important for electrical part of exam
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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.。
考试对大学生学术的影响英语作文The Impact of Exams on Academic Performance of College StudentsIntroductionExams are an integral part of the academic life of college students. They serve as a measure of the students' understanding of the course material as well as their ability to apply that knowledge. However, the impact of exams on the academic performance of college students is a topic of much debate. Some argue that exams are a necessary tool for assessing students' knowledge and skills, while others believe that exams can have a negative impact on students' mental health and overall academic performance.In this essay, I will explore the various ways in which exams can influence the academic performance of college students. I will discuss the positive and negative effects of exams, as well as provide some strategies for students to cope with the stress and pressure of exams.Positive Effects of ExamsExams can have several positive effects on the academic performance of college students. Firstly, exams provide studentswith an opportunity to demonstrate their understanding of the course material. By testing their knowledge and skills in a controlled environment, exams allow students to showcase their abilities and receive feedback on their performance.Secondly, exams can motivate students to study and work harder. The prospect of taking an exam can encourage students to engage with the course material, review their notes, and seek help from their professors or classmates. This can lead to a deeper understanding of the subject matter and improved academic performance.Finally, exams can help students develop important skills such as time management, critical thinking, and problem-solving. By preparing for exams, students learn how to organize their study time effectively, analyze complex information, and apply their knowledge to solve problems. These skills are valuable not only for academic success but also for future careers and personal development.Negative Effects of ExamsDespite these positive aspects, exams can also have negative effects on the academic performance of college students. One of the main drawbacks of exams is the stress and anxiety they can cause. The pressure to perform well on exams can beoverwhelming, leading to feelings of fear, self-doubt, and insecurity. This can have a detrimental impact on students' mental health and well-being, affecting their ability to concentrate, retain information, and perform to the best of their abilities.Furthermore, exams can sometimes be a poor measure of students' true abilities. Some students may excel in exams but struggle with real-world application of knowledge, while others may perform poorly on exams but demonstrate a deep understanding of the material through other means such as class participation, projects, or essays. This can create an unfair assessment of students' abilities and may not accurately reflect their true potential.Strategies for Coping with Exam StressTo mitigate the negative effects of exams on their academic performance, college students can employ several strategies to cope with exam stress. Firstly, students should prioritize self-care and well-being by getting enough sleep, eating healthy foods, exercising regularly, and practicing relaxation techniques such as deep breathing or meditation. Taking care of their physical and emotional health can help students feel more confident, focused, and resilient in the face of exam pressure.Secondly, students should create a study schedule and stick to it. Breaking down the material into manageable chunks, setting goals, and creating a study plan can help students stay organized and focused during their exam preparation. Additionally, seeking help from professors, tutors, or classmates can provide students with additional support and resources to aid their learning.ConclusionIn conclusion, exams play a significant role in the academic performance of college students. While exams can have positive effects such as assessing students' knowledge, motivating them to study, and developing important skills, they can also have negative effects such as stress, anxiety, and unfair assessment. By adopting strategies to cope with exam stress and prioritize their well-being, college students can improve their academic performance and achieve success in their studies. It is important for students to recognize the impact of exams on their academic performance and take proactive steps to manage exam stress effectively.。
Examinations are an integral part of the educational system,and their importance cannot be overstated.They serve as a tool for assessing the knowledge and skills that students have acquired throughout their academic journey.Here are some key reasons why examinations are crucial:1.Assessment of Learning:Examinations provide a means to evaluate the effectiveness of teaching and learning.They help educators understand what students have grasped and where they may need additional support.2.Motivation for Students:The prospect of an exam can motivate students to study and prepare more diligently.It encourages them to review their coursework and deepen their understanding of the subject matter.3.Standardization:Exams help to standardize the evaluation process across different educational institutions.This ensures that students are assessed fairly and consistently, regardless of where they study.4.Recognition of Achievement:Successful performance in exams can lead to the recognition of a students academic achievements,such as the awarding of degrees, diplomas,or certificates.5.Career Opportunities:Many professional fields require proof of competency,which is often demonstrated through examination results.High scores on standardized tests can open doors to further education or employment opportunities.6.SelfDiscipline and Time Management:Preparing for exams requires students to develop selfdiscipline and time management skills.These are valuable life skills that extend beyond the classroom.7.Benchmarking:Exams can serve as benchmarks for educational institutions to compare their performance against national or international standards,leading to improvements in curriculum and teaching methods.8.Adaptability:The process of preparing for and taking exams can help students develop the ability to adapt to different types of questions and problemsolving scenarios,which is essential in the real world.9.Incentive for Continuous Learning:The competitive nature of exams can encourage a culture of continuous learning and selfimprovement among students.10.Personal Growth:Overcoming the challenges of exam preparation and achieving success can be a source of personal growth and confidence for students.In conclusion,while examinations can be stressful,they play a vital role in the educational process.They not only measure academic progress but also contribute to the development of essential skills that are beneficial for personal and professional life.It is important for students to approach exams with a positive attitude and view them as opportunities for growth and learning.。
电大统考英语作文The Importance of English Learning。
English is one of the most widely spoken languages in the world. It is the official language of over 50 countries and is spoken by more than 1.5 billion people. In today's globalized world, English has become the lingua franca of business, science, and technology. As a result, the ability to speak and understand English has become an essentialskill for anyone who wants to succeed in the modern world.There are several reasons why learning English is important. First and foremost, English is the language of international communication. Whether you are traveling abroad, studying at a foreign university, or doing business with international partners, being able to speak English will open up a world of opportunities for you. In addition, many of the world's top universities and research institutions use English as their primary language of instruction and publication. This means that if you want toaccess the latest knowledge and research in your field, you will need to be able to read and understand English.Furthermore, English is the language of the internet. The majority of online content is in English, including websites, social media, and online courses. If you want to fully participate in the digital age, you will need to have at least a basic understanding of English. Finally, English is the language of global business. Many multinational companies use English as their official language, and being able to speak English will greatly increase your chances of landing a job with one of these companies.Given the importance of English in today's world, it is essential for students to start learning English at an early age. Fortunately, there are many resources available to help students learn English, including textbooks, online courses, and language exchange programs. In addition, many schools and universities offer English language classes as part of their curriculum. By taking advantage of these resources, students can develop the skills they need to succeed in the globalized world.In conclusion, English is an essential skill for anyone who wants to succeed in the modern world. Whether you are a student, a professional, or a traveler, being able to speak and understand English will open up a world ofopportunities for you. Therefore, it is important for students to start learning English at an early age and to continue developing their skills throughout their lives. With the right resources and dedication, anyone can become proficient in English and reap the benefits of being able to communicate with people from all over the world.。
汽车检测与维修技术专业V ehicle Inspection and Maintenance Technology专业培养计划英文翻译人才培养方案专业代码:580402 专业:汽车检测与维修技术一、培养目标本专业培养德、智、体、美全面发展,具有良好的职业道德、熟练的职业技能,可持续发展的基础能力,掌握“必需、够用”的专业知识,面向汽车维修、汽车检测、汽车使用、汽车制造等生产一线,从事汽车制造、使用、维修、检测和管理等工作的高端技能型专门人才。
严格推行多证书制度,学生在毕业前,本专业毕业生至少获取汽车维修工中级证书、汽车维修电工中级证书、汽车驾驶证书其中的至少一种。
所开设的汽车电控系统检修、汽车行驶转向制动系统检修、汽车电气系统检修等课程为英国轻型车辆维修和保养三级技工证书课程,将提高学生在国内和国际相关行业企业的就业能力。
Ⅰ. Cultivation GoalThis specialty provides well-developed & skillful talents who have good professional ethics, competent skills, and master enough theoretical expertise, and own the ability for actual automobile maintenance, automobile inspection, automobile application, automobile manufacturing etc.This specialty strictly implements the multi-certificate system. The graduates should get at least one of the following certificates before graduation: middle-rank certificate for automotive maintenance, middle-rank electrician certificate for automotive maintenance, car-driving license. Besides, one must obtain an exquisite skills certificate in vehicle inspection and maintenance specialty of the Y ellow River Conservancy Technical Institute.Students of this specialty will learn the courses of UK Light V ehicle Repair and Maintenance Level Three Certificate including Inspection & Maintenance of Automotive Electrical-control System, Inspection & Maintenance of Automobile Running/Steering and Braking System, Inspection & Maintenance of Automotive Electric System, which willimprove the employability of students in domestic and foreign enterprises.二、基本规格1.掌握一定的自然科学、人文和社会科学基础知识,辩证唯物主义的思想方法。
1、(contrary) to your prediction,..........2、(To be honest), I didn’t agree with what you said3. (What ) is the price of that T-shirt4. (For) what reason did she quarrel with you?5(Looking) back into history, we can see that Chinese people have achieved 6(Given) their inexperience,they ‘ve done a good job7, (Now that ) I have lost the notes ,...have to go without them .8(Welcome ) to beijing ”,Mr. Li. said to the foreign friends.A:1 a baby learns to ( creep ) before it learns to walk .2 A co-generation power plane not ...( steam ) to heat users3 a dog was ( run over ) by a bus and killed .4 A desperate man will stop at nothing to get (what ) he wants5 A generator is a machine to ( convert ) mechanical energy.6 A good dictionary is ( indispensable ) to the study ...7 A really powerful speaker can work ( up//into )...the feeling of the crowd to a8 A thunder came down (while) they were listening to light music9 According to distribution equipment ( location ) ,...10 accuracy is ( fundamental ) to the programming of computers.11 ( Far from ) admiring his paintings, ...12 After ( flying ) across the Atlantic , bacame famous .( overnight ).13 After a good deal of trial and error, we..............(succeeded) in making our but14 After being rescued from the wrecked ship, ...( be thankful for ) .15 After my mother finished cleaning up the house ,she went on (to read ) ......16 After the lecture,...had a heated (discussion) on it .17 After Tom passed his driving test he ( filled out ) ...18 after being rescued from the ...( be thankful for ).19 All the desks in the room ( are covered with ) dust .20 All the worker s in the factory ( are provided )with protective hats.21 All those expensive shoes are made ( by ) hand22 although many young people are (keen).....23 Although my friends ...( insisted) on ...24 Although she was disabled when ...( high ) ,for ...( highly ) of her.25 Although the three workers got very tired ...( none of ) ...26 although her marriage was very unhanppy,...( sake ) of...27 although the family trusted her , she ( let )...28 american blacks were ( denied ) equal rights with ...29 American school children should learn .......address by ( heart ).30 An argument was inevitable because they disliked each other so much31 ( by way of ) an excuse he said he was tired ..32 Any body who has broken the law ..(being punished ).33 Anything that is dropped falls towards the centre ...( gravity ).34 apart from blue eyes , blonde hair is also ( characteristic ) of ...35 ( pumps ) are widely used in ...36 As a result of the neglect of all safety rules,.( were exposed )to37 As a result , he succeeded in ( reaching ) the castle but (failed to find )...38 As soon as his party came into ( power ).39 as they crossed broadway , he came ( face to face ) with a new ...40 as winter ( gave way to ) spring the days ...B:1 Because he stammered slightly , ...boys made ( fun ) of him2 Because I don’t like these shoes, show me ( some others,)please3 before moving to another city, brenda ( disposed ) of the house ...4 but, because barbary is all that ( matters ) to me ...5 By ( improveing ) his learning habits an average student can be ...6 By th e year 2000 , he ( had worked ) in the power ...7 ( Followed ) by his students ,the teacher entered the classroomC:1 Camping ( facilities ) include a tent,...2 can we hold ( out ) for another day without water?3 can you run ( off ) 200 copies of this ...4 Can you tell me where shanghai power company is ?( down ) ...( at )...5 Can you tell me who can best ( handle ) the problem?6 Candidates who wish to ( withdraw ) from ...7 Carbon is ( an element ) , while carbon dioxide is a compound8 carbon is ( an element ) , while ...9 children are very curious ( by nature ).10 China is famous ( for ) the great wall .11 chinese brown medicine has little side ( effect ).12 ‘’( enjoy youselves ) , children , ‘’...D:1 Despite all the heated ( arguments ) they had,...2 despite all the heated ( arguments ) they had ,...3 ( for what reason ) did she quarrel with you4 Do not touch anything that runs ( electricity ) with ...5 do you enjoy listening to ....(as good as ) or ...6 do you enjoy reading about the ( adwentures ) .....7 Do you know when and where the sports meeting( is to be held ).8 do you like these shors? ...( some others )9 don’t ( quarrel about ) small things10 Don’t eat anything ( that ) will spoil ... for dinner.11 Don’t judge a crime until you know all the ( circumstances ).12 don’t make ( fun ) of her ...(tears)...13 Don’t worry . It is not difficult ...( treat ) the fault .E:1 Egypt is famous for the ...( associate ) Egypt ...2 einstein was a mathematical ( genius ).3 Elephants don’t live wild in North A,erica , but in India they (do)4 Engineer Zhou has ( learned ) english for five years5 Every evening after supper, .my parents would go out (for )a walk..6 Every government that refuses to meet the needs ...(consequences ).7 everyone seemed to (turn). a blind...8 Excuse me , is this the way to shanghai ... No, ( i’m afraid it isn’t ) ....F:1 failure to communicate ( resulted in)...2 First the curtain ( caught ) fire and then ...3 Five years ago the pox ( wiped out ) the population of the island.4 For almost an hour the workers were just hanging about (waiting ).5 From his appearance we may safely ( conclude ) that he is a heavy sm6 From then on , she was never late ..means.( after that ) .7 from his appearance we may safely ( conclude ) that ...8 Fruit sells in best and cheapest when it is ( in season ).G:1 George left in such ...( hardly ) had time ...2 good work ( deserves ) good pay .3 Grown-ups should be (responsible) for............H:1 ( hardly ) had i got home ( than )...2 ( the rains ) have started ...3 handwriting is how you (present)...4 Harris turned a page to two and the title of a poem ( caught ) his eye5 have you ( heard about ) Mr.J...6 He rose early the next day , finding the river (had risen) on foot.7 He ( has been studying ) the problem in his study for a long time ...8 He ( is said) to be very generous with his money .9 He ( left) his mobile phone in the meeting ...10 He ( made up ) his mind not to speak a word .11 He (has been keeping ) up a correspondence with his friend in12 he (shot at) the bird ...13 he came to inspect the house (with a view to ) buying it .14 he consididers himself a great singer, and ( in a )...15 he decided to attend the conference ( in person ) .16 He did it out of gratitude for (what) she had done for him17 He didn’t ( live up to ) what had been expected of him .18 He didn’t tell me (when ) he was planning to return.19 he had hoped to get ...( for )$100.20 he has ( kept ) up a correspondence with ....21 He has a ( peculiar ) habit of biting his lips ...22 He hoped the firm would ( transfer ) him to...23 He hurried to the railway station ( in the hope of )...24 he intends to ( replace ) the secretary he has dismissed by ...25 he is always ( respectful ) to older people.26 He is an expert at making himself (understood) in foreign languages.27 He is the best candtidate to carry( out ) the plan .28 he is the manager of the company . He’s ( in charge of ) it .29 he is always kind,......(either)...(or).to animals.30 He lay awake all night , (thinking over ) the problem .31 He left ( out ) an important detail in his report of the accident.32 He read a book on the plane to ( while ) away the time .33 He restricts himself ( to ) smoking two cigarettes a day .34 he sai he had every ( confidence ) in his secretary;...35 He said the situation there was not ...(painted ).36 he said it in such clear terms...( doubt )...37 he shows no ( respect for ) at this rules.38 He slipped and broke his leg. (As a result)......39 He supplies them (with ) food and clothing .40 he touched ( on ) many points without ...41 He was finally convinced after a heated ( debate ).42 He was saving as much as possible in order (to repay .)..43 He was very ill but a good doctor and careful nursing brought( hin through ).44 He was very near- sighted, almost helpless (without) glasses.45 He was very near-sighted, almost ( helpless ) without glasses.46 he was ( only too ) willing to accept ...47 he’s ( taken to ) drink and never ...48 He’s a nice person , ( to be sure ) ...but49 her deeds are worthy ( of being praised ).50 her family ( sympathized ) with her desire to be a lawyer .51 Her father flew into a ( passion ) when he learned ...want (to get married)52 her husband wasn’t (content )with ...53 his ( total ) income of a year is ...( totals )...( a total ).54 His ( ambition ) had always been to become an architect.55 his ( ambition ) had always been to ...56 His business is growing so fast (that) he must take on more workers.57 His energetic efforts met with only..( partial ) success.58 His failure in the examination is ( fatal ) to the plan .59 his failure to pay ..(confirmed)...60 His father ( paved ) the way for him to climb to the top.61 His fatness renders him unable ( to touch ) his toes.62 his heart stopped (beating ) just ...63 His heart stopped (beating) just when we64 His honesty is ( out of ) question : nodody can doubt it65 his job was to ( sort ) out the good ...66 His opinions are always different from his girl-friend, ....(get along)67 His parents couldn’t get ( through ) to him ...68 his father is very busy ...( pace )of modern life .69 ( look back into ) history, and...70 ( For all ) his knowledge and academic ...71 ( thanks to ) his expert driving, we reached ...72 (considering) his age, he is ...73 How about ( staying ) at home if we meet with a stormy day ...74 how about jim ? He came home ( with safety )75 How can I get to your office ?...You can take a( walking ) there .76 how can i have put ( on ) a pound since ...77 How can you avoid mistakes when you ‘re in (such a) worry.?78 How is Mary getting on (with ) her study79 Hundreds of students (attended) the party last night.I:1 I heard her talking. About Mrs.White ........never.( heard of ).2 I received the invitation to the conference, but have no idea ( if )accept it .3 I recognized him the moment I ( saw) him.4 I would rather have noodles (than) rice5 i ( ran across ) her in ...6 I ( disapproved ) of the way he behaved. at the meeting.7 i (long for) an opportunity .....8 I am ( tired ) of your stupid conversation.9 I am ( tired ) of your ...10 I am glad to (have been given ) a chance to.......11 I am very sleepy because I ( stayed up ) ...12 I can still ( remember us walking ) hand in hand through the street...13 I couldn’t find a large enough coat , so I took this one .14 ( to be honest ), i didn’t agree with ...15 I didin’t expect that these few books (would come) to so much..16 I don’t see that i have deserved any fame and i have no ( taste ) for it ,...17 i don’t know how i,d ( react ) to a situation ...18 I found the little boy sitting on the steps ,crying.( bitterly ).19 I found the little boy ...( bitterly ) ...20 I gave him a present , but he (gave ) me nothing in return21 I had my shoes ( shined ) just before I went out22 I had not even a vague idea ( of what ) was going to happen23 I had to stand in a ( queue ) for hours to get tickets ...24 I happened ( to see ). him in the department store ...25 I have always (considered) you ...26 i have been to beijing ( scores of ) eggs in the basket.27 I have tried ( in vain ) to account for the riddle .28 i much prefer factual books such as biogrephies to ( fiction ) of any kind .29 I must admit my whole heart is ( set on )..being a medical biologist.30 I never expected you to turn ( up ) at the meeting; ...31 I often look back ( on ) my stay in England as32 I prefer staying at home to ( playing ) outside.33 I promise you shall see her again before (long ) .34 I recommended ( buying ) this dictiongary ..,but he could not afford it.35 I speak ( for ) my colleagues as well as ...36 ( What ) is the price of ...37 (baby-sitting) is part of ....38 I think english is quite different ( from ) chinese .39 i tired to call you ( up ) last night , but ...40 I tried to remember to put everything in good order..( to do ) so .41 I tried to call you (up) last night , but no one answered the phone42 I wanted to make a dress, but I didn’t know how to (set about ) it43 i wanted to make a dress, but ...(set about ) it .44 i was ashamed (of) when i lived.45 I was learning my lessons ( in the radio ) when she called me up46 I was told that this sweater. would .not. ( shrink ) ...47 i will have him come and keep ( me company ).48 I would rather he ( came ) tomorrow than today.49 I ‘ll vist my friend and stay with him for (some time ) next week.50 I must count ( up ) how much money ...51 i suppose 由have 。
UNIT 4HUMOURPart 4LESSON 3 MY FAVOURITE COMEDIAN基础过关练Ⅰ.单词拼写1.The girl sat in the front and(咀嚼)the candy slowly.2.She fixed a lot of(镜子) on the wall for her daughter to practice dancing.3.(2022新高考Ⅱ)We offer (奖学金) to low-income schools and youth organizations.4.They(遇到) many difficulties but they didn't give up.5.There are many g in the field of music, for example, Beethoven and Mozart.Ⅱ.选词填空1.Nowadays some students don't their teachers for help but do some research on their own.2.The difficulties didn't stop us. , they improved our ability to solve problems.3.The girl preparing for the exam and made much progress.4.Wang Yaping to walk out of the spaceship and became the first female Chinese astronaut to walk in space.Ⅲ.单句语法填空1.(2024河南开封七中月考)The world today is moving at a so high pace that we (rare) stop to listen to each other.2.We set up the organization (pure) to provide voluntary service.3.A smile is a (universe) language around the world.4.This comic book is designed(most) for children under six.5.When my mother saw my(create), she told me how creative my design was.6.We were(astonish) to find that the oil price had been going up in recent months.7.Beijing has a reputation hosting the Summer Olympics and the Winter Olympics.8.He graduated from Peking University in 2001,(obtain) his master's degree.Ⅳ.完成句子1.昨天学生们被他们的老师讲的有趣的故事逗笑了。
电能的重要性英文作文高中下载温馨提示:该文档是我店铺精心编制而成,希望大家下载以后,能够帮助大家解决实际的问题。
文档下载后可定制随意修改,请根据实际需要进行相应的调整和使用,谢谢!并且,本店铺为大家提供各种各样类型的实用资料,如教育随笔、日记赏析、句子摘抄、古诗大全、经典美文、话题作文、工作总结、词语解析、文案摘录、其他资料等等,如想了解不同资料格式和写法,敬请关注!Download tips: This document is carefully compiled by theeditor. I hope that after you download them,they can help yousolve practical problems. The document can be customized andmodified after downloading,please adjust and use it according toactual needs, thank you!In addition, our shop provides you with various types ofpractical materials,such as educational essays, diaryappreciation,sentence excerpts,ancient poems,classic articles,topic composition,work summary,word parsing,copyexcerpts,other materials and so on,want to know different data formats andwriting methods,please pay attention!Electricity is an essential part of our daily lives. We use it to power our homes, our businesses, and our technology. Without electricity, our world would be a very different place.In our homes, electricity allows us to cook our food, keep our food cold, and light up our rooms. We use it to power our televisions, computers, and smartphones. Without electricity, we would be living in darkness and unable to communicate with the outside world.In our businesses, electricity is crucial for running machinery, lighting up offices, and powering up computers. Many industries rely on electricity to keep their operations running smoothly. Without it, productivity would come to a halt.Electricity also plays a crucial role in the healthcare industry. Hospitals rely on electricity to power life-saving equipment such as ventilators, dialysis machines,and heart monitors. Without electricity, many lives wouldbe at risk.Furthermore, electricity is important for the environment. It can be generated from renewable sourcessuch as wind, solar, and hydro power, which helps reduceour reliance on fossil fuels and decrease harmful emissions.In conclusion, electricity is an indispensable part of modern life. Its importance cannot be overstated, and we must continue to find ways to generate and use it in a sustainable and responsible manner.。
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.。