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DELMAR

C E N G A G E Learning'

Electrical Transformers and Rotating

Machines, Third Edition

Stephen L. Herman

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© 2012,2006 Delmar, Cengage Learning

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Library of Congress Control Number: 2011921637

ISBN-13:978-1-1110-3913-4

ISBN -10:1-1110-3913-5

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Printed in the U nited States o f A m erica

3 4 5 6 7 14 13

Contents

Preface

Unit 1

Unit 2

Unit 3

ix

Magnetism 1

The Earth Is a Magnet 2

Permanent Magnets 3

The Electron Theory of Magnetism 3

Magnetic Materials 6

Magnetic Lines of Force 6

Electromagnetics 8

Magnetic Measurement 11

Magnetic Polarity 13

Demagnetizing 13

Magnetic Devices 15

Magnetic Induction 17

Magnetic Induction 17

Moving Magnetic Fields 20

Determining the Amount of Induced Voltage 20

Lenz’s Law 22

Rise Time of Current in an Inductor 24

The Exponential Curve 26

Inductance 28

Induced Voltage Spikes 30

Inductance in Alternating-Current Circuits 38

Inductance 39

Inductive Reactance 42

Schematic Symbols 47

Inductors Connected in Series 47

Inductors Connected in Parallel 48

Voltage and Current Relationships in an Inductive Circuit 49

Power in an Inductive Circuit 50

Reactive Power 53

Q of an Inductor 53

Mi

iv CONTENTS

Unit 4

Unit 5

Unit 6

Unit 7

Unit 8

Unit 9

Unit 10

Unit 11

Single-Phase Isolation Transformers 58

Transformer Formulas 59

Isolation Transformers 61

Autotransformers 116

Determining Voltage Values 117

Using Transformer Formulas 118

Current Relationships 121

Current Transformers 127

Clamp-On Ammeters 132

Three-Phase Circuits 137

Three-Phase Circuits 138

Wye Connections 139

Delta Connections 143

Three-Phase Power 146

Watts and VARs 146

Three-Phase Circuit Calculations 147

Load 3 Calculations 156

Load 2 Calculations 157

Load 1 Calculations 158

Alternator Calculations 159

Power Factor Correction 159

Three-Phase Transformers 165

Three-Phase Transformers 166

Closing a Delta 171

Three-Phase Transformer Calculations 172

Open-Delta Connection 178

Single-Phase Loads for Three-Phase Transformers 191

Closed Delta with Center Tap 196

Closed Delta without Center Tap 197

Delta-Wye Connection with Neutral 197

Transformer Installation 201

Transformer Protection 201

Determining Conductor Size for a Transformer 210

Transformer Cooling 216

Air-Cooled Transformers 216

Liquid-Cooled Transformers 220

CONTENTS V

Unit 12 Transformer Maintenance 226

Safety Procedures 227

Entering a Transformer Tank 227

Maintenance of Small Control Transformers 228

Large Control Transformers Supplying a Motor Control Center 231

Small Dry-Type Transformers for Commercial and Industrial Use 231

Large Industrial Dry-Type Transformers for Use in Unit Substations 232

Media-Cooled Transformers 233

Pad-Mounted Oil-Cooled Transformers 234

Internal Inspection and Maintenance 235

Insulation Testing 235

Oil Testing 237

Unit 13 Harmonics 240

Harmonic Effects 241

Circuit Breaker Problems 244

Buss Ducts and Panel Problems 245

Determining Harmonic Problems on Single-Phase Systems 245

Determining Harmonic Problems on Three-Phase Systems 248

Dealing with Harmonic Problems 248

Determining Transformer Harmonic Derating Factor 249

Unit 14 Direct Current Generators 252

What Is a Generator? 253

Armature Windings 263

Brushes 264

Pole Pieces 264

Field Windings 264

Series Generators 265

Shunt Generators 268

Compound Generators 274

Compounding 275

Countertorque 278

Armature Reaction 279

Setting the Neutral Plane 281

Fleming’s Left-Hand Generator Rule 283

Paralleling Generators 284

Unit 15 Direct Current Motors 289

DC Motor Principles 289

Shunt Motors 293

Series Motors 295

Compound Motors 297

VI CONTENTS

Terminal Identification for DC Motors 299

Determining the Direction of Rotation of a DC Motor 300

Speed Control 303

Field Loss Relay 305

Horsepower 306

Brushless DC Motors 309

Converters 312

Permanent Magnet Motors 312

Right-Hand Motor Rule 320

Unit 16 Alternators 325

Three-Phase Alternators 325

The Rotor 328

The Brushless Exciter 330

Alternator Cooling 331

Frequency 333

Output Voltage 334

Paralleling Alternators 335

Sharing the Load 337

Field Discharge Protection 337

Unit 17 Three-Phase Motors 341

Three-Phase Motors 342

The Rotating Magnetic Field 342

Synchronous Speed 342

Connecting Dual-Voltage Three-Phase Motors 346

Squirrel-Cage Induction Motors 353

Three-Speed Consequent Pole Motors 367

Four-Speed Consequent Pole Motors 372

Wound-Rotor Induction Motors 372

Synchronous Motors 378

Selsyn Motors 385

Unit 18 Single-Phase Motors 392

Single-Phase Motors 392

Split-Phase Motors 393

Resistance-Start Induction-Run Motors 395

Capacitor-Start Induction-Run Motors 402

Dual-Voltage Split-Phase Motors 404

Determining Direction of Rotation for Split-Phase Motors 407

Capacitor-Start Capacitor-Run Motors (Permanent-Split Capacitor Motors) 408

Shaded-Pole Induction Motors 411

Multispeed Motors 414

CONTENTS VN

Single-Phase Synchronous Motors 416

Repulsion Motors 418

Construction of Repulsion Motors 418

Repulsion-Start Induction-Run Motors 421

Repulsion-Induction Motors 424

Stepping Motors 424

Universal Motors 432

Unit 19 Motor Maintenance and Troubleshooting 440

Motor Bearings 441

Direct Current Machines 441

Testing Alternating Current Motors 446

Testing Dual-Voltage Motors 447

Unit 20 Motor Nameplate Data 458

Manufacturer 460

Horsepower 460

Torque 462

RPM 462

Frequency 463

Phase 464

Motor Type 465

Frame 466

Voltage 468

Full-Load Amps (FLA) 468

Enclosure 471

Duty Cycle 472

Temperature Rise 473

Service Factor (SF) 473

Locked Rotor Code Letter 475

NEMA Design Code 476

Motor Efficiency 478

Insulation Temperature Rating 478

Model Number 479

Serial Number 480

Connection Diagrams 480

Unit 21 Motor Installation 484

Determining Motor Current 484

Determining Conductor Size for a Single Motor 490

Overload Size 493

Determining Locked-Rotor Current 496

Short-Circuit Protection 499

viii CONTENTS

Starter Size 501

Multiple Motor Calculation 506

Laboratory Experiments 515

Experiment 1 Transformer Basics 516

Experiment 2 Single-Phase Transformer Calculations 521

Experiment 3 Transformer Polarities 528

Experiment 4 Autotransformers 537

Experiment 5 Three-Phase Circuits 543

Experiment 6 Three-Phase Transformers 548

Experiment 7 Rotating Magnetic Field 565

Experiment 8 Three-Phase Dual-Voltage Motors 567

Experiment 9 Single-Phase Dual-Voltage Motors 575

Appendix A Greek Alphabet 581

Appendix B Metals 583

Appendix c Full-Load Current Direct Current Motors 585

Appendix D Full-Load Current Alternating Current Single-Phase

Motors 586

Appendix E Full-Load Current Alternating Current Three-Phase

Motors 587

Appendix F NEMA Design Codes for Three-Phase Squirrel-Cage

Motors 589

Answers to Practice Problems 591

Glossary 594

Index 602

Preface

Transformers a n d Rotating Machines, Third Edition combines theory and practical applications

for those desiring employment in the industrial electrical field. This text assumes the student has

knowledge of basic electrical theory. The text begins with a study of magnetism and magnetic

induction and progresses through single-phase isolation transformers, current transformers, and

autotransformers. A unit on three-phase power refreshes the student’s knowledge of basic three￾phase connections and calculations before proceeding into three-phase transformers. All the

basic types of three-phase transformers are covered, such as delta-wye, delta-delta, wye-delta,

wye-wye, and open-delta. Special transformer connections such as the Scott, T, and zig-zag are

also presented. Examples of adding single-phase loads to three-phase transformers are included.

Transformers a n d Rotating M achines also provides information on direct current generators and

motors. The basic types of DC machines (series, shunt, and compound) are discussed. The text also

provides information on brushless motors, printed circuit motors, and permanent magnet motors.

Alternating current machines covered in this text include alternators, three-phase motors, and

single-phase motors. The operating characteristics of squirrel cage, consequent pole, wound

rotor, and synchronous motors are explained. Diagrams and explanations provide students with

thorough coverage of both wye and delta high- and low-voltage connections for three-phase

motors.

Single-phase alternating-current motors include split phase, repulsion, universal, and shaded

pole. The operating characteristics of each type of motor are discussed.

Transformers an d Rotating Machines includes a set of hands-on laboratory experiments for

single-phase transformers, three-phase transformers, three-phase motors, and single-phase motors.

All the transformer experiments require the use of common equipment such as 0.5 kVA control

transformers, 100 watt lamps, voltmeters, ohmmeters, and ammeters.

NEW FOR T H E THIRD ED ITION

Since motors and transformers are magnetic devices, three units have been added on basic mag￾netism and magnetic induction. The units covering the installation of transformers and motors

have been updated to reflect the changes in the 2011 National Electric Code (NEC)®.

The third edition also includes a unit on motor nameplate data. This unit is included to aid the

student in understanding the information listed on a motor nameplate. The third edition also con￾tains information on the series and parallel connection of transformers.

S U P P LEM EN TS

An Instructor Resource CD for this text includes the Instructor’s Guide, chapter presentations, and

topical presentations done in PowerPoint, and a computerized testbank. (ISBN 1111039143)

ix

viii CONTENTS

Starter Size 501

Multiple Motor Calculation 506

Laboratory Experiments 515

Experiment 1 Transformer Basics 516

Experiment 2 Single-Phase Transformer Calculations 521

Experiment 3 Transformer Polarities 528

Experiment 4 Autotransformers 537

Experiment 5 Three-Phase Circuits 543

Experiment 6 Three-Phase Transformers 548

Experiment 7 Rotating Magnetic Field 565

Experiment 8 Three-Phase Dual-Voltage Motors 567

Experiment 9 Single-Phase Dual-Voltage Motors 575

Appendix A Greek Alphabet 581

Appendix B Metals 583

Appendix c Full-Load Current Direct Current Motors 585

Appendix D Full-Load Current Alternating Current Single-Phase

Motors 586

Appendix E Full-Load Current Alternating Current Three-Phase

Motors 587

Appendix F NEMA Design Codes for Three-Phase Squirrel-Cage

Motors 589

Answers to Practice Problems 591

Glossary 594

Index 602

Preface

Transformers a n d Rotating Machines, Third Edition combines theory and practical applications

for those desiring employment in the industrial electrical field. This text assumes the student has

knowledge of basic electrical theory. The text begins with a study of magnetism and magnetic

induction and progresses through single-phase isolation transformers, current transformers, and

autotransformers. A unit on three-phase power refreshes the student’s knowledge of basic three￾phase connections and calculations before proceeding into three-phase transformers. All the

basic types of three-phase transformers are covered, such as delta-wye, delta-delta, wye-delta,

wye-wye, and open-delta. Special transformer connections such as the Scott, T, and zig-zag are

also presented. Examples of adding single-phase loads to three-phase transformers are included.

Transformers a n d Rotating M achines also provides information on direct current generators and

motors. The basic types of DC machines (series, shunt, and compound) are discussed. The text also

provides information on brushless motors, printed circuit motors, and permanent magnet motors.

Alternating current machines covered in this text include alternators, three-phase motors, and

single-phase motors. The operating characteristics of squirrel cage, consequent pole, wound

rotor, and synchronous motors are explained. Diagrams and explanations provide students with

thorough coverage of both wye and delta high- and low-voltage connections for three-phase

motors.

Single-phase alternating-current motors include split phase, repulsion, universal, and shaded

pole. The operating characteristics of each type of motor are discussed.

Transformers an d Rotating M achines includes a set of hands-on laboratory experiments for

single-phase transformers, three-phase transformers, three-phase motors, and single-phase motors.

All the transformer experiments require the use of common equipment such as 0.5 kVA control

transformers, 100 watt lamps, voltmeters, ohmmeters, and ammeters.

NEW F O R TH E THIRD ED ITIO N

Since motors and transformers are magnetic devices, three units have been added on basic mag￾netism and magnetic induction. The units covering the installation of transformers and motors

have been updated to reflect the changes in the 2011 N ational Electric Code (NEC)®.

The third edition also includes a unit on motor nameplate data. This unit is included to aid the

student in understanding the information listed on a motor nameplate. The third edition also con￾tains information on the series and parallel connection of transformers.

S U P P LEM EN TS

An Instructor Resource CD for this text includes the Instructor’s Guide, chapter presentations, and

topical presentations done in PowerPoint, and a computerized testbank. (ISBN 1111039143)

ix

2 ELECTRICAL T R A N S F O R M E R S A N D ROTATING M A C H IN E S

TH E EA R TH IS A M AGNET

The first compass was invented when it was noticed that a piece of mag￾netite, a type of stone that is attracted to iron, placed on a piece of wood

floating in water always aligned itself north and south (Figure 1-1). Because

they are always able to align themselves north and south, natural magnets

became known as “leading stones” or lodestones. The reason that the lode￾stone aligned itself north and south is because the earth itself contains mag￾netic poles. Figure 1-2 illustrates the position of the true North and South

poles, or the axis, of the earth and the position of the magnetic poles. Notice

Magnetite North

Figure 1-1 The first com pass.

Flgirt 1-2 The earth Is a magnet.

© 2012 DelmatCengage Learning ©2012 Oelmar Cengage Learning

U N IT 1 M A G N E T I S M

that what is considered as m agnetic north is not located at the true North

Pole of the earth. This is the reason that navigators must distinguish between

true north and magnetic north. The angular difference between the two is

known as the angle of declination. Although the illustration shows the mag￾netic lines of force to be only on each side of the earth, the lines actually sur￾round the entire earth like a magnetic shell.

Also notice that the magnetic north pole is located near the southern polar

axis and the magnetic south pole is located near the northern polar axis. The

reason that the geographic poles (axes) are called north and south is because

the north pole of a compass needle points in the direction of the north geo￾graphic pole. Since unlike magnetic poles attract, the north magnetic pole of

the compass needle is attracted to the south magnetic pole of the earth.

P E R M A N E N T M AG N ETS

Perm anent magnets are magnets that do not require any power or force

to maintain their field. They are an excellent example of one of the basic

laws of magnetism, which states that Energy Is required to create a mag￾netic field, but no energy is required to maintain a magnetic field.

Man-made permanent magnets are much stronger and can retain their mag￾netism longer than natural magnets.

T H E ELEC TR O N T H E O R Y O F M AGNETISM

There are actually only three substances that form natural magnets: iron,

nickel, and cobalt. Why these materials form magnets has been the subject

of complex scientific investigations, resulting in an explanation of magnetism

based on electron spin patterns. It is believed that each electron spins on

its axis as it orbits around the nucleus of the atom. This spinning motion

causes each electron to become a tiny permanent magnet. Although all elec￾trons spin, they do not all spin in the same direction. In most atoms, electrons

that spin in opposite directions tend to form pairs (Figure 1-3). Since the elec￾tron pairs spin in opposite directions, their magnetic effects cancel each other

out as far as having any effect on distant objects. In a similar manner two

horseshoe magnets connected together would be strongly attracted to each

other, but would have little effect on surrounding objects (Figure 1-4).

An atom of iron contains twenty-six electrons. Of these twenty-six, twenty￾two are paired and spin in opposite directions, canceling each other’s mag￾netic effect. In the next-to-the-outermost shell, however, four electrons are not

paired and spin in the same direction. These four electrons account for the

magnetic properties of iron. At a temperature of 1420°F, or 771.1°C, the elec￾tron spin patterns rearrange themselves and iron loses its magnetic properties.

4 ELECTRICAL T R A N SF O R M ER S AND ROTATING M ACH IN ES

When the atoms of most materials combine to form molecules, they

arrange themselves in a manner that produces a total of eight valence elec￾trons. The electrons form a spin pattern that cancels the magnetic field of the

material. When the atoms of iron, nickel, and cobalt combine, however, the

magnetic field is not canceled. Their electrons combine so that they share

valence electrons in such a way that their spin patterns are in the same direc￾tion, causing their magnetic fields to add instead of cancel. The additive

effect forms regions in the molecular structure of the metal called magnetic

domains or magnetic molecules. These magnetic domains act as small

permanent magnets.

A piece of nonmagnetized metal has its molecules in a state of disarray

as shown in Figure 1-5. When the metal is magnetized, its molecules

© 2012 Delmar Cengage Learning © 2012 Delmar Cengage Learning

U N IT 1 M A G N E T I S M 5

|N s| |N s| N s| |N s|

N s| |N s| In S| |N s|

N s| N S| |n S| N S|

In S| |N s| |N S| |N S|

Figure 1-e The m olecules are aligned In an orderly fashion in a piece of m agnetized metal.

align themselves in an orderly pattern as shown in Figure 1-6. In theory,

each molecule of a magnetic material is itself a small magnet. If a perma￾nent magnet were cut into pieces, each piece would be a separate magnet

(Figure 1-7).

© 2012 Delmar Cengage Learning © 2012 Delmar Cengage Learning © 2012 Delmar Cengage Learning