Comparison of Vacuum Arc Behaviors Between Axial-Magnetic-Field Contacts

We compared vacuum arc behaviors of three slottype axial-magnetic-field (AMF) contacts at two fixed contact gaps using high-speed digital photography and arc-voltage measurements. Solid angle is subtended by anode at the center of cathode or the ratio of the anode diameter to the gap length $(D/g)$ . With coupled influence of the AMF, we observed that with the $D/g$ ratio 48 mm/14 mm, the arc kept in low current modes until higher current compared with the $D/g$ ratio 48 mm/8 mm case. With the $D/g$ ratio 58 mm/14 mm, the arc-mode transitions with the increase of current were similar as the $D/g$ ratio 58 mm/8 mm case. However, with the $D/g$ ratio 66 mm/14 mm, the arc became high current modes at lower current compared with the $D/g$ ratio 66 mm/8 mm case. Correspondingly, with the $D/g$ ratio 48 mm/14 mm, arc voltages at current peaks were close to the $D/g$ ratio 48 mm/8 mm case, whereas arc voltages at current peaks with the $D/g$ ratio 58 mm/14 mm were close to that of the 58 mm/8 mm case. However, with a contact diameter of 66 mm, the arc voltages with the $D/g$ ratio 66 mm/14 mm were higher than the 66 mm/8 mm case. The arcs with the $D/g$ ratio 66 mm/8 mm were more diffuse than the 48 mm/8 mm case. Moreover, the arcs with the $D/g$ ratio 66 mm/14 mm were also more diffuse than the 48 mm/14 mm case when the current peak was 17.8 kA and above. The relationship between interrupting capacity of vacuum interrupters with the slot-type AMF contacts could be expressed as $I = k \times (D/g) + b$, where $k$ and $b$ are constants.

I. Introduction

Vacuum ARC is a high-current low-voltage electrical discharge between two electrodes situated in a vacuum ambient [1]. The vacuum arc has many important applications. One of them is vacuum switching of high current at high voltage. In the process of current interruption in vacuum under axial magnetic field (AMF), the arc appears diffuse, when arc current does not exceed a certain limiting current value, which depends on the ratio of contact radius to gap distance , AMF, and contact material [2], [3]. With increase of current, an anode spot forms, and the arc develops finally into a columnar arc at further increase of the current [4]. There are critical currents at which significant changes take place in the vacuum arc, and one of the specific critical currents is , the current at which an anode spot forms [4]. depends on the geometry of the gap and contact diameter, the contact materials, the current waveform [3], [4], and the AMF [5], [6].