Arc Welding Equipment - the guts of the machine

Arc Welding Equipment

equipment used for arc welding typically consists of a constant current welding power supply and an electrode, with an electrode holder, a work clamp, and welding cables (also known as welding leads) connecting the two.

Arc Welding Equipment Power supply

A high output welding power supply for SMAW and GTAW

The power supply used in SMAW has constant current output, ensuring that the current (and thus the heat) remains relatively constant, even if the arc distance and voltage change.

This is important because most applications of SMAW are manual, requiring that an operator hold the torch.

Maintaining a suitably steady arc distance is difficult if a constant voltage power source is used instead, since it can cause dramatic heat variations and make welding more difficult.

However, because the current is not maintained absolutely constant, skilled welders performing complicated welds can vary the arc length to cause minor fluctuations in the current.

Arc Welding Equipment

The polarity of the SMAW system depends primarily upon the electrode being used and the desired properties of the weld.

Direct current with a negatively charged electrode (DCEN) causes heat to build up on the electrode, increasing the electrode melting rate and decreasing the depth of the weld.

Reversing the polarity so that the electrode is positively charged and the workpiece negatively charged increases the weld penetration.

With alternating current, the polarity changes over 100 times per second, creating an even heat distribution and providing a balance between electrode melting rate and penetration.

Arc Welding Equipment

Typically, the equipment used for SMAW consists of a step-down transformer and a rectifier, for converting alternating current into direct current.

Because the power normally supplied to the welding machine is high-voltage alternating current, the welding transformer is used to reduce the voltage and increase the current.

As a result, instead of 220 V at 50 A, for example, the power supplied by the transformer is around 17-45 V at currents up to 600 A.

A number of different types of transformers can be used to produce this effect, including multiple coil and inverter machines, with each using a different method to manipulate the welding current.

The multiple coil type adjusts the current by either varying the number of turns in the coil (in tap-type transformers) or by varying the distance between the primary and secondary coils (in movable coil or movable core transformers).

Inverters, which are smaller and thus more portable, use electronic components to change the current characteristics.

Generators and alternators are frequently used as portable welding power supplies, but because of lower efficiency and greater costs, they are less freqently used in industry.

Maintenance also tends to be more difficult, because of the complexities of using a combustion engine as a power source.

However, in one sense they are simpler: the use of a separate rectifier is unnecessary, because they can provide either AC or DC.

WELDING WITH CONSTANT CURRENT

The power source is the heart of all arc welding process. Two basic types of power sources are expressed by their voltage-ampere output characteristics.

The constant current machine is considered in this paragraph. The other power source, the constant voltage machine, is discussed in paragraph 10-3.

The static output characteristic curve produced by both sources is shown in figure 10-1. The characteristic curve of a welding machine is obtained by measuring and plotting the output voltage and the output current while statically loading the machine.

a. The conventional machine is known as the constant current (CC) machine, or the variable voltage type.

The CC machine has the characteristic drooping volt-ampere curve, and has been used for many years for the shielded metal arc welding process.

A constant-current arc-welding machine is one which has means for adjusting the arc current.

It also has a static volt-ampere curve that tends to produce a relatively constant output current.

The arc voltage, at a given welding current, is responsive to the rate at which a consumable electrode is fed into the arc.

When a nonconsumable electrode is used, the arc voltage is responsive to the electrode-to-work distance.

A constant-current arc-welding machine is usually used with welding processes which use manually held electrodes, continuously fed consumable electrodes, or nonconsumable electrodes.

If the arc length varies because of external influences, and slight changes in the arc voltage result, the welding current remains constant.

b. The conventional or constant current (CC) type power source may have direct current or alternating current output.

It is used for the shielded metal-arc welding process, carbon arc welding and gouging, gas tungsten arc welding, and plasma arc welding.

It is used for stud welding and can be used for the continuous wire processes when relatively large electrode wires are used.

c. There are two control systems for constant current welding machines: the single-control machine and the dual-control machine.

(1) The single-control machine has one adjustment which changes the current output from minimum to maximum, which is usually greater than the rated output of the machine.

The characteristic volt-ampere curve is shown by figure 10-2. The shaded area is the normal arc voltage range.

By adjusting the current control, a large number of output curves can be obtained.

The dotted lines show intermediate adjustments of the machine. With tap or plug-in machines, the number of covers will correspond to the number of taps or plug-in combinations available.

Most transformer and transformer-rectifier machines are single-control welding machines.

(2) Dual control machines have both current and voltage controls. They have two adjustments, one for coarse-current control and the other for fine-current control, which also acts as an open-circuit voltage adjustment.

Generator welding machines usually have dual controls. They offer the welder the most flexibility for different welding requirements. These machines inherently have slope control.

f. The constant-current type welding machine can be used for some automatic welding processes. The wire feeder and control must duplicate the motions of the welder to start and maintain an arc. This requires a complex system with feedback from the arc voltage to compensate for changes in the arc length. The constant-current power supplies are rarely used for very small electrode wire welding processes.

WELDING WITH CONSTANT VOLTAGE

The second type of power source is the constant voltage (CV) machine or the constant potential (CP) machine. It has a relatively flat volt-ampere characteristic curve.

a. The static output characteristic curve produced by both the CV and CC machine is shown by figure 10-1 above. The characteristic curve of a welding machine is obtained by measuring and plotting the output voltage and the output current while statically loading the machine. The constant voltage (CV) characteristic curve is essentially flat but with a slight droop. The curve may be adjusted up and down to change the voltage; however, it will never rise to as high an open-circuit voltage as a constant current (CC) machine. This is one reason that the constant voltage (CV) machine is not used for manual shielded metal arc welding with covered electrodes. It is only used for continuous electrode wire welding. The circuit consists of a pure resistance load which is varied from the minimum or no load to the maximum or short circuit. The constant current (CC) curve shows that the machine produces maximum output voltage with no load, and as the load increases, the output voltage decreases. The no-load or open-circuit voltage is usually about 80 volts.

b. The CV electrical system is the basis of operation of the entire commercial electric power system. The electric power delivered to homes and available at every receptacle has a constant voltage. The same voltage is maintained continuously at each outlet whether a small light bulb, with a very low wattage rating, or a heavy-duty electric heater with a high wattage rating, is connected. The current that flows through each of these circuits will be different based on the resistance of the particular item or appliance in accordance with Ohm’s law. For example, the small light bulb will draw less than 0.01 amperes of current while the electric heater may draw over 10 amperes. The voltage throughout the system remains constant, but the current flowing through each appliance depends on its resistance or electrical load. The same principle is utilized by the CV welding system.

c. When a higher current is used when welding, the electrode is melted off more rapidly. With low current, the electrode melts off slower. This relationship between melt-off rate and welding current applies to all of the arc welding processes that use a continuously fed electrode. This is a physical relationship that depends upon the size of the electrode, the metal composition, the atmosphere that surrounds the arc, and welding current. Figure 10-7 shows the melt-off rate curves for different sizes of steel electrode wires in a C02 atmosphere. Note that these curves are nearly linear, at least in the upper portion of the curve. Similar curves are available for all sizes of electrode wires of different compositions and in different shielding atmospheres. This relationship is definite and fixed, but some variations can occur. This relationship is the basis of the simplified control for wire feeding using constant voltage. Instead of regulating the electrode wire feed rate to maintain the constant arc length, as is done when using a constant current power source, the electrode wire is fed into the arc at a fixed speed. The power source is designed to provide the necessary current to melt off the electrode wire at this same rate. This concept prompted the development of the constant voltage welding power source.

d. The volt-ampere characteristics of the constant voltage power source shown by figure 10-8, was designed to produce substantially the same voltage at no load and at rated or full load. It has characteristics similar to a standard commercial electric power generator. If the load in the circuit changes, the power source automatically adjusts its current output to satisfy this requirement, and maintains essentially the same voltage across the output terminals. This ensures a self-regulating voltage power source.

e. Resistances or voltage drops occur in the welding arc and in the welding cables and connectors, in the welding gun, and in the electrode length beyond the current pickup tip. These voltage drops add up to the output voltage of the welding machine, and represent the electrical resistance load on the welding power source. When the resistance of any component in the external circuit changes, the voltage balance will be achieved by changing the welding current in the system. The greatest voltage drop occurs across the welding arc. The other voltage drops in the welding cables and connections are relatively small and constant. The volt-age drop across the welding arc is directly dependent upon the arc length. A small change in arc volts results in a relatively large change in welding current. Figure 10-9 shows that if the arc length shortens slightly, the welding current increases by approximately 100 amperes. This change in arc length greatly increases the melt-off rate and quickly brings the arc length back to normal.

f. The constant voltage power source is continually changing its current output in order to maintain the voltage drop in the external portion of the welding circuit. Changes in wire feed speed which might occur when the welder moves the gun toward or away from the work are compensated for by changing the current and the melt-off rate briefly until equilibrium is re-established. The same corrective action occurs if the wire feeder has a temporary reduction in speed. The CV power source and fixed wire feed speed system is self-regulating. Movement of the cable assembly often changes the drag or feed rate of the electrode wire. The CV welding power source provides the proper current so that the malt-off is equal to the wire feed rate. The arc length is controlled by setting the voltage on the power source. The welding current is controlled by adjusting the wire feed speed.

g. The characteristics of the welding power source must be designed to provide a stable arc when gas metal arc welding with different electrode sizes and metals and in different atmospheres. Most constant voltage power sources have taps or a means of adjusting the slope of the volt-ampere curve. A curve having a slope of 1-1/2 to 2 volts per hundred amperes is best for gas metal arc welding with nonferrous electrodes in inert gas, for submerged arc welding, and for flux-cored arc welding with larger-diameter electrode wires. A curve having a medium slope of 2 to 3 volts per hundred amperes is preferred for CO2 gas shielded metal arc welding and for small flux-cored electrode wires. A steeper slope of 3 to 4 volts per hundred amperes is recommended for short circuiting arc transfer. These three slopes are shown in figure 10-10. The flatter the curve, the more the current changes for an equal change in arc voltage.

h. The dynamic characteristics of the power source must be carefully engineered. Refer again to figure 10-9. If the voltage changes abruptly with a short circuit, the current will tend to increase quickly to a very high value. This is an advantage in starting the arc but will create unwanted spatter if not controlled. It is controlled by adding reactance or inductance in the circuit. This changes the time factor or response time and provides for a stable arc. In most machines, a different amount of inductance is included in the circuit for the different slopes.

i. The constant voltage welding power system has its greatest advantage when the current density of the electrode wire is high. The current density (amperes/sq in.) relationship for different electrode wire sizes and different currents is shown by figure 10-11. There is a vast difference between the current density employed for gas metal arc welding with a fine electrode wire compared with conventional shielded metal arc welding with a covered electrode.

j. Direct current electrode positive (DCEP) is used for gas metal arc welding. When dc electrode negative (DCEN) is used, the arc is erratic and produces an inferior weld. Direct current electrode negative (DCEN) can be used for submerged arc welding and flux-cored arc welding.

k. Constant voltage welding with alternating current is normally not used. It can be used for submerged arc welding and for electroslag welding.

l. The constant voltage power system should not be used for shielded metal-arc welding. It may overload and damage the power source by drawing too much current too long. It can be used for carbon arc cutting and gouging with small electrodes and the arc welding processes.