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 KiloAmperes (KA  one KiloAmp 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 lowcarbon 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 1520 Amps (0.0150.020 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.
Power
Power is Voltage multiplied by Current, and is measured in Watts, or
KVA (KVA stands for KiloVoltAmperes. Watts and KVA will be used interchangeably
in this text). This means that the amount of current flowing times the
pressure that's causing it to flow equals the amount of power generated.
A basic law to bear in mind is that the power going into a transformer
will always equal the power coming out of it. Returning to the transformer
example, 200 Amps coming in at 500 Volts (200 x 500 = 100,000 KVA) on
the primary with a 50 to 1 turns ratio in the transformer will be converted
into 10,000 Amps at 10 Volts (10,000 x 10 = 100,000 KVA) going out.
As the math illustrates, the results are the same. The initial and final
amperage and voltage may be different, but because the ratio is the
same, the total amount of power is also the same.
Resistance
As mentioned earlier, resistance is defined as the opposition that a
substance offers to the flow of electric current. Resistance is calculated
by dividing the Voltage by the Current, and is measured in Ohms. (When
written, Ohms are represented by the Greek letter Omega: .). Since resistance
to the current is what generates the heat in the workpiece, it is critically
important that the area with the greatest resistance be at the interface
between the two parts being joined. This interface is also known as
the faying surfaces. Remember that the heat is where the resistance
is, and the resistance is where the heat will be. If the area with the
most resistance is, for example, where the lower bus bar connects to
the transformer of the welder and not at the faying surfaces of the
workpieces, then that's where the heat will go. Likewise, if the greatest
resistance is at the contact area between the electrode tip and the
workpiece, the heat generated there will cause the tip to weld directly
to the workpiece.
