M.N. Bandyopadhyay

Theory and Practice

ELECTRICALMACHINESELECTRICALMACHINES

ELECTRICAL MACHINESTheory and Practice

Delhi-110092 2014

M.N. BandyopadhyayDirector

National Institute of TechnologyKurukshetra

Copy R 608

ELECTRICAL MACHINES: THEORY AND PRACTICEM.N. Bandyopadhyay

© 2007 by PHI Learning Private Limited, Delhi. All rights reserved. No part of this book may be reproduced in any form, by mimeograph or any other means, without permission in writing from the publisher.

ISBN-978-81-203-2997-3

The export rights of this book are vested solely with the publisher.

Fourth Printing … … November, 2014

Published by Asoke K. Ghosh, PHI Learning Private Limited, Rimjhim House, 111, Patparganj Industrial Estate, Delhi-110092 and Printed by Rajkamal Electric Press, Plot No. 2, Phase IV, HSIDC, Kundli-131028, Sonepat, Haryana.

ToProfessor Nirmal Kanti Chatterjee

andProfessor Basudev Barman

Ex Vice Chancellor

v

Contents

Preface xiiiIntroduction xv

1. DC Machines 1–1071.1 Introduction 11.2 Principle of Operation of DC Machine 11.3 Components of DC Machine 61.4 Armature Windings 7

1.4.1 Lap Winding 71.4.2 Wave Winding 111.4.3 Method of Connection of Windings in the Slot 14

1.5 Voltage Equation of a DC Machine 141.6 Armature Reaction in DC Machines 18

1.6.1 Calculation of Demagnetization and Cross MagnetizationAmpereturns Per Pole 20

1.6.2 Compensating Winding 211.6.3 Commutation 22

1.7 Excitation of DC Machine 321.7.1 Build-up of a DC Shunt Generator 341.7.2 Critical Resistance and Critical Speed of DC Machine 35

1.8 Characteristics of DC Generator 381.8.1 Characteristics of Separately Excited DC Generator 381.8.2 Characteristics of DC Shunt Generator 391.8.3 Determination of Internal and External Characteristics from

Open-Circuit Characteristic for DC Shunt Generator 391.8.4 Characteristics of DC Series Generator 421.8.5 Characteristics of DC Compound Generator 42

vi Contents

1.9 Parallel Operation of DC Generators 461.9.1 Conditions Necessary for Parallel Operation of

DC Generators 461.9.2 Procedure for Connecting DC Shunt Generators in Parallel 461.9.3 Parallel Operation of Compound Generators 47

1.10 Principle of Operation of DC Motor 551.11 Different Types of DC Motors 561.12 Characteristics of DC Motors 561.13 Starting of DC Motor 64

1.13.1 Manual Starters 641.13.2 Automatic Starters 67

1.14 Speed Control of DC Motors 701.14.1 Shunt Motor Speed Control 701.14.2 Series Motor Speed Control 741.14.3 Speed Control with the Power Electronic Devices 78

1.15 Braking of DC Motors 861.15.1 Plugging 861.15.2 Dynamic or Rheostatic Braking 871.15.3 Regenerative Braking 881.15.4 Application of Various Types of Braking on

Different Categories of Motors 881.16 Testing of DC Machines 93

1.16.1 Swinburne’s Test 931.16.2 Hopkinson’s Test 95

Summary 102Rreview Questions 105

2. Transformers 108–1882.1 Introduction 1082.2 Construction of a Transformer 109

2.2.1 Windings and Their Insulation from the Iron Core 1092.2.2 Oil of the Transformer 1132.2.3 Main Tank and Conservator Tank 1132.2.4 Silicagel Breather 1132.2.5 Cooling Arrangements 1142.2.6 Buchholz Relay 1162.2.7 Bushings 1172.2.8 Tap-Changing Device 118

2.3 Principle of Transformer Operation 1212.4 Equivalent Circuit of Single-Phase Transformer 1272.5 Phasor or Vector Diagram of Single-Phase Transformer 1292.6 Regulation of Transformer 132

Contents vii

2.7 Tests on Transformer 1342.7.1 Polarity Test 1342.7.2 Open-Circuit Test or No-Load Test 1352.7.3 Short-Circuit Test 1362.7.4 Determination of Eddy Current Loss and Hysteresis

Loss in Transformers 1372.7.5 Sumpner’s Test or Back-to-Back Test 138

2.8 Efficiency Calculation of Transformer 1412.9 All-Day Efficiency of Transformer 1422.10 Autotransformer 1432.11 Three-Phase Transformer 146

2.11.1 Vector Grouping or Phase Group ofThree-Phase Transformer 147

2.11.2 Methods of Connections of Three-Phase Transformer 1502.12 Parallel Operation of Single-Phase Transformers 158

2.12.1 Parallel Operation of Transformers with+30° and –30° Phase Shift 159

2.12.2 Load Sharing by Two Transformers in Parallelwith Equal Voltage Ratios 160

2.12.3 Load Sharing by Two Transformers in Parallelwith Unequal Voltage Ratios 161

2.12.4 Per Unit and Percentage Impedance of Transformer 1632.13 Three Winding Transformers 168

2.13.1 Suppression of Triple Frequency Currents byTertiary Winding 169

2.14 Open-Delta Connection 1702.15 Three-Phase to Two-Phase Conversion by Scott Connected

Transformer 1702.15.1 Three-Phase to One-Phase System 173

2.16 Induction Regulator 1752.17 Transformer Diagnostics 177Summary 185Review Questions 187

3. Three-Phase Induction Motor 189–3043.1 Principle of Rotating Field in Three-Phase Induction Motor 1893.2 Reason for Not Attaining the Rotating Field Speed by

the Three-Phase Induction Motor Rotor 1903.3 Slip of Three-Phase Induction Motor 1903.4 Main Difference with the Principle of Transformer 1923.5 Types of Three-Phase Induction Motors 192

3.5.1 Comparison between Squirrel Cage and Wound Rotors 193

viii Contents

3.6 Construction of Three-Phase Induction Motor 1933.6.1 Stator 1943.6.2 Stator Winding 1943.6.3 Voltage Equation of Rotating Machine 202

3.7 Equivalent Circuit of Three-Phase Induction Motor 2073.8 Vector Diagram or Phasor Diagram of Three-Phase Induction Motor 2103.9 Approximate Equivalent Circuit of Three-Phase Induction Motor 2123.10 Torque Equation of Three-Phase Induction Motor 214

3.10.1 Determination of Maximum Torque ofThree-Phase Induction Motor 215

3.10.2 Determination of Approximate Value of the Maximum Torqueof Three-Phase Induction Motor 218

3.10.3 Determination of Starting Torque of Three-PhaseInduction Motor 219

3.10.4 Approximate Value of Starting Torque 2213.10.5 Maximum Power Output 2213.10.6 Torque–Slip Characteristics of Three-Phase Induction Motor 222

3.11 Tests of Three-Phase Induction Motor 2313.11.1 Separation of Core Loss from the Windage and

Friction Loss 2313.11.2 No-Load Test 2323.11.3 Blocked Rotor Test 234

3.12 Circle Diagram 2383.13 Starting of Three-Phase Induction Motor 244

3.13.1 Direct-on-Line Starting 2453.13.2 Reduced Voltage Starting 2453.13.4 Star-Delta Starting 2473.13.5 Application of External Resistance to the Rotor

Circuit During Starting 2493.14 Deep Bar Squirrel Cage Motor 2583.15 Double Cage Rotor 2603.16 Crawling of Three-Phase Squirrel Cage Induction Motor 2613.17 Cogging of Three-Phase Squirrel Cage Induction Motor 2633.18 Vibration Torques on Three-Phase Induction Motor 2633.19 Impact of Higher Harmonics on Three-Phase Induction Motor 2643.20 Some Generalized Rules for Selecting Rotor Slots 2643.21 Speed Control of Three-Phase Induction Motor 265

3.21.1 Voltage Control 2653.21.2 Rotor Resistance Control 2653.12.3 Pole Changing 2653.21.4 Frequency Control 270

3.22 Induction Generator 2733.22.1 Induction Generator without Capacitor

Connection Across It 277

Contents ix

3.23 Dynamics of Induction Machine 2783.23.1 Starting at No-Load 280

3.24 Injection of Voltage at Slip Frequency in the Rotor Circuitof Three-Phase Induction Motor 281

3.25 Cascade Connection of Three-Phase Induction Motors 2823.26 Braking of Three-Phase Induction Motor 285

3.26.1 Dynamic Braking 2863.26.2 Regenerative Braking 293

3.27 Linear Induction Motor 2933.27.1 Introduction 2933.27.2 Mathematical Analysis of Operation of Linear

Induction Motor 2943.27.3 Principle of Operation 2963.27.4 Characteristic of Linear Induction Motor 298

Summary 299Review Questions 302

4. Single-Phase Induction Motor 305–3414.1 Principle of Operation of Single-Phase InductIon Motor 305

4.1.1 Double-Revolving Field Theory 3074.1.2 Mathematical Representation of Double-Revolving

Field Theory 3084.1.3 Tests of Single-Phase Induction Motor 309

4.2 Starting of Single-Phase Induction Motor 3194.2.1 Resistance Split-Phase Motor 3224.2.2 Capacitor Split-Phase Motor 3244.2.3 Shaded-Pole Motor 327

4.3 Optimum Starting Characteristic 3294.3.1 Resistance Split-Phase Motor 3294.3.2 Capacitor Split-Phase Motor 332

Summary 339Review Questions 340

5. AC Commutator Motor (and Some Special Motors) 342–3885.1 Introduction 3425.2 Single-Phase AC Series Motor 342

5.2.1 Analysis of the Application of AC Single-PhaseSupply to a DC Series Motor 342

5.2.2 Modification Required in the AC Series MotorDesign and Construction 344

5.2.3 Compensating Winding and Interpole WindingArrangements in AC Series Motor 344

5.2.4 Concept of Universal Motor 346

Electrical Machines: Theory AndPractice

Publisher : PHI Learning ISBN : 9788120329973Author :BANDYOPADHYAY, M. N.

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