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合格考试的重要性英语作文Passing exams is crucial for many reasons. It demonstrates our knowledge and understanding of the subject matter, showing that we have grasped the key concepts and are able to apply them effectively in different situations.Exams also help us to develop important skills such as critical thinking, problem-solving, and time management. By preparing for exams, we learn how to analyze information, evaluate different perspectives, and make informed decisions.Furthermore, passing exams opens up new opportunities for us. It can lead to better job prospects, higher salaries, and increased job security. Employers often look for candidates who have a proven track record of academic success, so passing exams can give us a competitive edge in the job market.In addition, passing exams can boost our confidence andself-esteem. It gives us a sense of accomplishment and validation, showing that our hard work and dedication have paid off. This can motivate us to set new goals and strive for even greater success in the future.Overall, passing exams is essential for our personal and professional growth. It helps us to acquire knowledge, develop skills, seize opportunities, and build confidence. So let's keep working hard and aim for success in all our exams!。
大学英语四级模拟试卷721(题后含答案及解析)题型有：1. Writing 2. Reading Comprehension (Skimming and Scanning) 3. Listening Comprehension 4. Reading Comprehension (Reading in Depth) 5. Cloze 7. V ocabulary and Grammar 8. TranslationPart I Writing (30 minutes)1．For this part, you are allowed 30 minutes to write an essay entitled On Conserving Energy. You should write at least 120 words following the outline given below in Chinese: 1．能源短缺问题已经非常严重；2．可以采取节能措解决能源短缺问题；3．你的看法。
正确答案：Nowadays there is growing concern on the serious problem of energy shortage. Many people worry that energy resources on the earth will be exhausted if we use them in an unchecked way. And they believe that the energy shortage will cause serious problems and even crisis. Conserving energy is one of the measures which have been taken to solve the problem of energy shortage. For example, some countries have a daylight saving system to use less electricity for lighting. Also, some cities have enforced some facilities for saving water at public places such as rest rooms and bars. Still more, some have enforced serious penalties on overuse of electricity, water, and farming land. By these means, people hope the drain on energy resources could be slowed down. In a word, it is high time that serious consideration was given to conserving energy. Only through immediate and effective action, can we human beings survive and enjoy life as desired.解析：节能是当前备受关注的一个话题，因此可说的内容很多，紧紧围绕提纲要求进行阐述是本文的鲜明特点。
电的重要性四级作文英语英文回答：Electricity plays a pivotal role in our modern world, serving as the backbone of numerous technological advancements and societal infrastructure. Its transformative impact has revolutionized the way we live, work, and interact with the world around us.Essential for Modern Life:Electricity has become an indispensable part of our daily lives, powering countless devices that enhance our comfort, convenience, and productivity. From lighting our homes to powering our appliances, electricity has made tasks that were once laborious or impractical effortless and efficient. It enables us to access information, communicate instantaneously, and enjoy entertainment at our fingertips.Economic Engine:Electricity is a crucial driver of economic growth and development. It fuels industries, powers transportation systems, and supports a wide range of businesses. By providing reliable and affordable energy, it creates jobs, stimulates innovation, and raises living standards. The development of new technologies, such as electric vehicles and renewable energy sources, is further boostingelectricity's economic significance.Environmental Sustainability:Electricity can play a key role in achieving environmental sustainability. The transition to renewable energy sources, such as solar and wind power, can reduceour reliance on fossil fuels and mitigate climate change impacts. Electric vehicles can significantly reduce carbon emissions compared to traditional gasoline-powered vehicles. By embracing electrification, we can create a cleaner and greener future.Transforming Healthcare:Electricity has revolutionized healthcare, enabling advanced medical technologies and treatments. Medical imaging techniques, such as X-rays and MRIs, rely on electricity to provide valuable insights into the human body. Robotic surgery systems offer increased precision and accuracy, leading to improved patient outcomes. Electricity powers life-saving devices, such as pacemakers and ventilators, ensuring the well-being of countless individuals.Challenges and Opportunities:While electricity has brought tremendous benefits, it also presents challenges that must be addressed. The increasing demand for electricity requires responsible management and sustainable energy practices. Climate change and extreme weather events can disrupt electricity supply, highlighting the need for infrastructure resilience and backup systems. Ensuring equitable access to electricity remains a global concern, especially in underservedcommunities.Despite these challenges, the future of electricity holds immense potential. Technological advancements, such as smart grids and artificial intelligence, are improving efficiency and reliability. The integration of renewable energy sources is reducing our carbon footprint and promoting sustainability. By harnessing electricity's transformative power, we can build a more prosperous, sustainable, and equitable world.中文回答：电的重要性。
考试的必要性英语作文The Necessity of Exams。
Exams are an essential part of the education system. They are used to evaluate a student's understanding of the material and their ability to apply that knowledge. While some people may argue that exams are unnecessary or even harmful, the reality is that exams serve several important purposes and are crucial for the development of students.First and foremost, exams provide a way for teachers to assess the progress of their students. By testing their knowledge and skills, teachers can identify areas where students are struggling and need additional help. This allows them to tailor their instruction to meet the needs of each individual student, ensuring that everyone has the opportunity to succeed.Exams also provide students with a clear understanding of where they stand academically. By receiving feedback on their performance, students can identify their strengths and weaknesses and take steps to improve. This can help them develop important study skills and work habits that will serve them well in the future.Furthermore, exams are an important part of the college admissions process. Universities use exam scores as a way to evaluate the academic abilities of potential students. By performing well on exams, students can improve their chances of being accepted into the college of their choice. This can have a significant impact on their future opportunities and career prospects.In addition to these practical benefits, exams also help to instill a sense of discipline and responsibility in students. By requiring them to prepare and study for exams, students learn the importance of hard work and dedication. They also learn to manage their time effectively and prioritize their responsibilities, skills that will serve them well in all areas of their lives.Despite these important benefits, some people argue that exams are unnecessary or even harmful. They claim that exams create unnecessary stress and anxiety for students,and that they do not accurately measure a student's true abilities. While it is true that exams can be stressful, it is also true that life is full of challenges and students need to learn how to cope with stress and pressure. Additionally, while exams may not provide a complete picture of a student's abilities, they are still an important tool for evaluating academic progress.In conclusion, exams are an essential part of the education system. They provide a way for teachers to assess student progress, help students understand where they stand academically, and play a crucial role in the college admissions process. Additionally, exams help to instill important skills and values in students, such as discipline and responsibility. While exams may not be perfect, they are still an important and necessary part of the educational experience.。
合肥市中考英语英语语法填空训练经典题目(含答案)一、英语语法填空1．阅读下面短文，按照句子结构的语法性和上下文连贯的要求，在空格处填入适当的词或使用括号中单词的正确形式填空。
Exam time! Most kids don't like exams, ________ we can't live without them. They play an important role in our lives.Exams are not all the same. There are ________ kinds. One kind tests our knowledge and skills. The other kind helps find excellent students in different fields. For example, universities ________ (use) it to choose students.At middle school, exams are about answering ________ (question) on paper. At university, doing an experiment or even ________ (draw) a picture can also be exams.Kids in different ________ take different exams. In the USA, many school children do a book report or a science report ________ part of their exams. In Australia, middle school students need to use ________ they have learnt in class to make speeches. Sometimes, teamwork is part of an exam.So exams are our lifetime "friends". Be nice and ________ (patient) with them. They ________(help) to make US better than before.【答案】 but；two；use；questions；drawing；countries；as；what；patient；will help 【解析】【分析】本文介绍了考试是我们一生的“朋友”，善待他们，耐心对待他们，他们会帮助我们变得比以前更好。
考试是必不可少的英语作文I think exams are a necessary evil. They stress me out so much, but I know they're important for testing my knowledge and skills. It's like a necessary evil, you know?When I sit down to take a test, my mind goes blank sometimes. It's like all the information I studied just disappears. I hate that feeling, but I guess it's just part of the whole exam experience.I remember one time I didn't study at all for a test and I totally bombed it. I felt so stupid afterwards. But I learned my lesson and now I always make sure to study beforehand, even if it's just a little bit.Sometimes I wish exams didn't exist. They just add so much stress to my life. But then I remember that they're necessary for evaluating my progress and understanding of the material. So I guess I'll just have to suck it up and deal with it.Overall, exams are a necessary evil that I can't avoid.I may not like them, but I know they're important for my education. And who knows, maybe one day I'll actually start to enjoy taking tests.。
专业英语对电气工程的帮助英语作文Professional English in Electrical EngineeringProfessional English plays a crucial role in the fieldof electrical engineering. It not only helps engineers to communicate effectively with colleagues and clients, but also allows them to access the latest technological developments and research findings. In this essay, I will discuss the ways in which professional English benefits electrical engineers and how it contributes to their success in the industry.First and foremost, professional English enables electrical engineers to access a wide range of technical information and resources. Most of the cutting-edge research and development in the field of electrical engineering is published in English, and proficiency in the language is essential for engineers to stay updated with the latest advancements. This includes academic journals, technical reports, conference papers, and online resources that provide valuable insights into new technologies, methodologies, and best practices. Without a strong command of professional English, engineers may struggle tounderstand and apply these advancements in their work, putting them at a disadvantage in the competitive industry.In addition, professional English is essential for effective communication within engineering teams and with clients and stakeholders. Engineers often work in multinational and multicultural environments where English serves as the common language for collaboration. Whetherit's discussing project requirements, presenting technical solutions, or reporting on progress, the ability to express ideas clearly and accurately in English is crucial for successful teamwork and project management. Furthermore, when dealing with clients and stakeholders, clear and professional communication in English helps to build trust and credibility, which are essential for securing contracts and maintaining long-term business relationships.Moreover, professional English is important for engineers to participate in international conferences, workshops, and seminars, where they can network with peers, share their expertise, and learn from others in the field. These events provide valuable opportunities for professional development, knowledge exchange, and exposureto new trends and innovations. By being proficient in English, engineers can fully engage in these activities and contribute to the global discourse on electrical engineering, enhancing their professional reputation and staying at the forefront of the industry.In conclusion, professional English is a valuable asset for electrical engineers, enabling them to access technical information, communicate effectively, and participate in international networking and professional development opportunities. As the field of electrical engineering continues to evolve and expand globally, proficiency in professional English will be essential for engineers to succeed and thrive in their careers.专业英语在电气工程领域起着至关重要的作用。
职称英语理工类A、B、C级综合试卷-14(总分：100.00，做题时间：90分钟)一、{{B}}第1部分：词汇选项{{/B}}(总题数：15，分数：15.00)1.Have you got a spare pen?（分数：1.00）A.a shortB.an extra √C.a thinD.a long解析：2.When he was the director of the company, his first accomplishment was to improve the working conditions.（分数：1.00）A.planB.purposeC.achievement √D.task解析：3.How do you account for your absence from the class last Thursday?（分数：1.00）A.explain √B.examineC.chooseD.expand解析：4.Their marketing policy for the product involves obtaining as much free publicity as possible. （分数：1.00）A.strategy √B.researchC.arrangementD.agreement解析：5.Apart from their differences, the couple were developing an obvious and genuine affection for each other.（分数：1.00）A.Except allB.Except for √C.But forD.Above all解析：6.He told the police that he had thought honestly about the ethics of what he was doing. （分数：1.00）A.behaviorB.wayC.morality √w解析：7.The fact that he has been quite frank in admitting his mistakes counts a lot to his credit. （分数：1.00）A.matters √B.estimatesC.explainsD.suspects解析：8.He seized the opportunity to present his proposal to the teacher.（分数：1.00）A.realizedB.took √C.followedD.kept解析：9.I was impressed by the way he could formulate his ideas, for he was only two years old. （分数：1.00）A.deviseB.handleC.express √D.demonstrate解析：10.The visitors to the island enjoyed themselves very much, but they refused to eat the raw fish. （分数：1.00）A.freshB.ripeC.burntD.uncooked √解析：11.Dick warned you that he might turn you down, didn't he?（分数：1.00）A.upset youB.defeat youC.reject you √D.trick you解析：12.They had a far better yield than any other farm miles around this year.（分数：1.00）A.goodsB.soilC.climateD.harvest √解析：13.The town is famous for its magnificent church towers.（分数：1.00）A.ancientB.oldC.modernD.splendid √解析：14.There is a limited number of books on this subject in the library.（分数：1.00）rgeB.totalC.small √D.Big解析：15.They had no way to dispose of the hazardous waste they produced in the process of the car manufacture.（分数：1.00）A.riskyB.fatalC.poisonousD.dangerous √解析：二、{{B}}第2部分：阅读判断{{/B}}(总题数：1，分数：7.00)阅读下面这篇短文，短文后列出7个句子，请根据短文的内容对每个句子做出判断。
考试是必要的英语作文Examinations are an integral part of the educational process, serving as a tool to evaluate a student's understanding and retention of knowledge. They are necessary for several reasons.Firstly, exams provide a standardized measure of performance. They allow educators to compare students' progress and ensure that the curriculum is being taught effectively. By setting clear expectations through exams, students are given a benchmark to strive for, which can motivate them to work harder.Secondly, exams encourage discipline and time management. Preparing for an exam requires students to organize their study schedules, prioritize topics, and manage their time efficiently. These skills are not just important for academic success but are also valuable in professional and personal life.Thirdly, exams help identify areas of weakness. By analyzing the results of an exam, students can identify the topics they struggle with and seek additional help or resources to improve. This self-awareness is crucial for personal growth and academic development.Moreover, exams foster critical thinking and problem-solving skills. They often require students to apply knowledge in newand challenging ways, which can enhance their analytical abilities and prepare them for complex real-world situations.Lastly, exams are a means of accountability. They ensure that students are held responsible for their learning and thatthey are committed to achieving their best. This sense of responsibility can carry over into other aspects of life, promoting a strong work ethic and a dedication to excellence.In conclusion, while exams can be stressful, they are a necessary component of education. They offer a structured way to assess and improve learning, develop important life skills, and hold students accountable for their academic progress.。
Linear Circuit Analysis Basics Second Edition CourseDesign (English Version)IntroductionLinear circuit analysis is an important part of electrical engineering, and it is critical for anyone who wishes to design and develop electrical systems. The purpose of this course is to provide students with a thorough understanding of linear circuit analysis andthe mathematics necessary to apply the concepts learned to practicalreal-world systems.In this second edition of the course, we will expand on the material from the first edition and provide more in-depth analysis of linear circuits. Additional topics covered in this course include operational amplifiers, frequency response, and advanced circuit analysis techniques.Course ObjectivesThe objectives of this course are as follows:1.To provide students with a strong understanding of linearcircuit analysis, including the mathematics necessary to apply the concepts learned.2.To teach students how to use operational amplifiers incircuit design and analysis.3.To introduce students to the concept of frequency responseand its importance in circuit analysis.4.To provide students with advanced circuit analysistechniques and problem-solving skills.Course OutlineUnit 1: Review of Linear Circuit AnalysisIn this unit, we will review the concepts from the first edition of the course, including:•Voltage, current, and power in resistance, capacitance, and inductance circuits•Kirchhoff’s laws•Mesh and nodal analysisUnit 2: Operational AmplifiersIn this unit, we will introduce the concept of operational amplifiers, including:•Ideal operational amplifier characteristics•Different configurations of operational amplifiers•Operational amplifier applications, such as voltage followers and amplifiersUnit 3: Frequency ResponseIn this unit, we will introduce the concept of frequency response and its importance in circuit analysis. Topics covered include: •AC steady-state analysis•Phasors and complex impedance•Frequency response of circuitsUnit 4: Advanced Circuit Analysis TechniquesIn this unit, we will cover advanced circuit analysis techniques, including:•Laplace transforms and their applications in circuit analysis•Two-port networks and their analysis•State-space analysisLearning OutcomesBy the end of the course, students will have achieved the following learning outcomes:•An understanding of linear circuit analysis and its applications in electrical engineering.•An understanding of operational amplifiers and their applications in circuit design.•An understanding of frequency response and its importance in circuit analysis.•The ability to use advanced circuit analysis techniques to solve complex circuit problems.Resources RequiredStudents will require the following resources:•Textbook: Linear Circuit Analysis Basics, Second Edition by John Doe (avlable for purchase online)•Scientific calculator•Computer with MATLAB or equivalent software for circuit analysisAssessmentAssessment for this course will consist of the following:1.Assignments (30%): Students will complete four assignmentsthroughout the semester to reinforce the concepts learned in class.2.Midterm exam (30%): A midterm exam will be held halfwaythrough the semester, covering material from the first half of the course.3.Final exam (40%): A final exam will be held at the end ofthe semester, covering all material from the entire course.ConclusionThis second edition of Linear Circuit Analysis Basics is designed to provide students with a thorough understanding of linear circuitanalysis and the mathematics necessary to apply the concepts learned to practical real-world systems. By the end of the course, students will have the skills and knowledge necessary to design and develop electrical systems and solve complex circuit problems.。
A.C. Motor Protection and FaultsAimUnderstand the safe operation of a.c. induction motorLEARNING OUTCOMES∙Explain the principle of operation of thermal and magnetic types of motor overload protection.∙Explain the advantage of using thermistor for motor protection.∙Describe how overload setting for a motor is determined.∙Describe the usual cause of single phasing.∙Explain the difficulties of protection against single phasing and how these might be overcome.∙Explain the effect of low voltage on a.c. induction motor.∙Explain the effect on an a.c. motor which is supplied with a current of 50Hz when its designed frequency is 60Hz and vice versa.∙Explain the steps to be taken to counteract the effects of operating an a.c. motor at a frequency other than its designed frequency.∙Explain the causes of overheating in induction motors.∙State the preventive measures to counteract overheating.OverviewThis lesson provides student with an understanding of the need for protection against over-current fault during the operation of an induction motor. The operation of protection devices found in motor circuits is explained.Motor Protection and FaultsNecessity for Over-current ProtectionProtection of motors is required mainly to prevent overheating which can cause deterioration of winding insulation and burn-out, which is a severe case of overheating. Overheating as a result of overload, stalling, single-phasing orprolonged starting period can be detected by a rise in line current and temperature increase in the windings.A device which monitors only the line current cannot detect overheating due to high ambient temperature or poor cooling due to block air passages, star-delta starter remaining in the star connections, and starting and stopping too often. Overheating in this case can only be detected by temperature rise within the windings. a) Magnetic Circuit BreakerThe circuit breaker is used to protect circuits against short circuit faults in the same manner like a fuse. A fuse burn out but a circuit breaker trip. It can be reset and use again.In normal operation, current flow through contacts that are held closed by an armature. Normal circuit current which flows through the coil is insufficient toproduce a significant magnetic flux to attract the armature. When too much current flow through the coil, the armature is attracted, initiating an unlatching action,causing the circuit to open. After the fault that causes the excessive current flow is rectified, the circuit breaker can be reset and used again.Figure 1b) Thermal Overload RelayMotors need to be protected against light overload. The overload protection will operate at 10% to 20% above the rated full load current.The thermal overload relay has three bi-metallic elements, one for each phase of the three phase lines. The bi-metallic strip consists of two metals sandwiched together. One end of the bi-metal is fixed while the other end which is free, bends whenheated. The motor line current passes through the heater. When the motor load current is normal, the bi-metal elements bends slightly. Excess current passing through the bi-metals causes them to bend much more. The three bi-metal elements are linked together through a plastic slide which moves to the right. A series of lever movement causes a normally close trip contact to open. In the motor control circuit, this n.c. trip contact is in series with the main contactor coil of the motor starter. With the opening of the trip contact, the coil gets de-energised, the main contactors on the supply lines open, and the motor stops.In this way, the motor is prevented from being overheated. If there is not overload protection, the excess heat would have over a period of time, damage the insulation so much that the motor would have burnt out.Figure 2Off Load PositionTripped Positionc) FusesShort circuit protection is also a requirement for motors of over 0.5 kW. Fuses of the cartridge/HRC design may be employed to provide the necessary rapid interruption of high fault current. Since short circuit current may be high enough to damage normal motor contacts, the fuses may be arranged to break fast first in the event of short circuit. The secondary function of fuses is to provide back-up for the other protective devices.d) Thermistor Protection of Motor WindingThe thermistor is thermal device which can be used in conjunction with an electro-magnetic overload trip. One of these inserted in each of the three windings would detect overheating from any cause. Thermistor are available with either a positive or negative characteristic. The former type are more definitive in operation because there is very sharp rise in resistance at a particular temperature (as opposed to gradual drop in resistance of the other sort). Positive thermistor can be connected simply in series and the very small current which passes through them normally is cut off by the effect of overheating in any one of them. Cessation of the minute checking current is used as the signal to operate the motor trip.Figure 3e) Electronic Overload RelaysElectronic overload relays are another option for motor protection. The features and benefits of electronic overload relays vary but there are a few common traits. One advantage offered by electronic overload relays is a heaterless design. Electronic overload relays can detect a phase loss and disconnect the motor from the power source. This feature is not available on mechanical types of overload relays.For more info, please browse:/index.htmlEOCR-ARELECTRONIC OVERLOAD RELAY WITH AUTO RESETIntroduction1) Adjustable Auto reset Time(0 - 120 seconds)2) Single Timer for starting and trip Delay3) Overload and Phase Loss Protection4) Visual Current Setting Aid and Trip Indication(LED)5) Flexible Power Supply(90 to 260VAC, 180 to 480VAC)6) Fail-safe ProtectionDescriptionTwo of the three phase motor currents are monitored by current transformers.The CTs form integral part of the relay, and external CTs are required for currents above 60Amps. internal solid state circuitry compares the actual motor current with the preset load current, set by the Load Knob(Load). When current level is greater than the preset point. due to overload or single phasing,red LED illuminated and after the elapse of the preset delay time,SPDT relay switches contact. The red LED remains illuminated, indicating an overload trip has occurred. After the elapse of preset Reset Delay Time(maximum 120sec), it resets automatically. immediate reset with of Reset button or remote electrical reset by interrupting control power, is possible. the starting delay and trip delay times are simultaneously set with single O-Time Knob and these timing functions are not independent.Test button provides means of testing the operation and integrity of the device with or without motor running. The actual load current can be determined with visual aid of LED(no Ammeter required).Simply adjust the the Load Knob slowly anti-clockwise until red LED flickers.The flickering red LED indicates 100% of actual motor current.How to set up1) Set the trip delay time at the O-Time Knob,not less than normal run-up time of the motor.2) Set the desired reset time on the Reset time Knob(R-time)3) With the control power on, depress Test button,and hold. Verify the red LED illuminates and tripping of therelay after the preset time. Red LED remains on until relay resetsautomatically after the preset delay.4) When commissoning relay or motor, set the load knob current tothe full load amperes, and engage the load.Then turn the load knob slowly CCW, until the LED flashes,where the 100% of actual load current is indicated.Set the Load Knob to the desired trip current.5) Periodic testing of the Test facilty is required to ensure full protection and regularly as Preventive Maintenance.ORDERING INFORMATIONDimensionTypical applicationSpecificationSingle PhasingCauses and Effects of Single-PhasingThe loss of current through one phase in a 3-phase supply is described as single phasing. This open circuit in a phase is often caused by:a) blown fuseb) faulty contactc) broken wire or loose connectionWith single phasing fault a motor will stall during starting. A running motor continues to run with the fault.A motor running with a single phase fault will carry excess current in the remaining supply cables. There will be an unbalanced distribution of current in the windings.It will be seen that at full load, the current in winding A is nearly 3 times its full load current and in windings B and C it is about 1.3 times above full load current. The line current is about 2.5 times the rated full load current.Single phasing in a running motor can be detected by overload devices in the supply lines.A single phased motor arranged for automatic starting will not self-start and if overload protection fails to operate, will result in overheating. Overheating in a stalled or running motor will cause burn-out of the overload windings.It is possible for single phasing in a lightly loaded motor to remain undetected by electro-magnetic trip (or thermal overload trip) on the supply lines which monitors the line current. Improved protection is given by thermistors placed in the windings to measure thermal effects. Alternatively, the differential type of overload relay may be used.The table below shows typical values of the line and phase currents under various loading during a single phasing fault.Examine the condition when the motor is running at 60% of full load rating when single phasing occurs. The line currents are 102% of the full load value whereas the current in winding A is 31% above its full load value. The line current will probably not activate a line-connected overload relay. The motor continues to run. However, local overheating in winding A of the motor will quickly result in damage.Single Phasing - Full Load ConditionL1L2L3Figure 4General Effect of Voltage and Frequency Variation of Induction MotorInduction motors are at times operated on circuits of voltage or frequency other than those for which the motors are rated. Under such conditions, the performance of the motor will vary from the standard rating.Voltage variations of 10% on power circuits are allowed in most commission rules. However, changing the voltage applies to an induction motor has the effect of changing its proper rating in proportion to the square of the applied voltage. This a 5-hp motor operated at 10% above the rated voltage would have characteristics proper for a 6-hp motor (6.05 to be exact), and operated at 10% below the rated voltage those of a 4-hp motor (4.05-hp). It is obvious that if the rating of a motor were greatly increased in this way, safe heating would frequently be exceeded. Relation of Torque to Supply VoltageAt constant frequency, the flux in the stator of the motor is directly proportional to the supply voltage, just as in a transformer. If the voltage is halved, the flux of the rotating field is also halved. At normal voltage, the motor develops a definite torque at a curtain slip. At half voltage, the halved stator flux sets up a halved emf and current in the rotor at the same slip.The rotor poles of half strength now react against the halved stator poles to produce approximately one quarter the former torque. It can be said that for a given slip the torque is proportional to the square of the supply voltage.The curves are shown for a motor having 300% breakdown torque at 20% slip and a starting torque of 150% at normal supply voltage. Accordingly, at half voltage, the breakdown torque is (½)2 or ¼ that at normal voltage and equal to ¼ of 300 or 75%. The starting torque is also ¼ of 150% or 37.5%. At 70% of normal supply voltage, both breakdown and starting torque are therefore 0.702 or practically 0.5 of normal supply voltage, both breakdown and starting torque are therefore 0.702 or practically 0.5 of normal, giving a breakdown torque or 0.5 x 300 or 150%, and a starting torque of 0.5 x 150 or 75% of that at normal voltage, as shown in Figure 5.1. Slip-Torque at Rated Voltage2. Slip-Torque at 70% Rated Voltage3. Slip-Torque at 50% Rated VoltageFigure 5Now, consider a 3-phase squirrel cage induction motor driving a blower or centrifugal pump. At rated voltage, the speed-torque characteristic of the motor is given by curve 1 of Figure 6. To simplify the drawing the curve is given as two straight lines. If we apply half the voltage the curve is given by curve 2. Because the torque is0 10 20 30 40 50 60 70 80 90 50 100 150 200 250 300proportional to the square of the applied voltage, the torques in curve 2 are only ¼ of the corresponding torque in curve 1.The load torque of the blower varies nearly as the square of the speed. This typical characteristic, shown by curve 3, is superimposed on the motor torque-speedcurves. Thus at rated voltage, the intersection of curve 1 and 3, is superimposed on the motor torque-speed curves. Thus at rated voltage, the intersection of curve 1and 3 shows the blower runs at 90% of the synchronous speed. On the other hand, at half rated voltage, the blower rotates at only 60% of synchronous speed.In general, a higher voltage will generally cause equipment to take excess current and overheat. It will also cause motors to accelerate more rapidly and this may overstress driven loads. A lower voltage is generally not so serious but may cause motor to run slower and may cause motor to stall. Windage is poor and motor may overheat.Figure 6When the frequency of the supply system is reducedIn a 60-Hz motor running on 50-Hz supply, the stator flux increases and the motor magnetising current increases so that heating of the windings and of the iron also increases. Thus a motor on reduced frequency will tend to run hotter.10040206080Torque %SpeedSynchronous SpeedHowever, a motor driving a fan also experience a reduction speed and torque which bring about a reduction in load.Any temperature rise may be compensated by a reduction in supply voltage.A reduction in frequency brings about an increase in torque. The breakdown torque and starting torque of a 60-Hz motor operating on a 50-Hz supply are about the same if the voltage is reduced to 5/6 of its name-plate value.In general, the full load torque of a 60-Hz motor operated at 50-Hz will be approximately 6/5 of the 60-Hz full load torque.When the frequency of the supply system is increasedIn a 50-Hz motor running on a 60-Hz supply, the stator flux reduces and the motor magnetising current reduces.However, a motor drivings a fan also experience an increase in speed and torque which bring about an increase in load. The load current increases on account of the higher load. Thus a motor on increased frequency will run hotter.Any temperature rise may be compensated by a reduction in supply voltage.An increase in frequency brings about a reduction in torque. The breakdown torque and starting torque of a 50-Hz motor operating on a 60-Hz supply are about the same if the voltage is increased to 6/5 of its name-plate value. This will ensure that the load torque does not exceed the driving torque.In general, the full load torque of a 50-Hz motor operated at 60-Hz will be approximately 5/6 of the 50-Hz full load torque.Figure 7 Effect of Changing stator frequencyShort Summary of Effect of Voltage and Frequency Variation1. A motor will operate successfully on any voltage within ±10% of normalvoltage and for any frequency within ±5% of norminal frequency. If thevoltage and frequency both vary, the sum of the two percentages must notexceed 10%.2. Changing the voltage applied to an induction motor has the effect of changingits proper rating as to power factor and efficiency in proportion to the squareof the applied voltage.3. Thus a 5 kW motor operated at 10% above the rated voltage would havecharacteristics proper for a 6 kW motor (i.e. 1.12 x 5 = 6.05). Operated at10% below the rated voltage it would have those of a 4 kW motor (i.e. 0.92 x 5 = 4.05).4. It is of course obvious that if the rating of a motor were greatly increase in thisway, safe beating would frequently be exceeded.5. If the line voltage is too high, the flux per pole will be high. This increasesboth the iron losses and the magnetising current with the result that thetemperature increases slightly and the power factor getting worse.6. Starting current will be higher with excessive voltage and causes overheat.7. If the voltage is less than 10% of the rated voltage, overheating, stalling andburn-out can result. The torque generated is reduced and for a fixed load(centrifugal pump or fan), the current will increase to maintain the requiredoutput. Such operation will generally cause overheating.8. If the line voltage is considerably below the rated voltage of the motor, themaximum torque that the motor can develop may be so reduced that themotor will stall on small overload.9. When the frequency is reduced, it causes a reduction in speed. The startingand running torque which is inversely proportional to the synchronous speed increases. With a reduction in frequency, the full load current also increases because stator flux increases and magnetising current increases. Hence,this iron loss increase results in overall increase in temperature.10. When the frequency is increased, it caused, it causes an increase in speed.Stator flux reduces resulting in the lowering of magnitising current. However,the higher speed will results in a higher load being driven. Hence the loadcurrent will increase to cause overheat.11. A 60 Hz motor can also operate on a 50Hz line but its terminal voltage shouldbe reduced to 5/6 of its rated value. The breakdown torque and startingtorque are then about the same as before and power factor, efficiency andtemperature rise remain the same.12. A 50Hz motor operates well on a 60Hz line but its terminal voltage should beraised to 6/5 of its rated value. The new breakdown torque is then equal to the original breakdown torque and starting torque slightly reduced. Powerfactor, efficiency and temperature rise remains satisfactory.Motor Overload ProtectionDOL starting introduce problems for overload and short circuit protection because of the initial inrush of starting current (5 - 8 times f.L. current). The protective device must be inoperative during the starting period but operative on normal moderate overloads and on short-circuit and under stalled conditions.Motors are now rated on CMR basis i.e. they are not required to have any margin or capacity for prolonged overload operation. This entails overload protection at a low level say 15% to 25% overloads for short periods.Overload protection should be set preferably at the working load current and not according to the motor current shown on motor rating plate.Under-voltage Protection for Motors.AC motor which is stopped due to interruption of supply should be disconnected from supply because on restoration it should not immediately switched on for the following reasons.(a) Operating personnel may be injured due to motor starting unexpectedly.(b) With certain ac motors a definite sequence of starting operations must begone through.(c) If all motors were to restart simultaneously the generators might be seriouslyoverloaded.(d) Certain adjustments to the driven machinery may be necessary before re-starting.U/V (or no-volt) protection is provided by spring loaded motor contactor because it will drop out when the supply volt is lost. When the supply returns the motor will not restart until its contact coil is re-energised.(a) Automatic starters under the control of float switches, pressure switches,thermostat will restart immediately. If necessary these can be locked after a supply failure.(b) Contactors will function to close a circuit with the supply voltage reduced to80% of normal for DC and 85% for ac as required by BS. However oncethey are closed, they will hold on with very much lower voltages down to two-thirds of normal and in some cases much lower. With an induction motor,the available torque is proportional to the square of the voltage so that at 2/3 voltage the torque available will have fallen to 40%. It will therefore mostprobably stall, causing considerable increase in stator current.Cartridge FusesHRC fuses have silver wire enclosed in a quartz powder filled ceramic tube with metal end caps. Arcing when this type of fuse blow is buried in the powder, fusion of which in the arc path helps to extinguish it.HRC fuses can be used for very high fault levels, determination is negligible, they have accurate time/current characteristics and reliability for discrimination; they are safer. If accidentally inserted on a fault; there is not issue of smoke or flame; cartridges are sized to ensure that the correct value fuse is inserted.Replacing FusesIt is prudent and sound practice when replacing fuses to replace all three. If any of the replaced fuses are still intact they can be transferred to another use such as lighting circuit where matching is not vital. Fuse clips and connections should be examined when renewing fuses as an overload contact may cause operation below the correct value.Sample SolutionExplain the necessity of keeping electrical contactors clean and closed with a firm contact pressure.Describe briefly the functions of auxiliary contacts.State how copper contactors are kept free of oxide formations.The basic requirements of any pair of contacts are:a) Low resistance of the contact material.b) Low resistance between the two contact surfaces when they meet to makethe circuit.Contacts in use are usually of copper for the sake of economy as well as its low resistance. Copper oxide film readily forms on the copper tips. Since copper oxide is not a good conductor, it must be eliminated by keeping contactors clean.Also, a certain amount of pressure is required to keep two contact surfaces closed sufficiently, to reduced what is called contact resistance.A switch for example, which is operated many times will eventually reach a stage when its spring weakened, with the result that pressure of the contacts is lost to such an extent that heat is generated and a breakdown of the switch follows:Within limits, the contact resistance varies inversely with spring pressure. The higher the resistance of contact material the more the heat there will be when a current pass along it. There is danger of contact welding occurring.An example of auxiliary contacts are found in the contactors of a main switchboard. Auxiliary contacts are operated by link mechanism and are of low power rating. Auxiliary contacts are used for switching, interlocking e.g. operating relays, indication lamps, etc in the control circuit.Copper contactors are kept free of oxide formation by regular cleaning with cleaning fluids which are available for this purpose. Very fine emery cloth can be used. Some Contactors are designed with a wiping action to remove copper oxide film. Sample SolutionGive 3 applications of wound rotor type of induction motor on board ship and justify reasons for its use.1) electric winch2) electric crane3) electric windlassesIn order to minimise the surge of starting current to the wound rotor motor due to high starting load, full resistance is inserted in the star-connected speed controller of the rotor circuit. As the motor controller can be cut out of the rotor circuit until the motor accelerates to its rated speed.The insertion of resistance in the motor circuit also produce high starting torque and also provides a means of adjusting the speed.Figure below illustrates that the torque performance of a wound-rotor induction motor is good whenever the full resistance of the speed controller is inserted in the rotor circuit. The large amount of resistance in the rotor circuit causes the rotor current to be almost in phase with the induced voltage of the rotor. As a result, the field set up by the rotor is almost in phase with the stator field. If the two fields reach a maximum value at the same instant there will be a strong magnetic reaction resulting in a high torque output.In all three cases given, a high starting and running torque is required. Hence for this reason and a reduction in starting current, justify the use of a wound rotor induction motor.Sample SolutionGive 4 application of squirrel cage type of induction motor on board ship justifying the reason for its use.1) feed water pump2) fresh water pump3) bilge pump4) auxiliary pumpsThe 3-phase, squirrel-cage induction motor is relatively small in physical size for a given horsepower rating when compared with other types of motors. It has good speed regulation under varying load conditions. A minimum of maintenance is required with is type of motor because:a) the rotor has no windings to become shorted.b) there are no commutators or slip-rings to service.c) there are not brushes to replace.。
