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单击此处编辑母版标题样式,单击此处编辑母版文本样式,第二级,第三级,第四级,第五级,*,Chapter 4,Electric Machinery,Section 4 Transformer,Text,New Words and Expressions,Exercises,End,Addition or Omission of Chinese words,Section 4 Transformer,Types and construction of transformer,A transformer is a device that changes AC electric energy at one voltage level into AC electric energy at another voltage level through the action of a magnetic field. It consists of two or more coils of wire wrapped around a common ferromagnetic core. These coils are (usually) not directly connected. The only connection between the coils is the common magnetic flux present within the core.,Section 4 Transformer,One of the transformer windings is connected to a source of AC electric power, and the second (and perhaps third) transformer winding supplies electric power to loads. The transformer winding connected to the power source is called the primary winding or input winding, and the winding connected to the loads is called the secondary winding or output winding. If there is a third winding on the transformer, it is called the tertiary winding.,Section 4 Transformer,Power transformer are constructed on one of two types of cores. One type of construction consists of a simple rectangular laminated piece of steel with the transformer windings wrapped around two sides of the rectangle. This type of construction is known as core form. The other type consists of a three-legged laminated core with the windings wrapped around the center leg. This type of construction is known as shell form. In either case, the core is constructed of thin laminations electrically isolated from each other in order to reduce eddy currents to a minimum.,Section 4 Transformer,The primary and secondary windings in a physical transformer are wrapped one on top of the other with the low-voltage winding innermost. Such an arrangement serves two purposes: (i) It simplifies the problem of insulating the high-voltage winding from the core. (ii) It results in much less leakage flux than would be the two windings were separated by a distance on the core.,Section 4 Transformer,Power transformers are given a variety of different names, depending on their use in power systems. A transformer connected to the output of a generator and used to step its voltage up to the to transmission levels is sometimes called a unit transformer. The transformer at the other end of the transmission line, which steps the voltage down from transmission levels to distribution levels, is called a substation transformer. Finally, the transformer that takes the distribution voltage and steps it down to the final voltage at which the,Section 4 Transformer,the power is actually used is called a distribution transformer. All these devices are essentially the same in their construction, the only difference among them is their intended use.,In addition to the various power transformers, two special-purpose transformers are used with electric machinery and power systems. The first of these special transformers is a device specially designed to sample a high voltage and produce a low secondary voltage directly proportional to it. Such a transformer is called a potential transformer.,Section 4 Transformer,A power transformer also produces a secondary voltage directly proportional to its primary voltage; the difference between a potential transformer and a power transformer is that the potential transformer is designed to handle only a very small current. The second type of special transformer is a device designed to provide a secondary current much smaller than but directly proportional to its primary current. This device is called a current transformer.,Section 4 Transformer,Equivalent circuit of transformer,The losses that occur in real transformers have to be accounted for in any accurate model of transformer behavior. The major items to be considered in the construction of such a model are,：,（,1,）,Copper (,R,r,) losses. Copper losses are the resistive heating losses in the primary and secondary windings of the transformer. They are proportional to the square of the current in the windings.,Section 4 Transformer,（,2,）,Eddy current losses. Eddy current losses are resistive heating losses in the core of the transformer.,（,3,）,Hysteresis,losses. These losses are associated with the rearrangement of the magnetic domains in the core during each half-cycle.,（,4,）,Leakage flux. The fluxes which escape the core and pass through only one of the transformer windings are leakage fluxes. These escaped fluxes produce a self-inductance in the primary and secondary coils, and the effects of this inductance must be accounted for.,Section 4 Transformer,It is possible to construct an equivalent circuit that takes into account all the major imperfections in real transformers. Each major imperfection will be considered in turn, and its effect will be included in the transformer model.,The easiest effect to model is the copper losses. Copper losses are resistive losses in the primary and secondary windings of the transformer core. They are modeled by placing a resistor,R,P,in the primary circuit of the transformer and a resistor,R,S,in the secondary circuit.,Section 4 Transformer,The leakage flux in the primary windings,ip,produces a voltage e,ip,and the leakage flux in the secondary windings,s,produces a voltage,e,is,. Since much of the leakage flux path is through air, and since air has a constant reluctance much higher than the core reluctance, the flux,ip,is directly proportional to the primary circuit current,i,P,and the flux,i,s,is directly proportional to the secondary current,i,S,Section 4 Transformer,where,L,P,is the self-inductance of the primary coil, and,L,S, is the self-inductance of the secondary coil. Therefore, the leakage flux will be modeled by primary and secondary inductors.,How can the core excitation effects be modeled? The magnetization current,i,m,is a current proportional (in the unsaturated region) to the voltage applied to the core, and lagging the applied voltage by 90, so it can be modeled by a reactance,X,m,connected across the primary voltage source. The core-loss current,i,n+e,is a current proportional to,Section 4 Transformer,the voltage applied to the core that is in phase with the applied voltage, so it can be modeled by a resistance,R,m,connected across the primary voltage source. (Remember that both these currents are really nonlinear, so the reactance,X,m,and the resistance,R,m,are at best approximations of the real excitation effects.),Section 4 Transformer,The resulting equivalent circuit is shown in Fig.4-4. Notice that the elements forming the excitation branch are placed inside the primary resistance,R,P and the primary inductance,L,P,. This is because the voltage actually applied to the core is really equal to the input voltage less the internal voltage drops of the winding.,Fig.4-4 The model of a real transformer,Return,New Words and Expressions,transformer,n.,变压器,wrap,v.; n.,包裹，包扎，缠绕，卷,ferromagnetic core,铁磁芯,primary winding,一次绕组，原绕组，初级绕组,secondary winding,二次绕组，副绕组，次级绕组,tertiary winding,三次绕组，第三绕组,power transformer,电力变压器,core form,铁芯式,New Words and Expressions,shell form,壳式,innermost,a.,最内部的，最里面的，最深处的,leakage flux,漏磁通,unit transformer,单元（机组）变压器,substation transformer,配电变压器,distribution transformer,配电变压器,potential transformer,电压互感器,current transformer,电流互感器,New Words and Expressions,copper loss,铜耗,hysteresis,loss,磁滞损耗，磁滞损失,reluctance,n.,磁阻,self-inductance,自感应,core-loss,铁芯损耗,Return,Addition or Omission of Words,在翻译过程中，根据“省词不省意”的原则，将原文中多余的词在译文中省略，使译文更加严谨、精练、明确。注意它不是删繁就简，也不是略去自己不懂的内容。用词的省略一是由于两种语言的语法不同而引起，二是可能由于技术上和修辞上的原因引起。,1.,冠词的省略,一般来说，英语的不定冠词,a,和,an,、定冠词,the,在句中用作泛指，在译文中常可省略。对于定冠词,the,只有在特指前文中所提的人、物或事件时，才是例外，这时一般译成“这”、“那”、“该”等。而不定冠词,a,和,an,在表示数量，强调类别时不可省略，要译成“一个”、“一种”、“一类”等，表示单位时要译成“一”、“每”等。,Addition or Omission of Words,1.,冠词的省略,(1) With,the,development of electrical engineering, power can be transmitted over long distance.,随着电气工程学的发展，电力能被输送到非常遥远的地方去。,(2) With,an,equation we can work with,an,unknown quantity.,利用方程，我们可以求解未知量。,(3) The greater,the,resistance of a wire, the less electric current will pass through it under,the,same pressure.,在电压相同的情况下，导线的电阻越大，流过的电流就越小。,Addition or Omission of Words,2.,代词的省略,英语中表示泛指的人称代词、用作定语的物主代词、反身代词以及用于比较句中的指示代词，在翻译时，可根据汉语的表达习惯省略。,(1) When the signal,we,pick up has increased by 10 times as the gain may have been reduced by 8 times.,信号增大到,10,倍，增益降低到,1/8,。,(2) There is not any established theory about how people store and retrieve information when,t,hey,need,it,.,关于人们如何储存信息以及需要时如何检索信息，目前还没有已经确定的理论。,Addition or Omission of Words,2.,代词的省略,(3) Turn the knob until,you,get a clear picture on the screen.,请转动旋钮直到屏幕上图像清晰为止。,(4) Like silicon, gallium arsenide is a semiconductora material whose conductivity is between,that,of a conductor and an insulator.,像硅一样，砷化镓也是一种半导体材料，其导电率介于导体和绝缘体之间。,Addition or Omission of Words,2.,代词的省略,(5) If,one,halves a bar magnet, each of the two halves will be a complete magnet.,如果把一根磁铁棒分成两段，那么其中每一段都仍是完整的磁铁。,Return,Exercises,I. Choose the best answer into the blank,1. A transformer is a device that can converter one ac voltage to another ac voltage through the action of,.,A. an electric circuit B. a magnetic field,C. an electric field D. a coupling circuit,2. The primary winding is connected to,.,A. the power source B. the load,C. the measurement instrument D. the motor,Exercises,I. Choose the best answer into the blank,3. The core construction consisting of a three-legged laminated core with the windings wrapped around the centre leg is called,.,A. core formB. shell form,C. cylindrical formD. global form,4. The transformer that takes the distribution voltage and steps it down to the final voltage at which the power is actually used is called a,.,A. distribution transformer B. unit transformer,C. substation transformer D. potential transformer,Exercises,I. Choose the best answer into the blank,5. Copper losses are,losses in the primary and secondary windings of the transformer core.,A. inductiveB. capacitive,C. resistiveD. reactive,Exercises,II. Answer the following questions according to the text,1. How many types of core construction do power transformers have?,2. Why are thin laminations electrically isolated from each other used to construct the core ?,3. What function does a potential transformer have?,4. Are a power transformer and a potential transformer essentially the same in their con-,struction,?,5. What are the major losses items to be considered in the construction of such a equivalent circuit model?,Exercises,III. Translate the following into Chinese,There are sometimes occasion when it is desirable to change voltage levels by only a small amount. For example, it may be necessary to increase a voltage from 110V to 120V or from 13. 2kV to 13. 8kV. These small rises may be made necessary by voltage drops that occur in power systems a long way from the generators. In such circumstances, it is wasteful and excessively expensive to wind a transformer with two full windings, each rated at about the same voltage. A special-purpose transformer, called an autotransformer, is used instead.,Return,Chapter 4,Section 4,End,
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