Arc In Transverse Magnetic Field Research Articles

Experimental study of the vacuum arc motion-diffuse transition in cup-shaped transverse magnetic field contacts

The arc in transverse magnetic field (TMF) contacts transitions to diffusion mode before current zero, affecting arc morphology and motion. TMF-generated force competes with local overheating's stagnation effect, impacting contact surface ablation. Cup-shaped TMF contacts exhibit slow, fast, and unstable transitions. Arc voltage declines gently, rapidly, or in repeated steps. Peak current, contact diameter, contact piece structure, and iron core influence transition characteristics. Transition energy increases linearly with peak current. Larger contact diameter and iron cores make transition constant earlier and transition current higher. Compared to circular contacts, annular contacts delay transition constant and widen transition current range. Scanning electron microscope (SEM) images showed that the ablation cathode surfaces exhibited five morphologies: melting pit, periphery, lamellar, particle, and pore. Energy dispersive spectrum (EDS) analyses show that the periphery has a higher Cu content than the interior, suggesting less Cu loss during fracture and less droplet splashing. The peripheral Cu content is comparable between the two contacts, 62.94 % vs. 62.37 %. However, the interior Cu content of contact NO.4 is higher than NO.6 at 60.54 % vs. 56.26 %. This indicates that the difference in the effect of fast and slow transitions on the cathode surface activity is mainly in the interior region.

Cooling Mechanisms of Switching Arcs Under Transverse Magnetic Fields in Comparison With Arcs Without Magnetic Blast

The importance of low-voltage devices to interrupt dc currents under load and fault conditions has been considerably growing recently. From experimental work, it is known that arcs in strong transverse magnetic fields (e.g., flux density 1 T) can generate arc voltages considerably higher than those generated without field. In a recent study, it was shown by the author in 2-D simulations that in arrangements where the arc is magnetically squeezed against a chamber wall, the dominant mechanism of power removal is convection propelled by the JxB force (“electromagnetic pump”). Immediately in front of the wall the convective heat flux is converted to heat conduction toward the cold wall. This subsequent work is concentrated on the mechanism of free-moving arcs in transverse magnetic fields. Here, a similar effect of voltage increase has been known, but the cold wall as final target of power transport is missing. For differentiation, the cooling mechanism of arcs in chambers without magnetic blast field is also studied. While the temperature profiles and consequently the arc field strength for still arcs in chambers are dominated either by heat conduction or by radiation, depending on the current, free-moving arcs in cross fields are mainly cooled by the convective JxB pump. Heat conduction is negligible. Power is consumed by the heating of the plasma gas masses, which can be visualized by an equivalent flux. Values of the arc field strength, i.e., voltage per arc length, are also presented for dc currents between 4 and 1000 A. They increase in the sequence “chamber without field”, “free arc with cross field”, and “chamber with cross field”. In comparison with air, hydrogen yields clearly higher voltages.

Experimental investigation on the initial expansion stage of vacuum arc on cup-shaped TMF contacts

Arc behavior and measures to control it directly affect the properties of vacuum circuit breakers. Nowadays, transverse magnetic field (TMF) contacts are widely used for medium voltages. A magnetic field perpendicular to the current direction between the TMF contacts makes the arc move, transmitting its energy to the whole contact and avoiding excessive local ablation. Previous research on TMF arc behavior concentrated mainly on the arc movement and less on the initial stage (from arc ignition to an unstable arc column). A significant amount of experiment results suggest that there is a short period of arc stagnation after ignition. The duration of this arc stagnation and the arc characteristics during this stage affect the subsequent arc motion and even the breaking property of interrupters. The present study is of the arc characteristics in the initial stage. Experiments were carried out in a demountable vacuum chamber with cup-shaped TMF contacts. Using a high-speed camera, both single-point arc ignition mode and multiple-point arc ignition (MPAI) mode were observed. The experimental data show that the probability of MPAI mode occurring is related to the arc current. The influences of arc-ignition mode, arc current, and contact diameter on the initial expansion process were investigated. In addition, simulations were performed to analyze the multiple arc expansion process mechanically. Based on the experimental phenomena and simulation results, the mechanism of the arc expansion motion was analyzed.

Modeling Arc in Transverse Magnetic Field by Using Minimum Principle

A method to calculate the properties of a high-current arc exposed to a transverse magnetic field is proposed. Using only the arc current and the magnetic field, the method predicts the arc size, electric field, temperature, and velocity of the arc. Comparison is presented for the calculated arc velocity V as a function of the magnetic field B with the available experimental data. The computed velocity of a 100-A arc at one atmosphere showed that: 1) the functional dependence of $V(B) \sim B^{\mathrm {0.63}}$ is close to that experimentally observed and 2) the difference between the computed velocity and the experimental one does not exceed the factor of 2 in a wide diapason of magnetic fields. The prediction of the velocity of an arc at superhigh pressure (100 atm) is made.

Vacuum arcing behavior between transverse magnetic field contacts subjected to variable axial magnetic field

The objective of this work is to reveal the effects of an axial magnetic field (AMF) on the vacuum arc characteristics between transverse magnetic field (TMF) contacts. These vacuum arc characteristics include the vacuum arcing behavior and the arc voltage waveform. In the experiments, an external AMF was applied to a pair of TMF contacts. The external AMF flux density BAMF can be adjusted from 0 to 110 mT. The arc current in the tests varied over a range from 0 to 20 kA rms at 45 Hz. The contact material was CuCr25 (25% Cr). A high-speed charge-coupled device video camera was used to record the vacuum arc evolution. The experimental results show that the application of the AMF effectively reduces the TMF arc voltage noise component and reduces the formation of liquid metal drops between the contacts. The diffuse arc duration increases linearly with increasing AMF flux density, but it also decreases linearly with increasing arc current under application of the external AMF. The results also indicate that the diffuse arc duration before the current zero is usually more than 1 ms under the condition that the value of the AMF per kiloampere is more than 2.0 mT/kA. Finally, under application of the AMF, the arc column of the TMF contacts may constrict and remain in the center region without transverse rotation. Therefore, the combined TMF–AMF contacts should be designed such that they guarantee that the AMF is not so strong as to oppose transverse rotation of the arc column.

Numerical Simulation of a Moving High-Current Vacuum Arc Driven by a Transverse Magnetic Field (TMF)

This paper deals with the numerical simulation of the constricted high-current vacuum arc (>15 kA), driven by a transverse magnetic field (TMF), as found in vacuum circuit breakers applying the TMF arc control. The magnetohydrodynamic approach, together with the detailed heat transfer and evaporation equations for the electrodes, is used to describe the arc behavior self-consistently, restricted to 2-D. A newly developed model describes the cathode attachment of the constricted arc, as a large laterally extended foot points, instead of regular cathode spots. The arc maintains itself when the electrode temperatures are higher than 3400 K on the cathode and 2900 K on the anode. This model leads to the characterization of the physical quantities of the arc plasma and describes the arc motion. A stepwise movement of the arc results due to different instantaneous velocities of the current attachment areas at the cathode and anode.

Analysis of open-flame electric arc in transverse magnetic field

The distribution of temperature and pressure, electromagnetic fields, and the structure of flow under the effect of an external transverse magnetic field are determined using a three-dimensional magnetogasdynamic model of a steady-state curved arc. The calculation results are consistent with the experimentally observed properties of the arc, such as stabilizing effect of current strength and the magnetogasdynamic mechanism of arc extinction.

Magnetically driven high current switching arcs in vacuum and low pressure gas

The behavior of high current switching arcs in transverse magnetic fields is essential for the design of switching elements, in which arc motion is used to reduce contact erosion. In this paper magnetically driven arcs in vacuum and low pressure gases are investigated at currents of several tens of kA. Arcs driven in a low pressure environment exhibit a peak arc velocity of more than 1/spl times/10/sup 5/ m/s, exceeding those of comparable vacuum arcs by at least two orders of magnitude. The influences of current amplitude, gas pressure, and the kind of gas filling on the arc velocity are investigated. A threshold behavior concerning the minimum requirements for a nonstationary arc is found. An improved model is presented, which explains the observed peak arc velocities in vacuum and in low pressure gases.

Temperature Dependence of Retrograde Velocity of Vacuum Arcs in Magnetic Fields

Retrograde velocity of vacuum arcs in transverse magnetic fields is known to depend on, among other things, the magnetic induction, the arc current, the electrode spacing, the cathode material, and the cathode surface condition, and was also found to depend on the cathode temperature. Using the optical method, the retrograde velocity was measured as a function of the cathode temperature with copper, aluminum, and stainless steel as cathode materials. The optical measurement shows that by increasing the cathode temperature, the arc velocity decreases. It appears that with the increase in the cathode temperature, the decrease of the arc velocity is related to the increase of the cathode crater radius. The experimentally measured temperature dependence of the retrograde velocity of vacuum arcs can be explained by the ion jet model for retrograde motion of vacuum arcs [10]. The relative decrease of retrograde velocity as a function of the cathode temperature calculated according to this model agrees quantitatively with the reported measurements.

High-current arcs in transverse magnetic fields in air at atmospheric pressure

This paper gives measurements of the velocity and voltage, and estimates of the size (‘effective drag width’) of electric arcs driven by transverse magnetic fields in air at atmospheric pressure. Ranges of parameters covered are: electrode spacing up to 10cm, magnetic field up to 1.75Wb/m2, and current up to 20000A. Several different electrode configurations and dimensions, and various electrode materials, are used, and conditions have been found where arc movement is virtually independent of the electrode material and spacing. Some comments on the interaction of electrode and column influences are made. In particular, it is noted that electrode influences can be significant under some conditions, and should then be accounted for, in the analysis of experimental results by similarity parameters based on column theory. Although the condensation of data achieved by similarity parameters is very great and highly illuminating in that under many conditions it is possible to predict arc velocity and column voltage gradient within about ±50%, given only the arc current, magnetic field, and electrode spacing and material, there is still much variation within these limits which is difficult to predict. Thus the dependence of the behaviour of an arc on the precise nature of its environment should not be overlooked.

Further correlation of experimental data for electric arcs in transverse magnetic fields

Data obtained for short arcs moving in a transverse magnetic field in argon and helium, together with some results for an arc held stationary in an argon flow, have been correlated by the use of similarity parameters presented in a recent paper. The results obtained in this form are compared with previous results for arcs moving in air, and a very close similarity between the magnetic characteristic for all three gases is found. The range of conditions over which arc velocity may be predicted is thereby considerably extended. It is also shown that, although the arc-column electric field can be successfully correlated with other main variables for a single gas, it is not yet possible to extend this correlation from one gas to another.

Correlation of experimental data for electric arcs in transverse magnetic fields

Correlation of experimental data for electric arcs in transverse magnetic fields