Evaluation of AC axial magnetic fields needed to prevent anode spots in vacuum arcs between opening contacts

Abstract

The influence of AC axial magnetic (AM) fields on the development of half-cycle drawn vacuum area is described. A Helmholtz coil provided an independent AC AM field B/sub ax//sup ac/ in the opening gap between flat profile, butt type, Cr-Cu contacts. For peak arc currents I/sub p/ up to 20 kA, critical fields B/sub crit//sup ac/ were determined which prevented anode involvement. The dependence of B/sub crit//sup ac/ on I/sub p/ is compared with the analogous dependence using constant magnetic fields B/sub ax//sup dc/. The results are used to extend the usefulness of the B/sub crit/ parameter to the design of contact structures which produce self-generated axial magnetic fields from the AC arcing current. A similar direct transition to the diffuse arc mode occurs without an imposed B/sub ax/ for separation currents I/sub s/<7 kA and I/sub p/<10 kA.