N. Ben Jemaa

Make arc erosion and welding in the automotive area

A make arc has been investigated using a new testing apparatus. This apparatus which includes a piezotranslator, enables controlled bouncing to be studied at make under mechanical conditions similar to those encountered in automotive relays. Material transfer and welding tendency of silver, silver alloys and silver metal oxide contact materials have been studied under inductive, resistive and lamp loads at 14 V DC and 10-70 A. With all loads, anode to cathode material transfer has been observed, AgCdO, Ag and AgNi under lamp and resistive loads show the highest material transfer, as opposed to non doped and doped AgSnO/sub 2/ under inductive loads. This well known transfer is due to the anodic arc, which occurs during bounces when the gap between the contacts is shorter than /spl sim/5 /spl mu/m, at the beginning and the end of the bounce. We have demonstrated that welding occurs in the beginning of the bounce, i.e. when the contacts begin to separate under arc, and not when the contacts close under arc at the end of the bounce. It was found that welding occurs mainly with Ag, AgCdO, AgZnO, AgFeRe and AgFeOx contact materials, and under high inrush current produced by lamp loads.

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.

Some investigations on slow and fast arc voltage fluctuations for contact materials proceeding in various gases and direct current

Arc voltage fluctuations with control of the main experimental parameters are studied, taking into account the excitation and ionization potentials of both contact materials and gases. The actual parameters of the experiments are the contact material (Au, Ag, etc.), and the environmental (air, N/sub 2/, He, Ar, etc.), electrical (50 V, 0.1-3 A DC), and mechanical (opening speed 1 to 50 cm/s) parameters. The accurate voltage values of the consecutive arc plateaus included in arc phases are determined by statistical measurements and voltage histogram drawings. Each of these voltage plateaus is attributed to a specific combination of contact metal ionization potential and excitation metastable levels of the surrounding gases. It has been found that each phase can enclose more than one plateau, such as in helium where the two last plateaus occur in gaseous phase, while in ambient the metallic phase is constituted by the two earlier plateaus. According to the plateau attribution, arc phase duration could be the sum of the corresponding plateau durations. Moreover, during each plateau, fast potential fluctuations occur. The fast Fourier transform (FFT) analysis of arc voltage samples shows that the maximum of the spectrum of these fluctuations corresponds to a frequency which is as high as 60 MHz in the metallic phase and only a few megahertz in the gaseous phase. >

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.

Statistical Study of Voltage Fluctuations in Power Connectors During Fretting Vibration

It is well known that vibration of contact interfaces is the main cause of contact degradation by the so called fretting corrosion phenomenon. In fact the process of generating particles by mechanical wear produces an increase of contact voltage, frequently manifested by a high contact resistance in low power and signal connectors. In high power connectors, this degradation is expected to be accentuated by thermal effects generated by increasing the contact voltage up to melting and arcing voltages. The main objective of this work is to examine conjointly the electrical behavior and amount of wear of a connector terminal under vibration and higher current stress. In order to reproduce harsh vibration conditions, spring with lamella shape from an automotive connector, made from a copper alloy and coated with 2 mum of tin, are submitted to a high number of oscillations (103 to 106) where the pin is firmly attached to a fixed support. High frequency oscillations (100 Hz) with an amplitude of 50 mum is used. The contact is inserted into a resistive circuit supplied by 20 V and 3 A. The contact voltage is measured with a fast sampling oscilloscope and the voltage histogram is built up in real time. The losses of mass in the erosion track are evaluated by micro weighing. The main results show that at the low level of wear observed during the earlier stage of fretting, the contact voltage corresponds to pure constriction voltage < 20 mV. However, after this initiation period as the wear is increased, the contact voltage is increased and reaches a few hundred millivolts (melting and fritting voltage). Finally in last stage of degradation, an arcing voltage due to bounce phenomena is detected. These short arcs, observed for the first time in fretting under power, could accentuate the wear and the degradation process by erosion and mass transfer.

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.

Time and Level Analysis of Contact Voltage Intermittences Induced by Fretting in Power Connector

It is well known that vibrations of contact interfaces are the main cause of contact degradation by the so called fretting corrosion phenomena. In fact the process of generated particles by mechanical wear produce either the increase of contact voltage accompanied with rapid fluctuations. The main objective of this work is to examine the electrical behaviour of contact interface under vibration and characterize theses fluctuations during the fretting. A single point contact is submitted to high number of vibrations (103 to 107) at 100 Hz with an amplitude of 50 mum. The contact is in a resistive circuit supplied by 14 V and 10 A. The contact voltage is acquired with fast sampling oscilloscope and fluctuations are analysed by real time FFT module. We have found that, depending on the degradation stage, the apparition of these fluctuations is attributed to electro-mechanical phenomena. Some slow fluctuations are well correlated to vibration period while the rapid ones are linked to electrical conduction perturbation in granular interface by movement. Furthermore, the self-heating by such high contact voltage at high current levels is examined.

Experimental and theoretical investigations on connector insertion phase

Inside a connector, an interface with low insertion force and contact resistance is required for low cost materials and coating such as copper alloys with tin coating. In addition, related to the application, the operating parameters have a wide range of values of currents, forces and materials. In this paper, we present a new experimental method based on nonintrusive probing of the deflection of the spring terminal, using a laser technique. The main feature is that the reflection of the laser beam on the spring allows the determination of the contact force of the lamella-spring inside the female part. The technique acquires the following insertion parameters during the insertion stroke: contact deflection /spl delta/ which allows contact force Fc, insertion force Fi and contact resistance Rc. It was found that the insertion force takes a maximum value and decreases to the stable value, which depends on the size and the material of the pin. However contact resistance decreases sharply on first insertion and tends towards stable values on completing the insertion process, which are less sensitive to the pin diameter. Finally, discrepancies were observed between the experimental and calculated data with simple numerical models. More complex models are in progress, which should improve the convergence of the theoretical approach to experimental results and proceed to the optimization of the connector parameters.

Minimization of arc extinction gap in the opening speed range 1 cm/s - 1 m/s

Speed separation in switches and relays is a fundamental parameter to improve breaking devices performances by reducing extinction gap and arc duration. It is commonly admitted, that with resistive load, high speed separation produces lower arc duration and keeps constant extinction gap, whereas with inductive load, the extinction gap is enlarged. The objective of this paper is to minimize the gap in a wide opening speed range (from 1 cm/s to 1 m/s) covering the main application of breaking and disconnecting devices. The results reveal a minimum arc extinction gap that occurred at an opening speed close to a few cm/s. Contrary to the common idea, this new result shows that high separation speeds are not always required to minimize the gap. In addition, this minimum gap value is highly affected by load and current, while contact materials seem to have a minor action. Based on arc shape and plasma examination, some attempts are made to explain this gap contraction.

Theoretical and experimental determination of erosion rate due to arcing in electrical contacts

Erosion rate of electrical contacts is a very important parameter to understanding and evaluating the performances of contact materials and switching devices such as the life time. The most common measurement of arc erosion is the mass change of the contacts, determined by the weighing of the anode and the cathode, before and after the electrical tests. The erosion is well known to be a transfer from the anode to the cathode then from the cathode to the anode and finally a bilateral loss takes place. In this work, and to avoid the transfer phenomena, the mass of electrode after a large number of breaking operations is performed in the domain of bilateral erosion of the anode and cathode. Calculation of the evaporated mass by finite element simulation of thermal input and heat transfer to the electrode from the arc is made. It is found that the evaporated amount per unit of duration arc is in agreement for silver materials but for metal oxide it needs the knowledge of material properties and namely data of physical heat conduction.