EP1122983B2 - Induction domestic cooking appliance - Google Patents

Description

The invention relates to a cooking device.

There are cooking devices known with a cooktop plate with two or four cooking zones, in which below at least one of the cooking zones known by a generator high-frequency controllable induction coil for inductive heating of placed on the cooking zone cookware. The induction coil is fixed on a metallic carrier device.

The invention has for its object to provide a special cooking device with induction heating.

This object is achieved according to the invention with the features of claim 1.

1. a) wenigstens eine Induktionsheizeinrichtung zum Erzeugen eines zeitlich veränderlichen magnetischen Feldes mit wenigstens einer Induktionsspule (mit wenigstens einer Windung) und wenigstens einer Steuereinheit zum Anlegen eines zeitlich veränderlichen elektrischen Feldes an die Induktionsspule sowie
2. b) eine auf ein im Wesentlichen konstantes elektrisches Potential gelegte Trägereinrichtung (Gehäuse, Chassis) aus elektrisch leitfähigem Material mit einer Trägerfläche, auf der die Induktionsspule, im Allgemeinen elektrisch isoliert, angeordnet ist.

The cooking device initially comprises

1. a) at least one induction heating device for generating a time-varying magnetic field with at least one induction coil (with at least one turn) and at least one control unit for applying a time-varying electric field to the induction coil and
2. b) a laid to a substantially constant electrical potential carrier means (housing, chassis) of electrically conductive material having a support surface on which the induction coil, generally electrically isolated, is arranged.

The invention is based on the surprising observation, based on measurements, that during operation of the induction heating device, high-frequency interference currents may occur which exceed predetermined limit values according to the EN 55011 standard, when the carrier device is switched to constant electrical potential, in particular earthed.

The invention further proceeds from a conclusion deduced from the measurements that an excessively high electrical capacitance between the induction coil and the carrier device is responsible for the interference currents. Even with an electrical insulation of the induction coil of the support device, which at least largely prevents a direct charge flow, still made of induction coil and carrier capacitor, especially in the typical for induction heating high frequency range, as a parasitic capacitance to a certain extent a weak point for displacement currents to the constant potential represents.

According to the invention, it is therefore proposed to provide means for reducing the electrical capacitance between the induction coil and the support device according to claim 1.

Advantageous embodiments and further developments of the cooking device according to the invention will become apparent from the dependent claims of claim 1.

Accordingly, the carrier device consists in particular of metal, for example of aluminum, an aluminum alloy, copper or steel, at least in the region of the carrier surface, wherein the metal is preferably used in the form of a metal sheet.

The invention comprises at least one spacer (spacer, spacers) of dielectric material, preferably with a low relative dielectric constant (as close as possible to 1), for increasing the distance between the induction coil and the support means.

• wenigstens einen Zwischenraum aus dielektrischem Gas wie Luft,
• eine oder mehrere Öffnungen (Löcher) in der Trägerfläche der Trägereinrichtung zum Verringern von deren kapazitiv wirksamer Fläche,
• eine oder mehrere Ausnehmungen oder Einbuchtungen in der Trägerfläche der Trägereinrichtung zum Vergrößern von deren Abstand zur Induktionsspule in diesen Bereichen, wobei die dadurch gewonnenen Zwischenräume mit dielektrischem festen oder gasförmigen Material gefüllt werden.

The means for reducing the electrical capacitance between the induction coil and the carrier device may comprise in advantageous embodiments, which can also be combined with each other:

• at least one space of dielectric gas such as air,
• one or more openings (holes) in the support surface of the support means for reducing its capacitive area,
• one or more recesses or indentations in the support surface of the support means to increase their distance from the induction coil in these regions, the interspaces obtained thereby being filled with dielectric solid or gaseous material.

The openings may be punched in the carrier surface. The recesses or recesses may be embossed in the support surface.

The cooking device may be formed as an induction cooking and then includes at least one hob plate for setting up cookware, each induction coil is disposed below an associated cooking zone of the hob plate. Alternatively, of course, a cooking oven can be equipped with a closable cooking chamber with the induction heating according to the invention.

The frequency components of the electric field of the generator are generally above a fundamental frequency between about 20 kHz and about 100 kHz, preferably about 25 kHz, and range in the harmonic frequencies up to the range of 10 MHz.

In a particularly advantageous embodiment, the control unit comprises a common-mode filter unit which can be connected to a mains voltage and is electrically coupled to the constant potential for suppressing common-mode interference. The common-mode filter unit preferably comprises two inductances connected in opposite directions, in each case one pole of the mains voltage, in particular a current-compensated toroidal choke with two coils arranged in opposite directions on a toroidal core, wherein preferably each inductor is connected via an associated capacitor is electrically coupled to the constant potential.

The induction coil also includes on its underside field guide body made of a magnetically conductive material, in particular a ferritic or densitic material, for guiding the magnetic field of the induction coil. There are then the induction coil with their Feldleitkörpern on the support device. The parasitic capacitance between the induction coil and the support means can now also be reduced by reducing the field guide bodies and the additional spaces filled with air or other dielectric material formed thereby.

FIG 1

eine Induktionsgareinrichtung mit einer Trägereinrichtung mit Prägungen,

FIG 2

eine Induktionsgareinrichtung mit einer Trägereinrichtung mit Öffnungen,

FIG 3

eine Induktionsgareinrichtung mit einer Trägereinrichtung und einer zusätzlichen dielektrischen Schicht zwischen Trägereinrichtung und Feldleitmitteln,

FIG 4

eine Induktionsgareinrichtung, die dem Prinzip nach dem Stand der Technik entspricht,

FIG 5

eine elektrische Schaltung der Steuereinheit für eine Induktionsgareinrichtung,

FIG 6

eine Induktionsspulenanordnung gemäß dem Stand der Technik in einer perspektivischen Ansicht.

Einander entsprechende Teile sind in den FIG 1 bis 6 mit denselben Bezugszeichen versehen.The invention will be explained below with reference to exemplary embodiments. Reference is made to the drawing, in each of which shows in a schematic representation:

FIG. 1

an induction cooking device with a carrier device with imprints,

FIG. 2

an induction cooking device with a carrier device with openings,

FIG. 3

an induction cooking device with a carrier device and an additional dielectric layer between carrier device and field-guiding means,

FIG. 4

an induction cooking device, which corresponds to the principle of the prior art,

FIG. 5

an electric circuit of the control unit for an induction cooking device,

FIG. 6

an induction coil assembly according to the prior art in a perspective view.

Corresponding parts are in the 1 to 6 provided with the same reference numerals.

In FIG. 4 An Induktionsgareinrichtung is shown, the structure of which is known in principle. The Induktionsgareinrichtung comprises a hob plate 1, on which in a cooking zone a cooking product 2 is placed, in which food is 6. In the bottom of the Gargutbehälters 2 a susceptor coil 5 is shown. Below the cooking zones of the hob plate 1, an induction coil 3 is arranged, which is designed as a flat coil with a substantially parallel to the hob plate 1 aligned plane.

The induction coil 3 is arranged on one or more field-guiding bodies 4, preferably by means of an adhesive connection. The one or more field guide 4 are arranged on a support surface 70 of a support means 7, in particular also glued, and are preferably made of a material having a high magnetic permeability and with the lowest possible electrical conductivity, such as a ferritic or densitic material.

The adhesive layers are not only intended for fixing, but also isolate the components electrically from each other. Instead of an adhesive connection, other fixing means for connecting the induction coil 3 to the field conducting bodies 4 or the carrier device 7 or the field guide 4 may be provided with the support means 7, for example a releasable connection such as a screw or latching connection. Electrical isolation is then generally achieved by separate means.

A special, known per se embodiment of the induction heater is in FIG. 6 shown. The induction coil 3 is enclosed together with six star-shaped field guide bodies 4 in a housing 15, which consists in particular of a heat-resistant plastic. The side up in the view is the bottom, with which the whole unit is placed on the carrier device.

The carrier device 7 consists of an electrically conductive material, in particular metal, for example aluminum or an aluminum alloy, copper or a steel, in particular of a metal sheet. The support means 7 is in the illustrated embodiment, a box-shaped with the continuous support surface 70 on the induction coil 3 side facing and laterally thereof downwardly projecting side walls. On a bottom wall 71, an electric control unit 8 is worn, which drives the induction coil 3 electrically. In particular, the control unit 8 is fastened to the carrier device 7 by means of one or more screws 11. The screw 11 at the same time establishes an electrical connection between the control unit 8 and the carrier device 7, which is connected directly to the earth (ground) or, more generally, to a substantially constant potential. The electrical connection of the carrier device 7 to the ground can also be realized independently of the mechanical fastening with a separate electrically conductive connection, for example an electrical cable and / or a clip wiring.

The control unit 8 supplies during operation of the Induktionsgareinrichtung the induction coil 3 with a time-varying electric field (electrical voltage) containing high-frequency components in the range from a fundamental frequency (fundamental frequency) of 25 kHz. By induction, the induction coil 3 now generates a time-varying magnetic field whose flux density in the FIG. 4 is denoted by B and whose field lines are shown in dashed lines, with a frequency spectrum corresponding to the electric field of the control unit 8. The field guide 4 concentrate and concentrate the magnetic field B, so that it is largely shielded down. At the top, the magnetic field B penetrates the cooking field plate 1, which consists of a dielectric heat-resistant material, for example a glass ceramic, glass or a ceramic, and induces an electric current in the susceptor coil 5 of the food container 2 which heats the food container 2 by Joule losses. The heating effect is generally reinforced by the fact that a ferromagnetic material is provided, whereby additional thermal energy is released by Ummagnetisierungsprozesse in the material of the Gargutbehälters 2. It can then be the bottom of the Gargutbehälters 2 simply made of ferromagnetic material. A susceptor coil 5 is not essential.

In addition to the mechanical carrier function for the induction coil 3, the carrier device 7 additionally has the functions of dissipating heat from the induction coil 3 and homogenizing the temperature distribution and adapting the resonance frequency. Furthermore, the carrier device 7 shields the electronic components of the control unit 8 from the magnetic field of the induction coil 3.

A measurement according to EN 55011 has now shown that in this known Induktionsgareinrichtung according to FIG. 4 undesired interference currents (and interference fields caused thereby) occur in a specific high-frequency range, in particular in a range between 150 kHz and 500 kHz. It has also been found that these parasitic currents are essentially due to a parasitic capacitance C _D between the induction coil 3 and the carrier device 7. According to the invention, it is therefore proposed to reduce this parasitic capacitance C _D or even eliminate it as completely as possible.

wobei ε₀ die Elektrizitätskonstante im Vakuum ist, ε_r die relative Elektrizitätskonstante des zwischen der Induktionsspule 3 und der Trägereinrichtung 7 befindlichen dielektrischen Materials, A die wirksame Fläche des Plattenkondensators sowie d der Abstand zwischen den beiden Kondensatorplatten, also der Induktionsspule 3 und der Trägereinrichtung 7, sind. Ausgehend von dieser Formel gibt es nun eine Reihe von Möglichkeiten, die parasitäre Kapazität C_D zwischen der Induktionsspule 3 und der Trägereinrichtung 7 zu verringern:

• Vergrößern des Abstandes zwischen Induktionsspule 3 und Trägereinrichtung 7 durch Einbringen zusätzlichen dielektrischen Materials, insbesondere einer dickeren Schicht oder einer zusätzlichen Schicht, oder eines zusätzlichen Luftzwischenraumes,
• Verringern der einander gegenüberliegenden Flächen des aus Induktionsspule 3 und Trägereinrichtung 7 gebildeten Kondensators, insbesondere Verringerung der Trägerfläche 70,
• Verwendung eines dielektrischen Zwischenmaterials zwischen Induktionsspule 3 und Trägereinrichtung 7 mit einer kleineren relativen Dielektrizitätskonstante.

Now, an electric capacitance of a plate capacitor, which is applicable here to a good approximation, is defined by the following formula $$\mathrm{C}=\left(\begin{array}{ccc}{\epsilon }_0& {\epsilon }_{\mathrm{r}}& \mathrm{A}\end{array}\right)/\mathrm{d}.$$

where ε _{0 is} the dielectric constant in a vacuum, ε _{r is} the relative dielectric constant of the dielectric material between the induction coil 3 and the carrier device 7, A is the effective area of the plate capacitor and d is the distance between the two capacitor plates, ie the induction coil 3 and the carrier device 7 , are. Starting from this formula, there are now a number of possibilities for reducing the parasitic capacitance C _D between the induction coil 3 and the carrier device 7:

• Increasing the distance between induction coil 3 and carrier device 7 by introducing additional dielectric material, in particular a thicker layer or an additional layer, or an additional air gap,
• Reducing the opposing surfaces of the capacitor formed from induction coil 3 and carrier device 7, in particular reduction of the carrier surface 70,
• Use of a dielectric intermediate material between induction coil 3 and carrier device 7 with a smaller relative dielectric constant.

FIG. 1 shows a possibility in which the support surface 70 of the support means 7 is further spaced from the induction coil 3 by introducing indentations 40 in these areas. The field guide 4 are then only in the lying outside of the indentation 40 areas of the support surface 70 of the support means 7, while between the Feldleitkörpern 4 and the downwardly bulging support surface 70 of the support means 7 now air-filled spaces are formed. The indentations 40 may in particular be produced by embossing in the metal sheet and may be formed in a variety of embodiments and design options. The depth of the indentations 40, which corresponds to the increase in the distance between the induction coil 3 and support means 7, may in particular be in the range between 0.2 mm and 5 mm, preferably in the range between 0.5 mm and 2 mm. This embodiment according to FIG. 1 has the advantage that the electromagnetic shielding effect of the carrier device 7 is essentially substantially preserved via the carrier surface 70.

FIG. 2 shows a further possibility for reducing the parasitic capacitance C _D between the induction coil 3 and the support means 7. There are now provided in the support surface 70 of the support means 7 below the field guide 4 more openings 41, which may be punched, for example. Through these openings 41, the induction coil 3 opposite, formed by the support surface 70 capacitor area is reduced and thus the parasitic capacitance C _D also reduced. The openings 41 can be designed differently. In the case of a circular induction coil 3, a radial arrangement of the openings 41 on radial rays offset relative to each other offers itself, wherein, as illustrated, in particular the inside width of the openings 41 can increase towards the outside. The field guide 4 are now again only in the intermediate regions between the openings 41 on the support surface 70. The shape and arrangement of the openings 41 in the carrier device 7 is preferably dependent on the desired shielding effect, which in turn is dependent on the used frequency spectrum of the induction coil 3 and control unit 8 induction heating device, and the other properties, in particular the thermal properties of the carrier device. 7 selected.

According to the invention, a reduction of the parasitic capacitance C _D between the induction coil 3 and the support means 7, which in FIG. 3 is illustrated. In this embodiment, an additional dielectric layer 10 is arranged as a spacer between the carrier surface 70 of the carrier device 7 and the field-guiding bodies 4. This dielectric layer 10 is dependent chosen from the desired parasitic capacitance C _D , in particular with regard to their layer thickness and their relative dielectric constants. The layer thickness of the dielectric layer 10 can be selected in particular in the range between 0.2 mm and 5 mm, preferably in the range between 0.5 mm and 2 mm. The relative dielectric constant is preferably chosen as close to 1 as possible. The dielectric layer 10 may in particular be a foil or even a sheet of high-temperature paper-like material which is glued to the field guide body 4 and the carrier surface 70.

Of course, in the 1 to 3 shown various possibilities for reducing the parasitic capacitance C _D between the induction coil 3 and the support means 7 are combined with each other in different variants.

FIG. 5 shows an electrical circuit diagram for the control unit 8 of Induktionsgareinrichtung according to the 1 to 4 , On the left side of the circuit diagram is the mains voltage U _N , which corresponds to the normal power supply 230 V with a frequency of 50 Hz. Between the two phases of the mains voltage U _N two capacitors C1 and C2 and a rectifier bridge G are connected. Serially in one of the two phases, two inductors L1 and L3 and a main switch S3 are connected and in the other phase, a further inductance L2. Furthermore, a capacitor C3 or C4 departs from each phase into a branch which is connected to earth 9.

With the two capacitors C1 and C2 and the inductance L3, a differential filter is formed which suppresses differential mode noise in the two phases of the mains voltage UN.

The two inductors L1 and L2 and the two capacitors C3 and C4 form a filter for suppressing common mode noise in the phases of the supply voltage U _N (common mode noise filter). The two inductors L1 and L2 can be formed in particular with a current-compensated toroidal core choke with two opposing coils on a common ring core, but of course also be realized with two separate components. Since the inductors L1 and L2 at the same time must be able to pass the full mains voltage U _N in push-pull operation (antiphase normal operation), the requirements for the inductors L1 and L2 representing coils with the toroidal core are quite high. In order to avoid saturation of the core by the push-pull interference, namely quite high-quality magnetic materials are required.

The parasitic capacitance C _D from the inductance L of the induction coil 3 to the carrier device 7 and thus to the earth 9 is connected via the capacitors C3 and C4 to the filter for the suppression of common-mode noise and brings by the common-mode noise caused by it or the cores of the coils for the inductors L1 and L2 are even faster in saturation. The reduction of the parasitic capacitance C _D achieved in accordance with the invention now has the great advantage that the component or components for the inductors L1 and L2 must or must meet lower requirements and, as a result, a considerable cost saving is achieved.

The on the right side of the diagram according to FIG. 5 shown part of the circuit is one of several possible standard circuits for the implementation of the mains voltage U _N in the required for the operation of the induction coil 3 with the inductance L high frequency voltage U _HF . This control circuit, also referred to as a half-bridge, contains two separately controllable electronic switches S1 and S2, for example semiconductor switching elements such as thyristors, bipolar power transistors, IGBTs, Darlingtons circuits or MCTs, as well as three capacitors C5, C6 and C7 and the already mentioned rectifier bridge G. The high-frequency voltage U _HF is preferably in a frequency range above the fundamental frequency 25 kHz. Other possibilities, not shown but standard, for generating the high frequency voltage U _HF are a single switch and a full bridge with four switches. The advantages of the measures according to the invention also apply to these control circuits.

Instead of the illustrated embodiments, the Induktionsgareinrichtung, with respect to the mechanical structure and the materials used as also be formed with respect to the electrical conditions, in another known embodiment, for example, according to one of the in WO 97/20451, WO 98/41061, WO 98 / 41062 , WO 98/41063 or WO 98/41064 disclosed embodiments, then according to the invention again corresponding means for reducing the capacitance between the carrier and coil are provided.

Claims (10)

1. Cooking device having

   a) at least one induction heating device for producing a temporally changeable magnetic field with at least one induction coil (3) and at least one control unit (8) for applying a temporally changeable electric field to the induction coil,

   b) a support device, in particular an earthed support device, set to an essentially constant electric potential made from electrically conductive material having a support surface (70) for the induction coil,

   c) means (10, 40, 41) for reducing the electrical capacitance (C_D) between the induction coil and the support device,

   d) in which field conductors (4) made from a magnetically conductive material, in particular a ferritic material, are arranged on an underside of the induction coil,

   e) in which the means for reducing the electrical capacitance comprise at least one intermediate layer (10) made from dielectric material, preferably having a low relative dielectric constant, arranged between the induction coil and the support device,

   f) wherein the layer thickness of the intermediate layer (10) of dielectric material is chosen in the range between 0.2 mm and 5 mm, preferably in the range between 0.5 mm and 2 mm,

   g) wherein the intermediate layer (10) is a film or a sheet of high-temperature-paper-like material that is adhesively bonded to the field conductors (4) and the support surface (70).
2. Cooking device according to Claim 1, in which the means for reducing the electrical capacitance additionally comprise one or more openings (41) in the support surface of the support device.
3. Cooking device according to either of the preceding claims, in which the means for reducing the electric capacitance additionally comprise one or more recesses or indentations (40) in the support surface of the support device.
4. Cooking device according to one of the preceding claims, in which the support device, at least in the region of the support surface, consists of metal or a metal alloy, in particular of aluminium, copper or steel, in particular of a sheet metal.
5. Cooking device according to Claim 2 and Claim 4, in which the openings are punched into the support surface.
6. Cooking device according to Claim 3 and 4, in which the recesses or indentations are embossed into the support surface.
7. Cooking device according to one of the preceding claims, having at least one hob plate (1) for placing cookware, wherein each induction coil is arranged below an associated cooking zone of the hob plate.
8. Cooking device according to one of the preceding claims, in which the frequency portions of the electric field of the control unit lie between about 20 kHz and about 100 kHz, preferably above about 25 kHz.
9. Cooking device according to one of the preceding claims, in which the control unit comprises a common-mode filter unit, which is electrically coupled to the constant potential and can be connected to a mains voltage, for suppressing common-mode interferences.
10. Cooking device according to Claim 9, in which the common-mode filter unit comprises two inductances connected in opposite senses which can be electrically connected to in each case one pole of the mains voltage, in particular a current-compensated toroidal core inductor having two coils arranged in opposite senses on a toroidal core, wherein each inductance is preferably electrically coupled to the constant potential via an associated capacitor.

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