Electrical transmission or interconnection systems – Switching systems – With operation facilitating feature
Patent
1995-04-07
1997-04-01
Shoop, Jr., William M.
Electrical transmission or interconnection systems
Switching systems
With operation facilitating feature
200 5112, 218 74, 218123, 361 12, H01H 930
Patent
active
056169728
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention relates to a circuit arrangement with a pair of switching contacts made from silver and/or a silver alloy and with an inductive load which can be connected to a DC voltage source via the pair of switching contacts.
Inductive loads in DC circuits occur, for example, in automobiles where ever more DC motors are being used owing to the increase in comfort and safety. In particular, in safety systems, for example the anti-skid system, there is a need for a long lifetime in operating cycles, but also for a high switching reliability, that is to say a low failure rate, in the range of use of the switching device, specifically the relay or switch. Both criteria, lifetime and failure rate, are strongly influenced by the material migration of the contacts during switching operation under load in the case of direct current. Of importance in this case is the formation of tips and holes on the contacts which, from the statistical point of view, are highly likely to lead to premature mechanical sticking of the contacts.
The phenomenon of material migration has been known for a long time. It has already been proposed several times to keep this effect as slight as possible by selecting specific alloys and specific pairs of contacts. Such silver alloys are described, for example, in EP 0 448 757 A1. However, in the case of selecting the contact materials purely from the point of view of material migration there is the risk that other contact properties cannot be selected optimally.
Another known possibility of avoiding material migration consists in employing circuit technology to intercept the arc by means of spark-quenching elements. However, such a supplementary circuit is costly.
SUMMARY OF THE INVENTION
The object of the invention is to employ circuit engineering in the case of a circuit arrangement with switching contacts, made from silver or a silver alloy, and an inductive load so as to avoid material migration in the simplest possible way without the contact material having to be specially selected and without the need for expensive additional circuit elements in the load circuit.
According to the invention, this object is achieved when the arcing time of the breakdown arc is determined by the circuit-specific time constant T from the ratio of the inductance L and the ohmic resistance R in the load circuit as a function of the predetermined design breaking current i according to the following relationship: ##EQU1##
Additional features and advantages of the present invention are described in, and will be apparent from, the detailed description of the presently preferred embodiments and from the drawings. ##EQU2##
Thus, the invention provides for the range, which is most interest in practice, of the specific, expected breaking current from 1 to 30 A a design rule for the load circuit, specifically for the ratio of the inductance to the ohmic resistance, and thus for the arcing time, by means of which the material migration can be completely suppressed or at least greatly reduced. The invention makes use in this regard of the following finding:
The material migration is the result of asymmetric evaporation processes on the two contacts, specifically the anode and the cathode. These are essentially produced by the breakdown arc, in particular in the case of inductive loads. It is the arcing time of this breakdown arc which is decisive in this regard. In certain regions of the arcing time, the asymmetry of the evaporation process can be compensated owing to special physical effects in such a way that the material migration virtually vanishes in the final analysis, that is to say no tips and holes are produced on the contact surfaces. Such a relatively flat surface profile of the contacts decisively reduces the risk of mechanical sticking, lengthens the lifetime and increases the operating reliability of the contacts.
Since the arcing time of the breakdown arc is essentially given by the time constant, that is to say the cut-off inductance L of the inductive component, for
REFERENCES:
patent: 3621275 (1971-11-01), Santi
patent: 3697774 (1972-10-01), Pascente
patent: 4348566 (1982-09-01), Baba et al.
patent: 5072328 (1991-12-01), Dvorak et al.
patent: 5285035 (1994-02-01), Williams et al.
Paladini Albert W.
Shoop Jr. William M.
Siemens Aktiengesellschaft
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