FastROOT - Kemppi

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Kemppi ProNews 2 • 2006
welding arc voltage is low, the above problem
rarely has any real impact. Current AC TIG machines
use a squarewave or modifi ed squarewave, whose rising
and falling edges are extremely steep. As a result an
AC TIG can in some circumstances be sensitive to cable
length, for reasons which are addressed in connection
with pulse MIG welding.
MIG/MAG welding and pulse MIG welding
Conventional MIG/MAG welding uses a constant voltage
rectifi er, whose generated voltage depends very little
on the current. In addition to this, the conventional MIG/
MAG process only works well with a constant voltage
rectifi er. As a result, the MIG/MAG process is considerably
more sensitive to cable loss than the MMA process.
Short arc welding in particular is, merely due to its
low voltage, more sensitive to cable loss than a spray arc
process, which allows signifi cant cable losses in practical
use. In either case cable loss can be compensated by
increasing the voltage of the power source. Flux-cored
welding works well with very long cables; it is here that
fl ux-cored welding is more reminiscent of MMA welding
than short arc welding.
In pulse MIG welding cable impact differs considerably
from other processes. It is for this reason that the matter
will be discussed in greater detail below.
Dynamic losses
If pure DC is running through the wires or the current
changes slowly, the wires’ voltage loss can be calculated
as a product of the wire resistance and current. Indeed
this is precisely the point in MMA and DC TIG welding.
On the other hand, current changes in short arc MIG/
MAG and pulse MIG welding are so fast that the impact
of the changes must be given attention. The physical
explanation of the phenomenon is that current running
through a wire generates a magnetic fi eld, whose changing
requires energy that must fed into the fi eld with a cable.
This always takes time. If the welding current needs
to be altered suddenly, such as with a pulse MIG, the
current and the magnetic fi eld energy must be altered at
the same time.
The longer the cables are, the greater the amount of energy
that needs to be moved and the more time it takes.
The technical term describing the slowness of the abovementioned
change in current is called inductance. It is
determined by not only the cable length, but also its diameter
and placement. Inductance is at its lowest level
when the cable is routed in a hairpin loop or a single
large loop. If the cable is instead reeled, inductance can
rise to a very high level. This is illustrated in Figure 4.
According to one laboratory measurement, a 25 m cable
routed as shown in Figure 3B generated an inductance
of 20 μH, but when reeled as shown in Figure 3C the
same cable’s inductance was 110 μH, i.e. over fi ve times
more. Although this kind of increase in inductance will
cause a noticeable change in short arc welding, it will
not make welding impossible. However, the impact this
has on pulse MIG welding is so powerful that it is for all
intents and purpose no longer possible to weld properly.
The inductance of a reeled cable depends on the length
of the cable, primarily due to the fact that doubling the
length of a cable coiled around a reel effectively quadruples
the inductance. The impact that the reel dimensions
and placement have is minimal compared to cable
length. It makes no real difference whether a cable reel
is coiled around the bars of a transport cart or lying on
the fl oor.
Increasing the cable’s copper cross-section reduces inductance
only marginally. Where practical work is concerned,
it is recommended that welding cables be kept
as short as possible. A better alternative to reeling is to
shorten excess cable, provided that the power source is
close to the welding point. If it is for some reason impossible
to cut any excess cable, uncoiling the cable into
loops (as shown in Figure 3A or 3B) will signifi cantly
reduce inductance. An excellent way to reduce cable inductance
is to connect the return lead to the workpiece
next to the power source, if possible. According to fi ndings,
a workshop scale workpiece also serves as an excellent
return lead where inductance is concerned. In this
case only the torch cable has an impact on inductance.
Below is an examination of how cable inductance is
dependent on cable geometry. The external inductance
of the cable in Figure 3A can only be calculated using
the law of perforation, thus resulting in the following
inductance formula:
L = u0*l*ln(a/r0)/pi (1)
where
u0 vacuum permeance
l loop length (half the cable length)
a distance between the cables
r0 cable copper cross-section
In addition to this, the situation is also affected by frequency-dependent
internal inductance, for which the
formula is:
Z = (k*l)/(2*pi*r0*s)*J0(k*r0)/J1(k*r0) (2)
where
Z cable inductance
k (1-j)sqr(f*s*u)
s conductivity
J0 and J1 Bessel function of the first kind
At low frequencies (abs(k)