Patent Publication Number: US-6222715-B1

Title: System for protecting electrical devices against overheating

Description:
BACKGROUND OF THE INVENTION 
     1. FIELD OF THE INVENTION 
     The present invention is directed to an electrical with a device for the protection thereof against overheating. 
     2. DESCRIPTION OF THE PRIOR ART 
     Devices for protecting electrical devices against overheating are known in the art. Such devices are disclosed by DE 23 42 015 A1. 
     For example, in refrigerator cooling units, a so-called motor start-up PTC thermistor can be connected in front of the units&#39; electric motors, such that the drive shafts of the electric motors connect to the units&#39; cooling compressors. In each start-up phase of an electric motor, the current flowing through the PTC thermistor heats it very strongly, as a result of which the resistance of the PTC thermistor increases within a very short time, frequently within seconds, from a few ohms in the cold state to very high resistances. 
     Since the surroundings of motor start-up thermistors contain oil residues or a generally oily atmosphere, there is a risk that these residues may under unfavorable circumstances be ignited. In the worst case, this may lead to the onset of a smouldering fire in the vicinity of the refrigerator cooling unit to which a motor start-up PTC thermistor is assigned. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a device that protects electrical devices from overheating such that there is no longer even the risk of local smouldering fires being started. 
     One particular advantage of the present invention is that existing electrical devices need to be altered only slightly so that it is possible to fit a thermal fuse which, according to the present invention, is to be arranged directly next to a critical element. In this way, existing electrical devices can thus be retrofitted according to the present invention with a thermal fuse. 
     For example, in the case of a motor start-up devices having PTC thermistors, a thermal fuse is arranged directly next to the critical element, in this case directly next to the PTC thermistor. Accordingly, when there is a risk of overheating, immediate response of the thermal fuse is ensured and an electrical device equipped or retrofitted according to the present invention is protected from overheating with absolute reliability. 
     When the thermal fuse, arranged according the present invention, melts, the electrical supply to the electrical device to be protected is immediately interrupted which reliably avoids the risk of a possible smouldering fire. 
     Since the thermal fuse is fashioned U-shaped or V-shaped, when a thermal fuse is arranged and fitted according to the present invention inside the housing of an electrical device, for example a motor start-up device having a PTC thermistor, the vertex of the U-shaped or V-shaped fuse is positioned directly next to the critical element, i.e, the PTC thermistor. 
     When the thermal fuse which is advantageously designed according to the invention, is used and arranged directly next to the element to be made safe, for example a PTC thermistor, then the thermal fuse will melt particularly quickly because of the small distance between the thermal fuse and the critical element to be protected from overheating. 
     In an embodiment of the present invention, in order to prevent spreading of an incipient smouldering fire, as an additional safety-related provision, the housing enclosing the electrical device consists of self-extinguishing plastic and/or the housing is clad with self-extinguishing plastic. 
     After (albeit perhaps a short time later) the thermal fuse has melted and the electrical supply has been interrupted, it is no longer possible for the heat source (in the example currently referred to, the PTC resistor of the motor start-up device) to heat up. Further, the incipient smouldering fire is immediately extinguished because the housing enclosing the electrical device, or the entire housing, is clad with self-extinguishing plastic. Spreading of a smouldering fire is thereby prevented with absolute reliability. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a plan view of a motor start-up device having PTC thermistor. FIG. 1 a  is a partial cross sectional view of a modified housing having a cladding of self-extinguishing plastic. 
     FIG. 2 is a side view of the motor start-up device of FIG. 1 with a thermal fuse in position; 
     FIG. 3 is a side view of the motor start-up device of FIG. 1 with a thermal fuse in position; 
     FIG. 4 is a plan view of thermal fuse illustrated in FIG.  3 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the plan view of a motor start-up device  11  which is represented in FIG. 1, a PTC thermistor  3  is held in a housing  1  via supports  2   a  and  2   b . Spring contacts  5   a  and  5   b , via which current is fed, bear on the PTC thermistor  3  at opposite sides thereof. 
     The spring contacts  5   a  and  5   b  are conductively connected to connection parts  4   a  and  4   b , which are connected via extensions  4   a ′ and  4   b ′ to electrical plug-in contacts  7   a  and  7   b . The extension  4   b ′ is split from the connection part  4   b  by a discontinuity  6 , thereby interrupting current flow to connection part  4   b.    
     As represented as an enlarged detail in FIG. 2, the discontinuity  6  is bridged by a thermal fuse  8  by fastening two ends  81  and  82  of the thermal fuse  8 , which are angled by about 90°, to the extension  4   b ′ and the connection part  4   b , which are separated from one another by the discontinuity  6 , using for example rivets  9  and  10 . 
     In a preferred embodiment, the thermal fuse  8  in FIG. 2 has the shape of a U, and its vertex  83  which points downwards in FIG. 2 extends as close as possible to the PTC thermistor  3 . 
     The thermal fuse  8  is made of a low-melting point material whose melting point is chosen such that it is below a critical temperature of the PTC thermistor  3 . This ensures that the maximum permissible temperature for the PTC thermistor  3  or for the motor start-up device  11  in which the PTC thermistor  3  is fitted, is not exceeded. 
     FIG. 3 depicts a plan view which corresponds to FIG.  2  and is also rated by 90° relative to the plan view in FIG.  1 . In contrast, FIG. 4 depicts a plan view of a 6-branched spring contact  5   b  along a line IV—IV in FIG. 3, in the direction of the connection part  4   b  and its extension  4   b′.    
     As can be seen from the plan view in FIG. 3, according to the invention a modified continuation of the thermal fuse  8 ′ is of approximately V-shaped design, the vertex or turning point  83 ′ of the V-shaped thermal fuse  8 ′ being again arranged directly next to the PTC thermistor  3 . 
     In an embodiment of the device according to the invention which is represented in FIG. 4, the extension  4   b ′ has a circular or V-shaped indentation  41   b ′ at its end of the discontinuity  6  adjoining the plug-in contact  7   b . The opposite edge region of the discontinuity  6  is preferably designed with a shape complementary to the indentation  41   b ′. 
     As can be seen in FIGS. 3 and 4, modified extension of a thermal fuse  8 ′ is fastened by its left angled continuation  81 ′ to the extension  4   b ′ and by its other end  82 ′ between the spring contact  5   b  and the connection part  4   b , using a rivet  10  which is represented by a dashed line in FIG.  3  and by a dot in FIG.  4 . 
     Referring to the vertex or turning point  83 ′ of the V-shaped thermal fuse  8 ′ may also be arranged between two branches  5   b   1 , and  5   b   2  of the 6-branched spring contact  5   b . In this way, the distance between the PTC thermistor  3 , which is to be protected against overheating, and the thermal fuse  8 ′ can be kept particularly small. 
     This small distance between the thermal fuse  8 ′, or its vertex  83 ′, and the PTC thermistor  3  ensures immediate response, that is to say melting of the thermal fuse  8 ′, as soon as a temperature is reached which could become critical either for the PTC thermistor  3  itself or for the device  11  whose housing  1  accommodates it. 
     It is particularly advantageous in the case of the embodiments represented in FIGS. 3 and 4 that one end of the thermal fuse  8 ′, namely the angled continuation  82 ′, is held and secured using the same rivet  10  between the connection part  4   b  and the spring contact  5   b . In contrast to the embodiment depicted in FIG. 2, only one additional rivet point for the rivet  10  is needed in the case of the embodiment of the thermal fuse  8 ′ in FIG.  4 . 
     This is particularly advantageous if, for example, a motor start-up device having a PTC thermistor  3  is equipped from the start with thermal fuse  8 ′ provided according to the invention. 
     Although the way of fitting and fastening the V-shaped thermal fuse  8 ′ which is depicted in FIGS. 3 and 4 is also possible in the case of retrofitting, the embodiment and fitting method represented in FIG. 2 for the thermal fuse  8  are generally preferable in the case of retrofitting, even though two holes need to be provided in this embodiment for inserting the rivets  9  and  10 . 
     In the case of retrofitting with the thermal fuse  8 ′ according to FIGS. 3 and 4, however, it would be necessary to drill out a rivet used to fasten the 6-branched spring contact  5   b  to the connection part  4   b . After the continuation  82 ′ of the thermal fuse  8 ′ has been introduced between the connection part  4   b  and the spring contact  5   b , a new rivet  10  is used to connect the connection part  4   b , the continuation  82 ′ of the thermal fuse  8 ′ and the spring contact  5   b  firmly to one another. The housing  1  may be made of a self-extinguishing plastic or be a housing  1 ′, as shown in FIG. 1 a , which has a cladding layer  20  of self-extinguishing plastic on a housing wall  21 . The wall  21  can also be made of a self-extinguishing plastic material. 
     Although modifications and changes may be suggested by those skilled in the art, it is the intention of the inventors to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of their contribution to the art.