WO2000035248A1

WO2000035248A1 - Device for induction heating and method for controlling the same

Info

Publication number: WO2000035248A1
Application number: PCT/SE1999/002201
Authority: WO
Inventors: Göran Langstedt; Tord Cedell
Original assignee: Linlan Induction Ab
Priority date: 1998-11-26
Filing date: 1999-11-26
Publication date: 2000-06-15
Also published as: SE9804059D0; SE9804059L; SE513131C2; AU2014000A

Abstract

A device for induction heating of a workpiece (4) has two separate bodies (1). Each body (1) comprises at least one pole (6) and a coil (7) arranged round each pole (6). The device further has a voltage source (8) connected to said coils (7) for generating a magnetic field in and round the poles (6). The bodies (1) are arranged on each side of the workpiece (4) with opposite poles (6). The device also comprises a control unit (9) which is adapted to provide a variable phase difference between the currents flowing through the coils (7) of the opposite poles (6), thereby varying the penetration depth of the generated magnetic field in the workpiece (4) or a tool (3) enclosing the workpiece (4).

Description

DEVICE FOR INDUCTION HEATING AND METHOD FOR CONTROLLING THE SAME

The present invention relates generally to treatment of materials by electromagnetic heating thereof. More specifically, the invention relates to an induction heating device and a method for controlling the same. The invention also concerns a press provided with such a device and use of such a device.

When manufacturing products wholly or partially of plastic or composite material, use is made of a press, whose press tool must be heated. This normally takes place by removing the press tool from the press, placing it in a preheating device, such as a furnace, and after heating again mounting it in the press. A preheating device of this type is disclosed in e.g. US-A-5 , 023 , 419. The handling of the press tool is very time-consuming, which means that the number of finished products per hour will be small .

With a view to reducing the time consumed, it is known to heat the tool in situ in the press . This can be carried out, for example, by providing the press with hot plates, by means of which conduction heat is transferred to the tool. This process, however, is still far too time-consuming for economy in large-scale manufacture of components which are wholly or partially made of composite material . The above problem has been solved by means of an induction heating device as disclosed in WO 97/26776. This device is intended for heating a workpiece in a press and consists of a core of electric sheet steel enclosing the workpiece, a coil arrangement arranged round the core and a voltage source. The voltage source is connected to the coil arrangement to generate a magnetic field in and round the core. The core has two poles which between them receive a press tool, which encloses the workpiece to be treated. The poles are displaceable relative to each other to apply a press force to and conduct the generated magnetic field into the tool.

This device enables rapid heating of the tool/work- piece, but still has a number of drawbacks. The core and opposite poles of the device must in terms of size be adjusted to the tool which is being treated. Therefore the construction yields a fairly poor geometric flexibility in that it is difficult to use one and the same device to treat tools of different size. It has also been found that there is a risk of local overheating of the tool/workpiece . One more disadvantage arises when the penetration depth of the magnetic field in the tool/ workpiece has to be very small, such as when treating thin tools/workpieces. It is true that the penetration depth decreases with an increase of the frequency of the applied voltage, but an increased frequency at the same time causes undesirably great losses in the core, i.e. a low degree of efficiency of the heating device. An object of the present invention is to obviate the above problems, i.e. to provide a more uniform and more controlled heating. It is also an object to provide induction heating with a high degree of efficiency independently of the thickness of the tool/workpiece. It is also desirable to be able to provide different temperatures in different parts of the tool/workpiece in one and the same process cycle. A further object is to increase the geometric flexibility.

According to the invention, these and other objects that will appear from the following specification are now wholly or partially achieved by means of a heating device and a press according to appended claims 1 and 22, respectively. Preferred embodiments are defined in the dependent claims. These objects are also achieved wholly or partially by methods according to appended claims 25-27. The invention is applicable to heat treatment of workpieces enclosed in a press tool. A non- limiting example of such workpieces is raw materials which wholly or partially consist of plastic or composite material . The invention is also applicable to direct heat treatment of workpieces, such as components comprising both metallic and polymeric materials, for the purpose of achieving a separation of these materials.

According to the invention, there is a variable phase difference between the currents flowing through the coils of the opposite poles, which preferably are arranged essentially in front of each other. The difference in phase between the opposite poles controls the penetration depth of the generated magnetic field in the workpiece. For instance, the penetration depth is at its maximum when two opposite poles are driven in opposition, i.e. with a phase difference of 180 degrees, and at its minimum when the phase difference is 0 degrees. Thus, the penetration depth can be varied with a maintained high degree of efficiency. Moreover, the invention can be used to heat a workpiece of an uneven thickness since each pair of opposite poles can be controlled individually to generate a magnetic field having a desired penetration depth. The device can thus be used to achieve a uniform temperature distribution in the workpiece independently of its form. Alternatively, different temperatures can be provided in different parts of the workpiece in one and the same process cycle.

According to a preferred embodiment, each pole on the respective bodies has an essentially constant distance to the nearest poles, below referred to as adjoining poles or neighbouring poles. By the distance between two poles is meant the minimum distance between the outer circumferences of the poles. Since the strength of the generated magnetic field decreases in proportion to the square of the distance from the respective poles, a constant pole distance within each body results in a more uniform heating of the tool/workpiece arranged under the body.

It is preferred for the poles to be placed in a two- dimensional pattern on each body since this results in a more uniform distribution of the generated magnetic field over the tool/workpiece and, thus, a more uniform heating thereof. When using the inventive device in a press, a more uniform distribution of the press forces over the tool/workpiece is also obtained. According to a preferred embodiment, the coils of each body are connected to the voltage source in such manner that the currents in the coils of poles neighbouring each other are phase-shifted relative to each other. This means that the magnetic flux will alternatingly be directed away from and to each pole. Seen over a longer period of time, the magnetic flux will be essentially homogeneously distributed over the tool/workpiece, which makes it possible to achieve good uniformity in heating. Moreover, a uniform load on the voltage source can be obtained.

It is further preferred that the poles connected to the respective phases have the same total cross-sectional area since this results in a uniform load on the voltage source and good uniformity in heating. According to one more preferred embodiment, the body of the device is composed of a number of separate units, which are designed to abut against the tool/workpiece. Since each unit has no core in the common sense of the word, i.e. a core enclosing the tool/workpiece, several units can be arranged essentially arbitrarily over the tool and independently of its thickness. This gives a great geometric flexibility since the number of units can be selected and positioned so that heating of the desired parts of the tool is achieved. It is preferred for the pattern of poles within each unit to be two-dimensional and symmetrical since this results in uniform distribution of both the magnetic field and any press forces over the tool.

To reduce the risk of local overheating, the cross- section of the poles is preferably formed without sharp or right-angled edges.

The inventive method comprises at least one of the steps of : a) determining the frequency at which a maximum active power transfer occurs from the device to the workpiece or the tool to be heated; b) tuning a resonance circuit so that a maximum active power transfer occurs to the workpiece/tool ; c) locking the phase of the voltage supplied to the respective coils to provide uniform heating in a plane extending parallel to the poles of the device; and d) controlling, by pulse width modulation, the duty cycle of the voltage supplied to the respective coils, so that the desired power transfer is obtained.

Each of the above steps gives in the preferred embo- diment the possibility of influence both the power transferred to the tool and the uniformity in heating.

If the device comprises a plurality of heating units distributed over the tool/workpiece, each unit is preferably individually controlled for optimum control of the heating, i.e. the temperature in the tool/workpiece.

The above method also enables experimental recording of a suitable period of time for current and voltage supply to achieve the desired heating in a large number of different applications. Thus, a number of different "heating schedules" can be recorded, which can then be run regularly.

The invention will now be described for the purpose of exemplification with reference to the accompanying drawings, which schematically and for the purpose of exemplification illustrate currently preferred embodiments . Fig. 1 is a sectional view of an inventive device for induction heating.

Fig. 2 is an exploded view of a heating unit included in an inventive device. Figs 3a-3f are top plan views of different embodiments of a body included in the heating device and comprising a plurality of units whose outer contour and pole pattern differ between the different embodiments. For the sake of clarity, only the poles and base plates of the units are shown.

Fig. 4 is a top plan view of three heating units which are arranged side by side and each have a centrally arranged pole of circular cross-section. For the sake of clarity, only the poles and base plates of units are shown.

Fig. 5a is a diagram of the time variation of the phases in a three-phase system, and Figs 5b-5c illustrate schematically the magnetic flux between the poles in a unit at two different points of time. Fig. 6 is a sectional view of two units which are arranged on each side of a workpiece . Description of Preferred Embodiments

Fig. 1 shows a device for induction heating of a tool according to an embodiment of the invention. The device has two separate bodies 1 each composed of a plurality of separate heating units or modules 2. The bodies 1 are placed on each side of a tool 3, which contains a workpiece 4 to be heated.

Each heating unit or module 2 comprises in the shown embodiment a disc-shaped base plate 5, three poles 6 and a coil 7 arranged round each pole 6. The poles 6 of the device are fixedly connected to the base plate 5 and are arranged so as to point at each other and abut against the tool 3. Each coil 7 is connected to a voltage source 8 and a control unit 9. For the sake of clarity, only the connection of two poles to the voltage source 8 and the control unit 9 is shown, which are adapted to generate via the coils 7 a magnetic field in and round each pole 6. The magnetic field heats the tool 3 and the workpiece 4, as will be described in more detail below. The inventive device can be mounted in, for example, a press so that opposite press forces are applied to the sides of the bodies 1 facing away from the tool 3.

Fig. 2 shows in more detail a module 2 included in a heating device according to the invention. For the sake of clarity, the parts of the module 2 are separated from each other. The disc-shaped base plate 5 of the module 2 consists of a bottom part 5' and a top part 5". In che embodiment shown, the bottom part 5' has milled grooves 10, in which a cooling medium can pass while flowing from an inlet tube 11 to an outlet tube 12. The cooling medium is adapted to carry off heat which is possibly conducted into the module 2 in the heating of the tool 3. The cooling medium can be water, air or some other fluid suitable for the purpose. Three poles 6 projecting perpendicularly from the base plate 5 are formed on the top part 5" of the base plate 5. A coil 7 is arranged round each pole 6. The coils 7 are via lines 13 connected to the voltage source 8 and the control unit 9 (not shown) .

In the embodiment shown in Fig. 2, the cross-section of the poles 6 is essentially circular. The cross-section of the poles 6 can alternatively be elliptic or polygonal, e.g. hexagonal. However, it is preferred for the cross-section of the poles 6 to have no sharp or right- angled corners since the generated magnetic field tends to be concentrated in such corners and cause local over- heating of subjacent portions of the workpiece 4. The poles 6 are arranged in a two-dimensional pattern on the base plate 5 of the module 2 to provide as uniform heating as possible of the workpiece 4. For the same reason, the poles 6 are arranged in a constant spaced-apart rela- tionship on the base plate 5. Moreover, the cross-sectional area of the poles 6 is maximised so that as large a contact surface as possible is provided to conduce the generated magnetic field into the tool. For good controllability of the temperature distribution in the workpiece 4, the poles 6 should cover a total of at least about 20% of the surface of the base plate 5. The modules are preferably so designed that two identical, juxtaposed modules 2 have an essentially constant distance between each pole 6 and its neighbouring poles 6 both within a module 2 and between adjoining modules 2. It is preferred for the base plate 5 of the module 2 to have such an outer contour that a portion of a module 2 can be brought into plane, direct abutment against a corresponding portion of another, identical module 2, so that these modules 2 without any mutual spacing cover a portion of the tool 3. Examples of par- ticularly preferred embodiments are given in the top plan views in Figs 3a-3f. In Figs 3a and 3b, each base plate 5 has an outer contour in the form of a triangle. More specifically, the outer contour of the base plate 5 is equilateral in Fig. 3a and isosceles in Fig. 3b. The outer contour of the base plate 5 can alternatively have portions which essentially have the form of an arc tan curve, as illustrated in Fig. 3c. In this case, the outer contour consists of two mutually symmetrical arc tan portions and two rectilinear portions, which at right angles connect the two arc tan portions with one another. This construction allows modules to abut against each other without any mutual spacing and also enables both an optimal positioning of the poles and a maximising of the cross-sectional surface of the poles. In fact it is desirable to avoid strong magnetic fields in the joint between two juxtaposed modules 2 since this may cause great losses. Thus, each pole 6 should have a maximum distance to the outer contour of the base plate 5 while at the same time the poles 6 should cover as much as pos- sible of the surface of the base plate 5. A good compromise between these contradictory requirements is achieved by means of the module shown in Fig. 3c. For reasons of manufacture, it is, however, preferred for the outer contour of the base plate 5 to be made up of rectilinear portions. An example of a "rectilinear" arc tan curve is given in Fig. 3d. Further examples are given in Figs 3e and 3f, where the outer contour of the base plate 5 has the form of a regular, i.e. equilateral, hexagon or a "honeycomb structure" .

A particularly preferred embodiment of the module is shown in Fig. 4, which illustrates three such juxtaposed modules 2. Each base plate 5 has the form of a regular hexagon and has only one centrally arranged pole 6 which is circular in cross-section. This outer contour and arrangement of the poles are easy to accomplish and guarantee that each pole 6 has a constant distance to adjoining poles 6 when the modules 2 abut against each other. Moreover, in this embodiment it is possible to provide both an adequate distance from the pole 6 to the outer contour of the base plate 5 and good coverage of the surface of the base plate 5. Below follows a description of the control of a heating device according to the invention. It will first be described how the magnetic field changes over time within a module with three poles. Then the interaction between two such opposite modules will be discussed. Finally, a method for automatic control of the heating will be described.

Each module is individually controlled in the preferred embodiment . This enables local control of the magnetic field that is being generated and, thus, heating of different parts of the workpiece or tool to different degrees. In the embodiment according to Fig. 2 each coil 7 is preferably connected to an associated phase of a voltage source 8 with three phases. The poles 6 abut against the tool 3 and thus form a magnetically closed circuit when the coils 7 are supplied with a voltage. The magnetic field which is generated when a current flows through the coils 7 causes different losses in the tool 3, which result in the tool 3 being heated. The loss mechanisms which cause the heating are hysteresis loss, eddy current loss and anomalous loss, also called micro- eddy current loss. Control of the phases makes it pos- sible to affect the magnetic field and, thus, the heating, both along the surface and in depth, as will be described below.

Fig. 5a shows how the three phases of the voltage source, which are designated R, S and T, vary over time, and Figs 5b and 5c illustrate the time variation of the magnetic flux between the poles of a module with three poles, as is illustrated in e.g. Figs 3a-3f, or an assembly of three modules abutting against each other and each having one pole, as shown in e.g. Fig. 4. Each pole 6 is via a coil connected to an associated phase of the voltage source 8. At the point of time ti (Fig. 5a) the R phase has its maximum current intensity, and the magnetic flux flows from the R-phase-connected pole R, in equal parts, to the poles S, T that are connected to the S and T phases, respectively, of the voltage source. To facilitate understanding, a vectorial description of the flux is conceivable, where two vectors of the same value point from the R pole at the S and T pole, respectively. The resulting vector will then be directed straight downwards in the Figure. At a later point of time t₂ (Fig. 5a) , the T phase has its minimum current intensity, and the magnetic flux flows in equal parts from the R and S poles of the modules to the T pole. The resulting vector will at this point of time instead be directed obliquely down- wards to the right in the Figure. Thus, the resulting flux vector has been turned between the points of time ti and t₂. This discussion can be continued in the same way and leads to the conclusion that the resulting flux vector has turned counterclockwise through a full revolution when a full period has been passed and the R phase is once more at its maximum. This results in a distribution of the flux between the different poles R, S, T which is very uniform over time and, thus, also a very uniform heating along the surface of the workpiece. If also a number of modules are placed quite close to each other, a synergistic effect is achieved between the poles 6 on different modules 2 and not only within the individual modules, especially when each pole 6 has a constant distance to and is connected to another phase than the neighbouring poles 6, both within and between the modules . In a further, not shown embodiment of the invention, the module has two poles, one central pole and one concentrically arranged, annular pole. The poles are surrounded by at least one coil each. In this concentric arrangement, the coil of the annular pole is supplied with a voltage which is phase- inverted relative to the voltage supplied to the coil of the central pole. Thus, the magnetic flux moves back and forth in the radial direction between the two poles so as to provide uniform heating . In all cases described above, the poles associated with the respective phases have essentially the same cross-sectional area since this results in uniform load on the voltage source and good uniformity in heating. By placing modules on each side of the tool/work- piece, the heating thereof can be controlled very accurately since a magnetic field can be generated and conducted through the tool/workpiece. Fig. 6 shows schematically a first module 2a, which with its poles 6 is arranged on a first side of a workpiece 4 and a second module 2b which is arranged on a second, opposite side of the workpiece . Thus the poles are facing each other and aligned in pairs. Each module is individually controllable .

First assume, as indicated in Fig. 6, that the modules are arranged so that the R pole of the first module 2a is positioned in front of the R pole of the second module 2b, that the S pole of the first module 2a is positioned in front of the S pole of the second module 2b, and that the T pole of the first module 2a is positioned in front of the T pole of the second module 2b. If the two modules 2a, 2b are driven by the same voltage, two opposite magnetic fluxes are generated. This results in the fluxes deflecting before having time to penetrate far into the workpiece 4. Consequently, only surface heating of the workpiece 4 is provided. If instead the voltage supply to the second module 2b is phase- inverted by 180 degrees compared with the first module 2a, the opposite condition is achieved, viz. that the magnetic flux is driven straight through the workpiece 4, which causes uniform heating of the entire workpiece 4, not only its surface. By a variable phase difference existing between the modules 2a, 2b, which is between 0 and 180°, different penetration depth in the workpiece 4 can thus be achieved. This can be performed, for example, by a simple phase locking loop.

The heating of the workpiece 4 can also be control - led by physically turning the opposed modules 2a, 2b relative to each other, for example so that the poles opposing each other are R-S, S-T and T-R, respectively. The poles need not necessarily be positioned exactly in front of each other, but can also be partially displaced in the lateral direction relative to each other.

In the currently preferred embodiment of the invention, the heating is automatically controlled. The tool 3 is equipped with a number of temperature sensors, which are of a type known per se and whose output signals func- tion as input signals of the control unit 9 which controls the voltage supply to the heating modules.

The control unit 9 can be described as a circuit which can be affected on at least one of essentially three levels. On the outermost level, where the slowest control is performed, the supplied voltage is controlled by a simple type of regulator, for example an on/off regulator or a P/D regulator. By changing, depending on the measured temperature, the duty cycle of the voltage supply, i.e. by pulse width modulation controlling the length of the periods with voltage supply to the module, the heating can be controlled. The energy supplied to the tool in fact depends on, inter alia, the length of the periods with voltage supply to the coils.

On the next lower level there are components for controlling the active power transfer to the tool . This results in more rapid control than the one that can be carried out by means of the temperature sensors. These components comprise a resonance circuit, which comprises a capacitor having a variable capacitance or alternatively an inductor having a variable inductance. The active or real power transfer to the tool is calculated by measuring current, voltage and phase angle. Subsequently, the capacitance of the capacitor or alternatively the inductance of the inductor is controlled until the power factor { cosφ) , and thus also the active power transfer to the tool, reaches its maximum. In ideal terms, this means that the driving stage is only affected by a resistive load, i.e. cosφ = 1.

On the third and lowest level, there are components which, if possible, control the frequency until electric resonance has been obtained. This means that the capaci- tor is tuned with the coil and that the power factor is at its maximum. Consequently, also the active power transfer to the tool is maximised, and the reactive (or "useless") effect is minimised. This means that the heating of the tool is maximised. This is the quickest and most basic control method.

Only a few possible embodiments of the invention have been described above. A person skilled in the art can, on the basis of the description, provide a number of variants which are suitable for the application in question and which are within the scope of protection of the appended claims. For example, the design of the control unit may be varied to achieve the maximum power transfer to the tool . Also other pole patterns and base plates than those described above can be provided. For instance, each base plate can have more than three poles. Also in this case, it is preferred for neighbouring poles to be connected to different phases of the voltage source. Moreover the poles should be arranged in a rota- tionally symmetrical pattern on the base plate, so that a turning of the module through 120 degrees maximum transfers the pole pattern to itself. In the case of six poles on the base plate and three phases of the voltage source, the pattern should be rotationally symmetrical for a turning of the module through 60 degrees.

Although it is preferred for the units to have an outer contour which allows the units to abut directly against each other without any spacing, the units may have an arbitrary outer contour. In this case, the units are suitably arranged in and mutually fixed by a frame structure to form said body.

It should also be pointed out that the bodies of the inventive device need not be composed of modules but can be made in one piece. Also in this case, a certain degree of geometric flexibility is achieved since the device is not limited to a certain thickness of the tool.

Preferably the poles of the modules are made of a material other than electric sheet steel since it may be difficult and costly to arrange poles in electric sheet steel in a suitable pattern and with a suitable cross- section, i.e. without sharp or right-angled corners. The poles are preferably made by compacting a metal powder, preferably of pure iron or an iron alloy containing silicon or nickel, and a binder, such as epoxy or phenolic resin. This method of production is per se known in other fields, see e.g. US-A-2 , 937 , 964. Before compaction, each metal powder particle is preferably coated with a thin surface layer of an insulating material, such as an oxide. Thus the metal powder particles are isolated from each other. When the powder is compacted at high pres- sure, a material forms, which has magnetic properties that are convenient in the context and a strong structure that can be worked to the desired shape by machining. According to a preferred embodiment, the base place and the poles are made in one piece of this material . Alternatively, the poles can be manufactured separately and then fixed to the base plate by gluing, bolt joincs or some other equivalent technique. The base plate can be made of this material, but also other materials are con- ceivable, for instance electric sheet steel. The base plate can also be composed of a number of separate pares.

Claims

1. A device for induction heating of a workpiece (4), said device having two separate bodies (1) each comprising at least one pole (6) and a coil (7) arranged round each pole (6) , and a voltage source (8) connected to said coils (7) for generating a magnetic field in and round the poles (6) , the bodies (1) being arranged on each side of the workpiece (4) with opposite poles (6) , c h a r a c t e r i s e d by a control unit (9) which is adapted to provide a variable phase difference between the currents flowing through the coils (7) of said opposite poles (6) .

2. A device as claimed in claim 1, wherein each body (1) has at least two poles (6) , and each pole (6) of the respective bodies (1) has an essentially constant distance to the nearest pole or poles (6) .

3. A device as claimed in claim 1 or 2 , wherein the poles (6) are arranged in a two-dimensional pattern on each body (1) .

4. A device as claimed in any one of the preceding claims, comprising a temperature sensor for measuring the temperature of the workpiece (4) , wherein the output sig- nal of the temperature sensor is an input signal of the control unit (9) .

5. A device as claimed in any one of the preceding claims, wherein poles (6) neighbouring each other on the respective bodies (1) are connected to different phases of the voltage source (8) .

6. A device as claimed in claim 5, wherein poles (6) are allocated to the voltage source (8) so that the poles

(6) for the respective phases have the same total cross- sectional area.

7. A device as claimed in any one of the preceding claims, wherein the poles (6) are made by compaction of a metal powder and a binder.

8. A device as claimed in any one of the preceding claims, wherein cooling coils (10) are arranged in at least one body (1) to be passed by a cooling medium.

9. A device as claimed in any one of the preceding claims, wherein at least one of said bodies (1) is composed of at least two separate heating units (2) each comprising a base plate (5) , at least one pole (6) and a coil (7) arranged round each pole (6) .

10. A device as claimed in claim 9, wherein each heating unit (2) is individually controllable in respect of the phase of the current through the associated coil (7) .

11. A device as claimed in claim 9 or 10, wherein each heating unit (2) has at least two poles (6) , which are arranged in a two-dimensional pattern on the base plate (5) .

12. A device as claimed in claim 11, wherein said pattern is such that a turning of the unit (2) through an angle of 120 degrees maximum essentially transfers the pattern to itself.

13. A device as claimed in claim 11 or 12, wherein said pattern has the shape of a regular polygon.

14. A device as claimed in claim 11 or 12, wherein the poles of each heating unit (2) are arranged in a con- centric pattern on the base plate (5) .

15. A device as claimed in any one of claims 9-14, wherein the outer contour of the unit (2) has the form of a triangle.

16. A device as claimed in claim 15, wherein said triangle is equilateral.

17. A device as claimed in claim 15, wherein said triangle is isosceles.

18. A device as claimed in any one of claims 9-14, wherein the outer contour of the unit (2) has the shape of a hexagon, preferably a honeycomb structure.

19. A device as claimed in any one of the preceding claims, wherein the poles (6) in cross-section have no sharp or right-angled corners.

20. A device as claimed in any one of the preceding claims, wherein the poles (6) are essentially elliptic in cross-section.

21. A device as claimed in any one of the preceding claims, wherein the poles (6) are essentially circular in cross-section.

22. A press for manufacturing products wholly or partially of plastic or composite material, comprising a heating device as claimed in any one of claims 1-21.

23. Use of a heating device as claimed in any one of claims 1-21 for heat treatment of components comprising both metallic and polymeric materials for the purpose of separating the metallic and polymeric materials from each other .

24. Use of a heating device as claimed in any one of claims 1-21 for heating a press tool which is mounted in a press for manufacturing products wholly or partially of plastic or composite material.

25. A method for controlling a device as claimed in any one of claims 1-21, comprising at least one of the steps of : a) determining the frequency at which a maximum active power transfer occurs from the device to the workpiece (4 ) ; b) tuning a resonance circuit so that a maximum active power transfer occurs to the workpiece (4); c) locking the phase of the voltage supplied to the respective coils (7) for providing uniform heating in a geometric plane extending parallel to the poles (6) of the device; and d) controlling, by pulse width modulation, the duty cycle of the voltage supplied to each coil (7) so that the desired power transfer is obtained.

26. A method for controlling a device as claimed in any one of claims 9-21, wherein each heating unit (2) is controlled individually.

27. A method for controlling a device as claimed in any one of claims 1-21, wherein a phase difference is provided between opposite poles (6) to control the magnetic flux between the opposite poles (6) .