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Lesson 1
The Basics of Arc Welding
Lesson 2
Common Electric
Arc Welding Processes
Lesson 3
Covered Electrodes for Welding
Mild Steels
Lesson 4
Covered Electrodes for Welding Low Alloy Steels
Lesson 5
Welding Filler Metals for Stainless Steels
Lesson 6
Carbon & Low Alloy
Steel Filler Metals -
Lesson 7
Flux Cored Arc Electrodes Carbon Low Alloy Steels
Lesson 8
Hardsurfacing Electrodes
Lesson 9
Estimating & Comparing Weld Metal Costs
Lesson 10
Reliability of Welding Filler Metals
© COPYRIGHT 1998 THE ESAB GROUP, INC. LESSON II 3.   Distance from Work - If the distance from the work to the power source is great, AC is the best choice since the voltage drop through the cables is lower than with DC.  Even though welding cables are made of copper or aluminum (both good conductors), the resistance in the cables becomes greater as the cable length increases.  In other words, a voltage reading taken between the electrode and the work will be somewhat lower than a reading taken at the output terminals of the power source.  This is known as voltage drop. 4.   Welding Position (See Appendix A - Glossary of Terms) - Because DC may be operated at lower welding currents, it is more suitable for overhead and vertical welding than AC.  AC can successfully be used for out-of-position work if proper electrodes are selected. 5.   Arc Blow - When welding with DC, magnetic fields are set up throughout the weldment.  In weldments that have varying thickness and protrusions, this magnetic field can affect the arc by making it stray or fluctuate in direction.  This condition is especially troublesome when welding in corners.  AC seldom causes this problem because of the rapidly reversing magnetic field produced. Combination power sources that produce both AC and DC are available and provide the versatility necessary to select the proper welding current for the application. When using a DC power source, the question of whether to use electrode negative or positive polarity arises.  Some electrodes operate on both DC straight and reverse polarity, and others on DC negative or DC positive polarity only.  Direct current flows in one direction in an electrical circuit and the direction of current flow and the composition of the electrode coating will have a definite effect on the welding arc and weld bead.  Figure 3 shows the connections and effects of straight and reverse polarity. Electrode negative (-) produces welds with shallow penetration; however, the electrode melt-off rate is high.  The weld bead is rather wide and shallow as shown at "A" in Figure 3.  Electrode positive (+) produces welds with deep penetration and a narrower weld bead as shown at "B" in Figure 3. FIGURE 3 DC POWER SOURCE ELECTRODE DC POWER SOURCE ELECTRODE A HIGHER BURN-OFF RATE, LESS PENETRATION DEEP PENETRATION, LOW BURN-OFF RATE WORK PIECE B STRAIGHT POLARITY REVERSE POLARITY WORK PIECE