Alexandre Vassa

French National Centre for Scientific Research, Lutetia Parisorum, Île-de-France, France

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Publications (5)0 Total impact

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    ABSTRACT: Contact resistance is usually determined by the ratio of the voltage to the current and the increase of this value means that the contact interface is degraded. However for high currents, this ratio is not constant so Ohm's law does not apply. In this paper, we study the electrical behaviour through degraded surface materials (Ag and Ag with various metal oxides) aged by break arcing up to 90A at 42VDC with a contact force of 14N. In order to make a current voltage characteristic, the samples are tested with increasing current (I) from 50mA to 75A and then decreasing from 75A to 50mA. The main result is that the proportionality between the current and the voltage is ensured up to 70mV. Beyond this value this proportionality is lost and voltage break down takes place at different level of fritting voltage. However this phenomenon is frequently observed during the first applied current ramp (increasing), and disappears at the decreasing current ramp and during further tests. Discussions here are given concerning the constant fritting voltage over a wide current range being related to the breakdown of thin oxide films on the degraded surface and/or the creation of additional contact spots.

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    ABSTRACT: The behavior of the arc formed during electrical circuit opening is examined, in the presence of a pulsed magnetic field. The field is applied in the inter-electrode gap where the arc develops. The opening sequence may be separated into two temporal phases. During the first phase, the arc remains stuck at the same position as under zero field. This is ascribed to the existence of a sticking force, which overcomes the Laplace force. The sticking force is related to the high pressure existing in the plasma. During the second phase, the Laplace force dominates. The arc is blown out of the contact region in a time duration of 1 to 1.5 ms. A model is developed which provides a semi-quantitative account of the experimental behavior.
    Electrical Contacts (ICEC 2012), 26th International Conference on; 05/2012
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    ABSTRACT: The phenomenon of magnetic blowing allows drastic reduction in arc duration. It is of specific interest in DC applications, more specifically in automotive applications. Blowing exploits the action of the magnetic force on the transitory arc formed during circuit opening. A new contact was described in a previous paper in which the magnetic field required for blowing is generated by the contact material itself. In the present study, aimed at a more thorough analysis of magnetic blowing, an external adjustable magnetic field (up to 320 mT) was produced by placing magnets on each side of the contact. The experiments were performed for voltage values from 42 V<sub>DC</sub> to 360 V<sub>DC</sub> and currents up to 100 A, using a resistive circuit. For considered magnetic field values, no magnetic blowing was observed below a certain critical current. At high current, the arc duration was found to be approximately current independent, following a phenomenological 1√B variation. Under 42 V<sub>DC</sub> and the maximum current of 100 A, the arc duration under 320 mT was reduced by a factor of 100 as compared to the arc duration under zero field. Qualitative account of the present phenomena is obtained by considering that blowing is governed by the competition between the magnetic force acting on the arc and the force acting against arc motion, originating from the enthalpy expense needed to move the arc from a heated region at the contact surface to a colder one.
    Electrical Contacts (HOLM), 2010 Proceedings of the 56th IEEE Holm Conference on; 11/2010