Ohm's
Law and Joule's Laws. The following laws are widely thought
to be what make or break resistance welding. While it is true that these
laws are very important to resistance welding, there are a few details
that should be clarified.
Ohm's
Law states that V (Voltage) = I (Current) x R (Resistance).
What does
this mean in realworld terms? Returning to the pipe example, the more
water pressure there is in a pipe (more voltage), the more water can
flow through that pipe (more current). If the size of the pipe decreases
(more resistance), then the water flow will decrease (less current)
but the pressure drop along the pipe will increase (more voltage).
Joule's
Law states that H (Heat) = I (Current) x V (Voltage) x T (Time the current
is allowed to flow).
Or, written
differently,
H (Heat)
= I2 (Current squared) x R (Resistance) x T (Time the current is allowed
to flow).
Note:
V (Voltage) = I (Current) x R (Resistance), so the two equations are
the same, just stated differently. The second version of this law is
probably more common in the field.
Joule's
Law is an equation that gives the amount of heat (energy) delivered
to something. It would seem sensible to assume that it's the amount
of heat delivered to the weld. However, it is important to consider
all the factors in the equation: Current, Voltage, and Time. Joule's
Law assumes that each of these factors remains constant in the secondary
of the welding transformer. A weld controller or weld timer may indeed
provide a constant amount of current at the electrodes, but recall Ohm's
Law: Voltage equals Current times Resistance, or written differently,
Current equals Voltage divided by Resistance.
Factors
like pitting or mushrooming of the electrodes, dirty workpieces, changes
in force, etc. all have an effect on the surface area (the area of contact)
between the electrode and the workpiece. Since changes in the surface
area affect the contact resistance (resistance of the surface area),
it is reasonable to say that the resistance at the workpiece is not
constant, but rather a factor that can change depending on a number
of other conditions.
If Resistance
is not constant, then according to Ohm's Law, Current is not constant
either. This means that the Isquared version Joule's Law will not reveal
the amount of heat generated at the workpiece unless the resistance
at the tips is known.
Simply
put, to determine how much heat is being generated at the workpiece
using Joule's Law, current, voltage or resistance must be measured at
the workpiece. Although a weld controller may be programmed to deliver
20 KA at 10 Volts, if there is significant resistance in the secondary
weld loop, the heat will go there and not to the workpiece. Likewise,
if the electrodes are worn or the workpiece is dirty, resistance and
current density will be affected. In such a situation, a controller
might indicate 10 Volts at the secondary, however there might actually
be only 5 Volts at the weld tips.
Such a
disparity could easily cause bad welds.
