PTC element

Abstract: PTC element comprises at least one body (10), provided with two parallel large surfaces, of an electrically conductive polymer composition with a positive temperature coefficient and two electrodes. At least one of the parallel surfaces on the body, or on one of the bodies, of polymer composition is adapted to make free contact with an electrode (11) or with a parallel surface of another body of electrically conductive polymer composition. A pressure device is provided to exert on the electrodes a pressure directed perpendicular to the parallel surfaces on the body, or bodies, of polymer composition. The pressure device is preferably provided with pressure-exerting devices (15, 16, 17, 17a, 17b) with the ability to be resilient. After changing from a low resistance to a high resistance state, the PTC element returns to the initial resistance state and is thus reusable after having been subjected to short-circuit currents. The parallel surfaces on the body, or the bodies, of polymer composition may be concentric.

Description: The invention relates to a PTC element according to the precharacterising part of claim 1. A PTC element often comprises a body, provided with two parallel end surfaces, of an electrically conductive polymer composition with a *restivity with a positive temperature coefficient and two electrodes, arranged in contact with the end surfaces, for carrying current through the body, the polymer composition comprising a polymer material and an electrically conductive powdered material distributed in the polymer material. The expression PTC element is the accepted term for an element whose resistivity has a positive temperature coefficient. PTC elements are used in electric circuits as overcurrent protection. The resistance of a PTC element of the above described kind is low, for example a few mΩ, in the normal operating range of the element, which may extend to, for example, 80°C, and increases slightly with the temperature. If the temperature of the element exceeds this value, for example because of an overcurrent, the resistance increases more rapidly, and when exceeding a certain temperature, the element suddenly changes from a low resistance to a high resistance state, in which the resistance may amount to one or a few tens of kOhm. It is well-known that the resistance of the PTC element, after changing from low resistance to high resistance state, does not return to the initial resistance. In more serious cases when it is subjected to very great electrical stresses, such as short-circuit currents, blistering and crack formation occur in central or other parts of the polymer composition of the PTC element, so that the PTC element can no longer function, that is, the element becomes destroyed. In the known PTC elements, the aim has been to secure the electrodes as efficiently as possible to the body of polymer composition to achieve the best possible electrical contact and hence minimize the contact resistance. To achieve the good securing of the electrodes, they are normally formed with an uneven surface structure on the side facing the body of polymer composition, so that the polymer composition during the manufacturing of the PTC element is able to melt and penetrate into cavities in the electrode surface. Usually, the electrodes consist of metal foils and are applied by being pressed onto the body under heating. A PTC element of the afore-mentioned kind is known from the EP-A-O 240 447. This document is specifically concerned with avoiding the application of tension or shear forces on the polymer body because such forces have a much more distorting effect on the characteristic of the PTC element than compression forces. For this reason the outer terminals of the PTC element, which may introduce forces into the PTC element, are not connected to the thin metal layers attached to the polymer material, but to stiff plates which contact the outer sides of said metal layers. These stiff plates are firmly pressed against the polymer body with its metal layers by pressure-generating means which at the same time forms a mechanical bridge between said plates to shunt possible shear or tension forces past the polymer body. From the EP-A-O 026 456 a PTC heating element is known consisting of a PTC disk with an electrode on either large side. These electrodes do not consist, as usually, of metal foils firmly fixed to the PTC disk by metal coating, but of loose thin plates of ductile material, which plates are permanently press with adequate pressure against the large surfaces of the PTC heating disk. The idea of this design is to facilitate mass production by avoiding the application of a metal foil layers to the PTC heating disk. To achieve the desired properties of the PTC heating element two conditions have to be observed: Firstly, the electrodes must consist of a soft material, such as unalloyed aluminium or lead, and secondly, during manufacture a relatively high pressure must be applied for a short period which pressure is about seven times as high as the permanent pressure which has to be upheld after manufacture. By this manufacturing process it is made use of a hysteresis effect of the material. The high pressure during manufacture is meant to squeeze the soft electrode material into irregularities on the surface of the PTC heating disk. The invention aims at developing a PTC element of the above-mentioned kind, which does not loose its favourable properties after it has changed from the low resistance state to high resistance state under operation. To achieve this aim the invention suggests a PTC element according to the introductory part of claim 1, which is characterized by the features of the characterizing part of claim 1. Further developments of the invention are characterized by the features of the additional claims. According to the invention it has proved to be possible to counteract or completely eliminate the above-described unfavourable processes during operation of the PTC elements and to produce a PTC element which, after transition from a low resistance to a high resistance state, returns to the initial resistance, and which is reusable also after having been subjected to short-circuit currents. Further, the PTC element changes its resistance in case of a lower energy development, that is, its current limiting properties are improved. According to the invention, the above result is obtained by producing at least one electrical contact between an electrode and a body of electrically conductive polymer composition or between two bodies of electrically conductive polymer composition in the PTC element by the electrode making free contact with the mentioned body or that the mentioned bodies make free contact with each other while maintaining a pressure directed perpendicular to the contact surface in question by means of a pressure device. The expression "to make free contact" here and in the following means that the respective elements making free contact are not fixed to each other but make contact with each other only by abutment. A feasible explanation of the result obtained according to the invention may be the following. During normal passage of current, a low contact resistance is maintained between the elements which make free contact with each other due to the pressure exerted on the contact surface. In case of short-circuit currents, electrodynamic repulsion forces occur between the elements making free contact with each other, which leads to a separation of the elements and hence a reduction of the number of contact points between electrode and conducting particles in the body of the polymer composition making free contact therewith or between conducting particles in bodies of the polymer composition making free contact with each other. This leads to a current concentration at the remaining contact points, which causes molten phases to occur in the polymer composition at the contact surface and the PTC element to trip at the contact surface without the rest of the polymer composition being subjected to any stress with ensuing unfavourable effects. Since the pressure against the contact surface remains, when the surface is still hot after the short circuit, the original contact and the original contact resistance may be reestablished between the elements making free contact with one another. The polymer composition may be of a known kind and its composition or constituents constitute no part of the present invention. Thus, the polymer material may consist of thermoplastic resins, *elstomers, thermosetting resins or mixtures thereof, used in prior art polymer compositions with PTC behaviour. As examples of suitable polymer materials may be particularly mentioned polyolefins such as polyethylene, crosslinked polyethylene, polypropylene, polybutene and copolymers or ethylene and propylene. The polymer material preferably has a crystallinity of at least 5%. The conducting powdered material preferably consists of conducting carbon black or conducting soot. However, it is possible, per se, to use, together with or instead of the mentioned powdered material, conducting particles of another kind, such as particles of metallic materials, for example nickel, cobalt, copper and silver. A suitable particle size of the powdered material is 0.01 to 10 µm and a suitable content of the powdered filler is 10 to 60 per cent of the total volume of the powdered filler and the polymer material. The *restivity of the polymer composition is preferably within the range of 10 mΩcm to 100 Ωcm and has the ability, after a transition, to exhibit a resistivity of 1 Ωcm to 1 kΩcm. If more than one body of electrically conductive polymer composition is included in the PTC element, the bodies may be of the same or different polymer composition and then with the same or different resistivity. In those embodiments of the present invention in which at least one of the electrodes is firmly secured to a body of conducting polymer composition, such an electrode or such electrodes may be of a conventional kind. They may consist of metal foils or of thin metal plates or metal nettings which are rolled on the polymer composition in heated state or fixed in some other way thereto, such as by spraying or vapour deposition. Usually, the surface facing the polymer composition has an irregular structure. The electrodes may also consist of a combination of two or more elements, for example a thin foil of nickel and a thicker plate of copper fixed thereto. According to one embodiment of the present invention, such fixed electrodes may consist of a tight plate or foil or metal, which on the side facing the polymer composition is coated with a layer of metal with an irregular surface structure, applied by thermal spraying such as plasma spraying, flame spraying or arc spraying, which metal layer comprises protruding portions with a height of 1 to 50 µm and a width of 1 to 50 µm. In those embodiments of the present invention where at least one of the electrodes makes free contact with a body of conducting polymer composition, such an electrode may in itself, that is, apart from the fact that it is not fixed to the body of conducting polymer composition, be of the same kind as indicated in the preceding paragraph. It may also consist of a more or less thick plate with a surface with an even structure facing the body of conducting polymer composition. It is also possible, per se, to use other high conductivity materials than metallic materials in the electrodes, such as polymer material containing very high contents of electrically conductive powdered fillers, such as conducting carbon black, conducting soot, copper or nickel. The pressure which is maintained on the electrodes perpendicularly to the parallel surfaces on the body, or bodies, of polymer composition preferably amounts to at least 0.1 MPa. Especially preferred is a pressure of 0.1 MPa to 10 MPa. The pressure on the electrodes of the PTC element may be achieved purely mechanically or by utilizing forces generated by electric currents. When utilizing purely mechanical forces, the pressure device may, for example, consist of two plates arranged in parallel with the parallel surfaces of the body, or bodies, of electrically conductive polymer composition and arranged outside the electrodes, and of drawing devices or clamping devices arranged in or adjacent the plates. When utilizing forces generated by electric currents, the pressure device may comprise plates of high conductivity material making contact with the electrodes, in which the current paths are adapted to be substantially parallel to the parallel surfaces of the body, or bodies, and directed in substantially the same direction as the plates, or yokes of a ferromagnetic material arranged outside the plates of high conductivity material. According to a particularly preferred embodiment of the invention, the pressure device is provided with pressure-exerting devices with the ability to be resilient. Such a design of the pressure device significantly facilitates a separation of the electrode and the body of polymer composition making free contact with each other, or of bodies of polymer composition making free contact with each other, at short-circuit currents. In a preferred embodiment the pressure device comprises two pressure-exerting parts, making contact with the electrodes of the PTC element by abutment, and a layer of rubber or another elastic material, for example an elastic plastic, is arranged between one of the electrodes and one of the pressure-exerting parts. According to another preferred embodiment the pressure device comprises parts exerting pressure against the electrodes of the PTC element, and at least one resilient element arranged between the pressure-exerting parts outside an electrode. According to an additional embodiment of the invention, a PTC element according to the invention is stacked on top of another PTC element according to the invention in the same pressure device, which is thereby adapted to exert a pressure directed perpendicularly to the parallel surfaces of the body, or the bodies, of polymer composition in each one of the PTC elements, and the outwardly-facing electrodes are electrically parallel-connected as are the inwardly-facing ones, which may possibly be designed to form one single electrode. Preferably, the device is formed with separate inwardly-facing electrodes and with a layer of rubber or other elastic material, such as elastic plastics, arranged between the electrodes. These embodiments with parallel-connected outwardly-facing and inwardly-facing electrodes have very good current-limiting properties also in case of a total area of the PTC elements which is relatively large. According to yet another embodiment of the invention, the body of polymer composition, if only one such body is included in the PTC element, or the bodies of polymer composition, if more than one such body is included in the PTC element, at surfaces exposed between the electrodes is/are surrounded by a casing of insulating material, which may advantageously be of the same kind and thus have the same physical properties as the polymer material which is included in the polymer composition. The casing of insulating material increases the dielectric strength of the PTC element. The reason is that the casing prevents flashover between particles of the conducting powdered material, which may be exposed at surfaces of the polymer composition, which are exposed between the electrodes, when the PTC element is subjected to short-circuit currents. According to a particularly preferred embodiment of the invention, the pressure device comprises a pressure-absorbing frame arranged around the electrodes and the body, or bodies, of electrically conducting polymer composition, with wedge-shaped elements insertable into the frame. The frame may have the shape of an apparatus housing provided with a lid and at least one of the wedge-shaped elements may be arranged in the apparatus housing itself and at least one of the wedge-shaped elements may be secured to the lid. Another embodiment of the invention is characterized in that the parallel surfaces on the body, or bodies, of electrically conductive polymer composition consist of concentric surfaces. The electrodes are thereby concentric with the mentioned concentric surfaces of the body, or bodies, of polymer composition. This embodiment requires an extremely small space. A PTC element of the kind mentioned in the preceding paragraph may be designed such that, inside an electrode which is arranged at an inwardly-facing concentric surface on the body, or on that body of polymer composition which is arranged furthest towards the interior, there is arranged a body which is expansible in a direction perpendicular to the concentric surface and that, outside an electrode which is arranged at an outwardly-facing concentric surface of the body, or on that body of polymer composition which is located furthest towards the exterior, there is arranged a counter-support or this electrode is itself designed to constitute a counter-support. By way of example, the invention will now be described in greater detail with reference to the accompanying drawings showing in Figure 1 a curve of the resistivity as a function of the temperature for an electrically conductive polymer composition included in a PTC element according to the invention; Figures 2 and 3 PTC elements consisting of two electrodes and respectively one or two bodies of an electrically conductive polymer composition arranged between the electrodes; Figures 4 and 5 cross sections of two different embodiments of a PTC element according to the invention with a pressure device acting purely mechanically; Figures 6 and 7 two cross sections, perpendicular to each other, of a PTC element according to the invention with a pressure device in which electric currents are utilized to generate compressive forces; Figure 8a device comprising two PTC elements with parallel-connected electrodes; Figure 9a PTC element which is provided with an insulating casing at surfaces exposed between the electrodes; Figures 10a, 10b and 10c perspective views of different parts of a device according to the invention in disassembled state; Figure 11a perspective view of a part of the same device in a vertical section in assembled state; Figures 12 and 13 PTC elements according to the invention consisting of two concentric electrodes and respectively one and two tubular concentric bodies of electrically conductive polymer composition arranged between the electrodes, and with a pressure device comprising an expansible body arranged inside the inner electrode as well as a counter-support arranged outside the outer electrode; Figure 14 a modification of the PTC element according to Figure 12; Figure 15 a PTC element according to the invention with a pressure device comprising a drawing or clamping device arranged outside the outer electrode; Figure 16 a modification of the PTC element according to Figure 15; Figure 17 a circuit diagram of a device in which a PTC element according to the invention is used as overcurrent protective device. Figure 1 shows the resistivity R as a function of the temperature T for an electrically conductive polymer composition which is included in the PTC element according to the present invention. T t is the transition temperature, at which the polymer composition changes from a low resistance to a high resistance state. The PTC element according to Figure 2 comprises a centrally arranged body 10 of an electrically conductive polymer composition with a positive temperature coefficient, for example consisting of 67 per cent by volume polyethylene (e.g. LUPOLEN 6031 M from BASF, Germany) and 33 per cent by volume carbon black (e.g. N 550 from Degussa AG, Germany), in the form of a rectangular 1 mm thick plate as well as two electrodes 11 and 12 with associated terminals 13 and 14, respectively, arranged at the parallel large end surfaces 10' and 10'' of the body (the flat sides of the plate). The electrode 11 consists of an 0.5 mm thick plate of nickel with an even surface structure on both sides. On the outside, the plate is coated with a thin layer of copper. The plate makes free contact with the body, that is, makes contact only by abutting the body, and thus not being fixed to the body. The electrode 12 consists of an 0.3 mm thick copper foil which on the side facing the body 10 is coated with an 0.1 mm thick layer of copper with uneven surface structure, applied by plasma spraying. The electrode 12 is fixed to the polymer by being pressed against the body 10, when the body has been heated, so that the polymer material in liquid state may penetrate into irregularities on the inwardly-facing side of the electrode. The device according to Figure 3 includes two bodies, 10a and 10b, of the same electrically conductive polymer composition as that in the body 10 in Figure 1, in the form of 1 mm thick rectangular plates with the parallel surfaces 10a', 10a'' and 10b', 10b'', respectively. These plates make electrical contact with each other only by abutment without being fused or otherwise fixed to each other, that is, they make free contact with each other. The electrodes 11 and 12 may be of the same kind as the electrodes 11 and 12 in Figure 1. However, the electrode 11 may also be of the same kind as the electrode 12 and, like this electrode, be fixed to the body 10a. In the device according to Figure 3, the bodies 10a and 10b may be of different electrically conductive polymer compositions and have different resistivities to modify the properties of the PTC element. The device may also be modified so as to comprise more than two bodies (10a and 10b) of conductive polymer composition with the same or a different resistivity. If the electrodes are fixed to adjacent bodies of electrically conductive polymer composition, at least one of the bodies must make free contact with another one of the bodies. If one or both of the electrodes make free contact with an adjacent body of electrically conductive polymer composition, all the polymer bodies may be fixed to each other, for example by fusion. Figure 4 illustrates a device according to the invention in which a PTC element according to Figure 2 is arranged in a pressure device comprising two plates 15, 16, which are parallel to the large parallel surfaces of the body 10 (the flat sides of the plate 10) and to the electrodes 11 and 12. The terminals 13 and 14 are not shown in the figure. The plates are of electrically insulating material, for example resin reinforced with glass fibre. The pressure against the electrodes and against the large surfaces of the body 10 is brought about by tightening a number of bolts 17. Between the bolt heads 17a and one of the plates 15,16 of the pressure device, stiff springs 17b are arranged which are not completely compressed when the PTC element is pressurized and prepared for normal operation. In case of a short circuit, the springs may therefore be further compressed, which permits the electrode 11 to be separated from the body 10. The device according to Figure 5 differs from the device according to Figure 4 in that the springs 17b are not included and in that a 1 mm thick mat 30 of rubber is arranged between the electrode 11 and the plate 15. Such a mat of rubber may possibly be arranged between the electrode 12 and the plate 16 as well. Instead of the PTC element according to Figure 2, a PTC element according to Figure 3 may be arranged, in an analogous manner, in the devices according to Figures 4 and 5. In the device illustrated in Figures 6 and 7, a PTC element according to Figure 2 or 3 is provided with plates 32, 33, arranged outside the electrodes 11,12, of copper or other high conductivity material between two opposite yokes 18 and 19, respectively, of iron or other ferromagnetic material. The iron yokes are U-shaped with a base 18a and 19a, respectively, of flat shape and with short legs 18b and 19b, respectively. Alternatively, only plates corresponding to the base 18a and 19a, respectively, may be used. By connecting the PTC element to the network in such a way that the current paths 20 and 21 become parallel to the end surfaces of the body 10 and directed in the same direction, as shown by dashed lines and arrows in Figure 6, an attractive force is achieved between the yokes, which is illustrated by dashed arrows in Figure 7. In this way, the body 10 is exposed to pressure between the end surfaces when current is conducted through the PTC element. In the device according to Figure 8, two PTC element of the kind shown in Figure 2 are arranged one above the other with an intermediate mat 30a of rubber. The two electrodes 11 are parallel-connected as are the two electrodes 12. The PTC element can be subjected to pressure in a manner analogous to that of the PTC elements according to Figure 2 in the devices according to Figures 4 and 5 with the two electrodes 12 making contact with the plates 15 and 16. When arranging a rubber mat 30a between the two electrodes 11 in accordance with Figure 8, no advantage is gained by utilizing springs 17b according to Figure 4 or a rubber mat 30 according to Figure 5. If the PTC element according to Figure 8 is formed without a rubber mat 30a, that is, with the two electrodes 11 making contact with each other or replaced by one single electrode which on both sides makes contact with bodies 10 of electrically conductive polymer composition and this PTC element is arranged in the pressure devices according to Figures 4 and 5, it is an advantage to retain the springs 17b and the rubber mat 30. Instead of the PTC element according to Figure 2, a PTC element according to Figure 3 may be used in the cases describes with reference to Figure 8. As explained before, a PTC element according to Figure 8 has particularly good current limiting properties. The PTC element according to Figure 9 is of the same kind as that shown in Figure 2 but is provided with a casing 35 of insulating material, preferably of polyethylene, which is the same polymer material as included in the body 10 of electrically conductive polymer composition. The casing 35 surrounds the surfaces 10''' on the body 10 which are exposed between the electrodes 11 and 12, which, as explained above, increases the dielectric strength of the PTC element. The PTC element according to Figure 9 may be arranged in the same way as the PTC element according to Figure 2 in the pressure devices according to Figures 4 and 5. Instead of a PTC element according to Figure 2, a PTC element according to Figure 3 may be used, in an analogous manner, in a PTC element according to Figure 9, the casing surrounding the surfaces 10a''' and 10b''' on the bodies 10a and 10b. The casing 35 of insulating material may be applied around the body 10 or the bodies 10a and 10b by applying a ring of insulating material around the body 10 or the bodies 10a and 10b in a tool suitable therefore and be brought to fuse together, by heating, with the edges on the body 10 or the bodies 10a and 10b. The device according to Figures 10a to 10c and 11 comprises three identical PTC elements, one of which being shown with the parts separated in Figure 10b. The body 10 of electrically conductive polymer composition, for example of the same kind as in Figure 2, is arranged in a plastic frame 40 which extends around the body. The two electrodes 11 and 12 consist of 1 mm thick silver-plated plates of copper, which make free contact with the body 10, that is, only by abutting it, and thus not being fixed to it. Each electrode is provided with alternative conductor terminals 13a and 14a for cable connection and 13b (not shown) and 14b for connection to a bar, respectively. Outside the electrodes 11 there is arranged a wedge-shaped, loose plate 41 and outside the electrode 12 there is arranged a loose spring plate 42 of thin sheet metal. The spring plate is provided with a plurality of tongues 42a cut out and bent out from the plate, which give the plate its resilient properties. A package comprising the plates 41 and 42 and electrodes 11 and 12 arranged therebetween and with an intermediate body 10 of conducting polymer composition is arranged in each one of three compartments 43a, 43b, and 43c in an apparatus housing 43 with two partitions 43d and 43e forming the compartments. The partitions are parallel to two opposite, parallel walls 43f and 43g in the apparatus housing. The apparatus housing is provided with hollows 44a and 44b for alternative connection of a conductor to the attachment 14a or 14b in the compartment 43a, with hollows 45a and 45b for alternative connection of a conductor to the attachment 14a or 14b in the compartment 43b, and with hollows 46a and 46b for alternative connection of a conductor to the attachments 14a or 14b on an electrode 12 in the compartment 43c. On the opposite, not visible wall of the apparatus housing, there are corresponding recesses for connection of conductors to the attachments 13a and 13b on each electrode 11. The pressure between the electrode 11 and 12 and the body 10 in each one of the packages in the compartments 43a, 43b and 43c is brought about by applying a lid 47 with wedge-shaped plates 48, 49 and 50, fixed to the lid, on the apparatus housing. A wedge-shaped plate is thereby inserted into each one of the compartments 43a, 43b and 43c adjacent a plate 41 located therein. The lid is provided with mechanical attachments 47a, 47b, 47c and 47d for mounting. As will be clear from Figure 11, the apparatus housing is provided with plane-parallel reinforcing walls 51 and 52 on the pressure-absorbing sides. The device according to Figures 10a, 10b, 10c and 11 is intended to be connected into a three-phase cable with a PTC element connected into each of the three phase conductors. The apparatus housing 43 with outer walls 43f and 43g, reinforcing walls 51 and 52, lid 47 and wedge-shaped plates 41, 48, 49 and 50 are all manufactured form an electrical insulating material, for example a polyamide, to which a filler, for example in the form of short glass-fibres, has been added. The PTC element according to Figure 12 comprises a tubular body 10 of an electrically conductive polymer composition with a positive temperature coefficient, for example of the same composition as that used in the body 10 in Figure 2. The interior cylindrical envelope surface is designated 10' and the exterior also cylindrical envelope surface is designated 10''. In a cross section perpendicular to the centre line of the tubular body, each one of the envelope surfaces forms a circle. The wall thickness of the tubular body amounts to at least 1 mm. The PTC element also has two electrodes 11 and 12 which are arranged in contact with the envelope surfaces 10' and 10'' and concentric therewith. The electrodes are provided with terminals 13 and 14, respectively. The electrode 11 consists of an 0.5 mm thick tube of nickel with an even surface structure on both sides. On the outside the tube is coated with a thin layer of copper. The tube is slitted in the longitudinal direction so that one edge at the slit is able to slide over the other. The electrode 11 makes free contact with the body 10, that is, makes contact only by abutting the body along the envelope surface 10' under pressure, without being fixed to the body. The electrode 12 consists of an 0.3 mm thick copper foil which on the side facing the body 10 is coated with an 0.1 mm thick layer of copper with an uneven surface structure, applied by plasma spraying. The electrode 12 is fixed to the polymer by being pressed against the body 10, after the body has been heated so that the polymer material in liquid state may penetrate into irregularities in the inwardly-facing side of the electrode. Inside the electrode 11 there is arranged a body 31, which is expansible in a direction perpendicular to the envelope surface 10' and the electrode 11. Outside the electrode 12 there is a counter-support 32, for example in the form of a tube of resin reinforced with fibre-glass, which is manufactured in situ, for example by applying the glass-fibre material and the resin in uncured state onto the electrode 12, and thereafter curing the resin. The expansible body 31 consists in the exemplified case of a tubular body 31a of rubber or other elastic material, which by means of a bolt 31b, arranged centrally in the body, with a bolt head 31c and a nut 31d and with washers 31e and 31f arranged at the end surfaces, inside the bolt head and the nut, may be brought to expand while achieving a desired pressure against the electrode 11. Instead of providing the PTC element with a separate counter-support, the electrode 12 may be formed as a tube with sufficient thickness to serve itself as a counter-support, such as in the form of a tube of copper which has been drawn down, on the spot, to the intended dimension to make contact directly with the envelope surface 10'' on the body 10. In this case both electrodes of the PTC element are adapted to make free contact, under pressure, with the envelope surfaces 10' and 10'' on the body 10. Such a copper tube may alternatively be adapted to serve only as a counter-support and then to make contact with the electrode 12. Also when using a separate counter-support, the electrode 12 may, instead of being fixed to the body 10, be adapted to make free contact with the body 10. In a manner analogous to that of a copper tube, a sleeve of memory metal may be used as a separate counter-support or both as a counter-support and an electrode, the sleeve in expanded state being arranged on the spot around the PTC element and thereafter brought to resume its smaller dimension before the expansion. If the expansible body 31 consists of a body of metallic material, it may be suitable or necessary to arrange a layer of rubber or other electrically insulating material between the expansible body and the electrode 11. The device according to Figure 13 includes two bodies 10a and 10b, of the same electrically conductive polymer composition as that in the body 10 in Figure 12, in the form of concentric tubes with a wall thickness of 1 mm. The concentric envelope surfaces are designated 10a', 10a'' and 10b', 10b'', respectively. The two bodies 10a and 10b make free contact with each other, that is, make contact only by abutment under pressure, and thus not by being fused or otherwise fixed to each other. The electrodes 11 and 12 may be of the same kind as the electrodes 11 and 12 in Figure 12. However, the electrode 11 may also be of the same kind as the electrode 12 in the first exemplified case, when this is fixed to the body of polymer composition, and thus be fixed to the body 10a. In the device according to Figure 13, the bodies 10a and 10b may be of different electrically conductive polymer composition and have different resistivity to modify the properties of the PTC element. The device can also be modified so as to contain more than two concentric bodies (10a and 10b) of conductive polymer composition with the same or with different resistivity. If the electrodes are fixed to adjacent bodies of electrically conductive polymer composition, at least one of the bodies must make free contact with another one of the bodies. If one or both of the electrodes make free contact with an adjacent body of electrically conductive polymer composition, all polymer bodies may be fixed to one another, for example by fusion. The device 31 with its parts for maintaining the electrodes 11 and 12 and the bodies 10a and 10b pressed against each other in the PTC element according to Figure 13 is in the exemplified case the same as that in the PTC element according to Figure 12. The PTC element according to Figure 14 differs from the PTC element according to Figure 12 in that a 1 mm thick layer 33 of rubber or other elastic material is arranged between the electrode 12 and the counter-support 32 to increase the elasticity of the pressure-exerting parts. As in the case according to Figure 12, instead of being fixed to the body 10, the electrode 12 may make free contact with the body, that is, only by abutting the body along the envelope surface 10''. In the device according to Figure 15, the electrode consists of a solid wire of copper or nickel with a thin coating of nickel. The electrode 11 makes free contact with the body 10, that is, only by abutting the body along the envelope surface 10'. The body 10 and the electrode 12 are of the same kind as the corresponding elements in the device according to Figure 12. The electrodes 11 and 12 and the body 10 are maintained pressed against each other by a drawing or clamping device 34, which, for example, may consist of a plate 34a arranged around the electrode 12 with an intermediate elastic insulating layer (not shown), tha plate being provided with overlapping edges in the axial direction of the body 10, so that the diameter of the tubular body formed by the plate can be reduced by tightening a screw device 34b, in a manner analogous to that of a hose clamp, or by compressing two jaws with an internally semicylindrical shape in a direction perpendicular to the envelope surfaces 10'and 10'' of the body 10. In the illustrated case, the electrode 11 itself serves as a counter-support. It is, of course, possible, instead of using an electrode 11 in the form of a solid wire, in accordance with Figure 15, to use an electrode 11 in the form of a layer of the kind illustrated in Figures 12 to 14 and a counter-support arranged inside such an electrode in the form of a rigid body or an elastic device, for example of the kind designated 31 in Figure 12 to 14. The PTC element according to Figure 16 differs from the PTC element according to Figure 15 in that a 1 mm thick layer 33 of rubber or other elastic material is arranged between the electrode 12 and the plate 34a to increase the capacity of the pressure-exerting parts to be elastic. Instead of the drawing of clamping device 34, it is possible to use a sleeve of memory metal, the sleeve of memory metal in expanded state being arranged in place around the PTC element and thereafter being brought to assume a smaller diameter. Such a sleeve may at the same time serve as an electrode in the case illustrated in Figure 15. Figure 17 exemplifies the use of the PTC element according to the invention as overcurrent and short-circuit protective device in an electric circuit 26 comprising a motor 25. The PTC element 22, for example of the kind shown in Figure 4, 5, 12 or 13, is series-connected to a contact device 23. In parallel with the PTC element there is an excitation coil 24, which is included in a fast magnetic tripping device for the contact device. The tripping device is adapted to influence the contact device for opening this at an overcurrent.

Claims: PTC element comprising at least one body (10, 10a, 10b) of an electrically conductive polymer composition with a positive temperature coefficient, each said body being provided with two large preferably parallel surfaces (10', 10", 10a', 10a", 10b', 10b"), whereby at least two surfaces (10', 10") of one single body (10) or at least two surfaces (10a', 10a", 10b', l0b") belonging to different bodies (10a, 10b) are arranged in electrical contact with electrodes (11, 12) included in the PTC element for carrying current through the body/bodies of polymer composition, whereby in the case of more than one of said electrically conductive bodies comprised in the PTC element and a greater number of body surfaces than electrodes the bodies contact each other via their surfaces, and whereby in the case of more than one of said electrically conductive bodies these bodies consist of the same or of different polymer compositions with the same or different positive temperature coefficient, characterized in that in the case that a body (10) of polymer composition contacts with each of its large surfaces an electrode, at least one of said surfaces (10') is adapted to make free contact with its associated electrode (11), in the case that there are two or more bodies (10a, 10b) of polymer composition which contact each other via their large surfaces (10a', 10a", 10b', 10b"), at least one pair of these contacting surfaces is adapted to make free contact with each other and/or at least one of the surfaces which contact an electrode is adapted to make free contact with its associated electrode (11), and that a pressure device is adapted to exert on the electrodes a pressure directed perpendicular to the large surfaces on the body, or the bodies, of polymer compooition. PTC element according to claim 1, characterized in that the pressure on the electrodes (11, 12) amounts to at least 0.1 MPa. PTC element according to claim 1 or 2, characterized in that the pressure on the electrodes (11, 12) amounts to 0.1 MPa - 10 MPa. PTC element according to any of claims 1 to 3, characterized in that the pressure device is provided with pressure-exerting devices (15, 16, 17, 17a, 17b) with the ability to be elastic. PTC element according to any of the preceding claims, characterized in that the pressure device comprises two parts (15, 16) exerting pressure on the electrodes (11, 12) of the PTC element and that a layer of rubber (30) or another elastic material is arranged between one of the electrodes and one of the pressure-exerting parts. PTC element according to any of claims 1 to 4, characterized in that the pressure device comprises parts (43, 41, 48, 49, 50) exerting pressure on the electrodes (11, 12) of the PTC element and that at least one resilient layer-formed element (42) is arranged between the pressure-exerting parts. PTC element according to any of claims 1 to 4, characterized in that it is stacked on another PTC element of the kind described in any of claims 1 to 4 in the same pressure device, which is adapted to exert a pressure directed perpendicular to the parallel Surfaces (10', 10'', 10a', 10a'', 10b', 10b'') on the body (10), or bodies (10a, 10b), of polymer composition in each of the PTC elements, and that the outwardly-facing electrodes (12) are electrically parallel-connected as are the inwardly-facing ones (11), which latter ones may be designed to form one single electrode. PTC element according to claim 7, characterized in that a layer (30a) of rubber or other elastic material or a resilient layer-formed element is arranged between the inwardly-facing electrodes (11). PTC element according to any of the preceding claims, characterized in that at least one of the electrodes (11, 12) consists of a tight plate or foil of metal, which on the side facing the body of polymer composition is coated with a layer of metal, applied by thermal spraying, with an uneven surface structure comprising projecting portions with a height of 1 to 50 µm and a width of 1 to 50 µm. PTC element according to any of the preceding claims, characterized in that the body (10) or bodies (10a, 10b) of polymer composition at surfaces (10''', 10a''', 10b''') exposed between the electrodes (11, 12) are surrounded by in insulated casing (35). PTC element according to any of the preceding claims, characterized in that the pressure device comprises two plates (15, 16) arranged outside the electrodes (11, 12) and being parallel to the parallel surfaces (10', 10'', 10a', 10a'', 10b', 10b'') of the body (10), or bodies (10a, 10b), of electrically conductive polymer composition, as well as drawing devices (17) or clamping devices arranged in or at the plates. PTC element according to any of claims 1 to 10, characterized in that the pressure device comprises plates (32, 33) of a high conductivity material arranged outside the electrodes (11, 12), in which plates the current paths (20, 21) are substantially parallel to the parallel surfaces (10', 10'', 10a', 10a'', 10b', 10b'') on the body (10), or bodies (10a, 10b), and substantially directed in the same direction, as well as plates or yokes (18, 19) of a ferromagnetic material arranged outside said plates (32, 33). PTC element according to any of claims 1 to 10, characterized in that the pressure device comprises a frame (43), arranged around the electrodes (11, 12) and the body (10), or bodies (10a, 10b), of electrically conductive polymer composition, as well as wedge-shaped elements (41, 48, 59, 50) insertable into the frame. PTC element according to claim 13, characterized in that the frame has the shape of an apparatus housing (43) provided with a lid (47) and that at least one of the wedge-shaped elements (41) is arranged in the apparatus housing itself and at least one of the wedge-shaped elements (48, 49, 50) secured to the lid. PTC element according to any of claims 1 to 6 or 9, characterized in that the parallel surfaces (10', 10'') on the body (10) of electrically conductive polymer composition, or the parallel surfaces ( 10a', 10a'', 10b', 10b'') on the bodies (10a, 10b) of electrically conductive polymer composition, consists of concentric surfaces. PTC element according to claim 15, characterized in that inside an electrode (11) which is arranged at an inwardly-facing concentric surface (10', 10a') on the body (10), or on that body (10a) of polymer composition which is located furthest towards the interior, there is arranged a body (31) which is expansible in a direction perpendicular to the concentric surface, and that outside an electrode (12) which is arranged at an outwardly-facing concentric surface (10", 10b') on the body (10), or on that body (10b) of polymer composition which is located furthest towards the exterior, there is arranged a counter-support (32) or this electrode is itself designed to constitute a counter-support. PTC element according to claim 16, characterized in that the expansible body (31) is elastic. PTC element according to claim 16, characterized in that between the electrode (11), arranged at the inwardly-facing concentric surface (10', 10a'), and the expansible body there is arranged a layer of rubber or other elastic material or a resilient layer-formed element. PTC element according to any of claims 16 to 18, characterized in that between the electrode (12), arranged at the outwardly-facing concentric surface (10", 10b'), and the counter-support (32) there is arranged a layer of rubber (33) or other elastic material or a resilient layer-formed element. PTC element according to claim 15, characterized in that outside an electrode (12), which is arranged at an outwardly-facing concentric surface (10", 10b') on the body (10), or on that body (lOb) of polymer composition which is located furthest towards the exterior, there is arranged a drawing or clamping device (34) and that inan electrode (11), which is arranged at an inwardly-facing concentric surface (10'', 10a') on the body (10), or on that body (10a) of polymer composition which is located furthest towards the interior, there is arranged a counter-support or the electrode (35) is itself designed to constitute a counter-support. PTC element according to claim 20, characterized in that between the electrode (12), arranged at the outwardly-facing concentric surface (10'', 10b'), and the drawing or clamping device (34) there is arranged a layer of rubber (33) or other elastic material or a resilient layer-formed element. PTC element according to claim 20 or 21, characterized in that the counter-support consists of an elastic body. PTC element according to claim 20 or 21, characterized in that between the electrode (11), arranged at the inwardly-facing concentric surface (10'', 10a'), and the counter-support there is arranged a layer of rubber or elastic material or a layer-formed resilient element.

Inventors: HANSSON TOMAS, KARLSTROEM PER-OLOF, HANSSON, TOMAS, KARLSTROEM, PER-OLOF, Karlström, Per-Olof
Applicants: ASEA BROWN BOVERI, ASEA BROWN BOVERI AB
Application year: 1991
Main classification: H01C1
IPC codes: H01C7, H01C1, H02H3