EP2027754B2 - High frequency induction heating device, and induction oven equipped with such device - Google Patents

Abstract

The induction heating device includes: a high frequency electric supply; an suitable inductor (3) for encircling, at least partially, an induced element (4) for heating, this inductor having an inductance of determined value (L) for the desired heating power; and a capacitive mounting used to connect the supply and the inductor, designed to increase the voltage in the inductor terminals in comparison with the voltage supplied. The inductor consists of at least two distinct elementary inductive sections (3a, 3b, 3c) inter-connected in series by at least one condenser (2, 2a, 2b), the voltage which appears between the connection points of the elementary inductive parts being reduced in comparison with the voltage required for the inductor to function for the desired heating power.

Description

• une seule alimentation électrique à haute fréquence,
• un inducteur propre à entourer, au moins partiellement, un élément induit à chauffer, l'inducteur ayant une inductance de valeur déterminée pour une puissance de chauffage souhaitée,
• et un montage capacitif de liaison entre l'alimentation et l'inducteur prévu pour augmenter la tension aux bornes de l'inducteur par rapport à la tension fournie par l'alimentation.

The invention relates to an induction heating device of the kind comprising:

• a single high frequency power supply,
• an inductor adapted to surround, at least partially, an inductive element to be heated, the inductor having a value inductance determined for a desired heating power,
• and a capacitive connection between the power supply and the inductor provided to increase the voltage across the inductor with respect to the voltage supplied by the power supply.

By the term "high frequency" used for the alternating current is meant a frequency equal to or greater than 10 kilohertz (kHz).

• amélioration de la productivité de four conventionnel ;
• séchage de produits de revêtement divers (peinture, galvanisation, etc...);
• traitement thermique (recuit, survieillissement...),

et d'autres encore.An application of induction heating concerns metal strips. Induction heating of metal strips is increasingly used by the iron and steel industry for various applications such as:

• improvement of conventional oven productivity;
• drying of various coating products (painting, galvanizing, etc.);
• heat treatment (annealing, over-aging ...),

and even more.

Progress in semiconductor performance is now making it possible to manufacture frequency converters with a unit power of several megawatts and an output frequency of several hundred kilohertz.

• L est égal à l'inductance de l'inducteur 3 (exprimée en henry),
• ω est la pulsation du courant alternatif du courant d'alimentation (en radians par seconde)
• I est l'intensité du courant dans l'inducteur 3 (en ampères).
• ω = 2πF, avec F égal à la fréquence du courant alternatif.

The principle of a conventional installation, according to the state of the art, is illustrated on the Figures 1 to 3 appended drawings in the case of a parallel oscillating circuit and a longitudinal flux inductor. Another example of induction heating is known from US 5,837,976 . A high frequency AC power supply 1 consists of a frequency converter. One or more compensation capacitors 2 are connected to the terminals of the converter. The inductor 3 surrounds a metal strip 4 to be heated; the length of the strip 4 is perpendicular to the plane of Fig.1 . Intensity currents of intensity I surround the band 4 so as to create induced currents whose circulation is schematically represented by a dotted line. The frequency converter 1 provides the active power required for heating the metal strip 4. The capacitor, or the capacitor bank, 2 provides the reactive power necessary for the magnetization of the space in which the magnetic field created by the inductor 3 develops. according to Fig.3 two link capacitors 2 'in series, combined with the capacitor 2, are provided to increase the voltage across the inductor. The circuit formed by the inductor 3 and the capacitors 2, 2 'is an oscillating circuit, and the voltage U (expressed in volts) which develops at the terminals of the inductor 3 (apart from the ohmic resistance). writes: U = LωI, formula in which:

• L is equal to the inductor inductor 3 (expressed in henry),
• ω is the alternating current pulsation of the supply current (in radians per second)
• I is the intensity of the current in the inductor 3 (in amperes).
• ω = 2πF, with F equal to the frequency of the alternating current.

• au carré du champ magnétique, lui-même proportionnel au nombre d'ampères tours n.l , où n est le nombre de spires de l'inducteur 3 ou circuit primaire,
• à la racine carrée de la pulsation ω du courant.

The power Pt transferred by induction to the band 4 is proportional:

• the square of the magnetic field, itself proportional to the number of ampere turns nl, where n is the number of turns of the inductor 3 or primary circuit,
• at the square root of the pulsation ω of the current.

To transmit a high power, it is necessary, all things being equal, that the current I is important, of the order of a few thousand amperes to several tens of thousands of amperes.

The inductance L because of the geometrical size of the inductor 3, often connected to that of the armature 4 that it surrounds, can not fall below a minimum value of the order of a few tenths of a microhenry for band heating inductors, even by constructing inductors called "monospires", that is to say, a single turn.

Finally, ω reaches high values to allow heating of thin strips 4 and avoid a drop in system efficiency.

The equation U = LωI directly shows that the large current I, necessary for the transmitted power to be high, can only be obtained by applying a voltage U of several thousand volts. This voltage is most often reached by a capacitive assembly "voltage multiplier or capacitive elevator". according to Fig.3 this capacitive elevator involves two capacitors 2 'in series with the inductor.

To ensure good oscillation conditions of the circuit formed by the inductor 3 and capacitors 2, 2 'of the inductor circuit, it is generally endeavored to satisfy the resonance condition between the values of inductance L and the capacitance C _eq equivalent to those of the capacitors of the circuit, according to the formula: LC _eq ω ² = 1

In the example of Fig.3 ; considering the case where the three capacitors 2, 2 'each have the same capacity value C, the equivalent capacity C _eq of these three capacitors in series in the closed loop comprising the inductor is C _eq = C / 3.

• d'où : L ω = 3/ Cω

The relation: LC _eq ω ² = 1 becomes, by replacing C _eq , by C / 3: $$\mathrm{LC}/\mathrm{3}{\mathrm{<figure-callout\; id="2"\; label="\omega "\; filenames="imgf0001.png,imgf0003.png"\; state="\{\{state\}\}">\omega </figure-callout>}}^{\mathrm{2}}=\mathrm{1}$$

• hence: L ω = 3 / Cω

The voltage across the inductor LωI is therefore equal to 3 I / Cω, while the voltage across the compensation capacitor 2, which corresponds to the supply voltage, is equal to I / Cω. The voltage across the inductor is multiplied by three.

The very high voltage applied to the inductor poses many equipment realization problems, which must avoid an undesirable voltage drop between the capacitor, or the capacitor bank, and the inductor. The connection connections must be "compensated", that is to say have a minimum connection inductance. This reduction in the inductance of the connections can only be obtained by bringing the supply conductors of the inductor and the internal conductors of the capacitor bank as close as possible. As illustrated schematically on Fig.2 , the distance e between the connection connections must be minimal to limit the parasitic inductance of the connection.

But we come up against the presence of a very high voltage. The short distance e between the connections makes it particularly difficult to achieve reliable electrical insulation to avoid a breakdown of the insulation (flash).

The same constraint applies to the capacitor bank, often consisting of elementary capacitors with low resistance in unit voltage, connected in series-parallel.

The object of the invention is, above all, to provide an induction heating device which makes it possible to transfer high power through the inductor while reducing the construction difficulties created by the voltages, in particular at the level of the inductor connections. and capacitors.

According to the invention, an inductive heating device of the kind defined above is characterized in that the inductor consists of at least two distinct inductive elements connected together in series by at least one capacitor, the voltage which appears between the connection points of the elementary inductive parts being reduced with respect to the voltage necessary for the operation of the inductor, for a desired heating power.

The device may comprise a capacitor connected between the terminals of the power supply and three distinct inductive elements, having or not even elementary inductance, interconnected in series by two connecting capacitors.

According to one variant, the device may comprise a capacitor connected between the terminals of the power supply and two distinct inductive elements, interconnected in series by a connecting capacitor.

In a more general manner, the device comprises a capacitor connected between the terminals of the power supply and N distinct inductive elements, interconnected in series by N-1 connecting capacitors.

Advantageously, the connection capacitor or capacitors have capacitance values equal to each other, and equal to the capacitance of the capacitor connected to the terminals of the supply.

Nevertheless, it is possible to obtain particular and non-integer voltage rise factors by using one or more capacitor banks of equivalent value different from that of the capacitor bank connected across the power supply. In this case, the capacitors, or capacitor banks, have capacitance values different from those of the capacitor, or of the capacitor bank, connected to the terminals of the high frequency power supply, to obtain elevation ratios. non-integer voltage.

Preferably, the connection capacitor (s) are located in a zone opposite to the supply with respect to the armature.

The capacitors or capacitor banks can be distributed over the perimeter of the armature in order to optimize the occupation of the space in the vicinity of the armature.

The or each connecting capacitor may consist of a capacitor bank.

Preferably, the capacity C _eq equivalent to those of the capacitors of the circuit in which the elementary inductors are located is related to the inductance L equivalent to that of the inductive parts by the LC relation _eq ω ² = 1.

The invention also relates to an induction heating furnace.

A heating furnace for metal strips, according to the invention, is characterized in that it comprises an induction heating device, with one or more turns, as defined above. Advantageously, the capacitors, or capacitor banks, of connection can be placed inside the furnace envelope, in the atmosphere of the furnace.

Another application of induction heating according to the invention is the melting of glass or oxide in a direct coil.

The invention consists, apart from the arrangements described above, in a certain number of other arrangements which will be more explicitly discussed hereinafter with regard to exemplary embodiments described with reference to the appended drawings, but which are not in no way limiting.

• Fig.1 est un schéma d'un dispositif de chauffage par induction selon l'état de la technique.
• Fig.2 est un schéma semblable à Fig.1, avec un inducteur monospire représenté en perspective, selon l'état de la technique.
• Fig.3 est un schéma d'un circuit avec batterie de condensateurs multiplicateurs de tension, selon l'état de la technique.
• Fig.4 est un schéma d'un dispositif de chauffage selon l'invention.
• Fig.5 est un schéma équivalent du dispositif de Fig.4.
• Fig.6 est une variante du schéma de Fig.5.
• Fig.7 est un schéma en perspective illustrant le déplacement d'une bande à chauffer dans un inducteur monospire.équipé d'un condensateur de liaison
• Fig.8 est un schéma d'un four de fusion de verre, à une seule spire, selon l'état de la technique, et enfin
• Fig.9 est un schéma d'un four de fusion de verre monospire selon l'invention.

On these drawings:

• Fig.1 is a diagram of an induction heating device according to the state of the art.
• Fig.2 is a diagram similar to Fig.1 , with a monosphere inductor shown in perspective, according to the state of the art.
• Fig.3 is a diagram of a circuit with capacitor voltage multiplier battery, according to the state of the art.
• Fig.4 is a diagram of a heating device according to the invention.
• Fig.5 is an equivalent diagram of the device of Fig.4 .
• Fig.6 is a variant of the schema of Fig.5 .
• Fig.7 is a perspective diagram illustrating the displacement of a strip to be heated in a monospire inductor.equipped with a connection capacitor
• Fig.8 is a diagram of a glass melting furnace, with a single turn, according to the state of the art, and finally
• Fig.9 is a diagram of a monospire glass melting furnace according to the invention.

Referring to Fig.4 an inductive heating device H according to the invention can be seen which comprises a frequency converter 1 constituting the high frequency power supply equal to or greater than 10 kHz. A capacitor, or a capacitor bank, of compensation 2 is connected between the terminals of the power supply 1.

The inductor, connected to the terminals of the capacitor 2 and the power supply, consists of at least two distinct inductive elements. According to the exemplary embodiment of Fig.4 , the inductor consists of three elementary inductive parts 3a, 3b, 3c interconnected in series by at least one capacitor, or capacitor bank, 2a, 2b. The inductance equivalent to that of the elementary inductive parts 3a, 3b, 3c, in series is equal to the determined value L of the inductor 3 of a conventional assembly according to Fig.1 to 3 .

According to the example of Fig.4 the voltage U1 develops between the terminals 6a and 7c of the respective elementary inductive parts 3a and 3c; the voltage U2 between the terminals 7a and 6b of the elementary inductive parts 3a and 3b and the voltage U3 between the terminals 7b and 6c of the elementary inductive parts 3b, 3c. The capacitor 2 is connected to the terminals 6a, 7c whereas the capacitors 2a, 2b are respectively connected to the terminals 7a, 6b and to the terminals 7b, 6c.

The voltages U1, U2 and U3 being reduced relative to the operating voltage of the inductor 3 according to Fig.1 to 3 , the electrical insulation requirements and the precautions to be taken for the realization of the equipment are also reduced and the drivers can be brought closer to one another with a reduction of the risk of breakdown.

According to the exemplary embodiment of Fig.4 the inductor formed by all of the elementary inductive parts 3a, 3b, 3c can be installed inside an enclosure 8 of a heating furnace, for example under a protective atmosphere, in particular H2 + N2, to keep separate from the presence of air or oxygen to avoid the risk of explosion. The envelope 8 is waterproof, and made for example of sheet steel. The capacitors 2a, 2b can be housed inside the envelope 8 because induction heating does not cause excessive temperatures in the interior volume of the envelope 8. Under these conditions, a single watertight passage 9 is to make, through the wall of the casing 8, for the passage of end branches 10a, 10c of the inductors 3a, 3c in order to make the connection terminals 6a, 7c accessible from the outside of the casing 8. The realization of a tight passage 9 being relatively expensive, it is particularly interesting to be able to avoid such a sealed crossing at the terminals 7a, 6b and / or 7b, 6c.

Fig.5 is a circuit diagram equivalent to the installation of Fig.4 . The same numerical references have been taken from the diagram of Fig.5 showing that inductor, of inductance L, is divided into several parts, three in the example in question, connected by capacitors 2a, 2b, 2.

Fig.6 is a diagram of another embodiment for a series oscillating circuit in which the inductor is divided into two parts 3a, 3b connected by a capacitor 2a. The other end terminals of the elementary inductive parts 3a, 3b are respectively connected to a plate of a capacitor 2 or 2b, itself connected by its other plate to a terminal of the power supply. This assembly can be provided with or without a transformer 11.

This concept can easily be generalized to a number of connection points different from those illustrated by the examples of Fig.4 to 6 and / or with inductors with two turns or more than two turns.

Fig.7 is a simplified diagram in perspective illustrating the case of a monospiral inductor 3 for metal strip 4 which scrolls vertically according to the representation of Fig.7 .

Fig.8 illustrates another example of application of induction heating in the case of a glass melting furnace or oxides, in direct turn. In an induction glass melting furnace there is a high-frequency power supply constituted by a frequency converter 21, with a capacitor, or a capacitor bank, 22 of compensation connected between the terminals of the converter. An inductor 23, for example single-core, connected to the terminals of capacitor 22 surrounds a mass of molten glass G, which is electrically conductive.

II. It is known that the weak point of induction furnaces melting glass or refractory oxides (asbestos, silicate ...) is located in a zone A closing the turn and connection to the power supply. This zone A is subject to electrical ignition between the conductors because of the large potential difference between the supply terminals 26a, 26b of the inductor.

The currents induced in the mass G of glass are schematically represented by a dotted line 25.

According to the invention, as illustrated in Fig.9 the inductor is divided into two elementary inductive parts 23a, 23b, represented here symmetrically and the equivalent inductance of the two inductors 23a, 23b is equal to that of the inductor 23 of Fig.8 .

The ends of the elementary inductors 23a, 23b opposite to the power supply are connected to a capacitor 22a connected to the respective terminals 27a, 27b of the elementary inductors.

With this arrangement, the voltages between the points 26a, 26b on the one hand and between the points 27a, 27b on the other hand, elementary inductive parts, are equal to half the voltage which was applied between the points 26a, 26b according to the assembly of the state of the art of Fig.8 , all things equal otherwise.

Thus, the voltage between the points 26a, 26b, according to Fig.9 , is halved, while the transferred heating power is retained.

according to Fig.8 the inductor 23 having an inductance L, and the capacitor 2 having a capacitance C, the pulsation of the current being ω, the relation LCω ² = 1 is satisfied for the resonance. The voltage across the inductor is equal to LωI equal to I / Cω, where I = intensity of the current in the circuit.

according to Fig.9 each elementary inductive part 23a, 23b has an inductance U2, while each capacitor has a double capacitance 2C so that the equivalent capacitance C _eq is equal to 2C / 2 = C. The resonance condition is expressed by: LC _eq ω ² = 1, ie LC ω ² = 1. For the same intensity I, the voltage at the terminals of each elementary inductive part is U2 ω I (apart from the ohmic resistance). The transferred power, equal to the sum of the powers transferred by each elementary inductive part, is the same as for Fig.8 . On the other hand, the voltage across the capacitor 22 and between the points 26a, 26b is equal to 1 / 2Cω, which is half of what it is on Fig.8 for the same power transferred in the armature to be heated.

The risks of initiation between insulated conductors at points 26a, 26b are substantially reduced.

The example of Fig.9 is not limiting, and the inductor could be decomposed into more than two elementary inductive parts.

It should be noted that the resonance condition L Cω ² = 1 can only be satisfied in an approximate manner.

Claims (12)

1. Induction heating device consisting of

   - a single high frequency electricity supply,

   - an inductor (3, 23) suitable for encircling, at least partially, an armature element (4, G) to be heated, said inductor having a specific inductance value (L) for a desired heating power;

   - and a capacitive assembly for connecting the power supply and the inductor provided to increase the voltage at the inductor terminals in relation to the voltage supplied by the power supply,

   the inductor being constituted of at least two distinct elementary inductive parts (3a, 3b, 3c; 23a, 23b) connected in series by at least one capacitor, or a battery of capacitors (2, 2a, 2b; 22, 22a), the voltage which appears between the connection points of the elementary inductive parts being reduced in relation to the voltage required for the functioning of the inductor for the desired heating power.
2. Device according to claim 1, comprising a capacitor or battery of capacitors (2) connected between the supply terminals and three distinct elementary inductive parts (3a, 3b, 3c) that are connected in series by two connecting capacitors or a battery of capacitors (2a, 2b).
3. Device according to claim 1, comprising a capacitor or battery of capacitors (22) connected between the supply terminals and two distinct elementary inductive parts (23a, 23b) connected in series by a capacitor or battery of connection capacitors (22a).
4. Device according to claim 1, comprising a capacitor or battery of capacitors connected between the supply terminals and N distinct elementary inductive parts, connected in series by N-1 capacitors or a battery of connection capacitors.
5. Device according to one of claims 2 to 4, the connection capacitor or capacitors (2a, 2b; 22a) having capacity values that are equal, and also equal to the capacity of a capacitor (2, 22) connected to supply terminals.
6. Device according to one of claims 2 to 4, the connecting capacitors or batteries of capacitors having different capacity values than the capacitor or the battery of capacitors connected to high frequency supply terminals to obtain non-integer ratios of voltage elevation.
7. Device according to any one of the preceding claims, the connecting capacitor or capacitors (2a, 2b; 22) being located in a zone opposite to the supply in relation to the armature (4, G).
8. Heating device according to any one of the preceding claims, the capacitors or batteries of capacitors being distributed around the perimeter of the armature (4, G) in order to optimise the occupation of space adjacent to the armature thus limiting parasitic inductances.
9. Heating device according to any one of the preceding claims, the capacitor or each connecting capacitor being formed by a battery of capacitors.
10. Heating furnace for metal strips (4) comprising an induction heating device with one or more coils according to any one of the preceding claims.
11. Heating furnace according to claim 10, the connecting capacitors, or batteries of capacitors (2a, 2b) being arranged on the inside of the furnace shell (8), in the atmosphere of the furnace.
12. Direct coil glass or oxide fusion furnace (G) comprising an induction heating device according to one of claims 1 to 9.

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