Effects of Optimized Switching Contact Materials, Aging, and Superimposed Axial Magnetic Field on Chopping Current in Vacuum Interrupters

Abstract

During opening and interruption operations in vacuum chambers, an arc is created during mechanical contact separation, consisting of vaporized contact material. With decreasing current, it becomes an unstable situation, and current can chop before the natural current zero crossing. This transient process, with a high current gradient, can cause high overvoltage in the network, particularly at inductive loads due to resonances. This may result in irreversible damages to the insulation of connected equipment, such as transformers or motors, as well as electromagnetic compatibility (EMC) issues. Switching contact materials for applications in contactors, such as tungsten carbide silver (WC-Ag), show chopping current values ranging from 0.7 to 2.5 A. To reduce chopping currents, different approaches are needed. Specifically, a proper selection of contact materials can minimize chopping currents or, at the very least, shift them closer to the zero crossing. Easily emitting contact materials at low temperatures are favorable. Additional tests cover the superimposition of externally applied magnetic fields during the arc burning phase. This research paper analyzes both the combination of different known and optimized contact materials and the superimposition of axial magnetic fields during interruption operations. Using materials comparable to WC-Ag reduces the chopping current by about 52% without a superimposed magnetic field. Other contact materials show field-dependent characteristics, such as copper chromium (Cu-Cr). Regarding interruption speed, an improvement in chopping behavior is observed with slower movements. Finally, the focus is set on the aging behavior. More than 10 000 switching opening operations are performed in this work, and, with the exception of one material combination, no aging can be observed.