 © COPYRIGHT 1998 THE ESAB GROUP, INC. LESSON II 2.4.4 Power Source - A direct current, constant voltage power source is recommended for gas metal arc welding.  It may be a transformer-rectifier or a rotary type unit.  The lower open circuit voltage and self-correcting arc length feature, as described in Lesson I, makes it most suitable.  Constant voltage power sources used for spray transfer welding and for flux cored electrode welding (to be covered later) are the same.  However, if the unit is to be used for short-circuiting arc welding, it must have "slope" or slope control. Slope control is a means of limiting the high short-circuit current that is characteristic of this type welder.  Figure 13 shows the effect of slope on the short- circuiting current. 2.4.4.1 If we were short-arc welding at approximately 150 amperes and 18 volts, as shown in Figure 13, and had no slope components in the power source, the current at short-circuit or when the wire touches the work, would be over 1400 amperes.  At this high current, a good length of the wire would literally explode off the end, cause much spatter, and the arc would be erratic.  With the slope components in the circuit, the short-circuiting current is in the neighborhood of 400 amperes, and the molten ball is sort of pinched off the end of the wire more gently.  For those with an electrical background, it might be added that in some machines, slope is achieved by adding a reactor in the AC secondary of the power source.  In others, a slope resistor is added in the DC output portion of the circuit.  Slope may be adjustable for varying wire diameters or it may be fixed, giving a good average value for .035" and .045" diameter wires, the two most popular sizes. 2.4.4.2 Another factor influencing the arc in short-circuiting welding is the rate that the amperage reaches the short-circuiting current level.  Using the example in Figure 13, we know that the current goes from 150 amperes to 400 amperes during each shorting period.  If we were to plot the current rise on a graph, as in Figure 14, we would see that the current rise if very rapid, as shown by the broken line. FIGURE 13 25 20 15 10 5 200 400 600 800 1000 1200 1400 OPERATING POINT CONSTANT VOLTAGE V/A CURVE SHORT CIRCUITING CURRENT NO SLOPE SHORT CIRCUITING CURRENT WITH SLOPE EFFECT OF SLOPE ON SHORT CIRCUITING CURRENT V O L T S  Turn Pages