電氣工程專業(yè)英語(yǔ)+unt1

1、1 電氣工程及其自動(dòng)化專業(yè)英語(yǔ)電氣工程及其自動(dòng)化專業(yè)英語(yǔ) 主編主編 楊勇楊勇 鄧秋玲鄧秋玲 2 CONTENTS Unit 1 Fundamentals of Electronic Circuits Unit 2 Power Electronics Unit 3 Electrical Machines Unit 4 Electric Power Systems Unit 5 Electric Power Generations Unit 6 Automatic Control Unit 7 Sensoring Technology Unit 8 Computer Network and Net

2、worked Control Unit 9 Signal Processing 3 Unit 1 Fundamentals of Electronic Circuits Unit 1 Fundamentals of Electronic Circuits 1.1 Introduction to Circuit Theory 1.1 Introduction to Circuit Theory 1.2 Analog and Digital Circuits 1.2 Analog and Digital Circuits 1.3 Three-Phase Circuits1.3 Three-Phas

3、e Circuits 1.4 Further Reading1.4 Further Reading 4 1.1 Introduction to Circuit Theory New Words and Expressions e element n. 成分成分；元件元件 i interconnect vt. 使互相連接使互相連接 n node n n. 節(jié)點(diǎn)節(jié)點(diǎn) b branch n n. . 分枝；分部；支流分枝；分部；支流 v v. . 出現(xiàn)分歧出現(xiàn)分歧 l loop n. 循環(huán)，回路循環(huán)，回路; ; vt. 使成環(huán)；使成環(huán)； t topology n n. . 拓?fù)?；布局；拓?fù)鋵W(xué)拓?fù)?；?/p>

4、局；拓?fù)鋵W(xué) c configuration n n. . 構(gòu)造；結(jié)構(gòu)；配置；外形構(gòu)造；結(jié)構(gòu)；配置；外形 t terminal n n. . 終點(diǎn)站；終端；接線端終點(diǎn)站；終端；接線端 5 1.1 Introduction to Circuit Theory New Words and Expressions resistor n. 電電電阻器電阻器 independent adj. 獨(dú)立自主的；不受約束的獨(dú)立自主的；不受約束的 series n. 連續(xù)；系列；級(jí)數(shù)；串聯(lián)連續(xù)；系列；級(jí)數(shù)；串聯(lián) parallel adj. 平行的；并聯(lián)的平行的；并聯(lián)的 n. 平行線；平平行線；平 行面；行面；v.

5、相應(yīng)；平行相應(yīng)；平行 impedance n. 電電阻抗；全電阻；阻抗；全電阻；物物阻抗阻抗 theorem n. 數(shù)數(shù)定理；法則定理；法則 6 NoteNote：providing one or more closed paths為為 network的后置定語(yǔ)。的后置定語(yǔ)。 1.1 Introduction to Circuit Theory text NoteNote：differentiate在此處為動(dòng)詞，在此處為動(dòng)詞，differentiate between A A and B B表示區(qū)分表示區(qū)分A和和B；To differentiate between a circuit and

6、a network是目的狀語(yǔ)從句是目的狀語(yǔ)從句. . NoteNote：regard as表示表示“把把認(rèn)為認(rèn)為” (1) Nodes, Branches and Loops Since the elements of an electric circuit can be interconnected in several ways, we need to understand some basic concepts of network topology. To differentiate between a circuit and a network, we may regard a ne

7、twork as an interconnection of elements or devices, whereas a circuit is a network providing one or more closed paths. 7 1.1 Introduction to Circuit Theory text In network topology, we study the properties relating to (關(guān)于關(guān)于.的特性的特性)the placement of elements in the network and the geometric configurat

8、ion of the network. Such elements include branches, nodes, and loops. A branch represents a single element such as a voltage source or a resistor. In other words, a branch represents any two-terminal element. The circuit in Fig.1.1 has five branches, namely, the 10V voltage source, the 2A current so

9、urce, and the three resistors. A node is the point of connection between two or more branches. 8 1.1 Introduction to Circuit Theory text A node is usually indicated by a dot in a circuit. If a short circuit (a connecting wire) connects two nodes, the two nodes constitute a single node. The circuit i

10、n Fig.1.1 has three nodes a, b, and c. Notice that the three points that form node b are connected by perfectly conducting wires and therefore constitute a single point. Note:Note: t three points that form node b 表示表示形成節(jié)點(diǎn)形成節(jié)點(diǎn) b b的的3個(gè)點(diǎn)個(gè)點(diǎn).此句可譯為此句可譯為: :注意到形成節(jié)點(diǎn)注意到形成節(jié)點(diǎn)b b的的3個(gè)點(diǎn)是通過理個(gè)點(diǎn)是通過理 想導(dǎo)線相連接的，因此組成了想導(dǎo)線相

11、連接的，因此組成了1個(gè)節(jié)點(diǎn)個(gè)節(jié)點(diǎn). . 9 1.1 Introduction to Circuit Theory text The same is true of the four points forming node c. We demonstrate that (證明 )the circuit in Fig.1.1 has only three nodes by redrawing the circuit in Fig.1.2. The two circuits in Fig.1.1 and Fig.1.2 are identical. However, for the sake of

12、clarity, nodes b and c are spread out with perfect conductors as in Fig.1.2. Note:：the same is true of :這也適用于這也適用于，也是如此。也是如此。 Note: ：for the sake of clarity:為清楚起見為清楚起見 ;：spread out: 伸展伸展;spread out with perfect conductors ：用理想導(dǎo)線分散開。：用理想導(dǎo)線分散開。 10 1.1 Introduction to Circuit Theory text A loop is any

13、closed path in a circuit. A loop is a closed path formed by starting at a node, passing through a set of nodes, and returning to the starting node without passing through any node more than once. A loop is said to be （可認(rèn)為是）independent if it contains a branch which is not in any other loop. Independe

14、nt loops or paths result in（ 導(dǎo)致導(dǎo)致,結(jié)果是）結(jié)果是） independent sets of equations. Note:：回路回路是一個(gè)是一個(gè)閉合閉合的的路徑路徑，它是，它是從一個(gè)從一個(gè)節(jié)節(jié)點(diǎn)開點(diǎn)開 始，經(jīng)過若干始，經(jīng)過若干節(jié)節(jié)點(diǎn)，再回到起始點(diǎn)，再回到起始節(jié)節(jié)點(diǎn)，點(diǎn)，任意一個(gè)節(jié)點(diǎn)只經(jīng)任意一個(gè)節(jié)點(diǎn)只經(jīng) 過一次過一次。 11 1.1 Introduction to Circuit Theory text For example, the closed path abca containing the 2 resistor in Fig.1.2 is a loo

15、p. Another loop is the closed path bcb containing the 3 resistor and the current source. Although one can identify six loops in Fig.1.2, only three of them are independent. A network with b branches, n nodes, and l independent loops will satisfy the fundamental theorem of network topology: b = l + n

16、-1(1.1) Note：containing the 2 resistor 作定語(yǔ)修飾作定語(yǔ)修飾 closed path abca ，意為包含意為包含2電阻的閉合路徑電阻的閉合路徑abca。 12 1.1 Introduction to Circuit Theory text As the next two definitions show, circuit topology is of great value （=is very valuable）to the study of voltages and currents in an electric circuit. Two or mor

17、e elements are in series （串聯(lián)）（串聯(lián)）if they are cascaded or connected sequentially （接順序連接）（接順序連接）and consequently carry the same current（流過相同電流流過相同電流）. Two or more elements are in parallel （并聯(lián)并聯(lián)）if they are connected to the same two nodes and consequently have the same voltage across them. Note:Note:：v

18、oltage across them表示關(guān)于兩節(jié)點(diǎn)之間的電壓，表示關(guān)于兩節(jié)點(diǎn)之間的電壓， “them”指代指代“two nodes ”。 13 1.1 Introduction to Circuit Theory text Elements may be connected in a way that they are either in series or in parallel. In the circuit shown in Fig.1.1, the voltage source and the 5 resistor are in series because the same curr

19、ent will flow through them. The 2 resistor, the 3 resistor, and the current source are in parallel because they are connected to the same two nodes (b and c) and consequently have the same voltage across them. The 5 and 2 resistors are neither in series nor in parallel with each other. Note：修飾電流的介詞短

20、語(yǔ)用修飾電流的介詞短語(yǔ)用through，：修飾電壓修飾電壓 用用across。them代表代表 resistors 14 1.1 Introduction to Circuit Theory text (2) Ohms Law，Kirchhoffs Current Law (KCL) and Kirchhoffs Voltage Law (KVL) Ohms Law states the voltage across a resistor, R (or impedance, Z) is directly proportional to （與.成比例） the current passing t

21、hrough it (the resistance/impedance is the proportionality constant). (1.2) Note:Note:：proportionality constant是比例系數(shù)。是比例系數(shù)。 ：DC:表示表示direct current; AC表示表示alternating current。 DC: AC: viR VIZ 15 1.1 Introduction to Circuit Theory text Ohms Law by itself is not sufficient to analyze circuits. However,

22、 when it is coupled with Kirchhoffs two laws, we have a sufficient, powerful set of tools for analyzing a large variety of electric circuits. Kirchhoffs laws were first introduced in 1847 by the German physicist Gustav Robert Kirchhoff (18241887). These laws are formally known as Kirchhoffs Current

23、Law (KCL) and Kirchhoffs Voltage Law (KVL). Note:Note: by itself ：本身；：本身； is coupled with： 加上，與加上，與.結(jié)結(jié) 合起來。合起來。 are formally known as ：被正式稱為。：被正式稱為。 16 1.1 Introduction to Circuit Theory text Kirchhoffs First Law is based on the law of conservation of charge, which requires that the algebraic sum （代

24、數(shù)和代數(shù)和）of charges within a system cannot change. Kirchhoffs Current Law (KCL) states that the algebraic sum of currents entering a node (or a closed boundary) is zero. Kirchhoffs First Law：基爾霍夫第一定律；：基爾霍夫第一定律； law of conservation of charge：電荷守恒定律：電荷守恒定律. 17 1.1 Introduction to Circuit Theory text Math

25、ematically, KCL implies that: (1.3) where N is the number of branches （支路數(shù)） connected to the node and in is the nth current entering (or leaving) the node. By this law, currents entering a node may be regarded as positive, while currents leaving the node may be taken as （可認(rèn)為是）negative or vice versa.

26、 Note:Note: By this law: 根據(jù)這個(gè)定律；根據(jù)這個(gè)定律； vice versa：反過來也一樣；反之亦然。：反過來也一樣；反之亦然。 1 0 N n n i 18 1.1 Introduction to Circuit Theory text Consider the node in Fig.1.3. Applying KCL gives: i1 - i2 + i3 + i4 - i5 = 0 (1.4) since currents i1, i3, and i4 are entering the node, while currents i2 and i5 are lea

27、ving it. By rearranging the terms（項(xiàng)）, we get: i1 + i3 + i4 = i2 + i5(1.5) Eq.(1.5) is an alternative form （另一種形式）of KCL. The sum of the currents entering a node is equal to （等 于）the sum of the currents leaving the node. 19 1.1 Introduction to Circuit Theory text Note that KCL also applies to a close

28、d boundary. This may be regarded as a generalized case, because a node may be regarded as a closed surface shrunk to a point. In two dimensions, a closed boundary is the same as a closed path. As typically illustrated in the circuit of Fig.1.4, the total current entering the closed surface is equal

29、to the total current leaving the surface. applies to：適用于；適用于； generalized case：一般的情況；一般的情況； As illustrated ：如圖所示：如圖所示. 20 1.1 Introduction to Circuit Theory text Kirchhoffs voltage law (KVL) states that the algebraic sum of all voltages around a closed path (or loop) is zero. Expressed mathematicall

30、y, KVL states that: (1.6) where M is the number of voltages in the loop (or the number of branches in the loop) and vm is the mth voltage. Note:Note:式中，式中，M是回路中的電壓個(gè)數(shù)（或回路中的支路數(shù)），是回路中的電壓個(gè)數(shù)（或回路中的支路數(shù)）， vm是第是第m條支路上電壓。條支路上電壓。 1 0 M m m v 21 各個(gè)電壓的符號(hào)就是沿著環(huán)路繞行方向首先遇到的端子的極各個(gè)電壓的符號(hào)就是沿著環(huán)路繞行方向首先遇到的端子的極 性。繞行可以從任一支路開始

31、，繞行方向可以是順時(shí)針，也可性。繞行可以從任一支路開始，繞行方向可以是順時(shí)針，也可 以是逆時(shí)針。以是逆時(shí)針。 1.1 Introduction to Circuit Theory text To illustrate KVL, consider the circuit in Fig.1.5. The sign on each voltage is the polarity of the terminal encountered first as we travel around the loop. We can start with any branch and go around the l

32、oop either clockwise or counterclockwise. Suppose we start with the voltage source and go clockwise around the loop as shown; then voltages would be v1,+v2,+v3,v4, and +v5, in that order. 假設(shè)從電壓源開始，順時(shí)針繞回路一圈，那么按順序，各個(gè)假設(shè)從電壓源開始，順時(shí)針繞回路一圈，那么按順序，各個(gè) 電壓符號(hào)分別為電壓符號(hào)分別為v1、+v2、+v3、v4、+v5。 22 1.1 Introduction to Cir

33、cuit Theory text For example, as we reach branch 3, the positive terminal is met first; hence we have +v3. For branch 4, we reach the negative terminal first; hence, （此處省略了 we have）v4. Thus, KVL yields: -v1 + v2 + v3- v4 + v5 = 0(1.7) Rearranging terms gives: 重新整理各項(xiàng)可以得到：重新整理各項(xiàng)可以得到： v2 + v3 + v5 = v1

34、 + v4(1.8) which may be interpreted as: Sum of voltage drops= Sum of voltage rises(1.9) Note：voltage drops：電壓降；：電壓降； voltage rises：電壓升電壓升 23 1.1 Introduction to Circuit Theory text This is an alternative form of KVL. Notice that if we had traveled counterclockwise, the result would have been +v1,-v5

35、, +v4,-v3, and-v2, which is the same as before except that the signs are reversed. Hence, Eqs.(1.7) and (1.8) remain the same. When voltage sources are connected in series, KVL can be applied to obtain the total voltage. The combined voltage is the algebraic sum of the voltages of the individual sou

36、rces. Note:Note: combined voltage ：總電壓。：總電壓。 24 1.2 Analog and Digital Circuits New Words and Expressions analog n. 類似物類似物；模擬模擬 digital adj. 數(shù)字的數(shù)字的； n. 數(shù)字?jǐn)?shù)字；數(shù)字式數(shù)字式 thermometer n. 溫度計(jì)溫度計(jì)；體溫計(jì)體溫計(jì) Fahrenheit adj. 華氏溫度計(jì)的華氏溫度計(jì)的 n. 華氏溫度計(jì)華氏溫度計(jì) drum n.鼓形圓桶鼓形圓桶 v. 打鼓打鼓 discrete adj. 不連續(xù)的不連續(xù)的；離散的離散的 original a

37、dj. 最初的最初的；原始的原始的； n. 原物原物；原作原作 remote adj. 遙遠(yuǎn)的遙遠(yuǎn)的；偏僻的偏僻的；細(xì)微的細(xì)微的 bulb n. 鱗莖鱗莖；球形物球形物 Morse code 莫莫爾斯電碼爾斯電碼 pulse n. 脈搏脈搏；脈沖脈沖 25 1.2 Analog and Digital Circuits New Words and Expressions buzzer n. 嗡嗡作聲的東西嗡嗡作聲的東西；蜂鳴器蜂鳴器；信號(hào)手信號(hào)手 manipulate vt.（熟練地熟練地）操作操作；利用利用； 應(yīng)付應(yīng)付 destination n. 目的地；目的單元格目的地；目的單元格 hu

38、midity n. 濕氣；潮濕；濕度濕氣；潮濕；濕度 interface n. 接觸面；界面接觸面；界面 comparator n. 比較儀比較儀 trigger vt.引起；觸發(fā)引起；觸發(fā) vi. 轉(zhuǎn)移；轉(zhuǎn)移； n. 扳機(jī)扳機(jī) sequence n. 次序；順序；序列次序；順序；序列 serial adj. 連續(xù)的；串行的；順次連續(xù)的；串行的；順次 decoder n. 解碼器解碼器 reassemble vt. 重新召集重新召集 vi. 重新集合重新集合 26 (1) Analog Analog quantities vary continuously, and analog system

39、s represent the analog information using electrical signals that vary smoothly and continuously over a range. A good example of an analog system is the recording thermometer, the actual equipment is shown in Fig. 1.6. 1.2 Analog and Digital Circuits TEXT Note：模擬系統(tǒng)通過在某一范圍內(nèi)平滑而連續(xù)地變化模擬系統(tǒng)通過在某一范圍內(nèi)平滑而連續(xù)地變化

40、 的電信號(hào)反映模擬信息。的電信號(hào)反映模擬信息。 27 An ink pen records the temperature in degrees Fahrenheit (F) and plots it continuously against time on a special graph paper attached to a drum as the drum rotates. Note that the temperature changes smoothly and continuously. There are no abrupt steps or breaks in the data

41、. 1.2 Analog and Digital Circuits TEXT Note：用一張?zhí)厥獾姆礁窦埾翟谝粋€(gè)鼓上，當(dāng)鼓轉(zhuǎn)動(dòng)用一張?zhí)厥獾姆礁窦埾翟谝粋€(gè)鼓上，當(dāng)鼓轉(zhuǎn)動(dòng) 時(shí)，墨水筆連續(xù)記錄下溫度（華氏溫度時(shí)，墨水筆連續(xù)記錄下溫度（華氏溫度F F），并把溫度隨），并把溫度隨 時(shí)間的變化曲線描繪在這張專用的方格紙上。時(shí)間的變化曲線描繪在這張專用的方格紙上。 28 (2) Digital Digital quantities vary in discrete levels(離散級(jí)）. In most cases, the discrete levels(離散電平） are just two valu

42、es ON and OFF. Digital systems carry information using combinations of ON-OFF electrical signals that are usually in the form of codes that represent the information. The telegraph system is an example of a digital system. 1.2 Analog and Digital Circuits TEXT Note：in the form of ：以以.的形式。數(shù)字系統(tǒng)通過導(dǎo)通和的形式

43、。數(shù)字系統(tǒng)通過導(dǎo)通和 截止的電信號(hào)組合來表示信息，通常以編碼的形式反映出來。截止的電信號(hào)組合來表示信息，通常以編碼的形式反映出來。 29 The system shown in Fig.1.7 is a simplified version of the original telegraph system, but it will demonstrate the principle and help to define a digital system. 1.2 Analog and Digital Circuits TEXT 30 The electrical circuit (as sho

44、wn in Fig.1.7(a) is a battery with a switch in the line at one end and a light bulb at the other. The person at the switch position is remotely located from the person at the light bulb. The information is transmitted from the person at the switch position to the person at the light bulb by coding t

45、he information to be sent using the international Morse telegraph code. 1.2 Analog and Digital Circuits TEXT Note： is remotely located from （遠(yuǎn)離） Note： transmitted from . to；從.傳遞到.； Note： information to be sent ：要傳遞的信息 31 Morse code uses short pulses (dots) and long pulses (dashes) of current to form

46、 the code for letters or numbers as shown in Fig.1.7(b). As shown in Fig.1.7(c), combining the codes of dots and dashes for the letters and numbers into words sends the information. 1.2 Analog and Digital Circuits TEXT Note： code for letters or numbers：字符碼或數(shù)字碼； Note： combining the codes of dots and

47、dashes for the letters and numbers into words 是動(dòng)名詞詞組作主語(yǔ)。意為： 對(duì)代表字母和數(shù)字的點(diǎn)和破折號(hào)進(jìn)行組合來發(fā)送信息。 32 The sender keeps the same shorter time interval between letters but a longer time interval between words.This allows the receiver to identify that the code sent is a character in a word or the end of a word itsel

48、f. 1.2 Analog and Digital Circuits TEXT Note：發(fā)送者在字母之間保持著相同的較短時(shí)間間隔，而在 詞之間保持較長(zhǎng)時(shí)間間隔。這就使接收者能分辨出傳送碼是一個(gè) 詞中的字符，還是一個(gè)單詞本身的結(jié)束。 33 The T is one dash (one long current pulse). The H is four short dots (four short current pulses). The R is a dot-dash-dot, and the two Es are a dot each. The two states are ON and

49、OFFcurrent or no current. The person at the light bulb position identifies the code by watching the glow of the light bulb. In the original telegraph, this person listened to a buzzer or “sounder” to identify the code. 1.2 Analog and Digital Circuits TEXT T表示破折號(hào)（一個(gè)長(zhǎng)電流脈沖）。H是4個(gè)短點(diǎn)（4個(gè)短電 流脈沖）。R是點(diǎn)-破折號(hào)-點(diǎn)的組

50、合。兩個(gè)E時(shí)，則一個(gè)點(diǎn)代表 一個(gè)E。ON 和 OFF兩種狀態(tài)分別表示有電流和無電流。 34 Coded patterns of changes from one state to another as time passes carry the information. At any instant of time the signal is either one of two levels. The variations in the signal are always between set discrete levels, but, in addition, a very importa

51、nt component of digital systems is the timing of signals. 1.2 Analog and Digital Circuits TEXT Note：隨時(shí)間從一個(gè)狀態(tài)變化到另一個(gè)狀態(tài)的編碼圖隨時(shí)間從一個(gè)狀態(tài)變化到另一個(gè)狀態(tài)的編碼圖 攜帶信息。攜帶信息。At any instant of time：任何一個(gè)時(shí)刻 35 In many cases, digital signals, either at discrete levels, or changing between discrete levels, must occur precisely

52、 at the proper time or the digital system will not work. Timing is maintained in digital systems by circuits called system clocks. This is what identifies a digital signal and the information being processed in a digital system. 1.2 Analog and Digital Circuits TEXT Note：通過系統(tǒng)時(shí)針電路來維持?jǐn)?shù)字系統(tǒng)的計(jì)時(shí)。通過系統(tǒng)時(shí)針電路來維

53、持?jǐn)?shù)字系統(tǒng)的計(jì)時(shí)。 Note：這就是在數(shù)字系統(tǒng)中識(shí)別被處理的數(shù)字信號(hào)和這就是在數(shù)字系統(tǒng)中識(shí)別被處理的數(shù)字信號(hào)和 信息的方法。信息的方法。 36 (3) Quantities The temperature, pressure, humidity and wind velocity in our environment all change smoothly and continuously, and in many cases, slowly. Instruments that measure analog quantities usually have slow response and l

54、ess than high accuracy. To maintain an accuracy of 0.1% or 1 part in 1000 is difficult with an analog instrument. 1.2 Analog and Digital Circuits TEXT Note： 0.1% or 1 part in 1000 ：百分之零點(diǎn)一或千分之一。：百分之零點(diǎn)一或千分之一。 37 Digital quantities, on the other hand, can be maintained at very high accuracy and measure

55、d and manipulated at very high speed. The accuracy of the digital signal is in direct relationship to the number of bits used to represent the digital quantity. For example, using 10 bits, an accuracy of 1 part in 1024 is assured. 1.2 Analog and Digital Circuits TEXT Note：is in direct relationship t

56、o.與.有直接的關(guān)系； Note： number of bits位數(shù)； 38 Using 12 bits gives four times the accuracy (1 part in 4096), and using 16 bits gives an accuracy of 0.0015%, or 1 part in 65 536. This accuracy can be maintained as digital quantities are manipulated and processed very rapidly, millions of times faster than an

57、alog signals . 1.2 Analog and Digital Circuits TEXT Note：對(duì)數(shù)字量進(jìn)行高速操作和處理時(shí)，仍然能保持對(duì)數(shù)字量進(jìn)行高速操作和處理時(shí)，仍然能保持 這樣的精度，因?yàn)閿?shù)字信號(hào)的處理速度要比模擬信號(hào)的快這樣的精度，因?yàn)閿?shù)字信號(hào)的處理速度要比模擬信號(hào)的快 數(shù)百萬倍。數(shù)百萬倍。 39 As a result, if analog quantities are required to be processed and manipulated, the new design technique is to first convert the analog q

58、uantities to digital quantities, to process them in digital form, to reconvert the result to analog signals and to output them to their destination to accomplish a required task. 1.2 Analog and Digital Circuits TEXT 40 The complete procedure is indicated in Fig.1.8, and the need for analog circuits,

59、 digital circuits and the conversion circuits between them is immediately apparent. Fig.1.8 A typical system describing the functions in the analog-to-digital and digital-to-analog chain 41 (4) Analog-to-Digital Conversion (A/D) Fig.1.9 separates out the analog-to-digital portion of the Fig.1.8 chai

60、n to expand the basic functions in the chain. Most of natures inputs such as temperature, pressure, humidity, wind velocity, speed, flow rate, linear motion or position are not in a form to input them directly to electronic systems. They must be changed to an electrical quantitya voltage or a curren