AWS welding handbook VOL 2 9th ed (2004) welding processes part 1

Welding
Handbook
Ninth Edition
Volume 2

WELDING PROCESSES, PART 1
Prepared under the direction of the
Welding Handbook Committee
Annette O’Brien
Editor

American Welding Society
550 N.W. LeJeune Road

Miami, FL 33126


02004 by American Welding Society
All rights reserved

No portion of this book may be reproduced, stored in a retrieval system, or transmitted in any form or by any
means, including mechanical, photocopying, recording, or otherwise, without the prior written permission of the
copyright owner.
Authorization to photocopy items for internal, personal, or educational classroom use only, or the internal, personal, or educational classroom use only of specific clients, is granted by the American Welding Society (AWS)provided the appropriate fee is paid to the Copyright Clearance Center, 222 Rosewood Drive, Danvers, MA 01923;
telephone: (978) 750-8400; Internet: www.copyright.com.
Library of Congress Control Number: 2001089999
ISBN: 0-87171-729-8
The Welding Handbook is the result of the collective effort of many volunteer technical specialists who provide
information to assist with the design and application of welding and allied processes.
The information and data presented in the Welding Handbook are intended for informational purposes only. Reasonable care is exercised in the compilation and publication of the Welding Handbook to ensure the authenticity of
the contents. However, no representation is made as to the accuracy, reliability, or completeness of this information, and an independent substantiating investigation of the information should be undertaken by the user.

The information contained in the Welding Handbook shall not be construed as a grant of any right of manufacture, sale, use, or reproduction in connection with any method, process, apparatus, product, composition, or system, which is covered by patent, copyright, or trademark. Also, it shall not be construed as a defense against any
liability for such infringement. Whether the use of any information in the Welding Handbook would result in an
infringement of any patent, copyright, or trademark is a determination to be made by the user.
Printed in Canada


PREFACE
Welding Processes, Part 2 is the second of the five volumes of the 9th edition of the Welding Handbook. The
fifteen chapters of this volume provide updated information on the arc welding and cutting processes, oxyfuel gas
welding and cutting, brazing, and soldering. Volume 3, Welding Processes, Part 2 will cover resistance, solid state,
and other welding and cutting processes. Volumes 4 and 5 of the Welding Handbook will address welding materials and applications. These volumes represent the practical application of the principles discussed in the chapters
of Volume 1, Welding Science and Technology, published in 2001.
This peer-reviewed volume of the Welding Handbook reflects a tremendous leap forward in welding technology.
While many basics of the welding processes have remained substantially the same, the precise control of welding
parameters, advanced techniques, complex applications and new materials discussed in this updated volume are
dramatically changed from those described in previous editions. In particular, advancements in digital or computerized control of welding parameters have resulted in consistently high weld quality for manual and mechanized
welding and the repeatability necessary for successful automated operations.
Chapter 1 of Welding Processes, Part 2 is a compilation of information on arc welding power sources. Subsequent
chapters present specific information on shielded metal arc welding, gas tungsten arc welding, gas metal arc welding, flux cored arc welding, submerged arc welding, plasma arc welding, electrogas welding, arc stud welding, electroslag welding, oxyfuel gas welding, brazing, soldering, oxygen cutting, and arc cutting and gouging.
Appendix A and B address safety issues. Appendix A reproduces the American Welding Society Lens Shade Selector. Appendix B is a list of national and international safety standards applicable to welding, cutting, and allied
processes. Although each chapter in this volume has a section on safe practices as they pertain to the specific process, readers should refer to Chapter 17, “Safe Practices,” of Volume 1 and to the appropriate standards listed in
Appendix B. Appendix C is a list of American Welding Society filler metal specifications and related documents.
An index of this volume and a major subject index of previous volumes are included.
This volume was compiled by the members the Welding Handbook Volume 2 Committee and the Chapter Committees, with oversight by the Welding Handbook Committee. Chapter committee chairs, chapter committee
members, and oversight persons are recognized on the title pages of the chapters. An important contribution to
this volume is the review of each chapter provided by members of the Technical Activities Committee and the
Safety and Health Committee of the American Welding Society.
The Welding Handbook Committee welcomes your comments and suggestions. Please address them to the Editor,
Welding Handbook, American Welding Society, 550 N.W. LeJeune Road, Miami, Florida 33126.
Harvey R. Castner, Chair

Welding Handbook Committee

Ian D. Harris, Chair
Volume 2 Committee

Annette O’Brien, Editor
Welding Handbook

xiii


ACKNOWLEGMENTS ......................................................................................................................................xii
...
PREFACE ...........................................................................................................................................................xlii
REVIEWERS ......................................................................................................................................................
xiv
CONTRIBUTORS ............................................................................................................................................. xv
CHAPTER 1 A R C POWER SOURCES..................................................................................................1
Introduction .........................................................................................................................................................
2
Fundamentals ....................................................................................................................................................
2
Principles of Operation ..........................................................................................................................................4
Volt-Ampere Characteristics ...............................................................................................................................12
Duty Cycle .......................................................................................................................................................16
Open-Circuit Voltage.........................................................................................................................................17
NEMA Power Source Requirements ....................................................................................................................19
Alternating-Current Power Sources .....................................................................................................................20
Direct-Current Power Sources ..........................................................................................................................30
Economics ......................................................................................................................................................42

Safe Practices .......................................................................................................................................................44
Conclusion .....................................................................................................................................................48
Bibliography ....................................................................................................................................................... 48
CHAPTER 2-SHIELDED METAL ARC WELDING .................................................................................51
Introduction ...................................................................................................................................................52
Fundamentals .................................................................................................................................................... 52
Equipment ...........................................................................................................................................................60
Materials ...........................................................................................................................................................
68
Applications ......................................................................................................................................................80
Joint Design and Preparation ...........................................................................................................................82
Welding Variables ..............................................................................................................................................85
Weld Quality ......................................................................................................................................................96
Economics ........................................................................................................................................................
98
Safe Practices ...................................................................................................................................................... 99
Conclusion ...................................................................................................................................................101
Bibliography ......................................................................................................................................................101
CHAPTER 3 - G A S TUNGSTEN ARC WELDING ..................................................................................103
Introduction ...................................................................................................................................................
104
Fundamentals .................................................................................................................................................104
Applications ......................................................................................................................................................107
Equipment .......................................................................................................................................................109
Techniques.......................................................................................................................................................128
Materials ...........................................................................................................................................................
135
Joint Design .......................................................................................................................................................
139
Weld Quality .................................................................................................................................................140

Economics ......................................................................................................................................................... 142
Safe Practices .................................................................................................................................................... 142
Conclusion ......................................................................................................................................................
144
Bibliography ...................................................................................................................................................... 144
CHAPTER &GAS METAL ARC WELDING .......................................................................................... 147
Introduction .................................................................................................................................................148

vii


Fundamentals .....................................................................................................................................................148
Principles of Operation ......................................................................................................................................................
150
Equipment ..........................................................................................................................................................160
Materials and Consumables ............................................................................................................................... 171
Process Variables ................................................................................................................................................178
Weld Joint Designs ............................................................................................................................................. 188
Inspection and Weld Quality ..............................................................................................................................
189
Troubleshooting .................................................................................................................................................
195
Economics ..........................................................................................................................................................
199
Safe Practices......................................................................................................................................................
201
Conclusion .........................................................................................................................................................
203
Bibliography.......................................................................................................................................................204

CHAPTER !%-FLUX CORED ARC WELDING ......................................................................................... 209
Introduction .....................................................................................................................................................-210
Fundamentals ....................................................................................................................................................-210
Applications .......................................................................................................................................................
211
Equipment ..........................................................................................................................................................
215
Materials ...........................................................................................................................................................
-219
Process Control ..................................................................................................................................................
237
Joint Designs and Welding Procedures ...............................................................................................................
241
Weld Quality ......................................................................................................................................................
247
Troubleshooting .................................................................................................................................................
247
Economics.........................................................................................................................................................
-247
Safe Practices......................................................................................................................................................
250
Conclusion ........................................................................................................................................................
-252
Bibliography.......................................................................................................................................................
252
CHAPTER 6. SUBMERGED ARC WELDING .........................................................................................
255
Introduction .......................................................................................................................................................
256
Fundamentals .....................................................................................................................................................

256
Equipment .........................................................................................................................................................
-258
Materials ............................................................................................................................................................
268
Process Variables ...............................................................................................................................................
-278
Operating Procedures .........................................................................................................................................
282
Process Variations and Techniques .....................................................................................................................287
Applications ....................................................................................................................................................... 294
Weld Quality ...................................................................................................................................................... 297
Economics......................................................................................................................................................... -299
Safe Practices...................................................................................................................................................... 299
Conclusion ......................................................................................................................................................... 300
Bibliography....................................................................................................................................................... 300
CHAPTER 7-PLASMA ARC WELDING .................................................................................................. 303
Introduction ....................................................................................................................................................... 304
Fundamentals .....................................................................................................................................................
305
Equipment ..........................................................................................................................................................
310
Materials ............................................................................................................................................................
319
Application Methods ..........................................................................................................................................
324
Process Variations ..............................................................................................................................................
326
Welding Procedures ............................................................................................................................................
332

Weld Quality ......................................................................................................................................................
332
Economics..........................................................................................................................................................
332


Safe Practices .....................................................................................................................................................
334
Conclusion ........................................................................................................................................................335
Bibliography ......................................................................................................................................................
33.5
CHAPTER 8-ELECTROGAS WELDING ................................................................................................ 337
Introduction ......................................................................................................................................................338
Fundamentals ....................................................................................................................................................
338
Equipment .........................................................................................................................................................
343
Materials ...........................................................................................................................................................348
Process Variables ...............................................................................................................................................
350
Applications ......................................................................................................................................................
366
Joint Design .......................................................................................................................................................
367
Inspection and Weld Quality .............................................................................................................................369
Economics .........................................................................................................................................................
387
Safe Practices .....................................................................................................................................................
387
Conclusion ..................................................................................................................................................390

390
Bibliography ......................................................................................................................................................
CHAPTER 9A
.
RC STUD WELDING .......................................................................................................
393
Introduction ......................................................................................................................................................
394
Fundamentals ....................................................................................................................................................
394
Applications ......................................................................................................................................................
395
Equipment and Technology ...............................................................................................................................
398
406
Designing for Arc Stud Welding.........................................................................................................................
Special Process Techniques ...............................................................................................................................
416
Capacitor Discharge Stud Welding ....................................................................................................................
417
Stud Welding Process Selection..........................................................................................................................
423
427
Weld Quality, Inspection, and Testing................................................................................................................
Economics .........................................................................................................................................................
430
432
Safe Practices .....................................................................................................................................................
Conclusion ........................................................................................................................................................433
433

Bibliography ......................................................................................................................................................
CHAPTER 10-ELECTROSLAG WELDING............................................................................................
435
Introduction .....................................................................................................................................................
436
Fundamentals ................................................................................................................................................... 4 3 6
Equipment ........................................................................................................................................................
441
444
Materials ..........................................................................................................................................................
Welding Variables ..............................................................................................................................................
446
Welding Procedures ...........................................................................................................................................
448
Applications ......................................................................................................................................................
455
Inspection and Quality Control .........................................................................................................................457
Weld Quality .....................................................................................................................................................
459
Economics .........................................................................................................................................................
460
Safe Practices .....................................................................................................................................................463
Conclusion ........................................................................................................................................................
464
464
Bibliography .....................................................................................................................................................
CHAPTER 11 4 X Y F U E L GAS WELDING ........................................................................................... 467
468
Introduction ......................................................................................................................................................
Fundamentals of Oxyfuel Gas Welding........................................................................................................... 468

471
Materials ...........................................................................................................................................................


Oxyfuel Gas Welding Equipment ....................................................................................................................... 479
Process Variables and Operating Procedures ......................................................................................................
489
Applications .......................................................................................................................................................491
Weld Quality ......................................................................................................................................................494
Welding with Other Fuel Gases ..........................................................................................................................495
Economics ......................................................................................................................................................... -495
Safe Practices......................................................................................................................................................495
Conclusion .........................................................................................................................................................
498
Bibliography.......................................................................................................................................................498
CHAPTER 12-BRAZING ...........................................................................................................................501
Introduction .......................................................................................................................................................502
Fundamentals .....................................................................................................................................................502
Applications ....................................................................................................................................................... 503
Principles of Operation ...................................................................................................................................... 503
Processes, Equipment, and Techniques ............................................................................................................... 504
Automation ........................................................................................................................................................ 515
Materials ............................................................................................................................................................
517
Joint Design .......................................................................................................................................................
532
Procedures..........................................................................................................................................................
541
Inspection...........................................................................................................................................................544
Troubleshooting .................................................................................................................................................

546
Braze Welding ....................................................................................................................................................
546
Economics .......................................................................................................................................................... 550
Safe Practices...................................................................................................................................................... 550
Conclusion .........................................................................................................................................................555
Bibliography .......................................................................................................................................................
555
CHAPTER 13.4 OLDERING ......................................................................................................................
559
Introduction .......................................................................................................................................................
560
Fundamentals ..................................................................................................................................................... 560
Applications
561
Process Variations .............................................................................................................................................. 563
Equipment..........................................................................................................................................................
570
Materials ............................................................................................................................................................
571
Procedures.......................................................................................................................................................... 584
Process Variables ...............................................................................................................................................
-588
Inspection and Testing........................................................................................................................................
590
Economics ..........................................................................................................................................................592
Safe Practices...................................................................................................................................................... 593
Conclusion ......................................................................................................................................................... 594
Bibliography .......................................................................................................................................................
594

.......................................................................................................................................................

CHAPTER 1&OXYGEN CUTTING .........................................................................................................
597
Introduction .......................................................................................................................................................
598
Fundamentals of Oxygen Cutting .......................................................................................................................
598
Oxyfuel Gas Cutting ......................................................................................................................................... -599
Equipment......................................................................................................................................................... -602
Gases.................................................................................................................................................................. 608
Operating Procedures.........................................................................................................................................
612
..............................................................................................................................................
Process
Variations
617
. .
Appllcatlons .......................................................................................................................................................
620
Quality ...............................................................................................................................................................
626


Oxygen Arc Cutting ..........................................................................................................................................630
Oxygen Lance Cutting....................................................................................................................................... 630
Metal Powder Cutting .......................................................................................................................................
631
632

Flux Cutting ......................................................................................................................................................
Economics .........................................................................................................................................................632
633
Safe Practices ....................................................................................................................................................
Conclusion ........................................................................................................................................................635
Bibliography......................................................................................................................................................635

CHAPTER 15-ARC CUTTING AND GOUGING...................................................................................
637
Introduction ...................................................................................................................................................... 638
Plasma Arc Cutting ...........................................................................................................................................638
Plasma Arc Gouging ..........................................................................................................................................648
Air Carbon Arc Cutting..................................................................................................................................... 651
Other Arc Cutting Processes ..............................................................................................................................
659
Economics ........................................................................................................................................................
662
Safe Practices ..................................................................................................................................................... 665
Conclusion ........................................................................................................................................................
669
Bibliography .................................................................................................................................................. 670
APPENDIX A-LENS

SHADE SELECTOR........................................................................................... 673

APPENDIX B-HEALTH AND SAFETY CODES AND OTHER STANDARDS.................................
675
APPENDIX C-FILLER

METAL SPECIFICATIONS ..............................................................................

679

INDEX OF MAJOR SUBJECTS:
681
Eighth Edition and Ninth Edition. Volume 1 and Volume 2 ...........................................................
INDEX OF NINTH EDITION. Volume 2 ............................................................................................ 699


CHAPTER 1

ARC WELDING
POWER SOURCES

Prepared by the
Welding Handbook
Chapter Committee
on Arc Welding Power
Sources:
S. P. Moran, Chair
Miller Electric
Manufacturing Company
D. J. Erbe
Panasonic Factory
Automation
W, E. Herwig
Miller Electric
Manufacturing Company
W. E. Hoffman
ESA B Welding and Cutting
Products

C. Hsu
The Lincoln Electric
Company
J. 0.Reynolds
Miller Electric
Manufacturing Company
Welding Handbook
Committee Member:
C. E. Pepper
ENGlobal Engineering
Contents
Introduction
Fundamentals
Principles of Operation
Volt-Ampere
Characteristics
Duty Cycle
Open-Circuit Voltage
NEMA Power Source
Requirements
Alternating-Current
Power Sources
Direct-Current
Power Sources
Economics
Safe Practices
Conclusion
Bibliography
Supplementary
Reading List

2 CHAPTER1

ARC WELDING POWER SOURCES

CHAPTER 1

ARC WELD1NG
POWER SOURCES
INTRODUCTION
This chapter presents a general overview of the
electrical power sources used for arc welding. It
explores the many types of welding power sources
available to meet the electrical requirements of the
various arc welding processes.
Welding has a long and rich history. Commercial arc
welding is over a hundred years old, and scores of processes and variations have been developed. Over the
years, power sources have been developed or modified
by equipment manufacturers in response to the changes
and improvements in these processes. As welding processes continue to evolve, power sources continue to
provide the means of controlling the welding current,
voltage, and power. This chapter provides updated
information on the basic electrical technologies, circuits, and functions designed into frequently used
welding power sources. Topics covered in this chapter
include the following:
1. The volt-ampere (V-A) characteristics required
for common welding processes,
2. Basic electrical technologies and terminology
used in power sources,

3. Simplified explanations of commonly used
power source circuits, and
4. An introduction to useful national and international standards.
A basic knowledge of electrical power sources will
provide the background for a more complete understanding of the welding processes presented in the other
chapters of this book.

FUNDAMENTALS
This section introduces the fundamental functions of
welding power sources and the concepts of constant-

voltage (CV) and constant-current (CC) characteristics
required for welding processes.
The voltage supplied by power companies for industrial purposes-120 volts (V), 230 V, 380 V, or 480 Vis too high for use in arc welding. Therefore, the first
function of an arc welding power source is to reduce the
high input or line voltage to a suitable output voltage
range, 20 V to 80 V. A transformer, a solid-state
inverter, or an electric motor-generator can be used to
reduce the utility power to terminal or open-circuit
voltage appropriate for arc welding.
Alternatively, a power source for arc welding may
derive its power from a prime mover such as an internal
combustion engine. The rotating power from an internal combustion engine is used to rotate a generator or
an alternator for the source of electrical current.
Welding transformers, inverters, or generator/
alternators provide high-amperage welding current,
generally ranging from 30 amperes (A) to 1500 A. The
output of a power source may be alternating current
(ac), direct current (dc) or both. It may be constant
current, constant voltage, or both. Welding power

sources may also provide pulsed output of voltage or
current.
Some power source configurations deliver only certain types of current. For example, transformer power
sources deliver ac only. Transformer-rectifier power
sources can deliver either alternating or direct current,
as selected by the operator. Electric motor-generator
power sources usually deliver dc output. A motoralternator delivers ac, or when equipped with rectifiers,
dc.
Power sources can also be classified into subcategories. For example, a gas tungsten arc welding power
source might be identified as transformer-rectifier,
constant-current, ac/dc. A complete description of any
power source should include welding current rating,
duty cycle rating, service classification, and input power


CHAPTER 1

ARC WELDING POWER SOURCES

I

3

WELDING POWER SOURCE
-ARC-

FUSED DISCONNECT SWITCH (OPEN)

CONTROLLING

--

-

CHARACTERISTIC

-

CHASSIS GROUND CONNECTION

--

ELECTRICAL CONNECTION
MECHANICAL STRUCTURE AND CHASSIS

I

Figure 1.1-Basic

Elements of an Arc Welding Power Source

requirements. Special features can also be included such constant. Constant-current power sources are also known
as remote control, high-frequency stabilization, current- as variable-voltagepower sources, and constant-voltage
pulsing capability, starting and finishing current versus power sources are often referred to as constanttime programming, wave balancing capabilities, and potential power sources. These fast-response, solidline-voltage compensation. Conventional magnetic con- state power sources can provide power in pulses over a
trols include movable shunts, saturable reactors, mag- broad range of frequencies.
netic amplifiers, series impedance, or tapped windings.
Solid-state electronic controls may be phase-controlled
silicon-controlled rectifiers (SCRs) or inverter-controlled
semiconductors. Electronic logic or microprocessor cir- CONSTANT-CURRENT ARC WELDING
POWER SOURCES

cuits may control these elements.
Figure 1.1 shows the basic elements of a welding
The National Electrical Manufacturers Association
power source with power supplied from utility lines.
(NEMA)
standard Electric Arc- Welding Power Sources,
The arc welding power source itself does not usually
EW-1:
1988
(R1999), defines a constant-current arc
include the fused disconnect switch; however, this is a
power
source
as one “which has means for adjusting
necessary protective and safety element.
the
load
current
and which has a static volt-ampere
An engine-driven power source would require elecurve
that
tends
to
produce a relatively constant load
ments different from those shown in Figure 1.1. It
current.
At
a
given
load current, the load voltage is

would require an internal combustion engine, an engine
responsive
to
the
rate
at which a consumable metal
speed regulator, and an alternator, with or without a
electrode
is
fed
into
the
arc. When a tungsten electrode
rectifier, or a generator and an output control.
Before the advent of pulsed current welding pro- is used, the load voltage is responsive to the electrodecesses in the 1 9 7 0 ~
welding
~
power sources were com- to-workpiece distance.”lg2 These characteristics are
monly classified as constant current or constant
voltage. These classifications are based on the static 1. National Electrical Manufacturers Association (NEMA), 1988
(R1999),Electric Arc-Welding Power Sources, EW-1: 1988, Washingvolt-ampere characteristics of the power source, not the ton,
D.C.:National Electrical Manufacturers Association, p. 2.
dynamic characteristic or arc characteristics. The term 2. At the time this chapter was prepared, the referenced codes and other
constant is true only in a general sense. A constant- standards were valid. If a code or other standard is cited without a date
voltage output actually reduces or droops slightly as the of publication, it is understood that the latest edition of the document
arc current increases, whereas a constant-current out- referred to applies. If a code or other standard is cited with the date of
publication, the citation refers to that edition only, and it is understood
put gradually increases as the arc length and arc voltage that any future revisions or amendments to the code or standard are not
decrease. In either case, specialized power sources are included; however, as codes and standards undergo frequent revision,
available that can hold output voltage or current truly the reader is advised to consult the most recent edition.

4

ARC WELDING POWER SOURCES

CHAPTER1

such that if the arc length varies because of external
influences that result in slight changes in arc voltage,
the welding current remains substantially constant.
Each current setting yields a separate volt-ampere curve
when tested under steady conditions with a resistive
load. In the vicinity of the operating point, the percentage of change in current is lower than the percentage of
change in voltage.
The no-load, or open-circuit, voltage of constantcurrent arc welding power sources is considerably
higher than the arc voltage.
Constant-current power sources are generally used
for manual welding processes such as shielded metal arc
welding (SMAW), gas tungsten arc welding (GTAW),
plasma arc welding (PAW), or plasma arc cutting
(PAC), where variations in arc length are unavoidable
because of the human element.
When used in a semiautomatic or automated application in which constant arc length is required, external
control devices are necessary. For example, an arcvoltage-sensing wire feeder can be used to maintain constant arc length for gas metal arc welding (GMAW) or
flux cored arc welding (FCAW).In GTAW, the arc voltage
is monitored, and via a closed-loop feedback, the voltage
is used to regulate a motorized slide that positions the
torch to maintain a constant arc length (voltage).

CO NSTANT-VOLTAGE ARC WELD ING
POWER SOURCES
The NEMA EW-1 standard defines a constantvoltage power source as follows: “A constant-voltage
arc welding power source is a power source which has
means for adjusting the load voltage and which has a
static volt-ampere curve that tends to produce a relatively constant load voltage. The load current, at a
given load voltage, is responsive to the rate at which a
consumable electrode is fed into the arc. 7’3 Constantvoltage arc welding is generally used with welding
processes that include a continuously fed consumable
electrode, usually in the form of wire.
A welding arc powered by a constant-voltage source
using a consumable electrode and a constant-speed wire
feed is essentially a self-regulating system, It tends to
stabilize the arc length despite momentary changes in
the torch position. The arc current is approximately
proportional to wire feed for all wire sizes.

“A constant-currendconstant-voltage arc welding power
source is a power source which has the selectable
characteristics of a constant-current arc welding
power source and a constant-voltage arc welding power
source. ’’4
Additionally, some power sources feature an automatic change from constant current to constant voltage
(arc force control for SMAW) or constant voltage to
constant current (current limit control for constantvoltage power sources).

PRINCIPLES OF OPERATION
The basic components of welding power sourcestransformers, series inductors, generators/alternators,
diodes, silicon-controlled rectifiers, and transistors-are
introduced in this section. Simple circuits of reactancecontrolled, phase-controlled, and inverter power

sources are discussed as examples.
Most arc welding involves low-voltage, high-current
arcs between an electrode and the workpiece. The
means of reducing power-system voltage, as shown in
Figure 1.1, may be a transformer or an electric generator or alternator driven by an electric motor.
Electric generators built for arc welding are usually
designed for direct-current welding only. In these
generators, the electromagnetic means of controlling
the volt-ampere characteristic of the arc welding power
source is usually an integral part of the generator and
not a separate element. Unlike generators, alternators
provide ac output that must be rectified to provide a dc
output. Various configurations are employed in the
construction of direct-current generators. They may
use a separate exciter and either differential or cumulative compound winding for selecting and controlling
volt-ampere output characteristics.

WELDING TRANSFORMER

A power source that provides both constant current
and constant voltage is defined by NEMA as follows:

A transformer is a magnetic device that operates on
alternating current. As shown in Figure 1.2, a simple
transformer is composed of three parts: a primary
winding, a magnetic core, and a secondary winding.
The primary winding, with N1 turns of wire (in
Equation l.l),is energized by an alternating-current
input voltage, thereby magnetizing the core. The core
couples the alternating magnetic field into the secondary winding, with N2 turns of wire, producing an output voltage.

3 . See Reference 1, p. 3 .

4. See Reference 1, p. 2.

CONSTANT-CURRENT/CONSTANT-VOLTAGE
POWER SOURCES


ARC WELDING POWER SOURCES

CHAPTER1

t

DC

AC
OUTPUT

I

Figure 1.2-Principal

5

-

Electrical Elements of a Transformer Power Source

Figure 1.2 also illustrates the principal elements of a
welding transformer, with associated components. For a
transformer, the significant relationships between voltages and currents and the turns in the primary and
secondary windings are as follows:

TAPPED

VOLTAGE
INPUT

! il1€-

where
N1 = Number of turns on the primary winding of
the transformer;
N2 = Number of turns on the secondary winding;
El = Input voltage, V;
E2 = Output voltage, V;
Il = Input current, A; and
I2 = Output (load) current, A.

Figure 1.5-Welding mansformer
with Tapped Secondary Winding

Taps in a transformer secondary winding may be
used to change the number of turns in the secondary
winding, as shown in Figure 1.3, to vary the opencircuit (no-load) output voltage. In this case, the tapped
transformer permits the selection of the number of
turns, N2, in the secondary winding of the transformer.
When the number of turns decreases on the secondary

winding, output voltage is lowered because a smaller
proportion of the transformer secondary winding is

in use. The tap selection, therefore, controls the ac
output voltage. As shown in Equation 1.1, the primarysecondary current ratio is inversely proportional to the
primary-secondary voltage ratio. Thus, large secondary
welding currents can be obtained from relatively low
line input currents.

TRANGORMEF


6

ARC WELDING POWER SOURCES

CHAPTER1

Eo essentially equals the no-load (open-circuit) voltage

SERIES REACTOR
A transformer may be designed so that the tap selection directly adjusts the output volt-ampere slope characteristics for a specific welding condition. More often,
however, an impedance source is inserted in series with
the transformer secondary windings to provide this
characteristic, as shown in Figure 1.4. The impedance is
usually a magnetic device called a reactor when used in
an ac welding circuit and an inductor when used in a dc
welding circuit. Reactors are constructed with an electrical coil wound around a magnet core; inductors are
constructed with an electrical coil wound around a
magnet core with an air gap.

Some types of power sources use a combination of
these arrangements, with the taps adjusting the opencircuit (or no-load) voltage, Eo, of the welding power
source and the series impedance providing the desired
volt-ampere slope characteristics.
In constant-current power sources, the voltage drop
across the impedance, E (shown in Figure 1.4)
increases greatly as the loa2 current is increased. This
increase in voltage drop, Ex, causes a large reduction in
the arc voltage, EA. Adjustment of the value of the series
impedance controls the Ex voltage drop and the
relation of load current to load voltage. This is called
current control, or in some cases, slope control. Voltage

of the power source.
As shown in Figure 1.5, the series impedance in
constant-voltage power sources is typically small, and
the transformer output voltage is very similar to that
required by the arc. The voltage drop, Ex, across the
impedance (reactor) increases only slightly as the load
current increases. The reduction in load voltage is
small. Adjustment in the value of reactance gives slight
control of the relation of load current to load voltage.
This method of slope control, with simple reactors,
also serves as a method to control voltage with saturable reactors or magnetic amplifiers. Figure 1.5 shows an
ideal vector diagram of the relationship of the alternating voltages for the circuit of Figure 1.4, when a reactor
is used as an impedance device. The no-load voltage
equals the voltage drop across the impedance plus the
load voltage when these are added vectorially. Vectorial
addition is necessary because the alternating load and
impedance voltages are not in time phase. In Figure 1.5,

the open-circuit voltage of the transformer is 80 V, the
voltage drop across the reactor is 69 V and the arc load
voltage is 40 V.
The voltage drop across the series impedance, Ex, in
an ac circuit is added vectorially to the load voltage, EA,
to equal the transformer secondary voltage, Eo. By varying the voltage drop across the impedance, the load or

EX

I1
IMPEDANCE

L - - J
AC
INPUT
VO LTAGE

I
I

TRANSFORMER

I
I

OUTPUT
VOLTAGE
EO

ARC

VOLTAGE
EA

Key:
EA = Arc voltage
Eo = No-load voltage
Ex = Voltage drop across impedance

Figure 1.4-Typical

Series Impedance Control of Output Current

8

ARC
8


CHAPTER1

ARC WELDING POWER SOURCES

EX
IOLTAGE
DROP
69 V

*
EA ARC VOLTAGE
40 V

Key:
E, = Arc voltage
E, = No-load voltage
Ex = Voltage drop across impedance

Figure 1.ti-ldeal Vector Relationship
of the Alternating-Voltage Output
Using Reactor Control

arc voltage may be changed. This distinctive characteristic of vectorial addition for impedance voltages in ac
circuits is related directly to the fact that both reactance
and resistances may be used to produce a drooping
voltage characteristic. An advantage of a reactor is that
it consumes little or no power, even though a current
flows through it and a voltage is developed across it.
When series resistors are used, power is lost and the
temperature of the resistor rises. Theoretically, in a
purely resistive circuit (no reactance), the voltage drop
across the resistor could be added arithmetically to the
load voltage to equal the output voltage of the transformer. For example, a power source with an approximately constant-current characteristic, an 80-V open
circuit, and powering a 25-V, 200-A arc would need to
dissipate 55 V x 200 A, or 11,000 watts (W), in the
resistor to supply 5000 W to the arc. The reason is that
the voltage and current are in phase in the resistive circuit. A resistance and reactance circuit phase shift
accounts for the greatly reduced power loss.

7

Another major advantage of inductive reactance is
that the phase shift produced in the alternating current

by the reactor improves ac arc stability for a given
open-circuit voltage. This is an advantage with the
GTAW and SMAW processes.
The inductive reactance of a reactor can be varied by
several means. One way is by changing taps on a coil or
by other electrical or mechanical schemes. Varying the
reactance alters the voltage drop across the reactor.
Thus, for any given value of inductive reactance, a
specific volt-ampere curve can be plotted. This creates
the required control feature of these power sources.
In addition to adjusting series reactance, the mutual
inductance between the primary and secondary coils of
a transformer can also be adjusted. This can be done by
moving the coils relative to one another or by using a
movable magnetic shunt that can be inserted or withdrawn from between the primary and secondary windings. These methods change the magnetic coupling of
the coils to produce adjustable mutual inductance,
which is similar to series inductance.
In ac/dc welding power sources incorporating a rectifier, the rectifier is located between the magnetic control
devices and the output terminal. In addition, transformerrectifier arc welding power sources usually include a
stabilizing inductance, or choke, located in the dc welding circuit to improve arc stability.

GENERATOR AND ALTERNATOR
Rotating machinery is also used as a source of power
for arc welding. These machines are divided into two
types-generators that produce direct current and alternators that produce alternating current.
The no-load output voltage of a direct-current generator can be controlled with a relatively small variable
current in the winding of the main or shunt field. This
current controls the output of the direct-current generator winding that supplies the welding current. The
output polarity can be reversed by changing the interconnection between the exciter and the main field. An
inductor or filter reactor is not usually needed to

improve arc stability with this type of welding equipment. Instead, the several turns of series winding on the
field poles of the rotating generator provide more than
enough inductance to ensure satisfactory arc stability.
These generators are described in greater detail in
following sections of this chapter.
An alternator power source produces alternating
current that is either used in that form or rectified into
direct current. It can use a combination of the means of
adjustment previously mentioned. A tapped reactor can
be employed for gross adjustment of the welding output, and the field strength can be controlled for fine
adjustment.


8

CHAPTER1

ARC WELDING POWER SOURCES

SOLID-STATE DIODES

high reverse-voltage transient will damage it. Most
rectifier power sources have a resistor, capacitor, or
other electronic devices, commonly called snubber networks,to suppress voltage transients that could damage
the rectifiers.

The term solid-stute is related to solid-state physics
and the study of crystalline solids. Methods have been
developed for treating crystalline materials to modify
their electrical properties. The most important of these

materials is silicon.
Transformer-rectifier and alternator-rectifier power
sources rely on rectifiers, or groups of diodes, to con- SILICON-CONTROLLED RECTIFIER
vert alternating current to direct current. In earlier (THYRISTOR)
times, welding circuits relied on vacuum tube and
Solid-state devices with special characteristics can
selenium rectifiers, but most modern rectifiers are made
of silicon for reasons of economy, current-carrying also be used to control welding power directly by altering the welding current or voltage wave form. These
capacity, reliability, and efficiency.
A single rectifying element is called a diode, which is solid-state devices have replaced saturable reactors,
a one-way electrical valve. When placed in an electrical moving shunts, moving coils, and other systems as concircuit, a diode allows current to flow in one direction trol elements in large industrial power sources. One of
only, when the anode of the diode is positive with the most important of these devices is the siliconrespect to the cathode. Using a proper arrangement of controlled rectifier (SCR), sometimes called a thyristor.
diodes, it is possible to convert alternating current to
The SCR is a diode variation with a trigger, called a
direct current. An example of a diode symbol and a gate, as shown in Figure 1.7. An SCR is non-conducting
stud diode is shown in Figure 1.6.
until a positive electrical signal is applied to the gate.
As current flows through a diode, a voltage drop When this happens, the device becomes a diode and
across the component develops and heat is produced conducts current as long as the anode is positive with
within the diode. Unless this heat is dissipated, the respect to the cathode. However, once it conducts, the
diode temperature can increase enough to cause failure. current cannot be turned off by a signal to the gate.
Therefore, diodes are normally mounted on heat sinks Conduction ceases only if the voltage applied to the
(aluminum plates, many with fins) to remove the heat.
anode becomes negative with respect to the cathode.
Diodes have limits as to the amount of voltage they Conduction will not take place again until a positive
can block in the reverse direction (anode negative and voltage is applied to the anode and another gate signal
cathode positive). This is expressed as the voltage rating is received.
of the device. Welding power-source diodes are usually
Silicon-controlled rectifiers are used principally in
selected with a blocking rating at least twice the open- the phase-control mode with isolation transformers and

circuit voltage in order to provide a safe operating in some inverter configurations. The output of a weldmargin.
ing power source can be controlled by using the action
A diode can accommodate repetitive current peaks of a gate signal to selectively turn on the SCR. A typical
well beyond its normal steady-state rating, but a single single-phase SCR circuit is shown in Figure 1.8.

ANODE

--

CATHODE

GATE\+
ANODE

Figure I.6-Stud Diode (A)
and Diode Symbol (B)

Figure 1.7-Silicon-Controlled Rectifier (A)
and Silicon-Controlled Rectifier Symbol ( 8 )


ARC WELDING POWER SOURCES

T

T

B
Key:
T =

A =
B =
T =
Z =

Isolation transformer
Top or start of the transformer secondary winding
Bottom or end of the transformer secondary winding
Isolation transformer
DC inductor, with reactance and resistance

Figure 1.8-SinglemPhase DirecbCurrent
Power Source Using an SCR Bridge for Control

In Figure 1.8, during the time that Point A is positive
with respective to Point By no current will flow until
both SCR 1 and SCR 4 receive gate signals to turn on.
At that instant, current will flow through the load. At
the end of that half-cycle, when the polarity of A and B
reverses, a negative voltage will be impressed across
SCR 1 and SCR 4,and they will turn off. With Point B
positive relative to Point A, gate signals applied to SCR
2 and SCR 3 by the control will cause these two to conduct, again applying power to the load circuit. To
adjust power in the load, it is necessary to precisely time
when, in any given half-cycle, the gate triggers the SCR
into conduction. With a 60-hertz (Hz) line frequency,
this arrangement produces direct current with a 120-Hz
ripple frequency at the arc or load.
The timing of the gate signals must be precisely controlled. This is a function of the control block shown in
Figure 1.8. To adapt the system satisfactorily for welding service, another feature, feedback, is necessary. The

nature of the feedback depends on the welding parameter to be controlled and the degree of control required.
To provide constant-voltage characteristics, the feedback (not shown) must consist of a signal that is proportional to arc voltage. This signal controls the precise
arc voltage at any instant so that the control can properly time and sequence the initiation of the SCR to hold
a voltage pre-selected by the operator. The same effect

CHAPTER1

9

is achieved with constant current by using feedback and
an operator-selected current.
Figure 1.8 shows a large inductance, Z , in the load
circuit. For a single-phase circuit to operate over a significant range of control, Z must be a large inductance
to smooth out the voltage and current pulses. However,
if SCRs were used in a three-phase circuit, the nonconducting intervals would be reduced significantly.
Since three times as many output pulses are present in
any time period, the inductance would also be significantly reduced.
When high power is required, conduction is started
early in the half-cycle, as shown in Figure 1.9(A).If low
power is required, conduction is delayed until later in a
half-cycle, as shown in Figure 1.9(B). This is known as
phase control. The resulting power is supplied in pulses
to the load and is proportional to the shaded areas in
Figure 1.9 under the wave form envelopes. Figure 1.9
illustrates that significant intervals may exist when no
power is supplied to the load. This can cause arc outages, especially at low power levels. Therefore, wave
filtering is required.
Most intermediate-sized or commercial SCR phasecontrolled welding power sources are single-phase.
Larger industrial SCR phase-controlled power sources
are three-phase. Single- and three-phase power sources

are the constant-current or constant-voltage type. Both
constant-current and constant-voltage types have distinct features because the output characteristics are
controlled electronically. For example, automatic linevoltage compensation is very easily accomplished,
allowing welding power to be held precisely as set, even
if the input line voltage varies. Volt-ampere curves
can also be shaped and adapted for a particular
welding process or its application. These power sources
can adapt their static characteristic to any welding
process, from one approaching a truly constant voltage
to one having a relatively constant current. They are
also capable of producing a controlled pulsed arc
voltage and a high initial current or voltage pulse at the
start of the weld.
An SCR can also serve as a secondary contactor,
allowing welding current to flow only when the control
allows the SCRs to conduct. This is a useful feature in
rapid cycling operations, such as spot welding and tack
welding. However, an SCR contactor does not provide
the electrical isolation that a mechanical contactor or
switch provides. Therefore, a primary circuit breaker or
some other device is required to provide isolation for
electrical safety.
Several SCR configurations can be used for arc
welding. Figure 1.10 depicts a three-phase bridge with
six SCR devices. With a 60-HZ line frequency, this
arrangement produces direct current, with a 360-Hz
ripple frequency at the load. It also provides precise
control and quick response; in fact, each half-cycle of

10

CHAPTER 1

ARC WELDING POWER SOURCES

(A) High-Power Conduction of SCR Early in Each Half-Cycle

I

(8)Lower-Power Conduction of SCR Late in Each Half-Cycle

Figure 1.9-Phase

T

THREE-PHASE
AC FROM
TRANSFORMER
0
SECONDARY

I

Control Using an SCR Bridge

I

I

TO ARC

Figure 1.lo-Three=Phase Bridge Using Six SCRs (Full-Wave Control)

each of the three-phase output is controlled separately.
Dynamic response is enhanced because of the reduced
size of the inductor needed to smooth out the welding
current.
Figure 1.11 is a diagram of a three-phase bridge rectifier with three diodes and three SCRs. Because of
greater current ripple, this configuration requires a
larger inductor than the six-SCR unit. For that reason it
has a slower dynamic response. A fourth diode, termed
a freewheeling diode, can be added to recirculate the
inductive currents from the inductor so that the SCRs
will turn off, or commutate. This offers some economic
advantage over the six-SCR unit because it uses fewer
SCRs and a lower-cost control unit.

TRANSISTORS
The transistor is another solid-state device used in
welding power sources. Transistors differ from SCRs in
several ways. First, conduction through the device is
proportional to the control signal applied. With no signal, no conduction occurs. The application of a small
signal from base to emitter produces a correspondingly
small conduction; likewise, a large signal results in a
correspondingly large conduction. Unlike the SCR, the
control can turn the device off without waiting for
polarity reversal or an off time. Since transistors lack
the current-carrying capacity of SCRs, several may be
required to yield the output of one SCR.

ARC WELDING POWER SOURCES

CHAPTER1

11

t
THREE-PHASE
AC FROM
TRANSFORMER
SECONDARY
0

41

21. FREEWHEELING

tl

DIODE

TO ARC

41

2iDlODE

2iDlODE

21 DIODE

--

-

-

Z

0

Figure 1.l1-Three-Phase Hybrid Bridge Using Three SCRs and Four Diodes (Half=WaveControl)

Several methods can be used to take advantage
of transistors in welding power sources. These include
frequency modulation or pulse-width modulation. With
frequency modulation, the welding current is controlled
by varying the frequency supplied to a high-frequency
transformer. Since the frequency is changing, the
response time varies also. The size of the transformer
and inductor must be optimized for the lowest operating frequency. With pulse-width modulation, varying
the conduction time of the switching device controls
welding current output. Since the frequency is constant,
the response time is constant and the magnetic components can be optimized for one operating frequency.

Inverter circuits control the output power using the
principle of time-ratio control (TRC) also referred to as
pulse-width modulation (PWM). The solid-state devices

(semiconductors)in an inverter act as switches; they are
either switched on and conducting, or switched off and
blocking. The function of switching on and off is sometimes referred to as switch-mode operation. Time-ratio
control is the regulation of the on and off times of the
switches to control the output. Figure 1.12 illustrates a
simplified TRC circuit that controls the output to a
load such as a welding arc. It should be noted that conditioning circuits include components such as a transformer, a rectifier, and an inductor, as represented
previously in Figure 1.8.

SOLID-STATE INVERTER
An inverter is a circuit that uses solid-state devices
called metal oxide semiconductor field effect transistors
(MOSFETs), or integrated gate bi-polar transistors
(IGBTs), to convert direct current into high-frequency
ac, usually in the range of 20 kHz to 100 kHz. Conventional welding power sources use transformers operating from a line frequency of 50 Hz or 60 Hz.
Since transformer size is inversely proportional to
line or applied frequency, reductions of up to 75% in
power source size and weight is possible using inverter
circuits. Inverter power sources are smaller and more
compact than conventional welding power sources.
They offer a faster response time and less electrical loss.
The primary contributors to weight or mass in any
power source are the magnetic components, consisting
of the main transformer and the filter inductor. Various
efforts have been made by manufacturers to reduce the
size and weight of power sources, for example, substituting aluminum windings for copper.

I

TRC SWITCH

CONDITIONING 7

U
Figure 1.l%Simplified Diagram of an Inverter
Circuit Used to Demonstrate the Principle of
Time.Ratio Control (Pulse Width Modulation)


12

ARC WELDING POWER SOURCES

CHAPTER1

When the TRC switch is on, the voltage out ( V O ~ ) semiconductors takes place between 1kHz and 50 kHz,
equals voltage in (VIN). When the switch is off, Vow depending on the component used and method of control.
equals zero. The average value of Vom is calculated as
This high-frequency voltage allows the use of a
follows:
smaller step-down transformer. After being transformed, the alternating current is rectified to direct current for welding. Solid-state controls enable the
VIN tON
operator to select either constant-current or constantvour =
$ON + $OFF
voltage output, and with appropriate options these
sources can also provide pulsed outputs.
where
The capabilities of the semiconductors and the particular circuit switching determine the response time
Vow= Voltage out, V;
and switching frequency, Faster output response times

t o N = On time (conducting), seconds (s);
are generally associated with the higher switching and
VIN = Voltage in, V;
control frequencies, resulting in more stable arcs and
toFF = Off time (blocking), s;
superior arc performance. However, other variables,
such as the length of the weld cables, must be considthus,
ered because they may affect the performance of the
power source. Table 1.1 compares inverter switching
devices and the frequency applied to the transformer.
Inverter technology can be used to enhance the performance in ac welding power sources and can also be
where
applied to dc constant-current power sources used for
Tp = toN + toFF = Time period total, s.
plasma arc cutting.
Variable VoUT is controlled by regulating the ratio of
on time to off time for each alternation tONITp. Since
the on/off cycle is repeated for every Tp interval, the
frequency (f) of the on/off cycles is defined as follows:
f=-

1

(1.4)

VO LT-AMPERE
CHARACTERISTICS

The effectiveness of all welding power sources is
determined by two kinds of operating characteristics,

where
static and dynumic. Each has a different effect on weldf = Frequency,Hz
ing performance. Both affect arc stability, but they do
so in different ways depending on the welding process.
thus, the TRC formula can now be written as:
Static output characteristics are readily measured
under steady-state conditions by conventional testing
VOUT=VINX~ONX~
(1.5) procedures using resistive loads. A set of output-voltage
curves versus output-current characteristic curves (voltThe TRC formula written in this manner points to ampere curves) is normally used to describe the static
two methods of controlling an inverter welding power characteristics.
The dynamic characteristic of an arc welding power
source. By varying t o N , the inverter uses pulse-width
source is determined by measuring the transient variamodulated TRC.
Another method of inverter control, frequency- tions in output current and voltage that appear in the
modulation TRC, varies the frequency, f. Both fre- arc. Dynamic characteristics describe instantaneous
quency modulation and pulse-width modulation are variations, or those that occur during very short intervals, such as 0.001 second.
used in commercially available welding inverters.
Most welding arcs operate in continually changing
Figure 1.13 presents a block diagram of an inverter
used for direct-current welding. A full-wave rectifier con- conditions. Transient variations occur at specific times,
verts incoming three-phase or single-phase 50-Hz or 60- such as the following:
Hz power to direct current. This direct current is applied
1. During the striking of the arc,
to the inverter, which inverts it into high-frequency
2. During rapid changes in arc length,
square-wave alternating current using semiconductor
3. During the transfer of metal across the arc, and
switches. In another variation used for welding, the
4. In alternating current welding, during arc extincinverter produces sine waves in a resonant technology

tion and reignition at each half-cycle.
with frequency-modulation control. The switching of the

TP


ARC WELDING POWER SOURCES

INPUT
BRIDGE
RECTIFIER

INVERTER

TRANSFORMER

1@OR 3 @PRIMARY-

-

13

CHAPTER1

77

I
:

I

f

:
:

OUTPUT
BRIDGE
RECTIFIER

-

INDUCTOR

O+

-

INVERTER
CONTROL
CIRCUIT

Figure I.I%Inverter Diagram Showing Power Source Sections
and Voltage Wave Forms with Pulse-WidthModulation Control

Table 1.1
wpes of Inverter Switching Devices and
Frequency Ranges Applied to the Transformer
Switching Device

Frequency Range

SCR devices
Transistor devices

1 kHz to 10 kHz

10 kHz to 100 kHz

The short arc-transient time of 0.001 second is the
time interval during which a significant change in ionization of the arc column occurs. The power source
must respond rapidly to these demands, and for this
reason it is important to control the dynamic characteristics of an arc welding power source. The steady-state
or static volt-ampere characteristics have little significance in determining the dynamic characteristics of an
arc welding system.
Among the arc welding power source design features
that do have an effect on dynamic characteristics are
those that provide local transient energy storage such as

parallel capacitance circuits or direct-current series
inductance, feedback controls in automatically regulated systems, and modifications of wave form or
circuit-operating frequencies.
Improving arc stability is typically the reason for
modifying or controlling these characteristics. Beneficial results include improvement in the uniformity of
metal transfer, reduction in metal spatter, and reduction
in weld-pool turbulence.
Static volt-ampere characteristics are generally published by power source manufacturers. No universally
recognized method exists by which dynamic characteristics are specified. The user should obtain assurance
from the manufacturer that both the static and dynamic
characteristics of the power source are acceptable for
the intended application.

CONSTANT-CURRENT
Volt-ampere curves show graphically how welding
current changes when arc voltage changes and power
source settings remain unchanged, as illustrated in Figure 1.14 for a drooper power source. Constant-current


14

CHAPTER1

ARC WELDING POWER SOURCES

LIVE GRAPH
Click here to view

CURRENT, A

Figure 1.14--‘Fypical Volt-Ampere Characteristics of a
“Drooper” Power Source with Adjustable Open=CircuitVoltage

welding power sources are sometimes called droopers
because of the substantial downward (negative) slope of
the volt-ampere curves they produce. A constantcurrent V-A characteristic is suitable for shielded metal
arc welding, gas tungsten arc welding, and other processes that use voltage-sensing wire feed systems.
The conventional constant-current output characteristic describes a power source that will produce a relatively small change in output current when a relatively
large change in arc voltage occurs. Arc voltage is
affected by arc length and process parameters such as
electrode type, shielding gas, and arc current. Reducing
the slope or the droop of a constant-current power

source gives the operator a degree of real-time control
over arc current or electrode melting rate. The power
source might have open-circuit voltage adjustment in
addition to output current control. A change in either
control will change the slope of the volt-ampere curve.
The effect of the slope of the V-A curve on power
output is shown in Figure 1.14. With Curve A, which

has an 80-V open circuit, a steady increase in arc voltage from 20 V to 25 V (25%) would result in a decrease
in current from 123 A to 115 A (6.5%). The change in
current is relatively small. Therefore, with a consumable electrode welding process, the electrode melting
rate would remain relatively constant with a slight
change in arc length.
By setting the power source to Slope Curve B in
Figure 1.14 the open circuit voltage is reduced from
80 volts to 50 volts. Curve B shows a shallower or
flatter slope intercepting the same 20-V, 123-A output.
In this case, the same increase in arc voltage from 20 V
to 25 V would decrease the current from 123 to 100 A
(19%), a significantly greater change. In manual welding, the flatter V-A curve would give a skilled welder
the opportunity to substantially vary the output current
by changing the arc length. This is useful for out-ofposition welding because a welder can control the
electrode melting rate and weld pool size in real time by
simply changing the arc length. A flatter slope also


CHAPTER1

ARC WELDING POWER SOURCES

provides increased short-circuit current. This helps
reduce the tendency of some electrodes to stick to the
workpiece during arc starts or times when the arc
length is reduced to control penetration. Generally,
however, less skilled welders would prefer the current to
stay constant if the arc length should change. The
higher open-circuit voltage of constant-current or
drooping output curves also helps reduce arc outages
with certain types of fast-freezing electrodes at longer
arc lengths or when weaving the arc across a root
opening.
Output current control is also used to provide lower
output current. This results in volt-ampere curves with
greater slope, as illustrated by Curves C and D in Figure
1.14. They offer the advantage of more nearly constant
current output, allowing greater changes in voltage
with minor changes in current.

CONSTANT-VOLTAGE CHARACTERISTICS
The volt-ampere curve in Figure 1.15 shows graphically how the output current is affected by changes in
the arc voltage (arc length). It illustrates that this power
source does not have true constant-voltage output. It
has a slightly downward (negative) slope because internal electrical impedance in the welding circuit causes a
minor voltage droop in the output. Changing that
impedance will alter the slope of the volt-ampere curve.
Starting at Point B in Figure 1.15, the diagram shows
that an increase or decrease in voltage to Points A or C
(5 V or 25%), produces a large change in amperage
(100 A or .SO%), respectively. This V-A characteristic is
suitable for maintaining a constant arc length in constant-speed electrode processes, such as GMAW, SAW,

40
35

-

and FCAW. A slight change in arc length (voltage)
causes a relatively large change in welding current. This
automatically increases or decreases the electrode melting rate to regain the desired arc length (voltage). This
effect is called self-regulation. Adjustments are sometimes provided with constant-voltage power sources to
change or modify the slope or shape of the V-A curve.
Typical adjustments involve changing the power source
reactance, output inductance, or internal resistance. If
adjustments are made with inductive devices, the
dynamic characteristics will also change.
The curve shown in Figure 1.16 can also be used to
explain the difference between static and dynamic characteristics of the power source. For example, during gas
metal arc welding short-circuiting transfer (GMAW-S),
the welding electrode tip touches the weld pool, causing
a short-circuit. At this point, the arc voltage approaches
zero, and only the circuit resistance and inductance limits the rapid increase of current. If the power source
responded instantly, very high current would immediately flow through the welding circuit, quickly melting
the short-circuited electrode and freeing it with an
explosive force, expelling the weld metal as spatter.
Dynamic characteristics designed into this power source
compensate for this action by limiting the rate of
current change, thereby decreasing the explosive force.

COMBINED CONSTANT-CURRENT AND
CONSTANT-VOLTAGE CHARACTERISTICS

Electronic controls can be designed to provide either
constant-voltage or constant-current outputs from single

LIVE GRAPH

A

-

Click here to view

C

10

I

I

I

250

300

350

5 -

I

0
0

50

I
100

150

15

200

CURRENT, A

Figure 1.I5-Volt-Ampere Output Relationship for a Constant-VoltagePower Source


16

ARC WELDING POWER SOURCES

CHAPTER1

VOLTAGE
CURRENT, A

Figure I.I64ombination Volt=AmpereCurve

power sources, making them useful for a variety of
welding processes.
Electronically controlled outputs can also provide
output curves that are a combination of constantcurrent and constant-voltage, as shown in Figure 1.16.
The top part of the curve is essentially constant-current;
below a certain trigger voltage, however, the curve
switches to constant voltage. This type of curve is beneficial for shielded metal arc welding to assist starting
and to avoid electrode stubbing (sticking in the weld
pool) if the welder uses an arc length that is too short.

DUTY CYCLE

vals without exceeding a predetermined temperature
limit. In the United States, for example, the National
Electrical Manufacturers Association (NEMA) specifies duty cycles based on a test interval of 10 minutes in
an ambient temperature of 40°C (104°F). Some agencies and manufacturers in other countries use shorter
test intervals, such as 5 minutes. Thus, a 60% NEMA
duty cycle (a standard industrial rating) means that the
power source can deliver its rated output for 6 out of
every 10 minutes without ~verheating.~
A 100% dutycycle power source is designed to produce its rated
output continuously without exceeding the prescribed
temperature limits of its components.
Duty cycle is a major factor in determining the type
of service for which a power source is designed. Industrial units designed for manual welding are normally
rated at a 60% duty cycle. For automatic and semiautomatic processes, the rating is usually a 100% duty
cycle. Light-duty power sources usually have a 20%
duty cycle. Power sources with ratings at other duty
cycle values are available from the manufacturers.

An important point is that the duty cycle of a power
source is based on the output current and not on a
kilovolt-ampere load or kilowatt rating. Manufacturers
perform duty-cycle tests under what NEMA defines as
usual service conditions. Caution should be observed in
basing operation on service conditions other than usual.
Unusual service conditions such as high ambient temperatures, insufficient cooling air, and low line voltage
are among the factors that contribute to performance
that is lower than tested or calculated.
Equation 1.6 presents the formula for estimating the
duty cycle at other than rated outputs, as follows:

KJ

Ta= - XT

(1.6)

Internal components of a welding power source tend where
to heat up as welding current flows through. The amount
T, = Required duty cycle, %;
of heat tolerated is determined by the breakdown temI = Rated current at the rated duty cycle, A;
perature of the electrical components and the meda used
la = Maximum current at the required duty cycle,
to insulate the transformer windings and other compoA; and
nents. These maximum temperatures are specified by
T
=
Rated duty cycle, %.
component manufacturers and organizations involved

with standards in the field of electrical insulation.
Equation 1.7 presents the expression for estimating
Fundamentally, the duty cycle is a ratio of the loadon time allowed in a specified test interval time. other than rated output current at a specified duty
Observing this ratio is important in preventing the cycle, as follows:
internal windings and components and their electrical
insulation system from heating above their rated temperature. These maximum temperature criteria do not
change with the duty cycle or current rating of the
power source.
5. It should be noted that a power source specified for uninterrupted
Duty cycle is expressed as a percentage of the maxi- operation at a rated load for 36 minutes out of one hour would have
mum time that the power source can deliver its rated a 100% duty cycle, rather than a 60% duty cycle, because it could
output during each of a number of successive test inter- operate continuously for the test-interval of 10 minutes.


CHAPTER1

ARC WELDING POWER SOURCES

where
I, = Maximum current at the required duty cycle,
A;
I = Rated current at the rated duty cycle, A;
T = Rated duty cycle, %; and
T, = Required duty cycle, YO.
The power source should never be operated above its
rated current or duty cycle unless approved by the manufacturer. For example, Equation 1.8 applies Equation
1.6 to determine the duty cycle of a 200-A power
source rated at a 60% duty cycle if operated at 250 A
output (provided 250 A is permitted by the manufacturer), as follows:

(3

T, = - ~ 6 0 %
= (0.8)2~ 0 . =
6 38%
Therefore, this unit must not be operated more than
3.8 minutes out of each 10-minute period at 250 A. If
used in this way, welding at 250 A will not exceed the
current rating of any power source component.
The output current that must not be exceeded when
operating this power source continuously (100% duty
cycle) can be determined by applying Equation 1.7, as
shown in Equation 1.9:

17

Table 1.2
Maximum Open-Circuit Voltages for Various
Types of Arc Welding Power Sources
Manual and Semiautomatic Applications
Alternating current
Direct current-over 10% ripple voltage*
Direct current-10% or less r i m l e voltaae

80 V root mean square
(rms)
80 V rms
100 V averaae

Automatic Applications

Alternating current
Direct current-over 10% ripple voltage
Direct current-10% or less ripple voltage
*Ripple voltage, % =

100 V rms
100 V rms
100 V average

Ripple voltage, rrns
Average total voltage, V

NEMA EW-1,Electric Arc- Welding Power SourcesY6
contains specific requirements for maximum opencircuit voltage. When the rated line voltage is applied
to the primary winding of a transformer or when a
generator arc welding power source is operating at
maximum-rated no-load speed, the open-circuit voltages are limited to the levels shown in Table 1.2.
112
NEMA Class I and Class I1 power sources normally
I, = 200 x
= 200 x 0.775 = 155 A
(1.9)
have open-circuit voltages at or close to the maximum
specified. Class I11 power sources frequently provide
If operated continuously, the current should be limited two or more open-circuit voltages. One arrangement is
to have a high and low range of amperage output from
to an output of 155 A.
the power source. The low range normally has approximately 80-V open circuit, with the high range somewhat lower. Another arrangement is the tapped
secondary coil method, described previously, in which
PEN-CIRCUIT VO LTAGE

the open-circuit voltage changes approximately 2 V to
4 V at each current setting.
In the United States, 60-Hz power produces reversals
Open-circuit voltage is the voltage at the output terminals of a welding power source when it is energized in the direction of current flow each 1/120 second
but current is not being drawn. Open-circuit voltage is (60 Hz). Typical sine wave forms of a dual-range
one of the design factors influencing the performance of power source with open-circuit voltages of 80 V and
all welding power sources. In a transformer, open- 55 V root mean square (rms) are diagrammed in Figure
circuit voltage is a function of the primary input voltage 1.17. (The rms of alternating current or voltage is the
and the ratio of primary-to-secondary coils. Although a effective current or voltage applied that produces the
high open-circuit voltage may be desirable from the same heat as that produced by an equal value of direct
standpoint of arc initiation and stability, the electrical current or voltage).
The current must change direction after each halfhazard precludes the use of higher voltages.
The open-circuit voltage of generators or alternators cycle. In order for it to do so, the current flow in the arc
is related to design features such as the strength of the ceases for an instant at the point at which the current
magnetic field, the speed of rotation, the number of wave form crosses the zero line. An instant later, the
turns in the load coils, and so forth. These power current must reverse its direction of flow. However,
sources "
eenerallv have controls with which the oDen6 . See Reference 1, p 91.
circuit voltage can be varied.

(g)

0