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current peak at the moment the tip of the wire short-circuits with the base metal.
Improved arc initiation contributes to the operating efficiency of GMAW robots.
2.2.2.1 Volt-ampere characteristics
Irrespective of design, it is well established that the performance of power
sources depends on their static and dynamic characteristics. These are commonly
referred to as slope and inductance and are normally fixed by the power source
manufacturers.
The static characteristic describes the relationship between mean output
current and the corresponding voltage available from the power source [ref. 51]. A set
of output-voltage versus output-current characteristic curves (volt-ampere curves) are
used to describe the static characteristics [ref. 2]. The slope of these curves is used to
control and limit the amount of short circuit current which is attainable. The steeper
the slope, the smaller is the available short circuit current [ref. 51]. Hence, the slope
can be used to reduce spatter in dip transfer mode. It must, however, be optimised
since a too little magnitude might result in very high currents during short circuiting,
leading to explosive transfer and spatter, whereas too much slope would lead to arc
ignition problems, mainly caused by inadequate current during short circuiting [ref.
40].
The dynamic characteristic of an arc welding power source describes its
response to instantaneous
variations in the load across its terminals. These transients
normally occur [ref. 2] during the striking of the arc, during rapid changes in arc
length, during the transfer of metal across the arc and, in dip transfer, during arc
extinction and reignition in a short-circuiting cycle. The power source design features
that have an effect on the dynamic characteristics are those that provide
[ref. 2]:
1. Local transient energy storage, such as parallel capacitance circuits or dc
series inductance;
2. Feedback controls in automatically regulated systems;
3. Modifications of waveform or circuit-operating frequencies.
In a conventional power source design, the dynamic characteristics are mainly
determined by a dc series inductor, whose inductance can generally be adjusted by
electromagnetic means [ref. 3]. In the modern inverter-controlled power sources, the
inductance effect can be achieved by electronic means [ref. 61]. The higher the
inductance is, the longer the current takes to rise to its maximum value in a short
circuiting situation. This effect can be used to control spatter in dip transfer [refs. 62,
63], since a long short-circuiting time implies that the current level necessary for
breaking the molten metal bridge and, therefore, the explosion energy, will be smaller.
However, an excessively high inductance may result in a short-circuiting current
which is smaller than the minimum necessary for rupturing the molten bridge, leading
to wire stubbing (see section 2.1.5.2). Furthermore, it may result in erratic starting
and in a sluggish unstable arc [ref. 51]. Its value must, consequently, be optimised for
the different current levels [ref. 61].
The power sources used in GMAW can be classified into two main types,
namely: a) constant-voltage power source and b) constant-current power source.
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