Direct current is not used because there is no way of reducing the voltage
conveniently without placing resistance wires in the circuit and this uses
power without producing useful work. Direct current may be changed to
alternating by having a direct current motor running an alternating current
dynamo, or the change may be made by a rotary converter, although this last
method is not so satisfactory as the first.
The voltage used in welding being so low to start with, it is absolutely
necessary that it be maintained at the correct point. If the source of
current supply is not of ample capacity for the welder being used, it will
be very hard to avoid a fall of voltage when the current is forced to pass
through the high resistance of the weld. The current voltage for various
work is calculated accurately, and the efficiency of the outfit depends to
a great extent on the voltage being constant.
A simple test for fall of voltage is made by connecting an incandescent
electric lamp across the supply lines at some point near the welder. The
lamp should burn with the same brilliancy when the weld is being made as at
any other time. If the lamp burns dim at any time, it indicates a drop in
voltage, and this condition should be corrected.
The dynamo furnishing the alternating current may be in the same building
with the welder and operated from a direct current motor, as mentioned
above, or operated from any convenient shafting or source of power.
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