Heating Systems 311
This internal friction produces a large amount of heat in the nonconduc-
tive material. Frequencies in the 50-Mhz range may be used for dielectric
heating. Dielectric heating produces rapid heating, which is spread even-
ly throughout the heated material. Common applications of this heating
method are the gluing of plywood and the bonding together of plastic
sheets.ELECTRICAL WELDING LOADS
Electrical welding is another common type of a heat-producing pow-
er conversion system. The types of electrical welding systems include re-
sistance welding, electric arc welding, and induction welding.Resistance Welding
Several familiar welding methods, such as spot welding, seam weld-
ing, and butt welding, are resistance welding processes. All of these pro-
cesses rely upon the resistance heating principle. Spot welding, illustrated
in Figure 12-4A, is performed on overlapping sheets of metal, which are
usually less than 1/4-inch thick. The metal sheets are clamped between
two electrodes, and an electrical current is passed through the electrodes
and metal sheets. The current causes the metals to fuse together. The in-
stantaneous current through the electrodes is usually in excess of 5000 am-
peres, while the voltage between the electrodes is less than 2 volts.
Seam welding, shown in Figure 12-4B, is accomplished by passing
sheets of metal between two pressure rollers, while a continuously inter-
rupted current is passed through the electrodes. The operational principle
of seam welding is the same as that of spot welding. Several other similar
methods, which are referred to as butt welding, edge welding, and projec-
tion welding, are also commonly used.Electric Arc Welding
While resistance welding utilizes pressure on the materials to be
welded, electric arc welding produces welded metals by localized heat-
ing without pressure, as shown in Figure 12-5. An electric arc is created
when the electrode of the welder is brought in contact with the metal to be
welded. Carbon electrodes are used for DC or AC arc welding of nonfer-
rous metals and alloys. Not all metals can be welded by the arc welding
process. When metals are welded together, part of the metals to be welded