AWS WHB-2 9-2004 Welding Handbook - Welding Processes Part 1 (Volume 2 Ninth Edition)《焊接手册 焊接工艺 第1部分第2 第9版》.pdf

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1、ARC WELDING POWER SOURCES CHAPTER 9 CHAPTER 1 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 ESAB Weldin

2、g and Cutting Products C. Hsu The Lincoln Electric Company J. O. Reynolds Miller Electric Manufacturing Company Welding Handbook Committee Member: C. E. Pepper ENGlobal Engineering Contents Introduction 2 Fundamentals 2 Principles of Operation 4 Volt-Ampere Characteristics 12 Duty Cycle 16 Open-Circ

3、uit 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 Supplementary Reading List 49 Photograph courtesy of NASA2 CHAPTER 1 ARC WELDING POWER SOURCES INTRODUCTION This chapter

4、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 year

5、s old, and scores of pro- cesses 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 pro- cesses continue to evolve, power sources continue to provide t

6、he means of controlling the welding current, voltage, and power. This chapter provides updated information on the basic electrical technologies, cir- cuits, and functions designed into frequently used welding power sources. Topics covered in this chapter include the following: 1. The volt-ampere (V-

7、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 inter- national standards. A basic knowledge of electrica

8、l power sources will provide the background for a more complete under- standing 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-curre

9、nt (CC) characteristics required for welding processes. The voltage supplied by power companies for indus- trial purposes120 volts (V), 230 V , 380 V , or 480 V is 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 voltag

10、e 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

11、 prime mover such as an internal combustion engine. The rotating power from an inter- nal 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

12、 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 deliv

13、er only cer- tain 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 motor- alternator deliver

14、s ac, or when equipped with rectifiers, dc. Power sources can also be classified into subcate- gories. 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

15、 rating, duty cycle rating, service classification, and input power ARC WELDING POWER SOURCES CHAPTER 1ARC WELDING POWER SOURCES CHAPTER 1 3 requirements. Special features can also be included such as remote control, high-frequency stabilization, current- pulsing capability, starting and finishing c

16、urrent versus time programming, wave balancing capabilities, and line-voltage compensation. Conventional magnetic con- trols include movable shunts, saturable reactors, mag- netic amplifiers, series impedance, or tapped windings. Solid-state electronic controls may be phase-controlled silicon-contro

17、lled rectifiers (SCRs) or inverter-controlled semiconductors. Electronic logic or microprocessor cir- cuits may control these elements. Figure 1.1 shows the basic elements of a welding power source with power supplied from utility lines. The arc welding power source itself does not usually include t

18、he fused disconnect switch; however, this is a necessary protective and safety element. An engine-driven power source would require ele- ments different from those shown in Figure 1.1. It would require an internal combustion engine, an engine speed regulator, and an alternator, with or without a rec

19、tifier, or a generator and an output control. Before the advent of pulsed current welding pro- cesses in the 1970s, welding power sources were com- monly classified as constant current or constant voltage. These classifications are based on the static volt-ampere characteristics of the power source,

20、 not the dynamic characteristic or arc characteristics. The term constant is true only in a general sense. A constant- voltage output actually reduces or droops slightly as the arc current increases, whereas a constant-current out- put gradually increases as the arc length and arc voltage decrease.

21、In either case, specialized power sources are available that can hold output voltage or current truly constant. Constant-current power sources are also known as variable-voltage power sources, and constant-voltage power sources are often referred to as constant- potential power sources. These fast-r

22、esponse, solid- state power sources can provide power in pulses over a broad range of frequencies. CONSTANT-CURRENT ARC WELDING POWER SOURCES The National Electrical Manufacturers Association (NEMA) standard Electric Arc-Welding Power Sources, EW-1: 1988 (R1999), defines a constant-current arc power

23、 source as one “which has means for adjusting the load current and which has a static volt-ampere curve that tends to produce a relatively constant load current. At a given load current, the load voltage is responsive to the rate at which a consumable metal electrode is fed into the arc. When a tung

24、sten electrode is used, the load voltage is responsive to the electrode- to-workpiece distance.” 1, 2These characteristics are 1. National Electrical Manufacturers Association (NEMA), 1988 (R1999), Electric Arc-Welding Power Sources, EW-1: 1988, Washing- ton, D.C.: National Electrical Manufacturers

25、Association, p. 2. 2. At the time this chapter was prepared, the referenced codes and other standards were valid. If a code or other standard is cited without a date of publication, it is understood that the latest edition of the document referred to applies. If a code or other standard is cited wit

26、h the date of publication, the citation refers to that edition only, and it is understood that any future revisions or amendments to the code or standard are not included; however, as codes and standards undergo frequent revision, the reader is advised to consult the most recent edition. Figure 1.1B

27、asic Elements of an Arc Welding Power Source ELECTRICAL CONNECTION MECHANICAL STRUCTURE AND CHASSIS FUSED DISCONNECT SWITCH (OPEN) ARC WELDING POWER SOURCE ELECTRODE CHASSIS GROUND CONNECTION WORKPIECE LEAD FUSES EXTERNAL GROUND X MEANS OF REDUCING POWER SYSTEM VOLTAGE MEANS OF CONTROLLING OUTPUT CH

28、ARACTERISTIC INPUT AC FROM PLANT POWER LINE4 CHAPTER 1 ARC WELDING POWER SOURCES 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 curv

29、e when tested under steady conditions with a resistive load. In the vicinity of the operating point, the percent- age of change in current is lower than the percentage of change in voltage. The no-load, or open-circuit, voltage of constant- current arc welding power sources is considerably higher th

30、an 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 el

31、ement. When used in a semiautomatic or automated applica- tion in which constant arc length is required, external control devices are necessary. For example, an arc- voltage-sensing wire feeder can be used to maintain con- stant arc length for gas metal arc welding (GMAW) or flux cored arc welding (

32、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). CONSTANT-VOLTAGE ARC WELDING POWER SOURCES The NEMA EW-1 standard defines a constant- voltage power s

33、ource 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 rela- tively constant load voltage. The load current, at a given load voltage, is responsive to the rate at

34、 which a consumable electrode is fed into the arc.” 3Constant- voltage 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 constan

35、t-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. CONSTANT-CURRENT/CONSTANT-VOLTAGE POWER SOURCES A power source that provides

36、 both constant current and constant voltage is defined by NEMA as follows: 3. See Reference 1, p. 3. “A constant-current/constant-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 weld

37、ing power source.” 4 Additionally, some power sources feature an auto- matic change from constant current to constant voltage (arc force control for SMAW) or constant voltage to constant current (current limit control for constant- voltage power sources). PRINCIPLES OF OPERATION The basic components

38、 of welding power sources transformers, series inductors, generators/alternators, diodes, silicon-controlled rectifiers, and transistorsare introduced in this section. Simple circuits of reactance- controlled, phase-controlled, and inverter power sources are discussed as examples. Most arc welding i

39、nvolves 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 genera- tor or alternator driven by an electric motor. Electric generators built for arc welding are usually designed

40、 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 rectif

41、ied 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 cumula- tive compound winding for selecting and controlling volt-ampere output characteristics. WELDING TRANSFORMER A transform

42、er 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 N 1turns of wire (in Equation 1.1), is energized by an alternating-current inp

43、ut voltage, thereby magnetizing the core. The core couples the alternating magnetic field into the second- ary winding, with N 2turns of wire, producing an out- put voltage. 4. See Reference 1, p. 2.ARC WELDING POWER SOURCES CHAPTER 1 5 Figure 1.2 also illustrates the principal elements of a welding

44、 transformer, with associated components. For a transformer, the significant relationships between volt- ages and currents and the turns in the primary and secondary windings are as follows: (1.1) where N 1 = Number of turns on the primary winding of the transformer; N 2 = Number of turns on the sec

45、ondary winding; E 1 = Input voltage, V; E 2 = Output voltage, V; I 1 = Input current, A; and I 2 = Output (load) current, A. 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 open- circuit (no-load) output

46、voltage. In this case, the tapped transformer permits the selection of the number of turns, N 2 , 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 =

47、 112 221 NEI NEI in use. The tap selection, therefore, controls the ac output voltage. As shown in Equation 1.1, the primary- secondary current ratio is inversely proportional to the primary-secondary voltage ratio. Thus, large secondary welding currents can be obtained from relatively low line inpu

48、t currents. Figure 1.2Principal Electrical Elements of a Transformer Power Source INPUT AC FROM PLANT POWER LINE N 1 TURNS N 2 TURNS C D B A I 1 E 1 E 2 I 2 A B C D METAL CORE TRANSFORMER AC OUTPUT DC OUTPUT ARC IF A PRIMARY SOLID-STATE INVERTER IS USED, THIS BOX IS THE LOCATION OF SOLID-STATE CONTR

49、OL PARTS. RECTIFIER CONTROL, IF DC OUTPUT IS PROVIDED BY POWER SOURCE. LOCATION OF SERIES CONTROL COMPONENTS, IF USED. SECONDARY SWITCHING SOLID-STATE DEVICE FOR PULSED CONTROL IF USED. ALSO LOCATION OF SLOPE RESISTOR, IF USED, OR INDUCTOR FOR AC CIRCUIT. Figure 1.3Welding Transformer with Tapped Secondary Winding TA P SELECTOR AC INPUT VOLTAGE PRIMARY WINDING TAPPED SECONDARY WINDING CORE TRANSFORMER CASE N 2 TURNS N 1 TURNS SELECTOR CASE AC OUTPUT VOLTAGE6 CHAPTER 1 ARC WELDING POWER SOURCES SERIES REACTOR A transformer may be designed so that the tap selec- tion directly adjusts the