Basic Electrical Engineering

Basic Electrical Engineering

Basic Electrical Engineering Revised Edition Dr. K Uma Rao Professor, Department of Electrical Engineering, R V College of Engineering, Bangalore Dr.A.Jayalakshmi AssociateProfessor, DepartmentofElectricalEngineering, JNTU,Kukatpally, Hyderabad SANGUINE Sanguine Technical Publishers Bangalore. 2014

Price: `425.00 ISBN 978 9383506 23-1 9 789383 506231 > Basic Electrical Engineering Revised Edition Dr. K Uma Rao & Dr. A Jayalakshmi This book contains information obtained from authentic and highly regarded sources. Reprinted material is quoted with permission, and sources are indicated. Reasonable efforts have been made to publish reliable data and information, but the author and the publisher cannot assume responsibility for the validity of all materials or for the consequences of their use. Neither this book nor any part may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, microfilming and recording, or by any information storage or retrieval system, without prior remission in writing from the publishers. The consent of SANGUINE TECHNICAL PUBLISHERS does not extend to copying for general distribution, for promotion, for creating new works, or for resale. Specific permission must be obtained in writing from SANGUINE for such copying. The export rights of this book are vested solely with the publisher. Direct inquiries: E-mail info@sanguineindia.com. Visit our website at www.sanguineindia.com 2014 by Sanguine Technical Publishers, Bangalore 560 016. Published by Lal Prasad for Sanguine. Production Editor: R.Subramanian. Printed in India.

To umesh, arathi & amrutha

PREFACE This book is primarily intended for a course in Basic Electrical Engineering. A strong need was felt for such a book, which would address the needs of Indian Students hailing from different backgrounds across the country. The book is written in two parts. Part A covers the important topics of circuit analysis and basic instrumentation. Part B deals with the concepts of Electrical Machines. Part A has six chapters. Chapter 1 of Part A, introduces the student to the two fundamental laws of Electrical Engineering which form the premise for all other topics, namely Ohm s law and Kirchhoff s laws. Chapter 2 deals with the fundamental concepts in Electromagnetism definitions of fundamental quantities, Biot-Savart s law, Faraday s laws, Self Inductance and mutual Inductance. Chapter 3 covers the various methods of circuit analysis nodal analysis, mesh analysis, superposition theorem, Thevenin s theorem, Norton s theorem, maximum power transfer theorem, reciprocity theorem, star-delta transformation applied to simple dc circuits. Chapter 4 introduces the students to the fundamentals of ac circuits. Solution of simple series and parallel ac circuits is presented. Chapter 5 presents concepts of three phase circuits. Solution of balanced three phase star and delta connected networks is dealt with in this chapter. The chapter also discusses the various methods for measurement of three phase power. Chapter 6 exposes the student to common electrical appliances and electrical engineering practices we come across in day to day life. The chapter covers the topics of methods of electrical wiring, choice of cables, simple wiring schemes, working principle of fuses, principle of MCBs, earthing methods, common lighting sources, estimation of energy requirement for simple loads, causes and prevention of electric shocks. Finally, chapter 7 covers basic concepts of electrical measurement. Moving coil and moving iron instruments, dynamometer wattmeter and energy meter are discussed in detail. Part B is devoted to the treatment of Electrical Machines. Chapter1 presents the fundamental concepts of electric machines. Chapter 2 deals with dc generators. The construction, principle of operation and derivation of induced emf are presented in detail. Chapter 3 vii

viii Preface deals with the operation, types and characteristics of dc motors. It also covers the important topics of testing and speed control of dc motors. Chapter 4 is devoted to transformers. The constructional features, operation, phasor diagram, testing, computation of efficiency and regulation are discussed in detail. Chapter 5 deals with three phase induction motors, which are the most rugged and widely used in the industry. Chapter 6 presents three phase alternators. The construction, emf equation and different methods of regulation are covered in detail. Chapter 7 covers single phase induction motors. Chapter 8 gives the experimental procedure for some of the common experiments conducted in electrical machines laboratory. All chapters are presented in a simple and lucid manner to facilitate an easy understanding of the subject. Important concepts are highlighted to emphasize the need to remember them. Every chapter contains a number of solved examples to strengthen the learning of the student and help the student, apply the concepts in various contexts. The examples presented help the students to relate abstract concepts in Electrical Engineering to practical applications which they see in their daily lives. Every chapter also gives a number of questions and unsolved problems for the student to work out. The book can be used as a text book for a course in Basic Electrical Engineering, a fundamental course in Electric circuits and also for a basic course in Electrical Machines. Dr. K. Uma Rao

CONTENTS Part I Circuits and Measurements 1 1 Basic laws 3 1.1 Introduction 4 1.2 Basic definitions 4 1.2.1 Charge 4 1.2.2 Current 4 1.2.3 Voltage 5 1.2.4 Power 6 1.2.5 Energy 7 1.3 Ohm s law 7 1.3.1 Resistances connected in series 9 1.3.2 Resistances connected in parallel 9 1.3.3 Division of current in parallel branches 10 1.3.4 Voltage division in series circuit 11 1.3.5 Conductance G (The unit is mhos or Seimens) 12 1.4 Nodes, branches and loops 13 1.5 Kirchhoff s laws 13 1.5.1 Kirchhoff s current law (KCL) 14 1.5.2 Kirchhoff s voltage law (KVL) 14 ix

x Contents 2 Electromagnetism 45 2.1 Introduction 46 2.2 Magnetic force 46 2.2.1 Biot-Savart Law 47 2.2.2 Force on a current carrying conductor 48 2.2.3 Force between two current carrying conductors 50 2.2.4 Magnetic flux 50 2.3 Magnetomotive force and magnetic field strength 52 2.4 Reluctance 55 2.5 Comparison of magnetic and electric circuits 56 2.6 Composite magnetic circuits 56 2.6.1 Composite series circuits 57 2.6.2 Composite parallel circuits 58 2.7 Electromagnetic induction Faraday s law 64 2.7.1 Fleming s right-hand rule 65 2.7.2 Lenz s law 65 2.7.3 Dynamically induced emf 67 2.7.4 Statically induced emf 67 2.7.5 Self induced emf 67 2.7.6 Inductance 67 2.7.7 Mutually induced emf 68 2.7.8 Mutual inductance 69 2.8 Energy stored in magnetic field 72 2.9 Dot convention 73 2.10 Inductance in series 75 3 Methods of Circuit Analysis 87 3.1 Introduction 88

Contents xi 3.2 Circuit Elements 88 3.2.1 Voltage source 89 3.2.2 Current Source 90 3.2.3 Source transformation 92 3.3 Nodal analysis 96 3.4 Mesh analysis 105 3.5 Linearity 110 3.5.1 Homogeneity 111 3.5.2 Superposition 111 3.6 Superposition theorem 112 3.7 Thevenin s theorem 116 3.7.1 Thevenin s resistance 117 3.8 Norton s theorem 123 3.9 Maximum power transfer theorem 126 3.10 Reciprocity theorem 135 3.11 Star-Delta Conversion 137 4 Single Phase ac Circuits 151 4.1 Introduction 152 4.2 Sinusoids 153 4.3 Phasors 156 4.3.1 Transformation of derivatives into phasor domain 159 4.3.2 Complex number representation 159 4.3.3 Mathematical operations with complex numbers 162 4.4 Effective or RMS value 165 4.4.1 Effective value of an arbitrary current 168 4.4.2 RMS value of a complex wave 169

xii Contents 4.5 Average value 170 4.5.1 Average value of a sine wave 170 4.5.2 Average value of an arbitrary current 170 4.6 Form factor 171 4.7 Crest factor or peak factor or amplitude factor 171 4.8 Voltage current relationship in circuit elements 176 4.8.1 AC through pure ohmic resistance 176 4.8.2 AC through a pure inductance 179 4.8.3 AC through a pure capacitor 181 4.9 Impedance and admittance 183 4.10 Series ac circuits 186 4.10.1 Series R-L circuit 187 4.10.2 Series R-C circuit 198 4.10.3 Series R-L-C circuit 203 4.10.4 Parallel circuits 206 5 Three phase circuits 233 5.1 Balanced three-phase voltage 235 5.2 Star-connected system 236 5.2.1 Star-connected voltages 236 5.2.2 Phase and line quantities in star-connected systems 238 5.2.3 Power in star connected systems 240 5.3 Delta-connected system 242 5.3.1 Phase and line quantities in delta-connected systems 242 5.3.2 Power in delta connected systems 244 5.4 Measurement of three phase power 251 5.4.1 Measurement with one wattmeter 251

Contents xiii 5.4.2 Measurement with two wattmeter method 252 5.4.3 Three wattmeter method 256 6 Electrical Wiring and Estimation 265 6.1 Introduction 266 6.2 List of Symbols 266 6.3 Supply Voltages 268 6.4 Service Connection 269 6.5 Interior wiring 270 6.5.1 Cleat wiring 270 6.5.2 Wood Casing Wiring 271 6.5.3 Batten wiring 271 6.5.4 Conduit wiring 272 6.5.5 Cable Specification 273 6.6 Guidelines for sub-circuits and fittings 275 6.7 Simple Wiring Schemes 275 6.7.1 Control with one switch 275 6.8 Two way control of lamps 277 6.9 Three way control of lamps 278 6.10 Lighting Sources 281 6.10.1 Incandescent lamp 281 6.10.2 Fluorescent lamp 282 6.10.3 CFL Bulbs 283 6.10.4 Light emitting diode 284 6.10.5 Comparison of different light sources 284 6.11 Fuses 285 6.11.1 Advantages and disadvantages of fuses 286

xiv Contents 6.11.2 Desirable properties of a fuse element 287 6.11.3 Important terms related to fuse 287 6.11.4 Types of fuses 289 6.12 Miniature circuit breaker 292 6.13 Earth Leakage Circuit Breaker (ELCB) 294 6.14 Earthing 294 6.14.1 Earthing systems 296 6.14.2 Methods of earthing 298 6.14.3 Uses of earthing 301 6.15 Computation of Energy consumed 301 6.15.1 Typical power ratings for appliances 303 6.15.2 Standard sockets and plugs 303 6.15.3 BEE star rating 304 6.16 Electric Shock 306 6.16.1 First aid for electric shock 307 6.16.2 Causes for leakage currents 307 6.16.3 Prevention to be taken to prevent electrical shocks 308 7 Electrical Instruments 311 7.1 Introduction 312 7.2 Classification of instruments 312 7.2.1 Mechanical, electrical and electronic instruments 312 7.2.2 Absolute and secondary instruments 313 7.2.3 Indicating, recording and integrating instruments 313 7.3 Principles of operation 313 7.4 Essential operating forces in indicating instruments 314 7.4.1 Deflecting or operating torque 314

Contents xv 7.4.2 Controlling or restoring torque 314 7.4.3 Damping Torque 317 7.5 Ammeters and voltmeters 320 7.5.1 Types of ammeters and voltmeters 321 7.6 Permanent Magnet Moving Coil instruments (PMMC) 321 7.6.1 Construction 321 7.6.2 Deflecting torque in PMMC meters 322 7.6.3 Advantages and disadvantages of PMMC 323 7.6.4 Extension of range as an ammeter 323 7.6.5 Extension of range as a voltmeter 324 7.7 Moving iron instruments 328 7.7.1 Attraction type instruments 328 7.7.2 Deflecting torque in attraction type MI instruments 329 7.7.3 Repulsion type MI instruments 330 7.7.4 Advantages and disadvantages of MI instruments 331 7.7.5 Extension of range 331 7.8 Errors in meters 332 7.8.1 Errors in PMMC meters 332 7.8.2 Errors in MI meters 333 7.9 Electrodynamometer Wattmeter 333 7.9.1 Construction 334 7.9.2 Deflecting torque 334 7.10 Energy meter 338 7.10.1 Construction 338 7.10.2 Deflecting torque 339 7.10.3 Errors in energy meters 340

xvi Contents Part II Electric Machines 343 1 Introduction to Electric Machines 345 1.1 Electric Energy 346 1.2 Basic principles of electric machines 347 1.3 Basic types and constructional features 347 1.3.1 D.C. machines 348 1.3.2 Synchronous machines 348 1.3.3 Induction machines 349 1.3.4 Single phase a.c. motors 349 1.3.5 Transformers 349 1.4 Losses in Machines 350 1.4.1 Copper losses 350 1.4.2 Iron losses 350 1.4.3 Mechanical losses 350 2 DC Machines Generators 353 2.1 Introduction 354 2.2 Principle of working 355 2.3 Construction of a d.c. generator 356 2.3.1 Yoke 358 2.3.2 Armature core 358 2.3.3 Armature windings 359 2.3.4 Pole Core and Shoe 361 2.3.5 Field coil (winding) 362 2.3.6 Commutator 362 2.3.7 Brushes and bearings 362 2.4 E.m.f. equation of d.c. generator 363

Contents xvii 2.5 Types of d.c. generator 366 2.5.1 Separately excited generator 366 2.5.2 Self-excited generator 366 2.5.3 Shunt generator 367 2.5.4 Series generator 368 2.5.5 Compound generator 369 2.6 Losses in d.c. generator 378 2.6.1 Iron loss 378 2.6.2 Copper losses 379 2.6.3 Mechanical losses 379 2.7 Power stages and efficiency 380 2.8 Armature reaction 386 2.9 Generator characteristics 386 2.9.1 Magnetisation characteristics 386 2.9.2 Load characteristics of d.c. shunt generator 387 2.9.3 Load characteristics of d.c. series generator 389 2.9.4 Load characteristics of d.c. compound generator 391 2.9.5 Critical resistance of a shunt generator 391 2.9.6 O.C.C. at different speeds 393 2.10 Voltage build-up of shunt generator 397 2.11 Application of DC generators 398 3 DC Motors 401 3.1 Introduction 402 3.2 Generator action in a motor 403 3.3 Condition for maximum power 404 3.4 Torque developed in a motor 405

xviii Contents 3.5 Motor characteristics 413 3.5.1 Types of d.c. motors 413 3.5.2 Characteristics of motors 413 3.5.3 Characteristics of shunt motor 414 3.5.4 Characteristics of dc series motor 416 3.5.5 Characteristics of d.c. compound motors 419 3.5.6 Performance curves of shunt motor 419 3.5.7 Performance curves of d.c. series motor 420 3.5.8 Applications of d.c. motors 421 3.6 Losses and efficiency of d.c. motors 421 3.7 Testing of d.c. motors 431 3.7.1 Brake test 432 3.7.2 Swinburne s test 435 3.8 D.C. motor starters 440 3.8.1 Three point starter 441 3.8.2 Series motor starter 443 3.9 Speed control of d.c. shunt motors 444 3.9.1 Flux control 445 3.9.2 Armature control 445 3.10 Speed control of series motors 453 3.10.1 Flux control 453 3.10.2 Rheostatic control 454 4 Transformers 459 4.1 Introduction 460 4.2 Working principle of a transformer 460 4.2.1 Transformer ratings 460

Contents xix 4.3 Transformer construction 461 4.3.1 Core-type transformers 461 4.3.2 Shell-type transformers 462 4.4 Single phase transformer 463 4.4.1 Ideal transformer on no-load 464 4.4.2 E.M.F. equation 464 4.4.3 Voltage transformation ratio 467 4.5 Transformer with resistance and magnetic leakage (Practical transformer) 469 4.5.1 Phasor diagram of transformer on no-load 470 4.5.2 Transformer on load 473 4.5.3 Phasor diagram on-load 474 4.5.4 Equivalent resistance and reactance 476 4.6 Equivalent circuit 485 4.6.1 Approximate equivalent circuit 486 4.7 Performance of transformers 486 4.7.1 Efficiency of transformer 487 4.7.2 Regulation of a transformer 488 4.8 Transformer tests 490 4.8.1 Open-circuit test or no-load test 490 4.8.2 Short-circuit text 492 4.8.3 Predetermination of efficiency and regulation from OC and SC test data 493 4.8.4 Condition for maximum efficiency of a transformer 495 4.8.5 Separation of core losses 496 4.9 All-Day Efficiency 513 4.10 Auto transformer 515

xx Contents 5 Three phase induction motors 519 5.1 Introduction 520 5.2 Construction 520 5.2.1 Squirrel - Cage rotor 520 5.2.2 Wound rotor or phase-wound rotor 521 5.3 Production of rotating magnetic field 522 5.4 Principle of operation 525 5.5 Frequency of rotor current 526 5.6 Torque developed 529 5.6.1 Starting torque 529 5.6.2 Condition for maximum starting torque 531 5.6.3 Torque under running conditions 531 5.6.4 Condition for maximum torque under running conditions 532 5.6.5 Break down Torque 533 5.6.6 Full load torque 533 5.7 Torque-slip characteristics (or Torque-Speed Characteristics) 542 5.8 Power stages in an induction motor 543 5.8.1 Relationship between various power stages 545 5.8.2 Synchronous watt 549 5.9 Starting of induction motors 554 5.9.1 Direct-on-Line [DOL] starting 555 5.9.2 Stator rheostat starter or primary resistor starter 556 5.9.3 Auto transformer starter 557 5.9.4 Star-delta starter 559 5.9.5 Rotor rheostat control 559 5.10 Equivalent circuit of induction motor 565

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