CHARACTERISTICS OF WELDING POWER SOURCES
3.2.1 Arc Welding Power Sources
The various welding processes described in Chapter 2 require special power sources (having low voltage and high current for arc welding) to produce energy sufficient to make a good weld. Power sources could be a. c. (transformers), d. c. (generator/rectifiers) with constant current or constant voltage characteristics having current rating 70-400 amperes at 60% or 80% duty cycle. Heat input to the weld is a function of arc voltage, arc current and travel speed. Arc length is related to arc voltage.
The voltage supplied by the electrical generating stations for industrial use is 240 or 480 volt and the open circuit voltage for arc welding is between 50-80 V. Once the arc is struck the working voltage falls down to 10 to 30 V. As arc is the source of welding energy its study is, therefore, important.
3.2.2 Arc Characteristics
When the arc operates in a stable manner, the voltage and current are related. The relationship is shown in Fig. 3.1. It can be seen from this graph that the arc does not follow Ohm’s law.
Fig. 3.1 Typical arc characteristic compared with Ohm's law
The arc voltage varies only slightly over a wide range of currents.
• The curve does not pass through the origin.
• The slope of the curve depends upon:
(i) metals involved (ii) arc atmosphere (iii) arc length
3.2.3 Arc-length Control
For this discussion consider arc characteristics for four arc-lengths between tungsten and copper electrodes in argon atmosphere (Fig. 3.2). From this data we can plot a relation between arc-length and arc-voltage (Fig. 3.2). Suppose a welder uses GTA Welding process to weld copper sheets and makes a current setting of 150 A. The arc-characteristics (Fig. 3.2) show that for a 2 mm arc to be operating stable, the voltage should be 15 V. This value of arc voltage will be maintained as long as the power source delivers 150 A and the welder maintains an arc length of 2 mm. This is practically not possible during manual welding operation as the arc length may change, and consequently the voltage will rise or fall accordingly and the operating point will, therefore, shift from one characteristics to another. For arc to remain stable, the power-supply unit must allow the voltage to vary while keeping the current substantially constant (Fig. 3.3). Thus, the power-supply unit must meet the practical requirements for a specific process. A typical characteristics curve for manual GTA Welding operation is shown in Fig. 3.4.
Arc length (mm)
Fig. 3.2 Arc characteristics for welding copper (G. T.A. welding)
Z= 156 A
Fig. 3.3 Variations in voltage and current with change in arc-length
When welding is not taking place, no output current is drawn from the circuit. The voltage at the output is called open circuit voltage (O. C.V.) and it is of the order of 50-80 V. As the welding arc is struck and welding operation is carried out the voltage falls and over an operating range of 10-30 V the current varies only a little. Power-sources of this type of volt - ampere output are known as “drooping characteristics” units or ‘constant-current’ machines.
Fig. 3.4 Typical power supply characteristics used in manual GTA welding operation
If the arc-characteristics and power-source characteristics are plotted on one graph (Fig. 3.3) their intersection gives the working voltage and current. Let us, consider the example of welding copper with GTAW process using 150 A, 15 V and 2 mm arc length. If the arc length changes to 3 mm, the voltage increases to 16.5 V but current falls to 143 A. (power input is increased to + 4.8%). Conversely if the arc length is decreased to 1 mm, the voltage falls to 13.3 V and current increased to 156 A (power input is reduced by — 7.8%). It is important here to note that as a manual arc welder makes a weld, as a result of inadvertent hand movements the power input remains within 8% of the preset value. This is much better than requiring them to maintain a consistent travel speed.
In SMA Welding the situation is similar with an additional requirement on the part of the welder to match the electrode feed rate with the burn-off rate. In manual metal arc welding (SMA Welding) the consistency of the weld depends on the skill of the operator in judging the arc length and adjusting the electrode feed rate.