D. Jeannot

Break arc study for the new electrical level of 42 V in automotive applications

In order to fulfil increasing need for electric power in automobiles, satisfy environmental requirements and decrease car weight, the supply must change from 14 V to 42 V. In this work using 42 V, we studied arc parameters such as break arc duration and extinction gap for different materials, contact opening speeds and circuit loads (correlated with circuit time constant L/R). We found that these main arc parameters are greatly enhanced compared to the 14 V arc. In the case of inductive or resistive load, it was found that there are two domains: a low current domain where the material has no significant effect, and a high current domain where some materials (primarily AgSnO/sub 2/) induce high arc durations and large extinction gap. In addition, increasing opening speed reduces arc duration in all cases. The increased opening speed can however increase the extinction gap for inductive loads and decrease it for resistive circuits. We found that reducing arc duration is always beneficial with regards to erosion, but in some cases it may increase the extinction gap. At 42 V, erosion and material transfer from cathode to anode are similar to that found at low extinction gaps (<1 mm) at 14 V. At large extinction gap, however, these transfer phenomena are considerably modified by a new mechanism that induces erosion of the cathode and anode. It was found that classification of material behavior versus erosion is different at this voltage. For example AgSnO/sub 2/, which is claimed as a best compromise at 14 V for all loads, cannot be used at 42 V as it exhibits high erosion and requires a larger contact gap to ensure successful break.

Electrical arc phenomena and its interaction on contact material at 42 volts DC for automotive applications

In order to fulfill the increasing needs of electrical power in automobiles, the electrical network, currently at 14 VDC, must be changed to 42 VDC. This involves that switching DC devices have to be improved to insure reliability and electrical safety. Therewith, for improve comprehension of arc phenomena and its interactions with contact materials under 42 VDC, the authors undertook a study on this subject. This work summarizes the whole of work and tackles the arc phenomena at break and make, the contact material transfer and the problems of welding at make. Thus, we studied break arc duration, arc length (gap) and material transfer versus the effect of material, opening speed and loads. We have established that the arc duration and its length are extremely extended by a few millimeters compared to the arc in 14 VDC. At 42 VDC and low current, erosion and transfer direction is similar to the previous one obtained at low arcing length in 14 VDC. However at large gap, with 42 VDC inductive loads or high current, new mechanism erosion mode takes place which induce the cathode and the anode loss. Erosion and arc length of AgSnO/sub 2/ contacts is higher than other material: for this particular behavior, some complementary investigations, using X chemical analysis, have been undertaken. It seems that electrical arc enriches in tin element the contacts surfaces leading to this behavior. In other way, we have investigated make arc erosion, welding tendency and welding forces for all materials. It was found that contrary to the 14 VDC case, the make arc takes place during the total bounce period. The rating material performances at make seem to be reversed than at break. This study enables to understand the material effect at this higher voltage and thus should allow the improvement of the material itself. Anyway, the commutation devices of 42 VDC should employ improved materials and also additional systems of arc reduction.

Erosion and contact resistance performance of materials for sliding contacts under arcing

In previous work, we have investigated copper sliding switching contacts for automotive power applications. In order to improve their reliability, we have studied, in this paper, alternative materials to copper such as silver based materials (Ag, AgSnO/sub 2/, AgC and AgCNi). Their performance was evaluated by measuring mass variations and contact resistance stability during sliding. The contacts are operated in a test machine during 50,000 sliding operations, under inductive loads which produce long arcs, or under lamp loads which produce short arcs. In most cases, we have seen significant wear of the anode compared to the cathode. We believe that the wear process for the sliding contact is abrasion of the track by a rough contact surface. This roughness is produced and renewed by material transfer due to arcing. With regard to this wear, we show medium performance with Ag and Cu contacts, while the worst performance is obtained with AgC and AgCNi, making these materials unsuitable. With regard to contact resistance, we have measured low values <1 m/spl Omega/ for AgC, AgCNi and Ag. With AgSnO/sub 2/ and Cu contacts, resistance can reach high values, especially with an inductive load, making these materials unsuitable. With regard to the effect of operating parameters, we show that polarity may emphasize the already poor performance of a high wear anode by disturbing the sliding motion. In addition, contact force and shape size are found to act in opposite ways on material performance. Low force and large shape (cylinder) reduce wear and enhance resistance, whereas high force and small shape increases wear and lowers contact resistance.

Modification in the microstructure of materials with air-break switching at high currents

An understandable interpretation of eroded contacts is provided by giving a systematic description of the various types of subsurface microstructures encountered and by suggesting mechanisms to explain how they are formed under the influence of an electric arc. Sections and surfaces of disrupted areas are examined by a scanning electron microscope. AgZnO, AgC, AgNi, AgSnO/sub 2/, and AgCdO are tested for high current separation (2000 A) on a testing apparatus which simulated the operation of a low-voltage circuit-breaker. With respect to AgZnO, the structures after a single separation at 2000 A or after AC4-type electrical life tests are similar and can be reproduced by simulating the thermal effect of an arc on the superficial layer by laser fusion or by melting in a furnace. The results obtained in these tests explain the structural modifications by correlating them to composition, granulometry, and the physical and chemical properties of the material, and thus a better understanding the electrical properties of AgZnO is gained.<<ETX>>

An arc study at high DC current levels in automotive applications

In order to fulfill the increasing electric power needs in automobiles and to satisfy new environmental requirements and security, the currently used 14 VDC electrical power systems must either supply higher currents (up to 100 A) or be changed to 42 V. Considering the difficulties of operating with this new 42 V system, we have focused our interests on higher currents by extending our previous 14 VDC study. The arc parameters such as break arc duration and extinction gap for different materials and circuit loads are investigated. We have established that these main arc parameters are greatly influenced by load (inductance) and contact materials. The behavior of the material is investigated according to the composition, oxide content, and doping agent in silver. As found for low currents, the material transfer direction from the anode to the cathode is maintained at high current and neither opposite transfer direction or bilateral erosion was observed.

Make arc erosion and welding tendency under 42 VDC in automotive area [electric contacts]

In this paper, we give the progress of a research program on arc phenomena and their consequences on contact materials for the next car generation power system, 42 VDC. We investigated make arc erosion, welding tendency and welding forces for the materials Ag, AgZnO, AgNi and AgSnO/sub 2/ under resistive loads and with a current range from 10 to 90 A. Using previous testing apparatus, developed for 14 VDC, we have simulated a mechanical bounce observed in relays (600 /spl mu/s duration and 80 /spl mu/m height) at make. We have found that a low material transfer from the cathode to the anode takes place at low current. However, at higher current, the transfer is in a reverse direction and the amount of material transferred is higher but this amount depends mainly on the bounce profile. In addition, a welding phenomenon appears at high current levels and can reach up to 20% of the welding tendency. However, high welding tendency does not correspond to high welding forces: this is the case of AgZnO compared to Ag.

Make arc parameters and subsequent erosion under 42 VDC in automotive area

Studies the make arc parameters and their subsequent effect on contact material erosion. Based on the typical profile of bounce in a relay (1 ms of duration and 50 /spl mu/m height) we have developed a test machine, which can simulate and evaluate erosion and material transfer. It was found that contrary to the 14 V case, the make arc takes place during the total bounce period and the extinction gap is identical to the bounce height. Arc energy is therefore greatly increased and depends mainly on the electrical current and load. The main action of this arcing is mass transfer from the cathode to the anode at low current and in the opposite direction beyond 15 A. Furthermore, the rating material performances at make arc seems to be reversed at break arc. In fact AgSnO/sub 2/ has the best performance at make, whereas it had the worst at break.

The effects of metal oxide additions or dopants on the electrical performance of AgSnO/sub 2/ contact materials

The new contact material AgSnO/sub 2/, which is replacing AgCdO in many applications, requires the addition of doping agents to improve electrical contact performance. These additions effect thermodynamic stability, wettability of the SnO/sub 2/ surface by liquid silver as well as the microstructure of the area melted by the electric arc. This work considers the frequently used dopants WO/sub 3/, MoO/sub 3/, Bi/sub 2/O/sub 3/, CuO and RuO/sub 2/ as well as others: ZnO, Ta/sub 2/O/sub 5/, HgO, RuO/sub 2/, Sb/sub 2/O/sub 3/, TeO/sub 2/ and TaC. DTA and TGA analyses are performed on mixtures of these metal oxides with SnO/sub 2/, Ag and AgSnO/sub 2/. Reaction products, if present, are characterized by X-ray diffraction. The wetting angles of molten silver on surfaces of these mixtures are established. The results are used to interpret the microstructures found in the molten regions of AgSnO/sub 2/ materials doped with these substances. These structures are interpreted and related to performance in arcing contact applications.

Advanced AgSnO/sub 2/ contact materials for the replacement of AgCdO in high current contactors

New silver tin oxide contact materials for a low voltage application were developed. They exhibit excellent over-temperature behavior and a very long life compared to commercial AgSnO/sup 2/ contact materials. These new contact materials are produced by die compaction or extrusion using a newly developed precipitated powder. Defined particle size distibution and high shrinkage during sintering characterize these powders. The new contact materials have comparable electrical properties whether produced by die compaction or extrusion. This paper reports on the production and electrical properties of these new materials. Tests were conducted using a 250 A contactor originally optimized for AgCdO and a 250 A contactor optimized for AgSnO/sub 2/. The test results show these new contact materials have the potential to replace AgCdO without changing the design of AgCdO optimized contactors. This is due primarily to their superior over-temperature behavior. The contacts and contactors made with these new materials have considerable technical, economical and environmental benefits.