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^-α, where s and α were constant and D was in a range of 12~100 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 12~24 mm. Last, a numerical calculation suggested that anode current density radial distribution contributed to the above findings.