Cooling Mechanisms of Switching Arcs Under Transverse Magnetic Fields in Comparison With Arcs Without Magnetic Blast

Abstract

The importance of low-voltage devices to interrupt dc currents under load and fault conditions has been considerably growing recently. From experimental work, it is known that arcs in strong transverse magnetic fields (e.g., flux density 1 T) can generate arc voltages considerably higher than those generated without field. In a recent study, it was shown by the author in 2-D simulations that in arrangements where the arc is magnetically squeezed against a chamber wall, the dominant mechanism of power removal is convection propelled by the JxB force (“electromagnetic pump”). Immediately in front of the wall the convective heat flux is converted to heat conduction toward the cold wall. This subsequent work is concentrated on the mechanism of free-moving arcs in transverse magnetic fields. Here, a similar effect of voltage increase has been known, but the cold wall as final target of power transport is missing. For differentiation, the cooling mechanism of arcs in chambers without magnetic blast field is also studied. While the temperature profiles and consequently the arc field strength for still arcs in chambers are dominated either by heat conduction or by radiation, depending on the current, free-moving arcs in cross fields are mainly cooled by the convective JxB pump. Heat conduction is negligible. Power is consumed by the heating of the plasma gas masses, which can be visualized by an equivalent flux. Values of the arc field strength, i.e., voltage per arc length, are also presented for dc currents between 4 and 1000 A. They increase in the sequence “chamber without field”, “free arc with cross field”, and “chamber with cross field”. In comparison with air, hydrogen yields clearly higher voltages.