An electric arc is a form of a self-maintained gas discharge—i.e., a discharge which does not need an external gas ionization source for continuous burning. An electric arc burns between two electrodes: positive (anode) and negative (cathode). If an electric arc is fed from an (AC) power source with a given frequency, then the cathode and anode replace each other at the same frequency. The term “arc” is due to the fact that a sufficiently long discharge between the horizontal electrodes has an arc shape, caused by free-convective vertical gas motion. A long electric arc can be divided into three areas: a conducting column, the properties of which at some length apart from the electrodes are independent of physical phenomena near the electrodes; and two areas near the electrodes, namely, the near-anode and near-cathode areas. In near-electrode areas, a noticeable increase of electric field strength usually occurs compared with an electric arc column. Voltage drops in these areas are called cathode and anode voltage drops. Their values usually don’t exceed 10 volts.
In an electric arc column, gas is heated to a high temperature and its electrical conductivity is attributed mainly to thermal ionization processes. At pressures higher than atmospheric pressure, gas in an electric arc column is usually in local thermodynamic equilibrium state.
An electric arc which burns in a large gas volume and isn’t affected by external factors (e.g., by gas flow or applied magnetic field) is called a free-burning arc. Such an arc usually rapidly and randomly moves and changes its shape. In special devices, particularly in plasmatrons, it is possible to have a stationary electric arc (e.g., arc burning in a narrow, cylindrical, insulating channel) or to arrange its motion in an ordered fashion. Such electric arcs are called stabilized arcs.
The dependence of electric arc voltage on its current is called current-voltage characteristic (CVC). CVCs are classified into the static CVC, which is based on stationary current and voltage values and dynamic CVC’s, which connect the corresponding instantaneous values.
The CVC of most direct current (DC) electric arcs is such that a current rise leads to a voltage decrease (drooping characteristic, see Figure 1, curve 1) or to a constant voltage (independent characteristic). Thus, an electric arc doesn’t follow Ohm’s Law and represents a nonlinear element of an electric circuit. To keep a stable electric arc burning, an additional resistor is connected in series with an arc to increase a power source’s own CVC slope (see Figure 1: curve 2 is a CVC of a power source without resistor; curve 3 is a CVC of a power source with resistor). Point A corresponds to unstable electric arc burning because with an occasional increase of current Ia by a magnitude of ΔI, a positive potential difference, ΔV, arises which causes further current increases until point B is reached. This corresponds to stable arc burning at current Ib. An additional resistor substantially decreases the energy efficiency of an electric arc device. To avoid this disadvantage, special power sources are sometimes used. Certain stabilized electric arcs have rising CVCs; in this case, it is possible to substantially decrease resistor magnitude or to entirely remove it from a feeding circuit.
Figure 1. Current-voltage characteristics for electric arcs (1 - “drooping” characteristic, 2 - CVC for power source without resistor, 3- CVC with resistor).
For alternating current (AC) electric arcs, current-time dependence during each half-period is near sinusoidal; voltage-time dependence usually has a near-rectangular shape, with characteristic sharp voltage peak at the point of origin (so-called ignition peak). A dynamic AC CVC has a loop shape that indicates a hysteresis phenomenon caused by thermal inertia of the electric arc column. A CVC, plotted by the effective values of current and voltage, has the same shape as a DC arc under the same conditions. That is why for stable AC arc burning, an induction coil is connected to a circuit in series with arc (more seldom a resistor is used). An advantage of an induction coil over a resistor is that the coil has a low resistance and consequently doesn’t influence an electric arc device’s efficiency. On the other hand, this leads to a significant decrease in power factor.
An electric arc is a powerful, highly-concentrated source of heat and light. These electric arc properties determine the main areas of its application. Electric arcs are widely used in various welding devices, in steel-melting arc furnaces and in plasmatrons. Arc light sources are used in various lighting devices (e.g., in floodlights). In cinematographic projection equipment, high-pressure xenon arc lamps are used. The light spectrum of a xenon electric arc is close to sunlight, which is why such lamps provide “white” light and correct color transmission.