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into which the materials to be welded are placed. Tubular metal is often
welded in this way.POWER CONSIDERATIONS FOR ELECTRIC WELDERS
Electric welders are rather specialized types of equipment, since
they use very high amounts of current at low voltage levels. They have a
peculiar effect on the power system operation. They draw large amounts
of current for short periods of time. Silicon-controlled rectifiers (SCRs) are
commonly used to control the starting and stopping of the large currents
associated with electric welders. The current rating of these devices must
be very high, sometimes in the range of 1000 to 100,000 amperes, and the
power distribution equipment must be able to handle these high currents.
SCRs are discussed in Chapter 17.
Figure 12-6 illustrates a typical electric welding system. The AC pow-
er supplied from the branch circuit of the power system is either stepped
down by a transformer to deliver AC voltage to the welder, or rectified to
produce DC voltage for DC welders. In either type of machine, an SCR
contactor may be used to control the on and off time of the welder.Figure 12-5. Electric arc welding