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A problem that exists for resistance welding process is that the
mechanical and electrical characteristics of the tools that furnish
welding current can change over time. This time can be gradual or
sudden. Examples of gradual changes are the wearing out of electrical
cables or the corrosion of electrical contacts in the secondary
welding circuit. This would be seen as an increase of resistance
to current. An example of a sudden change is the variation of magnetic
materials being inserted in the secondary throat which affect the
inductive impedance. A specific example of this would be a large
welding gun that does repetitive welds on a part which feeds into
the throat of the welding gun. In either case, the resulting output
current will vary.
In the table above, you can see what happens to the output if it
is programmed to give a percentage of the maximum available current.
If in condition (f1) the maximum available output is 20,000 amperes
and we need 10,000 amperes for the weld, the user would program
50% current to obtain the desired response. If a change is introduced,
whether it be gradual or sudden, the same level will yield lower
current levels. Condition (f2) for example would have seen changes
in resistive or inductive impedance such that the maximum available
output would only by 14,500 amperes. Therefore if the user is still
using the 50% current level, the output would only be 7,250 amperes,
which may be shy of producing the desired weld nugget.
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