TEMPERATURE LIMITING DEVICE Technical field

[01] The present invention relates to a temperature limiting device for protecting electrical apparatuses and the like, as well as a method for manufacturing said device.

Prior art

[02] It is currently known, in the technical field concerning the manufacture of temperature limiters suitable for ensuring the thermal protection of electrical devices or components of apparatuses such as electric motors and the like, the use of anti-surge temperature limiters based on the use of a bimetallic disc adapted to switch the configuration of a spring-loaded disc that correspondingly closes or opens an electrical circuit of the device to be protected. Such temperature limiting devices, in the specified sector, are also referred to as bimetallic thermal switches.

[03] In particular, the bimetallic disc is capable of moving from a low temperature position to a high temperature position in which it acts on the spring-loaded disc to open the electrical circuit and thereby interrupt heating.

[04] A temperature limiting device of this type is shown for example in European patent EP 0 678891.

[05] Further examples of temperature limiters of the aforementioned type are shown in documents US 4,470,033, US 5,745,022 and DE 102007063650.

[06] This temperature limiter comprises an external container defining a cup-shaped housing, closed by a lid, inside which a bimetallic disc and a spring-loaded disc are arranged in a stack. The cover carries the fixed contact of the changeover switch, while the spring- loaded disc carries the movable contact. In the normal operating condition, i.e. at low temperature, the movable contact is pressed by the spring-loaded disc against the fixed contact and thus closes the electrical circuit, allowing the current to flow. On the other hand, when the temperature exceeds a predetermined threshold value, the bimetallic disc reverses its position, consequently moving the disc with spring release. In this way the movable contact is detached from the fixed contact and the electrical circuit opens, interrupting the flow of current and heating.

[07] In operation, the bimetallic thermal switch is usually inserted inside the stator winding of the electric motor, during the motor manufacturing step, so that the thermal switch is in direct thermal coupling with the winding.

[08] Basically, bimetallic thermal switches work thanks to the behaviour of a foil, cut into the form of a "snap-disc", composed of two metals that, as the temperature increases, expand in a different way, producing a tendency to flex. The particular configuration of the bimetallic disc limits the possible positions of use, as the "snap-disc" admits only two positions of equilibrium. For this reason, the bimetallic foil cut in the form of a "snap-disc" remains in the initial equilibrium position until a given temperature is reached, commonly identified as the upper snap temperature or TOS (obere Schnapptemperatur), and then suddenly snaps into the second equilibrium position, passing from the convex to the concave shape.

[09] The bimetallic disc remains in the second equilibrium position until, when it cools, it reaches the reset temperature, commonly called the lower snap temperature or TUS (untere Schnapptemperatur), at which it clicks again in the starting position, passing from the concave shape to the convex one.

[10] In bimetallic thermal switches, the passage of the bimetallic disc from the convex to the concave position is used to separate the contacts and open the circuit when the TOS is reached. Cooling up to the TUS causes the passage from the concave position to the convex one, with consequent closure of the contacts.

[11] The distance between the contacts, once the opening of the thermal switch is reached, is directly connected to the curvature of the bimetallic disc, understood as the distance between the base and the highest point of the dome that constitutes the bimetallic disc itself. If this distance is sufficient to guarantee a high dielectric strength, a complete opening of the contacts is obtained; otherwise, the voltage at the ends of the two contacts generates an electric arc between them, preventing the correct opening of the circuit and seriously damaging the bimetallic thermal switch.

[12] In practice, assuming a fixed model of bimetallic foil and keeping its thickness unchanged, for example equal to 0.14 mm, it is possible to cut and form bimetallic discs with different combinations of TOS and TUS, by means of an appropriate production process. However, it has been found that the curvature of the bimetallic disc increases with increasing TOS, starting from a minimum value for a TOS equal to 60° C up to maximum values for TOS above 180° C.

[13] This is problematic for lower TOS values, for example 60° C or 70° C, because the slight curvature of the bimetallic disc is not sufficient to separate the thermal switch contacts by a sufficient distance to avoid breaking the dielectric rigidity of air and the consequent generation of an electric arc.

[14] To overcome this drawback, the use of different bimetallic foils and different thicknesses is currently provided, in order to obtain sufficient curvatures even for low TOS values. However, this solution involves a considerable limitation, as it requires the management of numerous different codes and the constant modification of the parameters of the machines according to the different foils and different thicknesses.

Disclosure

[15] The aim of the present invention is to solve the aforementioned problems, devising a temperature limiting device capable of ensuring the opening of the electrical contacts even at the lowest values of the upper snap temperature or TOS.

[16] As part of this aim, a further object of the invention is to provide a temperature limiting device that permits to always use the same material and the same thickness for manufacturing the bimetallic disc.

[17] A further object of the invention is to provide a temperature limiting device of simple construction and functional design, having reliable operation, versatile use, as well as relatively inexpensive cost.

[18] The aforementioned objects are achieved, according to the present invention, by the temperature limiting device according to claim 1.

[19] The temperature limiting device comprises an external container defining a cup-shaped housing, closed by a lid, inside which a spring-loaded disc and a bimetallic disc are stacked.

[20] The external container has a lower base from which a containment wall which defines the upper mouth of said external container extends peripherally.

[21] Said spring-loaded disc faces the bottom of said cup-shaped housing defined by said base of the outer container.

[22] Said bimetallic disc is disposed above said spring-loaded disc.

[23] Preferably, said spring-loaded disc has a slightly arched shape.

[24] Preferably, in the normal operating condition, said spring-loaded disc rests peripherally on the bottom of said cup-shaped housing defined by said base of the external container.

[25] Preferably, said bimetallic disc has a more arched shape than said spring-loaded disc.

[26] Preferably, said bimetallic disc abuts peripherally on the edge of said spring-loaded disc.

[27] According to the present invention, the temperature limiting device comprises a metal material separation ring arranged above said bimetallic disc and able to engage a peripheral portion of the bimetallic disc itself.

[28] Said separation ring is tightened between said bimetallic disc and said lid of the external container or a spacer member of annular shape arranged inside said cup-shaped housing.

[29] The insertion of the separation ring inside the temperature limiting device is able to determine a sufficient separation of the contacts in any operative condition, even in the presence of a bimetallic disc with a limited curvature. This separation ring, in the opening position of the contacts, determines an additional thickness which increases the distance between the contacts themselves.

[30] Preferably said outer container is made of conductive material.

[31] Preferably said cover is made of insulating material.

[32] Alternatively, said cover is made of a combination of materials such as to be isolated with respect to said external container. [33] The bimetallic disc is axially associated with said spring-loaded disc.

[34] The temperature limiting device comprises at least one fixed contact integral with said lid of the outer container and protruding for a portion thereof above the same lid and at least one movable contact integral with said spring-loaded disc.

[35] Preferably, the temperature limiting device comprises a protective cover of insulating material which extends to cover at least said lid, said portion of the fixed contact protruding above said lid and a curved edge of said outer container.

[36] Preferably said protective cover is dome-shaped and at least partially embeds the connection cables of the temperature limiting device.

[37] Preferably said curved edge faces inwardly said outer container so as to retain said lid.

[38] Preferably said bimetallic disc is made up of a foil made by coupling iron and copper.

[39] Preferably said bimetallic disc is arch-shaped, so as to abut peripherally on the edge of said spring-loaded disc.

Description of drawings

[40] The details of the invention will become more evident from the detailed description of preferred embodiments of the temperature limiting device, illustrated by way of example in the accompanying drawings, wherein:

Figure 1 is an axial sectional view of the temperature limiting device according to the present invention;

Figure 2 is the same axial sectional view of the temperature limiting device in a different operative configuration.

Description of embodiments of the invention

[41] With particular reference to these figures, the temperature limiting device according to the present invention is indicated as a whole with the reference numeral 1.

[42] The temperature limiting device comprises an external container 2 defining a cup-shaped housing 3. In particular, the external container 2 has a substantially circular lower base 20, from which a containment wall 21 which defines the upper mouth of the container 2 extends peripherally.

[43] The housing 3 of the external container 2 is closed at the top by a lid 4, retained by a peripheral curved edge 5 of the external container 2.

[44] The external container 2 is made of conductive material, for example brass. The lid 4 is made of insulating material, such as for example ceramic material and the like, or alternatively of a combination of materials, including metal, but such as to be insulated with respect to the external container 2.

[45] Inside the housing 3 of the external container 2 a spring-loaded disc 6 and a bimetallic disc 7 are arranged in a stack. The spring-loaded disc 6 faces the bottom of the housing 3 defined by the base 20 of the external container 2; the bimetallic disc 7 is arranged above the spring-loaded disc 6. In the normal operative condition, the spring-loaded disc 6, having a slightly arched shape, rests peripherally on the bottom of the housing 3 defined by the base 20 of the external container 2; the bimetallic disc 7, having a more arched shape, rests peripherally on the edge of the same spring-loaded disc 6.

[46] The spring-loaded disc 6 is preferably made of stainless steel coated with a precious material, for example silver; the bimetallic disc 7 is preferably constituted by a foil made by coupling iron and copper.

[47] The lid 4 is tightened against a spacer member 8 of annular shape, which rests peripherally on an annular shoulder 9 internally shaped by the external container 2. In practice, the lid 4 and the spacer member 8 are packed against the annular shoulder 9 by the curved edge 5 of the outer container 2, suitably riveted. The spacer member 8 is made, for example, of brass or the like.

[48] The cover 4 carries a fixed contact 10 of the thermal switch, axially inserted, in a way known per se; the spring-loaded disc 6 carries in turn axially inserted a movable contact

11 of the thermal switch. The fixed contact 10 is protruding below the cover 4; the movable contact 11 is protruding above the bimetallic disc 7, through an axial hole made in the same bimetallic disc 7.

[49] The contacts 10, 11 are preferably made of precious metal, for example silver, or an alloy thereof, or a composition of a precious metal with a conductive metal, for example copper rod and silver head.

[50] In the normal operative condition, i.e. at low temperature, the movable contact 11 is elastically pushed by the spring-loaded disc 6 against the fixed contact 10 and thus closes the electrical circuit, allowing the passage of current, as specified below (see Fig. 1).

[51] The outer container 2 is coated above the lid 4 with a dome-shaped protective cover 12 of insulating material. The protective dome 12 extends to completely cover the lid 4, the portion of the fixed contact 10 protruding above the lid 4 and the curved edge 5 of the outer container 2, or at least the inner part of said curved edge 5.

[52] The protective dome 12 also embeds the cables 13, 14 connecting respectively to the outer container 2 and to the fixed contact 10. It should be noted that it is not necessary for the cable 13 of connecting to the outer container 2 to be covered by the protection dome

12 since the fundamental function of this coating is the protection of the internal components of the device from the penetration of liquids and not the protection of the welds.

[53] According to the present invention, the temperature limiting device comprises a metal material separation ring 15 arranged above the bimetallic disc 7 and able to engage a peripheral portion of the bimetallic disc 7 itself.

[54] The separation ring 15 is tightened between the bimetallic disc 7 and the lid 4 of the external container 2.

[55] In particular, in the illustrated case, the separation ring 15 is tightened between the bimetallic disc 7 and the spacer member 8 of annular shape. If, on the other hand, this spacer member 8 is not present, the separation ring 15 is tightened directly between the bimetallic disc 7 and the cover 4.

[56] The operation of the temperature limiting device according to the present invention is readily understandable from the foregoing description.

[57] In the normal operating condition, the spring-loaded disc 6 and the bimetallic disc 7 are centered and secured between the base of the housing 3 defined by the outer container 2 of conductive material and the lid 4 carrying the fixed contact 10 integrated. The spring- loaded disc 6 faces the bottom of the housing 3 defined by the base 20 of the external container 2.

[58] The spring-loaded disc 6 constitutes the element through which the electric current passes and supports the movable contact 11 which is kept under constant pressure against the fixed contact 10 (Fig. 1 ).

[59] The bimetallic disc 7, supported by the movable contact 11 that passes therethrough, is able to react to any changes in the temperature of the apparatus to be protected. When a predetermined switching temperature is reached, the bimetallic disc 7 snaps into the reverse position, with the concavity downwardly facing, and pushes the spring-loaded disc 6 downwards (Fig. 2).

[60] In this way, the contact between the fixed and movable contacts 10, 11 opens instantly and therefore interrupts the increase in temperature in the apparatus to be protected. Conversely, when the temperature drops and reaches the preset normal operative value, the bimetallic disc 7 snaps again to return to the initial position and the contact closes.

[61] The insertion of the metal separation ring 15 inside the housing 3 of the thermal switch, in a position between the bimetallic disc 7 and the spacer member 8, or directly between the bimetallic disc 7 and the lid 4, does not affect the correct operation of the device when the thermal switch is closed (Fig. 1), but the separation between contacts 10, 11 increases in the open configuration (Fig. 2).

[62] The temperature limiting device according to the present invention achieves the purpose of having a temperature limiting device capable of ensuring the opening of the electrical contacts even at the lowest values of the upper snap temperature or TOS.

[63] This result is achieved through the inventive idea of equipping the temperature limiting device with an auxiliary separation ring capable of determining sufficient separation of the contacts in any operative condition.

[64] This makes it possible to use a single material and a single thickness for manufacturing the bimetallic disc, regardless of the TOS required, as the addition of the separation ring in the low TOS switch models guarantees sufficient separation of the contacts in any case, even in the presence of a bimetallic disc having a limited curvature.

[65] It should be noted that the separation ring does not interfere with the operation of the switch in the closed position, but is essential in the open position because it determines an additional thickness that increases the distance between the contacts.

[66] Furthermore, the possibility of always using the same material and the same thickness makes both material supply and production machinery settings much easier.

[67] The temperature limiting device described by way of example is susceptible of numerous modifications and variations according to the various requirements.

[68] In the practical embodiment of the invention, the materials used, as well as the shape and the dimensions, may be modified depending on needs.

[69] Should the technical features mentioned in any claim be followed by reference signs, such reference signs were included strictly with the aim of enhancing the understanding of the claims and hence they shall not be deemed restrictive in any manner whatsoever on the scope of each element identified for exemplifying purposes by such reference signs.