202 Modern Welding Copyright Goodheart-Willcox Co., Inc. A lower average current level is used in pulsed spray than in spray transfer. This lower average current level makes it possible to weld out of position. Thin metal sec- tions can also be welded more easily with the pulsed spray. This transfer method creates very little spatter. An advantage of the pulsed spray transfer method is that it can use larger-diameter welding wire. Larger- diameter electrodes are cheaper. Also, nonferrous wires of a larger diameter can be fed through the wire drive unit more easily without kinking. The pulsed spray transfer method can be used with silicon bronze welding wire. This process is sometimes called GMAW brazing or MIG brazing. The pulsed spray transfer method allows thin steel parts to be brazed with very low heat inputs. Very low heat input from the pulsed spray transfer process reduces distortion and melt-through problems. 8.3 GMAW and FCAW Power Sources GMAW and FCAW welding power sources provide a constant voltage. A common type of power source is the transformer-rectifi er machine. GMAW is done with DCEP. DCEN is used for some special applica- tions, including surfacing. Alternating current (AC) is not used for GMAW or FCAW. FCAW–G for carbon steel requires DCEP. Many self-shielding carbon steel electrodes use DCEN. Refer to the electrode manufac- turer’s polarity requirements. Inverter power sources are much smaller and lighter than traditional transformer-rectifi er machines. An inverter can be built to provide constant voltage for GMAW and FCAW or constant current for SMAW and GTAW. Some inverter power supplies can be used for multiple welding processes. Inverter power sup- plies designed for multiple processes provide a choice of constant current (CC) or constant voltage (CV) from the same machine. The welder must select the con- stant voltage mode for GMAW or FCAW. An inverter machine performs much like a transformer-rectifi er constant voltage machine but with some advantages. Transformer-rectifi er machines are designed to con- trol voltage. Voltage is one of the two important vari- ables used to set the welding parameters for GMAW and FCAW. On the front of the machine, there is a control that sets the voltage. The second variable is the wire feed speed. On machines used for GMAW and FCAW, the wire feeder may be built into the power source, or it may be an external unit. See Figure 8-10. If the wire feeder is built into the power source, the wire feed speed con- trol is on the welding power source. If the wire feeder is not in the power source, the wire feed speed is set on the external wire feeder. When the welder changes the wire feed speed, the welding current changes to adapt to the new wire speed. A higher wire feed speed requires a higher cur- rent to melt the welding wire faster. A slower wire feed speed requires less current to melt the welding wire. Inverter machine controls are similar to those on a transformer-rectifi er machine. If the inverter machine has the ability to perform multiple welding processes, select GMAW, FCAW, or the constant voltage setting. After making this selection, set the voltage. Next, set the wire feed speed. The wire feed speed setting deter- mines the wire feed speed and also sets the appropri- ate current. The output and performance of an inverter are the same as those of a transformer-rectifi er type machine. Chapter 34, Technical Data, includes addi- tional information about inverter machines. 8.4 Setting Up the GMAW/FCAW Station A complete GMAW outfi t is shown in Figure 8-11. The same equipment can be used for fl ux cored arc weld- ing. Remember, self-shielding FCAW does not require any shielding gas and uses a gun that does not have a gas nozzle. Cable to welding gun Electrical contact to gun trigger Water connections for water-cooled torch Miller Electric Mfg. Co. Figure 8-10. An inverter power source with a separate wire feeder mounted on top of the welder.