DESCRIPTION

FIELD OF THE INVENTION

The present invention relates to the construction of magnetic cores for low and medium voltage electrical trans formers and reactances (hereafter indicated as a whole as "electrical trans formation devices" ) .

DESCRIPTION OF THE PRIOR ART

As is known, electrical trans formation devices are widely used in many sectors in which electrical energy must be trans ferred between circuit s in three-phase or single-phase mode at dif ferent voltage levels (these circuit s are called primary and secondary circuit s ) or to filter current harmonics ( reactances ) . For example , electrical trans formation devices are present in special transformer substations , in industrial production environment s , in hospital premises , at road boring sites , on large ships , in hydroelectric power stations , in substation rooms , in temporary facilities for sport s and/or cultural events , and so on .

Hereafter, for seek of description concisenes s , only the term "electrical trans formation device" will be used, which is understood to refer to both trans formers and reactances for use in a very wide-range of sectors .

In these contexts , electrical trans formation devices are electric machines intended to change the voltage and current ratios between primary and secondary circuits and to limit current harmonics . To this end, as schematically shown in Figure 1 , which represent s an example of the prior art , the electrical trans formation device has three primary coils and three secondary coils or windings produced with copper or aluminium electrical conductors (through which alternating electrical current flows ) . These windings are mounted concentrically with the vertical limbs of a ferromagnetic core through which the alternating magnetic flux flows .

The ferromagnetic core is formed by a set of metal sheet s of grain-oriented or non-grain-oriented ferromagnetic material (typically thin silicon steel sheets FeSi , FeNi , or amorphous or nano-crystalline laminations ) suitably shaped and assembled, according to a scheme of limbs and yokes , where the number of limbs can vary according to the type of electrical trans formation device and the dimensioning of the electrical design, such as the cross-sections of the limbs and of the yokes .

These choices contribute to determining the weight and size of the electrical trans formation device .

An electrical trans formation device is produced in accordance with the electrical and mechanical design from which individual details are defined, such as the coil , core and accessories , which are as sembled in accordance with the electrical and mechanical design parameters .

For a better understanding, the main structural arrangement of this kind of trans formation device is described hereafter . The construction of the ferromagnetic core assembly of the device provides for the following three steps :

Step 1 ) : separately cutting the individual metal sheets of the ferromagnetic core , coupling them in a "package" according to the design;

Step 2 ) : as sembling the entire ferromagnetic core , stacking, crossing or laying side by side the individual metal sheet s so as to form a structure made up of limbs and yokes ( for simplicity, reference is made to the arrangement of three limbs and two yokes as depicted in Figures 3A and 3B) ;

Step 3 ) : coupling a metal framework structure to the ferromagnetic core, defining a certain number of lifting plates ( for example , two for each limb) , two lower and upper yoke clamping frames which are clamped as a unit to the core by means of tierods and as sociated nuts and washers which are suitably isolated ( see Figures 4A and 4B) .

Next , the device structure anticipates the structure and assembly of the windings , in particular, of low voltage ( LV) or medium voltage (MV) coils , as represented as examples in Figures 5A and 5B . In order to be able to position the coils on the limbs of the core, the upper yoke clamping frames must be removed (Figure 6A) . The coils are then coupled to the respective limbs (Figures 6B and 6C) , the upper yoke is refitted in succes sive steps cros sing or laying side by side packages of magnetic metal sheets , to then tighten the yoke with the yoke clamping frame and with the associated tie-rods and locking nut s which are suitably isolated (Figure 6D ) .

The ferromagnetic core, as later the finished trans formation device, requires to be handled and hence lifted by grasping the upper yoke clamping frames , for example using a crane , an overhead travelling crane or other hoist . The yoke clamping frames are therefore equipped with technical solutions such as eyelet s , holes or eyebolts , to which the lifting means are hooked .

Given the pos sible high weight of the core and of the complete device, in order to ensure lifting takes place correctly, although the yoke clamping frames are clamped as a unit by means of the clamping tie-rods , provision must also be made for the use of lifting strips (normally, but not mandatorily, two for each limb ) which mechanically connect the upper yoke clamping frames with the lower ones .

Some examples from the prior art are illustrated in CN102360793 and W02014066075 .

At the same time , in addition to a correct mechanical dimensioning, electrical continuity must be ensured throughout the entire structure : the lifting strips and the upper and lower yoke clamping frames .

The main solutions currently employed are : a) use of lifting strips screwed or welded to the yoke clamping frames and clamped as a unit to the ferromagnetic core by screws ; this solution requires interposing plates of insulating material between the yoke clamping frame and the magnetic core to compensate for the gap corresponding to the thickness of the lifting strips ; this system is therefore complex and expensive due to the additional operations during the disassembly and reassembly of the structure as well as due to the use of packing material; b) use of lifting strips having T-shaped ends ( or similar shaped ends ) which lodge into the T-shaped ( or similarly shaped) seats produced in the yoke clamping frames ; to ensure that during lifting the weight of the device does not cause the T-shaped ends of the lifting strips to come out of their seats , it is customary to lock them using various solutions , the most common being a retaining brace ( see Figure 7 ) ; the T-shaped (or similarly shaped) ends of the strips enter transversally with a certain amount of play in the T-shaped ( or similarly shaped) seat s of the yoke clamping frames , thereby making assembly and disassembly of said clamping frames easy; in this solution the quantity of screws used is reduced and the use of compensating materials is rendered useless , in that the lifting strips are coupled within the thickness of the yoke clamping frame it self without leaving space between the ferromagnetic sheet s and the yoke clamping frames ; c) a configuration similar to that described in b ) , which provides for a mechanical securing member obtained directly as part of the yoke clamping frame ( see Figure 8 ) or as a member fixed to the yokes where no further securing is required for the purposes of mechanical retention of the strips .

An example of the prior art is also described in EP 477423 .

A crucial aspect of these constructional methods is the need to ensure electrical continuity between the lifting strips and both upper and lower yoke clamping frames . To address this problem of electrical continuity in the structure, currently ground cables are used, which link the lifting strips and the yoke clamping frames ; alternatively, provision is made to drill the retaining brace in which a point screw is screwed in, thereby creating the electrical contact with the lifting strips and the structures behind them .

Clearly however, these solutions require a specific as sembly intervention, time and the supply of additional material for the electrical connection .

SUMMARY OF THE INVENTION

The problem forming the basis of the invention is therefore to propose an improved yoke clamping frame design which ensures the mechanical contrast to keep the lifting strips in position and, at the same time, maintain stable electrical continuity between the lifting strips , produced with T-shaped (or similarly shaped) ends , without needing the intervention of additional items during installation .

This aim is achieved with a securing member, structurally obtained in the production of the yoke clamping frame , which acts by interposing with a certain amount of interference and therefore act s with an elastic compres sion force against the lifting strips . The geometric interference is calculated so as to keep the compres sion member in the elastic deformation range of the material once assembled at the yoke clamping frame , providing constant contact between the lifting strips and yokes .

This solution ensures easy as sembly and disassembly of said yoke clamping frames , without los s of functionality . This construction, carried out with the use of lifting strips and yoke clamping frames , which are produced from conductive material and/or using conductive coatings , provides electrical continuity between all the lifting strips and the upper and lower yoke clamping frames of the ferromagnetic core .

These characteristic features of the invention are defined in the accompanying main claims . The dependent claims describe preferred features of the invention .

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the invention will become clearer from the following detailed description of a preferred embodiment , given purely by way of example and in a non-limiting manner, and illustrated in the appended drawings in which :

Figure 1 is a perspective view of a finished electrical trans formation device of the prior art ;

Figures 2A and 2B are side and front elevation views of a magnetic core assembly of the prior art ;

Figures 3A and 3B are side and front elevation views of a ferromagnetic core of the prior art ;

Figures 4A and 4B are side and front elevation views of a metal structure for the magnetic core of the prior art ;

Figures 5A and 5B are schematic views of two examples of a set of windings of the prior art ;

Figures 6A- 6D are perspective and partially exploded schematic views of as sembly steps for the electrical trans formation device ;

Figures 7 and 8 are views of details of ends of lifting strips of the prior art ;

Figures 9A and 9B are views , respectively in front elevation and perspective , of a structure of an electrical transformation device, according to the invention;

Figure 10 is a photographic view in front elevation of a yoke- tie-rod combination produced on a device according to the invention ;

Figure 11 is a photographic view of a detail of Figure 10 ; and

Figure 12 is a partial cross-sectional view of the detail represented in Figure 11 .

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

To better understand the invention, in the introduction of the present specification there has already been illustrated, in its main details , an industrial type electrical transformation device, equipped with six lifting strips having T-shaped ends , which also forms the referenced subj ect of the present invention .

Figures 9A and 9B illustrate in greater detail a structure of a device which provides for, in a manner that is known per se, a ferromagnetic core 2 , formed by sheets placed side by side each other, as sembled and clamped as a unit to form the designed shape .

Two yoke clamping frames , lower 3a, 3b and upper 4a, 4b, are typically formed by a pair of metal beams , for example with L- shaped or C-shaped cros s sections (therefore equipped with at least a vertical web part and a horizontal flange part ) , held on the ferromagnetic core by tie-rods 5a and 6a . The yoke clamping frames are for example made of galvanised and/or painted iron .

The yoke clamping frames 4a and 4b of the upper yoke 4 exhibit eyelets 7 for lifting the device . In a manner that is known per se, there is provided between the yoke clamping frames 3 and 4 a plurality of lifting strips which are composed of a single strip 8a with T-shaped ends 8b, intended to be coupled, with a certain amount of play, in corresponding seat s or cut-out s 8 ' produced in all the yoke clamping frames .

As is well depicted in the drawings , according to the invention, at least the upper yoke clamping frame 4 (and preferably also the lower one 3 ) has an arch-shaped portion next to each cut-out 8 ' , intended to exert an elastic pressure on the head end 8b of the lifting strip 8 , when assembled in it s operational position .

In particular, as visible in Figures 10 and 11 , provision is made for a portion of the yoke clamping frame 4a, at the area of coupling with the lifting strips 8 , which is shaped so as to define an external convex arch 10a and a corresponding internal opening 10b, which define an arch-strip or a brace-type parenthesis shape 11 having a hump or cusp Ila that is faced inwards .

Terms such as "inwards" and "external" refer to a direction that is orthogonal to the ferromagnetic core .

In particular, the arch-shaped portion is made on a horizontal flange part of the yoke clamping frame .

As is well depicted in Figure 12 , the cusp I la of the archshaped portion extends on its plane beyond the vertical plane coincident with the outer surface of the web part of the yoke clamping frame 4a, therefore partly entering the cut-out 8 ' in which the T-shaped head end 8b of the lifting strip 8 is accomodated . For example , as illustrated, the cusp Ila extends over an interference depth of about 2-3 mm into the cut-out 8 ' . "Interference depth" is understood to mean the distance between vertical plane coincident with the outer surface of the web part of the yoke clamping frame 4a ( i . e . the entry plane of the cutout 8 ' ) and an imaginary plane parallel to it and tangential to the end of the cusp I la .

Thus , an interference between the cusp Ila and the end 8b of the lifting strip 8 is established . This brings about an elastic deformation of the arch-shaped portion, in particular of the archstrip 11 , when the yoke clamping frames 4a and 4b are coupled with one another against the ferromagnetic core and clamped as a unit using horizontal tie-rods for the metal structure .

The deformation of the arch-shaped portion produces an elastic reaction which ensures contact with pressure between the cusp Ila and the head 8b of the lifting strip 8 , ensuring a useful mechanical coupling upon lifting and the desired electrical continuity between the yoke clamping frame and the lifting strips 8 .

As can be understood, during as sembly/disas sembly no intervention is needed in any way in the engagement region of the ends 8b, in that the contact with pres sure is established automatically when the yoke clamping frames 4a and 4b of each yoke are clamped as a unit with one another by respective horizontal tie-rods or other suitable clamping means .

The galvanised iron of the yoke clamping frames 4a-4b also maintains over time an elastic effect which enables the archshaped portion to keep to maintaining the necessary interference with the lifting strips 8 during the various stages of life of the product .

By virtue of this mechanical interference which produces an elastic deformation, a firm contact is achieved, both from the mechanical and electrical perspectives for all the metal parts which form the framework structure and the core of the device, yet enabling easy disas sembly of the upper yoke 4 from the ferromagnetic core .

In the arrangement illustrated in the drawings , there are six contact points via arch-shaped portions (or twelve if the lower yoke clamping frames are also equipped with the same coupling system) , i . e . three for each yoke clamping frame, one at each lifting strip . It is clear, however, that this number can be varied according to requirement s .

According to a preferred embodiment , each beam frame 4a, 4b of the yoke clamping frame is obtained by laser-cutting or plasma- cutting a flat plate of galvanised iron, hence obtaining an accurate cut-out 8 ' , arch form 10a and inner opening 10b, and then bending 90 ° two parts along a longitudinal line , so as to obtain as one part the vertical web part and as the other part the horizontal flange part ( in which the arch-strip 11 is defined) of the yoke clamping frame 4a-4b . In particular, during lasercutting, the cut-outs 8 ' are defined as well as the corresponding arch-shaped portions , by cutting out the external convex arch 10a and the corresponding inner opening 10b adj acent to the as sociated cut-out s 8 ' .

As noted from the above description, the yoke clamping frame provided with arch according to the invention allows to perfectly achieve the aims disclosed in the preambles .

The advantages attained by the arrangement according to the invention can be summarised as follows :

- optimisation of the as sembly of the electrical trans formation device , by virtue of easy coupling between the heads of the lifting strips and the yoke clamping frame, without the use of tools or fastening means ;

- electrical continuity ensured purely through clamping of the metal framework of the core, without using additional screws , ground cables , etc;

- ef fective mechanical retention of the end 8 ' of the lifting strip 8b, to avoid any possible accidental releasing;

- reduction of the use of general purpose screws ;

- overall cost savings in metal sheet s , screws and labour .

It is understood that the invention must not be considered to be limited to the particular arrangement illustrated above, which forms only one example embodiment thereof , but that various variant s are possible, all within the capabilities of a technician skilled in the art , without thereby departing from the scope of protection of said invention, as defined in the appended claims .