Arc Welding Equipment - the guts of the machine
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.
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.
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.