The prime objective of an arc welding power source is to deliver controllable welding current at a voltage demanded by the welding process. The arc welding processes have different requirements with respect to the controls necessary to give the required welding conditions and these in their turn influence the design of the power source. In order to understand how the requirements of the processes affect the design of the power source it is necessary to understand the interaction of the power source and the arc characteristics.
If the voltage of a welding arc at varying arc lengths is plotted against the welding current the curves illustrated in Fig. 1 are obtained. The highest voltage is the open circuit voltage of the power source. Once the arc is struck the voltage rapidly falls as the gases in the arc gap become ionised and electrically conductive, the electrode heats up and the size of the arc column increases. The welding current increases as the voltage falls until a point is reached at which time the voltage/current relationship becomes linear and begins to follow Ohms Law. What is important to note from Fig. 1 is that as the arc length changes both the voltage and welding current also change – a longer arc giving higher voltage but with a corresponding drop in welding current and vice versa. This characteristic of the welding arc affects the design of the power source since large changes in welding current in manual metallic arc (MMA) and TIG welding is undesirable but is essential for the MIG/MAG and flux cored arc welding processes.
MMA, TIG and submerged arc power sources are therefore designed with what is known as a drooping output or constant current static characteristic, MIG/MAG and FCAW power sources with a flat or constant voltage static characteristic. On most power sources the slope of the characteristic can be changed either to flatten or make steeper the curves shown in Fig 2 and Fig. 3
Fig 2 shows drooping or constant current power source static characteristics, such as would be used for the MMA or TIG process, superimposed on the arc characteristic curves. When manual welding is taking place the arc length is continually changing as the welder cannot maintain a constant arc length. With a constant current power source as the arc length changes due to the welder’s manipulation of the welding torch there is only a small change in the welding current – the steeper the curve the smaller the change in current so there will be no current surges and a stable welding condition is achieved. Since it is primarily the welding current that determines such features as the penetration and electrode consumption this means that the arc length is less critical, making the welder’s task easier in achieving sound defect free welds. Typically, a ±5volt change would result in around a ±8 amp change at 150amp welding current.
In some situations – for example when welding in the overhead position or when the welder is faced with variable root gaps - it is an advantage if the welder has rather more control over deposition rates by enabling him to vary the rate by changing the arc length. In such a situation a flatter power source characteristic will be of benefit.
Submerged arc welding also uses a drooping characteristic power source where the welding current and the electrode feed rate are matched to the rate at which the wire is melted and transferred across the arc and into the weld pool – the “burn-off rate”. This matching of parameters is carried out by a monitoring system which uses the arc voltage to control the electrode feed speed – if the arc length/voltage increases the wire feed speed is increased to restore equilibrium.The constant voltage power source characteristic is illustrated in Fig. 3. This shows that as the arc length and hence the voltage changes there is a large change in the welding current – as the arc lengthens the welding current falls, as the arc shortens the current increases.
With MIG/MAG and FCAW power sources the welding current is controlled by the wire feed speed, the welding current determining the rate at which the welding wire is melted and transferred across the arc and into the weld pool – the “burn-off” rate. Therefore, as the current decreases the burn-off rate also falls, less wire is melted and the wire tip approaches the weld pool. In doing so, the voltage decreases, the welding current and hence the burn-off rate increase. Since the wire feed speed is constant there is a surplus of burn-off over wire feed such that the desired arc length, voltage and current are re-established. The converse also occurs – a shortening of the arc causes a reduction in voltage, the current rises, the burn-off rate increases, causing the arc to lengthen, the voltage to increase and the welding current to fall until the pre-set welding conditions are re-established. Again, a typical figure for the change in welding current for a constant voltage power source would be in the region of ±40amps for a change in arc length of ±5volts. This feature gives us what is known as a “self-adjusting arc” where changes in arc length, voltage and current are automatically returned to the required values, producing stable welding conditions. This makes the welder’s task somewhat easier when compared with MMA or TIG welding. Although in principle it may be possible to use a constant voltage characteristic power source for MMA welding it is far more difficult for the welder to judge burn-off rate than arc length so arc instability results and the method is not practicable.
In addition to this voltage control of the welding arc the speed at which the power source responds to short circuiting is important - this is known as the power source dynamic characteristic. Short circuits occur during arc striking and in MIG/MAG welding during dip transfer. As the voltage drops to zero when a short circuit occurs the current rises. If this increase in the current is fast and uncontrolled then the electrode tip blows like an electrical fuse resulting in excessive spatter – too slow a rise and the electrode may stub into the weld pool and extinguish the arc. This is not too significant when using the MMA process since the maximum current at zero voltage is controlled by the slope of the static characteristic curve and the welder can easily establish an arc gap. It is, however, important in the MIG/MAG process where a flat static characteristic power source is used and the current could rise to an extremely high value, in particular when welding in the dip transfer or short circuiting condition.
An electrical component called an inductor is therefore introduced into the power source electrical circuit. This device opposes changes in the welding current and hence slows the rate at which the current increases during a short circuit. The inductance is variable and can be adjusted to give a stable condition as shown in Fig. 4. Inductance in the welding circuit also results in fewer short circuits per second and a longer arc-on time - this gives a smoother better shaped weld bead. Too much inductance, however, may result in such a slow rise in the welding current that there is insufficient time for the arc to re-establish and melt the wire tip so that the welding wire then stubs into the weld pool. Inductance during spray transfer is also helpful in providing a better and less violent arc start.
This article was written by Gene Mathers.