Electricity: electrothermally or thermally actuated switches – Thermally actuated switches – With bimetallic element
Reexamination Certificate
2000-03-01
2002-09-10
Vortman, Anatoly (Department: 2835)
Electricity: electrothermally or thermally actuated switches
Thermally actuated switches
With bimetallic element
C337S372000, C337S375000, C337S342000
Reexamination Certificate
active
06448883
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a switch having a temperature-dependent switching mechanism that comprises a stationary contact element; a movable contact element coacting therewith; and a bimetallic element, having a predetermined functional service life, that as a function of its temperature lifts the movable contact element away from the stationary contact element, the two contact elements being, in one switch position, in contact with one another in order to carry or conduct a current that is to be guided through the switch, and in a second switch position, lifted away from one another so that the switch is open.
2. Related Prior Art
Switches of this kind, which are also called temperature controllers or temperature limiters, are commonly known from the prior art.
The known switches are used to protect an electrical load from overtemperature and/or excessive operating current. For this purpose it is connected, in series with the electrical load, to a voltage source for powering the load, and is arranged mechanically so that it is in close thermal contact with the load.
There are several design variants of such switches, which differ firstly in terms of whether the bimetallic part itself carries the current flowing through the switch or whether a spring element, which carries the current when the switch is in the closed state and is moved by the bimetallic element, is connected in parallel with the bimetallic element. It is furthermore known to equip the switch with series resistors and/or parallel resistors, the series resistor serving to heat the switch in the event of excessive current and the parallel resistor ensuring that when the switch is open, a residual current flows which generates heat in the parallel resistor such that the switch remains open.
As a rule, the switch is closed below the response temperature of the bimetallic element, so that the load is supplied with current. There are also designs, however, in which the switch activates, for example, a fan which is intended to cool the electrical load being protected if its temperature is too high.
The active switching elements in temperature-dependent switching mechanisms and switches of this kind are bimetallic or trimetallic elements that comprise two or three layers of different metals which have differing coefficients of thermal expansion, as is commonly known. When a temperature change occurs above or below the response temperature, switching elements of this kind snap back and forth, for example, between a convex and concave shape. The response temperature and the extent of the deformation upon switching are determined on the one hand by the material selection and material thickness, but also by mechanical dimples, etc.
An important criterion for the switching behavior of such bimetallic switching mechanisms is the speed at which the movable contact element is lifted away from the stationary contact element. If the switching speed is too slow, there can form between the two contact elements an undesirable arc that, in the most unfavorable case, welds the two contact elements to one another so that the switch is permanently closed.
Such a situation is of course undesirable, since it means that the switch can no longer perform its monitoring function. What is disadvantageous in this context is in particular the fact that this defect of the switch is initially not noticed, since the operating current of the load being protected continues to be carried through the switch. If the load then heats up to an excessive temperature, the switch can no longer open, which results in serious safety risks. The malfunction of the switch is not recognized at all until excessive heating of the load occurs, with the damage resulting therefrom.
For these reasons, in the known switches the bimetallic elements are adjusted—in terms of material selection, geometrical dimensions, and dimpling—so that they have a specific functional service life. The switches would then need to be replaced before the functional service life is reached.
A further measure for preventing welding between the two contact elements consists in designing them with an identical geometry that is matched to one another in such a way as to prevent the creation of arcs.
Since replacement of a switch upon reaching its functional service life cannot always be guaranteed, cut-out fuses that are connected in series between the switch and the load being protected are often additionally used in safety-relevant switches. While the switches with a temperature-dependent switching mechanism switch back on when the load cools off, the blow-out fuses irretrievably open the circuit. The cut-out fuses are designed so that they respond well above the response temperature of the temperature-dependent switch, so that they irretrievably open the circuit only if the bimetallic switch is no longer operating properly.
Such additional features are of course cost-intensive, so that they are disadvantageous for that reason alone. A further disadvantage is the fact that an additional component is necessary in order to monitor the load being protected; this requires physical space that often can be made available only with difficulty or not at all.
The functional service life of a bimetallic or trimetallic element is determined by fatigue phenomena resulting from frequent switching. Once a temperature-dependent switching mechanism of this kind has switched, for example, ten thousand times, the bimetallic element no longer snaps over abruptly when the response temperature is reached; instead the movable contact element at first moves only slightly away from the stationary contact element, and only when a greater temperature rise occurs the open position is reached, which might occur in the course of a creeping movement. When the switching capability of the bimetallic element has weakened in this fashion, there can form between the contact elements an arc which, while at first it is extinguished again, ultimately results—as the switching capability further weakens—in the aforementioned welding of the contact elements.
SUMMARY OF THE INVENTION
In view of the above, it is an object of the present invention to improve the switch mentioned at the outset such, that with a simple design, it reliably ends its service life in the open state.
With the switch mentioned at the outset, this object is achieved according to the present invention in that the two contact elements are arranged and coordinated with one another in such a way that when the functional service life of the bimetallic element is reached, the switching mechanism is welded in the second switch position; preferably the bimetallic element is welded in the second switch position, by an arc which forms when the functional service life of the bimetallic element is reached, to a preferably current-carrying part of the switch.
The object underlying the invention is completely achieved in this fashion.
Specifically, the inventor of the present application has recognized that by selecting the arrangement and the geometry of the two contact elements, it is possible to ensure that an arc which occurs welds the switch not—as in the prior art—in the closed state, but rather in the open state. This is therefore an departure from measures known from the prior art for preventing an arc; instead, according to the present invention, provision is made for a deliberately directed arc to weld the switch in the open position.
This exploits the fact that when the functional service life is reached—i.e. as the switching capability and spring force of the bimetallic element weaken because the speed with which the two contact elements move apart is now decreasing—an arc is formed between them; in the prior art, however, it is extinguished again by the geometry of the contact elements. According to the present invention, however, provision is made for this arc not to be extinguished, but rather to weld the switch in the open state.
It is particularly preferred in this context if, in the case of a swit
Harness & Dickey & Pierce P.L.C.
Vortman Anatoly
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