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 DE 102013102006 and DE 102004014028.

[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 lid 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 spring-loaded disc. 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] However, it should be noted that the need to ensure mechanical solidity and fastening to the winding often involves the need to impregnate the motor stator using chemical products, such as solvent-based paints, water-based paints, resins and the like. During this manufacturing step, there is therefore a high risk of penetration inside the bimetallic thermal switch of the chemicals used for impregnating the stator, with consequent malfunction of the switch itself. To prevent this risk, bimetallic thermal switches are usually protected by a resin cover. This resin cover, once solidified, covers the parts of the switch potentially at risk of infiltration and ensures safe use, even in the case of impregnation with the aforementioned chemicals.

[09] For this purpose, resins which polymerize under the effect of temperature, for example mono or bi-component epoxy resins, silicone resins and the like, are currently used for finishing bimetallic thermal switches. However, these resins have evident limitations both in terms of the production process and the quality of the final result.

[10] As for the production process, the used resins require the use of a kiln to complete the polymerization, or long curing times at room temperature. This involves a pause in the production flow, which is necessary for the the heat treatment to be completed, be it at room temperature or at high temperature. The production process is affected by this pause, as a continuous flow of production is not achieved.

[11] In addition, resins of this type are often very susceptible to room temperature, with high viscosity variations, due to the variation of the temperature. This peculiarity implies the need to regulate the temperature of the work areas, in order to avoid that the same resin is almost liquid in the hot periods of the year and very viscous in the cold periods, with evident complications in the dosing and process phase.

[12] Another problem complained of in the specified sector, in relation to the type of product and manufacturing process used in the prior art, is the so-called phenomenon of "capillarity". In fact, it has been noted that, once applied above the bimetallic thermal switch, the liquid resin infiltrates between the cracks between one component and the other, in particular between the lid and the outer container, in its riveted portion, or between the lid and the rivet that acts as an electrical contact.

[13] This phenomenon is the more definite, the more liquid the resin is and longer the first polymerization time, so-called "gel-time". If the resin penetrates inside the bimetallic thermal switch, it can prevent its correct operation, with serious consequences for the safety of the apparatuses in which it is installed.

Disclosure

[14] The aim of the present invention is to solve the aforementioned problems, by devising a temperature limiting device that has an outer container coating that achieves optimal protection from external agents.

[15] As part of this aim, it is a further object of the invention to provide a temperature limiting device provided with a coating that prevents the entry of chemicals and other liquid substances into the housing containing the operating components of the same device.

[16] 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.

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

[18] 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.

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

[20] Preferably said lid is made of insulating material.

[21] Alternatively, said lid is made of a combination of materials such as to be isolated with respect to said external container.

[22] The bimetallic disc is axially associated with said spring-loaded disc.

[23] 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.

[24] According to the present invention, the temperature limiting device comprises a protective cover of insulating material which extends to cover said lid, said portion of the fixed contact protruding above the lid and a curved edge of said outer container and is made of resin adapted to polymerize by exposure to a source of UV rays.

[25] Advantageously, said UV rays have a wavelength in the range 320-450 nm.

[26] Preferably said UV source consists of UV lamps with an intensity in the range 50-200 mW/cm ² .

[27] The use of a resin that polymerizes through exposure to a source of UV rays reduces considerably the time of the production process, allowing an application in line with continuous flow and without breaks due to long thermal cycles.

[28] Preferably said protective cover is made by means of an epoxy-based resin or epoxy polyester with a trigger and hardening mechanism by means of said UV rays.

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

[30] Preferably said curved edge faces the inside of said outer container so as to retain said lid.

[31] According to an embodiment of the invention, said movable contact comprises a movable bridge integral, by axial securing means, to said spring-loaded disc and to said bimetallic disc, tightened in pack.

[32] Preferably said movable bridge has a circular shape and it can be brought in abutment to a pair of fixed contacts.

[33] Preferably said bimetallic disc is made up of a foil made by coupling iron and copper. [34] Preferably said bimetallic disc is arch-shaped, so as to abut peripherally on the edge of said spring-loaded disc.

[35] The present invention also relates to a method for manufacturing a temperature limiting device which provides for coating at least the upper part of the external container of conductive material of the device with a protective cover made of resin suitable for polymerizing through exposure to a source of UV rays.

[36] Advantageously, said protective cover of insulating material extends to cover a lid of said external container of conductive material, a portion of a fixed contact integral with said lid of the external container and protruding above said lid and a curved edge of said external container.

Description of drawings

[37] 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 shows an axial sectional view of the temperature limiting device according to the present invention;

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

Figure 3 shows a similar axial sectional view of a different embodiment of the temperature limiting device according to the present invention.

Description of embodiments of the invention

[38] 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.

[39] The temperature limiting device comprises an external container 2 defining a cup-shaped housing 3. 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. 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.

[40] Inside the housing 3 of the external container 2 a spring-loaded disc 6 and a bimetallic disc 7 are arranged in a stack. In the normal operating condition, the spring-loaded disc 6, having a slightly arched shape, rests peripherally on the bottom of the housing 3 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.

[41] 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.

[42] The lid 4 is tightened against a spacer member 8 of annular shape, which in turn 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.

[43] The cover 4 carries axially inserted, in a way known per se, a fixed contact 10 of the thermal switch; 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 lid 4; the movable contact 11 is protruding above the bimetallic disc 7, through an axial hole made in the same bimetallic disc 7.

[44] 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.

[45] In the normal operating 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).

[46] The outer container 2 is coated above the lid 4 with a protective dome-shaped 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.

[47] 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 for 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.

[48] According to the present invention, the protective dome 12 is made of a resin adapted to polymerize by exposure to a source of UV rays.

[49] In particular, said resin adapted to polymerize by exposure to a source of UV rays consists of an epoxy-based resin or epoxy-polyester with triggering and hardening ultraviolet mechanism.

[50] The UV source preferably consists of UV lamps with a wavelength 320-450 nm, with intensity in the range 50-200 mW/cm ² .

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

[52] 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.

[53] 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 ).

[54] 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 clicks into the reverse position, with the concavity downwardly facing, and pushes the spring-loaded disc 6 downwards (Fig. 2).

[55] In this way, the contact between the fixed and mobile 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 operating value, the bimetallic disc 7 clicks again to return to the initial position and the contact closes.

[56] Fig. 3 shows a different embodiment of the temperature limiting device in which the lid 4 of the outer container 2 carries a pair of fixed contacts, for clarity indicated with reference numerals 10a and 10b. The fixed contacts 10a and 10b cross the lid 4 in parallel positions with respect to each other and are protruding for a portion thereof from the lid 4 itself.

[57] Inside the housing 3 defined by the outer container 2 is arranged a movable bridge 15, of circular shape, adapted to be brought in abutment with the fixed contacts 10a and 10b. The movable bridge 15 is made integral, by means of an axial rivet 16, to the tightly packed spring-loaded disc 6 and bimetallic disc 7. Basically, the movable bridge 15 performs the same function performed, in the previous solution, by the movable contact 11.

[58] The movable bridge 15 is preferably made of brass, bronze or the like, coupled with a precious metal or an alloy thereof such as silver or the like, with or without a precious metal coating.

[59] The spring-loaded disc 6 is secured against a circular seat obtained inside the housing 3 by a spacer ring 17.

[60] As in the previous case, if there is a change in the temperature of the apparatus to be protected, such as to exceed a predetermined switching temperature, the bimetallic disc 7 clicks into the reverse position, with the concavity downwardly facing, and pushes the spring-loaded disc 6 downwards, opening the contact between the movable bridge 15 and the fixed contacts 10a and 10b.

[61] The temperature limiting device according to the present invention achieves the purpose of having a coating of the outer container which provides optimal protection against external agents, in particular such as to prevent the entry of chemicals and other liquid