Arc Welding Power Supplies and Equipment

The article is about Arc Welding Power Supplies, Rectifier Welders, Direct Current Generator Welders, Selection of Welding Power Supplies. For new welders always ask What are the basic power source designs for arc welding? 

Arc Welding Power Supplies and Equipment

1. SCOPE
2. REFERENCES
3. GENERAL
4. ALTERNATING CURRENT WELDERS
5. ALTERNATING CURRENT – HIGH FREQUENCY STABILIZED WELDER
6. DIRECT CURRENT GENERATOR WELDERS
7. DIRECT CURRENT RECTIFIER WELDERS
8. ALTERNATING CURRENT – DIRECT CURRENT WELDERS
9. MULTIPLE ARC WELDERS
10. COMBINATION CONSTANT-VOLTAGE CONSTANT-CURRENT WELDERS
11. SELECTION CHARACTERISTICS AND APPLICATIONS
12. POWER SUPPLY

TABLE

I Guide for Selection of Arc Welding Power Supplies
II Combination Constant Current – Constant Voltage Power Supplies
III Guide for Selection of Welding Power Supplies

1. Scope of Arc Welding Power Supplies

This Article provides characteristics and selection guides for three basic types of arc-welding power supply: alternating current transformer, direct current generator, and direct current rectifier, together with variations of each.

2. References

3. General about Arc Welding Power Supplies

3.1 Welders in this article means the welding machines.

3.2 Duty cycle is the maximum percentage of time during a ten minute period that a welder can be operated at a specific load. A 60 percent duty cycle rated unit will provide 100 percent of its rated output for 6 minutes out of 10. The maximum current for 60 percent duty cycle rated welders is 25 percent more than their rated currents, although at a lower duty cycle. For example, a 400 A, 60 percent duty cycle, ac welder may be operated at its maximum current of 500 A on a 38 percent duty cycle. The maximum current at 100 percent duty cycle (continuous operation) is about 75 percent of the rated current at 60 percent duty cycle. For example, the 400 A ac welder may be operated continuously at currents below 300 A maximum. Welding power supplies are available at duty cycle ratings of:

a. 20 , 30 , and 50 percent duty cycle for farm, home, and limited duty industrial welders for ac welding.
b. 60 percent duty cycle for general-purpose industrial welding power supplies of all types.
c. 100 percent duty cycle for heavy duty, automatic welding power supplies of all types.

3.3 Arc force is determined by the momentary current delivered by the power supply on short circuiting of the electrode to the work. A forceful arc power supply delivers a great deal more current during shorting than normal, whereas a soft arc supply delivers only slightly higher current. 

3.3.1 A forceful arc is generally desirable for manual SMAW of thick sheet, plate, and pipe; especially in positions other than the flat position and also where the joint fit-up is poor. With a forceful arc, a welding operator is able to exercise an appreciable effect on the electrode melting rate and the heat input to the work by slightly lengthening or shortening the arc. Also, with a forceful arc, ‘stubbing’ or freezing of the electrode into the weld puddle is seldom a problem.

3.3.2 A soft arc is desirable for welding light-gage sheets and for welding in the presence of strong air currents. In such situations, it is desirable to have the welding current remain constant, thereby maintaining the electrode melting rate and heat input to the work relatively constant, regardless of any incidental changes in arc length.

4. Alternating Current Welders

4.1 All ac welders are of the single-operator, single-phase, static-transformer type.

4.2 Various methods are employed for adjusting the current output of ac welding machines. The smaller farm type and  limited use industrial type welders frequently use a tapped reactor in the secondary circuit of the transformer. General purpose industrial ac welders usually employ either a moving primary coil, which is cranked up or down to vary the spacing from the secondary coil, or a dc saturable reactor in the transformer secondary circuit. Remote control of the welding current is facilitated by the saturable reactor type of control, since the current supplied to the dc winding of the reactor can be easily varied with a manual or foot operated rheostat.

4.3 With the larger capacity ac welders (400 A and higher) a forceful arc is obtained over most of the available operating current range. That is, the instantaneous current delivered by the machine on short-circuit is appreciably higher than the current delivered at the normal operating arc voltage. With smaller capacity machines 300 A and lower, a relatively soft arc is obtained over most of the available operating current range; that is, the momentary short-circuit current is only slightly higher than the current delivered at the normal operating arc voltage.

4.4 Covered electrodes designed especially for use with ac welders are available. These electrodes contain easily ionizable ingredients in the coatings, which tend to prevent the arc from extinguishing as the voltage wave passes through zero 120 times per second. A deficiency of ac welding in this respect is that the amount of ionization of the electrode coating ingredients produced at low currents is not adequate to provide a stable, easily controllable arc, and frequent ‘stubbing’ of the electrode occurs. Steel electrodes, the AWS designation for which end in an odd digit, are designed for ac currents; even digits are for dc currents.

4.5 The major application of ac transformer welders is in flat position butt welding and flat and horizontal position fillet welding of heavy steel plate and structural shapes, either by manual welding using heavily coated ‘hot rods,’ or by submerged melt (Unionmelt) welding.

4.6 Alternating current welders require little maintenance other than a periodic lubrication of the bearings of the cooling fan and also the adjusting screw in a movable coil welder. Alternating current transformer welders are comparatively simple electrical machines, having a minimum of moving parts and no moving parts subject to appreciable wear.

4.7 Advantages of Alternating-current Welders: Low initial cost, high electrical efficiency, economical in power consumption, low maintenance cost, quiet operation, and absence of arc blow.

4.8 Disadvantages of Alternating current Welders: Application limited mainly to ferrous metals; not well suited to thin gage metals on which low currents are required, because of poor arc stability; inferior to dc arc welders for welding plate and pipe in positions other than flat, where a forceful arc at moderately low currents is required.

5. Alternating Current – High Frequency Stabilized Welder

5.1 All alternating current-high frequency (ACHF) stabilized welders in service are special modifications of a basic alternating current transformer type welder. ACHF machines are intended primarily for GTAW welding of aluminum and aluminum alloys, magnesium and magnesium alloys, aluminum bronzes, and beryllium copper. ACHF welders are available in ratings of 200, 300, 400, and 500 A at 60 percent duty cycle, and 200 and 300 A at 100 percent duty cycle.

5.2 ACHF welders have a spark-gap oscillator in the secondary circuit which superimposes low amperage – high voltage – high frequency power on the 60 cycle welding voltage to provide arc stability.

5.3 For a given current rating, the transformer in an ACHF welder is larger (contains more iron) than the transformer in an ordinary ac welder. This is necessary to accommodate the additional saturation and heating effects of the dc component of current which is produced during GTAW welding.

5.4 Many ACHF machines contain a switch to permit operation of the welder either with superimposed high-frequency power for GTAW or without the high frequency for SMAW. Provision frequently is made for selecting the use of high frequency only for arc starting or for continuous operation, as desired. There is usually provision also for varying the intensity of the superimposed high frequency power by adjusting the high frequency voltage and, in some cases, by shifting the phase of the high frequency voltage with respect to the 60 cycle welding voltage.

5.4.1 ACHF welders can be purchased either with or without automatic flow controls for shielding gas and torch cooling water. Enclosed water circulating systems also are available. Attachments are available for remote control of the primary contactor and the welding current by means of a manual operated or foot operated switch and rheostat. Attachment control units are also available for GTAW.

5.4.2 ACHF welders are desirable for GTAW welding of aluminum, magnesium, aluminum bronze, and beryllium copper, since they offer a satisfactory compromise between the advantages and disadvantages of welding these materials with dc reverse polarity (electrode positive). The dc reverse polarity is desirable because it provides an essential cleaning action on the surface of these materials. Direct current reverse polarity is undesirable because it causes intensive heating of the tungsten electrode, requiring the use of very large diameter electrodes even for low welding currents in order to avoid appreciable consumption of the electrode. The ac provides a compromise, inasmuch as during the half cycle that the work has a negative polarity, the cleaning action takes place; and during the half cycle that the work is positive, the electrode has some opportunity to cool. This makes it possible to use electrodes which are considerably smaller in diameter than would be required for equivalent values of current in welding with dc reverse polarity. Another factor of importance is that the depth of penetration of fusion is appreciably greater with ACHF power than it is with dc reverse polarity. 

5.5 Advantages of ACHF Welders: Suitable for welding aluminum, magnesium, aluminum bronze, and beryllium copper, where quantity of work is sufficient to justify a single purpose machine rather than combination ac-dc high frequency welder, see section 8; suitable for SMAW; suitable for manual GTAW welding of light gages of ferrous and nonferrous metals; suitable for inert-gas arc spot welding of light-gage metals.

5.6 Disadvantages of ACHF Welders: High initial cost and relatively limited application.

6. Direct Current Generator Welders

6.1 A dc generator welder consists of a specially designed generator with a suitable drive (electric motor, gasoline engine, or diesel engine), plus generator and motor or engine controls. The dc welding generators may be either self excited, in which case the field current is obtained from an independent brush; or separately excited, in which case the field current is obtained from a smaller generator (exciter) whose armature is on the same shaft as the main generatorarmature.

6.2 There are two general types of dc welding generators.

6.2.1 Conventional arc welding generators have a drooping volt-ampere characteristic, for manual SMAW, open arc  hard-surfacing, manual and mechanized GTAW, manual and mechanized GMAW, manual and mechanized submerged-melt welding, carbon arc-air gouging, stud welding, and GTAW.

6.2.1.1 Single-operator type, drooping volt-ampere characteristic generators are made in ratings of from 150 to 600 A.They are available in three classes: (1) 150 and 200 A, 30 V (suitable for light-duty SMAW and GTAW); (2) 200, 250 (gas engine drive), 300, and 400 A, 40 V, 60-percent duty cycle (manual and mechanized arc welding of all types); (3) 600 A, 40 V, 60-percent duty cycle (GMAW welding, stud welding, and carbon-arc cutting).

6.2.1.2 With separately excited generators, independent adjustment of the open-circuit voltage and the short-circuit current is possible, usually over wide ranges of these variables. Therefore, it is relatively easy to obtain ‘soft’ arc for sheet-metal welding or a forceful arc for fixed-position pipe welding. With self-excited generators, however, there is somewhat less freedom in selecting machine output characteristics to match the requirements of specific applications of arc welding. In the large majority of welding applications, there is no particular advantage of one type of machine over the other; however, there are occasional situations in which the versatility afforded by the separately excited generator is advantageous.

6.2.2 Constant-voltage generators are used for manual and mechanized GMAW and Submerged Arc Welding (SAW)6.2.2.1 Constant-voltage generators are available in ratings of 200 to 1,200 A at 100 percent duty cycle.

6.2.2.2 With the constant-voltage generator, the desired arc voltage can be selected in advance, and it will remain  virtually constant, independent of the amount of current delivered by the machine. This feature is advantageous for welding processes which employ continuously feeding electrode wires. Constant-voltage welders eliminate the need for feedback systems of arc-voltage control. In the constant-voltage generator, the brushes can be shifted slightly fromthe neutral position to provide either a slightly rising or a slightly drooping volt-ampere characteristic, which may be preferable for some applications. Constant-voltage generators cannot be satisfactorily employed for SMAW or for GTAW. The open-circuit voltage is too low for satisfactory arc starting with coated electrodes, and it also would be necessary to insert a high-wattage stabilizing resistor in the welding circuit, since these processes have negative resistance characteristics.

6.3 Special engine-driven combination dc welding generators and ac or dc power generators are used where auxiliarypower is required in remote locations for operation of power tools. Units can be mounted on a self-propelled tractor chassis.

6.4 Advantages of DC Generator Welders: Wide versatility with regards to the types and thicknesses of material and  positions of welding; usually ruggedly built for long life; output of induction-motor- driven generators is practically insensitive to line voltage fluctuations; moderate initial cost. 

6.5 Disadvantages of DC Generator Welders: Parts subject to wear; brushes require occasional replacement;  commutator subject to damage by chemical fumes, oil, and abrasive dust; noisy operation; and magnetic arc blow requires skill and experience to overcome lower electrical efficiency and higher power consumption than ac transformer welders.

7. Direct Current Rectifier Welders

7.1 The dc rectifier welder is basically a transformer plus a device for obtaining current rectification. This may be accomplished by means of a stack of selenium or silicon plates or silicon diode solid-state device. A three-phase transformer produces a more stable welding current than a single-phase transformer.  

7.2 The dc rectifier welders are of two general types.

7.2.1 Conventional drooping volt-ampere characteristic type for general-purpose welding similar to the use of dc  generator welders, see 6.2.1.

7.2.1.1 Conventional rectifier welders (drooping volt-ampere type) in ratings of 200, 300, 400, 500, and 600 A for single units, and 300/600 and 400/800 A for duplex units, can be used as two separate units of the lower rating, or as a single unit of the higher rating. These machines are rated at 60 percent duty cycle. 

7.2.2 Constant-voltage type for GMAW and SAW.

7.2.2.1 Constant-voltage rectifiers are available in ratings of 200 to 2,500 A at 100 percent duty cycle.

7.3 The current output of rectifier welders is adjusted by moving the primary coil up or down, or using a dc saturable reactor in the transformer secondary circuit.

7.4 Rectifier welders of the drooping volt-ampere type inherently provide a ‘soft’ arc; that is a decrease in voltage from  the normal arc voltage to zero voltage results in no appreciable increase in current delivered by the machine. Shortly after rectifier welders were introduced, this characteristic was found to be objectionable when welding with SMAW because of frequent stubbing of the electrodes into the weld puddle, especially in vertical, horizontal, and overhead welding. This led to the development of methods for automatically increasing the current delivered by the machine whenever the arc voltage tends to approach zero. This is achieved in one case by a feedback system using tertiary windings on the transformer. Other manufacturers use the magnetic amplifier principle, taking a portion of the slight increase in dc current which occurs as the arc voltage decreases to supply additional current to the dc windings of the saturable reactor and, thus, to further increase the amount of ac current delivered to the rectifier.

7.5 The ‘soft’ arc characteristic of dc rectifier welders is suitable for GTAW.

7.6 Constant-voltage rectifier welders may be employed in applications similar to constant-voltage generators for GMAW.

7.7 The output of rectifier welders is not subject to change due to heating up, as is the case with generator welders.  However, the output of rectifier welders is affected by line voltage fluctuations, whereas the output of induction motor-driven generators is unaffected by such fluctuations.  

7.8 Attachments are available to permit high-frequency starting of the arc for manual and automatic GTAW welding. Enclosed water circulation systems also are available. Control units are available to permit GTAW spot welding.

7.9 On some large construction projects, as in storage tank fabrication, it has been found feasible to use a large diesel generator to supply ac power to several rectifier welders rather than using individual engine-driven generator welders.

7.10 It is possible to add a control console to a dc rectifier power supply for conversion to Plasma Arc Welding (PAW). When anticipating the use of the plasma process, it is necessary that the dc power supply have a 3-phase primary input. It is also important that the high frequency in the power supply not be energized during PAW.

7.11 Advantages of DC Rectifier Welders. Higher electrical efficiency and greater operating economy than motor generators; quiet operation; and low maintenance cost.

7.12 Disadvantages of DC Rectifier Welders. Output is affected by line voltage fluctuations; slightly higher electrical power cost than motor generators. For welders equipped with plate-type rectifiers, rectifier life is seriously impaired by overheating due to dust and dirt on stacks, or to overloads. Equipment using silicon diodes does not have this disadvantage.

8. Alternating Current – Direct Current Welders

8.1 An ac-dc welder is basically a single-phase transformer plus a dry-plate rectifier stack of selenium or silicon plates. The ac-dc welders are available in ratings of 200, 300, and 500 A at 60 percent duty cycle.

8.2 The ac-dc welders are equipped with a selector switch which permits bypassing the rectifier stack when welding with ac current is desired.

8.3 The ac-dc welders may be obtained with built-in or attachment high frequency oscillators and automatic gas and water flow controls to permit both ac high frequency and dc GTAW. The ac-dc welders also can be obtained with built-in or attachment controls for GTAW spot welding.

8.3.1 The ac-dc welders containing high frequency oscillators, automatic gas and water flow controls, and arc-spot timers are the most versatile single unit welding power supplies currently available. A single welding machine of this type, having an adequate welding current rating, can be used for all of the presently available manual and automatic welding processes except atomic hydrogen welding.

8.4 The desirability of purchasing fully equipped ac-dc welders rather than individual machines of different types depends upon the quantity of work and the variety of types of work requiring different welding processes. An ac-dc
welder can supply only one operator using one welding process. Thus, where there is a sufficient quantity of different types of work requiring different processes; for example, GTAW of aluminum or SMAW, it is preferable to have appropriate individual power supplies for each type of work. On the other hand, for a shop doing mostly dc SMAW and GTAW, and only occasional fabrication of aluminum, an ac-dc high frequency welder would provide ideal facilities for aluminum work, and it also would be highly suitable for the dc work. The same reasoning applies to a shop which may require occasional use of arc spot welding.

8.5 Advantages of AC-DC Welders: High versatility for different processes, quiet operation, and low maintenance cost.

8.6 Disadvantages of AC-DC Welders: Relatively high initial cost; and combination machine can be used less  efficiently than individual machines of different types where quantity of different types of work is high.

9. Multiple Arc Welders

9.1 Multiple-arc welders furnish power from one source to as many as 30 welding arcs; more, if supplies are connected in parallel. Voltage and current are supplied to a series of resistor banks on the secondary side. The usual power supply is a rectifier unit which is available in amperage ratings of 500, 750, or 1,500. Frequently, these are connected in parallel to supply additional welding arcs.

9.2 Multiple arc welders are most useful where a large number of arcs are to be used.

9.3 The multiple arc system is based upon the knowledge that operators do not normally weld continuously, and that in  any group, not all welders will be welding at the same time. Hence, the duty cycle or use cycle can be considered to be relatively low. By calculation of arc time, welders duty cycle, and diversity of welding operations, the number of arcs per machine and rating of machine can be found. For example, with welders ‘arcing’ at 30 percent of the time using a 1,500 A multiarc rectifier, and each welder drawing 180 A [4 mm (5/32 in) electrode], 28 arcs could be drawn from the unit rather than utilizing 28 separate welding units. An example of this is;

1 500/ 0.30 * 180= 28

9.3.1 The multiple arc units are designed and built to operate near rated capacity 100 percent of the time and each unit has a two-minute 50 percent overload capacity.

9.4 Packaged equipment is available to provide power unit, distribution block, panels, resistors, and other components. Economy is obtained readily whenever multiple arcs are required for SMAW, GTAW, stud welding, and other welding operations. All manufacturers supply detailed service for particular requirements. 

9.5 Advantages of multiarc include reduced maintenance, less space, lower initial cost per arc, reduced operating costs, versatility, allowance for adjusting heat at arc, and portability.

9.6 Disadvantages include need for high utility, some require that all arcs be of the same polarity, and that all welding from one unit be relatively near the power source.

10. Combination Constant-voltage Constant-current Welders

10.1 This type of welding power supply can furnish power for either GMAW or SMAW welding.

10.2 This equipment can be purchased as motor-generator sets, rectifiers, or gasoline-driven units.

10.3 The duty cycle usually is rated at 100 percent for this type equipment.

11. Selection Characteristics and Applications

11.1 Alternating-current units are highly efficient, but are severely limited in application to rather special ferrous covered electrode uses. They are rarely used in industrial applications. Alternating current units, high frequency stabilized, are widely used for GTAW welding light metals and light gage ferrous alloys. 

11.2 Direct current generators can be portable and are noisy, require high maintenance, are inefficient, but can be used for most processes except GTAW of light metals.

11.3 Direct current rectifiers, more widely used for the same applications as the generators, are more recent developments with good power efficiency, no noise, and minimal maintenance concerns.

11.4 Combination ac-dc units with or without ac stabilization are available to increase application versatility with a wide variety of characteristics.

11.5 The multi-arc units are special rectifiers designed to reduce per-operator power cost for applications where several operators are working simultaneously.

11.6 Constant voltage units are used for GMAW and arc-air applications.

11.7 Combination constant voltage-constant current units are designed for automatic or semiautomatic GMAW and  SMAW applications.
11.8 Pulsed power units provide GTAW or PAW operators greater control latitude at considerable added expense.

12. Power Supply

12.1 The minimum and the name plate maximum current rating are important in selecting a welding power supply. For example, thin wall tubing requires 15-20 A.

12.2 A solid state pulsing unit can be added to conventional dc power supplies as well as purchased integrated in the power supply. This unit provides greater latitude for the operator than that afforded by a conventional constant current unit. This capability permits the use of pulsed GTAW and pulsed plasma arc welding.

Arc Welding Power Supplies

Rectifier Welders, Direct Current Generator Welders

Selection of Welding Power Supplies

TABLE III – Guide for Selection of Welding Power Supplies

Choosing a Welding Power Source is not as difficult as we think. Question arise How to pick a welding power source? Determine the following when purchasing a welding machine:

1. USAGE LOCATION (Check one)

 Shop
 Field

2. WELDING PROCESS TO BE USED (Check as many as applicable)

 SMAW
 GTAW
 PAW or cutting
 GMAW—solid fine wire (material—aluminum, steel)
 FCAW

3. OTHER USES (Check as many as applicable)

 Air arc gouging
 Thawing water pipe
 Heat treating

4. SIZE—CURRENT CAPACITY (Indicate anticipated need)

 Max covered electrode diameter (SMAW)
 Torch capacity or material thickness
 Max wire diameter and spool size (GMAW or FCAW)
 Current range (indicate both as required)
 Minimum
 Maximum

5. DUTY CYCLE REQUIRED (Check one)

 40 percent
 60 percent
 100 percent

6. CURRENT REQUIRED (Check one of each category)

 1 phase
 3 phase
 220/208 volt
 440/480 volt
 550 volt
 50 Hertz
 60 Hertz

7. METHOD OF CURRENT GENERATION (Check one)

 MG set
 Rectifier

8. ACCESSORIES REQUIRED (Indicate needs)

 Ampere Meter
 Volt Meter
 High frequency
 Polarity switch
 Pulsed power
 Gas/water solenoid
 Remote current control
 Remote contactor (on/off)
 Auxiliary 110 volt current
 Under carriage
 Cooling water recirculator

9. WELDING CURRENT TYPE (Indicate needs)

 AC/engine drive DC or
 AC/DC
 Constant potential (CP) or
 Drooper  or
 Combination
 Slope control (CP)

10. MULTIPLE ARC–SINGLE ARC (Indicate number)

 Multi-operators
 Single operator

Check out: WELDING IMPERFECTIONS

In this article we have discussed now Arc Welding Power Supplies, Rectifier Welders, Direct Current Generator Welders, Selection of Welding Power Supplies. For new welders always ask What are the basic power source designs for arc welding? 

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