Automatic switch with actuating electromagnet for short circuits

Abstract: A modular automatic switch in which the electromagnet (11) providing protection against short circuits and which is of the movable core type, consists of a core (25, 26) with a rectangular cross-section around which a coil (35) formed with rectangular turns of wire is wound, a rectangular yoke (31) ensuring closure of the magnetic circuit, and in which the movable part (26) of the core is "polarized" into a rest position, defined by an abutment (45) formed in the switch shell (1), by a spring (41) outside the yoke so as to optimize the working section for the magnetic flux in the core and the section of the copper, resulting in a reduction in the losses in the iron and in the copper and a flatter design of the electromagnetic structure.

Description: The present invention relates to an automatic switch with actuating electromagnet for short circuits. Miniaturized and modular automatic switches for installation on a rail, with the adjacent arrangement of several modules, are well known. A switch of this type is formed by a casing made of insulating material and having a generally parallelepipedal shape, with two broader parallel faces or sides and smaller faces corresponding, with reference to the installation conditions, to a rear face, a front face, an upper face and a bottom face. The body or casing is formed by two half-shells joined together in a plane parallel to the sides and houses the various mechanical and electrical components. The arrangement of the various components and access thereto from the exterior depends on their function and on the installation methods. For example, the rear face of the body is intended for mechanical installation of the switch on the support and has a recess for receiving a support rail onto which the switch is secured by means of sliding teeth arranged on the rear wall. The components for manual operation of the switch are located on the front side, and a first and second clamp for securing external electrical terminals emerge on the bottom side and top side, respectively. The sides of the switch must be flat, without raised areas preventing the adjacent arrangement of several modules, and must not have apertures for access to internal components, except for those necessary and specifically designed for establishing a mechanical interconnection between adjacent modules. For a more detailed description of these modular switches, reference may be made to international patent application WO96/12292. The essential components forming part of a switch of this type, which must be able to interrupt high-intensity currents, including currents in the region of several tens of amperes, include a rapid-action electromagnet for automatically opening the switch in the event of a major overload due in particular to a short circuit. The electromagnet must be constructionally simple, easy to install within the body and must have optimum characteristics in terms of reliability and operational repeatability. Moreover, with regard to the modular dimensions of the body, the latter must have compact dimensions, in particular in the direction perpendicular to the sides of the switch, and at the same time must not give rise to significant voltage drops or losses in the copper and the iron which are the cause, under normal operating conditions (variable depending on the load), of local heating which may alter the operating characteristics of the electromagnet itself or of the other devices present in the switch, such as, for example, the bimetallic thermal protection system for preventing prolonged overloads. These drawbacks are all the greater, the more compact the structure and, therefore, the smaller the heat dispersion. Moreover, the need for compact dimensions is difficult to reconcile with the need for a rapid opening action which requires a suitable magnetic force of attraction associated with a significantly long working stroke of the armature or the movable core and, essentially, a kinetic energy acquired as quickly as possible and sufficient to actuate the mechanism for tripping and opening the switch. Although automatic switches with an electromagnet of the movable armature type are available on the market, the most widely adopted solution, able to achieve the best possible performance in this application, is a switch with a movable-core or plunger-type electromagnet. In these electromagnets, the magnetic circuit is composed of a first fixed cylindrical core portion, integral with a yoke for closing the flow, and a second movable cylindrical core portion, axially aligned with the first portion and separated therefrom by an air gap. A hollow cylindrical plastic cap, which is force-fitted onto the first core portion, acts as an axial guide for the movable portion, which is freely inserted inside the cap, and with its closed end forms a bearing stop against which the movable core is pushed by an actuating stem, freely passing inside an axial cavity of the fixed core, and in turn biased or, in other words, "polarized" by a helical compression spring which is also housed in said cavity. Mounted on the cap is a helical winding of conducting wire with a relatively thick circular cross-section (2-2.5 mm) for exciting the electromagnet. Although relatively simple, this solution has various drawbacks: the fixed core, owing to the presence of the actuating stem and, even more so, of the biasing spring, has a relatively small working section for the magnetic flux, relative to the movable core, with a consequent uneven distribution of the magnetic induction within the section, in particular the air gap, resulting in additional losses (the electromagnet is excited with an alternating current) and, for the same excitation conditions, in the generation of a force of attraction which is less than that which would be achieved with a solid-section core; the electromagnet cannot be set for different operating conditions, in order to achieve these conditions it being necessary to diversify production using, in the various cases, biasing springs with different elasticity characteristics; in the copper also, owing to the dispersed flows which affect the winding, additional and not insignificant losses due to parasitic currents occur; finally, the constructional design does not allow a reduction in the dimensions and the production of modular automatic switches with a small thickness less than the currently defined standard of 17.5 mm. This latter requirement is particularly important in domestic installations where, for safety reasons, the installation must be divided up into as large a number of sections as possible, individually protected by automatic switches, each set according to the various expected possible maximum loads. An automatic switch according to the above cited features is disclosed in document EP-A-0 621 618. The object of the present invention is to provide an automatic switch with actuating electromagnet for short circuits, which has structural and functional characteristics such as to overcome the abovementioned drawbacks described with reference to the known art. This object is achieved by means of an automatic switch comprising, housed in an insulating shell, manual setting and resetting devices, opening and closing contacts actuated by said manual devices, tripping devices for automatically opening said contacts and a rapid-action electromagnet for activating said tripping devices when an overload occurs, characterized in that said electromagnet, which is of the movable core (plunger) type, consists of: a first fixed ferromagnetic core portion having an axial hole housing a thruster or striker for actuating said tripping devices and a rectangular cross-section transversely with respect to said axial hole; a second movable ferromagnetic core portion aligned with said first portion in the direction of said axial hole and movable in said direction, said second portion having a rectangular cross-section perpendicularly with respect to said direction; a tubular element made of non-magnetic material and having a hollow rectangular cross-section, for receiving and mutually aligning said first and second portions and for guiding said second portion movable in said direction; a flow-closing yoke which is integral with said first portion and formed by a ferromagnetic strip folded in the form of a rectangle around said tubular element and provided with recesses on opposite sides for insertion, in said yoke, of said first core portion and said tubular element; a coil formed by insulated conducting wire wound around said tubular element; resilient recall means, situated outside said yoke, for keeping said second movable core portion at a distance, when at rest, from said first fixed portion; and abutment means, situated outside said yoke, for providing said second movable core portion with a predefined rest position relative to said first portion, with the formation of an air gap of predefined width; said second movable core portion acting, via said air gap, on said actuating thruster when said electromagnet is energized by a current in said coil which is greater than a predefined intensity. The present invention overcomes the drawbacks of the known art, is able to satisfy the abovementioned requirement and provides an automatic switch which is simple and inexpensive to produce and is provided with an electromagnet for protection against short circuits which is safe, reliable, fast and effective for a vast range of currents, and which results in minimal additional losses and at the same time has small dimensions in the direction perpendicular to the sides of the switch. By way of a further advantage, the structure of the electromagnet allows, without diversification of the components during production, the preparation, during final assembly, of modular switches of the same type having different operating characteristics. The characteristic features and advantages of the invention will emerge more clearly from the following description of a preferred embodiment and from the accompanying drawings in which: Figure 1 is an overall view, sectioned in a direction parallel to the sides, of an automatic switch in accordance with the present invention; Figure 2 is an exploded perspective view of the various components which form the electromagnet of the switch according to Fig. 1. With reference to Figure 1, the automatic switch comprises a body 1 made of insulating material and consisting of two half-shells which are joined together and one of which is shown sectioned so as to reveal the internal structure of the two half-shells. The structure and the internal ribbing of the two half-shells fit together in a suitable manner and precisely position the two half-shells, with respect to each other, as well as the various components housed in the casing relative thereto and to each other. The two half-shells are fixed together by means of screws or rivets passing through holes 2,3,4,5,6 perpendicular to the plane of the drawing. The modular switch shown is provided with a recess 7 on the rear side for receiving a DIN standard rail onto which it is fixed by means of toothed slides, not shown. A plurality of mechanical and electrical components is housed and precisely positioned inside the casing, in particular: a first and a second terminal/clamp 8, 9, respectively, for electrical connection to external wires; a bimetallic strip 10 having an end connected electrically and mechanically to the terminal 8; an electromagnet 11; an electric-arc extinguishing labyrinth 12, also known as a deionizing cell; a manual setting lever 13 which is hinged on a pin 14 formed or engaged in the casing and connected to a setting rod 24; a fixed contact 15 which is supported by a rigid metal end-piece 16 of the electromagnet 11, electrically connected to one end of the electromagnet winding, the other end of the winding being connected to the terminal 9; a movable contact 17 at the end of a contact arm 18 electrically connected to the bimetallic strip 10 by a flexible copper braiding; a tripping device comprising the contact arm and other elements collectively identified by the reference number 19; a slide 20 for unidirectional connection to the bimetallic strip to the tripping device; an electric-arc guiding and switching electrode 21 electrically connected to the bimetallic strip. All these devices, except for the electromagnet 11, do not fall within the scope of the invention, but are mentioned in order to provide a general idea of the complexity of the switch, the difficulties of assembly and the requirements relating to compactness and minimum dimensions which the various components must satisfy in order to allow them to be housed in a flat small-size container. The operating principle of a switch of this type is known: when the tripping device 18, 19 is manually set, the two contacts 15, 17 are closed and electrical continuity is established between the terminals 8 and 9 such that a current flows in the electromagnet winding and in the bimetallic strip for supplying of a general external load. It is therefore obvious that these elements must have a minimum resistance (or, more generally, an impedance) so as not to draw power from the load and avoid, to a certain degree, the generation of heat inside the switch. In the case of a prolonged overload or short circuit, the respective thermal protection device (bimetallic strip 10) and magnetic protection device (electromagnet 11) activate the tripping device, causing the opening of the contacts 15 and 17. The electric arc which is produced with the opening of the contacts passes from the contact 17 onto the arc guiding electrode 21 and is extinguished in the deionizing cell. In order to ensure rapid extinction of the arc, opening of the contacts must be particularly rapid. This is achieved by means of a relatively powerful spring which is tensioned by the manual setting lever and part of the tripping device. This, however, is not sufficient: in order to activate the tripping device, it is necessary to have a certain energy which, in the event of a short circuit, must be made available as quickly as possible, as soon as the short circuit occurs. This energy is provided by means of excitation of the electromagnet 11, owing to the effect of the short-circuit current. The structure of the electromagnet 11 is shown in detail in the exploded view in Figure 2. The electromagnet comprises a magnetic core consisting of a first fixed portion 25 and a second movable portion 26 which are aligned with each other in an axial direction. The prism-shaped and non-cylindrical core has, perpendicularly with respect to the axial direction, a rectangular cross-section (having in a preferred embodiment, by way of guidance, side dimensions of 3.5 x 8 mm) with slightly rounded edges and is partially housed in a tubular aligning and guiding element 27 which also has a rectangular cross-section and is made by means of extrusion or moulding of plastic, for example with a thickness in the region of 0.3-0.4 mm. The fixed core portion has, in the axial direction, a cylindrical through-hole 28 with a diameter in the region of 1-1.4 mm, inside which an actuating thruster 29 made of a low-conductivity, metallic or plastic, non-magnetic material is freely inserted. Close to the end opposite that facing the movable portion 26, the fixed core portion 25 has an annular groove 30 with sides perpendicular to the core axis for fixing, by means of engagement, into a yoke 31. The yoke 31 is formed from a strip of ferromagnetic material with a cross-section of, for example, in the region of 7 x 1.5-2 mm folded so as to form a rectangular frame with dimensions of about 20 x 16 mm and a height equal to the width of the strip (7 mm). A recess 32 for joining, by means of engagement, with the end recess 30 in the fixed core 25 is formed in one of the shorter sides of the yoke. The opposite side of the yoke has, formed in it, a rectangular recess 33 for housing one end of the tubular guide element 27 which is arranged, in a similar manner to the core portions housed therein, inside the yoke, with the smaller dimensions oriented in the direction of the heightwise extension of the frame. The shorter side of the yoke, where the recess 32 is present, has fixed to it, by means of electric spot welding, a metal end-piece 16 for supporting the fixed contact of the switch, said end-piece also being provided with a recess 34 which is superimposed over the recess 32 for joining, by means of engagement, to the fixed core portion. The tubular element 27 has, arranged on it, a winding 35 formed by means of a preferably flat conducting wire having a rectangular cross-section (by way of guidance, 2.4 x 1.3 mm) and folded so as to form substantially rectangular turns with the smaller thickness of the copper oriented perpendicularly with respect to the axial direction of the core. In this way, the winding is completely housed inside the yoke with a dimension, in the direction of the heightwise extension of the yoke, not greater than its height. Suitable recesses 36, 37 in the yoke 31 allow, without an increase in the dimensions, the ends 38, 39 of the winding to pass outside the yoke so as to be fixed, respectively, by means of soldering, to the metal end-piece 16 provided, for this purpose, with a flange 40 which extends beyond the end of the yoke, and to the terminal 9 (Fig. 1) of the switch. The electromagnet structure is completed by a torsionally operating helical spring 41 with a working arm 42 hinged in a hole 43 formed in the end of the movable core 26 opposite that adjacent to the fixed core 25. The function of the spring is to bias or "polarize" the movable core 26 into the rest position and to oppose the plunging force which is exerted on the latter when the electromagnet winding has, passing through it, currents which fall within normal operating conditions and therefore do not cause opening of the switch. Returning to consider Figure 1, all of the parts of the electromagnet 11, shown in cross-section, can be clearly seen. In particular, it can be seen that the internal ribbing inside the casing, such as for example the rib 44, forms a seat for the stable and precise positioning of the yoke and of the end-piece 16 integral therewith (and therefore of the entire electromagnet) as well as an abutment 45 against which the end of the movable core rests and thus exactly defines the rest position relative to the fixed part and, consequently, the width of the air gap in the rest condition. The helical recall spring 41 is pivotably mounted on a pin 46 which may be formed integrally with the casing (or, if it is a separate component, fixed inside the casing in a predefined position) and with its working arm 42 pushes the movable core against the abutment surface 45. The force exerted by the spring depends on its pretensioned condition: this is set by fixing the end 47 of the spring in a suitable position by means of engagement in a notch formed inside the casing so that this produces a predefined angle of winding of the spring with respect to its rest configuration. As shown in Figure 1, the casing may be provided with several notches, such as 48, 49, so as to cause the spring 42 to assume one of several possible pretensioned conditions, which may be selected during assembly, so as to define one of various working capacities of the switch, which is generally expressed in amperes (for example 15-25 50 A) . The use of a helical spring with an arm and end portion, although preferred, is not essential: the recall means could also be structured in another manner, for example as a lever pivotably mounted on the pin 46 and wholly equivalent to the arm 42 and which is biased or "polarized" by means of an extension spring which is attached, on one side, to the lever and, on the other side, to a pin or notch (including one of several pins or notches) suitably arranged in the free space available between the manual setting device (13, 14) and the electromagnet 11. By way of a further improvement, which involves only minimal production diversification during the final assembly stage, it is also possible to set the width of the air gap in the rest condition so as to optimize the operating time of the electromagnet depending on the capacity assigned to it. For this purpose, the abutment 45, instead of being integral with the casing, may consist of a strip having a thickness defined as a function of the capacity, which is inserted into a seat in the casing. It is thus clear that, with the electromagnet structure described, it is possible to achieve substantial advantages: the working section for the flow, within the magnetic circuit, in particular within the core, is practically uniform and of a value suitable for achieving a high force of attraction, with uniform distribution of the flow; in this connection it should be recalled that, for the same magnetic driving force nI (n = number of turns; I = current intensity) acting within the circuit, the force of attraction exerted on the movable core is proportional to the area A of the section of the magnetic circuit in the region of the air gap; the additional losses due to hysteresis within the magnetic circuit are kept to a minimum owing to the uniform distribution of the flow; the additional losses due to parasitic currents in the winding are also kept to a minimum because the conductors have, in a direction perpendicular to the inevitable dispersed flows, a smaller thickness and therefore a minimum cross-section; the use of a biasing spring outside the magnetic circuit, in addition to ensuring, as already mentioned, a more uniform distribution of the magnetic flux, offers the possibility of setting the operating conditions of the electromagnet without the need for substantial diversification during production; owing to the small dimensions of the electromagnet in the direction perpendicular to the sides of the switch (relative to the arrangement inside the casing), it is possible to provide modular switches which have a reduced thickness.

Claims: Electromagnet for use in an automatic switch, said switch comprising, housed in an insulating shell (1), manual setting and resetting devices (13, 14, 24), opening and closing contacts (15, 17) actuated by said manual devices, tripping devices (18, 19) for automatically opening said contacts, said electromagnet (11) being of the rapid-action type for activating said tripping devices when an overload occurs, and comprising a first fixed ferromagnetic core portion (25), a second movable ferromagnetic core portion (26), a flow-closing yoke (31), a coil (35) formed by insulated conducting wire, resilient recall means (41) for keeping said second movable core portion at a distance, when at rest, from said first fixed position, characterized in that : said first fixed ferromagnetic core portion (25) has an axial hole (28) housing a thruster (29) for actuating said tripping devices (18, 19) and a rectangular cross-section transversely with respect to said axial hole; said second movable ferromagnetic core portion (26) is aligned with said first portion in the direction of said axial hole and movable in said direction, said second portion having a rectangular cross-section perpendicularly with respect to said direction; said electromagnet comprises a tubular element (27) made of non-magnetic material and having a hollow rectangular cross-section, for receiving and mutually aligning said first and second portions and for guiding said second portion movable in said direction; said flow-closing yoke (31) is integral with said first portion and formed by a ferromagnetic strip folded in the form of a rectangle around said tubular element (27) and provided with recesses (32, 33) on opposite sides for insertion, in said yoke, of said first core portion and said tubular element; said coil (35) is wound around said tubular element (27); said resilient recall means (41) are situated outside said yoke (31); said second movable core portion being provided with a predefined rest position relative to said first portion, with the formation of an air gap of predefined width, by means of abutment means (45), situated outside said yoke; said second movable core portion acting, via said air gap, on said actuating thruster (29) when said electromagnet is energized by a current in said coil (35) which is greater than a predefined intensity. Electromagnet according to Claim 1, in which said coil (35) is formed by insulated conducting wire with a rectangular cross-section, wound around said tubular element (27) with a greater side of said rectangular cross-section of the wire facing said tubular element. Automatic switch comprising, housed in an insulating shell (1), manual setting and resetting devices (13, 14, 24), opening and closing contacts (15, 17) actuated by said manual devices, tripping devices (18, 19) for automatically opening said contacts and a rapid-action electromagnet (11) according to claim 1, for activating said tripping devices when an overload occurs, said electromagnet comprising a first fixed ferromagnetic core portion (25), a second movable ferromagnetic core portion (26), a flow-closing yoke (31), a coil (35) formed by insulated conducting wire, resilient recall means (41) for keeping said second movable core portion at a distance, when at rest, from said first fixed position, said automatic switch characterized in that : said first fixed ferromagnetic core portion (25) has an axial hole (28) housing a thruster (29) for actuating said tripping devices (18, 19) and a rectangular cross-section transversely with respect to said axial hole; said second movable ferromagnetic core portion (26) is aligned with said first portion in the direction of said axial hole and movable in said direction, said second portion having a rectangular cross-section perpendicularly with respect to said direction; said electromagnet comprises a tubular element (27) made of non-magnetic material and having a hollow rectangular cross-section, for receiving and mutually aligning said first and second portions and for guiding said second portion movable in said direction; said flow-closing yoke (31) is integral with said first portion and formed by a ferromagnetic strip folded in the form of a rectangle around said tubular element (27) and provided with recesses (32, 33) on opposite sides for insertion, in said yoke, of said first core portion and said tubular element; said coil (35) is wound around said tubular element (27); said resilient recall means (41) are situated outside said yoke (31); said automatic switch comprising abutment means (45), situated outside said yoke, for providing said second movable core portion with a predefined rest position relative to said first portion, with the formation of an air gap of predefined width; said second movable core portion acting, via said air gap, on said actuating thruster (29) when said electromagnet is energized by a current in said coil (35) which is greater than a predefined intensity. Automatic switch according to Claim 3, in which said coil (35) is formed by insulated conducting wire with a rectangular cross-section, wound around said tubular element (27) with a greater side of said rectangular cross-section of the wire facing said tubular element. Automatic switch according to Claim 3 or 4, in which said abutment means (45) consist of a bearing surface for said second core portion, formed integrally in said shell (1) in a predefined position relative to means (44) for positioning said yoke, said means also being formed integrally in said shell. Automatic switch according to Claim 3 or 4, in which said abutment means (45) consist of a strip of suitable thickness separate from said shell (1) and housed in said shell (1) in a predefined position relative to means (44) for positioning said yoke, formed integrally in said shell. Automatic switch according to the preceding claims 3-6, in which said resilient recall means consist of a helical spring (41) with an end portion (47) and a working arm (42), mounted on a support pin (46) in said shell, said working arm being secured to an eyelet (43) in said second movable core portion (26), said end portion (47) being constrained in a notch (48, 49) formed inside said shell. Automatic switch according to Claim 6, in which said shell (1) is provided internally with a plurality of stop notches (48, 49) for said end portion (47) so as to provide said spring (41) with a selective degree of pretensioning depending on the notch in which said end portion is constrained.

Inventors: FABRIZI FABRIZIO, FABRIZI, FABRIZIO
Applicants: BTICINO SPA, BTICINO S.P.A.
Application year: 2000
Main classification: H01H71
IPC codes: H01H71