Page 184 - CITS - Welder - Trade Theory

WELDER - CITS

TOPIC - POWER SOURCE- high frequency unit,

D.C suppressor unit and uses

Objectives: At the end of this lesson you shall be able to
• describe the TIG welding power source
• explain the high frequency unit of TIG power source
• explain the working principal of TIG power source.

Selecting a Power Source

With the many types of welding machines available, certain considerations must be made in order to fit the right
machine to the job. Rated output of the welding machine is an important consideration. The ranges of voltage and
amperage needed for a particular process must be determined. Then, a welding machine can be selected to meet
these output needs. Remember, the output must be within a proper duty cycle range. Light welding, (low output
requirements of about 200 amps or less) can often be done with single-phase welding machines. Duty cycles are
often in the 60% or less range. These types of welding machines are especially suited for shops and garages
where only single-phase power is available. Some of these smaller single-phase machines may be capable of
using 115 volt AC primary power. Other machines may use 230 volt or higher primary power. Larger DC TIG
welding machines used for heavy plate, structural fabrication and high production welding generally need three
phase AC input power. Most industrial locations are supplied with three-phase power since it provides the most
efficient use of the electrical distribution system and it is required by many electric motors and other industrial
electrical equipment. These welding machines often have capacities of over 200 amps, and often have 100%
duty cycles.
Gas tungsten arc welding uses a constant current power source, meaning that the current (and thus the heat)
remains relatively constant, even if the arc distance and voltage change. This is important because most
applications of GTAW are manual or semiautomatic, requiring that an operator hold the torch. Maintaining a
suitably steady arc distance is difficult if a constant voltage power source is used instead, since it can cause
dramatic heat variations and make welding more difficult.
The preferred polarity of the GTAW system depends largely on the type of metal being welded. Direct current
with a negatively charged electrode (DCEN) is often employed when welding steels, nickel, titanium, and other
metals. It can also be used in automatic GTA welding of aluminum or magnesium when helium is used as a
shielding gas. The negatively charged electrode generates heat by emitting electrons which travel across the arc,
causing thermal ionization of the shielding gas and increasing the temperature of the base material. The ionized
shielding gas flows toward the electrode, not the base material. Direct current with a positively charged electrode
(DCEP) is less common, and is used primarily for shallow welds since less heat is generated in the base material.
Instead of flowing from the electrode to the base material, as in DCEN, electrons go the other direction, causing
the electrode to reach very high temperatures. To help it maintain its shape and prevent softening, a larger

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CITS : C G & M - Welder - Lesson 49 - 60