FR2473244A1 - Pulsed field induction heating for metals - using rotating DC coils or rotating yoke element to vary magnetic circuit reluctance - Google Patents

Abstract

The metal object (1) to be heated has an overall flat form positioned across the poles of two DC coils (3,4) forming a series magnetic circuit via the metal and a 'U' shaped yoke (2). The coil arrangement spins about its centre line (5) over the metal and a pair of fixed low reluctance magnetic material elements. The field in the metal varies as a function of rotation of the coil group. Alternatively the two coils may be fixed, the field variation being due to rotation of an inter-connecting yoke element. The method overcomes the poor power factor achieved in orthodox continuous systems.

Description

 The invention relates to a method and a device for electric heating by induction of metallic products, using pulsed magnetic fields.

 It is known that fixed magnetic fields of variable intensity are commonly used to heat metal products by induction, by disposing, for example, said products inside a Nordic soles traversed by an alternating current. The devices implementing this principle have a bad power factor because they consume a lot of reactive energy and this results in a poor efficiency which becomes unacceptable for high powers.

To avoid these drawbacks, induction heating methods have been proposed based on the principle of displacement of invariable magnetic fields.

Unfortunately, this displacement generates significant forces on the product which must be compensated.

 On the other hand, to obtain good heating efficiency it is necessary that the coils of electromagnets permanently cover the product, which leads to serious difficulties in heating with the same device products of different dimensions.

 The method according to the invention judiciously combines the two aforementioned methods while eliminating the respective drawbacks.

 It essentially consists in placing the product to be heated in fixed magnetic fields of variable intensity, or pulsed fields, generated by invariable magnetic fields. The transformation of the invariable magnetic fields into fixed magnetic fields of variable intensity is obtained by variation in time of the reluctance of the magnetic circuit. This temporal reluctance variation results from the relative displacement of the fields of invariable intensity with respect to a magnetic circuit with topologically variable reluctance.

 One can then indifferently, for the implementation of the method, move the invariable magnetic fields or move part of the magnetic circuit.

 The magnetic circuit has a variable reluctance in space through the use of very different magnetic permeability materials. The material of low permeability can be air, the material of high permeability can be a magnetic metal, possibly laminated.

 The invariable magnetic fields are produced by permanent magnets or by electromagnets supplied with direct current, which allows an excellent power factor and, consequently, a good efficiency.

 The variation in the reluctance of a part of the magnetic circuit makes it possible to vary the amplitude of the fields without them moving, the part of the magnetic circuit which is arranged facing the product being fixed. No significant force is therefore communicated to the product, and, with a fixed magnetic member, it is easier to adapt to different dimensions of the product.

The invention will be described in more detail with reference to the accompanying drawings by way of non-limiting example and in which:
- Figure 1 is a diagram illustrating the implementation of the method according to the invention by moving electromagnets in front of a variable reluctance magnetic circuit;
- Figure 2 is a diagram illustrating the implementation of the method according to the invention by displacement of a variable reluctance part of a magnetic circuit for closing the flux produced by electromagnets;
- Figure 3 shows in elevation a preferred embodiment of a magnetic pad used in a device according to the invention;
- Figure 4 is a side view of the pad according to Figure 3;
- Figures 5 to 7 show, seen from above, the arrangement of a pad according to Figures 3 and 4 with respect to products of different widths;
- Figure 8 is an elevational view of a first embodiment of a device according to the invention, of the type shown in Figure 1;
- Figure 9 is a top view along
IX-IX of the device according to FIG. 8;
- Figure 10 is a top view, very schematic, showing the relative positions of the pads and poles of a device according to Figures 8 and 9 comprising, in each heating section, two poles and two magnetic pads on each side of the product;
- Figure 11 is a view comparable to Figure 10, for a device comprising, in each heating section, four poles and four magnetic pads on each side of the product;
- Figure 12 is an elevational view of a second embodiment of a device according to the invention, of the type shown in Figure 1 ;;
- Figure 13 is a sectional view along
XIII-XIII of a device according to Figure 12;
- Figure 14 is a view identical to Figure 13, on which an integrated electric drive device of the cylinder head has been added;
- Figure 15 shows an elevational view of a third embodiment of a device according to the invention, of the type shown in Figure 2;
- Figure 16 is a top view of the device according to Figure 15;
- Figure 17 shows an elevational view of a fourth embodiment of a device according to the invention, of the type shown in Figure 2;
- Figure 18 is a view along XVIII-XVIII of the device according to Figure 17.

In Figure 1, there is shown a metallic product 1, flat, to be heated by induction.

An inductor device is arranged on one side of this product and is essentially constituted by two coils 3 and 4, traversed in opposite directions by continuous currents so as to form north and south poles respectively, and connected by a cylinder head 2 allowing the magnetic flux to close at their top.

The inducing device therefore produces invariable magnetic fields of opposite directions. It is rotated around a vertical axis 5, perpendicular to a large face of the product 1 to be heated. Fixed studs 6, made of magnetic material with low reluctance, are arranged between the product 1 to be heated and the inductor device constituted by the electromagnets 3 and 4 and the yoke 2. In a first position represented in FIG. 1, the studs 6 are fixed between the product 1 and the lower parts of the electromagnets 3 and 4. In this position, the magnetic flux circulates from the North pole 3 to the South pole 4 through the product 1, using the fixed pads 6 of low reluctance, to close by the yoke 2 as indicated diagrammatically by arrows in FIG. 1. When the inductor device 2, 3, 4, 5, has turned a quarter of a turn relative to the position shown, the coils 3 and 4 are no longer opposite the studs 6, the air gap existing between the electromagnets 3 and 4 and the product 1 then corresponds to the thickness of the studs and is too large for the flux to pass from one pole to the other through product 1. In this second positio n, the flow essentially flows horizontally from one pole to the other, short-circuited, by the studs 6, without passing through the product 1.

 Thus the flux passing through the product goes from a maximum value when the inductor device is in the first position to a substantially zero value when the inductor device is in the second position.

Then if the inductor still turns a quarter of a turn the South pole 4 will have taken the place of the North pole 3 and vice versa, the two poles being again above the pads 6, and the product 1 will be crossed by a flow maximum opposite direction to that passing through it when the inductor is in the first position.

Thus, step by step, during a rotation of the inductor device around the axis 5, the product 1 is subjected to an alternating flow, or pulsed flow, the variation of which generates eddy currents which heat it.

In practice, there is a second inductor device (not shown), identical to the first, symmetrically with respect to the product 1, studs made of magnetic material being arranged between the product and this second inductor device facing the studs 6 shown in FIG. 1. rotate these inductors so that a North pole of one is always placed facing a South pole of the other when they are facing the studs. In this way, when the poles are placed opposite the studs 6, magnetic flux passes from the pole
North 3 of the first inductor device to the South pole of the second by crossing the studs 6 arranged on either side of the product and crossing the product vertically in a first direction, then it closes by the cylinder head of the second inductor, before passing from the pole
North of it at the South pole 4 of the first through the studs 6 crossing the product 1 vertically in a second direction opposite to the first, the flow closing by the cylinder head 2 of the first inducing device When the inducing devices rotate a quarter turn the air gap is too large to allow the flow to pass through the product and the flow then closes, as previously, horizontally from one pole to the other of an inductor device, short-circuited by the corresponding studs.

 During a rotation, the product is thus subjected to a vertical bipolar alternating flow, the variation of which generates eddy currents which heat it.

This flow always retaining the same vertical direction, the product is not subjected to any torque.

The arrangement and the shape of the studs can be chosen so that the flow is always centered on the product, whatever the dimensions thereof.

 In FIG. 2, invariable magnetic fields are produced by fixed coils 3 and 4, supplied in opposite directions by continuous currents so as to define North 3 and South 4 poles. These coils respectively surround fixed polar cores 7. In in this case, only the cylinder head 2 is movable, driven in rotation about the vertical axis 5, perpendicular to a large face of the product 1. When the cylinder head 2 is disposed in the first position shown in FIG. 2, where it runs circulates the cores 7, the magnetic flux passing through the product 1 is maximum. When the cylinder head 2 rotates a quarter of a turn, the flow can no longer close as previously through the cylinder head and is therefore very reduced in the product. After an additional quarter turn of the cylinder head 2, the flow returns to its previous maximum value , with the same sign. Product 1 is therefore subject to a variation in the flux, of zero sequence nature, that is to say that the flux always remains in the same direction, between a maximum value and a minimum value.

As before, this variation in flow generates eddy currents in the product which heat it.

In practice, two inducing devices are arranged as previously symmetrically with respect to the product 1 so as to obtain vertical fluxes, thus eliminating the formation of undesirable couples.

 The devices represented in FIGS. 1 and 2 both allow induction heating of a metal product by producing pulsed magnetic fields, of fixed direction and of variable intensity, by relative displacement of magnetic fields of invariable intensity with respect to a part. of my variable reluctance magnetic circuit they are going through.

In the first case (FIG. 1) invariable magnetic fields are displaced with respect to a variable reluctance magnetic circuit (constituted by the pads 6 and the absence of pads), thus forming a heteropolar machine.

 In the second case (FIG. 2) a part (2) of a magnetic circuit with variable reluctance is displaced with respect to magnetic fields produced by fixed inductor coils, thus forming a machine of zero sequence type.

Figures 3 to 7 show a preferred embodiment of a pad 6 for maintaining good heating efficiency regardless of the width of the product 1 to be heated. Each pad 6 consists of two parts, one of which (8) is fixed and the other (9) movable relative to the previous one. As shown in FIG. 3, in elevation, and in FIG. 4, seen from the side, the upper part 8 of the stud, disposed opposite the inductors (not shown) is the fixed part, the lower part 9, constituting the part d polar expansion of the stud, being mobile around a vertical axis and having an elongated shape.

 Figures 5 to 7 show the various positions that can be taken by the movable part 9 of the pad depending on the width of the product 1 to be heated, so as to cover in all cases the product optimally.

 For a relatively narrow product (FIG. 5), of width 11, the longitudinal axis of the movable part 9 is parallel to the longitudinal axis of the product 1.

For a wide product (Figure 6), of width 13, the longitudinal axis of the movable part 9 is perpendicular to the longitudinal axis of the product 1. Finally, for a product of intermediate width 12 (Figure 7), with li 12 <13, the longitudinal axis of the movable part 9 is inclined relative to the axis of the product 1.

 it is thus possible to best adapt the position of the pad as a function of the width of the product to be heated.

Similarly, in the device shown in FIG. 2, it is possible to adapt the dimensions of the lower part of the magnetic cores 7 to the dimensions of the product to be heated.

 FIGS. 8 and 9 illustrate a first embodiment of a device implementing the method according to the invention by moving invariable magnetic fields so as to form a heteropolar machine of the type shown schematically in FIG. 1.

 In FIG. 8 is shown in elevation a section of a device for continuously heating a flat metallic product 1. it is understood that the number of sections is to be determined as a function of the dimensions of the product and the thermal power required .

The product 1 rests horizontally by its large underside on support and drive rollers 10 which, when driven in rotation make it possible to move the product relative to the induction heating device so as to heat it over any its length. In the case where the product 1 is a steel slab for example, the heating device can be placed between a continuous casting and a rolling mill so as to heat the slab continuously as part of a metallurgical process. The rollers 10 can be coated with a refractory and thermally insulating material so as to reduce the loss of thermal energy by conduction.

 Product 1 is thermally insulated by refractory walls II at the top and 12 at the bottom between the rollers.

 In the embodiment shown, invariable magnetic fields are produced by electromagnets with a vertical axis arranged in drums 13 with a vertical axis, driven by motors 14. In each section of the induction heating device two inducing devices (13 , 14) identical rotary, coaxial, are arranged on either side of the product.

The upper and lower electromagnets can advantageously be driven in the opposite direction and in such a way that poles of opposite polarities face each other when they are facing the studs.

Between the drums 13, on the one hand, and the refractory walls 11 and 12 surrounding the product 1, on the other hand, there are laminated magnetic pads 6.

 Each pad 6 is preferably constituted by a stack of magnetic sheets, arranged vertically to conduct the flow vertically when an electromagnet covers them, and bent along concentric portions of cylinder with the same vertical axis (see Figure 9) to conduct hori the flow between two opposite successive poles of the same.

inductor device when these are between the studs.

 The pads are supported by plates 15 preferably of non-conductive material so that they do not heat up. Figure 9 shows the device according to Figure 8, seen from above along IX-IX.

It highlights the concentric lamination of the pads 6 and a possible arrangement of the pads relative to the product. It can also be seen in this figure that the support and drive rollers 10, supported by bearings 16, are driven by motors 17 by means of couplings 18.

 In FIG. 10 is shown very schematically, seen from above, the relative positions of the studs and the poles of a device according to FIGS. 8 and 9, in which each inducing device comprises a pair of poles 3, 4, two studs 6 rectangular being arranged transversely on each face of the product 1. Three adjacent sections 10A, 10B and 10C of the device are shown with three successive positions taken by the inductive poles during a rotation in the clockwise direction.

In section 10A, the North 3 and South 4 poles are arranged between the pads 6. The flow passes from one pole to the other, horizontally, short-circuited by the pads 6, as shown by the arrows. It is the same for the inductor device, not shown, placed under the product 1. The flow through the product is then zero.

In section 10B the poles 3 and 4 are located just above the pads 6. The flow 4 from the North pole is channeled by the corresponding pad 6 and crosses the product vertically from top to bottom to reach the South pole of the device inductor opposite to it on the other side of the product i, by crossing the stud associated with it. The product is therefore crossed vertically by a maximum flow going from high effbas facing the North Pole 3 and by a maximum flow going from bottom to top facing the South Pole 4.

 In section 10C, the poles 3 and 4 are again between the studs 6, but in a diametrically opposite position relative to that which they occupied in section 10A. The flow through the product is again zero.

 In a fourth position, not shown, the poles cover the pads but are diametrically opposite the position they occupy in section 10B, so that the product is crossed by maximum flows but in the opposite direction.

 The product is therefore crossed, at locations arranged opposite the pads 6, by a flow of fixed, vertical direction, successively taking maximum values in one direction, zero, then maximum in the other direction.

 Figure 11 is comparable to Figure 10, but shows an inductor device comprising two pairs of poles, four pads 6 being arranged on each side of the product in each section (liA, 11B and liC).

As previously we see that the flow through the product is maximum in one direction when the pads 6 are located opposite the poles (section 11A), zero when the pads 6 are between the poles (section 11C, the flow is then horizontal) , then maximum in the other direction (section 11C).

 FIGS. 12 to 14 represent a second embodiment of a device, of heteropolar type, for implementing the method illustrated in FIG. 1.

This second embodiment is more particularly intended for the continuous heating of long products with regular round or polygonal section, square in the embodiment shown. it has already been proposed to heat tubes by rotating electromagnets around their longitudinal axis, but this poses problems due to the appearance of a torsional torque exerted on the product by the rotating electromagnets.

 By having, according to the present invention, magnetic pads around the product the mobile fields of invariable intensity, are transformed into fixed fields of variable intensity and the disadvantage mentioned above disappears.

Figure 12 shows a section of such a heating device, in elevation, partially in section. As shown, the long product 1 of square section is supported, guided and driven by rollers 19, arranged in pairs, alternately horizontally and vertically, between the sections of the induction heating device. The product is thermally insulated by tunnel sections 20 of refractory material. Magnetic pads 6, arranged in blocks 21 which rest on a fixed frame 22, are arranged opposite the various faces of the product 1. The fixed frame 22 also supports electromagnets 24, four in FIG. 13 fixed on the surface inner of a circular cylinder head 25. This is driven in a rotational movement around its main axis which is common with the longitudinal axis of the product and the device.

This rotational movement can be carried out using any known external drive device, for example with gear or belt. In FIG. 13, which is a sectional view along XIII-XIII of the device according to FIG. 12, there is a better distinction between the constitution of the electromagnets, each comprising a coil 24 surrounding a magnetic core 26, and their arrangement opposite screw of the studs 6.

When, as shown in Figure 13, the poles are arranged opposite the pads 6, flows channeled by the magnetic pads pass through the product 1 from the North poles to the South poles. When the poles are between two adjacent studs, the flow is largely short-circuited by the studs.

Consequently, the product is subjected to a substantially fixed directional flow, alternately maximum in one direction, then zero, then maximum in the other direction.

 FIG. 14 is a view identical to FIG. 13, to which an integrated electric drive device for the cylinder head 25 has been added. For this, electromagnets 27 are arranged outside the cylinder head 25 so as to realize a synchronous machine inductor.

This inductor is subjected to rotating fields created by the wound stator 28 of a synchronous motor so as to obtain, on the outside of the induction heating device, a coaxial synchronous drive motor.

 Figures 15 and 16 show a third embodiment of an induction heating device, implementing the method illustrated in Figure 2, in which there is a zero sequence type variation of a fixed flow.

 In this embodiment, seen in elevation in FIG. 15 and seen from above in FIG. 16, the flat metallic product 1 to be heated is supported, guided and driven by rollers 10 placed in the intervals between the sections (a single section being represented).

Refractory walls 11 and 12 thermally insulate the product at its upper and lower parts respectively. Fixed electromagnets 29 are arranged on either side of the product, poles of opposite polarities facing each other so as to force the flux to pass vertically through the product. As shown in Figure 16, four electromagnets are arranged on each side of the product. The magnetic circuit is interrupted to allow interposing between the external parts of the magnets of the movable magnetic members 30, with variable reluctance. Each of these organs is made up of a heterogeneous drum, with a horizontal axis, made up of a succession of parts with low and high reluctance. So when these drums are driven in rotation around their axes, the reluctance of the magnetic circuit varies and the intensity of the flux which crosses the product varies between maximum and minimum values, the flux always remaining in the same direction at a determined location.

 FIG. 16 shows a motor 31 for driving the drum 30.

Figures 17 and 18 show a fourth embodiment, very close to the third mode.

In this case, the variable reluctance drum 30 with horizontal axis is replaced by a plate 32, with variable reluctance, with vertical axis, driven by a motor 33 with vertical axis. In the embodiment shown, the plate 32 is divided by a diameter into two parts of very different reluctances. The same homopolar flux variation effect is thus obtained as with the device in FIGS. 15 and 16.

 It goes without saying that the present invention has been described for purely explanatory and in no way limitative and that any useful modifications in the field of equivalents may be made without departing from its scope.

This is how, in particular:
- that it applies to the heating of metallic products of all kinds and of all dimensions, and more particularly to semi-finished steel products, such as steel slabs, billets and blooms;
- it allows both fixed and continuous heating, the product running past the heating device;
- that the magnetic pads for the passage of fluxes can take different forms depending on the production constraints;
- That the inductors can be as well constituted by permanent magnets as by electromagnets supplied with direct current, and that their number is arbitrary;
- that the movements of the electromagnets, or parts of magnetic circuit with variable reluctance can be either movements of rotation, translation or a combination of such movements; ;
- that the number of inductor assemblies arranged opposite each side of the product to be heated is determined according to the dimensions of the product and the thermal power required.

Claims (12)

1. A method of induction heating of metallic products, characterized in that magnetic fields of fixed direction and of variable intensity are produced by relative displacement of magnetic fields, of invariable intensity, with respect to a topologically reluctant part. variable of the magnetic circuit they pass through, the product to be heated being placed in fixed magnetic fields of variable intensity.  2. Method according to claim 1, characterized in that said displacement consists of a displacement of magnetic fields of invariable intensity opposite a fixed magnetic circuit with topologically variable reluctance. 3. Method according to claim 1, characterized in that said displacement consists of a displacement of a topologically variable reluctance part of a magnetic circuit opposite fixed magnetic fields of invariable intensity. 4. Method according to any one of claims 1 to 3, characterized in that the movement is a rotational movement.  5. Method according to any one of claims 1 to 3, characterized in that the displacement is a translational movement.  6. Method according to any one of claims 1 to 5, characterized in that the product is moved relative to the heating device so as to allow continuous heating. 7. Device for implementing the method according to any one of claims 1 to 6, characterized in that it comprises a device for producing magnetic fields of invariable intensity, a magnetic circuit with variable reluctance, means producing a relative displacement of said fields with respect to the magnetic circuit so as to produce magnetic fields of fixed direction and of variable intensity, and means intended to place the product in the fixed fields of variable intensity.  8. Device according to Claim 7, characterized in that the device for producing magnetic fields of invariable intensity consists of permanent magnets. 9. Device according to claim 7, charac terized in that the device for producing the magnetic fields of invariable intensity is constituted by electromagnets supplied with direct current.  10. Device according to any one of claims 7 to 9, characterized in that the magnetic reluctance variable circuit comprises pads of magnetic material, fixed, arranged between the product and the mobile device for producing magnetic fields of invariable intensity .  11. Device according to claim 10, characterized in that the number and the arrangement of the pads are determined according to the dimensions of the product. 12. Device according to any one of claims 7 to 17, characterized in that the device for producing the magnetic fields of invariable intensity comprises two inductor devices arranged symmetrically on either side of the product.