapidly become problematic。

As a result it is important to ensure financial risks are identified and managed appropriately. Preparation is a key component of risk management。

【Key Words】Financial risk,Risk management,YieldsI. Financial risks arising1.1What Is Risk1.1.1The concept of riskRisk provides the basis for opportunity. The terms risk and exposure have subtle differences in their meaning. Risk refers to the probability of loss while exposure is the possibility of loss although they are often used interchangeably。

外文文献及翻译

外文文献及翻译

外文文献及翻译1. 文献:"The Effects of Exercise on Mental Health"翻译:运动对心理健康的影响Abstract: This article explores the effects of exercise on mental health. The author discusses various studies that have been conducted on this topic, and presents evidence to support the claim that exercise can have positive impacts on mental well-being. The article also examines the mechanisms through which exercise affects mental health, such as the release of endorphins and the reduction of stress hormones. Overall, the author concludes that exercise is an effective strategy for improving mental health and recommends incorporating physical activity into daily routines.摘要:本文探讨了运动对心理健康的影响。

作者讨论了在这个主题上进行的各种研究,并提出证据支持运动对心理健康有积极影响的观点。

该文章还探讨了运动如何影响心理健康的机制,如内啡肽的释放和压力激素的减少。

总的来说,作者得出结论,运动是改善心理健康的有效策略,并建议将体育活动纳入日常生活。

2. 文献: "The Benefits of Bilingualism"翻译:双语能力的好处Abstract: This paper examines the benefits of bilingualism. The author presents research findings that demonstrate the cognitiveadvantages of being bilingual, such as enhanced problem-solving skills and improved attention control. The article also explores the social and cultural benefits of bilingualism, such as increased cultural awareness and the ability to communicate with people from different backgrounds. Additionally, the author discusses the positive effects of bilingualism on mental health, highlighting its role in delaying the onset of cognitive decline and in providing a buffer against age-related memory loss. Overall, the author concludes that bilingualism offers a range of advantages and recommends promoting bilingual education and language learning. 摘要:本文研究了双语能力的好处。

外文文献翻译译稿

外文文献翻译译稿

外文文献翻译译稿1可用性和期望值来自Willliam S.Green, Patrick W.Jordan.产品的愉悦:超越可用性根据人机工程学会(HFES)的观点,人机工程学着眼于“发现和共享可用于各种系统和设备设计的、关于人的特点的知识”。

人们通常只是把它作为生物力学和人体测量所关注的内容,实际上它是从更广泛的意义上的一种对人(产品用户)的全面和综合的理解。

HFES从二战中有军方从事的系统分析中发展而来。

其中的三种主要研究的是人体测量、复杂信息的解释和管理,以及在部队和装备调配中应用的系统分析。

系统分析在尺度和复杂性方面跨度很大,大的系统分析有类似于诺曼底登陆准备的大型系统规划,小到去理解如何从合理性和规模的角度才最佳的布置和装备人员。

诺曼底登陆是20世纪最复杂的事件之一。

他要求建立一个在战斗开始之前还不确定的庞大的人员和物资的合理分配系统。

在更小的规模上,装备和军事人物的布置意味着如何去组织、训练和安排战士,最大限度的发挥他们的长处。

士兵必须迅速地接受训练,并且能够有效地使用和维护在二战中发展起来的一系列技术装备。

其中,对于飞行员、潜艇人员和坦克驾驶员有神采的限制。

复杂的新装备的开发要求找到最好的税收、密码便医院、破译人员、雷达和声纳操作员、轰炸机驾驶员和机组人员。

在战后,随着公司及其产品在尺度、领域和复杂性方面的增长,很多系统分析人员在商用领域找到了发展机会。

尽管是战后的发展才导致了1957年人机工程协会(HFES)的建立,但人机研究的起源可以追溯到大批量生产方式的成型阶段,是当时提高生产效率的要求。

随着工作方式从手工生产和农业生产中的转移,新的工厂工作的概念逐步发展起来。

福特的流水生产线和泰勒的效率理论开始对生产的规划和教育产生影响。

即使在家庭生活中,妇女们也开始接受了现代家庭管理理论,并运用这些理论来组织和规划家庭。

在20世纪末,一种涵盖面更广的人机工程正在发展之中。

新的人机工程学是为了适应已经被广泛意识到的对用户行为模式更深入的需求而诞生的,它开始应用定型研究方法,并探索人的情感和认知因素。

外文文献及翻译

外文文献及翻译

Yunnan Ethnic pattern in Packaging DesignAbstract: Art is a folk Mother of the arts,is the source of the new art, From which to draw a strong tradition of high—grade Nutrition。

The persons belonging to national folk arts Ethnic patterns,are folk arts Intraoperative a gem, its development Research,and with the means of modern art,art wind Grid,professional skills combine to form a unique Style and features a modern design There are important applications,this paper focuses on Minority Folk pattern in modern packaging design Meter applications are discussed.Keywords: Yunnan Ethnic. Pattern。

Packaging Design.IntroductionYunnan is a multi-ethnic province, Here multiply survive the Han, Yi, Bai More than twenty families, Zhuang, Miao, Dai, etc。

Nation. Long history of various ethnic groups in Yunnan Province, the source is far Long, creating a rich and colorful Folk art。

电气工程的外文文献(及翻译)

电气工程的外文文献(及翻译)

电气工程的外文文献(及翻译)文献一:Electric power consumption prediction model based on grey theory optimized by genetic algorithms本文介绍了一种基于混合灰色理论与遗传算法优化的电力消耗预测模型。

该模型使用时间序列数据来建立模型,并使用灰色理论来解决数据的不确定性问题。

通过遗传算法的优化,模型能够更好地预测电力消耗,并取得了优异的预测结果。

此模型可以在大规模电力网络中使用,并具有较高的可行性和可靠性。

文献二:Intelligent control for energy-efficient operation of electric motors本文研究了一种智能控制方法,用于电动机的节能运行。

该方法提供了一种更高效的控制策略,使电动机能够在不同负载条件下以较低的功率运行。

该智能控制使用模糊逻辑方法来确定最佳的控制参数,并使用遗传算法来优化参数。

实验结果表明,该智能控制方法可以显著降低电动机的能耗,节省电能。

文献三:Fault diagnosis system for power transformers based on dissolved gas analysis本文介绍了一种基于溶解气体分析的电力变压器故障诊断系统。

通过对变压器油中的气体样品进行分析,可以检测和诊断变压器内部存在的故障类型。

该系统使用人工神经网络模型来对气体分析数据进行处理和分类。

实验结果表明,该系统可以准确地检测和诊断变压器的故障,并有助于实现有效的维护和管理。

文献四:Power quality improvement using series active filter based on iterative learning control technique本文研究了一种基于迭代研究控制技术的串联有源滤波器用于电能质量改善的方法。

论文外文文献翻译

论文外文文献翻译

论文外文文献翻译以下是一篇700字左右的论文外文文献翻译:原文题目:The Role of Artificial Intelligence in Medical Diagnostics: A Review原文摘要:In recent years, there has been a growing interest in the use of artificial intelligence (AI) in the field of medical diagnostics. AI has the potential to improve the accuracy and efficiency of medical diagnoses, and can assist clinicians in making treatment decisions. This review aims to examine the current state of AI in medical diagnostics, and discuss its advantages and limitations. Several AI techniques, including machine learning, deep learning, and natural language processing, are discussed. The review also examines the ethical and legal considerations associated with the use of AI in medical diagnostics. Overall, AI has shown great promise in improving medical diagnostics, but further research is needed to fully understand its potential benefits and limitations.AI在医学诊断中发挥的作用:一项综述近年来,人工智能(AI)在医学诊断领域的应用引起了越来越多的关注。

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毕业设计(论文)外文文献翻译题目:基于DSP的单相恒压逆变器设计教学院:电气与电子信息工程学院专业名称:电气工程及其自动化学号:201040220221学生姓名:明寿指导教师:马学军2014 年5月10日逆变器SHI TingNa, W ANG Jian1引言逆变器是一种电动装置,转换成直流电(DC),交流电流转换的AC(交流)可以在任何所需的电压和频率使用适当的变压器,开关,控制circuits.Solid状态逆变器有没有移动部件,用于广泛的应用范围从小型计算机开关电源,高压大型电力公司电力,运输散装直接电流应用。

逆变器通常用于提供交流电源,直流电源,如太阳能电池板或电池。

逆变器的主要有两种类型。

修改后的正弦波逆变器的输出是类似方波输出,输出变为零伏前一段时间切换积极或消极的除外。

它是简单,成本低,是大多数电子设备兼容,除敏感或专用设备,例如某些激光打印机。

一个纯正弦波逆变器产生一个近乎完美的正弦波输出(<3%的总谐波失真),本质上是相同的公用事业提供电网。

因此,它是与所有的交流电的电子设备兼容。

这是在电网领带逆变器使用的类型。

它的设计更复杂,成本5或10倍以上每单位功率电逆变器是一个高功率的电子振荡器。

它这样命名,因为早期的机械AC到DC转换器工作在反向,因而被“倒”,将直流电转换AC.The变频器执行的整流器对面功能。

2应用2.1直流电源利用率逆变器从交流电力来源,如电池,太阳能电池板,燃料电池的直流电转换成。

电力,可以在任何所需的电压,特别是它可以操作交流电源操作而设计的设备,或纠正,以产生任何所需的voltage Grid领带逆变器的直流送入分销网络的能量,因为它们产生电流交替使用相同的波形和频率分配制度提供。

他们还可以关掉一个blackout.Micro逆变器的情况下自动转换成交流电电网的电流直接从当前个别太阳能电池板。

默认情况下,他们是格领带设计。

2.2不间断电源不间断电源(UPS),电池和逆变器,交流电源,主电源不可用时使用。

当主电源恢复正常时,整流提供直流电源给电池充电。

2.3感应加热逆变器的低频交流主电源转换到更高频率的感应加热使用。

要做到这一点,首先纠正交流电源提供直流电源。

逆变器,然后改变高频率的交流电源,直流电源。

2.4高压直流输电随着高压直流输电,交流电源经过整流和高压直流电源传输到另一个位置。

在接收的位置,在静态逆变器厂逆变器转换回交流电源。

2.5变频驱动器一个变频驱动控制向电动机提供电源的频率和电压控制交流电机的运行速度。

逆变器提供控制电源。

在大多数情况下,变频驱动,包括整流器,使逆变器的直流电源,可从交流主电源提供。

由于逆变器是关键部件,变频驱动,有时被称为逆变器驱动器,或只是逆变器。

2.6电动汽车驱动器目前使用的权力,在一些电动和柴油- 电动轨道车辆以及一些电池的电动汽车和混合动力电动公路车辆,如丰田Prius和菲斯克噶牵引电机调速电机控制逆变器。

变频技术的各种改进正在开发专门用于电动汽车的应用。

[2]在再生制动的车辆,逆变器也需要从电机(作为发电机)的权力,并储存在电池中。

2.7一般情况下一个变压器,使交流电源转换为任何所需的电压,但在相同的频率。

直流逆变器,加上整流器,可以被用来转换从任何电压,交流或直流,任何其他的电压,也交流或直流,在任何所需的频率。

输出功率不能超过输入功率,但效率高的余热消耗的功率小的比例。

3电路描述3.1基本设计在一个简单的逆变电路,直流电源连接到变压器初级绕组中心抽头通过。

一个正在迅速来回切换开关允许电??流流回直流电源后,两个备用路径,然后通过初级绕组的一端其他。

在变压器的初级绕组中的电流方向交替产生交流电(AC)在二次回路。

机电开关设备的版本包括两个固定触点和支持动触头弹簧。

春天拥有对固定触点之一的可移动的接触和电磁铁拉动产接触到相对固定的联系。

在电磁铁的电流被中断的开关,使开关不断来回切换迅速的行动。

这种机电逆变器开关的类型,称为一个振动器或蜂鸣器,曾一度被用于真空管汽车收音机。

类似的机制已被用于门铃,蜂鸣器和纹身枪。

当他们成为具有足够的额定功率,晶体管和其他各种类型的半导体开关逆变电路设计已纳入。

3.2输出波形如上所述,在简单的逆变器开关时不耦合到输出变压器,产生一个方形的电压波形作为反对的是平常的交流电源波形的正弦的波形,由于其简单的关闭和对自然的。

使用傅立叶分析,周期性波形表示作为正弦波无穷级数的总和。

原始波形正弦波具有相同的频率被称为基本组成部分。

其他正弦波,称为谐波,该系列包括有频率是基本频率的整数倍。

需要从变频器的输出波形的质量取决于所连接的负载的特点。

一些负载需要一个近乎完美的正弦波电源电压才能正常工作。

方波电压与其他负载可能工作得非常好。

3.3三阶段逆变器用于三相逆变器变频驱动应用和高功率应用,如高压直流输电。

一个基本的三个单相逆变器的三相逆变器组成,每个交换机连接到三个负载端子。

三层交换机的运作最基本的控制计划,协调,使一台交换机工作在60度的基本输出波形的每个点。

这将创建一个线到线输出波形有六个步骤。

六步波形方波是3的倍数的谐波消除如上所述的正面和负面的部分之间的零电压的一步。

当舰载PWM技术应用于六步波形,基本整体造型,或信封的波形,保留3次谐波及其倍数,使被消除。

4历史4.1早期的逆变器从十九世纪末到二十世纪中叶,直流 - 交流功率转换完成使用旋转器或马达发电机组(爵套)。

在二十世纪初,真空管和充满气体管开始被用于逆变器电路中的开关。

最广泛使用的管型晶闸管。

机电逆变器的起源解释源长期变频器。

早期的AC至DC转换器采用感应或同步交流电机直接连接到一台发电机(发电机),使发电机的整流子扭转在正确的时刻其连接生产直流。

后来的发展是同步的转换器,电机和发电机绕组结合成一个电枢,一端与滑环和整流子在其他只有一个领域的框架。

结果要么是交流,直流。

与MG 组,直流,可考虑将分别从AC生成,同步器,它在一定意义上可以认为是“机械纠正交流”。

由于正确的辅助设备和控制设备,MG集或旋转转换,可以“倒着跑”,将直流转换为交流电。

因此,逆变器是一个倒置的转换。

4.2可控整流逆变器自从1957年初年初以来,晶体管不能提供足够的电压和额定电流最逆变器应用,它是1957年的晶闸管或可控硅(SCR)的介绍,开始过渡到固态逆变电路。

可控硅的换相的条件是在可控硅电路设计的关键考虑因素。

不要关闭可控硅整流自动门控制信号被切断时。

他们只关闭当正向电流降至低于最低维持电流,每一种可控硅变化,通过一些外部进程。

对于连接到交流电源的可控硅,整流发生自然每次源??电压极性反转。

可控硅直流电源连接到通常需要强迫换,强制要求减刑时电流为零的一种手段。

最复杂的可控硅电路采用自然,而不是被迫换减刑。

此外被迫换电路,可控硅已被用于在以上所述的逆变器电路的类型。

在逆变器传输到AC电源由直流电源供电的应用程序,它可以使用交流 - 直流可控整流电路的反演模式经营。

在反演模式,可控整流电路整流逆变器行。

这种类型的操作,可用于高压直流输电系统和再生制动电机控制系统的操作。

另一种类型的可控硅逆变电路是电流源输入(CSI)逆变器。

一个CSI逆变器是一个六步的电压源逆变器的双。

用一个电流源逆变器,直流电源作为电流源而非电压源配置。

变频器可控硅开关在六步序列直接阶梯电流波形作为一个三相交流负载的电流。

沪深逆变器换方法包括整流负载和并联电容器减刑。

这两种方法,输入电流调节协助减刑。

带整流负载,负载是在领先的功率因数运行的同步电机。

因为他们已经成为在更高的额定电压和电流,如可以通过控制信号的晶体管或IGBT的半导体已成为首选开关元件逆变电路使用。

4.3整流器和逆变器的脉冲数整流电路往往流的每个周期的AC输入电压整流的直流侧电流脉冲的数量分类。

单相半波整流是一个脉冲电路和单相全波整流是两个脉冲的电路。

一个三相半波整流是一个三脉冲电路和三相全波整流是一个六脉冲电路。

两个或两个以上的整流器三相整流器,有时串联或并联连接以获得更高的电压或额定电流。

提供特种变压器提供相移输出整流器的输入。

这有相乘法效应。

六个阶段是从两个变压器,12个阶段从三变等。

12脉冲整流器,18脉冲整流器等相关的整流电路。

当可控整流电路的反演模式在运作,他们将分为脉冲数也。

整流电路具有较高的脉冲数减少交流输入电流和减少直流输出电压纹波的谐波含量。

在反演模式,有较高的脉冲个数的电路,在AC输出电压波形的谐波含量较低。

5 参考文献[1] R. Organti, K. Nagaswamy, L. Sang, Predicted equal charge creterion scheme forconstant frequency control of single phase boost-type AC-DC converter, IEEE Trans.Power Electron 13 (1) (1998) 47-57.[2] M. Ohshima, E. Masada, A single-phase PCS with a novel constantly sampledcurrent-regulated PWM scheme, IEEE Trans. Power Electron 14 (5) (1999) 823-830.[3] T. Toshida, O. Shiizuka, O. Miyashita, K. Ohniwa, An improvement technique for theefficiency of high-frequency switch-mode rectifiers, IEEE Trans. Power Electron 15 (6)(2000) 1118-1123.[4] R. Sriniuasan,R. Oruganti,A unity power factor converter using half-bridge boosttopology, IEEE Trans.Power Electron 13(3)(1998) 487-500.[5] K. Itako, T. Mori, A high performance rectifier control system with feedforward controland DC resonance filter, in: International Conference on Electrical Engineering 2 005, Kunming, China, July 10-14, 2005.[6] K. Itako, T. Mori. Full bridge PWM rectifier with load current feedforward, in:International Conference on Electrical Engineering, Hong Kong, 2009.[7] W. Leonhard, Control of Electrical Drives, 3rd ed.,Springer, 2001, pp. 160-162.InverterSHI TingNa, WANG Jian1 IntroductionAn inverter is an electrical device that converts direct current (DC) to alternating current (AC); the converted AC can be at any required voltage and frequency with the use of appropriate transformers, switching, and control circuits.Solid-state inverters have no moving parts and are used in a wide range of applications, from small switching power supplies in computers, to large electric utility high-voltage direct current applications that transport bulk power. Inverters are commonly used to supply AC power from DC sources such as solar panels or batteries.There are two main types of inverter. The output of a modified sine wave inverter is similar to a square wave output except that the output goes to zero volts for a time before switching positive or negative. It is simple and low cost and is compatible with most electronic devices, except for sensitive or specialized equipment, for example certain laser printers. A pure sine wave inverter produces a nearly perfect sine wave output (<3% total harmonic distortion) that is essentially the same as utility-supplied grid power. Thus it is compatible with all AC electronic devices. This is the type used in grid-tie inverters. Its design is more complex, and costs 5 or 10 times more per unit power The electrical inverter is a high-power electronic oscillator. It is so named because early mechanical AC to DC converters were made to work in reverse, and thus were "inverted", to convert DC to AC.The inverter performs the opposite function of a rectifier.2 Applications2.1 DC power source utilizationAn inverter converts the DC electricity from sources such as batteries, solar panels,or fuel cells to AC electricity. The electricity can be at any required voltage; in particular it can operate AC equipment designed for mains operation, or rectified to produce DC at any desired voltageGrid tie inverters can feed energy back into the distribution network because they produce alternating current with the same wave shape and frequency as supplied by the distribution system. They can also switch off automatically in the event of a blackout.Micro-inverters convert direct current from individual solar panels into alternating current for the electric grid. They are grid tie designs by default.2.2 Uninterruptible power suppliesAn uninterruptible power supply (UPS) uses batteries and an inverter to supply AC power when main power is not available. When main power is restored, a rectifier supplies DC power to recharge the batteries.2.3 Induction heatingInverters convert low frequency main AC power to a higher frequency for use in induction heating. To do this, AC power is first rectified to provide DC power. The inverter then changes the DC power to high frequency AC power.2.4 HVDC power transmissionWith HVDC power transmission, AC power is rectified and high voltage DC power is transmitted to another location. At the receiving location, an inverter in a static inverter plant converts the power back to AC.2.5 Variable-frequency drivesA variable-frequency drive controls the operating speed of an AC motor by controlling the frequency and voltage of the power supplied to the motor. An inverter provides the controlled power. In most cases, the variable-frequency drive includes a rectifier so that DC power for the inverter can be provided from main AC power. Since an inverter is the key component, variable-frequency drives are sometimes calledinverter drives or just inverters.2.6 Electric vehicle drivesAdjustable speed motor control inverters are currently used to power the traction motors in some electric and diesel-electric rail vehicles as well as some battery electric vehicles and hybrid electric highway vehicles such as the Toyota Prius and Fisker Karma. Various improvements in inverter technology are being developed specifically for electric vehicle applications. In vehicles with regenerative braking, the inverter also takes power from the motor (now acting as a generator) and stores it in the batteries.2.7 The general caseA transformer allows AC power to be converted to any desired voltage, but at the same frequency. Inverters, plus rectifiers for DC, can be designed to convert from any voltage, AC or DC, to any other voltage, also AC or DC, at any desired frequency. The output power can never exceed the input power, but efficiencies can be high, with a small proportion of the power dissipated as waste heat.3 Circuit description3.1 Basic designsIn one simple inverter circuit, DC power is connected to a transformer through the centre tap of the primary winding. A switch is rapidly switched back and forth to allow current to flow back to the DC source following two alternate paths through one end of the primary winding and then the other. The alternation of the direction of current in the primary winding of the transformer produces alternating current (AC) in the secondary circuit.The electromechanical version of the switching device includes two stationary contacts and a spring supported moving contact. The spring holds the movable contact against one of the stationary contacts and an electromagnet pulls the movable contact tothe opposite stationary contact. The current in the electromagnet is interrupted by the action of the switch so that the switch continually switches rapidly back and forth. This type of electromechanical inverter switch, called a vibrator or buzzer, was once used in vacuum tube automobile radios. A similar mechanism has been used in door bells, buzzers and tattoo guns.As they became available with adequate power ratings, transistors and various other types of semiconductor switches have been incorporated into inverter circuit designs3.2 Output waveformsThe switch in the simple inverter described above, when not coupled to an output transformer, produces a square voltage waveform due to its simple off and on nature as opposed to the sinusoidal waveform that is the usual waveform of an AC power supply. Using Fourier analysis, periodic waveforms are represented as the sum of an infinite series of sine waves. The sine wave that has the same frequency as the original waveform is called the fundamental component. The other sine waves, called harmonics, that are included in the series have frequencies that are integral multiples of the fundamental frequency.The quality of output waveform that is needed from an inverter depends on the characteristics of the connected load. Some loads need a nearly perfect sine wave voltage supply in order to work properly. Other loads may work quite well with a square wave voltage.3.3 Three phase invertersThree-phase inverters are used for variable-frequency drive applications and for high power applications such as HVDC power transmission. A basic three-phase inverter consists of three single-phase inverter switches each connected to one of the three load terminals. For the most basic control scheme, the operation of the three switches is coordinated so that one switch operates at each 60 degree point of the fundamental output waveform. This creates a line-to-line output waveform that has six steps. The six-step waveform has a zero-voltage step between the positive and negative sections ofthe square-wave such that the harmonics that are multiples of three are eliminated as described above. When carrier-based PWM techniques are applied to six-step waveforms, the basic overall shape, or envelope, of the waveform is retained so that the 3rd harmonic and its multiples are cancelled.4 History4.1 Early invertersFrom the late nineteenth century through the middle of the twentieth century, DC-to-AC power conversion was accomplished using rotary converters or motor-generator sets (M-G sets). In the early twentieth century, vacuum tubes and gas filled tubes began to be used as switches in inverter circuits. The most widely used type of tube was the thyratron.The origins of electromechanical inverters explain the source of the term inverter. Early AC-to-DC converters used an induction or synchronous AC motor direct-connected to a generator (dynamo) so that the generator's commutator reversed its connections at exactly the right moments to produce DC. A later development is the synchronous converter, in which the motor and generator windings are combined into one armature, with slip rings at one end and a commutator at the other and only one field frame. The result with either is AC-in, DC-out. With an M-G set, the DC can be considered to be separately generated from the AC; with a synchronous converter, in a certain sense it can be considered to be "mechanically rectified AC". Given the right auxiliary and control equipment, an M-G set or rotary converter can be "run backwards", converting DC to AC. Hence an inverter is an inverted converter.4.2 Controlled rectifier invertersSince early transistors were not available with sufficient voltage and current ratings for most inverter applications, it was the 1957 introduction of the thyristor or silicon-controlled rectifier (SCR) that initiated the transition to solid state inverter circuits.The commutation requirements of SCRs are a key consideration in SCR circuit designs. SCRs do not turn off or commutate automatically when the gate control signal is shut off. They only turn off when the forward current is reduced to below the minimum holding current, which varies with each kind of SCR, through some external process. For SCRs connected to an AC power source, commutation occurs naturally every time the polarity of the source voltage reverses. SCRs connected to a DC power source usually require a means of forced commutation that forces the current to zero when commutation is required. The least complicated SCR circuits employ natural commutation rather than forced commutation. With the addition of forced commutation circuits, SCRs have been used in the types of inverter circuits described above.In applications where inverters transfer power from a DC power source to an AC power source, it is possible to use AC-to-DC controlled rectifier circuits operating in the inversion mode. In the inversion mode, a controlled rectifier circuit operates as a line commutated inverter. This type of operation can be used in HVDC power transmission systems and in regenerative braking operation of motor control systems.Another type of SCR inverter circuit is the current source input (CSI) inverter. A CSI inverter is the dual of a six-step voltage source inverter. With a current source inverter, the DC power supply is configured as a current source rather than a voltage source. The inverter SCRs are switched in a six-step sequence to direct the current to a three-phase AC load as a stepped current waveform. CSI inverter commutation methods include load commutation and parallel capacitor commutation. With both methods, the input current regulation assists the commutation. With load commutation, the load is a synchronous motor operated at a leading power factor.As they have become available in higher voltage and current ratings, semiconductors such as transistors or IGBTs that can be turned off by means of control signals have become the preferred switching components for use in inverter circuits.4.3 Rectifier and inverter pulse numbersRectifier circuits are often classified by the number of current pulses that flow to the DC side of the rectifier per cycle of AC input voltage. A single-phase half-wave rectifier is a one-pulse circuit and a single-phase full-wave rectifier is a two-pulse circuit.A three-phase half-wave rectifier is a three-pulse circuit and a three-phase full-waverectifier is a six-pulse circuit。

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