Dr. Livingstone, I Presume WELDWISE 2400 - Manual (Page 12)

0428-INS-400 Rev. E
1-4
Power
Power is Voltage multiplied by Current, and is measured in Watts, or KVA (KVA stands for Kilo-
Volt-Amperes. 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: Ω). 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.
Electrodes
Typically made of copper alloys, electrodes actually have three separate functions: to conduct current
to the workpieces being welded, to transmit the proper pressure or force to those workpieces to
produce and forge a good weld, and to help dissipate heat from the area being welded. To ensure that
all three of these functions are executed properly, it is important to regularly maintain the electrodes,
keeping them clean and in good condition. A reprint of an RWMA chart describing various types of
electrode materials and their different uses may be found in Chapter 11, APPENDICES, of this
manual.
Conducting Current
The first of these functions is purely electrical— fire weld current through the workpiece. Taking into
account the relationship among current, voltage and resistance, it becomes important to pay attention to
the type of electrodes used. For example, it wouldn't be wise to select electrodes made entirely from a
high resistance material, since they would get so hot they'd melt before the current even had a chance
to flow to the workpiece. It is also important to make sure that the electrodes are the right size for the
application; proper electrode sizing is largely dependent on the amount of force being used on the
workpieces.
Transmitting Force
The second function of the electrodes is mechanical. The amount of force needed to make a good weld
varies, depending on the type of metal being welded and other factors, but a general figure would be
about 600-800 lbs. Because electrodes are typically on the small side— roughly from about the size of