专业英语第六章演示文稿
会计英语课件 Unit 6 Assets
In the balance sheet, cash in listed first among the current assets, because it is the most liquid of all current assets and the banker, credit manager, or investor who studies a balance sheet critically is always interested in the total amount of cash as compared with other balance sheet items, such as accounts payable.
会计英语
secondsຫໍສະໝຸດ secondsseconds
seconds
seconds
Unit 6 Assets
1
Current assets
22
Long-term assets
Learning objectives
1. To understand the definition of assets 2. To understand the classification of assets 3. To understand the methods of depreciation
Warming up
One hospital maintains an auxiliary generator for use in electrical outages. Such outages are rare, and the generator has not been used for the past two years. Should the generator be reported as a fixed asset on this hospital’s balance sheet?
学术英语综合-Unit-6PPT课件
.
3
Unit 6
Mathematics
Unit Objectives
1. Professional Knowledge
• Know more about applied mathematics; • Think critically about mathematics education in college; • Get acquainted with the Mayan system of numeration
Now work in pairs and share your answers with your partner.
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9
Unit 6
Text A Overview
Mathematics
Summarize the main ideas of Text A on the basis
of Task 1/Critical Reading and Thinking/Text A.
4. Speaking
• Analyze the audience and purpose of the speech
5. Writing
• Achieve coherence in research paper writing
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7
Unit 6
Mathematics
Unit Objectives
6. Researching
.
5
Unit 6
Mathematics
Unit Objectives
3. Listening
• Know how to use note-taking symbols and abbreviations
新视野大学英语unit6PPT课件
Text Study—Understanding
2. People’s response to greeting from a trash-man:
• nothing • a surprised stare • startled • retreat indoors • frightened • coldness
For more details
随笔
词汇学习
The Trashman
课文阅读
导入
预习
小结
写作
Part III
Part I Part II Part IV
Back
Main Idea of Part III
Devices for Developing It
T[Phaeraasu. t1h2otrod1e3s]crTihbeesauhtihsocrhraenalgiezeodftmhaitnad by
discarded toys small appliances cans
• Yard Trash:
It includes leaves, grass trimmings, shrubbery trimmings.
• Yard Bulky Household Trash:
Any waste accumulation of bulky items such as stoves, refrigerators, washers, dryers, water heaters, sofas, carpets and similar items.
About Jobs
大学英语课程课件lesson6textappreciationPPT课件
WB T L E
To be continued on the next page.
• Text Analysis
Lesson 6—The Man in the Water
Question: In the author’s view, do people feel
proud or sad about the disaster? Why?
problem, a difficult task
Last Wednesday, the bad weather, unconcerned about the consequences it might bring about as always, made Flight 90 fall down. On that same afternoon, human nature, groping for the flotation rings and struggling in the icy water, came to prove its greatness displayed in an unexpected tragedy.
Lesson 6—The Man in the Water
• Text Analysis
Question: The fight between the man in
the water and the natural forces ended up with the death of the man. Why does the author say he is not a loser?
They feel proud because man defeated the indifferent natural forces.
大学课程《会计英语》PPT课件：Chapter 6 Unit 1
1. Plant And Equipment 固定资产 2．Trade In 以旧换新 3．Betterments 改造投资 4．Additions 扩建 5．Disposal Of Plant And Equipment 固定资
产处置 6．Property Taxes 财产税
Continued
Only reasonable and necessary expenditures should be included.
Determining the Cost of Assets
Purchased for Cash
Cost is most easily determined when an asset is purchased for cash. The cost of the asset is then equal to the cash outlay necessary in acquiring the asset plus any expenditures for freight, insurance while in transit, installation, trial runs and any other costs necessary to make the asset ready for use.
大专英语教材Unit6-Section A(new)老师教学ppt
involve v.
Unit Six
1. 包含；需要 involve sth. 这个项目有许多困难。
This project involves much difficulty. 这项实验需要多年的工作。
This experiment involves years of work. involve doing sth. 秘书的工作包括安排会议和写报告。
Unit Six
broadcast
Unit Six
1. n. 广播 广播站, 广播电台 _________
broadcast station
The broadcast said no one was hurt in the accident.
2. v. 广播，播送 播送新闻 __________
broadcast news 正在播送新闻。(be broadcast)
fewer than ten percent of Americans carry out their New Year’s
resolutions.
Text A Para. 6
Chinese
Unit Six
evil 【1】adj. not good morally; wicked; harmful
electrical adj.与用电有关的; 电气科学的
电力设备 __________ electrical equipment
e.g. charger, battery, fuse, light, switch, wire
家用电器 __________ household electrical appliances
不道德的；邪恶的；有害的 evil spirit ________
B2 Unit 6英语专业 基础英语 第二学期课件
dilemma: a situation in which one has to make a difficult choice e.g. This is the industrialist’s dilemma: invest, and risk going bankrupt, or not invest and risk losing your share of the market.
illusion: something that deceives a person by seeming to be real
e.g. The mirrors all round the walls give an illusion of greater space
devastating: completely destructive e.g. The US then launched its most devastating attack of the entire war. devastate (v.); devastation (n.): destroy, damage (synonyms)
reassuring: make someone feel less worried or frightened e.g. It’s reassuring to know that the children are being looked after. reassuringly adv. e.g. Seeing his worried face, Betty smiled reassuringly.
商务英语课程课件(Unit 6 Business Presentations)
Tips to help your presentation skills
Know what you are going to say - make sure that you have practised and revised the content where necessary so that you are able to make a confident and polished presentation on the day. Practice with handouts and props - if you are using handouts or other props in your presentation, practice with them as part of your preparation so that your presentation flows. .
6.1. 2 Listen to the presentation and complete the sheet below
according to the information you hear.
In this part you are going to listen to a presentation introduction and a presentation given by Allan Swales about his company called Cultural Study Tour . Now listen and try to finish the exercises while listening.
◆ Warm-up Practice ◆ Listening Task ◆ Speaking Task
专业外语教学资料专业英语第6单元.docx
6 DC Motor And induction Motor6.1TYPES OF MOTORSThe types of commercially available DC motors basically fall into four categories: （Dpermanent-magnet DC motor,②series-wound （串励）DC motors, （§）shunt-wound（并励）DC motors, and ©compound-wound （复励）DC motors. Each of these motors has different characteristics due to its basic circuit arrangement and physical properties.6.1.1Permanent-magnet DC motorsThe penn an ent-magnet DC motor, shown in Fig.6.1, is constructed in the same manner as its DC generator counterpart. The permanent-magnet motor is used for low-torque applications. When this type of motor is used, the DC power supply is connected directly to the amiature conductors through the brush/commutator assembly. The magnetic field is produced by pennanent magnets mounted on the stator. The rotor of permanent magnet is a wound armature.This type of motor ordinarily uses either alnico （乍吕镰钻合金）or ceramic （陶瓷）permanent magnets rather than field coils. The alnico magnets are used with high-horsepower applications. Ceramic magnets are used for low-horsepower show-speed motors. Ceramic magnets are highly resistant to demagnetization, yet they are relatively low in magnetic-flux level. The magnets are usually mounted in the motor fame and thenmagnetized prior to the insertion of the armature.The permanent-magnet motor has several advantages over conventional type of DC motors. One advantage is reduced operational cost. The speed characteristics of the permanent-magnet motor are similar tothose of the shunt-wound DC motor. The direction of rotations of a permanent-magnet motor can be reversed by reversing the two power lines.6.1.2Series-wound DC motorsThe manner in which the armature and field circuits of a DC motors are connected determines its basic characteristics. Each of the type of DC motors are similar in construction to the type of DC generator that corresponds to it. The only difference, in most cases, is that the generator(a) Pictorial diagram (b) Schematic diagramFig.6.1 Permanent-magnet DC motoracts as a voltage source while the motor functions as amechano-巴powerconversiondevice.Theseries-wound mofor"showninFig.62has(hearmafureandfieldcircuits connes-edinaseriesarrangemenLThereisonlyonepathforcurrent:fo Howfrom (heDCvolfagesourceTherefore"thefieldiswoundofrelativelyfewfums (同)oflargediameferwire"givingfhefieldalowresistanceChangesin loadappliedfofhemoforshaftcauseschangesinfhecuirenfthroughfhefield.Iffhemechanic巴load increases" fhe current 巴so increases- TheincreasedcurrentcreafesastrongermagneticfiledThespeedofaseriesmoforvariesfhe low ’resisfancefiled"fhe series mofor producesahighdorqueoufpuhSeriesmotors are used where heavy loadsmust:bemovedandspeed巴mofors-(a)piaoa.aldiagramsschematicFig ・6・ 2 Series-woundDCm 2.o r6.1.3Shunt-wound DC motorsShunt-wound DC motors are more commonly used than any other type of DC motor. As shown in Fig.6.3, the shunt-wound DC motor has field coils connected in parallel with its armature. This type of DC motor has field coils which are wound of many turns of small-diameter wire and have a relatively high resistance. Since the field is a high-resistant parallel part of the circuit of the shunt motor, a small amount of current flows through the field. A strong electromagnetic field is produced due to the many turns of wire that from the field windings.A large majority （about 95%） of the current drawn （汲取）by the shunt motor flows in the armature circuit. Since the field current has little effect on the strength of the field, motor speed is not affected appreciably （略微）by variations in load current. The relationship of the current that flow through a DC shunt is as follows:Qla + IfWhere h—total current drawn from the power source;I a—armature current;I f—field current.The field current may be varied by placing a variable resistance in series with the field windings. Since the current in the field circuit is low, a low-wattage rheostat （变阻器）may be used to vary the speed of the motordue to the variation in field resistance （励磁回路阻抗）.As fieldresistance increases, field current will decrease. A decrease in field current reduces the strength of the electromagnetic field. When the field flux is decreased, the armature will rotate faster, due to reduced magnetic-field interaction. Thus the speed of a DC shunt motor may be easily varied by using a field rheostat.The shunt-wound DC motor has very good speed regulation. The speed does decrease slightly when the load increases due to the increase in voltage drop across the armature. Due to its good speed regulation characteristic and its ease of speed control, the DC shunt motor is commonly used for industrial applications. Many types of variable-speedmachine tools are driven by DC shunt motors.(a) Pictorial diagram (b) Schematic diagramFig.6.3 Shunt-wound DC motor6.1.4Compound-wound DC motorsThe compound-wound DC motors shown in Fig.6.4, has two sets of field windings, one in series the armature and one in parallel. This motorcombines the desirable characteristics of the series- and shunt-wound motors. There are two methods of connecting compound motors: cumulative and differential. A cumulative compound （积复励）DC motor has series and shunt fields that aid each other. Differential compound （差复励）DC motors have series and shunt fields that oppose each other. There are also two ways in which the series windings are placed in the circuit. One method is called a short shunt （短复励）（see Fig.6.4）, in which the shunt field is placed across （跨接）the armature. The long-shunt （长复励）method has the shunt field winding placed across both the armature and the series field （see Fig.6.4）.Compound motors have high torque similar to a series-would motor, together with good speed regulation similar to a shunt motor. Therefore, when good torque and good speed regulation are needed, the compound-wound de motor can be used. A major disadvantage of a compound-wound motor is its expense.Hich-resistance ^himr n^i/4（a） Pictorial diagramLong shuntShort shunt（b ） Schematic diagramFig.6.4 Compound-wound DC motor6.2 DC MOTOR ANALYSISA DC motor is a DC generator with the power flow reversed. In the DC motor electrical energy is converted to mechanical from. One the basis of foregoing discussion, there are three types of DC motors: the shunt motor, the cumulatively compounded motor, and the series motor. The compound motor is prefixed with the word cumulative in order to stress that the connections to the series field winding are such as to ensure that the series field flux aids the shunt-field flux. The series motor, unlike the series generator, finds wide application, especially for traction-type loads. Hence due attention is given to this machine in the treatment that follows.The performance of the DC motor operating in any one of its three modes can conveniently （常规）be described in terms of an equivalent circuit （等效电路）,a set of performance equations （特性方程）,a power-flow diagram （功率流程图）.And the magnetization curve. Thei —nnnnnnnr^ |Series fieldDC - sourccf^-nnnnnnnn^ Series fieldDC J C sourceT-Shunt fieldequivalent circuit is depicted in Fig.6.5. It is worthwhile to note that now the armature induced voltage is treated as a reaction (反作用)or counter emf (反电动势).By imposing some constraints (约束),we obtain thecorrect equivalent circuitfor the desired mode ofoperation. For example, fora series motor theappropriatecircuit Fig .6.5 Equivalent circuit of the DC motor equivalentresults upon removing R f from the circuitry of Fig.6.5.The set of equations needed to compute the performance is listed below.Ea=Ke ① n (6-1)T=K/Da (6-2)Ut=Ea+Ia(Ra+Rs ) (6-3)I 厶Mia (6-4)Note that the last two equations are modified to account for (说明，解释) the fact that for the motor Ut is the applied or source voltage and as such must be equal to the sum of the voltage drops. Similarly the line current is equal to the sum rather than the difference of the armature current and field currents.The power-flow diagram is illustrated in Fig.6.6. The electrical power input U t I£originating from the line supplies the field power needed to establish the flux filed as well as the armature-circuit copper loss needed to maintain the flow of la. This current flowing through the armatureconductors imbedded （嵌入）in the flux field causes torque to be developed. The law of conservation （守恒定律）of T（D m, where co m is the steady-state operating speed. Removal of the rotational losses （损耗）from the developed mechanical power yields the mechanical output power.The DC motor is often called upon （被迫）to do the really tough jobs in industry because of its high degree of flexibility and ease of control. These features cannot easily be matched by other electromechanical energy-conversion devices. The DC motor offers a wide range of control of speed and torque as well as excellent acceleration （力口速度）and deceleration （减速）.For example, by the insertion of an appropriate armature-circuit resistance, rated torque can be obtained at starting with no more than ratted current flowing. Also, by special design of the shunt-field winding, speed adjustments over a range of 4:1 are readily obtainable. If this is then combined with armature-voltage control, the range of speed adjustment speeds to 6:1. In some electronic control devices that are used to provide the DC energy to the field and armature circuits, a speed range of 40:lis possible. The size of the motor being controlled, however, is limited.6.3 DC MOTOR SPEED-TROOUE CHARACTERISTICSHow dose the DC motor react to the application of a shaft load (轴 荷)？ What is the mechanism by which the DC motor adapts itself to supply to the load the power it demands? The answers to these questions can be obtained by reasoning in terms of the performance equations. Initially our remarks are confined (限制) to the shunt motor, but a similar line of reasoning applies for the others. For our purposes the two pertinent (有关的) equations are those for torque and current. ThusT=K T ① laandNote that the last expression result from replacing Ea by Eq.(6.1) in Eq.(6.3). With no shaft load applied, the only torque needed is that which 总氏+Rs)M/r7o=ra;m =niechanical power outputla (6-5)fig.6.6 Power-flow diagram of the DC motorUt- KT^ novercomes the rotational losses. Since the shunt motor operates at essentially constant flux, Eq.(6.2) indicates that only a small armature current is required compared to its rated value to furnish these losses. Eq.(6.5) reveals the manner in which the armature current is made to assume just the right value. In this expression Ut, Ra, K E, and ① are fixed in value. Therefore the speed is the critical (重要的)variable. If, for the moment, it is assumed that the speed has too low a value, then the numerator of Eq.(6.5) takes on an excessive (额夕卜的) value and in turn makes la large than required. At this point the motor reacts to correct the situation. The excessive armature current produces a developed torque which exceeds the opposing torques of friction and windage (偏差).In fact this excess serves as (提供) an accelerating torque, which then proceeds (继续) to increase the speed to that level which corresponds to the equilibrium value of armature current. In other words, the acceleration torque becomes zero only when the speed is at that value which by Eq.(6.5) yields just the right la needed to overcome the rotational losses.Consider next that a load demanding rated torque is suddenly applied to the motor shaft. Clearly, because the developed torque at this instant is only sufficient to overcome friction and windage and not the load torque, the first reaction is for the motor to lose speed. In this way, as Eq.(6.5) reveals, the armature current can be increased so that in turn the electromagnetic torque can increase. As a matter of fact the applied loadtorque causes the motor to assume that value of speed which yields a current sufficient to produce a developed torque to overcome the applied shaft torque and the frictional torque. Power balance is thereby achieved, because an equilibrium condition is reached where the electromagnetic power, Eala, is equal to the mechanical power developed, TGj m.A comparison of the DC motor with the three-phase induction motor indicates that both are speed-sensitive devices in response to applied shaft loads. An essential difference, however, is that for the three-phase induction motor developed torque is adversely （相反的，不利的）influenced by the power-factor （功率因素）angle of the armature current Of course no analogous situation prevails in the case of the DC motor.Fig.6.7 Typical speed-torque curves of DC mo- torsOn the basis of the foregoing discussing it should be apparent that the speed-torque curve of DC motors is an important characteristic.Appearing in Fig.6.7 are the general shapes of the speed-torque characteristics as they apply for the shunt, cumulatively compounded, and series motors. For the sake of (为了) comparison the curves are drawn through a common point of rated torque and speed. An understanding of why the curves take the shapes and relative positions depicted in Fig.6.7 readily follows from an examination of Eq.(6.1), which involves the speed. For the shunt motor the speed equation can be written asThe only variables involved are the speed n and the armature current la. At rated output torque the armature current is at its rated value and so, too, is the speed. As the load torque is removed, the armature current becomes correspondingly smaller, making the numerator tern ofEq.(6.6) large. This results in higher speeds. The extent to which the speed increases depends upon how large the armature circuit resistance drop is in comparison to the terminal voltage. It is usually around 5%—10%. Accordingly, we can expect the percent change in speed of the shunt motor to be about the same magnitude. This change in speed is identified by a figure of merit (性能扌旨标) called the speed regulation(静差率 ---调速精度).It is defined as followspercent speed regulation=〔山同唧心-侦呼ad 用却)x 100 (6-7) Ea Ut-IaRa(6-6)£wll load speedThe speed equation as it applies to the cumulatively compounded motor takes the fromA comparison with the analogous expression for the shunt motor bears out (证实)two differences. One, the numerator tern also includes the voltage drop in the series-field winding besides that in the armature winding. Two, the denominator term is increased to account for the effect of the series-field flux ①s. Starting at rated torque and speed, Eq.(6.7) makes it clear that as load torque is decreased to zero there is an increase in the numerator tern which is necessarily greater then it is for the shunt motor. At the same time, moreover, the denominator tern decreases because ①s reduces to zero as the torque goes to zero. Both effects act to bring about an increase in speed. Therefore the speed regulation of the cumulatively compounded motor is greater than for the shunt motor.Fig.6.7 presents this information graphically.The situation regarding the speed-torque characteristic of the series motor is significantly different because of the absence of a shunt-field winding. Keep in mind that the establishment of a flux field in the series motor comes about solely as a result of the flow of armature current through the series-field winding. In this connection, then, the speed equation for the series motor becomesII = |Ut-Ia (Ra+Rg)]KEOsh 中 E (6-8)U一Ea LUt-Ia(Ra+Rs)] (6-9)KF和-Where K E' denotes (扌旨示) a new proportionality factor which permits O s to be replaced by the annature current la. When rated torque is being developed the current is at its rated value. The flux field is therefore abundant. However, as load torque is removed less annature current flows. Now since la appears in the denominator of the speed equation, it is easy to see that the speed will increase greatly. In fact, if the load were to be disconnected from the motor shaft, dangerously high speeds would result because of the small armature current that flows. The centrifugal forces at these high speeds can easily damage the armature winding. For this reason a series motor should never have its load uncoupled (分开).Because the armature current is directly related to the air-gap flux in the series motor, Eq.(6.1) for the developed torque may be modified to read asT=K T<DI a=K T I2a(6.10)Thus the developed torque for the series motor is a function of the square of the armature current. This stands to the linear relationship of torque to armature current in the shunt motor. Of course in the compound motor an intermediate relationship is achieved. It is interesting to note, too, that as the series motor reacts to develop greater torques, the speed drops correspondingly. It is this capability which suits the series motor so well to traction-type loads.One of the attractive features the DC motor offers over all other types is the relative ease which speed control can be achieved. The various schemes available for speed control can be from Eq.(6.6), which is repeated here with one modificationThe modification involves the inclusion of an external armature-circuit resistance Re. Inspection of Eq.(6.11) reveals that the speed can be controlled by adjusting any one of the three factors appearing on the right side of the equation: Ut, Re, or ①.The simplest is adjust ①.A field rheostat such as that shown in Fig.6.5 is used. If the field-rheostat resistance is increased, the air-gap flux is diminished, yielding higher operating speeds. General-purpose shunt motors are designed to provide a 200%increase in rated speed by this method of speed control. However, because of the weakened flux field the permissible torque that can be delivered at the higher speed is correspondingly reduced, in order to prevent excessive amiature current.A second method of speed adjustment involves the use of an external resistor Re connected in the armature circuit as illustrated inFig.6.8. The size and cost of this resistor are considerably greater than those of the field rheostat because Re must be capable of handling the full armature current. Eq.(6.11 )indicates that the large Re is made, the greater will be the speed change. Frequently the external resistor is selected to furnish as much as a 50%drop in speed from the rated value. The chief disadvantage of this method of control is the poor efficiency of operation. For example, a 50% drop in speed is achieved by having approximately half of the terminal voltage U t appear across Re. Accordingly, about 50%of the line input power is dissipated in the from of heat in the resistorRe. Nonetheless, armature-circuit resistance control is often used-especially |Ut-Ia ；Ra~Rg)]I ｛强 (6-11)for series motors.Fig. 6. 8 Speed adjustment of a shunt motor by an external armature-circuit resistanceA third and final method of speed control involves adjustment of theapplied terminal voltage. This scheme is the most desirable from the view-point of flexibility and high operating efficiency. But it is also the mostexpensive because it requires its own DC supply. It means purchasing amotor-generator set with a capacity at least equal to that of the motor to becontrolled. Such expense is not generally justified except in situations wherethe superior performance achievable with this scheme is indispensable （不可缺少的），as is the case in steel mill （钢厂）applications. Armature terminal voltage control is referred to as the ward-Leonard system （发电机-电动机系统）.6.4THREE-PHASE INDUCTION MOTOROne distinguishing feature of the induction motor is that it is a singly excited machine. Although such machines are equipped with both a field winding and an armature winding, in nonnal use an energy source is connected to one winding alone, the field winding. Currents are made toflow in the armature winding by induction, which creates an ampere-conductor distribution that interacts with the field distribution to produce a net unidirectional torque. The frequency of the induced current in the conductor is affected by the speed of the rotor on which it is located: however, the relationship between the rotor speed and the frequency of the armature current is such as to yield a resulting ampere-conductor （载流导体）distribution that is stationary relative to the field distribution of the stator. As a result, the singly excited induction machine is capable of producing torque at any speed below synchronous speed. For this reason the induction machine is placed in the class of asynchronous machines. In contrast, synchronous machines are electromechanical energy-conversion devices in which a net torque can be produced at only one speed of the rotor. The distinguishing characteristic of the synchronous machine （同步电机）is that it is a doubly excited device except when it is being used as a reluctance motor（磁阻电机一一凸极无励同步电机）.The salient construction features of the three-phase induction motor are described in Sec.5.3. Because the induction machine is singly excited, it is necessary that both the magnetizing current and the power component of the current flow in the same lines. Moreover, because of the presence of an air gap in the magnetic circuit of the induction machine, an appreciable amount of magnetizing current is needed to establish the flux per poledemanded by the applied voltage. Usually, the value of the magnetizing current for three-phase induction motors lies between 25 and 40% of rated current. Consequently, the induction motor is found to operate at a low power factor at light loads and at less than unity power factor in the vicinity of （在....附近）rated output.The primary functions of a controller are to furnish proper starting, stopping and reversing without damage or inconvenience to the motor, other connected loads, or the power system. However, the controller fulfills other useful purposes as well, especially the following:（1）It limits the starting torque. Some connected shaft loads may be damaged if excessive torque is applied upon starting. For example, fan blades （叶片）can be sheared off （剪断）or gears having excessive backlash （后座，后冲）can be stripped （剥离）.Thecontroller supplies reduced voltage at the start and as the speed picks up （力口速）the voltage is increased in steps to its full value. （2）It limits the starting current .Most motors above 2.38kW cannot be started directly across the three-phase line because of the excessive starting current that flows. Recall that at unity slip （转差率）the current is limited only by the leakage impedance（泄漏阻抗），which is usually quite a small quantity, especially in the larger motor sizes. A large starting current can be annoying because it causes light to flicker and may even cause other connected motors to stall （停转）. Reduced-voltage starting readily eliminates these annoyances.（3）It provides overload protection. All general-purpose motors are designed to deliver full-load power continuously without overheating.However, if for some reason the motor is made to deliver, say, 150% of its rated output continuously, it will proceed to accommodate the demand and bum itself up in the process. The horsepower rating of the motor is based on the allowable temperature rise that can be tolerated （忍受）by the insulation used for the field and armature windings.The losses produce the heat that raises the temperature. As long as these losses do not exceed the rated values there is no danger to the motor, but if they are allowed to become excessive, damage will result There is nothing inherent in the motor that will keep the temperature rise within safe limits .Accordingly, it is also the function of the controller to provide this protection. Overload protection is achieved by the use of an appropriate time-delay relay which is sensitive to the heat produced by the motor line currents.(4)It furnishes undervoltage protection. Operation at reduced voltage canbe harmful to the motor, especially when the load demands rated power. If the line voltage falls below some preset limit, the motor is automatically disconnected from the three-phase lie source by the controller.6.5 INDUCTION MOTOR TORQUE-SPEED CHARACTERISTICSThe variation of torque with speed (or slip) is an importantcharacteristic of the three-phase induction motor. The general shape of the curve can be identified in tenns of the basic torque equation Eq.(6.12) and knowledge of the performance computation procedure.T=0.177p © (Z2K W2I2)CO S W (6.12) Where p一number of poles;Z2一total number of conductors of the armature winding;一armature winding factor;KW2I2— armature winding current per phase.When the motor operates at a very small slip, as at no-load, the rotor current is practically zero so that only that amount of torque is developed which is needed to supply the rotational losses. As the slip is allowed to increase from nearly zero to about 10%, Eq.(6.13) shows that the rotor current increases almost linearly.I2=sE24- (r2+jsx2) (6.13)This is because the j part of the impedance, sx2, is small compared to r2. Furthennore, it can be shown that the space-displacement angle 血in Eq.(6.12)is identical to (等于) the rotor power-factor angle . That is= 9 2=tan-1—(6.14)r zTherefore, for values of s from zero to 10 percent, 血varies over a range of about 0 to 15. This means that the cos 血remains practically invariant over the specified slip, so the torque increases almost linearly in this region. Of course the quantity ① in Eq. (6.12) is essentially fixed since the applied voltage is constant.As the slip is allowed to increase still further, the current continues to increase but much less rapidly than at first. The reason lies in the increasing importance of the sx2 term of the rotor impedance. In addition, the space angle 血now begins to increase at a rapid rate which makes the cos t diminish more rapidly than the current increases. Since the torque equation now involves two opposing factors, it is entirely reasonable to expect that a point is reached beyond which further increases in slip culminate in decreased developed torque. In other words, the rapidly decreasing cos factor predominate (占优势) over the slightly increasing I2factor in Eq.(6.12). As 血increases, the field pattern for producing torque becomes less and less favorable becausemore and more conductors that produce negative torque are included beneath a given pole flux. Accordingly the composite torque-speed curve takes on （呈现） a shape similar to that shown in Fig.6.9.The starting torque is the torque developed when s is unity, i.e., the speed n is zero. Fig .6.9 indicates that for the case illustrated the starting torque is somewhat in excess of rated torque, which is fairly typical of such machines. The starting torque is computed in the same manner as torque is computed for any value of slip .Here it merely requires usings=l .Thus the magnitude of the rotor current at standstill isThe corresponding gap power （气隙功率）is then（6.15）”(6.16):r’+r； ) — (xx+xg )Where q一number of phases of the armature winding.It is interesting to note that higher starting torques result from increased rotor copper losses at standstill.At unity slip the input impedance is very low so that large starting currents flow. Eq.(6.15) makes this apparent. In the interest of limiting this excessive starting current, motors whose ratings exceed 2.38 kW are usually started at reduced voltage by means of line starters (直接启动). Of course, starting with reduced voltage also means a reduction in the starting torque. In fact if 50% of the rated voltage is used upon starting, then clearly by Eq.(6.16) it follows that the starting torque is only one-quarter of its full-voltage value.Another important torque quantity (定额) of the three-phase induction motor is the maximum developed torque. This quantity is so important that it is frequently the starting point in the design of the induction motor. The maximum (or breakdown) torque (止转转矩、失步转矩) is a measure of the reserve capacity of the machine. It frequently has a value of 200 to 300% of rated torque. It permits the motor to operate through momentary peak loads. However, the maximum torque cannot be delivered continuously because the excessive currents that flow would destroy the insulation.Since the developed torque is directly proportional to the gap power, it follows that the torque is a maximum when P g is a maximum. Also, P g is a。
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Part3 Reading Skills of Scientific
and Technical English
科技英语文章的阅读有以下五个方面的技巧: ❖ 获取主题思想(Reading for Main Idea)
获取主题思想的阅读技巧分四步： (1)辨认主题名词； (2)找出主题句； (3)获取主题思想； (4)避免不相关的内容。 ❖ 获取细节(Reading for Details)
Part2 Product Specification
❖ 简略性表现为内容简略，文字浅显，句式 简单，大量使用祈使句和省略句，避免使 用不必要的修辞手段。
❖ 技术性指说明书中文字涉及某方面的专业 知识。
❖ 描述性表现为文字具有层次感、程序性和 说服力。
❖ 客观性是指用在说明书中的词语应该恰当、 如实地反映产品的真实情况。
Unit Five Applied Writing
Part1 Notice of Invitation for Bids
招标通告通常由以下几部分组成: ❖ 标题(Headline) ❖ 招标单位名称(Name of Tendering Unit) ❖ 工程名称(Name of the Project) ❖ 工程招标号(Bid of Number) ❖ 正文(Body Copy) ❖ 附项(Supplementary Items)
❖ 理解句子(Understanding Sentence) (1)分析句子 (2)利用标点符号 (3)利用参照词 (4)利用信号词
Part3 Reading Skills of Scientific and Technical English
❖ 抓住段落核心，整体理解文章 (Understanding Article) (1)研读(study reading) (2)略读（skimming） (3)浏览（scanning）
Bylines Keywords Abstract
Introduction
Materials and Methods Results
Discussion Acknowledgements
References Appendix
Part3 Reading Skills of Scientific and Technical English
❖ 理解词义(Understanding Vocabulary ) (1)通过上下文线索猜测词义 (2)通过构词法掌握词汇前缀、后缀和词 根 (3)扩大知识面，克服背景知识方面的障 碍
பைடு நூலகம்
Part3 Reading Skills of Scientific and Technical English
Part4 How to Write a Scientific Paper
a Scientific Paper
Features Divisions
content the style of writing the system of documenting the sources Title of the Paper
Part2 Product Specification
❖ 产品说明书是帮助用户认识产品，指导用 户使用产品的书面材料 。
❖ 机械产品说明书的内容一般包括产品特点、 用途、规格、结构、性能、操作程序及注 意事项等 。
❖ 产品说明书大多都由标题、正文、落款三 部分组成。
❖ 产品说明书的文体特点为简略性、技术性、 描述性和客观性。