Low Current DC Vacuum Arcs Research Articles

Effects of Homopolar Magnetic Fields on Low-Current DC Vacuum Arcs

This article deals with the effects of homopolar magnetic fields on the lifetime of low-current dc vacuum arcs. A homopolar magnetic field is a distribution with the axially symmetric radially oriented field in the contact gap. It is produced when two dipole magnets are kept with the same poles facing each other. Switching experiments are conducted using an off-the-shelf ac vacuum contactor, and the ring magnets are used to set up a homopolar magnetic field in the contact gap. The arcing times are measured for currents varying from 1 to 30 A with voltages up to 100 V and magnetic surface strengths from 0.34 to 0.43 T. The tests are performed with the fixed contact of the vacuum contactor connected to positive as well as negative polarity. It is observed that the arcing time values are lesser in the presence of magnetic field with the fixed contact at the negative polarity due to asymmetry in the placement of the ring magnets. Finite element method (FEM) simulations are performed to examine the magnetic field distribution in the contact gap. The ratio of radial-to-axial magnetic field components in the contact gap decreases with increasing strength of magnets in the present experimental arrangement. It correlates well with the observed arcing time trends against the variation in the strength of magnets and the polarity of the fixed contact. The present article shows the potential of homopolar magnetic field in the development of dc vacuum contactors and interrupters without the need to create an artificial current zero using forced commutation.

Spectroscopic Analysis of the Effect of Magnetic Field on Low Current DC Vacuum Arc Plasma

AbstractEffects of a magnetic field on the characteristics of a low‐current DC vacuum arc plasma of less than 30 A were examined by arranging the electrodes in a straight line or perpendicularly. Two types of magnetic fields were applied to the gap by embedding a small column magnet inside the electrode and arranging a disc magnet at the back side of the electrode. The arc plasma forms a column depending on the magnetic field distribution and an anode spot is generated under strong magnetic field on the anode side. Spectroscopic analysis of emissions from cathode and anode showed that the cathode metal ions arrive at the anode and the anode metal ions arrive at the cathode along the magnetic field, but neutrals are not guided by the magnetic field and only a small part of the neutrals arrive at the opposite electrode in the perpendicular arrangement. Copyright © 2010 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

Effect of Axially Symmetric Magnetic Fields for Dynamics of Low-Current DC Vacuum Arc Plasma

In this paper, effects of four types of axially symmetric magnetic fields on the dynamics of low-current dc vacuum arc plasma, which is less than 30 A, were examined. Arcs were ignited by the opening of the butt contacts made of zinc. In the axial magnetic field, the arc plasma was restricted within a column, and the arc voltage rose with increasing gap length. When a magnet was arranged behind the cathode, the cathode spot rotated in the opposite direction to the Lorentz force (-J times B) on the edge of the circular-rod cathode. On the other hand, when a magnet was arranged behind the anode, the arc plasma was constricted on the anode side, and the lifetimes of the arcs became longer, although the arc voltage rose with increasing gap length. The compression effect of the axial component of the magnetic field was analyzed by introducing a simple magnetic piston model, and the experimental diameters of the arc column showed a good agreement with the model.

Effect of Magnetic Field on the Behavior and Sustaining Characteristics of Low Current DC Vacuum Arc

Effect of magnetic field on sustainment and dynamics of low current DC vacuum arcs, less than 20 A, were examined. Without magnetic field, the cathode spot moves at random on the cathode surface and the arc voltage is constant for increasing gap length up to 30 mm. In the axial magnetic field, the arc plasma was constricted and the arc voltage rose with increasing gap length. When a magnet was arranged behind the cathode, the cathode spot rotated in the opposite direction of the Lorentz force (J x B) on the edge of the cathode. On the other hand, when a magnet was arranged behind the anode, the arc column was constricted on the electrode axis and the lifetime of the arcs was extremely extended.

Optical investigations of dynamic vacuum arc mode changes with different axial magnetic field contacts

By using a high-speed charge-coupled device (CCD) video technique, three different axial magnetic field contact systems (i.e., unipolar, bipolar, and quadrupolar systems) are investigated at an arc current of 10 kA. Video recordings were compared to computer simulations of light emission emitted at the side-on of diffuse and diffuse columnar arcs. The computer images reproduced typical trends, such as stronger light intensities in front of the cathode caused by higher-plasma densities in this region. A low-current dc vacuum arc was initiated by contact separation before the high current was injected at a fixed contact distance of 10 mm. Videos were taken from two directions perpendicular to each other to localize the vacuum arc properly. From these investigations, the transient development of vacuum arc under different axial magnetic field profiles can be visualized. The results were interpreted with respect to the behavior of the vacuum arc in the second half cycle after an eventual reignition.

Ignition and instability of low-current DC vacuum arcs ignited by the opening of the electrodes

The ignition, lifetime, and instability of low-current DC vacuum arcs, which were ignited by the opening of the electrodes, were investigated experimentally. Copper electrodes were used and the arc current was less than 50 A. Waveforms of the voltage and current between the electrodes at the moment of arc ignition and during the arc continuation were measured. It was confirmed that, for the ignition of the arc, a voltage which was larger than a critical value should appear between the electrodes at the moment of the opening of the electrodes. The lifetime of the arc is short, less than 1 s in the present condition, and increases with an increase in arc current and output voltage of the current source. The arc voltage consists of a certain DC level, around 20 V, and an additive high-frequency component. The arc current consists of a certain DC level and a subtractive high-frequency component.

Voltage drop over a vacuum arc and the cathode-spot brightness

A spectral investigation of low-current DC vacuum arc voltage is presented. High-speed streak photographs of cathode-spot light emission with high time resolution were obtained and are discussed. Characteristic frequencies of cathode-spot instabilities are derived from average frequency spectra of the arc voltage.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

The influence of cathode surface microstructure on DC vacuum arcs

The experimental results of the arc lifetime, the arc voltage, the cathode spot velocity and the erosion structure influenced by the cathode surface microstructure for low-current DC vacuum arcs are presented. The correlation between the arc voltage instability peak and the discontinuity in the cathode erosion trace has been shown when applying a transversal magnetic field. With increasing cathode surface roughness, an increase (of about a factor of 100) in the arc lifetime and a 30% decrease in arc voltage at the most, has been found for a drawn arc. The increase of the cathode spot velocity of the retrograde motion by cathode roughness is significant (a hundred times different at most) for triggered arcs. The mechanism of the cathode surface microstructure influence on the DC arc stability is discussed.

Voltage Hash in Low-Current DC Vacuum Arcs

The nature and mechanism of the high-voltage pulses in the anode region of copper-vapor arcs have been investigated. Arcs are drawn between a 1.3-cm-diameter anode and a 5-cm-diameter cathode in the dc range of 30-400 A. Prior to anode spot formation and at constant current, the arc voltage exhibits HF pulses superposed on the dc voltage. The amplitude and frequency of repetition (= 500 kHz) of these pulses is highly erratic, and the total instability is evidenced as voltage hash. The hash amplitude increase with both arc current and contact spacing and, at 2.5 cm, varies from several tens of volts at 30 A to several hundreds of volts at 400 A. The hash is attributed to fluctuations in vapor and plasma density in the anode region. Although these fluctuations appear unrelated to the gross motion of the cathode spots, streak photographs indicate a time-variant cathode evaporation that may contribute to the hash phenomenon. A second contributory mechanism is suggested from experience of hash in the anode region of fluorescent lamps, and involves quasi-periodic emission from the anode.