The effect of the axial magnetic field structure on the threshold welding current for closed axial magnetic field vacuum interrupter contacts

Abstract

There are two designs of large area, vacuum interrupter contacts: the transverse magnetic field (TMF) contact and the axial magnetic field (AMF) contact. These contacts are required to perform a wide variety of roles within vacuum circuit breakers. One duty is to pass short-circuit currents with the vacuum interrupters' contacts closed for a period of time (1 to 4 seconds), after which the circuit breaker's mechanism must be able to open the contacts. Thus, the possibility of contact welding must be minimized. The flow of current through practical contacts generates a repulsive blow-off force, which has to be balanced by a closing force from the circuit breaker mechanism plus the force from atmospheric pressure acting on the vacuum interrupter's bellows. The axial magnetic field (AMF) vacuum interrupters have an additional attractive force because of the parallel currents flowing in the two AMF coils behind the contacts' faces. This force is calculated using three- dimensional finite element analysis (FEA) for three practical AMF designs using a contact diameter of 62 mm and a current of 31.5 kA (45.5 kA peak). The extra attractive forces are then combined with the other forces acting on the closed vacuum interrupter contacts to calculate the threshold welding current: the current above which the contacts will form a weld. Calculations of the total closing force compares the difference in the threshold welding current between the three AMF contact designs.