Cr and Mo Cathode Spots Distribution Characteristics with External Axial Magnetic Field

CuCr contact materials are dominant in vacuum circuit breakers. It is interesting to find superior material than the CuCr. Mo has a higher melting point and boiling point, so that it becomes a potential candidate to replace Cr. The objective of this paper is to compare the distribution characteristics of cathode spots (CSs) on Cr and Mo cathodes in drawn vacuum arcs with an external axial magnetic field (AMF). The experiments were carried out in a demountable vacuum chamber. A pair of Helmholtz coils were set in the chamber to supply a uniform AMF. The arc current in the experiments was 10 kA (rms) at 44.5 Hz. The AMF flux density were 75, 133, and 187 mT, respectively. The experimental results showed that, under the three levels of AMF, the constriction of the CSs of Mo cathode was more serious than those of Cr cathode, which would lead to the more severe erosion of Mo cathode. Especially, before current zero, much cathode spot plasma particles jetted from the Mo cathode contact, while that of Cr cathode was much less. Therefore, Mo could not replace Cr for vacuum circuit breakers, indicating that the thermal parameters could not be used alone as a criterion for selecting contact materials.