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Issa Batarseh • Ahmad Harb

Power Electronics
Circuit Analysis and Design

Second Edition
Issa Batarseh Ahmad Harb
University of Central Florida German Jordanian University
Orlando, FL, USA Amman, Jordan

ISBN 978-3-319-68365-2 ISBN 978-3-319-68366-9 (eBook)

https://doi.org/10.1007/978-3-319-68366-9

Library of Congress Control Number: 2017955223

1st edition: © John Wiley & Sons, Inc 2003

© Springer International Publishing AG 2018
 Preface

In the past three decades, the field of power electronics has witnessed unprece-
dented growth in research and teaching worldwide, and has emerged as a special-
ization in electrical engineering. This growth is due to expanding market demand
for integrated and networked power electronics-based circuits and systems in all
kinds of energy processing and conversion applications. Moreover, the need for
power electronics engineers and researchers equipped with knowledge of new
energy conversion technologies has never been greater.
Power Electronics is intended as a textbook to teach the subject of modern
power electronics to senior undergraduate and first-year graduate electrical engi-
neering students. Because of the breadth of the field of power electronics, teaching
this subject to undergraduate students is a challenge. This textbook is designed to
introduce the basic concepts of power electronics to students and professionals
interested in updating their knowledge of the subject. The objective of this textbook
is to provide students with the ability to analyze and design power electronic
switching circuits used in common industrial applications.
The prerequisites for this text are a first course in circuit analysis techniques and
a basic background in electronic circuits. Chapter 3 gives an overview of diode
switching circuits and basic analysis techniques that students will find useful in the
remaining chapters.

Material Presentation

Unlike many existing texts in power electronics, Power Electronics targets mainly
senior undergraduate students majoring in electrical engineering. Since the text is
intended to be used in a three-credit-hour course in power electronics, topics such as
power semiconductor devices, machine drives, and utility applications are not
included. Because of limited lecture times, one course at the undergraduate level
cannot adequately cover such topics and still present all power electronic circuits
used in energy conversion. This text contains sufficient material for a single-
semester introductory power electronics course while giving the instructor flexibil-
ity in topic treatment and course design.
The text is written in such a way as to equip students with the necessary
background material in such topics as devices, switching circuit analysis tech-
niques, converter types, and methods of conversion in the first three chapters. The
presentation of the material is new and has been recommended by many power
electronics faculty. The discussion begins by introducing high-frequency,
non-isolated dc-to-dc converters in Chap. 4, followed by isolated dc-to-dc con-
verters in Chap. 5. Resonant soft-switching converters are treated early on in
Chap. 6. The traditional diode and SCR converters and dc-ac inverters are presented
in the second part of the text, in Chaps. 7, 8, and 9, respectively.

Examples, Exercises, and Problems

Unlike many existing texts, this text provides students with a large number of
examples, exercises, and problems, with detailed discussions on resonant and soft-
switching dc-to-dc converters.
Examples are used to help students understand the material presented in a given
chapter. To drill students in applying the basic concepts and equations and to help
them understand basic circuit operations, several exercises are given within each
chapter. The text has more than 250 problems at different levels of complexity and
difficulty. These problems are intended not only to strengthen students’ understand-
ing of the materials presented but also to introduce many new concepts and circuits.
To help meet recent Accreditation Board for Engineering and Technology (ABET)
requirements for design in the engineering curriculum, special emphasis is made on
providing students with opportunities to apply design techniques. Such problems
are designated with the letter “D” next to the problem number, such as D5.32,
which is Design Problem # 32 in Chap. 5. Students should be aware that such
problems are open-ended without unique solutions.
A bibliography is included at the end of the text, and a list of textbooks is given
separately.

Web-Based Course Material

Ancillaries to this text are available on a dedicated website, www.fpec.ucf.edu,

where both faculty and students can access additional material such as a complete
set of lecture notes, additional sample quizzes and problems, up-to-date text
corrections, and the opportunity to submit new ones.

Orlando, FL, USA Issa Batarseh

Amman, Jordan Ahmad Harb
 Contents

1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.2 What Is Power Electronics? . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.2.1 Recent Growth in Power Electronics . . . . . . . . . . . . . . . . 3
1.3 The History of Power Electronics . . . . . . . . . . . . . . . . . . . . . . . 4
1.3.1 The History of dc and ac Electricity in the Late
Nineteenth Century . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.3.2 The History of dc and ac Electricity in the Late
Twentieth Century . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.3.3 History of Modern Power Electronics . . . . . . . . . . . . . . . 6
1.4 The Need for Power Conversion . . . . . . . . . . . . . . . . . . . . . . . . 7
1.5 Power Electronic Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
1.5.1 Classification of Power Converter Circuits . . . . . . . . . . . 10
1.5.2 Power Semiconductor Devices . . . . . . . . . . . . . . . . . . . . 17
1.5.3 Converter Modeling and Control . . . . . . . . . . . . . . . . . . 18
1.6 Applications of Power Electronics . . . . . . . . . . . . . . . . . . . . . . . 19
1.7 Future Trends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
1.8 About the Text and Its Nomenclatures . . . . . . . . . . . . . . . . . . . . 20
1.8.1 About the Text . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
1.8.2 Nomenclature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
2 Review of Switching Concepts and Power
Semiconductor Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
2.2 The Need for Switching in Power Electronic Circuits . . . . . . . . . 26
2.3 Switching Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
2.3.1 The Ideal Switch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
2.3.2 The Practical Switch . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
2.4 Switching Functions and Matrix Representation . . . . . . . . . . . . . 38
2.5 Types of Switches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
 2.6 Available Semiconductor Switching Devices . . . . . . . . . . . . . . . 49
2.6.1 Bipolar and Unipolar Devices . . . . . . . . . . . . . . . . . . . . . 49
2.6.2 Thyristor-Based Devices . . . . . . . . . . . . . . . . . . . . . . . . 69
2.7 Comparison of Power Devices . . . . . . . . . . . . . . . . . . . . . . . . . 76
2.8 Future Trends in Power Devices . . . . . . . . . . . . . . . . . . . . . . . . 77
2.9 Snubber Circuits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
2.10 Interest in High-Temperature Power Devices:
The Wide Band Gap . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
3 Switching Circuits, Power Computations,
and Component Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
3.2 Switching Diode Circuits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
3.2.1 Switching Diode Circuits Under dc Excitation . . . . . . . . . 93
3.2.2 Switching Diode Circuits with an ac Source . . . . . . . . . . 102
3.3 Controlled Switching Circuits . . . . . . . . . . . . . . . . . . . . . . . . . . 108
3.4 Basic Power and Harmonic Concepts . . . . . . . . . . . . . . . . . . . . . 111
3.4.1 Average, Reactive, and Apparent Powers . . . . . . . . . . . . 111
3.4.2 Sinusoidal Waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . 115
3.4.3 Non-sinusoidal Waveforms . . . . . . . . . . . . . . . . . . . . . . 120
3.5 Capacitor and Inductor Responses . . . . . . . . . . . . . . . . . . . . . . . 135
3.5.1 Capacitor Transient Response . . . . . . . . . . . . . . . . . . . . . 135
3.5.2 Capacitor Steady-State Response . . . . . . . . . . . . . . . . . . 139
3.5.3 Inductor Transient Response . . . . . . . . . . . . . . . . . . . . . 139
3.5.4 Inductor Steady-State Response . . . . . . . . . . . . . . . . . . . 142
Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144
4 Non-isolated Switch Mode DC-DC Converters . . . . . . . . . . . . . . . . . 173
4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173
4.2 Power Supply Application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174
4.2.1 Linear Regulators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174
4.2.2 Switch-Mode Power Supplies . . . . . . . . . . . . . . . . . . . . . 175
4.3 Continuous Conduction Mode . . . . . . . . . . . . . . . . . . . . . . . . . . 177
4.3.1 The Buck Converter . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182
4.3.2 The Boost Converter . . . . . . . . . . . . . . . . . . . . . . . . . . . 196
4.3.3 The Buck-Boost Converter . . . . . . . . . . . . . . . . . . . . . . . 205
4.3.4 The Fourth-Order Converters . . . . . . . . . . . . . . . . . . . . . 211
4.3.5 Bipolar Output Voltage Converter . . . . . . . . . . . . . . . . . 225
4.4 Discontinuous Conduction Mode . . . . . . . . . . . . . . . . . . . . . . . . 227
4.4.1 The Buck Converter . . . . . . . . . . . . . . . . . . . . . . . . . . . . 228
4.4.2 The Boost Converter . . . . . . . . . . . . . . . . . . . . . . . . . . . 234
4.4.3 The Buck-Boost Converter . . . . . . . . . . . . . . . . . . . . . . . 238
 4.5 The Effects of Converter Non-idealities . . . . . . . . . . . . . . . . . . . 243
4.5.1 Inductor Resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . 243
4.5.2 The Boost Converter . . . . . . . . . . . . . . . . . . . . . . . . . . . 246
4.5.3 Transistor and Diode Voltage Drops . . . . . . . . . . . . . . . . 248
4.5.4 The Effect of Switch Resistance . . . . . . . . . . . . . . . . . . . 250
4.6 Switch Utilization Factor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 257
Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 260
5 Isolated Switch-Mode DC-DC Converters . . . . . . . . . . . . . . . . . . . . 273
5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 273
5.2 Transformer Circuit Configurations . . . . . . . . . . . . . . . . . . . . . . 274
5.2.1 Transformer Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . 274
5.2.2 Circuit Configurations . . . . . . . . . . . . . . . . . . . . . . . . . . 275
5.3 Buck-Derived Isolated Converters . . . . . . . . . . . . . . . . . . . . . . . 277
5.3.1 Single-Ended Forward Converter . . . . . . . . . . . . . . . . . . 281
5.3.2 Half-Bridge Converters . . . . . . . . . . . . . . . . . . . . . . . . . 290
5.3.3 Full-Bridge Converter . . . . . . . . . . . . . . . . . . . . . . . . . . 291
5.3.4 Push-Pull Converter . . . . . . . . . . . . . . . . . . . . . . . . . . . . 297
5.4 Boost-Derived Isolated Converters . . . . . . . . . . . . . . . . . . . . . . 303
5.4.1 Single-Ended Flyback Converter . . . . . . . . . . . . . . . . . . 303
5.4.2 Half-Bridge Converter . . . . . . . . . . . . . . . . . . . . . . . . . . 312
5.4.3 Full-Bridge Converter . . . . . . . . . . . . . . . . . . . . . . . . . . 312
5.5 Other Isolated Converters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 319
5.5.1 Isolated Cuk Converter . . . . . . . . . . . . . . . . . . . . . . . . . 319
5.5.2 The Weinberg Converter . . . . . . . . . . . . . . . . . . . . . . . . 321
5.6 Multi-output Converter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 329
Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 330
6 Soft-Switching dc-dc Converters . . . . . . . . . . . . . . . . . . . . . . . . . . . . 347
6.1 Types of dc-dc Converters . . . . . . . . . . . . . . . . . . . . . . . . . . . . 347
6.1.1 The Resonant Concept . . . . . . . . . . . . . . . . . . . . . . . . . . 348
6.1.2 Resonant Versus Conventional PWM . . . . . . . . . . . . . . . 349
6.2 Classification of Soft-Switching Resonant Converters . . . . . . . . . 350
6.3 Advantages and Disadvantages of ZCS and ZVS . . . . . . . . . . . . 351
6.3.1 Switching Loci . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 351
6.3.2 Switching Losses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 351
6.4 Zero-Current Switching Topologies . . . . . . . . . . . . . . . . . . . . . . 352
6.4.1 The Resonant Switch . . . . . . . . . . . . . . . . . . . . . . . . . . . 352
6.4.2 Steady-State Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . 354
6.5 Zero-Voltage Switching Topologies . . . . . . . . . . . . . . . . . . . . . . 383
6.5.1 Resonant Switch Arrangements . . . . . . . . . . . . . . . . . . . 384
6.5.2 Steady-State Analyses . . . . . . . . . . . . . . . . . . . . . . . . . . 384
6.6 Zero-Voltage and Zero-Current Transition Converters . . . . . . . . 398
6.6.1 Switching Transition . . . . . . . . . . . . . . . . . . . . . . . . . . . 398
6.6.2 The ZVT Buck Converter . . . . . . . . . . . . . . . . . . . . . . . 400
 6.6.3 The ZVT Boost Converter . . . . . . . . . . . . . . . . . . . . . . . 408
6.6.4 The Practical ZVT Boost Converter . . . . . . . . . . . . . . . . 416
6.7 Generalized Analysis for ZCS . . . . . . . . . . . . . . . . . . . . . . . . . . 429
6.7.1 The Generalized Switching Cell . . . . . . . . . . . . . . . . . . . 430
6.7.2 The Generalized Transformation Table . . . . . . . . . . . . . . 431
6.7.3 The Basic Operation of the ZCS-QRC Cell . . . . . . . . . . . 433
6.7.4 The Basic Operation of the ZVS-QRC Cell . . . . . . . . . . . 437
Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 447
7 Uncontrolled Diode Rectifier Circuits . . . . . . . . . . . . . . . . . . . . . . . . 461
7.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 461
7.2 Single-Phase Rectifier Circuits . . . . . . . . . . . . . . . . . . . . . . . . . 463
7.2.1 Resistive Load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 463
7.2.2 Inductive Load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 467
7.2.3 Capacitive Load Rectifiers . . . . . . . . . . . . . . . . . . . . . . . 475
7.2.4 Voltage Source in the dc Side . . . . . . . . . . . . . . . . . . . . 478
7.3 The Effect of the ac-Side Inductance . . . . . . . . . . . . . . . . . . . . . 479
7.3.1 Half-Wave Rectifier with Inductive Load . . . . . . . . . . . . 479
7.3.2 Half-Wave Rectifier with Capacitive Load . . . . . . . . . . . 484
7.3.3 Full-Wave Rectifiers with an Inductive Load . . . . . . . . . . 489
7.4 Three-Phase Rectifier Circuits . . . . . . . . . . . . . . . . . . . . . . . . . . 492
7.4.1 Three-Phase Half-Wave Rectifier . . . . . . . . . . . . . . . . . . 493
7.4.2 Three-Phase Full-Wave Rectifiers . . . . . . . . . . . . . . . . . . 499
7.5 Ac-Side Inductance in Three-Phase Rectifier Circuits . . . . . . . . . 501
7.5.1 Half-Wave Rectifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . 501
7.5.2 Full-Wave Bridge Rectifiers . . . . . . . . . . . . . . . . . . . . . . 506
Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 510
8 Phase-Controlled Converters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 525
8.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 525
8.2 Basic Phase-Controlled Concepts . . . . . . . . . . . . . . . . . . . . . . . 526
8.3 Half-Wave Phase-Controlled Rectifiers . . . . . . . . . . . . . . . . . . . 528
8.3.1 Resistive Load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 528
8.3.2 Inductive Load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 530
8.4 Full-Wave Phase-Controlled Rectifiers . . . . . . . . . . . . . . . . . . . 535
8.4.1 Resistive Load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 535
8.4.2 Inductive Load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 537
8.5 Effect of AC-Side Inductance . . . . . . . . . . . . . . . . . . . . . . . . . . 550
8.5.1 Half-Wave Circuits . . . . . . . . . . . . . . . . . . . . . . . . . . . . 550
8.5.2 Full-Wave Rectifier Circuits . . . . . . . . . . . . . . . . . . . . . . 552
8.6 Three-Phase Phase-Controlled Converters . . . . . . . . . . . . . . . . . 557
8.6.1 Half-Wave Converters . . . . . . . . . . . . . . . . . . . . . . . . . . 557
8.6.2 Full-Wave Converters . . . . . . . . . . . . . . . . . . . . . . . . . . 557
Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 566
9 dc-ac Inverters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 575
9.1 Basic Block Diagram of dc-ac Inverters . . . . . . . . . . . . . . . . . . . 576
9.1.1 Voltage- and Current-Source Inverters . . . . . . . . . . . . . . 577
9.1.2 Inverter Configurations . . . . . . . . . . . . . . . . . . . . . . . . . 578
9.1.3 Output Voltage Control . . . . . . . . . . . . . . . . . . . . . . . . . 578
9.2 Basic Half-Bridge Inverter Circuit . . . . . . . . . . . . . . . . . . . . . . . 578
9.2.1 Resistive Load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 578
9.2.2 Inductive-Resistive Load . . . . . . . . . . . . . . . . . . . . . . . . 583
9.3 Full-Bridge Inverters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 593
9.3.1 Approximate Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . 599
9.3.2 Generalized Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . 600
9.4 Harmonic Reduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 609
9.4.1 Harmonic Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . 610
9.5 Pulse Width Modulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 616
9.5.1 Equal-Pulse (Uniform) PWM . . . . . . . . . . . . . . . . . . . . . 618
9.5.2 Sinusoidal PWM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 626
9.6 Three-Phase Inverters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 640
9.7 Current-Source Inverters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 648
Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 651

Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 663
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 667
Chapter 1
Introduction

1.1 Introduction

No doubt that power electronics is now considered one of the most vital enabling
technologies in electrical engineering. In fact, large part of all electrically powered
devices, circuits, or systems has close connection with the field of power electron-
ics. Its scope is broad and covers very wide spectrum, with the paramount among
them is its ever-increasing role in integrating renewable energy sources and electric
storage to the grid. Power electronics is the “glue” that makes the ushering of a new
kind of smart energy technology revolution possible. It is because of the engineer-
ing field of power electronics that we are able to encompass the efficient and cost-
effective use of electronic components, circuit and control theory, modern analyt-
ical tools, and design techniques to make this smart energy revolution possible. This
revolution will modernize our electric grid, give birth to massive electric transpor-
tation, allow for large solar energy penetration, help solve climate change, and
enable the deployment of the highest possible energy efficiency systems. In short,
power electronics has emerged as the enabling technology that transformed the field
of energy and power engineering from a high-tech frontier to smart-tech frontier.
Arriving at today’s remarkable important role of power electronics took more than
100 years of innovation and hard work by many scientists and engineers coupled
with strong partnerships between the private sector, professional societies, and
governments.
This chapter is intended to give the reader a broad introductory overview about
the field of power electronics and its applications. Basic block diagrams for a power
electronic system and its major functions will be given. We also present different
types of power electronic circuits used to achieve power conversion that will be
studied throughout the text.
2 1 Introduction

1.2 What Is Power Electronics?

To date, there is not a widely accepted statement that can clearly and specifically
define the field of power electronics. In fact, many experts in the academic and
industrial communities feel that the name itself does not do justice to the field that is
application oriented and multidisciplinary in nature that encompasses many sub-
areas in electrical engineering.1 Because of this multidisciplinary nature of the field
of power electronics, experts must have commanding knowledge in several elec-
trical engineering fields such as electronic devices, electronic circuits, signal
processing, magnetism, electrical machines, control, and power. In a very broad
sense, power electronic circuits have the task to process one form of energy
supplied by a source to a different form required at the load side. Hence, power
electronics can be closely identified with the following subdiscipline areas in
electrical engineering: electronics, power, and control. Here, electronics deal with
the semiconductor devices and circuit topologies for signal processing in order to
implement the control functions, and power deals with both static and rotating
equipment that uses electric power, whereas control deals with the steady-state
stability of the closed loop system during power conversion process. Hence, the
subject of power electronics deals specifically with the application of power
semiconductor devices and circuits for conversion and regulation of electric
power. In summary, power electronics is an enabling technology that brings
together three fundamental technologies: power semiconductor devices technology,
power conversion technology, and power control technology, as illustrated in
Fig. 1.1.
A final observation to make is that in power electronic circuits, there exist two
types of switching devices: one type exists in the power processing stage which
handles high power up to hundreds of gigawatts which represents the muscle of the
system and the other type located in the feedback control circuit which handles
low-power signal processing up to hundreds of milliwatts, representing the brain or
the intelligence of the system. Hence, today’s power electronic circuits are essen-
tially digital electronic circuits whose switching elements manipulate pulsed power
from the milliwatts to gigawatts range. As a result, one may conclude that the task
of power electronics is to convert and control power using low-power switching
devices to process power at much higher power levels of these devices (hundred
times or more).

1
Many universities today offer power electronic discipline either under the “power” or “electron-
ics” area, with limited number of universities have it separately.
1.2 What Is Power Electronics? 3

Fig. 1.1 Power electronics is a systems solution that encompasses three technologies: conversion,
power semiconductor, and power control technologies

1.2.1 Recent Growth in Power Electronics

The field of power electronics has recently experienced unprecedented growth not
only in terms of research and educational activities, but in diverse applications. Its
application has been steadily and rapidly expanded to cover many sectors of our
society. This growth is due to several factors, paramount among them is the
growing markets in renewable energy applications, coupled with technological
advancement made by the semiconductor device industry which led to the intro-
duction of very fast, high-power capabilities and highly integrated power semicon-
ductor devices. Other factors include (1) the revolutionary advances made in the
microelectronic field which led to the development of very efficient and highly
integrated circuits (ICs) used for generation of digital control signals for processing
and control purposes; (2) the ever-increasing demand for smaller size and lighter
weight power electronic systems; (3) the expand market demand for new power
electronic applications in wind and solar energy conversion and other applications
that require the use of variable-speed motor drives, regulated power supplies,
robotics, and uninterruptible power supplies; and (4) a result of this increasing
reliance on power electronic systems made it mandatory that all such systems have
radiated and conducted electromagnetic interference (EMI) be limited within reg-
ulated ranges. The industry’s interest in developing power systems with low
harmonic contents with improved power factor and reduced cost will continue to
place the field of power electronics on the top of the research and development
priority list.
4 1 Introduction

1.3 The History of Power Electronics

Before presenting the history of power electronics in this century, it might be useful
to the reader to know the history of the development of what is called the alternating
current (ac) and the direct current (dc) electricity in the last two decades of the
nineteenth century. This is because the inventions of the 1880s resulted in the
present worldwide ac electric power system, providing the energy form that must
be processed for any power electronic applications.

1.3.1 The History of dc and ac Electricity in the Late

Nineteenth Century

It was decided in the late nineteenth century that the electrical form of energy is the
most practical and economic way to produce energy for human use. This is because
electricity is an excellent form of energy when it comes to generation, transmission,
and distribution. However, this realization not before a heated debate was underway
among scientists and engineers whether the future of transmitting and distributing
electricity to industries and homes would be based on alternating type of current
flow known as (ac) or the direct type of current flow known as (dc). It was George
Westinghouse and Nikola Tesla (1856–1943), representing the ac camp, and
Thomas Edison (1847–1931), representing the dc camp. After more than 15 years
of intellectual debate, supported by new inventions and developmental and exper-
imental studies, the ac advocates won; consequently, the entire world today is using
an ac-based power distribution system.2
Thomas Edison was a self-educated inventor who was awarded 1033 patents
over 50-year period. He is best known for the invention of the phonograph and
incandescent lamp, which was invented in 1879 after many years of repeated
experiments. In 1878, he formulated the concept of a centrally located power
station from which power can be distributed to surrounding areas. In September
4, 1882, using dc generators (at that time called dynamos) driven by steam engines,
Edison opened Pearl Street Station in New York City to supply electricity to
59 customers in a one-square-mile area. It was the first dc-based power station in
the world with a total power load of 30 kW only. In fact, it was the beginning of the
electric utility industry that grew at a remarkable rate. In 1884, Frank Sprague
produced a practical dc motor for Edison’s dc systems. This invention, coupled with
the development of three-wire 220 VDC, Edison succeeded to distribute dc

2
Tesla and Edison worked together for a short time, and soon both developed hatred for one
another, resulting in Tesla opening his own business, believing in ac transmission systems. Several
rumors in the press stated that both were nominated for Nobel Prize in physics, and because of the
feud between them, the prize was given to a third party. These rumors were all false since no one is
asked whether to accept or decline a Nobel Prize!
1.3 The History of Power Electronics 5

electrical power to cover larger areas and supply heavier loads and consequently
more customers. By doing so, Edison prompted the dc-based power distribution
systems. As transmission distances and load demands start to increase, Edison’s dc
systems ran into troubles. The dc distribution lines suffered from very high-power
losses because of the high voltage and current that existed simultaneously. This
severely limited the transmission distance and resulted in highly inefficient sys-
tems. So in order to sustain power level, Edison had to build dc power station every
20 km! This was costly and very impractical. However, he did not give up the dc
transmission idea and insisted that these problems can be overcome.
George Westinghouse and Nikola Tesla did not waste time to develop ac-based
power distribution systems, despite Edison’s plans to continue to construct dc
transmission systems in New York. In 1885, a major step was taken by Westing-
house to develop ac systems when he bought the American patents of L. Gaulard
and J.D. Gibbs of Paris for the design of transformers. Westinghouse, backed by
Tesla’s patents and the new transformers designs, challenged the dc transmission
system and went ahead in developing it.
A major step in supporting ac systems was in 1885, when William Stanley, an
early associate of George Westinghouse, developed a commercially practical
transformer, allowing the possibility of distribution of ac-based electricity. This
was the first challenge to Edison’s idea of dc power systems. Using transformers, it
was possible to transmit high-level voltages at a very low-level current, resulting in
a very low voltage drop (low-power dissipation) in the transmission line. In winter
of 1886, Stanley installed the first experimental ac distributed system in Great
Barrington, Massachusetts, supplying 150 lamps in the covered area. In 1889, the
first single-phase distributed power system was operational in the United States
between Oregon City and Portland, covering a 21 km distance with 4 kV power
rating.
Second major step that boosted the potential of using ac systems took place on
May 16, 1888, when Tesla presented a paper at the annual meeting of the American
Institute of Electrical Engineers, discussing two-phase induction and synchronous
motors. Basically, he had shown that it is more practical and more efficient to use
polyphase systems to distribute power. The first three-phase ac transmission power
system was installed in Germany in 1891 rated at 12kv and transmitted over a
distance of 179 km. Two years later (1893), the first three-phase power transmission
system in the United States was installed in California, rated at 2.3 kV and a
distance of 12 km. Moreover, a two-phase distributed system was demonstrated
at the Colombian Exposition in Chicago in 1893. At this time, the apparent
advantages of ac, especially the three-phase systems, over the dc system lead to
the gradual replacement of dc by ac systems. Presently, the transmission of
electricity is done almost entirely by means of ac. However, dc transmission of
electric power is used in some locations in Europe and is rarely used in the USA.
Since the late nineteenth century, economic studies have shown that ac transmis-
sion is much economical, hence receiving worldwide acceptance.
6 1 Introduction

1.3.2 The History of dc and ac Electricity in the Late

Twentieth Century

Over the last two decades, the ever increase of deployment of renewable energy
sources to the power grid, coupled with technological advancement made by the
semiconductor device industry and the revolutionary advances made in the micro-
electronic communication and sensing technologies, has led to renewed interest in
using dc transmission systems which is renewed. This time, many experts believe
that because of the new technological advances, it is possible to develop dc
transmission electric power systems economically and efficiently. Today’s conver-
sion systems from ac to dc and back to ac can be done using very fast, high-power
rated, and highly integrated power semiconductor devices. What we can achieve
using today’s technology was not imaginable only 10 years ago. This is why many
power electronic researchers believe that the old debate between dc and ac camps is
coming back under a new set of technological rules.
Today, because of power electronics more than 100 years later, the argument
whether dc or ac should be the way to go in future home and industry is resurfacing.
In other words, in the turn of the twentieth century, ac was declared a winner over
dc, and in the turn of the twenty-first century, the dc promoters, mostly power
electronic experts, have had another shot at ac! Sounds familiar. . . history repeats
itself! A century later, the dc advocates might win, and the twenty-first century
might very well be friendlier to dc transmission system advocates! Who will win
the new century? Only time will tell!

1.3.3 History of Modern Power Electronics

Many agree that the history of power electronics began in 1900 when the glass bulb
mercury-arc rectifiers were introduced for the first time, signaling the beginning of
the age of vacuum tube electronics or what was also called glass tube-based
industrial electronics. This period remained until 1947 when the germanium tran-
sistor was invented at Bell Telephone Laboratory by the three physicists Bardeen,
Brattain and Shockley, signaling the end of the age of vacuum tubes and the
beginning of the age of transistor electronics. Between the 1930s and 1940s, several
new power electronic circuits (then known as industrial electronics) were intro-
duced including the metal-tank rectifier, grid-controlled vacuum-tube rectifier,
thyratron motor, and gas/vapor tubes switching devices such as hot cathode thyra-
trons, ignatrons, and phanotrons. In the 1940s and early 1950s, solid-state magnetic
amplifiers, using saturable reactors, were introduced.
The age of modern era of power electronics began in 1958 when General Electric
Company introduced a commercial thyristor, 2 years after it was invented by Bell
Telephone Laboratory. Soon, all the industrial applications that were based on
mercury-arc rectifiers and power magnetic amplifiers were replaced by SCRs. In
1.4 The Need for Power Conversion 7

less than 20 years after commercial SCR was first introduced, significant improve-
ments in semiconductor fabrication technology and physical operation were made,
and many different types of power semiconductor devices were introduced. The
growth in power electronics is made possible with the microelectronic revolution of
the 1970s and 1980s by which the low-power IC control chips provided the brain
and the intelligence to control the high-power semiconductor devices. Moreover,
the introduction of microprocessors made it possible to apply modern control
theory into power electronics. In the 25 years, the growth in power electronic
applications was noticeable because of the introduction of very fast, high-
temperature, and high-power switching devices, coupled with the utilization of
advanced digital control algorithms. Today, power electronics is a mature technol-
ogy. The future direction of the new era of power electronics is hard to predict, but
one is certain that as long as humans seek to improve the quality of life and cleaner
environment and implement energy-saving measures, the growing demand for
clean energy will continue. This in turn implies that power electronics must be
used to address the tremendous changes in the way we generate, transmit, and
distribute electricity as we cross the bridge into the new century. For more detailed
discussion of the modern history of the power electronics, see the paper by
D. Wyke, and see this web site that was originally written by Prof. Bimal k. Bose
http://ethw.org/Power_electronics.

1.4 The Need for Power Conversion

Since the invention of a practical transformer by Stanley in 1885 and polyphase ac

systems by Tesla in 1891, the advantages of low-frequency ac over dc were
compelling to power engineers at that time. The basis of utility power system
generation, transmission, and distribution since the beginning of this century has
been ac at fixed frequency either 50 or 60 Hz. The most outstanding advantage of ac
over dc was the high-voltage over long transmission lines and the simplicity of
designing distribution networks. However, the nature form of electricity being
distributed is totally different than the nature of energy required by the
electrical load.
At the consumer end, many applications may need dc power or ac at line, higher,
lower, or variable frequencies. Therefore, it became necessary to convert the
available ac systems into dc and must be controlled with precision. Furthermore,
in some cases, the generated power is from dc sources such as batteries, fuel cells,
or photovoltage, or in other cases, the available power is generated as variable
frequency ac from sources such as wind and gas turbine. This power conversion,
ac-dc, became more acute with the invention of vacuum tubes, transistors, ICs,
computers, servers, smart appliances, and data centers. Moreover, modern electric
conversion goes beyond ac-dc conversion, as we shall shortly address.
In the late 1880s, power conversion from ac-dc was done by using ac motors
along with dc generators in series (motor-generator set). Such motor-generator
8 1 Introduction

arrangement was still operational and was used in dc and 50/60 Hz motors and
generators. The difficulties of using the electromechanical conversion system
include large weight and size, noisy operation, servicing and maintenance prob-
lems, short lifetime, low efficiency, limited range of conversion, and slow recovery
time. To avoid problems of the electromechanical conversion systems, industrial
engineers turned into linear electronics in the late 1960s, where power semicon-
ductor devices were operated in their linear (active) region. To obtain electrical
isolation, input line-frequency transformers were used, resulting in bulky, heavy,
and large size power converters systems. Furthermore, since power devices are
operating in the linear region, the overall efficiency of the system was low. Unlike
electromechanical systems and linear electronic systems, power electronics has
many advantages including the following: (1) high energy conversion efficiency,
(2) results in highly integrated power electronic systems, (3) reduced EMI and
electronic pollution, (4) higher reliability, (5) utilizes environmentally clean volt-
age sources such as photovoltaic and fuel cells to generate electric power, (6) allows
for the integration of electrical and mechanical systems, and (7) allows for maxi-
mum adaptability and controllability.
In short, all forms of electrical power conversion will always be needed as long
as the consumers keep living in homes and use light, heat, electronic devices,
equipment, and interface with industry.

1.5 Power Electronic Systems

Most of the power electronic systems consist of two major modules: (1) the power
stage (forward circuit) and (2) the control circuit (feedback circuit). The power
stage handles the power transfer from the input to the output, whereas the feedback
circuit controls the amount of power transferred to the output.
Typical generalized block diagram of a power electronic system is given in
Fig. 1.2 below.

where.
x1, x2, . . .xn: Inputs signals (voltage, current, or angular frequency)
y1, y2, . . .yn: Output signals (voltages, currents, or angular frequency)
pin(t): Instantaneous input power in Watts
pout(t): Instantaneous output power in Watts
f1, f2, . . .fn are feedback signals: voltages or currents in electrical system or angular
speed or angular position in mechanical systems.
Efficiency, η, is defined as follows:
pout
η¼  100%
pin
1.5 Power Electronic Systems 9

Fig. 1.2 Simplified block diagram for a power electronic system

Fig. 1.3 Detailed block diagram of a power electronic system

Figure 1.3 shows a more detailed description of a block diagram for a power
electronic input system with electrical and mechanical output loads. The main
objection of the power electronic circuits is to process energy from a given source
to a required load. In many applications we illustrated earlier, the conversion
process concludes with mechanical motion.
10 1 Introduction

1.5.1 Classification of Power Converter Circuits

The function of the power converter stage is to perform the actual power conver-
sion and processing of the energy from the input to the output by incorporating a
matrix of power switching devices. The control of the output power is carried out
through control signals applied to these switching devices. Broadly speaking,
power conversion refers to the power electronic circuit that changes one of the
following: voltage form (ac or dc), voltage level (magnitude), voltage frequency
(line or otherwise), voltage waveshape (sinusoidal or non-sinusoidal such as square,
triangle, sawtooth, etc.), and the voltage phase (single or three phase).
Broadly speaking, there are four possible conversion circuits that are used in the
majority of today’s power electronic circuits:
(a) ac-ac
(b) ac-dc
(c) dc-ac
(d) dc-dc
In terms of the functional description, modern power electronic systems perform
one or more of the following conversion functions:
1. Rectification (ac-dc)
2. Inversion (dc-ac)
3. Cycloconversion (ac-ac different frequencies) or ac controllers (ac-ac same
frequencies)
4. Conversion (dc-dc)

1. Rectification (ac-dc)
The term “rectification” refers to the power circuit whose function is to alter
the ac characteristic of the line electric power to a “rectified” ac power at the
load site that contains dc value. Figure 1.4a, b shows the block diagram repre-
sentation of an ac-dc converter and its typical input and output waveforms,
respectively. To smooth out the output voltage by removing the unwanted ac
component, additional “filtering” circuit is added at the output side. Depending
on the switch implementations, these converters are further divided into two
types: diode converter circuits (uncontrolled) and thyristor converters (phase
controlled); each type can have either single-phase or three-phase input voltages.
Both types are extensively used in various offline applications as shown in
Table 1.1. Rectification circuits will be discussed in Chaps. 5 and 6. The
topologies that perform the rectification function include half-wave, full-wave
(full-bridge), semi-bridge, and transformer-coupled center-tapped. From the
beginning of the industrial electronics area, the ac-dc line commutation con-
verter class, utilizing thyristors, has been the largest among power electronic
converters because of their simplicity in design, efficiency, and higher current
and voltage ratings.
1.5 Power Electronic Systems 11

Fig. 1.4 ac-dc rectification

Table 1.1 Applications of power electronics by conversion functions

Conversion function Application
• Uncontrolled ac-dc con- – Front-end offline regulated dc-ac power supplies and
verters (diode circuits) dc-ac inverters
– Battery chargers
– Welding
– dc motor drives
• Phase-controlled converters – Regulated dc power supplies
(thyristor circuits) – ac and dc variable-speed motor control
– Battery chargers
– Flexible ac transmission system (FACTS)
– Utility interface of photovoltaic systems
– Regulated ac inverters
– Solid-state circuit breakers
– dc motor drives
– Induction heating
– Electromechanical processing (electroplating, anodizing,
metal refining)
– HVDC systems
– Light dimmers
– Active power line conditioning (APLC) (VAR compen-
sator, harmonic filters)
– Induction heating
(continued)
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slowed our speed suddenly to fall behind them they instantly did
likewise.
Meanwhile ships all about us were driving aimlessly away, reeling
blindly off into space or smashing into each other, as the pale death-
beams found more and more of them in that mad running fight. Not
for many minutes longer, I knew, could the unequal contest be kept
up. Already we were past the Cancer cluster, still racing along the
galaxy's edge, and then abruptly there came another sharp order
from the instrument beside me. Instantly, in obedience to that order,
all our racing, battling ships slowed, swiftly grouped themselves into
a triangular formation, its apex in turn pointing toward the long line
of the enemy's fleet, between us and the galaxy. Then, before they
could mass their own fleet again, our triangle of mighty cruisers had
leapt straight toward the galaxy, its apex tearing full into the long line
of their ships.

There was a moment of reeling, crashing shock, as our massed fleet

crashed into that line, and all about me in that moment, it seemed,
patrol-cruisers and oval ships were smashing into each other, colliding
and bursting wildly there in mid-space. Then suddenly we were
through, the mass of our fleet ripping through their line by main
force; but now, as we smashed on through, another order sounded
and we curved swiftly about, and still in that close-massed formation
rushed back upon the shattered enemy line of ships. Before they
could reform that broken line, before they could mass again in their
own close formation, we were upon them, and then again our
wedge-shaped mass was driving through them, shattering their
disorganized masses still further and sending scores of them into
annihilation now with our red rays as we flashed through.
"We've won!" shouted Jhul Din, at the window, as our massed fleet
again wheeled and sped back upon the disorganized mass of ships
before us. "We've won! We've broken up their fleet!"
Now, though, we were rushing back to strike another deadly blow,
and before us, I saw, the thousands of the invading ships were still
milling aimlessly there in space, their organization shattered by the
smashing blows we had dealt them. With red rays flashing we sped
upon them again, but now, from the disorganized mass before us, I
saw a score or more of ships rising, flashing upward with immense
speed, ships that were not oval like the rest but flat and round and
disk-like, ships that I had vaguely glimpsed in our first rush on the
enemy fleet and which through all the battle they had kept protected
from us at their fleet's center. Now, with all their terrific speed, the
disk-ships were flashing upward, and even in the instant that we
rushed again upon our enemies they had attained to a great height
above us.
In that instant I gave them but a glance, since again we were darting
upon the mass of oval ships, our own cruiser now toward the rear of
our fleet's formation. But in the next moment, even as we flashed on
in our swift charge, I saw the score of disk-ships hanging high above
suddenly glow and flicker with strange force, the whole great lower
side of their big disks alive with a flickering, rippling, viridescent light.
And at the same moment I saw the ships of our fleet ahead of us
suddenly breaking from their mad charge forward and lifting slowly
upward, saw them twisting and turning and reeling but still moving
steadily up, toward those score of disk-ships high above, as though
pulled upward by a mighty, unseen grip.
"Attraction-ships!" I shouted, as I saw what was happening. "Those
disk-ships above—they're pulling our cruisers up with some magnetic
or electrical attractive force, that affects the metals of our ships but
not of theirs!"
We were still racing forward, at the rear of our fleet, but as I saw
that all the thousands of our cruisers before us, almost, were in the
grip of the attractive forces from above, were being pulled helplessly
upward, I shouted to Korus Kan, and he shifted the controls swiftly
sidewise, sending our cruiser veering away before it came beneath
the disk-ships high above and was pulled up likewise. We had
escaped for the moment, but now from ahead all the disorganized
masses of the oval invading ships had gathered together again and
were leaping forward, springing upon our own helpless masses of
cruisers as they were pulled resistlessly upward. From all about those
masses of twisting, turning cruisers the pale death-beams smote
toward them, and only here and there could a few shafts of the red
ray answer them, caught as our ships were in that tremendous grip.
Swiftly the cruisers of our fleet were being wiped clean of all the
crews inside, as the death-beams swung and circled through them
from all about. But a few score of cruisers at the rear of our fleet, like
ourselves, had managed to escape the relentless grip of the disk-
ships above, and now upon ourselves other masses of the oval ships
were rushing. Wildly we battled there, the hordes of the invading
ships spinning and flashing about us, but swiftly our few score of
cruisers were sent reeling blindly off by the death-beams; and now,
looking back an instant, I saw that the last of our mighty fleet of
thousands of cruisers were being annihilated by the death-beams of
the oval ships that swarmed about them, as they were drawn
helplessly upward. We and a few other cruisers, struggling wildly
there against the encircling masses of the oval ships, were all that
remained of the galaxy's once mighty fleet!
Even as we fought there, with the mad energy of despair, I saw the
last of our companion cruisers whirling away as the death-beams
found it, and realized that except for a few stragglers here and there
like our own ship the great fleet was annihilated, and that our only
chance was in flight. With every moment the oval ships about us
were increasing in number, completely encircling us, now, and it was
only by a miracle of veering, twisting turns by Korus Kan that our ship
was able to avoid the death-beams that reached toward us from all
sides. Escape seemed impossible, so completely were we hemmed in
by the circling, striking ships, and another moment would see our
end, I knew; and so I wheeled, shouted hoarsely to Korus Kan.
"We'll have to break through them!" I shouted. "Give her full speed,
Korus Kan, and head straight in toward the galaxy!"
Instantly he jerked open the power-control to the last notch, and as
our ship leapt forward like a living thing toward the masses of ships
that surrounded us he sent it driving straight toward the galaxy, and
toward a spot where there showed a momentary gap between the
ships that hemmed us in. But a single instant it took us to reach that
gap, pale beams whirling all about us while our own red rays flashed
sullenly forth, but in the instant that we reached it one of the oval
ships had seen our intention and had leapt forward to close the gap.
An instant too late it was to close it completely, but the oval ship's
nose, containing its transparent-walled pilot room, lay across our path
as we reached the gap, and straight into it we crashed!

There was a terrific, rending shock as our great prow tore into the
transparent-walled nose of the enemy ship, and beneath that shock
we saw the whole fore portion of the oval ship crumpling up and
collapsing, reeling away a shattered wreck of metal. Our own cruiser
rocked and swayed crazily at the collision, and for a moment it
seemed that we too were doomed, but the next our battered ship
leapt forward, and in an instant was free of the masses of oval ships
that had encircled us, and was driving now in toward the galaxy's
suns, with a score of the oval ships behind in hot pursuit.
In we drove, speeding now past the great Cancer cluster as we
flashed at our utmost speed into the galaxy, its great ball of gathered
suns flaring in the black heavens to our left as we sped inward.
Behind came our pursuers, racing on close after us; and now,
glancing back beyond them, I saw the whole mighty fleet of the
invaders, still fully three thousand ships in number, moving in toward
the galaxy also, toward the great Cancer cluster, with its swarming
suns and thronging worlds, saw the great fleet slowing, slanting
down toward those suns, those worlds, and knew then that these
invaders, having annihilated the galaxy's fleet, were settling upon the
suns and worlds of the Cancer cluster as a first foothold in our
universe, a base from which they could subdue all that universe.
Then their fleet had vanished from our distance-windows as we fled
on, and of the score of our pursuers all but three had turned back to
rejoin that fleet.
The three remaining ships, though, drove straight on our track, and
swiftly were overhauling us, though inside the galaxy they dared not
use all their tremendous speed. Yet remorselessly after us they came,
and I knew that moments more would see our end unless we could
escape them. Directly ahead of us, though, there flamed a small
crimson sun, a dying, planetless star not far inward from the Cancer
cluster, largening each moment before us as we drove on toward it
with terrific speed. As I saw it a last plan flashed through my brain,
and I turned to Korus Kan.
"Head straight toward that sun!" I told him. "It's our only chance—to
get in close and lose them in its corona!"
He nodded grimly, swerving the ship a little, and now straight toward
the red star we raced, Jhul Din and I gazing out with him toward it as
we flashed on, and then behind to where the gleaming three ships of
the invaders drove after us. Swiftly they were overtaking us, two
close behind us and the remaining one a little behind the two, but
ahead the crimson star was filling almost all the heavens, now, a
great sea of fiery red flame that stretched above and beneath us,
ahead, as though occupying all the firmament. Its glare was awful,
now, for we were racing straight in toward the mighty corona of it,
the glowing outer atmosphere of radiant heat about it in which, I
knew, no ship, however heat-resistant, could live for more than a
moment. On we raced, our cruiser creaking and swaying still from the
effects of the collision with the ship we had smashed into, but
flashing on with unabated speed.
Behind us, the three gleaming shapes of our pursuers were following
with unslackened speed, too, gradually drawing nearer, the two
foremost of those ships just behind us, now. Another moment and
their death-beams would stab toward us, and though we might
destroy one or even two of them the other would surely destroy us
before we could turn to it, I knew. The heat, too, of the great star
before us was penetrating into our ship, and full before us, not a
dozen million miles ahead, glowed the great corona. On we flashed—
on—on—and then, just as we were about to burst into the terrible,
glowing corona, just as the two ships close behind us sprang closer
to stab with their beams toward us, Korus Kan jerked the controls
suddenly back, and instantly our ship shot upward in a great vertical
rush, while beneath, before they could see and follow our change of
course, the two racing oval ships pursuing us had flashed on and into
the mighty glare of the corona. Then we glimpsed them shriveling,
twisting, vanishing, in the awful heat there, while our own cruiser
turned now away from the red sun.
Beneath we saw the single remaining oval ship turning, too, since it
had been far enough behind the two to change its course in time to
avoid the terrible corona. It seemed to pause, hesitate, and then, as
though satisfied that our ship too had met death in the corona with
its own two companions, it began to flash backward toward the
galaxy's edge, toward the Cancer cluster where the mighty invading
fleet had settled. And now, burning for revenge, our own cruiser was
slanting back with it and down toward it, as it drove on
unsuspectingly beneath. Another moment and we would be above it,
would loose our red rays on it before ever it suspected our existence.
I was breathing with relief at our escape, now, and heard an exulting
cry from Jhul Din as he strode down into the cruiser's hull from the
pilot room, to direct the ray-tubes there, but the next moment all our
triumph vanished, for from our cruiser's hull, toward its battered
prow, there came suddenly a succession of appalling cracks.
Standing suddenly tense we listened, and then, as there came from
beneath a prolonged, cracking roar, I heard shouts of fear from our
crew, and then Jhul Din had burst up into the pilot room from
beneath.
"The cruiser's walls are giving!" he cried. "That collision with the oval
ship when we smashed our way out strained and wrenched loose the
whole prow and side-walls—the cruiser can't hold together for five
minutes more!"
There was a stunned silence in the little room then, a silence in which
it seemed that all the disasters that had befallen us were crowding
together upon us, overpowering us. This was the end, I knew. Within
minutes more the walls about us would collapse and in the infinite
cold and emptiness of interstellar space we would meet our deaths.
We were hours away from the nearest friendly planet, with all our
companion ships destroyed. It was the end, and for a moment I
bowed to the inevitable, stood in stunned despair awaiting that end.
But then, as my eyes fell upon the oval ship beneath, toward which
our collapsing cruiser was still slanting downward, I saw that upon its
broad metal back was the round circle of a space-door, like the
double space-doors of our own ship, and as I saw that, all the
ancient combativeness that has carried men out into the remotest of
the galaxy's depths surged up in me, and I wheeled around to the
other two.
"Order all our crew down to the cruiser's lower space-door," I cried,
"and have an emergency space-suit issued to each of them!"
They stared at me, strangely, tensely. "What are you going to do?"
asked Jhul Din, at last, and my answer came out in a shout.
"We're going to do what never yet has been done in all the battles
between the stars!" I told him. "We're going to put our lives on one
last mad chance and board that enemy ship in mid-space!"

4. A Struggle Between the Stars

A moment there was silence in the pilot room, a silence of sheer
surprize, in which my two lieutenants gazed at me in utter
amazement, and then from Jhul Din came a great shout.
"It's a chance!" he cried. "If we can do it we'll escape yet!"
"Down to the space-door at once, then!" I told him. "The ship can't
last for seconds now!"
For even then there had come to our ears another long, cracking roar
as our battered walls gave still farther. Now Jhul Din was racing down
from the pilot room to assemble the crew, and now our cruiser was
slanting still farther down toward the long, gleaming oval ship
beneath. Down we slanted, until our own swaying cruiser hung at a
distance of a score of feet above the enemy ship, which, believing us
destroyed, never dreamed of our presence as we raced on through
space at the same speed as itself. And now Korus Kan hastily set the
automatic controls in the pilot room that would hold our cruiser at the
same speed and course without guiding hand, and then we too
hurled ourselves down the narrow stair, through the big power room
where the great generators were still throbbing on, down through the
succession of compartments in the cruiser's hull until we had reached
the long, low room that lay at its very bottom, and in the floor of
which was set the cruiser's lower space-door.
In the long room all our crew was gathered now, with Jhul Din at
their head, a hundred odd in number, and a strange enough
aggregation they were, drawn as they were from the far-different
races of the galaxy's peopled stars. Octopus-beings from Vega, great
plant-men from Capella, spider-shapes from Mizar—these and a score
or more of differing forms and shapes stood before me, listening in
disciplined silence as I briefly explained our plan. About us the walls
were wrenching and cracking fearfully, but when I had finished those
before me raised a fierce shout, and then each of us was hastily
climbing into the emergency space-suits which were kept always in
all interstellar ships in case outside repairs to it were necessary in
mid-space.
A moment more and we all stood attired in the hermetically sealed,
clumsy-looking suits of thick, flexible metal, with head-pieces of
metal in which were transparent vision-plates. As we donned them
each pressed the button which set the little air-generators inside each
suit pouring forth their supply of fresh air and purifying the breathed
air; and then, with a swift glance around that showed each in his
suit, I motioned to Jhul Din and at once the big Spican pressed the
stud in the wall that sent the round space-door in the floor sliding
open.
We could not feel through our insulating suits the tremendous cold
that instantly invaded the ship, but we heard plainly the swift, terrific
swish of air about us as it rushed out of the ship into the mighty void
outside. Now, looking down through the open door, we could see a
score of feet beneath the broad metal back of the great oval ship, still
racing on unsuspectingly beneath us. I turned back to the crew about
me, saw that each had gripped one of the metal bars that were to be
our only weapons in this attempt, since the use of rays would destroy
the ship beneath, which was our only hope of life. Then, reaching
forth again to the switch-case on the wall, Jhul Din at my motion
threw off the cruiser's gravity-control, so that the attraction-plates
built into the floor beneath us, which pulled us always downward and
enabled us to walk upright and normally inside the cruiser, no longer
pulled us. Instead, though, we were being pulled down now by the
gravitational force of the racing ship beneath and a step through the
open door would send any of us hurtling down toward that ship.
Now I gave one last glance around, even while the cruiser's walls
cracked terribly again, and then swung myself over the edge of the
opening in the floor, hanging by my hands from it and swinging there
in the infinite void of interstellar space a score of feet above the oval
ship's broad metal back. It seemed, that moment that I swung there,
a time of endless length, and surely never before had any hung thus
between two ships racing on through the void. Then, as another
cracking roar came from the walls about me, I loosed my hold upon
the edge and hurtled down through empty space toward the back of
the ship below.
Down, down—that fall seemed endless as I rushed down through
space, but unimpeded as I was by air-resistance it was but an instant
before I had slammed down on the ship's broad back, lying
motionless for an instant and then rising carefully to a sitting
position. Just above me hung our racing cruiser, the opening in its
bottom directly overhead, and in another moment Korus Kan had
followed me, striking the ship's back beside me while I gripped him
and held him tightly. Then came one of the crew, and another, and
another, until in a moment the last of them was dropping down
among us, Jhul Din alone remaining above. He stepped toward the
opening, to lower himself and drop down to us likewise, but even as
he did so I saw the great walls of the cruiser above collapsing and
buckling inward as they gave at last. I motioned frantically to Jhul Din
as the walls collapsed about him, saw him give one startled glance
around, and then as the cruiser's sides crumpled up about him he ran
forward and leapt cleanly through the opening in the floor, hurtling
down toward us and striking full in our midst, just as the crumpled
cruiser above, the power of its generators gone with its collapse,
jerked sharply out of sight toward the crimson sun behind, hurtling
away from us a twisted wreck of metal.

It was with something of a tightness in my throat that I saw the

wreck of our familiar, faithful ship drive away from us, but I turned
toward our own desperate situation. We were clinging to the back of
the great oval ship as it drove on toward the Cancer cluster, with
above and all about us the blackness of the void, and the galaxy's
flaming suns. Ahead shone the gathered suns of the great cluster,
and I knew that we must capture the ship soon if at all; so now, half
creeping and half walking, we made our way along the great ship's
back toward the round space-door set midway along that back. In a
moment we were clustered about it, and found it closed tightly from
within, as I had expected. Instantly, though, we set to work on it with
the metal bars and tools we had brought with us, drilling down
through the thick metal of the door while we clung, like a hundred
odd tiny mites, upon the mighty ship's back as it flashed on and on.
What might lie in the ship beneath, what manner of beings might
these terrible invaders be, we could not even guess, but it was our
one chance to penetrate inside, and frantically we worked. Within
moments more we had drilled through in a dozen places, were
swinging aside the great bolts that held the door closed inside, and
then were sliding it open and dropping swiftly down inside. We heard
a little rush of air outward as the door opened, and knew that this
ship was inhabited by air-breathing beings, at least, and then we
found ourselves in the room beneath the space-door, a bare little
vestibule chamber in whose side was a single square door.
Before opening this, however, we closed the round space-door above
us, plugging the holes we had drilled in it by driving in sections of
metal bar, and then I turned toward the door in the wall, felt carefully
around it, and finally pressed a small white plate inset beside it, at
which it slid silently aside. We stepped through it, bars raised ready
for action. We were in a corridor, a long corridor apparently running
the length of the great oval ship, but quite empty for the moment.
The throbbing of great generators was loud in our ears, a throbbing
much like that in our own ships but with another unfamiliar beating
sound mingled with it. Silently we gazed about, then began to make
our way down the corridor toward the ship's front end, toward the
pilot room at its nose, stopping first to divest ourselves silently of the
heavy space-suits, and then starting on.
Now we had come to an open door in the corridor's side, and peering
cautiously through it we saw inside a long room holding a score or
more of great, cylindrical mechanisms from which arose the
throbbing and beating of the oval ship's operation. About these
mechanisms were moving some two dozen of the ship's occupants,
and as our eyes fell upon them we all but gasped aloud, so utterly
strange and alien in shape were they even to us, who held strange
shapes enough in our own gathering. Many and many a strange race
had we of the Patrol seen in our long journeys through the galaxy,
but all these were familiar and commonplace beside the shapes that
moved in the room before us. For they were serpent-people!
Serpent-people! Long, slender shapes of wriggling pale flesh, each
perhaps ten feet in length and a foot in diameter, without arms or
legs of any kind, writhing swiftly from place to place snake-like, and
coiling an end of their strange bodies about any object which they
wished to grip. Each end of the long, cylindrical bodies was cut
squarely off, as it were, and in one such flat end of each were the
only features—a pair of bulging, many-lensed eyes like those of an
insect, big and glassy and unwinking, and a small black opening
below that was the only orifice for their breathing. These were the
beings who had come out of outer space to attack our universe!
These were the beings who had annihilated the galaxy's fleet and
were preparing now to seize the galaxy itself!
I turned from my horror-stricken contemplation of them to Jhul Din
and Korus Kan, close behind me. "The pilot room!" I whispered.
"We'll make for it—get the ship's controls!"
They nodded silently, and silently we stole past the open door and
down the long corridor, toward the door at its end that we knew must
lead into the pilot room at the ship's nose. Past other doors we crept,
all of them fortunately closed, and as we stole on toward the door at
the corridor's end I began to hope that at last our luck had turned.
But ironically, even as I hoped, the door at the corridor's end, not a
score of feet ahead, slid suddenly aside, and out of it, out of the pilot
room beyond it, came one of the writhing serpent-creatures. It
stopped short on seeing us, then gave vent to a strange, hissing cry,
a high, sibilant call utterly strange to my ears, but at the sound of
which the doors all along the corridor behind us slid swiftly open,
while through them scores of the serpent-beings writhed out, and
upon us!
"The pilot room!" I yelled, above the sudden hissing cries of the
serpent-creatures and the shouts of our own crew. "Head for it, Jhul
Din!"
Down the corridor we leapt, and out from the pilot room there came
to meet us a half-dozen of the serpent-creatures, while one remained
inside at the controls still. Then they were rushing toward us, and as
they reached us were coiling about us, endeavoring to crush us by
encircling us with their bodies and coiling with terrific power about
us. As they did so, though, our own metal bars were crashing down
among them, sending them to the corridor's floor in masses of
crushed flesh as we plunged on toward the pilot room. Now we were
through them, had crushed them before us, and were leaping
through the door, the single serpent-creature inside wheeling to face
us. Before he could spring upon us, though, Jhul Din had lifted him
high above his head and then had flung him far down the corridor,
where he struck against the wall and fell crushed to the floor. Then
Korus Kan was leaping to the controls, swiftly scanning them and
then twisting and shifting them, heading the racing ship around in a
great curve, away from the Cancer cluster ahead and back in toward
the galaxy's center, while Jhul Din and I now sprang back down the
corridor to where our crew was struggling fiercely with the hordes of
serpent-creatures rushing up from all parts of the ship.
Down that corridor, and down another, through rooms and halls and
twisting stairways, down through all the great ship the battle raged,
the serpent-creatures leaping and coiling about us with the courage
of despair while we strode among them, metal bars smashing down
in great strokes, mowing them down before us. Despite their
overpowering numbers they were no match for us in such hand-to-
hand fighting, and they dared not use ray-tubes, like ourselves, lest
they destroy their own ship about them. So we forced them on, ever
sending them down in crushed, lifeless masses, as they gradually
gave way before us.
I will not tell all that happened in that red time of destruction, but
quarter there could be none for these things that had come to attack
our universe, that had destroyed our comrade ships in thousands;
and so within a half-hour more the last of the serpent-creatures had
perished and we were masters of the ship, though but a scant two
score of us were left to operate it, so fierce had been the battle.

Our first action was to clear the ship of dead, casting them loose into
space through the space-doors; then Jhul Din and I made our way
back into the pilot room, where Korus Kan was holding the ship to a
course inward into the galaxy. The controls, he had found, were very
much like those of our own cruisers, but the great generators, as we
found, were much different. Instead of setting up a vibration in the
ether to fling the ship forward, as in our own cruisers, they projected
a force which caused a shifting of the ether itself about the ship,
forming a small, ceaseless ether-current which moved at colossal
speed, bearing the ship with it. The speed could thus be raised or
lowered at will by controlling the amount of force projected, and as
the general nature of the generators was clear enough the remaining
engineers of our crew took charge of them while we fled on into the
galaxy.
"We'll head straight for Canopus," I said, indicating the great white
star at the galaxy's center far ahead. "We'll report at once to the
Council of Suns; our capture of this ship may be of use to them."
While I spoke Korus Kan had opened the power-control wider, and
now our newly captured prize was racing through the void toward the
mighty central white sun at thousands upon thousands of light-
speeds, though I knew that even this terrific velocity, all that we
dared use inside the galaxy, was but a fraction of what the ship was
capable of in outer space. Glancing about the pilot room, I
endeavored for a time to penetrate the purpose of some of the things
about me, as we flashed on. Above our window, as in our own
cruiser, was a great space-chart, functioning similar to ours, I had no
doubt, and showing the dot that was our ship flashing on between
the sun-circles that lay about us. There was a device for flashing vari-
colored signals, also, such as space-ships inside the galaxy use to
show their identity on landing. There was, too, a cabinet containing a
great mass of rolls of thin, flexible metal, inscribed with strange,
precise little characters that I guessed formed the written language
of the serpent-people, though they were beyond all comprehension
to me. I turned back to the windows about me, gazing forth into the
vista of thronging suns and worlds that lay all about us now as we
flashed on into the galaxy toward Canopus.
From all the suns about us, our space-chart showed, great masses of
interstellar ships were also flashing inward into the galaxy, the first
exodus of the galaxy's people from the outer suns and worlds, driven
inward by the fear of these mighty invaders from the outer void who
had already destroyed the galaxy's fleet, and were preparing now to
grasp all our universe. Far behind us I could see the great ball of
suns that was the Cancer cluster, glowing in supreme splendor at the
galaxy's edge, and I knew that even now, on the worlds of those
thronging suns, the great fleet of the invading serpent-creatures
would be settling, would be moving to and fro, wiping out the races
that thronged those worlds, wrecking and annihilating the civilizations
upon them and making of all the suns and worlds of the great cluster
a base for their future attacks upon and conquest of the galaxy.
Could we, in any way, save ourselves from that conquest? It seemed
hopeless, and now, weary as we were with crushing fatigue from the
swift succession of events that had crowded upon us in the last few
hours, since our discovery of the invading swarm's approach, it was
with a dull despair that I watched Canopus enlargening ahead as we
flashed on toward it.
On between the galaxy's thronging suns we raced, our vast speed
carrying us through them and through the swarming, panic-driven
ships about them before they could glimpse us. Onward, inward, we
flashed, veering here and there to avoid some star's far-swinging
planets, dipping or rising to keep clear of the masses of traffic that
were jamming the space-lanes leading inward, racing on at the same
unvarying, tremendous velocity while we three in the pilot room, and
the remainder of our crew beneath, strove to remain awake and
conscious against the utterly crushing oppression of fatigue that
pressed down upon us. At last we were flashing past the last of the
suns between us and Canopus, and the great white central sun lay
full before us, a gigantic globe of blazing, brilliant light. As we leapt
toward it I saw Korus Kan gradually decreasing our speed, our ship
slackening in its tremendous flight as we slanted down toward the
planets of the great sun, and toward the inmost planet that was the
center of the galaxy's government.
Down, down—our speed was dropping by hundreds of light-speeds
each moment, now, as we sped down through the terrific glare of the
vast white sun toward its inmost world. As we shot downward I saw
that Jhul Din, now, was lying on the floor beside me, overcome by
the fatigue that crowded down upon me also; only Korus Kan, of all
of us, holding to the controls untiring and unaffected, the metal body
in which his living organs and intelligence were cased being
untrammeled by any weariness. Beneath us now lay the great
masses of traffic, countless swarms of swirling ships, that had fled in
to Canopus from the outer suns at the invaders' attack, and as they
glimpsed our great oval craft these swarms broke wildly from before
us. They took us for a raiding enemy ship, we knew, but down
between them unheedingly we flashed, heading low across the
surface of the great planet, still at tremendous speed.
Moments more and there loomed far ahead and beneath the colossal
tower of the Council of Suns, toward which we were heading. By then
I felt all consciousness and volition beginning to leave me, as an utter
drowsy weariness overcame me, and I realized but dimly that Korus
Kan was slanting the ship down toward the great tower, until abruptly
there came from him a sharp cry. With an effort I raised my gaze and
saw that from below, as we sped downward, three long, shining
shapes were arrowing up to meet us. They were cruisers of our own
Interstellar Patrol, and as they flashed upward there suddenly leapt
from them a half-dozen brilliant shafts of the crimson rays of death,
stabbing straight toward us!

5. For the Federated Suns!

Half conscious as I was, it seemed to me in that dread instant that
the whole scene about us was but a strange, set tableau, racing ships
and flashing rays frozen motionless in mid-air. Then another cry from
Korus Kan jarred me back to realization.
"The signal!" he cried. "Flash the signal of the Interstellar Patrol
before they annihilate us!"
At his cry a flash of realization crossed my darkened brain, and I
understood that the Patrol cruisers beneath had recognized our craft
as an enemy ship, that Korus Kan himself dared not leave the
controls even for an instant to flash from the signal our identity. With
a last summons of my waning strength I rose, staggered blindly
across the room toward the switch, and then, as from beneath the
crimson rays flashed blindingly toward us, grasped the switch and
swept it around the dial, flashing from our ship's nose the succession
of colored lights that proclaimed us of the Patrol. I felt myself sinking
to the floor, then, seemed to see the three uprushing ships swerving
by us at the last moment as they glimpsed the signal, and then as
Korus Kan sent the ship slanting down and over the ground to land I
felt a bumping shock, seemed to sink still deeper into the drowsy
darkness, then knew no more.
How long it was that I had lain in that darkness, in a stupor of sleep
from the weariness of our hours of rushing action, I could not guess
when next I opened my eyes. I was lying upon a thick mat on a low
metal couch, in a small room lit by a flood of white sunlight that
poured through a tall opening in its side. On a similar couch beside
me lay Jhul Din, just waking like myself; and for a moment we stared
about in bewilderment. Then the sunlight, the brilliant pure white
glare of light that could never be mistaken for the light of any star
but Canopus, gave me my clue, and I remembered all—our discovery
of the approaching swarm while patrolling the galaxy's outer edge,
our flight inward and the great battle, our capture of the enemy ship
and our escape. I jumped to my feet, and as I did so Korus Kan came
into the room.
"You're awake!" he exclaimed, as his eyes fell on Jhul Din and me,
standing. "I thought you would be, by now; the Council of Suns is
waiting for us."
"The Council!" I repeated, and he nodded quickly as we strode with
him to the door.
"Yes. We've been here for many hours, Dur Nal—you and Jhul Din
sleeping—and in those hours the Council has been in almost constant
session, deliberating this invasion of our universe."
While he spoke we had been traversing a narrow, gleaming-walled
corridor, and now turned at right-angles into another, strode down it
and through a mighty, arched doorway, and were in the tremendous
amphitheater of the Council Hall, a room familiar to all in the galaxy,
the vast circle of its floor covered now by the thousands of seated
members. It was toward the central platform that we strode, where
Serk Haj, the present Council Chief, a great, black-winged bat-figure
from Deneb, stood before the vast assembly, behind him on the
platform the score of seated figures who were the heads of the
different departments of the galaxy's government. It was toward
seats among these that he motioned us, as we reached the platform,
and as we took our place in them I glanced about the great hall,
interested in spite of the cosmic gravity of the moment. It was with
something of a leap in my heart that I saw, among all those
dissimilar thousands of strange shapes from the galaxy's farthest
stars, the single human figure of the representative of my own little
solar system. Then, as Serk Haj went on with the address to the
assembly which our entrance had interrupted, I turned my attention
to his words.
"And so," he was saying, "it is clear to you how these strange
invaders from outer space, these serpent-creatures from outside our
universe, have been able to annihilate all but a few ships of our great
fleet, to settle upon the worlds of the great Cancer cluster as a base,
to set up clear around the edge of our galaxy the watchful patrol of
their ships that our scouts report. All this they have done with a fleet
of a few thousand ships, have shattered our galaxy's defenses and
sent wild panic flaming across that galaxy; yet these few thousand
ships, as we have now learned, are but the vanguard of the countless
thousands that are soon to follow, to pour down upon us in colossal,
irresistible hordes!
"It was through the feat of Dur Nal, here, and his companions, that
we have learned this. You have heard how, after the great battle, he
and his party were able to do what never before was done in all the
annals of interstellar warfare, to board and capture an enemy ship in
mid-space and bring it back, intact, to Canopus. That ship has been
thoroughly examined by the best of the galaxy's scientists, and in its
pilot room was found a collection of metallic sheets or rolls covered
with strange characters, the written records of these serpent-
invaders. Upon those records for hours our greatest lexicologists have
worked, and finally they have been able to decipher them, and have
found in them the facts of the history and purposes of these strange
invaders from outer space.
"These invaders, as the records show, are inhabitants of one of the
distant universes of stars like our own, lying millions of light-years
from our own in the depths of infinite outer space. So far are these
mighty galaxies like our own that they appear to us but faint patches
of light in the blackness of space, yet we recognize them as universes
like ours, and have given them names of our own, calling one the
Andromeda universe, and another the Triangulum universe, and so
on. The universe of these serpent-creatures, though, although one of
the nearest to our own, has never been seen or suspected by us
because it is invisible from our distance, being not a living universe of
flaming stars like our own and the ones we see, but a darkened,
dying universe.
"It is a universe in which the thronging stars have followed nature's
inexorable laws and have darkened and died, one by one, a great
universe passing into death and darkness and decay as our own and
all others, some time in the far future, will pass. For eons upon it had
dwelt the great masses of the serpent-people, thronging its countless
worlds, and as their suns began to fail them, one by one, as their
universe swept toward its final darkness and death, they saw that it
was necessary for them to migrate to another universe unless they
wished to pass also into death. So they constructed great space-ships
which were able to travel at millions of light-speeds, by causing an
ether-shift about the ship; space-ships in which it would be possible
to do what never had another done, to cross the vast gulf between
universes. Five thousand of these, when finished, they sent out with
serpent-crews and death-beam armament as an advance party which
was to locate a universe satisfactory for their races and then attack it,
gaining a foothold upon it while the rest of the countless serpent-
hordes would build a still mightier fleet of tens of thousands of ships,
which would transport all their great hordes to the universe they
meant to conquer.
"So the five thousand ships drove out from the dying universe into
the void, toward the Andromeda universe, the nearest to their own.
Down they poured upon it in swift attack, but up to meet them rose
the people of the Andromeda universe, a single race ruling all of it,
whose science and power were so great that with mighty weapons
they drove back and defeated the five thousand attacking ships,
forcing them back into outer space again. It was clear that for the
present the Andromeda universe could not be conquered, so they
turned at a right angle, and after flashing a message by some means
of etheric communication to the masses of their peoples in the dying
universe, struck out through the infinite void in a new direction,
toward our own universe.
"Across the void they came, toward our universe, and rushed in upon
it after the long days of their tremendous flight through space, met
and annihilated our own great fleet at the galaxy's edge, and have
settled upon the Cancer cluster, gaining the foothold they desired.
Soon from their dying universe will come their vast main fleet with all
their hordes, and with a mighty weapon which the records mention
as now being constructed in the dying universe, a weapon to
annihilate all life on our worlds with terrific swiftness. They will come,
in all their masses, and when they have annihilated the races of the
Federated Suns and hold all our galaxy in their grasp will then sail
back with renewed power to pour down upon the Andromeda
universe and conquer it also. A cosmic plague of conquest and
destruction, creeping through the infinite void from universe to
universe!"
Serk Haj was silent a moment, and all in the great room were silent,
a silence such as surely none ever experienced before. I was listening
tensely, Jhul Din and Korus Kan beside me, but no whisper broke that
stillness until the Council Chief's voice went calmly on.
"Doom creeps upon us," he said, "yet there is still one chance to stay
that doom. We know that before attacking us the serpent-creatures
attacked the Andromeda universe and were repulsed, that they plan
to return to that attack after they have conquered us. So if we could
send a messenger across the terrific void to the Andromeda universe,
to tell its peoples of the serpent-creatures' attack upon us and their
intention to invade the Andromeda universe once more, after
conquering us, there is a chance that those peoples would come to
our aid, with the powerful weapons with which they have already
once repulsed the serpent-creatures, and would help us to crush
these invaders before all their resistless hordes can pour down on our
galaxy. It is a chance—a chance only—but on that chance rests the
fate of our universe!
"This chance, a chance to seek the help that may save us, has been
given to us by Dur Nal and his companions, in their capture of the
enemy ship in mid-space; for this captured ship, with its colossal
speed, can do what none of ours can do: it can cross the mighty void
that lies between us and the Andromeda universe, and carry an
appeal for help to that universe. The captured ship has been
thoroughly studied by our scientists, for we plan to build a great fleet
of others with mechanisms like it, to help in crushing these invaders
whom we can not crush alone. A special crew of picked engineers
and fighters, from various of our stars, has been selected for it, and
now waits in it for the start of this great flight through the void that
they are to make for our galaxy. The command of it, though, can go
only to the one who captured it, to Dur Nal, who was first to warn us
of the oncoming peril, and to his lieutenants, Jhul Din and Korus
Kan!"
With the words we three snapped to our feet, the great assembly
rising likewise in their excitement, and now Serk Haj turned to face
us.
"Dur Nal," he said, steadily, "it is not for me to exhort you and your
friends to do now your best, who have done always your best. If you
can break through the enemy's patrol around the galaxy's edge, can
cross the mighty void which never yet has any of our galaxy crossed,
and can carry to the Andromeda universe our appeal for help, it may
be that you will save us all—it may be that you will save the races
and civilizations of all the Federated Suns from conquest and
annihilation and death. To you three, who have spent your lives in
the service of the Federated Suns, I need say no other word."
We saluted, and there was a moment of death-like silence, until I
spoke. "We start at once!" I said, simply.

The next moment we three were striding down the broad aisle across
the mighty hall, between the thousands of members who, still in the
grip of that strange silence, watched us go, the one chance of our
universe with us. Out of the great hall we strode, and down the big
corridor, out of the great tower into the white glare of Canopus' light,
and toward the long, gleaming oval shape of our waiting ship. Inside
it our crew awaited us, a full eight score of strange, dissimilar shapes
from every quarter of the galaxy, among them the two score who had
been of my cruiser's crew and had helped capture this ship. Swiftly I
gave to them our first orders, heard the space-doors clanging as we
ascended to the pilot room, and then as Korus Kan stepped to the
controls heard the mingled throbbing and beating of the great
generators beneath.
I gave a brief signal, and Korus Kan gently opened the mighty ship's
controls, its nose lifting now as it shot smoothly upward. Past us now
from beneath there rushed up two cruisers of the Patrol, speeding up
ahead of us and flashing signals that cleared swiftly from before us
the masses of swarming traffic above, that swept hastily to either
side as our long, grim ship drove up and outward. Up, up—and then
we were clear of the last of them, our escorting Patrol cruisers
dropping behind us now and turning back as with rapidly mounting
speed we shot out from the great planet and upward, mighty
Canopus blazing full behind us now, as we flashed out again from it,
out with our velocity increasing by leaps and bounds, out toward the
Cancer cluster once more, toward the galaxy's edge.
With the passing minutes our generators were throbbing faster and
faster, and we were leaping on through the galaxy at a speed that
equaled or exceeded that of our flight inward. Suns were flashing by
us on either side now, at a rate that was an index to our appalling
speed, but still we flashed on with greater and greater speed, racing
out between the thronging suns of the galaxy toward its edge, the
great ball of suns of the Cancer cluster expanding before us as we
raced on in its direction. On—on—until the mighty cluster lay full to
our right, until we were flashing past it, the blackness of outer space
stretching ahead, and in that far-flung blackness the dim little patch
of light that was the Andromeda universe. We were passing the
mighty cluster, now, heading straight out into the black abyss, and
my heart hammered with excitement as we flashed on. Could we
pass the patrol of enemy ships around the galaxy's edge without a
challenge, even? Could we—but suddenly there was a low
exclamation from Korus Kan, and I turned to see, racing up beside us
at our left, a close-massed squadron of five great oval ships!
They had glimpsed us on their space-charts, we knew, and now were
flashing beside us through space at a speed the same as our own,
drawing nearer toward us while from their white-lit pilot rooms their
serpent-pilots inspected us. A moment I held my breath, as they
flashed on at our side, peering toward us; then, apparently satisfied
that our great oval craft was but one of their own fleet, they began to
drop behind, to turn and resume their patrol. I breathed a great sigh,
but the next moment caught my breath again, for the foremost of the
five ships, as it dropped behind, had paused at our side, had veered
a little closer as though still unsatisfied. Closer it came, and closer,
until the serpent-creatures in its pilot room were clear to our eyes, as
it and the ships behind it raced on with ourselves through space.
Then suddenly from that foremost ship a signal of brilliant light
flashed to those behind it, and at once all five drove straight toward
us!
"They've seen us!" shouted Jhul Din. "They know we're not of their
own fleet!"
But as he shouted I had leapt to the order-tube, had cried into it a
swift command, and then as the five ships veered in toward us there
leapt from our vessel's sides long, swift shafts of crimson light, the
deadly red rays with which our captured ship had been equipped at
Canopus, narrow brilliant shafts that touched the two foremost of
those five racing ships and annihilated them even as they sprang
toward us. The other three were leaping on, though, their death-
beams reaching like great fingers of ghostly light through the void
toward us, and I knew that we could not hope to escape them by
flight, since they were as swift as our own craft; so in a moment I
made decision, and shouted to Korus Kan to head our ship about.
Around we swept, in one great lightning curve, and then were
rushing straight back upon the three racing ships. Into and between
them we flashed, death-beams and red rays stabbing thick through
the void in the instant that we passed them. I saw one of the great
pale beams slice down through the rear end of our ship, heard shouts
from beneath as those of our crew in that end were wiped out of
existence, and then we were past, were turning swiftly in space and
flashing back outward again, and saw that two of the three ships
before us were visible only as great crimson flares, the other ship
hanging motionless for the moment as though stunned by the
destruction of its fellows.
"Four gone!" yelled Jhul Din, as we flashed toward the last of the five
ships.
That last ship, though, paused only a moment as we raced toward it,
and then suddenly flashed away into the void to the right, vanishing
instantly from sight as it raced in flight toward the Cancer cluster. We
had destroyed and routed the squadron that had challenged us, had
broken through the enemy's great patrol! Korus Kan was opening our
power-controls to the utmost, and now the throbbing and beating of
the great generators beneath was waxing into a tremendous,
thrumming drone, as we shot outward into space, the Cancer cluster
falling behind us as we flashed out at a tremendous and still steadily
mounting speed.
Out—out—into the vast black vault of sheer outer space that lay
stretched before and about us now, the awful velocity of our great
craft increasing by tens of thousands, by hundreds of thousands of
light-speeds, as we shot out into the untrammeled void. Behind us
the mighty, disk-like mass of flaming stars that was our universe was
contracting in size each moment, dwindling and diminishing, but
before us there glowed out in the vast blackness misty little patches
of light, universes of suns inconceivably remote from our own.
Strongest among them glowed a single light-patch, full before us, and
it was on it that our eyes were fixed as our ship at utmost speed
plunged on. It was the Andromeda universe, and we were flashing
out into the mighty void of outer space toward it at a full ten million
light-speeds, to seek the help which alone could save our universe
from doom!
 6. Into the Infinite
Standing at the controls, his tireless metal figure erect as he gazed
out into the vast blackness of cosmic space that lay before us, Korus
Kan turned from that gaze toward me as I stepped inside the pilot
room. Silently I stepped over beside him, and silently, as was our
wont, we contemplated the great panorama before us. A stupendous
vault of sheer utter darkness it stretched about us, darkness broken
only by the misty light of the great universes of thronging suns that
floated here and there in this vast void through which we were
racing. Behind us our own galaxy lay, just another of those dim
glows; for hours had passed since we had launched out into outer
space from its edge, and in those hours our awful speed had carried
us on through the void through thousands of light-years of space.
But though in those hours of flight our own universe had dwindled to
a mere mist of light, those other misty patches that were the
universes ahead had hardly grown at all in size or intensity of light,
making us realize that even the vast expanse of space through which
our ship had already flashed was but a fraction of the gulf that lay
between us and the great Andromeda universe. Before us the soft
glow that was that universe seemed a little brighter, a little larger, but
even so I knew that more than a score of days must elapse before
even our ship's tremendous velocity would bring us to it. And even
were we able to secure the help we needed, it would still be many
days before we could flash back to our own galaxy, and in those
days, I well knew, the serpent-invaders would be completing their
last plans, tightening their grip on all the suns and worlds of the
Cancer cluster, and preparing the way for the vast hordes that soon
would cross the void to pour down on that cluster, spreading
resistlessly from it across all our galaxy.
It was with heavy heart that I gazed ahead, knowing these things,
but my gloomy thoughts were suddenly interrupted by an
exclamation from Korus Kan, who had been peering intently forward
into the tenebrous void, and who now pointed ahead, toward the
right.
"That flicker of light," he said: "you see it?"
I bent forward, gazing to where he was pointing in the heavens
before us, and then at last made out in the blackness, not far to the
right of the glowing Andromeda universe, another patch of light of
equal size, but one whose light was so dim as only to be seen with
straining eyes. A mere dim flicker of light it was, in that crowding
darkness, but as I gazed at it the nature of it suddenly came clearly
to my mind, and I uttered a low exclamation myself.
"The universe of the serpent-creatures!" I said. "It's the dying
universe from which they came to invade our own!"
He nodded. "Yes. It's nearer the Andromeda universe than our own,
too."
I saw that he was right, and that the two universes, that of
Andromeda and this dim, dying one, lay comparatively close to each
other, and at almost equal distances from our own, the two forming
the base of a long, narrow triangle of which our own universe was
the apex. Together we gazed toward that dim flicker of light, in a
thoughtful silence. We knew, even as we gazed, what great
preparations were going on in that dying universe for the conquest of
our own galaxy, what mighty efforts the serpent-races there were
making, to complete their vast fleet and the strange, huge weapon
which the records we had captured had mentioned, so that they
could flash through the void to pour down on our galaxy. The
knowledge held us wrapped in thought as our great ship raced on,
still holding to its tremendous utmost velocity, rocking and swaying a
little as it plunged through the vast ether-currents which swirled
about us here in outer space.
Gradually, as we two stood in silence with our great craft speeding
on, I became aware that during the last few minutes the air inside
the pilot room had become perceptibly warmer, and that its warmth
was still increasing. I glanced at the dial that registered the output of
our heat-generators, but it was steady at its accustomed position; yet
with each moment the warmth was increasing, until within a few
minutes more the heat about us had become decidedly
uncomfortable. Korus Kan, too, had noticed it, and had now swung
backward the control of the heat-generators; yet still the warmth
increased, the heated air in the pilot room rapidly becoming
unbearable. I turned to the Antarian, fully alarmed now, but as I did
so the door snapped open and Jhul Din burst up into the pilot room.
"What's happening to the ship?" he cried. "Its inner walls are getting
almost too hot to touch!"
In stunned surprize we gazed at each other, our heating-mechanisms
turned completely off now, yet the inside-temperature dial's arrow
was still moving steadily forward! The thing was beyond all reason,
we knew, and for an instant we stood in amazement, the heat
increasing still about us. Then suddenly Jhul Din pointed upward
toward the massed dials above the controls, his arm quivering.
"Look!" he cried. "The outside-temperature dial!"
Swiftly we raised our own eyes toward it, the dial upon which was
shown the temperature outside the ship. It should have shown
absolute zero, we knew, as always in the infinite cold of empty space.
But now it did not, and our eyes widened as we stared at it, in utter
astonishment and fear. For it registered a temperature of thousands
of degrees in the empty void about us!
"Heat!" I cried. "Heat in empty outer space! It's unthinkable!"
Unthinkable it was; yet, even as we stood and stared, the arrow on
the outside-temperature was still creeping steadily forward, showing
a swiftly increasing heat outside, while the air inside had become all
but unbreathable, parching to the lungs. At the same moment a faint
light began to appear about us, a dim-red glow that was intensifying
with each moment that we raced onward, and as we wheeled toward
the windows we saw, in the blackness of space before us, a great,
faintly glowing region of red light ahead, stretching across the
heaven before us. Ever stronger that crimson glow was growing as
we raced on, the heat about us mounting with it, and from beneath
came the cries of fear of our crew as they too glimpsed the awful
region of heat and light through which we now were racing.
I knew that not much longer could the heat about us increase thus if
our ship and ourselves were to survive, yet steadily the arrows on the
temperature-dials were moving forward, and as more and more of
the awful heat outside penetrated through the insulation of our heat-
resistant walls I felt my brain turning dizzily, saw big Jhul Din stagger
and sway against the wall, and saw Korus Kan, the heat penetrating
through his metal body even more than through our own, slumping
sidewise across the controls as he was overcome by it, only half
conscious. I sprang to his side, despite my own dizziness and
parching throat and lungs, grasped the controls and held our ship
straight onward, since all about us the vast glow of crimson light and
heat stretched, encircling us and beating upon us as we flashed
onward. No flame there was, nor incandescent gas, nor solid burning
matter of any kind, nothing but a titanic region of brilliant crimson
light, without visible source of any kind, glowing with terrific heat
there in the emptiness of outer space.

The glow about us was becoming more brilliant with each moment
that we raced on, and as the heat outside and inside increased still
more I saw Jhul Din fling open the pilot room's door in a vain search
for cooler air; heard from beneath a rumbling, ominous thumping and
cracking, as our heat-seared walls began to warp in the terrible
temperature to which they were being subjected. Far ahead in the
awful region of heat and light through which we were speeding I
glimpsed now a deeper spot of crimson light in the great red glow,
and as we raced on toward it I saw that it was the center of all the
great outpouring of red light and of heat, since it was all but blinding
in its brilliance, while our dials showed a temperature mounting each
moment that we neared it.
"It's the center of the whole thing!" cried Jhul Din, staggering toward
me and then slumping down to the floor, overcome. "Keep the ship
clear of it!" he shouted, collapsing as he did so, while beside me I
saw Korus Kan, completely unconscious, neither the great crustacean
Spican nor the metal-bodied Antarian possessing my own resistance
to the heat that now was smothering us, though I too knew that not
much longer could I hold to the controls.
Hold to them I did, though, but half conscious now myself; then as
there flamed dead ahead the heart of the whole great inferno, a
blazing area of brilliant crimson light that dazzled me, its terrific heat
pouring full down upon our plunging ship, I swung the controls
sidewise, swerving our craft to the left and around the great heat-
region's fiery heart. Along its side we flashed, our ship plunging and
reeling now as it shot through ether-currents that must have been of
unparalleled size and speed, but even in that darkness that was