Induction heating device having a uniform heating distribution
Patent Information
- Application Number
- EP2024717122
- Authority / Receiving Office
- EP · EP
- Patent Type
- Applications
- Current Assignee / Owner
- Priority Date
- 2023-03-30
- Filing Date
- 2024-03-26
- Publication Date
- 2026-02-11
AI Technical Summary
Conventional induction heating devices using flat spiral-shaped coils fail to provide uniform heating, leading to localized overheating and potential deformation of objects, and existing solutions like multi-coil designs or high-cost materials are expensive and inefficient.
An induction heating device with a spiral inductor having a resistance greater than or equal to 0.5 ohms and a quality factor less than or equal to 10, utilizing materials like iron for the inductor, which allows for homogeneous heating distribution by combining induction heating with Joule heating from the inductor, ensuring even temperature rise without deformation.
The device achieves rapid and homogeneous heating, preventing object deformation while being cost-effective and easy to manufacture, using materials like iron to maintain a low quality factor and high resistance for efficient Joule heating.
Smart Images
Figure EP2024058143_03102024_PF_FP_ABST
Abstract
Description
[0001] DESCRIPTION
[0002] TITLE: INDUCTION HEATING DEVICE WITH HOMOGENEOUS HEATING DISTRIBUTION
[0003] Technical field of the invention
[0004] The present invention relates to the general field of heating an electrically conductive part. The invention relates more particularly to an induction heating device having a homogeneous heating distribution.
[0005] State of the art
[0006] In the general field of heating electrically conductive parts, it is known to use the technique of induction heating using eddy currents induced in the part to be heated by a high-frequency magnetic field.
[0007] In this way, a solenoid-type inductor, commonly called a coil, is conventionally used, through which an electric current flows in order to create a magnetic field in which the part to be heated is placed, said coil being, for example, a flat spiral-shaped coil in the case of a device for heating suitable kitchen utensils such as saucepans or frying pans provided with at least a ferromagnetic base. Induction heating with this type of flat spiral-shaped coil allows rapid heating but has the disadvantage of not guaranteeing uniform heating of the object to be heated.Indeed, the temperature rise produced by the eddy currents circulating in the bottom of the object to be heated is only centralized in a generally circular zone whose radius is equal to approximately half the radius of the flat spiral-shaped coil because it is in this zone that the magnetic field produced by the alternating current passing through said flat spiral-shaped coil is the highest. This localized overheating can even cause deformation of the bottom of the object to be heated.
[0008] To overcome this drawback, it is known to use kitchen utensils made from a material with very good thermal conduction and inertia and of high thickness to distribute the heat as well as possible, such as, for example, a rolled ferromagnetic steel with an aluminum plate inside to distribute the heat as well as possible. It is also known to use several small flat coils in the shape of a spiral over the entire heating surface to distribute the heat more or less evenly. These solutions described above are certainly effective but have the major drawback of being particularly expensive.
[0009] Summary of the invention
[0010] The aim of the present invention is therefore to overcome the drawbacks mentioned above and to propose an alternative to known induction heating devices allowing rapid heating and homogeneous temperature distribution, in order to avoid any deformation of the objects to be heated, said alternative being easy and economical to manufacture and implement.
[0011] According to the invention, there is therefore proposed an induction heating device having a homogeneous heating distribution and comprising at least one inductor defined by at least one resistor, an object to be heated and an electrical power generator connected to the terminals of the inductor, remarkable in that, when the value f of the operating frequency of the electrical power generator is equal to the value fo of the resonant frequency of the induction heating device, the value R5 of the resistance of the inductor is greater than or equal to 0.5 ohms and the quality factor Q of the inductor is less than or equal to 10. According to a first embodiment, the inductor is made from a metal sheet in the form of a flat spiral with a thickness of the order of one millimeter, the electrical insulation function of the spiral being carried out by a sheet of electrically insulating material.
[0012] Said flat spiral shape of said inductor is advantageously obtained by cutting, punching or even stamping.
[0013] According to a second embodiment, the inductor is obtained by depositing a thin layer of electrically conductive metal in a spiral shape on an insulating substrate capable of withstanding high temperatures chosen from the following group: mica, imide-based polymer, ceramic, or silicone.
[0014] Said thin layer of metal is preferably deposited by screen printing.
[0015] According to a third embodiment, the inductor is produced on a printed circuit with a spiral-shaped layer of high-temperature aluminum.
[0016] According to a fourth embodiment, the inductor is in the form of a winding of an electrically conductive metal wire or strip associated with a suitable electrical insulator. According to a final embodiment, the inductor is obtained from an electrically conductive metal tube such as, for example, steel, bent into a spiral shape.
[0017] Brief description of the figures
[0018] Other advantages and characteristics will emerge more clearly from the following description of an embodiment of the invention, with reference to the appended figures in which:
[0019] [Fig 1] is a schematic view showing an induction heating device according to the invention,
[0020] [Fig 2] is a view of the equivalent electrical diagram of the induction heating device of Figure 1,
[0021] [Fig 3] is a schematic view showing an induction heating device according to the invention.
[0022] Description of the embodiments
[0023] In Figure 1, there is shown an induction heating device 1, advantageously for domestic use, having a homogeneous heating distribution and comprising a high-frequency inductor 2 in the form of a spiral arranged under a cooking hob 3, conventionally made of ceramic, for example, on which an object to be heated 4 of the kitchen utensil type is placed. Said inductor 2 is traversed by an alternating electric current coming from a high-frequency electric power generator, for example of the inverter type, not shown in Figure 1, in order to create a magnetic field in which the kitchen utensil 4 is placed, which has the effect of a rise in temperature produced by the eddy currents circulating in the bottom of the kitchen utensil 4.
[0024] It goes without saying that the induction heating device 1 may be of another type such as, for example, a griddle, without departing from the scope of the present invention. In this case, said induction heating device 1 will only comprise the inductor 2 and the object to be heated 4 which will then be a metal cooking plate on which a user can directly cook food.
[0025] It is known to model the induction heating device 1 by an equivalent electrical diagram, such as that shown in Figure 2, in which: - the hob 3 is considered as an inductive resistive load and the magnetic coupling inductor 2 - hob 3 is modeled by an ideal transformer T, that is to say a transformer free from hysteresis and eddy current loss and, consequently, having an efficiency of 100%,
[0026] - a resistance 5 and an inductance 6 of the inductor 2 model the ohmic losses of said inductor 2 and the leakage magnetic flux,
[0027] - a capacitor 7 models the fact that the induction heating device 1 is a resonant electrical system whose best energy efficiency is obtained when said system is excited at its resonant frequency,
[0028] - a resistance 8 and an inductance 9 of the object to be heated 4 model the ohmic and hysteresis losses, and the leakage flux due to eddy currents,
[0029] - an electric power generator 10 is modeled by a voltage source and represents an electrical power supply connected to the terminals of the inductor 2.
[0030] The electrical power generator 10 also comprises, in a conventional manner, an electrical converter, not shown, advantageously of the half-bridge type, well known to those skilled in the art.
[0031] It is understood that the induction heating device 1 can then be modeled by a minimal equivalent electrical diagram, such as that shown in Figure 3, in which:
[0032] - an equivalent resistance 11 corresponds to the grouping of said respective resistances 5, 8 of the inductor 2 and of the object to be heated 4,
[0033] - an equivalent inductance 12 corresponds to the set of said respective inductances 6, 9 of the inductor 2 and of the object to be heated 4,
[0034] - capacitor 7 is the same as that described previously,
[0035] - the voltage source 10 is the same as that described previously.
[0036] Said minimal equivalent electrical diagram of figure 3, corresponds to that of a series RLC circuit, is, in a known manner, such that the value Z of its complex impedance is given by the following formula: Z = Req + jx [Leq x 2 x jt x f- l / (C x 2 x jt x Q] in which;
[0037] - Leq is the value in Henry of said equivalent inductance 12, Leq corresponding to the sum of the values L6, L9 in Henry of the respective inductances 6, 9 of the inductor 2 and of the object to be heated 4,
[0038] - f is the value in hertz of the operating frequency of the electric power generator 10 of the induction heating device 1,
[0039] - C is the value in farads of the capacitance of capacitor 7, and - Req is the value in ohms of said equivalent resistance 11, Req corresponding to the sum of the values R5, R8 in ohms of the respective resistances 5, 8 of inductor 2 and of the object to be heated 4.
[0040] Furthermore, for this type of electrical diagram, it is known that when the value f of the operating frequency of the electric power generator 10 is equal to the value fo of the resonant frequency of the induction heating device 1, the current in the circuit is maximum and the value Z of its complex impedance is equal to Req. Consequently, when the value f of the operating frequency of the electric power generator 10 is equal to the value fo of the resonant frequency of the induction heating device 1, the latter behaves like a resistance and the current passing through said induction heating device 1 generates a rise in temperature by Joule effect of the inductor 2 and the object to be heated 4.
[0041] Furthermore, it is known to characterize the quality of an inductor by the quality factor Q, which is a dimensionless parameter that describes the relationship between the impedance of the circuit and the losses in the circuit and is determined by the following formula: Q = (L x 2 >< 7i; xf) / R, in which L is the value in Henry of the inductance of the inductor concerned, R is the value in ohms of the resistance of said inductor, and f is, as previously described, the value in hertz of the operating frequency of the electric power generator 10.
[0042] In the present invention, the quality factor Q of the inductor 2 is determined by the following formula: Q = (L6 / 2 / 71 / f) / R5, in which L6 is the value in Henry of the inductance 6 of the inductor 2, R5 is the value in ohms of the resistance 5 of said inductor 2, and f is, as previously described, the value in hertz of the operating frequency of the electric power generator 10.
[0043] Furthermore, as described above, it is known that if the value f of the operating frequency of the electric power generator 10 is equal to the value fo of the resonant frequency of the induction heating device 1, then the quality factor Q of the latter is maximum.
[0044] Likewise, it is known that in a minimal equivalent electrical diagram of the RLC type in series, the value fo of the resonant frequency depends on the characteristics of said minimal equivalent electrical diagram and is determined by the following formula: fo = 1 / (2 x K x (Leq x C)), in which Leq is the value in Henry of said equivalent inductance 12 and C is the value in Farad of said capacitor 7. Taking into account the previous formula, said quality factor Q can then also be determined by the following formula: Q = (1 / R5) x (Leq / C).
[0045] Furthermore, when developing the induction heating device 1 according to the invention, the designers cannot act on the respective values R8 and L9 of the resistance 8 and inductance 9 of the object to be heated 4, because the latter is interchangeable. However, these values R8 and L9 are, conventionally, very low or even negligible compared to the respective values R5 and L6 of the resistance 5 and inductance 6 of the inductor 2, so that only the variations of the value L6 are significant in the variations of the value Leq.
[0046] Furthermore, in state-of-the-art induction heating devices, it is sought that, on the one hand, the value of the operating frequency of the alternating current used is substantially equal to the value of the resonant frequency of the induction heating device and, on the other hand, the quality factor Q of the inductor of the latter is as high as possible (of the order of several hundreds), in order to promote the inductive aspect when heating the object to be heated.
[0047] Given the formula Q = (1 / R5) x (L6 / C), it is clear that to obtain the highest possible quality factor Q in state-of-the-art induction heating devices, the resistance value of the inductor must be as low as possible. This characteristic also prevents the inductor from overheating because this is a loss of power and ends up degrading the inductor, which typically uses materials resistant to a maximum temperature of around 220°C.
[0048] Concerning the induction heating device 1 according to the invention, the latter is also configured for the value f of the operating frequency of the alternating current used to be substantially equal to the value fo of its resonant frequency. However, unlike the induction heating devices of the State of the Art, the induction heating device 1 according to the invention is configured for its quality factor Q of its inductor 2 to be as low as possible, of the order of a few units, that is to say less than or equal to 10. For this, taking into account the above, the inductor 2 of the induction heating device 1 is such in particular that the value R5 of its resistance 5 must be high. This characteristic then has the consequence that the electric current passing through said resistance 5 of the inductor 2 generates a rise in temperature of the inductor 2 by Joule effect.This property is then used to homogenize the heating of the cooking utensil 4. Indeed, in addition to induction heating, the induction heating device 1 uses heating of the inductor 2. Furthermore, it is known that the resistance of a material will increase as a function of the temperature, which will have the consequence of further improving the homogeneous distribution of the heating of the induction heating device 1 according to the invention.
[0049] It is clear that, to modify the value of the quality factor Q of inductor 2, the most economically and technically accessible method is to vary the value R5 of its resistance 5.
[0050] Numerous and tedious tests have made it possible to show that, for an operating frequency of the voltage generator 10 having a value f conventionally between 20 kHz and 100 kHz, a value R5 of the resistance 5 of the inductor 2 greater than or equal to 0.5 ohms allows the induction heating device 1 to heat the object to be heated 4 homogeneously by using not only induction heating but also heating of the inductor 2. As a reminder, the minimum and maximum ends of the range of value f of the operating frequency of the voltage generator 10 correspond respectively to a frequency value generating noise nuisance for the user and to a frequency value beyond which the electronic components, for example of the insulated gate bipolar transistor (IGBT) type, suffer irreversible damage.
[0051] To obtain such a value R5 of the resistance 5 of the inductor 2, one can for example play on the section of the conductor of the inductor 2, or on the type of material used. Indeed, copper, traditionally used to produce the inductor of induction heating devices of the State of the Art, has an electrical resistance of the order of 17 pOhms per kilometer, which is very low and suitable for obtaining a high Q factor. Thus, for the inductor 2 of the induction heating device 1 according to the invention, a more resistive material will be preferred such as, for example, iron which has an electrical resistance of the order of 100 pOhms per kilometer.
[0052] In the following, several embodiments of the inductor 2 of the induction heating device 1 according to the invention will be described.
[0053] Thus, according to a first embodiment, the inductor 2 is made from a metal sheet with a thickness of the order of one millimeter (i.e. between 0 and 5 millimeters), the flat spiral shape of said inductor 2 being obtained by cutting, punching or even stamping. The electrical insulation function of the spiral is advantageously carried out by a sheet of electrically insulating material of the mica type, for example.
[0054] According to a second embodiment, the inductor 2 is obtained by depositing, by screen printing for example, a thin layer of electrically conductive metal in a spiral shape on an insulating substrate capable of withstanding high temperatures (the term "high temperatures" designating in the present application temperatures above 300°C) such as, for example, mica, imide-based polymer, ceramic, or even silicone.
[0055] Here, the term "thin layer of electrically conductive metal" refers to a layer of said metal whose thickness does not exceed 100 micrometers (i.e. 0.1 millimeter).
[0056] According to a third embodiment, the inductor 2 is produced on a printed circuit with a layer of high-temperature aluminum (above 300°C) in the shape of a spiral.
[0057] According to a fourth embodiment, the inductor 2 is in the form of a winding of a wire or strip of electrically conductive metal such as, for example, anodized aluminum.
[0058] According to a final embodiment, the inductor 2 is obtained from an electrically conductive metal tube such as, for example, steel, bent into a spiral shape.
[0059] It is understood that all the embodiments previously described are particularly easy and inexpensive to implement.
[0060] A person skilled in the art will know, whatever the embodiment of the inductor 2 of the induction heating device 1 according to the invention, how to choose the shape, dimensions and type of material to be used so that the value R5 of the resistance 5 of the inductor 2 is sufficient to obtain, when the value f of the operating frequency of the electrical power generator 10 is equal to the value fo of the resonance frequency of the induction heating device 1, a low quality factor Q, i.e. less than or equal to 10.
[0061] Therefore, it is clear that the induction heating device 1 according to the invention goes against the teachings of the induction heating devices of the State of the Art, by being configured so as, on the one hand, to have a low quality factor and, on the other hand, to guarantee homogeneous heating by using the temperature rise of the inductor 2 by Joule effect. In addition, it is also clear that the technical problem addressed by the present invention is solved by an induction heating device 1 that is easy and economical to manufacture and implement. The induction heating device 1 according to the invention finds a particular application in domestic use. However, it is obvious that the induction heating device 1 can be adapted, for example to have an inductor 2 in a cylindrical shape, in order to be used for industrial use.
[0062] Finally, it goes without saying that the examples of induction heating devices 1 in accordance with the invention which have just been described are only particular illustrations, in no way limiting the invention.
Claims
CLAIMS 1. Induction heating device (1) having a homogeneous heating distribution and comprising at least one inductor (2) defined by at least one resistor (5), an object to be heated (4) and an electric power generator (10) connected to the terminals of the inductor (2), characterized in that, when the value f of the operating frequency of the electric power generator (10) is equal to the value fo of the resonant frequency of the induction heating device (1), the value R5 of the resistor (5) of the inductor (2) is greater than or equal to 0.5 ohms and the quality factor Q of the inductor (2) is less than or equal to 10.
2. Induction heating device (1) according to claim 1 characterized in that the inductor (2) is made from a metal sheet in the form of a flat spiral with a thickness of the order of one millimeter, the electrical insulation function of the spiral being carried out by a sheet of electrically insulating material.
3. Induction heating device (1) according to claim 2 characterized in that the flat spiral shape of said inductor 2 is obtained by cutting, punching or even stamping.
4. Induction heating device (1) according to claim 1 characterized in that the inductor (2) is obtained by depositing a thin layer of electrically conductive metal in a spiral shape on an insulating substrate capable of withstanding high temperatures chosen from the following group: mica, imide-based polymer, ceramic, or silicone.
5. Induction heating device (1) according to claim 4 characterized in that the thin layer of metal is deposited by screen printing.
6. Induction heating device (1) according to claim 1 characterized in that the inductor (2) is produced on a printed circuit with a layer of high temperature aluminum in the form of a spiral.
7. Induction heating device (1) according to claim 1 characterized in that the inductor (2) is in the form of a winding of an electrically conductive metal wire or strip associated with a suitable electrical insulator.
8. Induction heating device (1) according to claim 1 characterized in that the inductor (2) is obtained from an electrically conductive metal tube such as, for example, steel, bent into a spiral shape.