EP0981931B1 - Induction oven for melting metals - Google Patents

Abstract

PCT No. PCT/FR98/00971 Sec. 371 Date Nov. 12, 1999 Sec. 102(e) Date Nov. 12, 1999 PCT Filed May 15, 1998 PCT Pub. No. WO98/53642 PCT Pub. Date Nov. 26, 1998An induction heating device which raises the temperature of a metal to be heated for one of melting or hot machining while providing considerably energy saving, increasing yield and observing current safety standards. The device (10) uses a cavity (11) to receive the metal to be heated and at least two magnetic yokes (13) arranged around a periphery of cavity (11), each yoke supporting an independent induction coil (14). The induction coils are mounted and wound in the same direction such that a north pole, of each coil, is located on one side of the cavity and a south pole is located on an opposite side of the cavity. The inductive coils are arranged so as to generate active non null magnetic field zones and inactive zones of null magnetic fields distributed about the periphery of the cavity. An inactive zone of null magnetic fields is located between each adjacent active non null magnetic field zone. The induced current is self-enclosed thereby producing high heating power and the invention is applicable to melting, forging, reheating, transforming, and working metals by induction.

Description

The present invention relates to an induction heating device for raising the temperature of metals with a view to their hot melting or transformation, this device comprising at least one cavity defined by a pocket arranged for receive metals intended to be brought to a temperature greater than or equal to their melting temperature or by an oven arranged to receive metal billets intended to be brought to a temperature below their melting temperature, this temperature being determined for forging metals, as well as means of induction heating of said pocket or said oven.

Induction heaters are well known in the art of metal smelting, forging metal billets for hot machining, the transformation or production of metals or alloys. However, in the known devices, the induction coil (s) are wound around the cavity receiving the metal and are usually cooled by a cooling circuit at water. There may be a risk of leaks in the cooling system, which is fully prescribed for working with molten metals. In addition, the yield obtained by this configuration does not generally exceed 40 to 60%. This efficiency is proportional to the ratio of the area of the inductor and the area of tank. In addition, the magnetic field created by the induction coils is a field open. Consequently, the losses are substantial and amount to approximately 1/3 of the total power applied.

• la protection des personnes contre les champs électromagnétiques, déterminée par les normes françaises et les directives européennes (CENELEC et DG5),
• le rendement, et
• la sécurité (il faut éviter absolument tout contact de l'eau avec le métal en fusion).

In this field of application, the main technical constraints to take into account are:

• the protection of people against electromagnetic fields, determined by French standards and European directives (CENELEC and DG5),
• performance, and
• safety (absolutely avoid any contact of water with the molten metal).

Other induction heaters have attempted to provide a solution to the first problem posed. Certain devices are described in publications DE-C-266 566, US-A-1 834 725 and BE-A-351 671 and have at least two cylinder heads arranged around the cavity receiving the metal to be heated and having an L-shape or C in such a way that the ends converge towards the interior of said cavity. Each cylinder head carries an electric coil creating a closing magnetic field through said cavity. An improvement to this type of construction is described in publication DE-C-277 870 which provides for three cylinder heads whose coils are supplied individually and shifted in phase so as to create a field turning. In all these realizations, all the magnetic fields are radial that is, the field lines cross the axis of the cavity and cross it axially right through. These magnetic fields create an induction current limited to the periphery of said cavity and generating a rise in temperature of the metal in this zone, the rest of the metal being heated by conduction. These different devices, and even the one providing for a rotating field, remain very efficient weak, the effective part of the field used for heating being reduced.

The present invention proposes to overcome the drawbacks of the prior art and to meet the requirements of the standards in force by means of a heating device by induction which achieves yields close to 80 to 95%, with smaller induction coils, with a higher power factor (cosine ϕ of 0.8 instead of 0.05 or 0.1) and requiring less consumption of electrical energy. In addition, the present invention makes it possible to accelerate the rise in temperature therefore the metal smelting or hot machining, thus also saving money energy. The energy savings obtained by the present invention allow to consider a return on investment in about two years, which is commercially very appreciable.

This object is achieved by a device as defined in the preamble and characterized in that the induction coils are mounted in the same direction so that their north pole is located on one side of the cavity and their south pole on the opposite side, in that they are arranged to generate zones of zero magnetic field arranged alternately between areas of non-zero field distributed around the periphery of the cavity, the non-zero field areas each having a maximum field area associated with two zones with decreasing field gradient arranged on either side of said maximum field area and extending to neighboring zero field areas, as well as a zero field zone located in the center of this cavity, the field zones non-zero forming active heating zones separated by said field zones null forming inactive zones.

Each cylinder head advantageously comprises an elongated branch extending from end to other end of the cavity, arranged substantially parallel to the axis of this cavity and carrying at least one induction coil arranged to generate a said active heating zones.

In a first embodiment of the invention, each cylinder head has a L-shaped profile and has said elongated branch and a side branch extending substantially perpendicular to said elongated branch and substantially radially with respect to one end of the cavity.

Said cavity may be a pocket, said lateral branch extending radially by compared to the bottom of this pocket towards its center.

In a second embodiment of the invention, each cylinder head has a U-shaped profile and has said elongated central branch and two branches lateral extending substantially perpendicular to said central branch elongated and substantially radially with respect to the two ends of the cavity.

In this variant, the cavity is preferably an oven and at least one of said lateral branches extends to near the longitudinal wall delimiting said cavity.

In a third embodiment, each cylinder head has a shaped profile of C and comprises said elongated central branch and two lateral branches extending substantially perpendicular to said elongated central branch and substantially radially with respect to the two ends of the cavity.

In this variant, at least one of said lateral branches extends as close as possible of the side wall delimiting said cavity.

Said cavity can be a pocket, one of the lateral branches extending radially relative to the bottom of this pocket and the other side branch being a free section attached to a cover intended to close said pocket and extending radially by relative to this cover until near the side wall delimiting said cavity.

In a fourth embodiment, the cylinder head has an I-shape and comprises said elongated branch and two lateral branches extending substantially perpendicular to said elongated branch and substantially radially with respect to at both ends of the cavity.

In this variant, at least one of said lateral branches extends radially up to the proximity of the side wall delimiting said cavity.

Preferably, each of said coils extends substantially over the entire length of the elongated branch of each cylinder head.

The heating means advantageously comprise a number n of cylinder heads evenly distributed around the periphery of the cavity.

Depending on the nature of the heating that one seeks to achieve, the coils can be supplied individually by an alternating electric current, this supply being shifted in phase from one coil to another, this shift in the supply of a reel to another which can be determined by an arithmetic progression.

The different coils can also be powered by several generators arranged to create a rotating field.

• la figure 1 est une vue en coupe axiale d'un dispositif selon l'invention destiné à la fusion du métal,
• la figure 2 est une vue de dessus en coupe suivant les flèches II-II du four de la figure 1,
• les figures 3 et 4 sont des vues en perspective du dispositif de la figure 1 respectivement vue de dessus et vue de dessous,
• la figure 5 est une vue en coupe longitudinale d'un dispositif selon l'invention destiné au forgeage du métal,
• les figures 6A et 6B sont des schémas représentant les lignes du champ magnétique respectivement du dispositif de l'invention et d'un dispositif classique,
• les figures 7A et 7B représentent la cavité en vue de dessus et schématiquement la circulation des courants induits respectivement du dispositif de l'invention et d'un dispositif classique, et
• les figures 8A et 8B sont des schémas représentant la répartition de la puissance de chauffage respectivement du dispositif de l'invention et d'un dispositif classique.

The present invention and its advantages will appear better in the following description of two exemplary embodiments, with reference to the appended drawings, in which:

• FIG. 1 is a view in axial section of a device according to the invention intended for melting metal,
• FIG. 2 is a top view in section along the arrows II-II of the furnace of FIG. 1,
• FIGS. 3 and 4 are perspective views of the device of FIG. 1 respectively seen from above and seen from below,
• FIG. 5 is a view in longitudinal section of a device according to the invention intended for forging metal,
• FIGS. 6A and 6B are diagrams representing the lines of the magnetic field respectively of the device of the invention and of a conventional device,
• FIGS. 7A and 7B show the cavity in top view and schematically the circulation of the currents induced respectively from the device of the invention and from a conventional device, and
• FIGS. 8A and 8B are diagrams showing the distribution of the heating power respectively of the device of the invention and of a conventional device.

Referring to Figures 1 to 4, the induction heating device 10 comprises a cavity 11 formed by a pocket 12 generally made of a material refractory and intended to receive the metal to be melted, as well as heating means by induction arranged to raise the temperature of the metal by a magnetic flux until it merges.

These heating means include independent magnetic yokes 13 arranged around the pocket 12 at a distance d sufficient to allow the setting places induction coils 14. Each cylinder head 13 has a general L shape comprising an elongated branch 13a substantially parallel to the axis of the pocket 12 and extending substantially over the entire height of said pocket 12 and a branch lateral 13b perpendicular to the elongated branch 13a and extending radially in direction of the bottom of said pocket 12. The ends of the branches 13a and 13b are curved to get as close as possible to the wall of the pocket 12.

Depending on the case, the cylinder heads 13 can have a C or I profile or even include that said elongated branch 13a. In all cases, the elongated branch 13a magnetic yokes 13 extends substantially over the entire height of the pocket 12 and the lateral branch is oriented radially and preferably extends up to proximity to the wall of the pocket 12. In certain cases, the lateral branches may constitute a free section arranged perpendicularly to the branch elongated by folding, for example near the bottom of the pocket or by folding close to the upper edge of the pocket if these branches are attached under a cover to close the pocket. In other cases, the side branches can extend to the center of the bottom of the pocket. Finally, these side branches can be profiled in such a way that they partially cover the bottom surface from the pocket. In all cases, the two lateral branches 13b of the same cylinder head 13 do not both extend to the center of pocket 12, one at less should stop near the edge of this pocket.

These magnetic yokes 13 are number n equal to eight (in FIGS. 1 and 2) and to six (in Figures 3 and 4) and are regularly arranged at equal distance one of the other all around pocket 12. This number is not limitative It may as well be lower than higher, even or odd, depending on the type of pocket and its notebook loads: capacity in tonnes of metal, heating power, etc.

The assembly described above and formed by the pocket 12, the magnetic yokes 13 and the induction coils 14, is housed in a tank 15 for protection and insulation may be provided with a cover or a door (not shown), this tank being tilting mount on a chassis or a stem not shown around an axis articulation 16 passing through two ears 17 secured to said tank 15. In the process of melting, this tank 15 can be hermetically sealed or not and can be placed under air vacuum to optimize the functioning of the induction heating means. After the metal has melted, the cover or the door opens, the tank 15 swings around of its articulation 16 to empty the pocket 12 of its molten metal content in molds for example and in the same way as in art devices prior.

Each induction coil 14 is arranged around the elongated branch 13a of each magnetic yoke 13 and extends substantially over its entire length. These induction coils 14 are supplied individually by an alternating current and generate a magnetic flux, the field lines of which are shown in the figure 6A. Thanks to the magnetic yokes 13, this magnetic flux is channeled, directed and closed in an inner peripheral zone of the pocket 12 near said cylinder head, through the metal to be heated. Only a small part of the flow goes to outside. The losses are then low. Figure 6B illustrates the field lines of a device of the prior art which is not equipped with a magnetic yoke and allows visualize very clearly the improvement in the concentration of the field lines around of the pocket 12 obtained using the device of the invention with reference to FIG. 6A.

In addition, in the device of the present invention, the coils14 are all oriented in the same direction, their north pole being placed on one side of the cylinder heads and their south pole on the other side. Thus the poles of the same nature repel each other by repelling their respective magnetic fields, thus creating zones of zero magnetic field 40 alternating with non-zero field areas 41, shown schematically on the Figure 7A. Therefore, the nonzero field areas are centered on radial planes 42 passing through the cylinder head axes 13 and extending on either side of a zone of maximum field, near the periphery of the cavity 11. These non-field fields null 41 thus include a central zone of maximum field and two zones with decreasing field gradient arranged on either side of the field area maximum up to the neighboring zero field areas 40. The field lines are arranged on either side symmetrically with respect to said radial planes 42 passing through the center of the cavity 11 and passing through the axis of the cylinder heads 13. Thus, the null field zones 40 delimit active zones 41, constituted by the zones of maximum field and the field areas with decreasing field gradient, corresponding metal heating zones. Therefore, unlike the devices of the prior art in which the heating zones 51 extend over the periphery of the ends of the cavity 11 as shown in FIG. 7B, the active heating zones 41 are delimited on defined angular portions of the periphery of said cavity 11. In in other words, in each active zone 41, the magnetic field induces a current 43 generating a heating power, this current being forced to close on it even forming a loop in this active area, whereas in the devices of the art anterior, the induced current 53 extends all around the periphery of the cavity. We know, moreover, that the induced current generates a heating power directly proportional to the volume of metal traversed by said current. Therefore, the fact that the currents induced 43 by the coils 14 are located in said zones active 41 significantly increases the volume of metal traversed by all of the induced currents, in comparison with the volume of metal traversed by the current induced on the periphery. The result is an increase in the volume of metal heated for the same induced current and therefore much better performance.

FIGS. 8A and 8B make it possible to compare the distribution of the power of heating between the devices of the prior art and that of the invention, the zones white representing the highest heating power, this power going decreasing in darker areas. At these different power levels of heating of course correspond to different temperature levels. These figures are illustrations of real tests carried out for the same induced current therefore the same magnetic field generated by each coil. In Figure 8B, which illustrates prior art, the white areas correspond to the heating areas 51 and are limited to the periphery of the ends of the cavity, the interior being completely dark. In FIG. 8A, which illustrates the present invention, the white areas are distributed on the periphery of the cavity and over its entire length. There are several areas white distributed on the periphery of the cavity, extending over its entire height and extending slightly inward. These white areas correspond to the active heating zones 41 delimited from each other by said zones of zero field 40. It is then easy to see that the entire surface covered by the white areas in Figure 8A is much larger than that in Figure 8B. This increase in surface area therefore has a direct impact on the yield of the induction heating which can reach 80 to 95%.

On the other hand, the distance d which separates the elongated branch 13a from the magnetic yoke 13 supporting the induction coil 14 of the pocket 12 can be relatively large to increase the thickness of the refractory walls of the pocket 12 and limit heat losses. Furthermore, induction coils 14 of smaller diameters, weaker powers and stronger power factors than those of art can be used. Therefore, the Joules effect losses are also limited and the induction coils 14 do not need to be cooled by specific water circulation. Air ventilation is sufficient to ensure their cooling.

Under the effect of the active heating zones 41, the temperature of the metal rises more quickly in certain areas causing displacement or mixing automatic between hot and cold metal masses, so that at their their temperature also rises until a homogeneous molten mixture is obtained.

This brewing is greatly improved and accelerated by individually feeding the induction coils 14 with an offset of the supply from one coil to the next and so on, clockwise or vice versa. This shift in feeding phase generates inside the pocket 12 a circumferential mixing and helical metal. The direct consequence of this form of mixing is a faster homogenization of the temperature gradient in the metal allowing significantly shorten the time required for softening and melting resulting in significant energy savings. This forced mixing can be also obtained by supplying each coil with an independent generator. All the generators can then be synchronized so as to obtain a rotating field creating the effect of a helix in the molten metal.

FIG. 5 illustrates an alternative embodiment of the invention in which the device 30 comprises a cavity 31 formed by an oven 32 generally produced in a refractory material and intended to receive metal billets 35 to be machined hot as well as induction heating means 33, 34 arranged to raise the temperature said billets at a temperature below their melting temperature, by a flow magnetic. These heating means include, as in the previous example independent magnetic yokes 33 arranged longitudinally around the oven 32 and at a distance d sufficient to accommodate induction coils 34. Each cylinder head 33 has an elongated central branch 33a and at least one branch lateral 33b, 33c perpendicular to the elongated central branch 33a. Branch elongated central unit 33a of the magnetic yokes 33 extends substantially over the entire length of the oven 32 and the two end branches 33b and 33c extend radially up to near the furnace 32. In the example shown, the cylinder heads 33 have a general U-shape. Each induction coil 34 is arranged around the elongated central branch 33a of each magnetic yoke 33 and extends substantially over its entire length.

The number n of magnetic yokes 33 and induction coils 34, their functionality and their advantages are identical to those described above. Of even, it is also possible to optimize the homogenization of the gradient of temperature inside and throughout the oven 32 by supplying the coils induction 34 with a phase shift from one coil to the next or by synchronized independent generators.

It is clear from this description that the invention achieves the intended purposes. His essential advantage is of course the energy savings that this device induction heating allows to achieve while respecting safety standards in force. Therefore, even if this device requires more overall investment high compared to a conventional and known device, the energy savings achieved allow to consider a return on investment over approximately two years.

The present invention is not limited to the embodiments described but extends any modification and variant obvious to a person skilled in the art. As precised, the number of magnetic yokes and induction coils is not limited. Of even, the shape of the cylinder heads can vary depending on the pocket or the oven. The cylinder heads can also be formed from several free sections. The management of the supply of the coils can also be delayed.

Claims (16)

1. Induction heating device (10, 30) to raise the temperature of metals with a view to melting or hot working them, this device comprising at least one cavity (11, 31) defined by a ladle (12) designed to receive metals to be brought up to a temperature greater than or equal to their melting point or by an oven (32) designed to receive billets of metal to be heated at a temperature which is lower than their melting point, this temperature being determined to forge the metals, as well as induction heating means (13, 14, 33, 34) for said ladle (12) or said oven (32), said induction heating means comprising at least two magnetic yokes (13, 33) arranged around this cavity (11, 31), extending over the height of the cavity (11, 31), each bearing at least one independent induction coil (14, 34), characterized in that the induction coils (14, 34) are fitted in the same direction so that their north pole is located on one side of the cavity (11, 31) and their south pole on the opposite side, in that they are arranged so as to generate null magnetic field zones (40) arranged alternately between non null field zones (41) spread out on the periphery of the cavity, the non null field zones each comprising a maximum field zone associated with two decreasing field gradient zones arranged on either side of said maximum field zone, extending as far as the neighbouring null field zones (40), as well as a null field zone (40) located in the center of this cavity, the non null field zones forming active heating zones separated by said null field zones forming inactive zones.
2. Device according to claim 1, characterized in that each yoke (13, 33) comprises an elongated branch (13a, 33a) extending from one end of the cavity to the other, which is arranged substantially parallel to the axis of this cavity (11, 31) and bearing at least one induction coil (14, 34) designed to generate one of said active heating zones (41).
3. Device according to claim 2, characterized in that each yoke (13) shows an L-shaped profile and comprises said elongated branch (13a) and a lateral branch (13b) extending substantially perpendicular to said elongated branch (13a) and substantially radially in relation to an end of the cavity (11).
4. Device according to claim 3, characterized in that said cavity (11) is a ladle (12), and in that said lateral branch (13b) extends radially in relation to the bottom of this ladle in the direction of its center.
5. Device according to claim 2, characterized in that each yoke (33) shows a U-shaped profile and comprises said central elongated branch (33a) and two lateral branches (33b, 33 c) extending substantially perpendicular to said central elongated branch (33 a) and substantially radially in relation to the two ends of the cavity (31).
6. Device according to claim 5, characterized in that the cavity (31) is an oven (32) and in that at least one of said lateral branches (33b, 33c) extends as far as the vicinity of the longitudinal wall delimiting said cavity (31).
7. Device according to claim 2, characterized in that each yoke shows a C-shaped profile and comprises said central elongated branch and two lateral branches extending substantially perpendicular to said central elongated branch and substantially radially in relation to the two ends of the cavity (11).
8. Device according to claim 7, characterized in that at least one of said lateral branches extends as far as the vicinity of the lateral wall delimiting said cavity (11).
9. Device according to claim 8, characterized in that said cavity (11) is a ladle (12), in that one of the lateral branches extends radially in relation to the bottom of this ladle and in that the other lateral branch is a free section directly attached to a cover designed to close said ladle and extending radially in relation to this cover as far as the vicinity of the lateral wall delimiting said cavity (11).
10. Device according to claim 2, characterized in that each yoke is I-shaped and comprises said elongated branch and two lateral branches extending substantially perpendicular to said elongated branch and substantially radially in relation to the two ends of the cavity.
11. Device according to claim 10, characterized in that at least one of said lateral branches extends radially as far as the vicinity of the lateral wall delimiting said cavity.
12. Device according to claim 2, characterized in that each of said coils (14, 34) extends substantially over the whole length of the elongated branch (13a, 33a) of each yoke (13, 33).
13. Device according to claim 1, characterized in that the heating means comprise a number n of yokes (13, 33) regularly spread out on the periphery of the cavity (11, 31).
14. Device according to claim 13, characterized in that the coils (14, 34) are fed individually by an alternating electric current, this power supply being phase-shifted from one coil to another.
15. Device according to claim 14, characterized in that the power supply shift from one coil to another is determined by an arithmetical progression.
16. Device according to claim 13, characterized in that the various coils (14, 34) are fed by several generators designed to create a rotary field.

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