Kenichi Koyama

Influence of vacuum arc behavior on current interrupting limit of spiral contact

The authors observed the behavior of an arc on spiral contacts using a high-speed video camera, and found three periods of arc motion. These periods consist of a slow arc period immediately after the arc ignition where the arc begins to move gradually at less than 10 m/s, an acceleration period, and a high speed period where the arc rotates faster than 50 m/s. The duration of the slow arc period and the speed during the high speed period showed a dependence on the shape of the contact. Their results were as follows: (1) the spiral contact with a short slow arc period has high interrupting capability; (2) there is a borderline between successful and failed interruption in the electric charge transferred into the contact during the slow arc period; and (3) the borderline depends on the contact material. From these results, they conclude that the transferred electric charge during the slow arc period influences the state of the gap at the current zero.

The Effect of contact material composition of AgWC and Axial Magnetic Field intensity on interrupting capability and chopping current

Silver tungsten-carbide (AgWC) has been used for contact materials of low surge type vacuum interrupter because of their properties such as low chopping current and low erosion. However the interrupting capability of AgWC is relatively less than CuCr. Generally, in order to increase the interrupting capability, axial magnetic field (AMF) is applied for large rated currents. On the other hand, it is known that the contact materials greatly affect the interrupting capability and chopping current. In this paper, in order to increase the interrupting capability of low surge type vacuum interrupters, the effect of the intensity of AMF and composition of AgWC under AMF on the interrupting capability were investigated. The contents of Ag and WC were varied and cobalt (Co) was added as a third element. Also the chopping currents were investigated. As a result, it was found that an increase of Co and a decrease of Ag content tended to increase the interrupting capability

Behavior of conductive microparticles under electric field in vacuum and their influence on breakdown characteristics

Impulse breakdown voltage (BDV) for the gap which was injected with copper microparticles with a diameter from 7 to 500 /spl mu/m is measured. Electrode materials are copper, stainless steel, and an alloy of copper and chromium. In the short gap (approximately <5 mm), the BDV does not depend on the particle size or electrode material. However, in the long gap (>5 mm), the BDV varies according to the particle size and electrode material. Compared with the short gap, incrementation of the BDV with an increase in gap length is gradual. For both gap ranges, the BDV under negatively charged particles is lower than the BDV under positive polarity. The effectiveness of electrode surface roughness on the BDV is small compared with the clear gap. Further, AC voltages are applied to a rod-plate gap which was injected in the particles in order to observe their dynamic behavior. The particles begin to move when the upper direction electrostatic force originated by the AC field competes to the gravity force acting on the particles, and are finally removed from the area where the electrostatic force exceeds the gravity force as a result of reflection movements between electrodes.

Observation of tungsten particles in vacuum arcs by laser induced fluorescence

The authors have clarified the relation between the decay of tungsten ion density in the vicinity of current zero and vacuum arc mode by using LIF method and a high speed video camera in tungsten vacuum arc of 60 Hz sinusoidal current with the peak value of 3.3, 6.7 and 9.8 kA. The tungsten ion density before current zero decreased as the current decreased. In the case of 6.7 kA, the transition to the anode spot mode occurred and an arc column was observed. The density near the anode before current zero was higher than near the cathode and the density near the anode was about 10 times as high as the case of 3.3 kA which was diffuse mode. In the case of 9.8 kA, the density near the anode was not significantly different from 6.7 kA. But the density near the cathode was higher than near the anode and the eruption from the cathode was observed by a high speed video camera. They concluded the arc at 9.8 kA was the intense arc mode. And the density at current zero was higher than the other case and tungsten ion was observed until about 30 /spl mu/s after current zero.