El principio de igualdad

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 relation-ship is shown in Fig. 3.1. It can be seen from this graph that the arc does not follow Ohm’s law.

Current Ohm's law

Arccharacteristic

Voltage

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 cop-per 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 operat-ing 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

Fig. 3.2 Arc characteristics for welding copper (G.T.A. welding)

Voltage

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.

O.C.V.

Voltage

Normal operating range

Current

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 weld-ing (SMA Weldweld-ing) the consistency of the weld depends on the skill of the operator in judgweld-ing the arc length and adjusting the electrode feed rate.

3.2.4 Self Adjusting Arc in GMA Welding

• Here the situation is different, the voltage setting of the power-source and not the welder controls the arc length.

• In GMA/GTA Welding the feed wire diameter is usually very small and the burn-off rates are far higher than in SMA or TIG Welding, and they vary much more with current. A small variation in current causes significant change in burn-off rate. Some typical burn-off curves for low-carbon-steel wires with carbon-di-oxide shielding are shown in Fig. 3.5. Change in burn-off rates with change in current are also shown.

We find that the electrode burn off rate changes rapidly with change in current. Thus we should have a power source which can accomodate these large changes in the

burn-off rates. For a small change in voltage, there should be a large change in current.

Special power-sources have been designed for this purpose.

0 2 4 6 8 10 12 14

Arc unstable 400

300

200

100

Weldingcurrent(A)

1.6 mm

dia 1.2 mm diameter wire electrode

0.8 mm dia

Wire feed speed m/min

Fig. 3.5 Wire feed rate Vs current for three electrodes in CO₂ welding

• Some welding power sources are designed to give a flat volt-ampere characteristics as shown in Fig. 3.6 with a voltage falling by 2 V for each 100 A fall in current. This type of characteristics is also known as constant potential characteristics.

100 200 300 400 500

40

30

20

10

Voltage(V)

Current (A) 35 V

B A

Slope 2 V/100 A

Fig. 3.6 Output characteristics for a constant-potential power-supply unit

• Consider an arc operating at 300 A, 35 V (point A in Fig. 3.6). If the arc length increases, voltage rises to point B (say). This causes significant decrease in current, giving lower burn-off rate. Arc length is immediately adjusted as the electrode tip in this situation will approach weld pool, and the arc length shortens. When this happens the current

increases and the burnoff matches with wire feed rate. The system returns to equilibrium.

• Conversely, if the arc-length shortens, the voltage falls, the current rises, burn-off rate increases, wire melts faster than it is being fed into the area, arc length thus increases continuously till it reaches the preset value. This is called self-adjustment of the arc.

• With electrode wires 0.8-1 .6 mm diameter, this requirement for rapid self-adjust-ment is readily met. For example, with 1 .2 mm wire using carbon dioxide shielding, a change in 20 A causes a change in burn of rate of 0.5 m/min. Thus a change of 1 mm in arc length will be adjusted in (60/500) seconds = 0.12 seconds. Proceeding in the same way we find that in manual metal-arc (MMA) welding a change in arc length of 1 mm

Table 3.1. Effect of change in current on burn-off rate

Welding Wire Change in Change in Time taken to adjust 1 mm Process diameter Current Burn-off rate change in arc length (sec)

CO₂ Welding 1.6 mm 20 A* 0.3 m/min** 0.20 sec.

CO₂ Welding 1.2 mm 20 A 0.50 m/min. (5.1–5.6) 0.12 sec CO₂ Welding 0.8 mm 20 A 1.1 m/min (10.4–11.5) 0.054 sec

SMA Welding 4 mm 20 A 0.02 m/min. 3.00 sec.

(200 Amperes oper. current)

*(200–to–220 Amp) **(2.5 to 2.8)

will require 3 seconds to self-adjust itself. This is too long as compared to the time taken by the operator to adjust it manually. Thus, for MMA Welding better results will be obtained if the current is kept constant by the use of drooping characteristics power supply.

Table 3.2. Control of welding parameters in TIG, MIG and MMA Welding

Welding Arc length Voltage Electrode Current

Process feed rate

TIG Welder Welder Not applicable Power supply

MIG Power supplyPower supplyWire feed Electrode speed via

via voltage wire feed motor

MMA Welder Welder via arc Welder Power supply

length