High power density induction heater

Abstract

An inductor unit for inductive heat treatment of a metal workpiece, comprising at least one coil unit (110) and at least one electrically conductive element (120) having a generative side (120a) and an active side (120b). The active side (120b) is configured to face the at least one metal workpiece (20) to be heated, and has a smaller cross-sectional surface area than the generative side (120a). The inductor unit also comprises at least one soft magnetic element (130). The at least one coil unit (110) is configured to induce currents in the electrically conductive element (120), and the at least one soft magnetic element (130) is arranged at least partly on the at least one electrically conductive element (120) such that the induced current is directed from the generative side (120a) to the active side (120b) of the electrically conductive element (120) and concentrated therein.

Description

[0001] HIGH POWER DENSITY INDUCTION HEATER

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to an inductor unit for heat treatment of a metal workpiece. The invention also relates to an induction heating system including the inductor unit, as well as a method of providing such a system. Furthermore, the present invention relates to a use of the inductor unit for heat treatment of a metal workpiece.

[0004] BACKGROUND

[0005] Industrial induction heating has been around for about a hundred years, with the vast majority of applications focused on heat treatment of metal objects. Nevertheless, the coils or inductors looks more or less the same today as they did in its early days. The concept is a coil made of copper tubing with one or a few turns, conducting large currents, typically in the order of kilo Amperes, and cooled with water inside the tube. The setup is robust and relatively cheap but suffers from several limitations. The bottlenecks are poor efficiency, often around 50%, limitations in geometrical complexity and thereby temperature uniformity for complex geometries. Furthermore, the high currents limit the size of the coil, and thereby the heated area, as well as the length of the extensions / terminals of the coil, meaning that a heavy and bulky workhead, typically containing a transformer and resonance capacitors, needs to be located close to the inductor.

[0006] Soft magnetic material is used in some applications to improve the efficiency to a certain extent, and also to improve the heating pattern. Modern additive manufacturing, often referred to as three-dimensional (3D) printing technologies for copper alloys have opened new opportunities in terms of geometrical complexity and inductor lifetime, which is particularly good for small geometries. Adding more turns of the coil reduces the current demand and thereby enables longer extensions to be used. However, the cooling flow is a limiting factor. Litz wires can be used to increase the efficiency of the inductor to more than 95%, commonly found in for example cook-tops, but with limited use for heat treatment applications due to its limited temperature resistance. Thus, there is room for improvements. SUMMARY OF THE INVENTION

[0007] An object of the present invention is to solve or at least mitigate the problems related to prior art. This object is achieved by means of the technique set forth in the appended independent claims; preferred embodiments being defined in the related dependent claims.

[0008] According to a first aspect, an inductor unit for inductive heat treatment of a metal workpiece is provided. The inductor unit includes at least one coil unit, and at least one electrically conductive element having a generative side and an active side, where the active side is configured to face the at least one metal workpiece to be heated. The active side of the electrically conductive element has a smaller cross-sectional surface area than the generative side. The inductor unit further includes at least one soft magnetic element. The at least one coil unit is configured to induce currents in the electrically conductive element; and the at least one soft magnetic element is arranged at least partly on the at least one electrically conductive element such that the induced current is directed from the generative side to the active side of the electrically conductive element and concentrated therein.

[0009] A major advantage of the inductor unit is that a combination of local and shallow heating of the surface of the metal workpiece can be achieved. At the same time, energy is saved as compared to other cumbersome techniques used in prior art. A particular industry that can benefit from the inductor unit above is the steel industry.

[0010] In an embodiment, the electrically conductive element further inlcudes an end portion extending at least partially between the generative side and the active side of the at least one electrically conductive element. The induced current is led from the generative side to the active side of the at least one electrically conductive element via this end portion. This way, a concentration of current can be achieved in a compact and efficient manner.

[0011] In an embodiment, the coil unit includes at least one coil forming at least two turns, where the turns are wound at least partially around the generative side of the electrically conductive element and / or the at least one soft magnetic element. This allows a large current to be produced and concentrated in the active side of the electrically conductive element, which benefits the fast heating of the metal workpiece. In an embodiment, the at least one soft magnetic element is arranged at least partially on a boundary surface of the inductor unit. This means that a major part of the periphery of the inductor unit may be, and is in fact preferably, covered by the soft magnetic element, to concentrate current density in the active side of the electrically conductive element.

[0012] In an embodiment, the inductor unit is in operative communication with a processing means configured to generate current in the coil unit. This way, the processing means can indirectly control the temperature induced in the workpiece to be heated.

[0013] In an embodiment, the coil unit comprises at least one litz wire. This is beneficial since litz wires reduce skin effects and proximity effects.

[0014] In an embodiment, the inductor unit further includes at least one cooling channel for fluid media. Preferably, the at least one cooling channel is provided in the electrically conductive element. The cooling channel is beneficial in that it can cool the workpiece and / or the inductor unit, which improves process efficiency.

[0015] According to a second aspect, a system for inductive heat treatment of a metal workpiece is provided. The system includes an inductor unit according to the above, a metal workpiece to be heated, at least one capacitor, and at least one processing means which is in operative communication with the inductor unit and / or the at least one capacitor.

[0016] In an embodiment, the at least one capacitor is arranged in series and / or in parallel with the at least one coil unit of the inductor unit. This is beneficial in that the system is adaptable to different kinds of applications where the configuration and the number of processing means, capacitors and coils of the inductor unit can be varied.

[0017] According to a third aspect, a method of providing a system for inductive heat treatment of a metal workpiece according to the above is provided. The method includes the steps of: providing an inductor unit with a coil unit according to the above; providing at least one metal workpiece to be heated; arranging the inductor unit in conjunction with the at least one metal workpiece to be heated; and providing a processing means to be in operative communication with the inductor unit. According to a fourth aspect, the use of an inductor unit according to the above is provided, with the purpose of inductively heating a metal workpiece to be heated.

[0018] An idea behind the invention is to combine soft magnetic material, an electrically conductive element and a coil unit that can be wound several times. By combining these features together with recent advancements made in the field of power electronics, a higher switching frequency can be used to further extend the benefits and applications of the invention.

[0019] Examples of applications that can benefit from the invention are heat treatment of static metal components of solid or powder-based type, but also continuous or semi- continuous processes where objects such as sheets, wires, bars, rods, tubes or powder streams are being heated while moving. The types of heat treatment may for example be hardening, tempering, annealing, softening, and melting. The major benefits can be found in applications benefiting from high excitation frequency, in the order of hundreds of kilohertz or more.

[0020] High-power density and high frequency in combination enables shallow heating due to the small skin depth, meaning that heat treatments otherwise only seen from laser sources is enabled, but in a more scalable and cost-effective manner. With a relatively short processing time in the order of sub milliseconds to seconds, modifications of the surface layer can be done to tailor the properties of the metal in a desired way in terms of hardness, ductility, micro- and crystallographic structure. Due to the power density relative to the thermal conductivity, shallow heat treatment is enabled, including surface melting. In the same way, small powder particles, that are usually not easily heated by induction, can be heated. Moreover, thin sheets can be heated also when the currents are going in counteracting directions on each side of the material. This applies also to non-magnetic metals, or magnetic workpieces above the Curie temperature, assuming sufficient frequency is used.

[0021] BRIEF DESCRIPTION OF THE DRAWINGS

[0022] By way of example, embodiments of the present invention will now be described with reference to the accompanying drawings, in which: Fig. l is a perspective view of a part of a system for heat treatment of a workpiece according to an embodiment,

[0023] Fig. 2 is a perspective view of a part of a system for heat treatment of a workpiece according to another embodiment,

[0024] Fig. 3 is a schematic block diagram of a system for heat treatment of a workpiece,

[0025] Fig. 4 is a cross section view of an inductor unit mirrored on both sides of a workpiece to be heated according to an embodiment,

[0026] Fig. 5A is a perspective view of an inductor unit according to another embodiment,

[0027] Fig. 5B is an alternative embodiment to the one illustrated in Fig. 5A,

[0028] Fig. 6 is a cross-section view of the embodiment of Fig. 5,

[0029] Fig. 7 is a perspective view of a part of an inductor unit according to yet an embodiment,

[0030] Fig. 8 is a cross-section view of the inductor unit partly shown in Fig. 7,

[0031] Fig. 9 is a schematic circuit diagram showing a system for inductive heating according to an embodiment,

[0032] Fig. 10 is a schematic circuit diagram showing a system for inductive heating according to another embodiment,

[0033] Fig. 11 is a schematic circuit diagram showing a system for inductive heating according to yet an embodiment,

[0034] Fig. 12 is a cross section view of an inductor unit according to an embodiment,

[0035] Fig. 13 is a cross section view of an inductor unit according to another embodiment, and

[0036] Fig. 14 is a schematic block diagram of a method of inductively heating a workpiece according to an embodiment.

[0037] DETAILED DESCRIPTION OF EMBODIMENTS

[0038] Embodiments of the invention will now be described with reference to the accompanying drawings. The invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. The terminology used in the detailed description of the particular embodiments illustrated in the accompanying drawings is not intended to be limiting of the invention. In the drawings, like numbers refer to like elements.

[0039] Fig. 1 illustrates a part of a system for heat treatment of a workpiece 20 according to the invention. In the following description, the workpiece 20 may be a metal workpiece. The workpiece 20 may also be referred to as an object to be heated. For instance, the metal object may be a large range of steel alloys, or other metals benefitting from thermal processing. Preferably, the workpiece 20 is ferromagnetic. More preferably, it is at least an area A of the workpiece 20 that is to be heated. Inductive power is caused by inductor unit 100 that induce a current into the workpiece 20 to be heated. In Fig. 1, four inductor units 100 are shown. However, depending on the application, and in particular the size of the workpiece to be heated, there may be only one inductor unit 100, or more. For instance, in order to process relatively large workpieces, such as sheets, it may be beneficial to incorporate more than one inductor unit 100 in the system.

[0040] The inductor unit 100 is preferably arranged in conjunction with, or close to, the workpiece 20 to be heat treated. The distance between the inductor unit 100 and the workpiece 20 is preferably less than 5 mm to achieve appropriate electromagnetic coupling, but may be as much as between 10 to 15 mm. Furthermore, the current in each inductor unit 100 of the system is preferably synchronized and operated based on a common feedback system (not shown). Alternatively, each inductor unit 100 may be operated independently.

[0041] It may be beneficial to have a system with a mirror-like configuration of inductor units 100, i.e. where the inductor units 100 in the system are mirrored in the plane corresponding to a central plane C of the workpiece 20 to be heated. This is exemplified in Fig. 2. Here, the current induced by each inductor unit 100 flows in opposite directions on each side of the workpiece 20, creating a well-defined heating pattern with less risk of undesired edge effects. The inductor units, as will be described further below, induces a current in the workpiece 20 which is susceptive to electromagnetic heating. Alternatively, for magnetic materials, such as steel, heat generation may be obtained through magnetic hysteresis losses, a dominating heating source mainly in thin workpieces such as powder or foils.

[0042] In Figs. 1 and 2, the workpieces 20 are rectangularly shaped. However, as should be understood by a person skilled in the art, they may have any shape. The workpiece(s) 20 to be heated may have complex geometries, and their thickness may vary from micrometer to more than a hundred millimeters. Moreover, the workpieces 20 are to be regarded as susceptors meaning that they have the ability to absorb electromagnetic energy and convert it to heat.

[0043] In Fig. 3, a schematic representation of a system 1 is shown, including an inductor unit according to the invention. The inductor unit 100, 200, 300, 400 includes a coil unit 110, 210, 310, 410, electrically conductive element(s) 120, 220, 320, 420 and soft magnetic element(s) 130, 230, 330, 430. Furthermore, the inductor unit 100, 200, 300, 400 is in operative communication with a processing means 30. The inductor unit 100, 200, 300, 400 is configured to heat a workpiece 20 included in the system 1. A capacitor unit 40 is also included in the system 1 and will be described in more detail in relation to Figs. 9-11. The processing means 30 is preferably also in operative communication with the capacitor unit 40.

[0044] The inductor unit may include at least one cooling channel (cf. Figs. 4, 8, 12 and 13) for fluid media. Preferably, the at least one cooling channel 125, 325, 326, 425, is provided in the electrically conductive element 120, 320, 420. The cooling channel(s) may have two purposes; firstly, to cool the inductor unit 100, 200, and secondly, to cool the workpiece 20 to be heated. Optionally, there may be separate cooling channels in the system that perform both of these functions. In many applications of inductive heat treatment, no active cooling of the workpiece is needed.

[0045] For most applications, water is the preferred cooling media. However, in some applications, liquid nitrogen, liquid carbon dioxide, or air may be beneficial as well or instead. In order to achieve heat generation with high power density, it is beneficial to be able to cool both the electrically conductive element 120, 220, 320, 420 as well as the soft magnetic element, 130, 230, 330, 430. To achieve sufficient cooling, solutions such as porous structures and / or heat pipe functionality may be beneficial to integrate in the inductor unit 100, 200, 300, 400.

[0046] A benefit of the inductor unit 100, 200, 300, 400 is that it can withstand high temperature radiation from the heated workpiece 20. It may be beneficial to insulate at least parts of the inductor unit 100, 200, 300, 400 electrically and / or to enhance process efficiency and prevent a short circuit in the case of direct contact between the electrically conductive element 120 and the workpiece 20. A large range of suitable materials, for example ceramic- or polymer-based types can be used for this, depending on the application.

[0047] The different parts of the inductor unit 100, 200, 300, 400 will now be described in more detail.

[0048] With reference to Figs 4-8 and 12-13, the inductor unit 100, 200, 300, 400 has a generative side 100a, 200a, 300a, 400a and an active side 100b, 200b, 300b, 400b. Both the generative side 100a, 200a, 300a, 400a and the active side 100b, 200b, 300b, 400b comprise, or form part of, an electrically conductive element 120, 220, 320, 420. The generative side 100a, 200a, 300a, 400a of the inductor unit 100, 200, 300, 400 may correspond to a generative side 120a, 220a, 320a, 420a of the electrically conductive element 120, 220, 320, 420. Correspondingly, the active side 100b, 200b, 300b, 400b of the inductor unit 100, 200, 300, 400 may correspond to an active side 120b, 220b, 320b, 420b of the electrically conductive element 120, 200, 300, 400.

[0049] In the active side 100b, 200b, 300b, 400b of the inductor unit 100, 200, 300, 400, the electrically conductive element 120, 220, 320, 420 preferably has a smaller cross-sectional surface area than the generative side 100a, 200a, 300a, 400a of the inductor unit 100, 200, 300, 400. The cross-sectional surface area relates to an area for the current to pass through.

[0050] The cross-sectional surface area may be defined by the width of where the current can flow, times the skin depth. Since the skin depth can be assumed to be the same everywhere on the electrically conductive element (valid if the electrically conductive element is made by the same material everywhere, for example copper), it is the same as the cross-sectional width of the surface where the current can flow. Hence, at least parts of the active side 120b, 220b, 320b, 420b may have a narrower surface width than the generative side 120a, 220a, 320a, 420a. In other words, the cross- sectional surface area may also be referred to as a cross-sectional surface width.

[0051] The electrically conductive element 120, 220, 320, 420 may have a curved or bent shape on the generative side 100a, 200a, 300a, 400a thereby increasing the cross- sectional surface area on the generative side 100a, 200a, 300a, 400a as compared to the surface area on the active side 100b, 200b, 300b, 400b. Conversely, the electrically conductive element 120, 220, 320, 420 may have a narrower shape towards the active side 100b, 200b, 300b, 400b of the inductor unit to create a smaller cross-section for the current and thereby a more concentrated electromagnetic flux density and power density.

[0052] The active side 120b, 220b, 320b, 420b of the electrically conductive element 120, 220, 320, 420 is configured to face the at least one workpiece 20 to be heated, and has, at least locally, a smaller cross-sectional surface area than the generative side 120a, 220a, 320a, 420a. If the active side 120b, 220b, 320b, 420b of the electrically conductive element 120, 220, 320, 420 has a varying width, the largest power density would be generated in the narrow section, with the highest current density. Exemplified with the shape of a sandglass, given the distance between the active side 120b, 220b, 320b, 420b of the electrically conductive element and the workpiece 20 is the same everywhere, the center part (where the current is concentrated) would be the warmest.

[0053] The cross-sectional surface area of the active side 100b, 200b, 300b, 400b may be described as the area of the inductor unit 100, 200, 300, 400 facing the workpiece 20 to be heated, and the cross-sectional surface area of the generative side 100a, 200a, 300a, 400a may be described as the area of the inductor unit 100, 200, 300, 400 facing away from the workpiece to be heated.

[0054] A part of the electrically conductive element 120, 220, 320, 420 linking the generative 120a, 220a, 320a, 420a and the active 120b, 220b, 320b, 420b sides together is referred to as a transfer part 120c, 220c, 320c, 420c. The transfer part is also referred to as a transfer side in the following. The transfer side(s) 120c, 220c, 320c, 420c may have virtually any shape. In certain inductor designs, the transfer sides may be negligible in size. The transfer sides are not covered by soft magnetic elements 130, 230.

[0055] The coil unit 110 may comprise one or more coils 111, 112, 113, ... , 1 In, together forming at least two turns, preferably more. The turns are wound at least partially around the generative side 120a, 220a, 320a, 420a of the electrically conductive element 120, 220, 320, 420 and / or the at least one soft magnetic element 130, 230, 330, 430. Each coil 111, 112, 113, ... , 1 In is preferably made of litz wire of low loss. The litz wire comprises a plurality of individual strands. The strands are typically of thin insulated wire arranged together.

[0056] Litz wires are used to reduce the skin effect. Furthermore, litz wires also reduce the so called proximity effect, which forces the current to concentrate in certain areas of a regular wire due to the currents in neighboring wires or conductors.

[0057] The electrically conductive element 120, 220, 320, 420 is made of a highly electrically conductive material. The material may for example be copper or aluminum. An advantage of using copper is the superior thermal and electrical conductivity, among the commonly used metals. An advantage of using aluminum is, except for the good electrical conductivity, the easiness of applying a high temperature resistant electrical insulation in terms of aluminum oxide through anodization, allowing the inductor to be in direct contact with the workpiece surface if desirable, without having direct electrical contact.

[0058] As mentioned, the inductor unit 100, 200, 300, 400 further comprises a soft magnetic element 130, 230, 330, 430. The soft magnetic element 130, 230, 330, 430 is made of a soft magnetic material of any sort, with a relative magnetic permeability of more than 10. A soft magnetic material should preferably also have high bulk electrical resistivity, many thousand or even million times higher than the electrically conductive element. The bulk electrical resistivity is defined as the global electrical resistivity in the directions in which induced currents can flow, rather than the properties on the micro level. A bulk resistivity of at least 0.01 Ohm*m is preferred in the soft magnetic material.

[0059] The soft magnetic material 130, 230, 330, 430 also referred to as a flux concentrating element, may be a composite or a ceramic, preferably with low magnetic hysteresis losses. It may be any one of a soft magnetic ferrite, and / or a powder-based core, soft magnetic composites of bundles or stacks of individually insulated soft magnetic wires, strips or laminates, including amorphous and semi-crystalline alloys. Preferably, the soft magnetic element 130, 230, 330, 430 is arranged at least partially on a boundary surface of the inductor unit 100, 200, 300, 400. More specifically, the soft magnetic element 130, 230, 330, 430 is preferably arranged at least partially on a surface of the electrically conductive element 120, 220, 320, 420. This is to enhance the magnetic flux and to prevent currents from being conducted on the areas of the conductive element covered by the soft magnetic element. The soft magnetic element 130, 230, 330, 430 arranged on the electrically conductive element 120, 220, 320, 420 preferably faces the surrounding environment rather than the workpiece 20 to be heated. The boundary surface of the inductor unit 100, 200, 300, 400 may be described as the outer surface of the inductor unit 100, 200, 300, 400 facing the environment.

[0060] A purpose of arranging the soft magnetic element 130, 230, 330, 430 on top of the electrically conductive element 120, 220, 320, 420 is to direct the current induced by the coil unit 110, 210, 310, 410 from the generative side 120a, 220a, 320a, 420a to the active side 120b, 220b, 320b, 420b of the electrically conductive element 120, 220, 320, 420 and to concentrate the current therein. Moreover, the soft magnetic element 130, 230, 330, 430 is provided to enhance the magnetic flux and to prevent currents from being conducted on the areas of the electrically conductive element 120, 220, 320, 420 covered by the soft magnetic element 130, 230, 330, 430. Further, the soft magnetic element 130, 230, 330, 430 may have the purpose of concentrating and enhancing magnetic flux in the desired regions of the inductor unit. Also, the soft magnetic element 130, 230, 330, 430 is configured to concentrate the induced current in the active side 100b, 200b, 300b, 400b of the inductor unit 100, 200, 300, 400 to heat the predetermined heating area A in the workpiece 20.

[0061] In one embodiment, the soft magnetic element 130, 230, 330, 430 is made of soft ferrite. In one alternative embodiment the soft magnetic element 130, 230, 330, 430 comprises a powder-based core of flux material or other similar type of soft magnetic composite. In yet one embodiment, the soft magnetic element 130, 230, 330, 430 comprises a laminated soft magnetic structure. In general terms, the soft magnetic element 130, 230, 330, 430 is configured to concentrate the electromagnetic flux and thereby help guide the current in a desired direction. It may be seen as a shield, which helps in concentrating electric currents.

[0062] The soft magnetic element 130, 230, 330, 430 might comprise a plurality of parts. Preferably, the soft magnetic element 130, 230, 330, 430 surrounds at least a part of the electrically conductive element 120, 220, 320, 420. The soft magnetic element 130, 230, 330, 430 is configured to act as a conductor for magnetic fields but not electrical currents.

[0063] The soft magnetic element 130, 230, 330, 430 may contribute to the concentration of current in predetermined parts of the inductor unit 100, 200, 300, 400 by providing a path for the magnetic flux. The induced current in the electrically conductive element 120, 220, 320, 420 of the inductor unit 100, 200, 300, 400 is enhanced by the flux concentration and in particular the current in the active side 100b, 200b, 300b, 400b of the electrically conductive element. This principle may be explained as follows.

[0064] The induced current in the electrically conductive element 120, 220, 320, 420 aims to minimize the stored energy of the circuit, i.e. the inductance of the circuit. Thus, the current prefers to flow on surfaces which are not covered by soft magnetic material 130, 230, 330, 430, such as the transfer sides 120c, 220c, 320c, 420c and avoids areas covered or delimited by soft magnetic materials 130, 230, 330, 430. The direction of the induced currents is defined by the coil unit 110, 210, 310, 410. The current is also forced to create closed current loops according to the laws of physics.

[0065] In some embodiments, the electrically conductive element 120, 220, 320, 420 may be formed with a surface area which tapers in a direction configured to be facing the workpiece 20 to be heated. Alternatively, or additionally, the electrically conductive element 120, 220, 320, 420 may have a cut out section, such as a slit 340, extending from the generative side 120a, 220a, 320a, 420a to the active side 120b, 220b, 320b, 420b of the inductor unit 100, 20, see Fig. 7. When the induced current reaches the slit 340, it is forced to pass through the transfer sides 320c to the active side 320b to form a close current loop, since the other surfaces of the inductor unit 300 are covered by the soft magnetic element 330. In either one of these two cases, or alternative configurations, a current induced in the electrically conductive element 120, 220, 320, 420 via the coil unit 110, 210, 310, 410 may be channeled through the inductor unit 100, 200, 300, 400 in a direction towards the workpiece 20 to be heated. The presence of the soft magnetic element 130, 230, 330, 430 enhances this current density formation.

[0066] As mentioned, with reference to Fig. 3, the inductor unit 100, 200, 300, 400 is in operative communication with a processing means 30, such as a frequency converter. The processing means 30 is configured to generate a high frequency current in the coil unit 110, 210, 310, 410. The processing means may also include a transformer for easy adaption of the impedance of the inductor unit to the power electronics. The system 1 shown in Figs. 3, 9, 10 and 11, comprising the inductor unit 100, 200, 300, 400 further includes the processing means 30. In particular, there may be more than one processing means 31, 32, ..., 3n in the system 1 (see Fig. 10). Each processing means 31, 32, ..., 3n may in turn be coupled to coils 111, 112, 1 In of the respective coil unit 110 in the respective inductor unit 100.

[0067] Preferably, the processing means 30 is or comprises a frequency converter. The processing means 30 is configured to generate an electromagnetic field, through the inductor unit 100, 200, 300, 400 by applying an alternating current to the inductor unit 100, 200, 300, 400 so as to inductively heat the workpiece 20 at the heating area A.

[0068] The processing means 30 may further comprise an interface (not shown) for transmitting data obtained by sensing the properties of the inductor. The properties of the inductor are influenced by the properties of the workpiece. This data can be used to control the heat treatment process, with or without other sensors monitoring the properties of the workpiece, such as temperature sensor, pyrometers, etc. The interface may be of any suitable type, including simple wiring, a serial interface such as Ethernet, RS485, USB, a wireless interface such as Bluetooth or WiFi, etc. The processing means 30 may comprise a programmable device, such as a microcontroller, central processing unit (CPU), digital signal processor (DSP) or field-programmable gate array (FPGA), discrete digital synthesizer (DDS) with appropriate software and / or firmware, and / or dedicated hardware such as an application-specific integrated circuit (ASIC). The processing means can be connected to or comprises a computer readable storage medium such as a disk or memory. The memory may be implemented using any commonly known technology for computer-readable memories such as ROM, RAM, SRAM, DRAM, FLASH, DDR, SDRAM or some other memory technology. The processing means 30 may further comprise a display unit to provide an operator or user with process information.

[0069] An advantage of the inventive inductor unit 100, 200, 300, 400 which is common for all embodiments is that it can be driven with a low current due to the multiturn structure of the coil unit 110, 210, 310, 410. By integrating the coil unit 110, 210, 310, 410 the electrically conductive element 120, 220, 320, 420 and the soft magnetic material 130, 230, 330, 430 as one unit, a relatively high power density may be achieved at the part A of the workipece to be inductively heated.

[0070] Compared to a traditional coil and workhead, the inductor unit 100, 200, 300, 400 is a compact, lightweight unit that can easily be mounted as the end effector of any type of robot. It can also handle a long, flexible wire between the processing means 30 and the inductor unit 100, 200, 300, 400 with stable operation, without substantial losses, and without EMC interference. The compact size in combination with high efficiency further means that higher power can be fed into the inductor unit 100, 200, 300, 400.

[0071] The inductor unit 100, 200, 300, 400 may be designed to have different shapes as will now be shown and discussed with reference to Figs. 4-8 and 12-13. Hence, the inductor unit 100, 200, 300, 400 is adaptable to different workpiece configurations and enables selective and well-defined heating of the workpiece. Non-exhaustive examples of inductor configurations are shown in the appended drawings. It should be noted that there are other embodiments that are not captured by the drawings.

[0072] With reference to Fig. 4, an inductor unit 100 according to a first embodiment is shown. In this embodiment, the inductor unit 100 has a longitudinally extending shape. The longitudinal shape is not shown in Fig. 4 since it shows a cross section of the inductor unit 100. The inductor unit 100 is mirrored on both sides of a workpiece 20 to be heated. The inductor unit 100 has a generative side 100a and an active side 100b, and comprises three major components; a coil unit 110, an electrically conductive element 120, and a soft magnetic element 130. As shown in Fig. 4, the coil unit 110 is preferably a litz wire wound multiple times into a coil. The coil unit 110 extends along the longitudinal length of the inductor unit 100 and is wound in recesses provided in the electrically conductive element 120 aimed to maximize the area for the induced current as well as to reduce the inductance of the circuit. As illustrated by crosses and dots in Fig. 4, the direction of the current passes in a positive and negative direction respectively on each side of the workpiece 20 to be heated.

[0073] The electrically conductive element 120 has a generative side 120a facing the coil unit 110 and an active side 120b corresponding to the active side 100b of the inductor unit 100. In practice, it is the active side 100b of the inductor unit 100 which faces the workpiece 20 during heating. As seen in Fig. 4, the generative side 120a of the electrically conductive element 120 has a larger surface area than the active side 120b. The electrically conductive element 120 tapers from the generative side 120a, via a transfer side 120c, to the active side 120b. In this embodiment, the electrically conductive element 120 tapers from the generative side 120a to the active side 120b in a direction perpendicular to the longitudinal extension of the inductor unit 100. Hence, an electric current flowing in the generative side 120a of the electrically conductive element 120 will be forced to flow to the active side 120b to form a closed current loop, utilizing the transfer side 120c, which is not covered by any soft magnetic material 130. The transfer side 120c may be seen as an end portion or gable of the inductor unit 100 in Fig. 4.

[0074] A smaller cross-sectional surface area of the active side 100b, 120b means a higher current density and thereby higher power density. Through transformer action and a minimal current leakage in the electrically conductive element 120 due to the soft magnetic element 130, the total current through a cross-section of the active side 100b, 120b is almost the size of the coil current times the number of turns in the coil unit 110.

[0075] Furthermore, the electrically conductive element 120 may be provided with a first opening 125 and a second opening 126. In Fig. 4, these openings 125, 126 are configured to extend within the electrically conductive element 120, along the majority of the length of the inductor unit 100 to form a U-shaped channel, with round holes in each end, where fittings can be attached. However, it may as well be that the first opening 125 and the second opening 126 are separate. The first and second openings 125, 126 are useful for cooling of the electrically conductive element 120. The first and second openings 125, 126 may also be referred to as cooling channels. The cooling channels are configured to receive fluid media, such as gas or liquid media.

[0076] The soft magnetic element 130 may be provided on one or more parts of the inductor unit 100, preferably on respective side portions of the inductor unit 100, surrounding the electrically conductive element 120. As previously mentioned, the soft magnetic element 130 is typically a soft magnetic ferrite or a powder-based core, configured to conduct magnetic fields but no current. Hence, the soft magnetic element 130 may be seen as a barrier for electrical currents. The main purpose of the soft magnetic element 130 is to help concentrate the induced current in the active side 100b of the inductor unit 100.

[0077] As a result of the inductor arrangement shown in Fig. 4, a concentration of a relatively small current entering the coil unit 110 can be achieved in the active side 100b of the inductor unit 100 configured to be arranged in the vicinity of the workpiece 20 to be heated. When the induced current travels through the electrically conductive element 120 (or its parts), it is forced down towards the tapered portion of the longitudinally extending funnel shaped electrically conductive element 120, i.e. the current travels from the generative side 100a, via the transfer side 120c to the active side 100b of the inductor unit 100. In other words, the current is concentrated from a relatively broad surface area (i.e. larger cross-sectional surface area) in the generative side 120a, to a relatively narrower surface area in the active side 120b of the electrically conductive element 120, corresponding to the active side 100b of the inductor unit 100. The heated area A of the workpiece 20 is illustrated in Fig. 4 as a pattern of crossed lines.

[0078] By completely encapsulating the coil unit 110 in the electrically conductive element 120, efficiency is improved and stray inductance of the circuit is reduced. In this case, the inductor unit 100 may include two different coils, which may be connected in parallel or in series for the best result in terms of impedance matching, voltage, etc.

[0079] Figs. 5 and 6 illustrate an inductor unit 200 according to another embodiment. This embodiment has benefits in some applications, for example where a very local heating is desired, such as the edge of a knife, teeth of a saw, the end of a tube or edge of a flange. Similarly to the inductor unit 100 shown in Fig. 4, the inductor unit 200 of Figs. 5 and 6 has a generative side 200a and an active side 200b, and comprises three major components; a coil unit 210, an electrically conductive element 220, and a soft magnetic element 230.

[0080] At the active side 200b of the inductor unit 200, which corresponds to the second side 220b of the electrically conductive element 220, the current density is substantially higher than at the generative side 200a of the inductor unit 200. In other words, the current is more concentrated at the active side 200b of the inductor unit 200 due to the difference in surface area between the generative and active sides of the electrically conductive element 220. As previously mentioned, the workpiece 20 to be heated is to be arranged in between the two active sides 220b, in a similar way as in Fig. 4, since the current (and thereby also the heat) is concentrated in this region of the inductor unit 200 during use.

[0081] In Figs. 5A and 5B, respectively, two of the transfer sides 220c of the electrically conductive elements 200 are visible, linking the current from the generative side 220a to the active side 220b to form closed current loops.

[0082] In Fig. 5 A, the windings of the coil unit 210 is shown having several turns. In Fig. 5B, an alternative visualization is shown in which the coil unit 210 is illustrated as having a substantially flat surface. In particular, the flat surface of the coil unit 210 of Fig. 5B illustrates what the coil unit 210 may look like after electrical insulation has been applied. Electrical insulation of the coil unit 210, between different turns and between the coil unit 210 and the electrically conductive element 220 and the soft magnetic element 230 is beneficial in all configurations and may be applied in different ways. In some configurations, insulation between different areas belonging to the same element may be feasible, such as in the slit in the electrically conductive element 320 shown in Fig. 7.

[0083] A workpiece 20 is arranged between the respective active sides 220b of each of the alternative inductor units 200 shown in Figs. 5A and 5B, respectively. The heated area A is achieved in the upper part of the workpiece 20 arranged between the transfer sides 220c. The embodiment of Fig. 5 is shown in cross-section in Fig. 6. The coil unit 210 induces current in the generative side 220a of the electrically conductive element 220. When the induced current reaches transfer sides 220c, it is transferred to the active side 220b to form a close current loop, since the other surfaces of the inductor unit 200 are covered by the soft magnetic element 230. The soft magnetic element 230 creates a path for the electromagnetic flux, flowing all around the coil unit 210 as well as the electrically conductive element 220 and inducing currents in the workpiece 20 to be heated.

[0084] Openings (not shown), may be provided in the electrically conductive element 220 shown in Figs. 5 and 6. These openings may extend into the inductor unit 200 and be integrated in the structure of the electrically conductive element 220. The openings may extend in the form of channels throughout the inductor unit 200. The openings may also be referred to as cooling channels. The cooling channels are configured to receive fluid media, such as gas or liquid media, as described above in relation to Fig. 4. Notably, the need of cooling of the inventive inductor unit 200 after use is reduced due to the high efficiency in heating of the workpiece 20. The cooling channel(s) may alternatively share the space with the coil unit 220 in the inductor unit 200.

[0085] The cooling channels are useful during cooling with liquid or gas of the electrically conductive element 220 as well as the coil unit 210, soft magnetic material 230 and potentially also the workpiece 20 to be heated. The openings may be equipped with fittings or other connection means to feed the cooling fluid. There may only one cooling channel, or more than one cooling channel, such as two cooling channels. The cooling channels, are configured to transfer cooling media to maintain a steady temperature of the entire inductor unit 200.

[0086] Figs. 7 and 8 illustrate an inductor unit 300 according to another embodiment. Similarly to the inductor units 100, 200 described above, the inductor unit 300 of Figs. 7 and 8 has a generative side 300a and an active side 300b, and comprises three major components; a coil unit 310, an electrically conductive element 320, and a soft magnetic element 330.

[0087] The inductor unit 300 of Fig. 7 may be referred to as a rotationally symmetric inductor unit 300. In particular, it is symmetric about a central axis CA. The inductor unit 300 of Fig. 7 may be useful for heating of for example wires, tubes or flowing powder. A slit 340 is provided in the electrically conductive element 320, creating a path for the current to pass from the generative side 300a to the active side 300b of the inductor unit 300. The slit 340 might be completely hidden under the soft magnetic element 330.

[0088] Fig. 8 shows a cross section of the inductor unit 300 of Fig. 7, including the electrically conductive element 320, the coil unit 310 and the soft magnetic element 330 as well as a cooling channel illustrated by two circles 325, 326 provided in the electrically conductive element 320. There may be more than one cooling channel in the inductor unit 300.

[0089] Fig. 9 illustrates a system 1 in which the inductor unit 100, 200, 300, 400 can be used. In its simplest form, as shown in Fig. 9, the system 1 includes a processing means 30, connected to a capacitor unit 40, built by one or several individual capacitors used to compensate the reactive power from the inductor unit 100, 200, 300, 400. The processing means 30 as well as the capacitor unit 40 are further connected to the coil unit 110, 210, 310, 410 of the inductor unit 100, 200, 300, 400. The system 1 further includes one or several workpieces to be heat treated, preferably metal workpieces. The capacitor(s) may for example be a plastic film capacitor or a ceramic capacitor type, able to handle bipolar voltages and high frequency currents. The capacitor unit 40 is preferably arranged in series with the coil unit 110, 210, 310, 410. However, the capacitor unit 40 may alternatively be connected in parallel with the coil unit 110, 210, 310, 410.

[0090] Referring now to Fig. 10, another exemplary system 1 is shown, based on the same features and connections as described in relation to Fig. 9. The system of Fig. 10 may be feasible in cases where an upscaling in size and power is required. Here, a single inductor unit 100, 200, 300, 400 is powered by a number of processing means 31, 32, ... 3n. Each processing means 31, 32, ... 3n is connected to an individual coil 111, 211, 311, 411, 112, 212, 312, 412 ..., l ln, 21n, 3 In, 41n of the inductor unit 100, 200, 300, 400, typically representing different turns of the coil unit 110, 210, 310, 410.

[0091] There may be other ways of connecting the processing means 31, 32, . . . , 3n, the capacitors 41, 41, ..., 4n and the coils 111, 211, 311, 411, 112, 212, 312, 412, ..., 1 In, 21n, 3 In, 41n together to obtain a similar behavior. For example, several processing means 31, 32, . . . , 3n may be connected in series with each other. The excitation frequency of each processing means 31, 32, . . . , 3n may as an example be the same on all of them, then preferably having a close to zero-degree phase shift between the currents of each circuit. Alternatively, the frequency may differ between the circuits, reducing the need for phase shift control.

[0092] Fig. 11 shows an alternative system 1 based on the same features and connections as described in relation to Fig. 9. The system of Fig. 11 is similar to the system of Fig. 10, except for that a single processing means 30 controls, or powers, the capacitor units 41, 42, . . . 4n and the inductor unit 100, 200, 300, 400. In many applications it is beneficial to use many turns in the coil unit 410, causing challenges with high voltage levels at high frequency operation, which mitigated by these kind of configurations shown in Figs. 9-11.

[0093] Figs. 12 and 13 show an inductor unit 400 having a coil unit 410, an electrically conductive element 420 and a soft magnetic element 430 similar to the inductor units 100, 200, 300 described above. The inductor unit 400 has a generative side 400b, and an active side 400b in which the current is concentrated. In particular, the inductor unit 400 shown in Figs. 12 and 13 includes a first electrically conductive element 421 and a second electrically conductive element 422, where the second electrically conductive element 422 surrounds the soft magnetic element 430. Both Figs. 12 and 13 show a cooling channel 425 in line with what has been described previously.

[0094] Referring to Fig. 12, the second electrically conductive element 422 is provided to enhance the power density. This is done by reducing undesired heating further away from the active side 400b of the inductor unit 400. If this element 422 is located close to the active side 400b of the inductor unit 400, counteracting currents will be induced in this element 422, resulting in a more concentrated heating pattern.

[0095] Referring to Fig. 13, in an induction heating setup where the inductor unit 400 provides induced current in only one direction of the workpiece 20, often referred to as a longitudinal flux inductor, heating only one side of the workpiece, then the return part of the coil unit 410 is preferably located close to an electrically conductive element 420 which aims to reduce the inductance. The design may look in many different ways, where one opportunity is to utilize the counteracting current in the electrically conductive element 420 in such a way that the heating pattern is further concentrated. This type of solution enables higher power densities that otherwise possible. The electrically conductive element 420 of the inductor unit 420 may include one single unit or several pieces, such as two pieces 421, 422 as shown in Fig. 13. Clever use of soft magnetic materials 430 may contribute to keeping the inductance low.

[0096] Fig. 14 illustrates a method 500 for inductively heating a workpiece 20 using any of the systems as described above. The method begins by providing 510 an inductor unit 100, 200, 300, 400 and arranging 515 it in conjunction with the workpiece 20 to be heated. A processing means 30 is also provided 520. An alternating voltage is applied 530 to the inductor unit 100, 200, 300, 400 by the processing means 30 and thereby induces an electric current in the electrically conductive element 120, 220, 320, 420 of the inductor unit 100, 200, 300, 400 via the coil unit 110, 210, 310, 410. The current in the electrically conductive element 120, 220, 320, 330 further induces currents in the at least partially susceptive workpiece 20. The aim is to inductively heat the workpiece 20 in the heating area A (cf. Fig. 1 and 4). The alternating voltage is an input signal to the inductor unit 100, 200, 300, 400 and is applied via the processing means 30. However, the method is not restricted to controlling the voltage level only, but also other electromagnetic properties may be controlled as well, such as a current I or a frequency F.

[0097] More specifically, the method of inductive heating is performed as follows. First, an inductor unit 100, 200, 300, 400 is provided 510 according to any of the embodiments described above. The inductor unit 100, 200, 300, 400 is arranged 515 in conjunction with the workpiece 20 to be heated. Next, a processing means 30 is provided 520 to control the overall functioning of the process. By applying 530 for instance an alternating voltage to the inductor unit 100, 200, 300, 400 an electromagnetic field is generated, inducing currents in the electrically conductive element 120, 220, 320, 420 of the inductor unit 100, 200, 300, 400. The current in the electrically conductive element 120, 220, 320, 420 further creates electromagnetic field that induces currents in and thereby heats the workpiece 20 in the heating area A. Optionally, a step 540 of providing a movement means configured to move the workpiece 20 or the inductor unit 100, 200, 300, creating a relative movement during or after the heating is provided, as well as a step 550 of providing a cooling means configured to cool the inductor unit 100, 200, 300 during the heating process.

[0098] The processing means 30 generates a current in the coil unit 110, 210, 310, 410. This current generates a magnetic field which induces opposite directed currents in the electrically conductive element 120, 220, 320, 420 preferably made of copper and / or aluminum or its alloys and thereby counteracting electromagnetic fields, reducing the magnetic flux density of the circuit. As mentioned, a soft magnetic element 130, 230, 330, 430 is provided around the electrically conductive element 120, 220, 320, 420 guiding the resulting electromagnetic flux to the workpiece 20 and forcing the current in the electrically conductive element 120, 220, 320, 420 to flow on desired surfaces, thereby improving efficiency and inducing the desired heating pattern. Since the soft magnetic element 130, 230, 330, 430 has a high electrical resistivity and small magnetic hysteresis losses, only a small amount of heat is generated in the inductor device 100, 200, 300, 400. Hence, in most setups, the efficiency in heating is improved by the provision of the soft magnetic element 130, 230, 330, 430 around the electrically conductive element 120, 220, 320, 420.

[0099] As a result of the different inductor units 100, 200, 300, 400 described above, the current induced in the electrically conductive element 120, 220, 320, 420 is guided from the generative side 100a, 200a, 300a, 400a to the active side 100b, 200b, 300b, 400b of the inductor unit 100, 200, 300, 400 which is located in close vicinity to the workpiece 20 to be heated.

[0100] In other words, by combining the elements of the inductor unit 100, 200, 300, 400 as described above, a rather small current first provided in the coil unit 110, 210, 310, 410 can result in a concentrated and highly efficient heating of the workpiece 20 in the heating area A.

[0101] Notably, the method of inductive heating may be performed by a continuous process or by a static process. Moreover, as long as a concentration of current can be achieved, the bottom part of the inductor unit 100, 200, 300, 400 facing the workpiece 20 may have different cross-sections. The cross-section may for instance be patterned and / or have a varying cross-section over the entire bottom surface. For instance, the surface may be rounded.

[0102] As an example, considering a heat treatment operation of a steel sheet in a continuous process, i.e. where the workpiece 20 is moving relatively to the inductor unit 100, 200, 300, 400, then the process may be performed on one side of the workpiece 10 or on both sides using a mirror-like setup. The double sided heat treatment is preferably done by having a currents circulating around the sheet, i.e. forming different directions on the two sides of the workpiece 20. In this way, undesired edge effects, commonly resulting in overheating of the edges can be avoided or at least mitigated. The distance between the inductor unit 100, 200, 300, 400 and respective surface of the workpiece 20 is preferably kept small. Furthermore, to maintain a constant gap between the objects, it is preferential to obtain uniform and repetitive power generation. For this, a steering between the inductor unit and the workpiece may be used. By using a flexible coil unit, for example made of flexible litz wire, the two halves of the inductor unit 100, 200, 300, 400 can move relatively to each other. Alternatively, by using two independent inductor units, the same flexible setup is obtained. A constant distance between the inductor unit and the workpiece can then be obtained on each side of the workpiece, compensating for deformations for example due to thermal effects or undesired dynamic effects caused by the movement means. The steering may be performed for example by sliding a part of the inductor unit 100, 200, 300, 400 towards the workpiece 20. This can be performed in the heated area or before the heating area is reached by the workpiece. The distance control may as an alternative be obtained by closed loop control using noncontact means. The same principle may be used also for single sided heating applications or other geometrical arrangements. Yet another benefit, in the case with different sheet thicknesses, the gap in the inductor unit may be changed to fit a thicker workpiece, without the need of replacing the entire inductor unit.

[0103] In all embodiments of the invention where there are air gaps in the inductor design, an isolation of the air gaps may be required to avoid short circuits.

[0104] Notably, the soft magnetic element is arranged and configured to concentrate the current induced in the electrically conductive material and lead the current in a predetermined direction throughout the inductor unit. This way, there will be small self-generated losses in the inductor unit 100, 200, 300, 400 heating the workpiece during use. Furthermore, there may be an insulation between the soft magnetic element 130, 230, 330, 430 and the electrically conductive element 120, 220, 320, 420.

[0105] For applications where different workpiece material are used in the same setup, alternatively where different geometries are being heated with the same inductor unit, a manual or automatic impedance matching adjustment may be obtained by changing the number of turns of the coil unit, for example by disconnecting one coil or by connecting two turns of a coil unit in parallel instead of series. Furthermore, similar manual or automatic adjustments of the resonance capacitors can be easily performed to the setup by connecting or disconnecting capacitor units or changing values.

[0106] The induction heating system may use temperature feedback for closed loop process control. Furthermore, it may use measurement signals from within the processing means, such as frequency, power, current, voltage, phase shift, etc, to control the process. For example, process control based on predicted properties of the material may be used to control material properties obtained from the heat treatment process. Temperature, material properties such as resistivity, magnetic permeability, etc. are influencing these signals. In turn, the combination of temperature and time history may change the material structure, such as crystallinity, grain structure, etc.

Claims

CLAIMS1. An inductor unit for inductive heat treatment of a metal workpiece, comprising: at least one coil unit (110, 210, 310, 410); at least one electrically conductive element (120, 220, 320, 420) having a generative side (120a, 220a, 320a, 420a) and an active side (120b, 220b, 320b, 420b), wherein the active side (120b, 220b, 320b, 420b) is configured to face the at least one metal workpiece (20) to be heated, wherein the active side (120b, 220b, 320b, 420b) of the electrically conductive element (120, 220, 320, 420) has a smaller cross-sectional surface area than the generative side (120a, 220a, 320a, 420a); and at least one soft magnetic element (130, 230, 330, 430); wherein the at least one coil unit (110, 210, 310, 410) is configured to induce currents in the electrically conductive element (120, 220, 320, 420); and wherein the at least one soft magnetic element (130, 230, 330, 430) is arranged at least partly on the at least one electrically conductive element (120, 220, 320, 420) such that the induced current is directed from the generative side (120a, 220a, 320a, 420a) to the active side (120b, 220b, 320b, 420b) of the electrically conductive element (120, 220, 320, 420) and concentrated therein.

2. The inductor unit according to claim 1, wherein the electrically conductive element (120, 220, 320, 420) further comprises an end portion extending at least partially between the generative side (120a, 220a, 320a, 420a) and the active side (120b, 220b, 320b, 420b) of the at least one electrically conductive element (120, 220, 320, 420), wherein the induced current is led from the generative side (120a, 220a, 320a, 420a) to the active side (100b, 200b, 300b, 400b) of the at least one electrically conductive element (120, 220, 320, 420) via said end portion.

3. The inductor unit according to any of claims 1 or 2, wherein the coil unit (110, 210, 310, 410) comprises at least one coil forming at least two turns, wherein the turns are wound at least partially around the generative side (120a, 220a, 320a, 420a) ofthe electrically conductive element (120, 220, 320, 420) and / or the at least one soft magnetic element (130, 230, 330, 430).

4. The inductor unit according to any of the preceding claims, wherein the at least one soft magnetic element (130, 230, 330, 430) is arranged at least partially on a boundary surface of the inductor unit (100, 200, 300, 400).

5. The inductor according to any of the preceding claims, wherein the inductor unit (100, 200, 300, 400) is in operative communication with a processing means (30) configured to generate current in the coil unit (110, 210, 310, 410).

6. The inductor unit according to any of the preceding claims, wherein the coil unit (110, 210, 310, 410) comprises at least one litz wire.

7. The inductor unit according to any of the preceding claims, further comprising at least one cooling channel (125, 126, 325, 326, 425) for fluid media, preferably wherein the at least one cooling channel (125, 126, 325, 326, 425) is provided in the electrically conductive element (120, 220, 320, 420).

8. A system for inductive heat treatment of a metal workpiece (20), comprising: an inductor unit (100, 200, 300, 400) according to any one of claims 1-7; a metal workpiece (20) to be heated; at least one capacitor (40); and at least one processing means (30) which is in operative communication with the inductor unit (100, 200, 300, 400) and / or the at least one capacitor (40).

9. The system according to claim 8, wherein the at least one capacitor (40) is arranged in series and / or in parallel with the at least one coil unit (110, 210, 310, 410) of the inductor unit (100, 200, 300, 400).

10. A method of providing a system (1) for inductive heat treatment of a metal workpiece (20), comprising: providing (510) an inductor unit (100, 200, 300, 400) with a coil unit (110, 210, 310, 410) according to any one of claims 1-7; providing at least one metal workpiece (20) to be heated; arranging (515) the inductor unit (100, 200, 300, 400) in conjunction with the at least one metal workpiece (20) to be heated; and providing (520) a processing means (30) to be in operative communication with the inductor unit (100, 200, 300, 400).

11. Use of an inductor unit according to any one of claims 1-7, for inductive heating of a metal workpiece (20) to be heated.

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