The Square Wave Welder
The goal of the Square Wave Welder was to combine all the good things from both conventional AC welding and DC welding. This meant a welder that was as easy to maintain as an AC welder but delivered the performance of a DC welder.
Square Wave Welding Theory
A simple square wave welder electrical schematic is shown in Figure 4. The main characteristic of a square wave welder is that only high voltage/low current is used until you reach the weld point. Electrically, the main difference between the DC and the square wave is that line current is rectified (through a full wave rectifier) without going through a step-down transformer. This is a high voltage/low current rectifier, and this high voltage/low current is then fed through a DC chopper and inverter which chops the DC into a special square wave AC.
Figure 4: Square Wave Welder Schematic
This high voltage/low current is then transmitted through conventional slip rings to the rotating unit on the tube mill and the high voltage/low current is transformed into low voltage/high current in a "vapor-cooled", single output transformer. The power is then transmitted to the tube through the rotating copper electrodes. The entire assembly is shown in Figure 5.
Figure 5: Square Wave Welder Assembly
A cross-section of a vapor-cooled welding transformer is shown in Figure 6. Although the term "vapor-cooled" is a coined term, it refers to the use of FC-40 freon coolant medium inside the transformer to take heat away from the transformer components. The absorption of heat causes the coolant to vaporize, hence the term vapor-cooled, and rise to the copper casing of the transformer. Here, it condenses thereby releasing its heat to the externally cooled casing.
Figure 6: Vapor-Cooled Transformer Cross Section
Square wave power units may be located wherever it is convenient to the plant layout. The square wave welder produces a square wave of power - not merely a square wave of voltage.
Square Wave Output Power
Figure 7 shows the power curve for a square wave welder. Voltage and current form a square wave and, even though they are changing direction approximately 200 times per second, they are in-phase and traveling in the same direction so the resulting power area is achieved. The little dip between cycles represents the amount of time (approximately 0.15 milliseconds) that it takes to go through zero which is further illustrated by Figure 8 which shows voltage across the electrodes.
Figure 7: Square Wave Welder Power Curve
Figure 8: "Scope Display" Square Wave Welder Power Curve
The angle of the lines going from plus to minus represents the switching time. The slight blip at the top and bottom of each half cycle represents a slight overshoot due to forced voltage introduced when passing through zero. This cuts down considerably on the amount of time it takes to change direction and keeps the area of power constant.
For all practical purposes, both the DC and square wave deliver constant power output.
How is power transmitted to the tube?
With the AC welder, current is traveling back and forth across the vertex at the operational frequency; with DC it is traveling across in one direction only and with square wave it is traveling back and forth at 400 times per second. With AC and square wave, there is a frequency effect which helps concentrate the current on the strip edges, and allows a variety of forming techniques to be used. DC requires special forming.
Tube diameter has no significant electrical affect on AC, DC or square wave efficiency. Wall thickness is generally the main factor determining speed versus power available; however, tube diameters from approximately 3/8" to 5/8" may present some speed restriction (mostly on thicker strip) due to a possible limitation transmitting full power from the electrodes to the tube. Mechanical forming variations, end use requirements, strip characteristics, etc. may also be a consideration. Sizes as small as 3/8" diameter can be run successfully on AC, DC and square wave welders.
What does the Tube see in the way of heat?
DC & Square Wave Welding
Since both the DC and square wave welders have a constant power rating, there is no compensation for periods of low power necessary. Consequently, the power can be set so that the temperature is maintained between the melting point and minimum good weld lines. Figure 9 shows this arrangement with a typical cross section of the weld shown at the right hand side. Typical of the DC and Square Wave welds are a uniform O.D. and I.D. weld bead upset, no periodic stitch and absence of ferrite balls or weld spatter inside of the tube.
Figure 9: DC & Square Wave Welder Heat Pattern
What about HF welding?
The main disadvantage to AC, DC and square wave welding is that it is a contact process that requires good electrical conductivity to achieve a weld. These welders are applicable only to low-to-medium carbon (0.05 to 0.20 carbon equivalents and 0.21 to 0.33 carbon equivalents with reduced speed and special procedures) clean surfaced steel. This can be either cold rolled or hot rolled pickled and oiled.
Consequently, HF induction welders employ a non-contact method of power transfer. This means they can weld any type of "weldable" material, which includes many grades of carbon or special alloy steels (cleaned or un-cleaned); many of the nonferrous such as aluminum, copper alloys, stainless steels; and many of the exotic materials.
What are characteristics of the different welds?
AC, DC & Square Wave Welds
Figure 10 shows a typical cross section of an AC, DC or square wave weld which typically is trapezoidal in shape with more of the weld bead upset to the outside than the inside. It is approximately 2/3 to the outside and 1/3 to the inside, but variations in the way the strip edges come together can vary the amount of bead extruded both inside and outside. The typical AC, DC or square wave weld has a wide heat-affected zone.
Figure 10: Weld Cross Section - AC, DC & Square Wave
HF Welds
Figure 11 shows a typical cross section of an HF weld which is characterized by an hourglass shape and a very narrow heat-affected zone.
Figure 11: Weld Cross Section - HF
