Contact Erosion Research Articles

Effects of ground surface deformability, trimming, and shoeing on quasistatic hoof loading patterns in horses.

To determine whether solar load distribution pattern on a solid nondeformable ground surface is the product of contact erosion and is the mirror image of load distribution on a deformable surface in horses. 30 clinically normal horses. Solar load distribution was compared among 25 clinically normal horses during quasistatic loading on a solid nondeformable surface and on a highly deformable surface. Changes in solar load distribution patterns were evaluated in 5 previously pasture-maintained horses housed on a flat nondeformable surface. Changes in solar load distribution created by traditional trimming and shoeing were recorded. Unshod untrimmed horses had a 4-point (12/25, 48%) or a 3-point (13/25, 52%) wall load distribution pattern on a flat solid surface. Load distribution on a deformable ground surface was principally solar and located transversely across the central region of the foot. Ground surface contact areas on solid (24.2 +/- 8.62 cm2) and deformable (69.4 +/- 22.55 cm2) surfaces were significantly different. Maintaining unshod horses on a flat nondeformable surface resulted in a loss of the 3- and 4-point loading pattern and an increase in ground surface contact area (17.9 +/- 2.77 to 39.9 +/- 12.77 cm2). Trimming increased ground surface contact area (24.2 +/- 8.60 to 45.7 +/- 14.89 cm2). In horses, the solar surface is the primary weight-loading surface, and deformability of ground surface may have a role in foot expansion during loading. Increased surface area induced by loading on deformable surfaces, trimming, and shoeing protects the foot.

The unusual electrical erosion of high tungsten content, tungsten copper contacts switching load current in vacuum

The electrical erosion of high tungsten content, tungsten-copper (7-10 wt.% Cu) was investigated. The contacts were placed in a vacuum interrupter envelope with a background pressure of about 10/sup -6/ torr. The contacts switched one half cycle of 60 Hz current per operation. The polarity of the current was changed on each operation. Six contact pairs were investigated. Each pair was subjected to an ever-increasing number of operations: 1 K, 5 K, 10 K, 20 K, 40 K, 50 K, and 60 K. The contact erosion was inferred by measuring the linear position of the moving contact terminal. On completing the electrical testing, the vacuum interrupters were dismantled and the contact surfaces were observed. Unlike the previous work on Ag-WC (50 wt.% Ag) and Cu-Cr (75 wt.% Cu), the W-Cu contacts showed a localized build up of erosion products on the contact surfaces even beginning at 1 K operations. The experiments were repeated switching a unidirectional current i.e., the contacts remained at the same polarity throughout the experiments. Here an anode pip and a cathode crater were formed immediately. The difference in the topographies of these contacts is discussed in terms of the metallographic analysis of the deposits on the contacts, the erosion deposits on the shields surrounding the contacts and the expansion of the vacuum arc.

Effects of Cr content on the interruption ability of CuCr contact materials

The effects of Cr content in CuCr contact materials, especially on high current interruption, have mostly been examined in experimental chambers rather than in actual vacuum interrupters (VIs). Possible interactions of the contacts with their surroundings, both electromechanical and gaseous, may require commercial VIs for a more realistic test determination of the effects of Cr content on the overall performance. In this work, the Cr content in CuCr contacts is varied from 5 wt% to 75 wt% and tested in our 38-kV axial magnetic field (AMF) type VIs. The material properties of electrical conductivity, hardness and total VI resistance are measured as a function of the Cr content. AC and impulse voltage withstand capabilities are tested before and after short-circuit current interruption tests. Contact erosion behavior is examined with VIs subjected to only a half cycle arcing duty. Results show that for this particular AMF VI design, a 30 wt% Cr content gives the best current interruption performance.

Erosion and resistance characteristics of AgW and AgC contacts

The effect of arc current, silver content, and density on contact erosion and contact resistance under break operation was measured for three types of AgW circuit breaking contacts in air. The arc current ranged from 2 kA/sub p/ to 22 kA/sub p/ and the percentage of silver in each of the contact materials was 40, 45, and 49% wt. Contact density and erosion measurements showed the closer the contact density was to 100% theoretical density the lower the erosion rate. Erosion rate results showed a gradual increase in the erosion rate over the 5-16 kA/sub p/ current ranges. The increase in erosion rate corresponded to an increase in contact resistance in all three materials. Above this range, the resistance dropped bark down near its original level as current density increased and the erosion rate leveled off. A relationship was established, through measurements of are diameter and surface composition, between arc current density and contact erosion/resistance.

Optimisation of the switching process by modelling the contact surface

AbstractThe aim of this paper is to show the design procedure of a disconnector starting at basic modelling up to a product used in practice. The relation between switchgear mechanics and electrophysics arc phenomena is discussed and the influence of short arcs on the opening contacts of a high voltage disconnector is analysed. With regard to contact erosion the most severe switching stress occurs during the bus‐transfer current switching procedure. Based on simplified models the influence of various parameters on the contact design and the disconnectors reliability is discussed. Finally a new contact design using main and auxiliary contact fingers is shown. The system uses the spring effect of flexible fingers mounted on the main contacts. The new disconnector type fulfils the type tests according to the appropriate international standards and is successfully used in practice.

The net zero mass loss phenomenon on opening switching contacts with AC loading

Studies are conducted on silver metal oxide contacts on a purposed built apparatus to investigate the extent of contact erosion under opening (break) operations. The contacts are opened between 0.1 and 0.8 ms/sup -1/ at particular point-on-wave, t/sub POW/, of the ac waveform. Current loading of up to 30 A rms, I/sub rms/ is investigated. Contact material deposition, as well as mass loss, is observed and is found to balance at particular current values and point-on-wave opening leading to a net zero mass loss on the contacts. Results are presented on mass change for each contact against the two parameters of current and point-on-wave. An arc duration characteristic, /spl zeta/, is plotted against current, I/sub rms/ which has a net zero mass loss. The characteristic is shown to be the same for both silver tin oxide and silver cadmium oxide but is influenced by the opening velocity. The metallic and gaseous ion transfer mechanisms of the arc discharge are presented as the main processes behind this phenomenon.

Comparison of the Switching Behaviour of Electrodeposited Silver and Silver-Cadmium Alloys Contact Materials

SUMMARYThe quality of contact materials depends on their physical properties, such as density, hardness and conductivity, as well as on their contact characteristics (contact resistance, weldability and erosion). An instrumentation system is described here, which measures the most important attributes of contact materials simultaneously. The measurements are taken under simulated service conditions. The rated electrical current is passed through contacts of reference and test materials in a relay controlled by an electronic module; the contact resistance, weldability and arc erosion are monitored. In this system two pairs of contact elements—one pair made of the test material, and the other made of reference material—are compared. The relay is an actual electrical device wich would use the test material as its contact elements in normal operation. The test material is silver-cadmium alloy electrodeposited from an iodide bath; and the reference material is electrodeposited silver from a cyanide bath.

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.

Modeling contact erosion in three phase vacuum contactors

A computer model was developed to simulate the unequal contact erosion measured in a vacuum contactor during three-phase AC-4 life testing. The model showed that the contact erosion rate was directly affected by the randomness of the opening time of the contacts. A probability distribution was created to simulate any degree of randomness in the opening. The model showed that repeated opening of the contactor at or near the same electrical phase angle over thousands of operations resulted in unequal contact erosion for each pole due to differences in are energy. If there is a consistent imbalance in the are energy one pole will erode at a significantly higher rate than the other two poles. The degree of unequal wear was dependent on the degree of randomness of contact part. The resulting unequal contact erosion would just amplify the unbalance with each subsequent operation leading to a run-away condition. A completely random opening resulted in even contact wear while synchronous opening resulted in unequal erosion. Synchronous opening of a contactor can occur from the inherent nature of the electromagnet used to operate the contactor. This model could also be used to simulate air contactor erosion.

Arc-contact-insulating wall interactions in low voltage circuit-breakers

This experimental study deals with the interaction between an electrical arc, the contacts and the insulating side walls in low voltage circuit breakers. The influence of this on the breaking capacity, dielectric strength and device overheating after breaking are shown. In a confined volume, the arc-wall interaction defined the gaseous atmosphere in which the arc develops and depends on the nature of the insulating wall material. The gaseous atmosphere therefore fixes, in part, the arc behavior (arc voltage and arc movement) and the physical-chemical properties of the breaking medium surrounding the metallic surfaces (reducing or oxidizing). The arc-electrode interaction influences the capacity of the arc to transfer from the contacts to the arc quenching region and introduces surface pollution, through contact erosion (droplets, metallic vapors and other components). These combined effects can have a negative influence on general circuit breaker behavior.

Arc mobility on bouncing contacts

The arc mobility on eroded Ag/CdO and three different Ag/SnO/sub 2/-contacts during bounces simulated in a model switch was investigated with the aid of a two-dimensional (2-D) optical arrangement. The high apparent arc mobility observed on two of the Ag/SnO/sub 2/-materials investigated was caused by arc commutation inside the contact gap. Ag/CdO and another Ag/SnO/sub 2/-material showed both arc creeping and arc commutation-but only very close to the site of arc ignition. No continuous fast arc motion occurred in short bounce gaps (0.18 mm, corresponding to bounce values in modern contactors) whether or not a magnetic blast field has been applied, There was no correlation between the amount of contact erosion at make and the apparent arc mobility observed on certain materials.

Arc erosion characteristics of Cu-W contact at load current range in SF6 gas

Development of SF6 gas-insulated switches that have a long service life and are highly reliable at medium voltage ratings require arc erosion of contacts in SF6 gas to be decreased. The effects of SF6 gas pressure and magnetic field for arc blow-out on erosion were studied experimentally in detail using sintered Cu-W contacts. The test current, SF6 pressure and magnetic field were 100 to 400 A, 0.1 to 0.6 MPa and 0 to 50 mT, respectively. The results show that the arc erosion rate increased linearly with the SF6 pressure. The magnetic field effectively reduced anode erosion (i.e., the application of a mere 50 mT lowered the erosion rate to 1/3 to 1/4). These phenomena may be attributed, respectively, to the increase in evaporation rate according to the shrinkage of arcing spot areas caused SF6 pressure and a decrease of the evaporation rate correlating to high-speed movement of the arcing spot on the anode by the magnetic field. However, the cathode erosion was insensitive to the magnetic field. The influence of both W grain size and composition ratio, Cu:W, on the erosion rate was also studied. The smaller W grain at Cu content of 50 to 70 wt.% was found to be best for suppression of erosion under the present conditions. Further, the increase of N2 mixture ratio to SF6 under constant total gas pressure greatly decreased contact erosion. © 1997 Scripta Technica, Inc. Electr Eng Jpn, 118 (1): 41–51, 1997

Break arc duration and contact erosion in automotive application

Switches and relays connected to complex circuits (motor, resistance, and lamp) and supplied by 14 V dc imposed by the battery voltage are currently used in the automobile field. Arc duration and subsequent erosion measurements for contacts made of pure silver, silver alloys (AgNi, AgCu), and silver metal-oxides (AgCdO, AgSnO/sub 2/) were carried out. Also, the influence of the opening speed, current, and loads on this duration is determined. It was found that inductive load causes high voltage and gaseous arc phase development as was confirmed by spectra emission of surrounding gas N/sub 2/. So, arc duration is enhanced but keeps the same values for silver alloys as well as for silver metal oxides. Nevertheless, in resistive and lamp loads, the metal oxide materials show large arc duration compared to pure silver or silver alloys. Opening speed acts on arc duration differently for each load: it decreases as root square of opening speed in the inductive load whatever the current, but follows a linear decrease at high current in the resistive case. The lowest erosion and mass transfer observed in resistive and lamps loads are reversed and enlarged when the load becomes inductive. We report the remarkable minimum erosion of AgSnO/sub 2/ when loads are free from inductance. These results associated with measurements of erosion will be useful to choose contact material, electrical and mechanical conditions to improve lifetime and reliability of the components.

The evaluation of arc erosion on electrical contacts using three-dimensional surface profiles

Three-dimensional surface profiling is a useful technique for the evaluation of electrical contact erosion. A review is given of the various methods by which a 3-D surface profile can be measured, and numerical techniques are described which can be used to analyze electrical contacts to define erosion in terms of volume and height changes. Experimental data on the rate of change of mass of Ag-CdO contacts are used to compare with volume and height changes over a number of switching cycles. Emphasis is given to the evaluation of 3-D surface profile in the particular condition where the change of mass of the contacts is nearly zero after a large number of break only switching cycles, and it is shown that in this condition the contacts exhibit volume changes both above and below the datum surface of a new contact.

SF<sub>6</sub>ガス中Cu-W接点の負荷電流域におけるアーク消耗特性

Development of SF6 gas-insulated switches that have a long service life and are highly reliable at medium voltage ratings require arc erosion of contacts in SF6 gas to be decreased. The effects of SF6 gas pressure and magnetic field for arc blow-out on erosion were studied experimentally in detail using sintered Cu-W contacts. The test current, SF6 pressure and magnetic field were 100 to 400 A, 0.1 to 0.6 MPa and 0 to 50 mT, respectively. The results show that the arc erosion rate increased linearly with the SF6 pressure. The magnetic field effectively reduced anode erosion (i.e., the application of a mere 50 mT lowered the erosion rate to 1/3 to 1/4). These phenomena may be attributed, respectively, to the increase in evaporation rate according to the shrinkage of arcing spot areas caused SF6 pressure and a decrease of the evaporation rate correlating to high-speed movement of the arcing spot on the anode by the magnetic field. However, the cathode erosion was insensitive to the magnetic field. The influence of both W grain size and composition ratio, Cu:W, on the erosion rate was also studied. The smaller W grain at Cu content of 50 to 70 wt.% was found to be best for suppression of erosion under the present conditions. Further, the increase of N2 mixture ratio to SF6 under constant total gas pressure greatly decreased contact erosion. © 1997 Scripta Technica, Inc. Electr Eng Jpn, 118 (1): 41–51, 1997

Make-and-break erosion of Ag/MeO contact materials

Make-only erosion, break-only erosion, and the combined make-and-break erosion of contacts made from powder metallurgically produced Ag/CdO and three differently produced Ag/SnO/sub 2/ materials were investigated employing both a commercial contactor and a model switch. The test conditions were chosen according to IEC H947-4 AC3 duty, yielding make and break are energies of the same order of magnitude. The model switch simulates both make and break operations with the same parameter values occurring in modern contactors, but in a much better reproducible way, and with the opportunity to vary the single parameter values in order to gain information about its influence on the erosion behavior. Make-only erosion tests demonstrated that Ag/SnO/sub 2/ materials suffered either three times higher or about 50% lower erosion losses than Ag/CdO depending on the manufacturing process and/or the additives of the materials. Therefore, the structure of the materials rather than the kind of the main metal oxide component (CdO or SnO/sub 2/) is decisive. The combined make-and-break tests yielded significantly lower erosion rates than make-only tests. This unexpected result was reproducible for all contact materials investigated, although the are energy per operation was more than doubled due to the additional break operation. Inspection of the micro-structure of the stressed contacts suggested that this reduction of the erosion rates is due to the motion of the break are and its effect on the eroded surfaces.

Effective erosion rates for selected contact materials in low-voltage contactors

Effective and absolute erosion rates are reported for contact materials from vacuum interrupters used for low voltage contactors. The effective erosion rates were determined from the linear erosion of both contacts with 60 Hz half cycle currents (from 630 to 685 A peak) and maximum gaps of /spl sim/2.2 mm. The contacts were opened at a present time in a half cycle, and the drawn arc continued to the next and final current zero. The polarity of the current automatically changed for each operation to ensure uniform erosion history for both contacts during many thousands of operations. Afterwards, the contacts were removed and placed in a demountable arcing chamber for determination of the absolute dc erosion rate of the cathode material. The absolute cathodic erosion rates were determined with applied dc current pulses on the order of 100 A and maximum gaps of 8.5 mm. With ac operation at a gap of 2.2 mm, a significant fraction of the ions and neutral vapor leaving one contact deposits on the other contact, so the effective erosion rate is markedly smaller than the absolute erosion rate. The materials investigated were Ag-WC, Cu-Cr, and Cu-Cr-Bi.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Restrikes in vacuum circuit breakers within 9 s after current interruption

AbstractAfter interruption of load or short‐circuit currents in the range (peak values) of 0.8 kA to 10 kA, restrikes were recorded within 9 s after current interruption when the peak value of the recovery voltage exceeded a certain critical fraction of the 10% breakdown voltage of the cold gap, independent of the break current, the absolute value of the breakdown voltage, the contact distance and the contact erosion, but influenced by the shape of the electrodes. The frequency of the restrikes increased progressively with increasing recovery voltage related to the “cold” breakdown voltage. Three different modes of breakdown were observed in different time intervals.

The effect of contact opening velocity and the moment of contact opening on the AC erosion of Ag/CdO contacts

This paper presents experimental results on the influence of contact opening velocity and the moment of contact opening with respect to the start of the AC current cycle on arc energy and contact erosion when breaking an AC resistive circuit. The test currents are 5.6 A, 14.7 A, and 26.3 A, and the contact material is Ag/CdO (90/10). A computer controlled test system allows the control of the contact velocity profiles with constant velocities up to 0.8 m/s and also enables the synchronization of the contact opening with a point on the AC current waveform. Contact erosion is evaluated by measuring the mass changes of the cathode and anode. The results show that increasing the contact velocity from 0.1 m/s to 0.8 m/s leads to an increase in the arc energy. The material transfer from one electrode to another is shown to depend on the current, the polarity of the moving contact, the moment of the contact opening with respect to the start of the AC current cycle, and the opening velocity of the contacts. >

The influence of bounce parameters on the make erosion of silver/metal-oxide contact materials

Make erosion of contacts made from powder metallurgically produced Ag/CdO and three differently produced Ag/SnO/sub 2/ materials was investigated employing both a commercial contactor and a new bounce model switch. Additional tests were executed in a fixed gap arrangement in order to study separately the influence of the thermal stress due to the bounce arc only. The bounce model switch was developed to simulate contact bounces at well-defined and reproducible but independently variable values of bounce duration, bounce height, and contact force. The influence of the bounce parameters (bounce arc duration, bounce height and static contact force, current and magnetic induction) on the make only erosion was investigated by systematic variation of each single parameter value in the bounce model switch. The Ag/SnO/sub 2/ materials suffered either three times higher or about 50% lower erosion losses than powder metallurgically produced Ag/CdO, depending on the manufacturing process (internally oxidized, powder metallurgically produced) and/or the additives of the materials, and therefore on the structure of the materials rather than on the kind of the main metal oxide component (CdO or SnO/sub 2/. The different make erosion rates of the materials investigated have been correlated with the micro structure of their stressed surfaces.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>