Dr. Livingstone, I Presume WELDWISE 2400 - Manual (Page 11)

0428-INS-400 Rev. E
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During the Hold Time, electrode force is still applied, even after the weld current has ceased. During
this period, the weld nugget cools and the metals are forged under the force of the electrodes. The
continuing electrode force helps keep the weld intact until it solidifies, cools, and the weld nugget
reaches its maximum strength.
Critical Factors in Welding
Understanding the resistance weld process requires an understanding of the main factors involved and
how they work together. This section will review current, voltage, resistance, and power, as well as the
various functions of the electrodes and how they affect surface contact and current density.
Current
Current, usually measured in Kilo-Amperes (KA — one Kilo-Amp is equal to 1,000 Amps), is one of
the most important factors. A resistance weld cannot be made unless there is sufficient weld current.
According to the RWMA, the typical amount of current needed to weld low-carbon steel, for example,
is about 10,000 Amps (10 KA) at about 5 Volts. To put this in perspective, a normal household or
office outlet provides a maximum of 15-20 Amps (0.015-0.020 KA) at 120 Volts, while a power
circuit in a factory may only be capable of providing 200 Amps (0.200 KA) at 500 Volts to a welder.
The factory's 200 Amps is then converted to the 10,000 Amps needed to weld by means of a welding
transformer.
A transformer consists of two coils of wire, called the primary and the secondary, wound around an
iron core. Power is transferred from primary to secondary via the magnetic properties of the iron. The
factor by which the current and voltage is stepped up or down is equal to the ratio between the number
of turns of wire in the coils forming the primary and secondary windings of the transformer. Consider
the steel that needs 10,000 Amps (10 KA) of current to be welded in a factory that can only provide
200 Amps (0.200 KA). If the welding transformer had 100 turns on the primary and 2 turns on the
secondary, the 'turns ratio' would be 100 to 2, or more simply, 50 to 1. The 200 Amp current in the
primary would then be converted (stepped up) to 10,000 Amps (200 Amps x 50 turns = 10,000 Amps)
in the secondary, which would yield enough amperage to make a weld.
Voltage
If current is the amount of electricity flowing, then Voltage (measured in Volts) is the pressure or force
that's causing the flow. A good analogy is water flowing through a pipe. A larger voltage will result in
greater water pressure, which will cause more water (current) to flow through the pipe. Using the
transformer example above, after the 200 Amps at 500 Volts on the primary passes through the
transformer coils, the secondary amperage increases to 10,000 Amps, but the voltage actually drops to
10 Volts. This decrease in voltage occurs because the amount of power coming out of a transformer
isn't actually increased, but more accurately exchanged.