The static volt ampere characteristic of the arc. The external characteristic of the power source.

The static characteristic of the arc is the dependence of the arc voltage on the current Uд = f(Icв)  at the constant length (lд = сonst).

The numerous studies have established that, although there are differences for different arcs (inessential), the common for all arcs is that the dependence for the arc Uд = f(Icв) – the static characteristic of the arc is nonlinear (Fig. 1). This arc distinguishes from other energy consumers, for example, the resistor that has a linear characteristic (U = f(I) the dependence in accordance with Ohm’s law is linear). This is due to another mechanism of electrical conductivity in the arc than in metals.

Mr. Leskov G. I, for example (see Fig. 1.) showed that the characteristic Uд = f(Icв) depends on the diameter of the electrode. However, if this graph is rearranged so that the current density in the electrode is plotted along the abscissa axis: j=Iсв/(πd2/4),

(where d is the diameter of the electrode), then the Uд = f(j) dependence for all values of the electrode diameter is the same. According to the data of other researchers, for analog arcs (welding in CO2, Ar, etc.), there is the Uд = f(j)  analogous dependence (see Fig. 2).

The current-voltage characteristic for all arcs is U-shaped. This form of arc characteristic is mainly due to phenomena in the arc column and is related to the dimensions of the arc diameter (its column), temperature and conductivity.

In I section the falling static characteristic of the arc as the current increases (more precisely j). The growth rate of the number of carriers of electricity in the arc is faster than the current growth rate (it increases more intensively). In II section, with increasing Icв(j) in proportion to the growth of I, the conductivity of the arc increases. In III section, the increasing static characteristic of the arc – the arc behaves like the normal resistive element. This is due to the fact that in the III section the number of carriers of electricity does not increase. All possible carriers of electricity were formed and to increase the current (I) it is necessary to increase the energy of the electric field between the electrodes, increase the arc voltage.

The static volt-ampere characteristic of the arc is removed with the steady burning of the arc (prolonged combustion with given parameters of the regime) at the constant length of the arc. The construct the characteristic by points, fixing the values Icв, Uд (changing Icв by adjusting the power supply). The volt-ampere characteristic of the arc characterizes the energy needs of the arc. The unit current in the I section – decreases, in the II – const, in the III – grow.

The external characteristic of the arc power supply (also volt-ampere) is removed when the active or inductive load resistance is switched on instead of the arc. This measures the voltage at the terminals of the power source and the current in the welding circuit. The external characteristics of the arc power sources can be increasing, stiff, gently falling, steeply falling and vertical (Fig. 3). The external characteristics of the arc power sources reflect the energy capabilities of the sources. With a rigid external characteristic, these possibilities are limited only by the design parameters of the welding chain. With a falling external characteristic, these possibilities of the source with increasing I first increase, and with a further increase of I, they decrease.

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Ivan Perfilyev
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