AC Welding
In Johnston's preferred version, the weld was produced by a sinusoidal AC welder. The output voltage and output current for an AC welder are illustrated in Figure 1.
Figure 1: AC Output Power Curve
By taking the instantaneous product of the output voltage and output current, the output power can be plotted. The bottom curve seen in Figure 1 shows the power "peaks" that result from an AC power supply. During the time when the power decrease towards and increases from zero, there is very little heat output. As this happens, the weld is colder. These cold areas are commonly referred to as "stitches".
To compensate for these cold periods, the maximum output power (i.e., the peaks) must be raised over what is actually necessary for a good weld. This excess heat is transferred by conduction to the stitches to obtain a constant weld. (refer to Figure 2)
During the power peaks, the material is in a molten condition and ferrite balls may be expelled to the inside of the tube due to the magnetic field set up in the tube (commonly referred to as "weld spatter"). The spacing between the peaks of high temperature is a function of the operating frequency of the welder and the line speed of the mill.
Figure 2: Heat Output and Weld Bead Condition
As a rule of thumb is that the line speed should not exceed 1½ times the operating frequency of the welder. For example, a 60-cycle welder should not be operating faster than 90 FPM (60 x 1½ = 90). Should the line speed exceed 1½ times the operating frequency of the welder, there is a risk that 100% grain growth will not be obtained every half cycle and a weld failure may occur (sometimes referred to as a "zipper weld"). Properly matching the operating frequency, line speed of the mill and power setting, would maintain a 100% weld. A change in strip characteristics may require an adjustment in one or more of the preceding variables.