With a
carbon electrode the voltage will vary from zero to forty-five volts. With
the metallic electrode the voltage will vary from zero to thirty volts. It
is, therefore, necessary for the welding machine to be able to furnish to
the arc the requisite amount of current, this amount being varied, and
furnish it at all times at the voltage required.
The simplest welding apparatus is a resistance in series with the arc. This
is entirely satisfactory in every way except in cost of current. By the use
of resistance in series with the arc and using 220 volts as the supply,
from eighty to ninety per cent of the current is lost in heat at the
resistance. Another disadvantage is the fact that most materials change
their resistance as their temperature changes, thus making the amount of
current for the arc a variable quantity, depending on the temperature of
the resistance.
There have been various methods originated for saving the power mentioned
and a good many machines have been put on the market for this purpose. All
of them save some power over what a plain resistance would use. Practically
all arc welding machines at the present time are motor generator sets, the
motor of which is arranged for the supply voltage and current, this motor
being direct connected to a compound wound generator delivering
approximately seventy-five volts direct current. Then by the use of a
resistance, this seventy-five volt supply is applied to the arc.
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