Electrical Power and Controls (2nd Edition)
PREFACEOver the past decade or so, communications, electronics, and power have converged to provide more efficient manufacturing processes for various types of industry. Many motors are installed with electronic drives; programmable logic controllers are finding widespread application; and communications networks at the device, control, and information levels are used to monitor many industrial processes. Engineers and technologists will require a broad background in all of these areas.Unfortunately, the time available in a typical curriculum is fixed, and in some cases the number of hours required for a degree has decreased. Thus, like many electric power instructors, we have been confronted by pressures to eliminate the required power course from our electrical engineering technology curriculum in favor of something that is "more worthwhile" in the view of many of our colleagues. The challenge was for us to show the value of our course for the majority of our graduates. Thus, we found it necessary to move away from a course that dealt almost exclusively with electric machines and transformer theory to one that includes the principles of operation and application of motors, motor controls, power quality, power electronics, motor circuit design, programmable logic controllers, and other topics that a typical graduate might encounter in the workplace.As we developed our course, it became evident that there was no particular text that covered all of the topics that we wanted to include. Thus, we began writing our own text. Although this text was developed to support a one-semester sophomore course in electrical engineering technology, we have included more material than could be covered in a single semester. This allows greater flexibility in designing a course using this text. This text uses primarily algebra and trigonometry for calculations and derivations; however, some calculus (primarily integrals) is used, particularly for Faraday's law. In general, the calculus can be skipped by those who have not taken an introductory calculus course. Throughout the book, we have made extensive use of figures and photos to reinforce the text discussion. We believe this is particularly important for today's traditional college students who are much more visually oriented and less inclined to read lengthy discussions. Recognizing the reluctance of students to read, in addition to problems, we have included review questions at the end of the chapters. This has a twofold benefit. First, it forces the student to read the material and, second, allows the incorporation of written assignments, which is emphasized by accreditation agencies and industry.   ORGANIZATION OF THE TEXTAlthough physics is a prerequisite course, we have found that our students often do not remember the concepts of force, torque, energy, and power and the relations between them. Thus, Chapter 1 presents a review of these basics, which allows us to introduce examples related to the electric power industry, such as pumped-water storage. Introducing the concept of energy storage in electric fields (capacitors) and magnetic fields (inductors) prepares the student for the discussion of reactive power in Chapter 2. Chapter 1 also introduces the electric power system and discusses some current events, such as deregulation and the projected decline of nuclear generation. We typically cover Chapter 1 in one to two lectures.Chapter 2 provides a review of basic electric power calculations, including a review of phasors and singlephase and three-phase electric power. The concepts of real, reactive, and apparent AC power are presented as they are used throughout the text. Because our students have seen this material before, we cover the material quickly in two lectures.Due to the widespread use of electronics in the commercial and industrial workplace, the issue of power quality has become extremely important. Every individual who uses electronic equipment contributes to the power-quality problem. Thus, we believe that every electrical engineer and technologist should be aware of the problems caused by the equipment they or others use. Chapter 3 introduces the concept of power quality using the CBEMAITIC curve. This curve includes not only steady-state phenomena but also transients. Thus, we have included a discussion of transient-voltage surge suppressors that can benefit every individual who wishes to provide protection for a personal computer, whether at home or in the workplace. We have also included a discussion of uninterruptible power supplies, as the advertising for these devices is often short on facts and long on claims. The heart of Chapter 3, however, is the discussion of harmonics in the power system. The concept of harmonics is introduced here and is continued in later chapters when transformers, induction motors, and variable-speed drives are discussed.Since one of our goals is for students to understand the operation of transformers and motors, some background in magnetic properties and materials is required. Chapter 4 introduces the concepts of flux, flux density, permeability, magnetic field intensity, reluctance, ferromagnetic materials, and losses due to hysteresis and eddy currents in magnetic materials. The emphasis is not to provide enough information for the students to become machine designers, but rather to give enough background so they can understand the operating principles of the devices.Chapter 5 introduces the first component of the power system—the transformer. The ideal transformer is introduced first and then the underlying assumptions are removed to produce an equivalent circuit for the transformer. Single-phase transformers are examined in detail, followed by a discussion of transformer bandwidth for electronics circuits, K-factor-rated (harmonics-rated) transformers, autotransformers, instrument transformers, and three-phase transformers. After the development of the transformer model, the topics are essentially independent, so the instructor can skip over sections that are not of interest.Because some instructors prefer to present AC machines before DC, and others prefer the opposite, Chapter 6 develops the basic concepts of motor and generator operation for both types of machines. Chapter 6 can be followed by Chapter 7, "The Induction Motor," Chapter 10, "The Synchronous Machine," or Chapter 11, "The DC Machine," depending on the desires of the instructor or reader. Because we believe that most students will encounter AC machines in the workplace, we present the AC machine first. The basic components of the synchronous machine are presented and the rotating magnetic field in the three-phase stator winding is developed prior to the discussion of the induction motor. Following the AC machine, the DC machine is presented in a similar manner.Chapter 7 presents the polyphase induction motor. It emphasizes practical, application-oriented material, beginning with construction features and nameplate information and concluding with wiring diagrams and reduced-voltage starting. The equivalent circuit for the induction motor is developed for those who feel it is important; however, we typically skip over that section and use the equivalent circuit primarily to discuss the losses in the induction motor. Following the calculation of motor efficiency, the requirements of the National Energy Policy Act (NEPACT) of 1992 for induction motors are discussed. Throughout the chapter, NEMA standards and requirements are discussed.Chapter 8 presents several types of single-phase induction motors and the universal motor. The primary emphasis is on understanding the differences in construction and performance of the various machines rather than on mathematical analysis. Speed control of the shaded-pole induction motor by varying the number of turns in the stator winding is discussed.Power electronics is an increasingly important topic due to the widespread use of variable-speed drives in industry. Chapter 9 begins with a discussion of a variety of solid-state power switches, including various types of diodes and transistors. This material is provided for completeness and to allow the practicing engineer/technologist to compare different technologies. Depending on the preferences of the instructor or reader, that material could be omitted. Following the discussion of switching technologies, the variable-frequency AC motor drive is discussed with primary emphasis on the induction motor. The chapter concludes with discussions of stepper motors and brushless DC motors, which are becoming increasingly important.The synchronous machine is presented in detail in Chapter 10. Following a description of the construction features, the synchronous generator is discussed first followed by the synchronous motor. Both round-rotor and salient-pole machines are discussed, although the primary emphasis is on the round-rotor case. Phasor diagrams are used extensively to illustrate the operation of the synchronous machine.Chapter 11 concludes the discussion of electric machines with the DC machine. The chapter begins with a discussion of the operation and construction of the DC machine, followed by a detailed discussion of commutation. Generator and motor operation of the various types of DC machines (series, shunt, compound) are presented, and the chapter concludes with a discussion of the efficiency of DC machines.Motors must be started and stopped and that is normally done with a control system. Chapter 12 presents the basics of control systems. A variety of control devices—relays, switches, pilot devices, motor starters, contactors, etc. —are discussed, followed by some basic considerations of ladder diagrams. Reduced-voltage starters and reversible starters for AC machines are presented, followed by starters for DC motors.Following the presentation of motor starters and overload sensors in Chapter 12, Chapter 13 presents the fundamentals of power circuit design. The use of National Electric Code tables is emphasized in the design of motor circuits and voltage drop calculations.Programmable logic controllers (PLCs) are being used for a wide variety of applications in the commercial, industrial, and institutional markets. As an example of how widespread they have become, our university has begun installing micro-PLCs in classrooms to control lighting levels via dimming ballasts on the fluorescent lights. As a result, we have devoted two weeks of the semester to PLCs. Chapter 14 provides some background on what a PLC is and what it is used for, a review of binary and octal arithmetic calculations, and a discussion of the scanning cycle. The Allen-Bradley PLC 5 is used as an example for discussing the programming commands, addressing schemes, and memory structure of PLCs. Much of the material would be similar for other PLCs and could be supplemented by PLC-specific information, such as addressing techniques.   ACKNOWLEDGMENTSWe would like to express our deepest appreciation to our wives, Carol Skvarenina and Nola DeWitt, for their support of our teaching and writing careers. Without their support, we could never have produced this book. We would also like to thank Tom Robertson of the EET Department at Purdue for his review and for providing suggestions for Chapter 14; our students for their suggestions while using various versions of our notes over the past several years; and the late Gil Rainey and his son, Alan Rainey of Learning Systems, who provided new versions of the text virtually every semester. Our appreciation also goes to our publisher, Charles Stewart, who provided strong encouragement to us to undertake this project; Alex Wolf, Prentice Hall production editor; and Gail Gavin of Clarinda Publication Services.T. L. Skvarenina and W. E. DeWitt--This text refers to an out of print or unavailable edition of this title.