Effect of Eddy Currents on the Current Interruption Performance of Axial Magnetic Field Type of Vacuum Interrupters

In the axial magnetic field (AMF) type of vacuum interrupters (VI), the design of their contact system is such that the magnetic field is oriented parallel to the arc. AMF keeps the arc diffused, and the value of the currentup to which the arc stays diffused depends on the contact diameter and the magnitude of the axial field. A certainminimum AMF is required to keep the arc diffused. AMF also induces eddy currents in the contact discs. This paperanalyses the effect of eddy currents induced in the discs on AMF between the contacts, especially near the currentzero region. This analysis incorporates the opening motion of the contacts along with the variation of the coil currentwith time. Initially, a 2-D axis symmetric model of the AMF contact is simulated to determine the radial variation ofthe magnetic field along a plane mid-way between the two separating contacts. A 3-D model of the AMF contact is then analyzed to include the effect of radial slots in the contact plate. The change in the magnetic field profile with the number of slots is studied.