FR2459539A1 - Car blower controlling resistor protection - includes short-circuiting conductor contacted by overheating resistor - Google Patents

Abstract

The protection of metal resistor controlling the speed of blower in the air conditioner of passenger space of cars eliminates overheating of the resistor which presents a hazard of fire for the dashboard. The protection is based on a short-circuiting conductor of the resistor (2) by a bar or a cylinder (5) separated from the resistor by air or a thin plastic layer. This avoids the unreliable safety switching since the resistor (2) tends to overheat and, therefore thermally dilate near the infeed terminal (6). If the resistor (2) is in the form of a wire spiral then the first turn of the latter expands more than the other turns and melts away the insulation to contact the short circuiting cylinder (5). As a result, the resistor would not carry a current and cool off to operate in normal conditions. Fast blower speed can be selected by operating switch (13) to connect cylinder (5) to the source (3).

Description

 The invention relates to a device intended to protect against thermal overloads a metallic electrical resistance, in particular an additional resistance serving to regulate by stages the speed of the motor driving a fan on motor vehicles, this device comprising a contact system capable to close when the metallic resistance expands under the effect of heat.

 Such a protective device is described in German Patent No. 537,785. In this case, the metal resistance is constituted by a rod which is extended, in the event of an electrical overload, under the effect of the heat developed by the Joule effect. In the event of a sufficient variation in length of this metallic resistance, a reference rod, fixed to the first rod so as to be thermally insulated beech, causes by means of a lever mechanism the closing of an electromechanical switch mounted separately at proximity, in order to trigger a cut-off function.

With such a protective device, the actuation of the lever mechanism requires, for sar and reproducible title, a strong elongation of the metal resistance in rod, that is to say a resistance of great length. The construction of this protective device is mechanically complicated and requires storytelling adjustment operations. Since the switch is actuated by a lever mechanism sensitive to an expansion effect, the device is highly exposed to operating incidents under the effect of vibrations.

It is therefore not suitable for use on a motor vehicle, in particular for protecting the additional resistances which are used to adjust the speed of the fan motors in stages.

German patent application no. 1 480 095 describes a device which is intended to protect against overloads an additional resistance associated with the electric motor of a fan and the operating principle of which is as follows: a temperature probe, constituted by a bimetallic strip or by a thermostat, reacts to a critical heating of the additional resistance and causes the shunting of this by means of a working contact switch so as to cause with certainty the triggering of an overload circuit breaker when the intensity of the current is excessive. Although this protective device has given satisfactory results in operation with the fan motors installed on motor vehicles and has thus been applied in large numbers for many years and although it is not very bulky, it presents for the assembly in large series of big disadvantages because of the time which it requires for its manufacture and so n adjustment. In fact, the admissible losses occurring during operation in the additional resistors which are used with the fan motors in order to adjust their speed in stages and which generally consist of windings of wire or by printed circuits, reach , depending on the characteristics of the fan, Up to 25% of the electrical power absorbed by the motor when operating under nominal voltage, that is to say without the additional resistor being connected in series. As such resistors are preferably placed in the air flow created by the fan, thus being subjected to forced ventilation, the heat of the Joule losses is dissipated without difficulty under normal circumstances. the high admissible losses however require that the response characteristics with regard to the shuntage of the additional resistance be adjusted with great precision by means of a working contact, for a current intensity which does not yet respond to the circuit breaker with overcurrent but which already leads to excessive heating of this resistance. Such operating states, which occur for example when the fan motor is seized or short-circuited, give rise to the risk of fire or at least the destruction of neighboring electrical devices by heat.

 In view of these drawbacks of the usual protective devices, the invention aims to create a protective device, of the type defined in the preamble, which can be easily manufactured and which, once put in place, operates as well as below than above critical operating conditions.

With a protective device of the kind in question, this object is achieved thanks mainly to the fact that the contact system is constituted, on the one hand, by the metallic resistance itself and, on the other hand, by a conductive element placed nearby. immediate of this resistance and forming a counter-contact for it.

With the solution according to the invention, it is not a separate switch installed which comes into action when a critical operating state is reached, but it is the additional resistance itself and a conductive element placed at its neighborhood which constitute working contacts in order to further increase the intensity of the current and thus to safely intervene the overcurrent circuit breaker, thanks to the fact that an electrical connection is established with the conductive element forming a counter contact, due to thermal expansion undergone by the additional metallic resistance.

 According to a preferred embodiment, the metal resistance is constituted by a cylindrical helix of wire extending between an input terminal and an output terminal and the conductive element, forming a counter-contact, extends in the form of 'a short-circuit conductor isolated from the wire helix, along the geometrical cylinder in which this helix is inscribed.

This embodiment is distinguished in particular by great mechanical robustness and by great resistance to heat, with a safe behavior with regard to the response to a thermal overload.

According to an advantageous development, the wire helix is electrically connected, by its current output, to the output end close to the short-circuit conducting element. It is thus the totality of the additional metallic resistance which is shunted in practice by the conductive counter-contact element and which is discharged of any current in the event of overload; it can therefore cool quickly, which safely avoids any risk of fire.

it is particularly judicious, from the practical point of view, to have a switch in front of the input terminal of the wire helix and in front of the input end near the short-circuit conducting element. In this case, in fact, it is the conductive counter-contact element itself which can be used to shunt the additional resistance, when the fan motor is running at non-reduced speed, and this without the need to complete. installation by additional contactors.

 The short-circuit conductive element may consist of a hollow cylindrical surface, electrically conductive, inside which the wire helix lies tangentially, with the interposition of a layer of thermoplastic insulating material. this results in particular in the advantage of a robust construction as well as a protective effect for the metallic resistance, and that of a high precision of the response due to the radial pressure which is exerted, during the heating , against the inner wall of the hollow cylinder. In this case, it is advantageous to constitute the short-circuit conducting element by a hollow metal cylinder, covered on its internal surface by a layer of thin insulating material, heat-deformable, in which the helix is adapted. This metal cylinder constitutes a heat trap which is used to evacuate, with an additional overloaded resistance, heat certainly increased but not yet critical and which thus prevents this resistance from being destroyed prematurely.

According to a solution related to the previous one, at least two hollow cylinders, in each of which a wire helix is housed tangentially, are juxtaposed so as to touch by their respective outer surfaces.

Heat dissipation is thus improved because the hollow cylinders in contact with each other together offer greater mass as a heat trap.

 Even if the additional resistance is enclosed in a hollow cylinder so that a radial back pressure is exerted during the radial expansion of the wire helix under the effect of heat, it is possible and advantageous to use the counter contact conductive element, which is mounted in parallel with respect to the additional metal resistance both geometrically and electrically in the event of reaction to the overload, as a short-circuit bridge between the input terminal and the current output terminal of such an additional metal resistance (or of a series connection of such resistors), when switching is made to additional resistors of different values in order to adjust the speed of the fan motor to steps of different values. To this end, at least one comia'tteur is connected on the one hand to a hollow cylinder and on the other hand, via an opening made in the wall of this hollow cylinder, to a socket that has the wire propeller inside the hollow cylinder.

By means of such short-circuit switches, the elements of a series connection of resistors can be shunted via the short-circuit conducting element.

According to an improvement, a stepped switch is connected by its input to a current source and by its output, either, in a first position, to the input terminal of a series mounting of metallic resistors, or, in a second position, at the socket located between two such metal resistors, or even, in a third position, at the entry end of the hollow cylinder. In order to adjust the fan speed in stages, this stepped switch switches on a more or less large part of the series connection of the resistors or closes the circuit on the short-circuit conducting element.

 Other characteristics and advantages of the invention will appear on reading the additional description which follows of preferred embodiments of the invention, which are shown in the accompanying drawings which have been limited to the essentials for the purpose of simplify them.

 Figure 1 shows, by a block diagram, the construction of the protective device according to invetsson.

 FIG. 2 represents, in axial section, a preferred embodiment of the block diagram of FIG. 1.

 Figure 3 shows on a larger scale the detail designated by III in Figure 2, before and after the response of the protective device.

4 shows, by a block diagram, a variant of the embodiment of Figure 2, on the left in longitudinal section and on the right in cross section.

FIG. 5 represents a variant of the block diagram of FIG. 4.

As is usual in particular on motor vehicles, the electric motor 1 of a fan serving for example for the ventilation or the air conditioning of the passenger compartment of a vehicle is powered by a current source 3 such as the battery of the vehicle by means of metallic resistors 2 which can be placed in series to adjust the supply voltage of motor 1 in stages, as shown in a simplified manner in FIG. 1.

In certain operating states, the high capacity of the current source 3 and the high power which can be drawn by the electric motor 1 have the effect that, in certain incidents te) that short-circuit or that seizure of the motor 1, the metallic resistor 2 heats up to red without reacting an overcurrent cut-out 4 connected in series with this resistor 2 and without consequently interrupting the circuit thus overloaded.

As the metal resistors 2 used to regulate the speed of the engine 1 are placed near the vehicle's electrical installations and frequently even very close to the dashboard lining, such excessive heating of the metallic resistors 2 risks causing damage to the electrical insulation from neighboring installations or even a fire at the dashboard.

 To put an end to such critical intermediate operating states which correspond to an overload of the metallic resistances but which are still incapable of immediately triggering the circuit breaker 4, each metallic resistance 2 can be shunted using a conductor short circuit 5. This increases the current intensity further, quickly and significantly, through the circuit breaker 4 which then reacts immediately by interrupting the circuit, so that the metal resistors 2 can cool down to non-critical temperatures.

To ensure this shunting, each metal resistor 2 is equipped with a thermally controlled contact device, one of the contacts of which is formed by a region of the external surface of this resistor 2 itself and whose counter-contact is constituted by the short circuit conductor 5.

These contacts are closed when, as a result of critical overheating, the metal resistance 2 expands enough to touch the short-circuit conductor 5 placed in its immediate vicinity. To this end, a contact can be provided over the entire length of the metal resistor 2 and therefore also over the entire length of the short-circuit conductor 5 to divert the current out of this resistor 2.

Tests have nevertheless shown that it is in the immediate vicinity of its input terminal 6 that the fastest and most significant heating of the metallic resistance 2 takes place, and consequently its expansion, in the event of an overload. the fastest and most important. As illustrated in FIG. 1, it is therefore advantageous to connect the output terminal 7 of the metal resistor 2 with the neighboring output end 8 of the short-circuit conductor 5 and to place this conductor 5 right next to the metallic resistance 2, with the interposition of an insulator 9. At the opposite end of the short-circuit conductor 5, that is to say at its input end 10, it therefore occurs, when the metallic resistance 2 expands under the effect of heat, a contact with the short-circuit conductor 5, shown diagrammatically in FIG. 1 by a zig-zag pattern 11, and consequently a shunting of the metallic resistance 2 which is therefore not no longer subject to the effect
Joule by passing electric current and which can therefore cool down to a non-critical temperature.

 The insulator 9 could consist of a narrow air gap between the short-circuit conductor 5 and the surrounding surface of the metallic resistor 2. It can safely avoid false contacts even if the device is subjected to vibrations, for example on a moving motor vehicle, without having to resort to a complicated construction to hold the elements of the device, it is n8anmdns more advantageous to constitute such an insulator 9 by a thin layer of insulating material 12, as will be described in more detail with the aid of FIG. 2.

 it is advantageous to mount a switch 13 in front of the input terminal 6 of the metallic resistance 2 and in front of the input end 10 of the short-circuit conductor 5 so as to be able to bypass the metallic resistance 2 by the conductor of short circuit 5, even in the absence of an overload, that is to say to operate the fan motor 1 at maximum voltage, therefore at maximum speed.

Preferably and as shown in FIG. 2, the metal resistance 2 is constituted by a wire helix 14 geometrically inscribed in a cylinder. This helix 14 is closely surrounded, being separated only by the thin layer of insulating material 12, by a short-circuit conductor formed by a hollow cylinder 15 whose inner surface 16 is electrically conductive. In the event of an electrical overload, the wire propeller 14 develops, as a result of its expansion, a force whose radial component is absorbed by the hollow cylinder 15. This component is maximum in the vicinity of the input terminal 6, c ' i.e. at the level of the first expiration 14.1. The thin layer of insulating material 12, with thermoplastic behavior, is melted where it is touched by the external surface 17 of the coil 14.1 and pressed outwards by this surface (see FIG. 3 on the left) until the connection shown diagrammatically by the arrow 11 in FIG. 3 (on the right) is established between the external surface 17 of the first turn 14.1 and the internal conductive surface 16 of the hollow cylinder 15 and that the other turns of the wire propeller 14 are thus shunted.

 It is in particular when, according to a preferred embodiment, the hollow cylinder 15 is formed by a metal tube that it constitutes a heat trap to quickly dissipate the strong heating produced in the first turn 14.f The surrounding elements, such as those made of plastic carried by the dashboard of a motor vehicle, are therefore protected against any risk of ignition or even sealerr1ent of alteration by fusion.

In addition, this hollow cylinder 15 provides mechanical protection to the wire propeller 14 so that one can dispense with adapting thereto the usual perforated protective sheath.

 The hollow cylinder 15 and the thin layer of insulating material 12 are preferably arranged, in particular with regard to their resistance to heat, so that in the event of minor operating incidents (damage to the bearings of the fan motor 1, FIG. 1, for example) causing an increase in the intensity of the current and consequently a rise in the temperature in the wire propeller 14, these do not yet cause the deactivation of this ice circuit due to the fact that an overheating insufficient to bring the metallic resistance 2 to red and to remove by fusion the thin layer of insulating material 12, is eliminated by evacuation of the heat in the hollow cylinder 15 due to the multiple contact established between the wire helix 14 and this cylinder hollow 15. This prevents the wire propeller 14 and the layer of insulating material 12 from being destroyed and the assembly visible in FIG. 2, consisting of the wire propeller 14 and the hollow cylinder 15, remains in a state of walk after ès replacement or repair of defective fan motor 1.

As shown in FIG. 2, there is interest in installing this assembly on an insulating plate 18; connection plates 19, between which the hollow cylinder 15 is held by its transverse ends, narrow so as to form contact pins 20 which pass through the plate 18 and are anchored therein. Via a crimp connection 21, the output terminal 7 of the wire helix 14 is mechanically and electrically connected to that of the pins 20 which is associated with the output end 8 of the hollow cylinder 15 The input terminal 6 of the wire helix 14 is connected to another contact pin 22 which is also anchored to the plate 18. When the switch 13 occupies the position symmetrical to that illustrated in FIG. 2, l supply of the fan motor 1 at maximum voltage, that is to say for the maximum speed, can be done via pins 20, 22 and for example a printed circuit board. This pin construction makes it easy to replace this assembly, even in the event of contact due to thermal overload of the wire propeller 14; until then, the circuit, once the cause of the incident and of the overload has been eliminated, continues to be able to function, without it being possible however to put in series an additional resistance 2.

 As shown in Figure 4, the wall of the hollow cylinder 15 may have an opening 23; a stepped switch 13 ′, comprising an additional stage with respect to the switch 13 in FIGS. 1 and 2, can be connected by one of its stages to a socket 25 placed between two wire helices 14 in series, by means of 'a conductor passing through the opening 23. The other stages of the switch 13' are connected, one to the input terminal 6 of the series of wire helices 14, and the other to the end input 10 of the short-circuit conductor 5, which is constituted by the hollow cylinder 15.-By its input, the step switch 13 'is connected to the current source (cf. FIG. 1) by the overcurrent cut-out 4 .This stepped switch 13 ′ therefore makes it possible to switch on either the series of two wire propellers 14 shown, or only one of these two wire propellers 14 (the one on the right in FIG. 4), or even none of these two wire propellers 14, which makes it possible to adjust the speed of a fan in stages. We could obtain the same effect by connecting short-circuit switches between on the one hand, the hollow cylinder 15 and, on the other hand, either input labor 6 or socket 25, but this would cause a great electromechanical complication .

FIG. 5 shows that with several wire helices 14 connected in series and provided with individual hollow cylinders 15 (for example three cylinders 15 as shown on the right side of FIG. 5), it is advantageous to arrange together so such that, as a result of the mutual contacting of the hollow cylinders 15 by their outer surfaces, a larger heat trap is obtained and therefore more effective in removing heat during heating which is not yet critical or during '' local overheating of the wire propeller (s) 14.

The invention is therefore not limited to the embodiments described and shown but it encompasses all the modifications and variants which are within the reach of specialists in the field.

Claims (9)

1. Device intended to protect against thermal overloads a metallic electrical resistance, in particular an additional resistance serving to adjust by stages the speed of the motor driving a fan on motor vehicles, this device comprising a contact system capable of closing when the metallic resistance expands under the effect of heat, characterized in that the contact system consists, on the one hand, of the metallic resistance (2) itself and, on the other hand, of a conductive element ( 5) disposed in the immediate vicinity of this resistor (2) and forming a counter-contact therefor. 2. Protective device according to claim 1, characterized in that the metallic resistance (2) is constituted by a cylindrical helix of wire (14) 'extending between an input terminal (6) and an output terminal (7) and in that the conductive element (5), forming a counter-contact, extends in the form of a short-circuit conductor insulated with respect to the wire helix (14), along the geometry cylinder in which fits this propeller (t4).  3. Protective device according to claim 2, characterized in that the wire helix (14) is electrically connected, by its current output, to the neighboring output end (8) of the short-circuit conductive element (5).  4. Protective device according to one of claims 2 and 3, characterized in that a switch (13) is disposed in front of the input terminal (6) of the wire helix (14) and in front of the end d neighboring input (10) of the short-circuit conducting element (5). 5. Protective device according to any one of claims 2 to 4, characterized in that the short-circuit conductive element (5) is constituted by a hollow cylindrical surface, electrically conductive, inside which lies tangentially l wire helix (14), with the interposition of a layer of thermoplastic insulating material (12). 6. Protective device according to claim 5, characterized in that the short-circuit conductive element consists of a hollow metal cylinder (15), covered on its inner surface by a layer of thin insulating material (12), deformable to heat, in which the wire propeller (14) is adapted. 7. Protective device according to any one of claims 2 to 6, characterized in that at least two hollow cylinders (15), in each of which a wire helix (14) is housed tangentially, are juxtaposed so as to touch by their respective exterior surfaces. 8. Protective device according to any one of claims 2 to 7, characterized in that at least one switch is connected on the one hand to a hollow cylinder (15) and on the other hand, via an opening (32 23) formed in the wall of this hollow cylinder (15), to a socket (25) that includes the wire helix (14) inside the hollow cylinder (15). 9. Protective device according to claim 8, characterized in that a stepped switch (13 ') is connected by its input to a current source and by its output, that is, in a first position, to the input terminal ( 6) of a series mounting of metal resistors (2), either, in a second position, to the socket (25) located between two such metal resistors (2), or even, in a third position, at the end inlet (10) of the hollow cylinder (15).