EP0150793B1 - Continuously working apparatus for heating of flat products by electromagnetic induction - Google Patents

Description

• - Un système de transfert pour maintenir le produit plat à chauffer dans un plan de chauffage et le faire défiler selon une direction longitudinale de transfert dans ce plan. La largeur du produit est disposée selon une direction transversale également dans ce plan et son épaisseur selon une direction de flux, chacune de ces trois directions étant perpendiculaire aux deux autres. La position transversale du produit à chauffer est réglable.
• - Des enroulements d'excitation d'inducteur.
• - Des moyens d'alimentation électrique pour fournir à ces enroulements une intensité électrique périodiquement variable selon le temps et commandable en amplitude, et pour leur faire ainsi produire un flux magnétique variant comme cette intensité,
• - et des circuits magnétiques d'inducteur pour canaliser ce flux et former un flux de chauffage traversant le produit à chauffer sensiblement selon ladite direction de flux.

The invention relates to the heating of electrically conductive flat products, this heating being carried out by electromagnetic induction. A known device used for this comprises the following essential elements:

• - A transfer system to hold the flat product to be heated in a heating plane and to scroll it in a longitudinal direction of transfer in this plane. The width of the product is arranged in a transverse direction also in this plane and its thickness in a direction of flow, each of these three directions being perpendicular to the other two. The transverse position of the product to be heated is adjustable.
• - Inductor excitation windings.
• - Power supply means for supplying these windings with an electric intensity that is periodically variable over time and controllable in amplitude, and thus to cause them to produce a magnetic flux varying like this intensity,
• - And magnetic inductor circuits to channel this flow and form a heating flow passing through the product to be heated substantially in said direction of flow.

The invention applies to the frequent case where the heating must be uniform over the entire surface of the product.

Among the known devices for obtaining such a heating, most use a magnetic field whose profile is uniform over most of the width of the product. This profile is corrected at the edges so as to obtain uniform heating over the entire width by adjusting the distribution of the closing currents. Uniformity along the length results from the longitudinal movement of the product.

This is particularly so according to British Patent No. 1,546,367. Edge corrections are obtained by various devices (additional magnetic coils or bridges, air gap modifications, etc.) which complicate the production and must be adapted to the armature reaction of the product, this reaction depending on the characteristics of the latter thickness, resistivity.

Another known device known as "square mesh" overcomes previous difficulties. It is described in document FR-A-2 538 665 (French patent n ° EN 82 21906 of December 28, 1982). The inductor constituted by the windings and the magnetic circuits realizes a square mesh of the heating flux with a sinusoidal distribution of the magnetic field in two directions parallel to the sides of the edges. If the product has a width containing an integer number of times the pitch of the mesh, the heating is then homogeneous without any correction of edges. But, if the width of the product does not contain an integer number of times the step of the mesh, a temperature heterogeneity appears on the two edges. To reduce this heterogeneity, it is possible on the one hand to modify the excitation intensity of the elements of the mesh facing these edges, on the other hand to provide a relatively small pitch of the mesh so that the heat diffusion by conduction provides acceptable homogenization in this area. But the small step of the mesh can be a handicap since the inducible surface power varies as the fourth power of this step, without ensuring sufficient homogenization by conduction in the product if the running speed is relatively high (short transfer time) . It is also possible to add additional inductors opposite these edges, but this introduces the complexity of embodiment indicated above.

The device according to the invention comprises the essential elements previously mentioned. Its inductor has a periodic constitution both in the longitudinal direction with a longitudinal pitch, and in the transverse direction with a transverse pitch, so that the variations in the amplitude of the heating flux in the heating plane draw a rectangular mesh constituted by a juxtaposition of rectangular tiles of lengths equal to this longitudinal step and widths equal to this transverse step. The magnetic circuit comprises in each of these tiles at least one central pole piece such that the amplitude of the heating flow is canceled out on the sides of the tile so that the average heating obtained after the product to be heated is the same in all tile widths fully included in the width of this product. The shape of this part is also chosen so that this amplitude is maximum in the center of the pane with a distribution substantially in the form of a sinusoid arch as well according to the longitudinal sections as according to the cross sections. The ratio of the transverse pitch to the longitudinal pitch is chosen to cancel the local heterogeneity of heating in each of the tiles which are entirely contained in the width of the product to be heated. This local heterogeneity is the difference in one direction or the other of the temperature of the middle of the width of the tile compared to that of the edges of the tile after the scrolling of this product.

The present inventors have noted that such local heterogeneity appears at the output of an inductor when its real transverse pitch deviates from an equilibrium value close to the longitudinal pitch of this inductor, this heterogeneity then increasing on the one hand with the difference from this equilibrium value and on the other hand with the intensity supplying this inductor.

A rectangle mesh is described in the aforementioned French patent application, which however only envisages that the ratio of the transverse pitch to the longitudinal pitch is equal to one, since the mesh proposed is square as previously indicated.

One could imagine realizing, at the price of a very great complexity and a prohibitive cost, an adjustment of the pitch of a square mesh, in the two directions to adapt exactly to all the widths of a family of products to heat.

The object of the invention is to allow homogeneous heating of a product of any width without special edge correction and using an acceptable manufacturing cost device.

The device according to the present invention is characterized in that it comprises first and second said inductors successively longitudinally, and having respectively a first and a second value different from the longitudinal pitch, and consequently a first and a second value d balance of the transverse step.

Each of these inductors is constituted by the juxtaposition of several inductor sections successively transversely in a regular manner according to said transverse pitch, each of these sections extending longitudinally by having its own inductor winding and its own magnetic circuit and by presenting said periodicity according to said longitudinal pitch.

Mechanical adjustment means control the spacing between said inductor sections and consequently said transverse pitch at the same value in these two inductors. This makes it possible to adapt the device to limited variations in the width of the product to be heated thanks to a variation of this spacing making this width equal to an integer number of transverse steps. The edges of this product can then be made to coincide with the edges of two so-called tiles in each inductor, so as to heat the edge areas of this product to the same temperature as its intermediate areas. The common transverse pitch is controllable between said first and second equilibrium values of the transverse pitch.

Electrical adjustment means control the ratio of electrical currents supplied to the two inductors. When a difference between the real transverse pitch and its equilibrium value in each inductor tends to cause a so-called local heating heterogeneity which is specific to this inductor, these adjustment means provide the two inductors with electrical intensities capable of canceling out the local heterogeneity of global heating of the device by compensation for the two heterogeneities specific to the two inductors.

More briefly, it appears that, in order to remain within acceptable costs, a configuration varying only in width is adopted according to the invention, so as to always have an integer number of transverse steps in this width, the device then comprising two inductors whose two longitudinal steps define, with the previous common transverse step, two rectangular meshes. The long side of the rectangle is in the direction of transfer for one of the inductors and in the direction perpendicular to the transfer for the other inductor. It is then possible, by adjusting the ratio of the excitation intensities in the two inductors, to obtain an exact compensation for the heterogeneities created by one inductor by heterogeneities equal in magnitude and opposite in sign and created by the other inductor.

Preferably, the magnetic circuit of each of said wafers comprises at least one longitudinal bar carrying said pole pieces successively longitudinally projecting towards the product to be heated. A winding specific to this bar follows a wavy path passing longitudinally to the right of a first pole piece, then transversely between this piece and a second, then longitudinally to the left of this second, then transversely between this second and a third, and so right now. This allows an easy realization of the inductor edge.

Furthermore, it is advantageous that the device also includes electrical switches to enable the excitation windings of the lateral sections of the two inductors to be connected or disconnected and thus to vary the number of transverse steps to adapt the width of the flow of heating to variations in the width of the product to be heated greater than said limited variations.

More preferably each section has two said bars arranged on either side of the product to be heated.

The present invention also relates to a method of heating flat products by electromagnetic induction on parade, according to which the product to be heated is passed longitudinally in the flow of an inductor having a double periodicity in a longitudinal step and in a transverse step , characterized by the fact that this product is passed through the flow of two successive inductors having a common transverse pitch which is adjustable substantially between the two longitudinal pitches of these two inductors, this transverse pitch is adjusted to make the width of this product with an integer number of transverse steps and thus obtain the same heating on the two edge zones of this product as on the intermediate zones, and we regulate on the one hand the ratio of the intensities feeding these two inductors to homogenize the heating in each not transverse, and on the other hand the total power to reach the desired temperature.

• La figure 1 représente un carreau de maillage rectangulaire de flux magnétique avec son grand côté dans la direction DX du transfert.
• La figure 2 représente l'hétérogénéité de chauffage obtenue dans la largeur du produit defilant dans le carreau de la figure 1, la température atteinte par les divers points de cette largeur étant portée en ordonnées.
• La figure 3 représente un carreau de maillage rectangle avec son grand côté dans une direction transversale DY perpendiculaire à la direction du transfert.
• La figure 4 représente l'hétérogénéité du chauffage obtenue dans la largeur du produit défilant face au carreau de la figure 3.
• La figure 5 représente les maillages des flux de chauffe créés par les inducteurs IL et IC de deux fours qui se suivent dans la direction du transfert, lorsque ces maillages sont tels que les deux fours doivent être utilisés à la même puissance, ce qui implique que les rectangles de maillage dans les deux fours présentent sensiblement le même rapport de longueur à largeur.
• La figure 6 et la figure 7 représentent les maillages des deux mêmes fours après diminution et augmentation du pas transversal de façon à aboutir à un maillage carré dans le premier et le deuxième fours, respectivement, un trait renforcé indiquant le four qui est seul alimenté électriquement et qui présente le maillage carre.
• La figure 8 représente une vue de face de l'inducteur d'un de ces fours, pour montrer le principe de bobinage des enroulements d'inducteur, les pôles magnétiques nord et sud étant désignés par les lettres N et S respectivement.
• Les figures 9 et 10 représentent, en vue de côté, les circuits magnétiques de pas longitudinaux fixes différents du deuxième et du premier indicteurs respectivement.
• La figure 11 représente une vue en perspective d'une portion extrême d'un barreau.
• La figure 12 représente un détail à échelle agrandie de la figure 8 pour montrer la disposition relative des pièces polaires et des enroulements.
• La figure 13 montre une vue en coupe transversale d'une tranche d'inducteur au niveau d'une pièce polaire selon un plan XII de la figure 12.
• La figure 14 représente un schéma de connexion des enroulements excitateurs d'un inducteur, pour montrer que les enroulements de plusieurs tranches latérales peuvent être déconnectés pour s'adapter à la largeur du produit à chauffer.
• La figure 15 représente une vue de côté du dispositif selon l'invention pour montrer le système de transfert du produit à chauffer défilant horizontalement.
• La figure 16 représente une vue en coupe transversale de l'inducteur court du dispositif selon la figure 15 pour montrer le mécanisme de réglage du pas transversal.

The advantages and features of the invention will be better understood on reading the following, illustrated by the appended figures.

• FIG. 1 represents a rectangular mesh of magnetic flux with its long side in the direction DX of the transfer.
• FIG. 2 represents the heating heterogeneity obtained in the width of the product scrolling in the pane of Figure 1, the temperature reached by the various points of this width being plotted on the ordinate.
• Figure 3 shows a rectangle mesh tile with its long side in a transverse direction DY perpendicular to the direction of transfer.
• FIG. 4 represents the heterogeneity of the heating obtained in the width of the scrolling product facing the pane of FIG. 3.
• FIG. 5 represents the meshes of the heating fluxes created by the inductors IL and IC of two ovens which follow one another in the direction of transfer, when these meshes are such that the two ovens must be used at the same power, which implies that the mesh rectangles in the two ovens have substantially the same ratio of length to width.
• FIG. 6 and FIG. 7 represent the meshes of the same two ovens after reduction and increase of the transverse pitch so as to result in a square mesh in the first and the second ovens, respectively, a reinforced line indicating the furnace which is only electrically powered. and which presents the square mesh.
• FIG. 8 represents a front view of the inductor of one of these ovens, to show the principle of winding of the inductor windings, the north and south magnetic poles being designated by the letters N and S respectively.
• Figures 9 and 10 show, in side view, the magnetic circuits of fixed longitudinal pitch different from the second and from the first indictors respectively.
• Figure 11 shows a perspective view of an end portion of a bar.
• Figure 12 shows an enlarged detail of Figure 8 to show the relative arrangement of the pole pieces and the windings.
• FIG. 13 shows a cross-sectional view of an inductor edge at the level of a pole piece according to a plane XII of FIG. 12.
• FIG. 14 represents a connection diagram of the excitation windings of an inductor, to show that the windings of several lateral sections can be disconnected to adapt to the width of the product to be heated.
• FIG. 15 represents a side view of the device according to the invention to show the system for transferring the product to be heated running horizontally.
• FIG. 16 represents a cross-sectional view of the short inductor of the device according to FIG. 15 to show the mechanism for adjusting the transverse pitch.

• si T désigne la température moyenne obtenue, a le petit côté et b le grand côté du rectangle.
• Si conformément à la figure 1, le côté a est disposé selon la largeur du produit (direction transversale DY) et le côté b dans la direction du transfert DX, les maxima de la sinusoïde ou surchauffes sont sur les bords des rectangles et le minima ou sous-chauffes dans l'axe desdits rectangles, comme sur la figure 2.
• Si conformément à la figure 3, a est dispose dans la direction du transfert DX et b selon la largeur du produit, les maxima de la sinusoïde ou surchauffes sont dans l'axe des rectangles et les minima ou sous-chauffes sur les bords des rectangles comme représenté sur la figure 4.

The inventors have established that if the mesh is rectangular instead of being square, the heterogeneity of heating in a transverse step has a sinusoidal distribution whose amplitude DT is given substantially by the expression

• if T denotes the average temperature obtained, has the short side and b the long side of the rectangle.
• If in accordance with Figure 1, side a is arranged along the width of the product (transverse direction DY) and side b in the direction of transfer DX, the maxima of the sinusoid or overheating are on the edges of the rectangles and the minimum or underheats in the axis of said rectangles, as in Figure 2.
• If according to figure 3, a is arranged in the direction of the DX transfer and b according to the width of the product, the maxima of the sinusoid or overheating are in the axis of the rectangles and the minima or underheats on the edges of the rectangles as shown in figure 4.

In the device according to the invention, the basic configuration comprises two ovens with respectively two inductors IL and IC, one of which forms a longitudinal pitch mesh PL and the other a longitudinal pitch mesh PC. These two steps are fixed by construction.

In the transverse direction DY, that is to say in the width of the product, the mesh has a variable pitch PT. This transverse step PT is between the longitudinal steps PL and PC and it must be contained an integer n of times in the width LA of the product LA = n PT.

Or the special case for which PL / PT = PT / PC.

The heating heterogeneities are in this case of the same amplitude in the two ovens if these are also excited, so as to each induce half of the total heating power, but have the configuration of FIG. 2 in the step oven PL, and the configuration of Figure 4 in the step oven PC, so that they compensate each other exactly.

This mesh is shown in Figure 5.

For all cases where the width of the product is between n.PL and n.PC, homogeneous heating can also be obtained. Having arranged as before, an integer n of steps PT in the width of the product, the two ovens are excited differently so as to demand more power from the one which leads to the least heterogeneity for a given intensity, and vice versa. The two borderline cases are shown in Figures 6 and 7.

In Figure 6, the product width is n.PC. One of the ovens then has a square mesh, leading to heating free of heterogeneity. It is therefore asked for full power while the second oven is not in use.

In figure T, the product width is n.PL. The oven not used in the previous case becomes a square mesh and it is now asked for full power, free of heterogeneity, while the other oven is not used.

In practice, in order to limit the sub use in these two extreme cases, therefore having a more favorable dimensioning economically, there is no unused oven, but an underload of that with a rectangular mesh so that the heating heterogeneity remains compatible with the tolerance admitted.

If the width of the product is less than n.PC, k longitudinal sections of the previous mesh are disconnected in the width of the product, k being determined by the smallest foreseeable width of products to be heated, this by opening switches such as 11, 12 (see figure 14).

It is clear that, since only the transverse pitch is variable, one can make all the meshes of the same section inseparable from each other. The windings Ei, Ei + represented in FIG. 8 belong to two successive sections of rank i and i + 1. Their shape is linked to this inseparability. The electrical conductor takes the form of a ripple around the alternating poles N and S of the same row determining three of the four sides CA, CB and CC of a rectangular turn. This same figure shows that the fourth side CE, of the turn is constituted by a conductor of the neighboring section, due to the directions of currents indicated, without preventing the relative displacement of the first section relative to the second to vary the pitch transverse.

It can however be noted that, when the transverse pitch deviates from its minimum value, one does not exactly obtain a perfectly closed turn of rectangular current because two intervals without current remain on the two transverse sides of the rectangle. In particular, when the transverse pitch is equal to the longitudinal pitch, the square current turn is not exactly obtained which would allow perfectly homogeneous heating to be obtained. This is one of the reasons why said equilibrium value of the transverse pitch is not exactly equal to the longitudinal pitch, this equilibrium value being that which in practice makes it possible to get as close as possible to a heater. homogeneous and being determined experimentally.

As an example of an embodiment, it is a matter of heating up to 480 ° C. strips of aluminum 1 mm thick traveling at a speed of 0.33 m / s and whose width is between 0 , 85 and 1.85 m. 800 kW must be induced for the maximum width.

The two ovens are produced one with a longitudinal pitch of 170 mm according to FIG. 9, the other with a longitudinal pitch of 240 mm according to FIG. 10. Each longitudinal row of the mesh is materialized by a section of inductor with a magnetic circuit in the form of a bar. The pole pieces are shown in P. The longitudinally extreme pole pieces PE have a length of half.

In both cases, each bar consists of a bed of magnetic sheets 1, clamped between cheeks 2 and braced by parts 3 and 4, as shown in Figure 11. The winding associated with a bar is shown in Figure 12. The conductor consists of two copper tubes 5, 25 mm outside diameter, and 19 mm inside diameter connected in parallel and describing around the poles the ripple previously explained.

Figure 13 is a cross section of a bar and shows in addition insulating wedges both electrical 6a and thermal 6b.

Figure 14 shows the principle of electrical connection of one of the inductors. The minimum width of 850 mm is covered by five transverse steps of 170 mm. To impose a zero value on the magnetic field on the two edges of the product, two additional slices are excited outside the product, one on each edge. These seven sections, each forming a row of poles, are supplied through a switch to which is only open when the inductor is not in service.

By varying the seven steps thus defined from 170 to 204 mm, the five active steps make it possible to heat all the widths between 850 and 1020 mm.

By adding an eighth row, by closing the switch 13, six active steps can be used to heat all the widths between 1020 and 1190 mm when these steps vary from 170 to 198.33 mm.

The addition of a ninth row (switch 14 closed) allows, with seven active steps to heat all the widths between 1190 and 1360 when these steps vary from 170 to 194.28 mm.

And so on until heating widths that can range from 1700 to 1870 mm by changing 12 transverse steps, including 10 active from 170 to 187 mm.

• 7 pas transversaux variant de 170 à 204 dont
• 5 actifs, largeurs chauffées 850 à 1040 mm 8 pas transversaux variant de 170 à 204 dont
• 6 actifs, largeurs chauffées 1040 à 1224 mm 9 pas transversaux variant de 170 à 204 dont
• 7 actifs, largeurs chauffées 1224 à 1428 mm 10 pas transversaux variant de 170 à 204 dont
• 8 actifs, largeurs chauffées 1428 à 1632 mm 11 pas transversaux variant de 170 à 204 dont
• 9 actifs, largeurs chauffées 1632 à 1836 mm 12 pas transversaux variant de 170 à 204 dont 10 actifs, largeurs chauffées 1836 à 2040 mm

We could also, following the same principle, do the following:

• 7 transverse steps varying from 170 to 204 including
• 5 active, heated widths 850 to 1040 mm 8 transverse steps varying from 170 to 204 including
• 6 active, heated widths 1040 to 1224 mm 9 transverse steps varying from 170 to 204 including
• 7 active, heated widths 1224 to 1428 mm 10 transverse steps varying from 170 to 204 including
• 8 active, heated widths 1428 to 1632 mm 11 transverse steps varying from 170 to 204 including
• 9 active, heated widths 1632 to 1836 mm 12 transverse steps varying from 170 to 204 including 10 active, heated widths 1836 to 2040 mm

This second device has the advantage of making it possible to heat wider widths than the first. However, for the maximum width of 1850 mm considered, the first device, not using for large widths the full variation of the transverse pitch, leads for these large widths to better distribute the total power between the two ovens and to minimize the power installation, without using an additional row.

This example does not limit the method of electrical connection. It is essentially illustrative. In practice, it is advisable to call in certain cases on paralleling. The supply voltages are chosen to obtain the same intensity in all the windings of the same inductor. Said intensity adjustment means RE are incorporated in the source SE.

According to FIG. 15, the system for transferring the product to be heated 7 comprises rollers R1, R2, R3, ensuring horizontal scrolling, the support of the product inside the ovens being provided either by the mechanical tension applied between the inlet and the outlet, or, if the product is not ferromagnetic by the magnetic levitation inseparable from the process (see for example the document FR-A-2 509 562). In the latter case, the rollers R1, R2 and R3 can be omitted, which is advantageous if the product must not be subjected to contact during its treatment. Each oven has two inductors symmetrical with respect to the plane of the product to be heated. The inductors with long pitch are represented in IL and the L, those with short pitch in IC and the C. Adjustable lateral guides G1, G2 ensure the transverse positioning of the product.

FIG. 16 represents the solution adopted in the embodiment to obtain the variation of the transverse pitch. It is the cross section of one of the two ovens and represents only a half width, the other being practically symmetrical. The oven is represented in the configuration corresponding to the minimum value of the transverse pitch.

The inductor bars BC1 to BC13 and B'C1 to B'C13, designed as described above and subject of Figures 9 to 13, are carried by mobile supports 21 to 27a, 21b to 27b, except that the bars 10a and 10b are identical to the others are carried by fixed supports 20a and 20b.

The supports 21a to 27b on the other hand, are mobile and driven by rotation of two screws 31 a and 31b, radially guided by guides 20a and 20b, the thread pitches being 2 mm for the supports 22a and 22b, as well as 21 a and 21 b, 4 mm for supports 23a and 23b, 6 mm for supports 24a and 24b, 8 mm for supports 25a and 25b, 10 mm for supports 26a and 26b and 12 mm for supports 27a and 27b . Guides not shown ensure the parallelism of the supports.

The configuration is practically symmetrical with respect to the fixed supports 20a and 20b, except that the part not shown has 5 pairs of movable bars (including 21a and 21b) instead of 6.

The two screws 31 a and 31 b are controlled by the same member comprising, on the same shaft 41, two bevel gears 51a a and 51 b and an operating wheel 61.

The product to be heated 7 circulates between the bars 11 a to 17a and 11 b to 17b perpendicular to the plane of the figure.

In phantom is shown the extreme position BC1 E of the bar BC1 after rotation of the screws 31 a and 31 b to obtain the maximum value of the transverse pitch.

Claims (5)

1. A device for heating flat moving products by electromagnetic induction, this device comprising:

- a transfer system (R1, R2, R3, G1, G2) for holding the flat product to be heated (7) in a heating plane and conveying it along a longitudinal transfer direction (DX) in this plane, the width of the product being arranged along a transverse direction (DY) also in this plane and its thickness along a direction of the magnetic flux (DZ), each of these three directions being perpendicular to the two others, the transverse position of che product to be heated being adjustable,

- inductor excitation windings (E1, E2, Ei, Ei +

- electric power supply means (SE, lo, 11, 12, 13, 14, 15) for supplying these windings with an electric current periodically variable according to the time and with amplitude control, and for thus making them produce a magnetic flux varying like this current,

- and magnetic inductor circuits (BCi) for channeling this flux and forming a heat flow through the product to be heated practically along the said direction of flux,

- these windings and these magnetic circuits forming an inductor (IC) having a periodical structure both along the longitudinal direction with a longitudinal pitch (PC) and along the transverse direction with a transverse pitch (PT), so that the variations in the, amplitude of the heat flow in the heating plane outline a rectangular meshing consisting of a juxtaposition of rectangular squares of lengths equal to this longitudinal pitch and of widths equal to this transverse pitch, and the magnetic circuit containing in each of these squares at least one central polar piece (P) such that the amplitude of the heat flow is cancelled out on the sides of the square so that the average heating obtained after having conveyed the product to be heated is the same in all the widths of a square comprised entirely in the width of this product, the shape of this piece being further chosen so that this amplitude is maximum in the center of the square with a distribution essentially in sinusoid arch form both in the longitudinal sections and in the transverse sections, and the ratio of the transverse pitch to the longitudinal pitch being selected to cancel out the local heating heterogeneity in each of the squares which are entirely contained in the width (LA) of the product to be heated, this local heterogeneity (DT) being the difference in one direction or the other between the temperature in the center of the width of the square and that at the edges of the square after the product has passed, such a local heterogeneity developing at the output of an inductor when its real transverse pitch diverges from an equilibrium value near the longitudinal pitch of this inductor, this heterogeneity increasing firstly with the divergence from this equilibrium value and secondly with the current supplied to this inductor,

- this device being characterized by the fact that it contains a first (IL) and a second inductor (IC) succeeding each other longitudinally and having respectively a first (PL) and a second longitudinal pitch (PC), which differ from each other and having consequently a first and a second equilibrium value of the transverse pitch,

- each of these inductors consist of the juxtaposition of several inductor sections (BCi, Ei, BCi + 1, Ei + 1) succeeding each other transversally and regularly according to said transverse pitch, each of these sections extending longitudinally and having its own inductor winding (Ei) and its own magnetic circuit (BCi), and offering the repetition rate according to said longitudinal pitch (PC),

- mechanical setting facilities (61, 51a, 31 a) controlling the spacing between said inductor sections (BC1, E1) and consequently said transverse pitch (PT) to the same value in these two inductors (IL, IC) so as to adapt the device to limited variations in the width of the product to be heated through a variation in this spacing making this width (LA) equal to an integer number of transverse pitches and thus making the edges of this products coincide with the edges of the two said squares in each conductor, so as to heat the edge areas of this product to the same temperature as its intermediate areas, this common transverse pitch being controllable between said first and second equilibrium values of the transverse pitch,

- and electrical setting means (RE) controlling the ratio of the electric currents supplied to the two inductors so that, when a difference between the real transverse pitch and its equilibrium value in each conductor tends to produce a said local heterogeneity in the heating specific to this inductor, the two inductors (IL, IC) are supplied with the electrical currents required to cancel the local global heating heterogeneity of the device by compensation between the two heterogeneities specific to the two inductors.

2. A device according to claim 1, characterized in that the magnetic circuit of each of said sections (BCi, Ei) contains at least one longitudinal bar (BCi) carrying said polar pieces (P) succeeding each other longitudinally and projecting towards the product to be heated (7), a winding (Ei) specific to this bar following an undulated path passing longitudinally to the right of a first polar piece, then transversally between this piece and a second one, then longitudinally to the left of this second one, then transversally between this second one and a third one, and so on, so as to provide for an easy production.

3. A device according to claim 1, characterized in that it further comprises electrical switches (11, 12, 13, 14, 15) for connecting or disconnecting the excitation windings (E1, E2, E3, E4, E5) of the lateral sections of the two inductors and thus varying the number of transverse pitches to adapt the width of the heat flow to variations of the width of the product to be heated (7) larger than said limited variations.

4. A device according to claim 2, characterized in that each section preferably contains two of said bars (B1, B'1) installed on either side of the product to be heated (7).

5. A process for heating flat moving products by electromagnetic induction, according to which the product to be heated (7) is conveyed longitudinally in the flux of an inductor (IC) offering a dual repetition scheme according to a longitudinal pitch and a transverse pitch, characterized in that this product is made to move in the flux of two successive inductors (PC, PL) with a common transverse pitch (PT) which is adjustable (31 a) substantially between the two longitudinal pitches (PC, PL) of these two inductors, this transverse pitch is adjusted so that the width of this product coincides with an integer number of transverse pitches, thus obtaining the same heating on the two edge areas of this product as on the intermediate areas, and firstly the ratio of the currents supplied to the two inductors is set to homogenize the heating in each transverse pitch, and secondly the total power is set to reach the temperature required.

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