Critical Axial Magnetic Field for Prevention of Anode Spot Formation in Vacuum Interrupters

The objective of this paper is to show an effective critical axial magnetic field (AMF) as a function of contact diameter and arc extinguishing contact gap in vacuum interrupters (VIs), based on prevention of anode spots formation. Experiments were conducted in a demountable vacuum chamber, in which an external Helmholtz AMF coil was installed coaxially between a pair of butt-type contacts. The AMF flux density B-AMF applied by the external coil was uniform and it can be adjusted from 0 to 110 mT. The experimental results showed that an effective critical AMF by arc current B-AMF(eff) (mT/kA) needed for prevention of anode spots formation followed a power function with contact diameter D as B-AMF(eff) = sD(-alpha), where s and alpha were constant and D was in a range of 12 similar to 100 mm. And B-AMF(eff) (mT/kA) increased as a linear function with the arc extinguishing contact gap l, even though the increasing of B-AMF(eff) was so slow with l in a range of 12 similar to 24 mm. Last, a numerical calculation suggested that anode current density radial distribution contributed to the above findings.