EP1155596B1 - Electromagnetic stirring of a melting metal - Google Patents

Abstract

An installation for processing by induction a metallic material is disclosed. The installation includes a first winding including, in series, at least one first coil of at least one turn and at least one second coil of at least one turn, wound in opposite directions around a container, the first winding having two end terminals connected to an A.C. supply source and across at first capacitor, and at least one second winding including, in series, at least one first coil of at least one turn and at least one second coil of at least one turn, wound in opposite directions around the container by being imbricated in the first winding, wherein the ends of the second winding being connected across a second capacitor (C').

Description

The present invention relates to installations for induction treatment of molten metal materials, like for example the brewing facilities, organization movements, and / or preparation of metallic materials. The invention relates more particularly to such installations using an inductive crucible and among these, the installations in which the crucible is intended, not only to organize the induction mixing of a molten metal, but also to heat this metal by induction. The invention applies to such inductive crucible installations, whether whether it is a continuous casting plant or not.

The present invention will be described below in relationship with an inductive cold crucible, but note that it more generally applies to any other installation in which of the molten metal is in a magnetic field. Among the inductive crucible installations, the use of a crucible cold is often preferred to a crucible made of refractory material when the metallic material to be produced requires a high degree of purity, insofar as a cold crucible minimizes pollution of the treated material.

Figure 1 shows, schematically, an installation classic of making a metallic material by induction from an inductive cold crucible.

Such an installation comprises a cooled crucible 1, for example, by circulation of water inside its wall, and intended to contain the material to be melted. In Figure 1, the constituent details of the crucible walls have not been shown for the sake of clarity. In particular, the means of circulation coolant in the thickness of the walls have not been illustrated.

An inductor, usually a coil 2, surrounds the crucible 1 and is connected, by its two ends, to the terminals of a single-phase alternating voltage generator 3. A capacitor C is connected in parallel on generator 3, i.e. across coil 2.

The metallic material to be melted in the crucible is introduced therein, for example, in the form of chips m. In the case of a continuous casting crucible, these chips m are introduced continuously, usually through a weir 4.

The magnetic field caused by the inductor 2 heats the material contained in the crucible 1. The frequency of the current alternator of excitation of inductor 2 depends, in particular, on diameter of crucible 1 and the electrical resistivity of material contained therein. How perfectly known, the electrical resistivity of the material and the frequency of excitation of the inductor 2 condition the skin thickness electromagnetic (δ). The desired skin thickness depends on the applications. For example, in the case of an ingot mold, the desired skin thickness is as small as possible while being sufficient not to harm the thermal efficiency in because of the cold walls of the crucible. For exemple particular, for a crucible with a diameter of around 10 centimeters, we generally want to have a thickness of electromagnetic skin on the order of 1 to 10 millimeters.

Note that, although this has not been shown in FIG. 1, a cold crucible is a sectored crucible, that is to say provided with vertical sectors isolated from each other to avoid looping of induced currents around the perimeter of the crucible. As illustrated in Figure 1, in a crucible fed by a single-phase generator, the free surface of the metal bath is present in the form of a liquid dome whose profile results the balance between hydrostatic pressure and pressure electromagnetic from the single-phase inductor 2. In addition, on this free surface there is an electromagnetic stirring force which is a force with a radial component always centrifugal on the surface of the cane bath illustrates the arrows in figure 1. Thus, the chips m coming from the supply system 4 and falling by gravity into the bath are systematically, at their arrival, trained from the center to the periphery, so towards cold walls of the crucible 1.

Such traffic is detrimental to performance thermal system and ingestion of m chips in the bath. This can even affect the surface condition of the solidified ingot obtained by continuous drawing in the lower part (not shown) of crucible 1.

It would be desirable to reverse the direction of mixing by surface of the bath so that the chips of the material to be melted are dragged towards the center of the bath, and thus improve the mixture. To do this, we can think of implementing, around the crucible, a system of polyphase inductors creating a magnetic field sliding, giving rise to an electromagnetic force ascending in the thickness of the electromagnetic skin, so in periphery of the crucible. This force then results in raise the liquid metal of the bath at the periphery in the thickness electromagnetic skin, and by conservation of the flow, to bring the liquid metal down to the center of the bath. We obtain thus a centripetal radial electromagnetic stirring force, which is conducive to the training of food shavings, their arrival on the bath, no longer towards the periphery, but at opposite towards the center of the bath where they are immediately ingested and melted.

However, the implementation of such a principle poses several problems which have led, so far, to this solution is not, in practice, industrially viable.

A first problem is related to the need to have a polyphase induction generator, therefore two generators voltage out of phase with each other.

For an installation with a crucible of a diameter sufficiently high (around thirty centimeters) allowing it to be powered by a working generator on the frequency of the AC grid (50 or 60 hertz), it is necessary to have a polyphase power supply (di or three-phase) to implement this principle inversion of bathing direction. Thus, this prohibits the simple connection of the installation to a power supply Single phase.

A similar problem arises for others containers for example the relatively diameter pipe small of an electromagnetic pump.

The problem is increased for induction generators medium frequency (around ten kilohertz) for which it is necessary to have an electronic circuit power to make the generator (3, figure 1) so to supply inductor 2 with a current of different frequency distribution frequency.

In such applications, which relate more particularly crucibles or small diameter containers it would then necessary to have an electronic power circuit for each phase, which greatly increases the cost of installation. In particular, this requires the multiplication of the number of power switches as a function of the number of phases. Of more, the synchronization of the generators with the phase of the feeding alternative provided by the distribution network is made all the more harder than the frequency of the induction generator and the number of phases are high.

Examples of induction treatment plants of a metallic material are described in the patents N ° 1 330 133 and N ° 1 986 353. These installations do not do not allow a reversal of the bread brewing direction.

The present invention aims to overcome the drawbacks conventional inductive crucible installations. The invention aims, in particular, to propose a new installation allowing to organize the stirring of the molten metal bath, to will, in a centripetal or in a centrifugal sense.

More generally, the invention aims to propose a new solution to induction brewing problems in containers of molten metal.

The present invention also aims to provide a new solution for performing a multiphase generation which is economically viable. The invention aims, in particular, to propose a solution that does not require multiplication power switches for applications requiring medium frequency induction.

The present invention also aims to provide a solution that can be fed from a source single phase electric.

The invention further aims to propose a solution which no problem of synchronization of the different phases between them.

un premier enroulement comprenant, en série, au moins un premier bobinage d'au moins une spire et au moins un deuxième bobinage d'au moins une spire, enroulés dans des sens opposés autour du contenant, le premier enroulement ayant deux bornes d'extrémités destinées à être raccordées à une source d'alimentation alternative et aux bornes d'un premier condensateur ; et

au moins un deuxième enroulement comprenant, en série, au moins un premier bobinage d'au moins une spire et au moins un deuxième bobinage d'au moins une spire, enroulés dans des sens opposés autour du contenant en étant imbriqués dans le premier enroulement, les extrémités du deuxième enroulement étant destinées à être reliées aux bornes d'un deuxième condensateur.

To achieve these objects, the present invention provides an installation for the induction treatment of a metallic material in a container, comprising:

a first winding comprising, in series, at least a first winding of at least one turn and at least a second winding of at least one turn, wound in opposite directions around the container, the first winding having two end terminals intended to be connected to an alternative power source and to the terminals of a first capacitor; and

at least a second winding comprising, in series, at least a first winding of at least one turn and at least a second winding of at least one turn, wound in opposite directions around the container while being nested in the first winding, the ends of the second winding being intended to be connected to the terminals of a second capacitor.

According to an embodiment of the present invention, the capacitor values are a function of the frequency of the generator and the desired skin thickness inside the container.

According to an embodiment of the present invention, applied to an induction heating installation in a inductive crucible forming said container, the combined inductors of the two windings are a function of the heating intensity inside the crucible.

According to an embodiment of the present invention, the installation also comprises at least a third winding whose terminals are connected to a third capacitor, the third winding being formed of at least two windings associated in opposition series.

la figure 1 décrite précédemment est destinée à exposer l'état de la technique et le problème posé ;

la figure 2 représente, de façon schématique et en coupe, un mode de réalisation d'une installation de brassage par induction selon la présente invention ;

la figure 3 illustre schématiquement le circuit électrique de l'installation de la figure 2 ; et

la figure 4 est une vue partielle en perspective d'un creuset froid inductif selon la présente invention en cours de fonctionnement.

These objects, characteristics and advantages, as well as others of the present invention will be explained in detail in the following description of particular embodiments given without limitation in relation to the attached figures, among which:

Figure 1 described above is intended to expose the state of the art and the problem posed;

Figure 2 shows, schematically and in section, an embodiment of an induction stirring installation according to the present invention;

Figure 3 schematically illustrates the electrical circuit of the installation of Figure 2; and

Figure 4 is a partial perspective view of an inductive cold crucible according to the present invention during operation.

The same elements have been designated by the same references to the different figures. For reasons of clarity, only elements of an induction heating installation which are necessary for the understanding of the invention have been shown in the figures and will be described later. In particular, the means for removing the liquid metal (for example, the ingot drawing means) have not been shown and do not are not the subject of the present invention. In this regard, we note that the invention applies equally well to a crucible (cold or refractory) of an ingot mold than a crucible intended to be emptied by overturning. More generally, it will be noted that the invention can be implemented in any installation using an inductive means around a metal container in fusion, for the purpose of organizing metal movements. he could be, for example, electromagnetic brewers (in which the metal is heated by induction, by arc, at using a plasma torch, or others), pumps electromagnetic, and more generally of any installation in which there are problems related to the diameters of the container (crucible, pipe, etc.)

A feature of the present invention is provide, around a container of molten metal, at least two windings each consisting of two windings in series opposite, only one of the windings being connected to the terminals of a single-phase generator. The other winding (s) constitute induced or secondary windings by being closed by means of a capacitor.

FIG. 2 schematically represents a mode production of an induction heating installation, for example continuous casting, according to the present invention. In the example of Figure 2, the container is a crucible cold.

As before, the installation is based on the use of a cold crucible 1 sectorized, that is to say comprising several cooled vertical sectors, for example, by circulation of water and which are assembled to each other to form a tubular structure. In Figure 2, the crucible has been shown in section and without showing the means conventional cooling for clarity.

A first winding 5 is surrounded around the crucible 1 and is connected by its two ends to the terminals of a single-phase alternating generator 3 in parallel with a capacitor C. According to the present invention, the winding 5 is consisting of at least two coils 51 and 52 associated in series-opposition, i.e. wrapped in opposite directions around crucible 1. Still according to the invention, at least a second winding 6 is also wound around crucible 1 and is connected at both ends to the terminals of a capacitor VS'. This second winding 6 comprises, like the first winding 5, at least two coils 61, 63 associated in series opposition.

In addition, windings 5 and 6 are nested one in the other, that is to say that the coils are arranged successively in the height of the crucible so as to alternate a coil from the first winding with a coil from the second winding.

So, in the example of a two-phase system such as shown in Figure 2, we find, from the top of the crucible 1, the first winding 61 of the winding 6, the first winding 51 of winding 5, the second winding 62 of the winding 6 and the second winding 52 of the winding 5.

According to the present invention, the second winding 6 plays the role of an induced circuit whose energy comes from the first winding 5.

Figure 3 shows the electrical circuit of the installation of figure 2. This figure shows the elements described in connection with FIG. 2 and illustrated by a view in perspective, the direction of the coils of windings 5 and 6 in opposition series. Note that, in Figure 2, the direction of traffic current in the respective coils has been indicated by the usual notations (x,.) in electromagnetism.

Winding 5 forms, with capacitor C, a first oscillating circuit connected to generator 3 and constituting a first phase of excitation of the polyphase system. The second winding 6, spatially offset from the first winding 5 forms, with the capacitor C ', a second oscillating circuit. This second oscillating circuit is finds in magnetic interaction through its mutual inductance with the first oscillating circuit. We can then, by dimensioning, ensuring that the magnetic field resulting from the superposition of the two phases in the presence of a sliding field likely to generate an electromagnetic driving force of pumping on the induced metal contained in crucible 1.

The respective dimensions of the windings and capacitors depend on the application and, in particular, the frequency of the generator 3, the diameter of the crucible 1, and the desired skin thickness in the metal. Preferably, the number of turns of the coils of the same winding is identical.

Optimization of the system according to the application is within the reach of those skilled in the art by implementing the electrical and electromagnetic operating rules to from the respective inductors, the respective resistances and respective capacitors of the oscillating circuits, as well than the mutual inductance of these two circuits and the pulsation of the single-phase generator.

To obtain a linear sliding field effect allowing the pumping effect at the periphery of the container, it will preferably be sought that the products LCω ² and L'C'ω ² , where L and L 'represent the respective inductances of the windings 5 and 6 and where ω represents the pulsation of the single-phase generator 3, are as close as possible to the unit in order to optimize the operation of the oscillating circuits.

An advantage of the present invention is that it allows to reverse the direction of mixing of the bath as illustrated by the arrows of Figure 2, by means of a single-phase generator. So, in the case of a low frequency corresponding to the frequency of the alternative power network (50 or 60 hertz), it is not now no longer necessary to have a polyphase power supply and an installation according to the invention can be connected directly on a single-phase connection to the distribution network. In the case of an installation that requires generation medium frequency, the present invention has the advantage of require only one single-phase generator, which reduces considerably the cost of installation by reducing the number of power switches required.

Another advantage of the present invention is that the synchronization of the induced phase (phase obtained by the winding secondary), or induced phases in the case where several secondary windings are used, no problem particular.

Another advantage of the present invention is that the system is particularly stable once adapted to the application. In fact, unlike the use of several generators separate to obtain an induction heating system polyphase, the elements (inductors and capacitors) used by the present invention to generate the additional phase or phases are not likely to go out of whack as it could be the case of active elements (high power switches).

Figure 4 illustrates, in a perspective view in section, the schematic structure of a cold inductive crucible according to the present invention. This figure shows the sectors s of the crucible 1 which are electrically isolated from each other. In the example of Figure 4, each coil 61, 51, 62, 52 has four turns.

The representation in Figure 3 illustrates that the number number of molten metal stirring wheels depends on the number of sectors from the crucible. So, not only the reversal of the sense of stirring by means of a polyphase system according to the present invention promotes the inclusion of particles in the center of the bath, but, in this application, mixing is also favored by the sectorized structure of the crucible which improves mixing.

Brewing speeds depend on the intensity of the currents i1 and i2, therefore of the intensity of the current delivered by the generator 3.

Note that, according to the present invention, it is not no need to have a 90 ° phase difference between the two circuits oscillating. A phase shift of the order of 20 to 40 ° is sufficient in terms of efficiency for the brewing operated by the system of the invention.

Note also that the phase angle between the two oscillating circuits is adjustable by the respective values of capacitors and inductors used. However, like this has been indicated previously, this phase angle is stable a times fixed by the dimensions of these elements.

In practice, when the application concerns a heating by induction, we will preferably start by fixing the values required for the respective inductances of the windings. These values condition the heating of the metal of the bath. However, according to the invention, the existence of the induced phase which also takes part in the heating.

Then set the respective values of the capacitors C and C 'as a function of the frequency of the single-phase generator and the desired skin thickness, which depends on the diameter of the crucible 1. It will be noted that the respective relationships between the inductances windings and capacitors C and C 'must be compatible with the output impedance of the single-phase generator 3.

As a particular embodiment, for a crucible with a diameter of around ten centimeters and for a single-phase generator having an operating frequency around twenty kHz, we can use capacitors with values on the order of 20µF with windings whose respective own inductances are around 2µH and whose resistances are around one thirty mΩ. In such an example, we get a phase shift of around 40 ° between the currents i1 and i2 of the respective windings, and a ratio of the amplitudes of the currents of the order of 1.1.

Of course, the present invention is capable of various variants and modifications which will appear to the man of art. In particular, although the invention has been described above in connection with a two-phase system, it can also be implemented with more than two phases. In this regard, we note that the greater the number of phases, the more the system is controllable, for example, to stir the metal in fusion on a higher height. Adaptation of the system described above at a larger number of phases is at the reach of the skilled person. We will however take care to respect the interweaving of the different windings in the height of the crucible as well as the associations in series opposition of the windings constituting the various windings.

In addition, the choice of the number of turns per coil, the number of coils per winding and arrangement of turns is within the reach of the skilled person from the indications data above. In particular, the section of the turns will depend of course the intensity of the currents, and the arrangement in the height of the crucible will depend on the height of the crucible and the number of coils. For example, referring to the embodiment described above in relation to Figure 4, the level liquid metal medium will be chosen to correspond approximately in the middle of the height of the first winding 51 of the first winding 5. The increase in the number of coils of the same winding increases (by cumulative effect due to the increase interaction height) the pumping force, so brewing efficiency.

Claims (4)

1. An installation for processing by induction a metallic material in a container, characterized in that it includes:

   a first winding (5) including, in series, at least one first coil (51) of at least one turn and at least one second coil (52) of at least one turn, wound in opposite directions around the container (1), the first winding having two end terminals intended for being connected to an A.C. supply source (3) and across a first capacitor (C); and

   at least one second winding (6) including, in series, at least one first coil (61) of at least one turn and at least one second coil (62) of at least one turn, wound in opposite directions around the container by being imbricated in the first winding (5), the ends of the second winding (6) being intended for being connected across a second capacitor (C').
2. The installation of claim 1, characterized in that the capacitances (C, C') depend on the frequency of the A.C. supply source (3) and on the desired skin depth inside the container (1).
3. The installation of claim 1 or 2, of heating by induction in an inductive crucible forming said container, characterized in that the combined inductances of the two windings (5, 6) are a function of the heating intensity desired inside the crucible (1).
4. The installation of any of claims 1 to 3, characterized in that it further includes at least one third winding, the terminals of which are connected to a third capacitor, the third winding being formed of at least two coils associated in series-opposition.

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