Michael Bruce Schulman

Visualization and characterization of high-current diffuse vacuum arcs on axial magnetic field contacts

We have investigated the behavior of drawn vacuum arcs for several designs of axial magnetic field (AMF) contacts using high-speed digital photography and arc voltage measurements, As the peak current was increased, a gradual transition occurred in the arc appearance from a multiple cathode-spot arc to the high-current diffuse mode, and then to a high-current diffuse columnar mode. Two relatively simple models based on the literature are used to explain the results. The first is an empirical criterion for using the arc voltage behavior to determine the maximum arc current for which an AMF geometry can produce a high-current diffuse mode from the initial bridge column arc. The second model predicts the highest arc current that can be forced into a fully diffuse mode for given values of the AMF and the contact arcing radius. The predictions of these models are compared to our experimental and analytical results.

Contact temperature and erosion in high-current diffuse vacuum arcs on axial magnetic field contacts

We have investigated the surface heating effects of drawn vacuum arcs for several industrial designs of axial magnetic field (AMF) contacts, using near infrared (IR) photography of the Cu-Cr arcing surfaces with an image-intensified charge-coupled device (CCD) camera and an IR pyrometer. This enables detailed contact temperature mapping immediately after a half-cycle of arc current. The very homogeneous temperature distribution observed at current zero stands in contrast to the visually nonhomogeneous high-current diffuse arc, which was studied in separately reported experiments using high-speed digital photography and arc voltage measurements. The peak temperature at current zero increased relatively linearly with the peak current I/sub P/, and reached well beyond the melting range. We combine the temperature maps with a heating model to determine the thermal sheath thickness after arcing and its dependence on I/sub P/. The results suggest that near the interruption limit of AMF contacts, the interaction of the stable high-current arc with the anode and cathode is dominated by processes induced by flowing liquid metal, which redistributes the heat input from the axially concentrated arc over most of the contact surface. Furthermore, the flow of liquid metal off the cathode and anode faces contributes to the overall contact erosion.

Modeling the effects of an axial magnetic field on the vacuum arc

A quantitative two-dimensional model of a vacuum arc in an applied magnetic field is presented. For the particular case, a uniform axial magnetic field B/sub Z/, it provides a unified condition for the two regimes of arc voltage versus B/sub Z/, i.e. ? the steeply falling voltage branch (low B/sub Z/), and the slowly rising voltage branch (high B/sub Z/, which characterizes the diffuse arc mode). The transition to the high-B/sub Z/ regime is complete when the average mixing distance of the individual expanding cathode-spot plasma jets, measured from the cathode plane, is equal to the contact gap length. The model also predicts the observed smooth transition and minimum arc voltage between the two branches, and the independence of the are voltage on the arc current in the high-B/sub Z/ regime. Calculated results are found to be in good agreement with reported experimental data from several independent investigations over a wide range of gap, B/sub Z/ and arc current I/sub ARC/.

Transition to the diffuse mode for high-current drawn arcs in vacuum with an axial magnetic field

The opening of electrical contacts while passing current generates a drawn arc. In vacuum, the arc begins as a bridge of molten metal connecting the contacts, which then ruptures to form a bridge column arc. Previous work observing the development of drawn arcs in vacuum with an imposed axial magnetic field (AMF) measured the time required for the bridge column to evolve into the high-current diffuse mode. Arc visualization experiments on Cu-Cr contacts with an AMF have now determined that the transition to the fully diffuse mode has a more complicated development. With high-speed photography, we characterized the appearance of the arc modes over half-cycles of power frequency short-circuit current. The opening sequence begins with the rupturing of the molten metal bridge, forming the bridge column. This column evolves into the transition mode, and then into the fully diffuse mode. This transition mode in an AMF consists of a region of concentrated cathode spots, similar to the transition mode for butt contacts at lower currents and no AMF. Over a few milliseconds, an increasing number of individual cathode spots begin to appear outside the concentrated region, until a diffuse arc forms. The transition mode produces a transient peak in the arc voltage. Increasing the AMF strength at a particular current can shorten the duration of the transition mode and reduces the arc voltage peak. Single or multiple half-cycle operations have been performed on Cu-Cr contacts to investigate the effect of the transition mode on contact melting. The melting patterns after a single half-cycle of high current are correlated with the behavior observed in the arc movies. Anode melting is confined to one or two regions of shallow melting, while individual cathode spot tracks covered most of the cathode surface. The combination of arc visualization and post-arcing contact examinations demonstrated that the transition arc mode was a significant source of contact melting.

Model of a diffuse column vacuum arc with cathode jets burning in parallel with a high-current plasma column

The behavior of the high-current diffuse columnar arc in vacuum is considered. A single cathode spot jet appearing to the side of a high-current plasma column is studied using a two-dimensional magnetohydrodynamic model. The plasma expansion and current flow in the jet are affected by the presence of the main column and the applied axial magnetic field (AMF). Increasing the current in the plasma column causes the arc voltage to increase, in turn affecting the displaced parallel jet. For instance, the current density near the anode for the parallel jet increases by factor of 1.8 in the case of a 15 kA plasma column. In this case, its anode sheath potential drop increases from a negative voltage drop of about 1 V toward zero. When the arc voltage exceeds a critical value, the anode sheath potential of the jet changes from negative to positive. The displaced jet can continue burning only if the anode supplies a significant fraction of its near-anode plasma. We apply this model to study experimentally observed changes in the high-current arc behavior in an AMF. The observed mode transitions of the arc column appear to be related to a critical arc voltage condition.

Influence of contact geometry and current on effective erosion of Cu-Cr, Ag-WC and Ag-Cr vacuum contact materials

Effective ac erosion rates were measured for Cu-Cr, Ag-WC and Ag-Cr contacts in vacuum contactors. Half-cycle arc currents of 450-600 A rms were used, with the full gap set to 10%-25% of the contact diameter. The contacts parted during the rising current, and the arc stopped at the next current zero. The polarity changed for each operation to ensure uniform effects for both contacts. The effective linear volume erosion rate [cm/sup 3//C] was determined by monitoring the axial thickness of the contacts versus the number of operations. When this was converted to an effective mass erosion rate [/spl mu/g/C], it was significantly smaller than the reported absolute cathode erosion rate based on measured loss of cathode mass. Both the dependence of the effective erosion on the gap and the distribution of metal droplets on the arc shield were studied. Droplets were a significant cathode material loss which left the gap close to the plane of the cathode, while a large fraction of the ionized vapor leaving the cathode spots was deposited onto the anode. The effective erosion rate increased when spiral-slotted contacts were used.

The behavior of vacuum arcs between spiral contacts with a fixed-polarity arc shield

A framing camera was used to study drawn vacuum arcs between spiral-type contacts. This contact design imposes a transverse (radially directed) magnetic field to drive the arc column along the edge of the gap. The fixed electrode was electrically connected to a cylindrical arc shield. With the fixed electrode and shield as cathode, the manner of ignition of cathode spots on the shield depended on the peak arc current I/sub p/. For I/sub p//spl gsim/25 kA, the arc entered an intense columnar mode, and cathode spots were produced on the shield when the arc column expanded radially outward from the gap for brief intervals during its running motion. For I/sub p/=12.6 kA to /spl sim/25 kA, cathode spots ignited on the shield when cathode spots moved over the adjacent edges of the fixed contact and large spikes occurred on the arc voltage. With the fixed electrode and shield as anode, there was less interaction of the arc column with the shield. In this polarity, there was no formation of an anodic arc root on the shield at least up to I/sub p//spl ap/38 kA, and a different mode of high-current arcing was observed.