EP0561208A2 - Induction cooking plate - Google Patents

Abstract

The inductive hot plate heater has an induction coil (30) which is supplied with radiofrequency. The thickness and wire thickness of the individual conductors are of the order of magnitude of 0.2 mm for a normal frequency between 20 and 30 kHz (25 kHz). This small wire thickness has been found to produce particularly low losses. The arrangement of the induction hot plate (10) consists of thermal insulation (32), which is arranged under a plate (12) which is suitable for cooking vessels (13) to be placed on, of a screen (33) having branched, earthed lead structures, and of an induction coil (30) which is in the form of a disc and is fed back to the underneath side by means of a ferrite feedback plate (31). <IMAGE>

Description

State of the art

The invention relates to an inductive hotplate heater for cooking vessels or the like.

Induction heating has the advantage of very low-inertia heat generation directly in the cooking vessel, namely in the bottom of the saucepan. The cooking appliance itself remains largely cold. Their disadvantage is the relatively high construction costs and the difficult controllability. Since electronic components are required for the necessary high-frequency generation and its control and, on the other hand, due to the heat loss in the electronics and the induction coil, the induction generating means nevertheless heat up more, it was necessary to arrange the conversion and control electronics separately from the hotplate. This hampered installation in normal cookers or hobs and induction hobs were therefore mostly installed in special appliances.

task

The object of the invention is to provide a particularly low-loss induction hob heating which is as free of disturbances as possible to the surroundings.

Solutions and explanations

(k = elektrische Leitfähigkeit; f = Frequenz; µ = Permeabilität) als untere Grenze der Drahtstärke für Hochfrequenzleiter galt. Ein weiteres Verkleinern des Drahtdurchmessers galt als nicht sinnvoll und ohne verlustmindernde Wirkung. Es hat sich jetzt erstaunlicherweise gezeigt, daß, insbesondere bei der vorgesehenen Verwendung als Induktionsspule für Kochstellenbeheizung, eine noch geringere Drahtstärke weitere erhebliche Verlustminderungen mit sich bringt, so daß eine Drahtstärke d zwischen einem Viertel und drei Vierteln des Basiswertes bevorzugt ist, wobei die kleineren Werte lediglich wegen der dann auftretenden mechanischen Herstellbarkeit in der Praxis kaum unterschritten werden können. Eine Verseilung der so bemessenen Einzelleiter zu einer Litze mit mehreren, beispielsweise sieben Kardeelen zu je einer Zahl von etwa ebenfalls sieben Einzelleitern schafft mechanisch und elektrisch optimale Verhältnisse.The solution to this problem according to claim 1 is astonishing in so far as, according to confirmed knowledge in the literature, the base value so far

$\text{D = 1 / √}\overline{\text{π * k * f * µ}}$

(k = electrical conductivity; f = frequency; µ = permeability) was the lower limit of the wire thickness for high-frequency conductors. A further reduction in the wire diameter was not considered sensible and had no loss-reducing effect. It has now surprisingly been found that, especially when it is used as an induction coil for hotplate heating, an even smaller wire thickness brings with it further considerable loss reductions, so that a wire thickness d between a quarter and three quarters of the base value is preferred, the smaller values only because of the mechanical producibility that then occurs can hardly be undercut in practice. A stranding of the individual conductors dimensioned in this way to form a strand with several, for example seven card cores, each with a number of about seven individual conductors as well, creates optimal mechanical and electrical conditions.

Induction heating is usually to be placed as close as possible under the cooking vessel. After another A feature of the invention is preferably a thermal insulation between the induction coil and the plate that supports the cooking vessels. Particularly in the case of the low-loss induction coil as a result of the aforementioned features, this prevents it from heating up from the cooking vessel side. It is therefore only possible to dissipate the relatively low inherent heat of the induction coil, which is easily possible via a heat sink, which can be cooled by a fan. The coil material and its insulation can be chosen optimally.

In order to prevent the spread and transmission of an electric field to the environment, in particular also to the cooking vessels (via influence), a shield can be provided between the induction coil and the glass ceramic plate according to a further feature. Such a grounded shield is designed according to the invention so as not to impede the spreading of the induction field, with low eddy currents, in that it has an inward or outward finger or radiation-shaped structure made of elements with a very small diameter, the diameter of which is far below the base value D for the corresponding one Frequency is. This structure can also be a layer of resistance material. On the underside, the ferrite plate forms a shield against the electrical field. Leakage currents and interference radiation can be avoided by the shielding.

As such, temperature monitoring is not necessary in an induction hob, because the heat is only generated outside the hob, namely in the cooking vessel. Nevertheless, heat can be transferred from there to the plate and thus overheating the glass ceramic plate and inadmissibly. It is difficult to sense this plate using conventional means. Therefore, according to the invention Novel optical measuring device used to measure the temperature of the plate. It contains an infrared sensor, for example a silicon photodiode, which carries out a temperature measurement using Planck's law of radiation. With increasing temperature of the glass ceramic plate, the maximum of the frequency of the emitted photons also increases (Wien's law of displacement). Above a certain temperature, the energy of the emitted photons corresponds to the spectral sensitivity of the sensor, so that an evaluable signal is produced which is used to switch off or reduce the heating power.

Since such overheating of the glass ceramic plate can practically only occur if the heating is used improperly, for example by setting up an empty pot, the temperature limitation should have a locking function, i. H. After the temperature limiting circuit has responded, the hotplate should remain switched off until it is switched off manually and then switched on again. This can easily be provided by the control electronics, for example a microcomputer.

These and further features of the invention emerge from the claims and also from the description and the drawings, the individual features being implemented individually and in groups in the form of subcombinations in one embodiment of the invention and in other fields, and being advantageous and Protectable versions can be represented, for which protection is claimed here.

Brief description of figures

Fig. 1

eine Draufsicht auf ein Bauelement zur induktiven Kochstellenbeheizung,

Fig. 2

einen schematischen Längsschnitt durch das Bauelement,

Fig. 3

einen Querschnitt,

Fig. 4

ein Blockschaltbild der Steuerung und Leistungsversorgung von zwei Induktionsspulen,

Fig. 5

ein teilweise detaillierteres Schaltbild für den Betrieb einer Induktionsspule,

Fig. 6 + 7

schematische Darstellungen einer Abschirmung,

Fig. 8a) - d)

Darstellungen "Strom über Zeit" verschiedener Impulsgrundmuster,

Fig. 9

eine tabellarische Darstellung der Zusammensetzung einzelnen Leistungsstufen aus Grundimpulsmustern,

Fig. 10

ein erläuterndes Diagramm eines Strom/Zeitverlaufes,

Fig. 11a) + b)

den Strom/Zeitverlauf und die zugehörigen Einschaltzeiten eines Topferkennungs-Prüfzyklus und

Fig. 12

einen Querschnitt durch eine Litze, aus der eine Induktionsspule aufgebaut ist.

Embodiments of the invention are shown in the drawing and are explained in more detail below. Show it:

Fig. 1

2 shows a plan view of a component for inductive heating of the cooking area,

Fig. 2

2 shows a schematic longitudinal section through the component,

Fig. 3

a cross section,

Fig. 4

a block diagram of the control and power supply of two induction coils,

Fig. 5

a partially more detailed circuit diagram for the operation of an induction coil,

Fig. 6 + 7

schematic representations of a shield,

8a) - d)

Representations of "current over time" of different basic pulse patterns,

Fig. 9

a tabular representation of the composition of individual power levels from basic pulse patterns,

Fig. 10

an explanatory diagram of a current / time curve,

11a) + b)

the current / time curve and the associated switch-on times of a pot detection test cycle and

Fig. 12

a cross section through a strand from which an induction coil is constructed.

Component

1 to 3 show a component 11 for two induction hotplates 10. It is provided for arrangement under a plate 12, for example a glass ceramic plate. The component forms a compact, relatively flat, manageable unit which, with the exception of the mains connection and an adjustment and control element, which can also include a power control device which contains all the elements necessary for operation. The component can be pressed against the plate 12 from below, for example, by spring elements, not shown. Due to this arrangement and the inclusion of all essential components, the induction heating can also be arranged in a glass ceramic cooktop instead of conventional radiant hotplates.

The component contains a heat sink 15 in a sheet metal shell 23, preferably an aluminum molded part with a largely closed surface at the top, and cooling fins 18 on the underside, which form cooling channels 19 between them. They run approximately along an axis 9 connecting the two hotplates 10. At the top, the heat sink has recesses 29 in which induction generating means 14 are arranged, each of which is assigned to a hotplate 10. A circuit board 16 is provided on the underside of the heat sink, for example screwed to the outer cooling fins, so that the cooling channels 19 and further larger spaces 28, which also serve as cooling channels, enclose on the underside of the heat sink 15. Electronic power control elements are in these, preferably in heat-conducting connection with the heat sink 15 21 arranged. The circuit board also carries electronic components, but predominantly the elements used for control, which operate with relatively small currents and therefore less heating. The whole thing is inserted in a sheet metal bowl. The board could also form the bottom cover itself. In the area of a short edge 24 of the elongated rectangular component 11, ventilation openings 25 are provided, through which a fan 37 arranged in a recess in the cooling body 15 sucks in air or blows it out after flowing through the cooling channels 19, 28. A fan arranged centrally on the heat sink and having an air outlet on two or more sides is also possible. As a result, the power control elements and the control electronics are cooled directly by the cooling air flow and the power control elements also give off their heat by conduction to the air-cooled heat sink.

Induction coil

The induction generating means 14 consist of an induction coil 30 in the form of a flat, disk-shaped or ring-shaped plate, magnetic return means 31 arranged underneath and thermal insulation 32 on the side facing the plate, in the area of which a shield 33 can be provided.

The induction coil 30 contains, as a helix and / or a spiral, stranded strands 38 which are constructed from individual conductors 39 (see FIG. 12). The strand 38 is made up of a plurality of, preferably five to nine, in the present case seven card cores 40 which are stranded together and in turn contain a number between five and nine, in the present case seven individual wires stranded together. The single ladder are electrically insulated from one another in the usual way, for example by a heat-resistant lacquer layer.

wobei D in Metern ermittelt wird. Die elektrische Leitfähigkeit k des Einzelleitermaterials ist in A/V*m, dessen Permeabilität µ in V*s/A*m einzusetzen ist und die Frequenz f in 1/s. Die bevorzugt verwendete Drahtstärke d liegt vorzugsweise zwischen einem Viertel und drei Viertel des nach dieser Formel berechneten Basiswertes D. Es hat sich erstaunlicherweise gezeigt, daß bei diesen geringen Durchmessern des Einzelleiters die Verlustleistung in der Induktionsspule 30 wesentlich gesenkt werden konnte.The individual conductors 39 made of copper have a diameter d between 0.1 and 0.4 mm, preferably 0.2 mm. This value applies to the preferred frequency of the current supplied to the induction coil between 20 and 30 kHz, preferably approximately 25 kHz. In particular, a base value D of the diameter of the individual conductor can also be determined for other frequencies using the following formula:

$\text{D = 1 / √}\overline{\text{π * k * f * µ}}$

where D is determined in meters. The electrical conductivity k of the individual conductor material is in A / V * m, the permeability µ is to be used in V * s / A * m and the frequency f in 1 / s. The preferred wire thickness d is preferably between a quarter and three quarters of the base value D calculated according to this formula. It has surprisingly been found that with these small diameters of the individual conductors, the power loss in the induction coil 30 could be significantly reduced.

According to the knowledge available so far, which was also proven by theoretical calculations, the coil losses should decrease with a reduction in diameter d up to a value equal to the base value D according to the above formula, but hardly afterwards. The theoretical knowledge, which was previously considered to be reliable, was based on the skin effect of a single conductor and determined an optimal size for the above-mentioned diameter D because the entire diameter was then flowed through evenly despite the current being displaced to the surface. The base value D corresponds to the depth of penetration of the current into a conductor surface, whereby Due to the round wire shape, penetration from all sides occurs simultaneously and thus a uniform current occupancy over the cross-section. The reasoning behind this theory has, however, surprisingly been refuted by experiments. Even a diameter of less than 0.2 mm, ie less than half of the base value D, would be preferred, but the mechanical possibilities of processing a diameter reduction put an end at some point.

Experiments have shown that the losses due to eddy currents and ohmic losses in the individual conductors due to the induction generated by the coil itself with wire thicknesses usually used based on the previous theory (equal to the base value D of 0.4 mm at 25 kHz frequency) at 70-100 W, while they are halved for a coil of the same power with a wire diameter d of 0.2 mm and are only about 40 W. As a result, the coil heating is significantly less and, in addition to considerable energy savings, problems that arise with the coil insulation and the heat dissipation from the coil can be eliminated.

Inference means

Under the coil, also as a flat, annular layer with a central opening 35, is the magnetic yoke means 31, which is made up of ferrite segments. It closes the magnetic field that arises on the underside of the induction coil with a low magnetic resistance but a high electrical resistance, so that the eddy current losses remain low there as well. Therefore, no essential induction field arises on the underside of the induction generating means 14. The magnetic yoke means 31 also form a thermal bridge between the induction coil 30 and the heat sink, on which they rest, so that the coil loss heat is dissipated directly into the heat sink.

Thermal insulation

The thermal insulation 32 is in the form of a plate covering the induction coil 30 with a central opening 35 between it and the glass ceramic plate 12. It consists of a very good heat-insulating and, if possible, also electrically insulating material, for example a pyrogenic silica airgel, which presses into a plate is.

It seems unusual to thermally shield the actual heating element, namely the induction coil, from the heat-absorbing cooking vessel. Even if one takes into account that the energy transfer takes place by induction and not by heat transfer itself, one should think that at least for the heat dissipation of the lost heat in the induction coil, the best possible heat connection to the consumer, the cooking vessel 13, would be advantageous. However, it has been shown that the induction coil generates so little heat, in particular in the case of the low-loss coil structure mentioned above, that heat bridge to the consumer tends to extract heat rather than supply it to the consumer. Thermal insulation keeps the induction coil at a lower temperature level, which has advantages for coil design and isolation. There is also an improvement in efficiency in that the heat of the cooking vessel 13 is not dissipated downward through the glass ceramic plate. The thermal insulation 32 advantageously also forms at the same time electrical insulation against the glass ceramic plate 12, which becomes electrically conductive at an elevated temperature.

Plate monitoring

In the area of the central opening 35, which passes through the insulation 32, induction coil 30 and yoke means 31, an optical sensor 36 is arranged which receives the radiation coming from the glass ceramic plate. It thus indirectly monitors the temperature of the cooking vessel that could become dangerous to the glass ceramic plate by means of non-contact measurement, which would otherwise be difficult to carry out in the magnetic field of an induction hob. It is therefore a measurement of the cause of the temperature hazard of the glass ceramic plate, since this is only heated by the cooking vessel. The glass ceramic allows the radiation to pass through to a large extent and can therefore hardly be measured without contact. With other plate materials, they can themselves be the radiation source.

The optical sensor is an infrared detector, whose spectral sensitivity is in the infrared range. As the temperature of the cooking vessel rises, the maximum of the frequency of the emitted photons increases according to Wien's law of displacement. From a predetermined temperature, the energy of the emitted photons corresponds to the spectral sensitivity of the IR detector, so that an evaluable signal is produced which is then used to switch off or reduce the power of the induction heating. For this purpose, the optical sensors 36 of each induction hotplate act on a microcomputer 42 via comparators 41 (FIG. 4), one of which is used to control and regulate one Induction hob is provided. It can be adjusted to a specific temperature or power level in each case by means of the setting element with the setting button 26. The optical sensors 36 can be silicon diodes.

Alternatively, measuring resistors could also be applied to the plate, e.g. B. between insulation and plate in the coil area if the measuring resistances are not or only slightly influenced by the magnetic field and an influence is compensated for by circuitry or in the measuring program.

shielding

The shield 33 is provided between the induction coil 30 and the glass ceramic plate 12. It can lie on the bottom or top of the thermal insulation 32 or advantageously be embedded in it. The shielding consists of a wire or ribbon structure, for example shown in FIGS. 4 and 6, which is of low eddy current design. On the one hand, this means that the thickness of the individual structural elements 45 (wires, strips or the like) is less than the current penetration depth at the frequency used and, on the other hand, the structures are in no way electrically closed. 6 there is therefore an open ring conductor 46 with inwardly projecting branches 45 which are of different lengths, so that the entire area is evenly occupied. The ring 46 is connected to an earth 34, for example by connection to the earthed sheet metal shell 23 of the component 11 (FIG. 1).

FIG. 7 shows a structure in which branches with conductor structures 45 come from a center at which the grounding acts reach outwards, which are also branched so that they shield the hob as evenly as possible.

By this shielding, without causing significant losses, the electrical field formed around the induction coil is shielded from above and thus the electrical interference radiation. The leakage currents from the cooking vessel can also be reduced. The shield could also be formed by a grounded layer of a resistance material. It is essential that the material is non-magnetic and, in order to avoid eddy current losses, has a relatively high electrical resistance compared to metallic conductors.

Basic circuit

The power supply, regulation and control of the induction coils 30 are shown in somewhat more detail in the block diagram in FIG. 4 and in FIG. 5. Fig. 4 shows that the alternating current coming from the mains connection 22 via a radio interference suppression 50 and rectifier 51 is fed to a common intermediate circuit 52, from which both converters 53, which could also be referred to as high-frequency generators, are supplied for each induction coil 30 . The intermediate circuit and converter are controlled by a controller 54, which in turn receives signals from the microcomputers (MC) 42.

5 shows the circuit of an induction coil 30 in greater detail, the control, converter 53 and induction coil 30 of a second hotplate, which is also connected to the intermediate circuit 52, not shown for the sake of clarity. For the details of the circuit, reference is expressly made to FIG. 5.

Each induction coil 30 lies in a resonant circuit with a half-bridge circuit, i. H. two branches 55, 56 are provided, in each of which a capacitor 57, 58 and an electronic switch 60, 61 are located. These can be IGBT components, i. H. Electronic semiconductor components that contain several transistor functions and, controlled by the controller 62, can switch extremely quickly. A free-wheeling diode 63, 64 and a resistor 65, 66 are connected in parallel with each of these circuit breakers 60, 61. These elements form the converter 53 designed as an oscillating circuit, to which the intermediate circuit 52 and the rectifier 51 are connected upstream. A rectifier bridge generates a pulsating DC voltage, in which sine half-waves of the same polarity are strung together by rectifying the mains alternating current. The outputs of the rectifier bridge 51 are connected to the two branches 55, 56. In the intermediate circuit there is a common capacitor 67 between the two branches and a resistor 68 switched by an electronic switch 69. The switch 69 can be a MOS-FET which, in cooperation with the resistor, prevents cracking noises when the converter is switched on. It discharges the DC link.

A control unit 80 is provided in the control path to the switches 60, 61, which contains a galvanic isolation between the low-voltage part 54 and the power side, for example by means of optocouplers. Furthermore, the switches are supplied with the control energy. This is supplied via supply units 81 which are located in the branches of the resistors 65, 66 and which each contain a Zener diode 82 and a diode 83 and a capacitor 84. The Zener Dionde limits the tension to that for the Switches 60, 61 required control voltage and diode and capacitor act as rectification. This creates a simple “power supply unit” for the switch drive energy, which draws its energy from the resistance branch, ie from an energy source that is available in any case. The resistors will thereby generate less energy and yet the other conditions will not be affected, e.g. B. the current value at point 70.

The resonant circuit shown in the symmetrical circuit structure could also be replaced by an asymmetrical structure in which only one is provided instead of the two resonant circuit capacitors 57, 58. The resonant circuit then only absorbs energy from the network on one side. In particular, in cases where it is not important to adhere to certain radio interference suppression values, this simpler circuit design could be advantageous.

At a tapping point 70 between the induction coil 30 and the capacitors 57, 58 of the resonant circuit, a switching control 71 for the converter 53 is connected, which contains a sample and hold element 72, a limit value memory 73, a comparator 74 and a yes / no memory 75.

This switching control is intended to switch off the induction heating immediately when there is no decrease in power, for example when the cooking vessel 13 is removed from the hotplate, and to switch it on again only when a cooking vessel is present. For this purpose, a check is carried out at relatively short intervals as to whether a customer is present. This is done by measuring the damping of the induction coil 30.

Power control

The resonant circuit is always switched on at the zero crossing of the mains voltage, in accordance with a certain scheme that is specified by the microcomputer 42 and will be explained in the following. The resonant circuit is controlled by the electronic circuit breakers 60, 61, specifically from the controller 62. Before each half-wave of the generated high-frequency voltage, which is of the order of 25 kHz, there is a switchover between the circuit breakers 60, 61 at the zero crossing. This creates a completely free-running converter or inverter 53 which has low switching losses. As will be explained below, no phase gating control, which would result in a forced oscillation, is used for power setting or control. The frequency is not constant and can be adjusted by frequency modulation according to the saturation effects. As a result, no oversizing of the electrical circuit breakers 60, 61 is necessary and there is also a small harmonic generation.

The power setting is done by a vibration package control. The converter is always switched on for a full line half-wave in normal operation. The basis of the power setting is that different power levels are determined by switch-on patterns, which consist of a sequence or combination of the same or different, in themselves symmetrical basic patterns of wave packets. The complete symmetry minimizes network interference.

• Fig. 8 a) zeigt ein Teilintervall T mit der Bezeichnung "*", in dem alle sechs Netzhalbwellen vorhanden sind. Es ist also ein "Volleistungs"-Intervall.
• Fig. 8b) zeigt ein Teilintervall T mit der Bezeichnung "X", bei dem insgesamt vier Netzhalbwellen so verteilt sind, daß sich insgesamt eine symmetrische Verteilung ergibt. Gegenüber dem "Volleistungs"-Muster nach Fig. 8 a) fehlt die dritte und sechste Netzhalbwelle (je eine positive und eine negative), so daß dieses Teilintervall "X" mit zwei Drittel Leistung belegt ist.
• Fig. 8 c) enthält insgesamt nur zwei Netzhalbwellen, und zwar die erste als positive und die vierte als negative. Auch hier ergibt sich eine symmetrische Aufteilung. Dieses Teilintervall T mit der Bezeichnung "Y" hat also einen Leistungsanteil von einem Drittel.
• Fig. 8 d) zeigt die Nulleistung, d. h. während dieses Teilleistungsintervalles "0" wird keine Leistung freigegeben.
• Fig. 9 zeigt nun die Belegungspläne unter Verwendung der insgesamt 35 Teilintervalle T, die zusammen das Zeitintervall Z von 2,1 Sekunden Dauer bilden. Es sind dort lediglich beispielsweise verschiedene Leistungsstufen, beispielsweise entsprechend der Knebelstellung des Einstellknopfes 44, dargestellt, denen die unterschiedlichsten Kombinationen der Grundmuster entsprechend Fig. 8, jeweils hintereinandergereiht, zugeordnet sind. Aus den dahinter angegebenen Prozentsätzen der Leistungsfreigabe ist zu erkennen, daß auf diese Weise die Leistungskennlinie bei einer leistungsgesteuerten Induktionskochstelle beliebig den Praxisforderungen angepaßt werden kann. So ist beispielsweise die Leistung in den unteren Einstellstufen viel feiner regulierbar als in den oberen, was den Anforderungen der Praxis entspricht. Da jedes Grundmuster "Y" nach Fig. 8 c) nur weniger als einem Prozent Leistung innerhalb des Zeitraumes Z entspricht, kann die Leistung also prozentweise angepaßt werden. Es können dabei auch durchaus völlig unregelmäßige oder auch unstetige Verläufe erzielt werden, wenn sich dies als zweckmäßig herausstellt. Trotzdem ist jeweils eine Schaltung im Nulldurchgang der Spannung sichergestellt.

FIGS. 8 and 9 show an example of a sample occupancy plan for such a vibration package control:
A total time interval Z of 2.1 seconds is divided into 35 subintervals T of 60 milliseconds each, ie six network half-waves at a frequency of 50 Hz. There are a total of four basic patterns of subintervals T, which are shown in FIGS. 8 a) to d) as “voltage versus time” diagrams:

• Fig. 8 a) shows a subinterval T with the designation "*", in which all six network half-waves are present. So it's a "full power" interval.
• 8b) shows a partial interval T with the designation "X", in which a total of four network half-waves are distributed in such a way that a symmetrical distribution results overall. Compared to the "full power" pattern according to FIG. 8 a), the third and sixth mains half-wave (one positive and one negative) are missing, so that this partial interval "X" is occupied by two thirds of power.
• Fig. 8 c) contains only two network half-waves, the first as positive and the fourth as negative. Here, too, there is a symmetrical division. This subinterval T with the designation "Y" therefore has a power share of one third.
• 8 d) shows the zero power, ie no power is released during this partial power interval "0".
• FIG. 9 now shows the occupancy plans using the total of 35 subintervals T, which together form the time interval Z of 2.1 seconds duration. Only various power levels are shown there, for example, corresponding to the toggle position of the setting button 44, to which the most varied combinations of Basic patterns corresponding to FIG. 8, each in a row, are assigned. It can be seen from the percentages of power release given below that the power characteristic curve in a power-controlled induction hob can be adapted to practical requirements in this way. For example, the output in the lower setting levels can be regulated much more precisely than in the upper ones, which corresponds to practical requirements. Since each basic pattern "Y" according to FIG. 8 c) corresponds to less than one percent of power within the time period Z, the power can therefore be adjusted as a percentage. Completely irregular or discontinuous courses can also be achieved if this turns out to be expedient. Nevertheless, a switching in the zero crossing of the voltage is ensured.

8 shows positive and negative network half-waves as they exist before rectification in order to demonstrate the absence of interference on the power network. Mains half-waves are present in the resonant circuit in the form of rectified alternating current.

In the time interval Z, which in the example explained is 2.1 seconds, but can be as long as desired and can be divided into subintervals T of any size, the basic patterns are mixed as desired under the control of the microcomputer and thus produce a control or control that is DC-free on the network side Control in relatively short pulses, but each containing an entire network half-wave. The setting can be purely performance-dependent via the setting elements 43, as shown in FIG. 9, but control influences from temperature sensors or the like can also act on the microcomputer, so that a control circuit is created.

The start of the resonant circuit for generating the high frequency feeding the induction coil 30 thus basically begins at the zero crossing of the mains voltage and the amplitude and frequency in the resonant circuit change with the rise and fall of current and voltage over the individual mains half-waves. The frequency is therefore higher at the beginning of each half-wave and decreases in the area of its maximum because the converter oscillates freely. Furthermore, the frequency changes not only with the current, but also with the pot material because, for example, the inductance is not constant due to magnetic saturation in the bottom of the pot. If the inductance of the overall arrangement becomes smaller, a higher frequency results. This arrangement also has advantages in terms of radio interference suppression because broadband interferers are easier to suppress. In addition, fewer harmonics are generated because phase control is not necessary.

Pot detection

The pot detection shown in FIG. 5, which also protects the surroundings against excessive induction fields and self-protection of the converter, works as follows:
If the cooking vessel is removed from it while the hotplate is switched on, the current in the resonant circuit rises sharply because the damping decreases. The current in the converter is tapped at point 70 and detected by the sample and hold element 72. If it exceeds a limit value stored in the limit value memory 73, the converter is switched off via the controller 62 in that the circuit breakers 60, 61 are closed or are no longer opened. This can also happen within a network half-wave. The energy then present in the resonant circuit is fed back into the intermediate circuit 52 via the freewheeling diodes 63, 64. Depending on the current in the resonant circuit, the shutdown works extremely quickly and without loss.

Even though the hotplate is switched on, no power is released until a suitable cooking vessel is put back on. This switch-on check takes place at the beginning of each time interval Z (2.1 seconds in the example). The test procedure is as follows:
In the controller 62, a phase-controlled loop circuit (PLL "phase locked loop") specifies the control clock frequency for the circuit breakers 60, 61. During the operation of the resonant circuit, it adjusts itself to the frequency of the main resonant circuit and switches the circuit breakers 60, 61 alternately. At idle, ie during the test phase, the loop circuit releases a semi-oscillation when one of the two power switches 60 or 61 is triggered by the microcomputer. Previously, the tapping point 70 was charged to a certain voltage via the resistors 65, 66 and thus a certain energy was present in the resonant circuit. When one of the circuit breakers is switched on, current therefore flows for a high-frequency half-wave. The sample and hold member, e.g. B. a peak detector, which also contains a current transformer to convert the actually flowing currents into measuring currents, measures the current during this oscillation and stores the result. It corresponds to the value i _max in FIG. 10. In the resonant circuit, the amplitude then decays according to the energy consumption due to the damping according to a specific function (corresponding to an e-function). If this decay is too slow, the damping is too low and the conditions are not available for power activation. This is demonstrated, for example, in FIG. 10, where a decaying oscillation is shown and the limit values G1, G2, G3 and G4 indicate, for example, the values that could be stored in the limit value memory 73. If they are exceeded, this means "insufficient damping" and a signal is sent to the microcomputer: "no activation".

The pan detection therefore works on the principle of damping measurement, with the test only working with one half of the converter, so that the power resonant circuit does not start, for which purpose an alternate activation of the two power switches 60, 61 would be necessary.

4 and 5, the test procedure takes place in such a way that the current value is measured from the first oscillation when one of the power transistors 60 or 61 is switched on for a very short period of time E, for example 20 microseconds (approximately one half oscillation in idle frequency), held by the sample-and-hold element 72 and the subsequent limit values, z. B. G1 to G5 can be derived. Under the control of the microcomputer, the loop circuit PLL then pauses P in the same order of magnitude and then switches on the power transistor again. From the current drop in the next oscillation (see FIG. 11 a) it can now be determined by comparison with the limit values, which is done via the comparator 74, whether the current exceeded these limit values (here G2 and G3). The result of this check is temporarily stored in the memory 75.

A second activation then takes place, where the limit values G4 and G5 are used for comparison. These second measurement is made for safety reasons, in order to prevent falsification by strong frequency deviation, e.g. B. to avoid errors in an aluminum or copper object instead of a cooking vessel. If this measurement also does not result in the limit values being exceeded, the damping is sufficient and the oscillating circuit is switched on by the control 62. Since the entire measurement took place in the microsecond range, the energy in the oscillating circuit decayed because it was above the circuit breakers 60, 61 high-resistance voltage dividers 65, 66 connected in parallel could not be replaced during this time. Until the next test cycle at the beginning of the next time interval Z (after 2.1 seconds), however, the resonant circuit is again supplied with the corresponding test voltage via this voltage divider and a new test can begin if the limit values are exceeded and thus "too little damping" was detected and the resonant circuit was not switched in power mode.

The test can take place with a very low test current, for example with a tenth of the nominal current during power operation. In addition, because the very short switch-on times of, for example, 20 microseconds within the test cycle of 2 seconds, the resonant circuit in test operation is only about 1 / 100,000th of the total time, the total power release during the test is only a completely insignificant fraction of the total power of the hotplate and can be neglected both energetically and in terms of influencing the environment. For example, with a 2,000 W hotplate, it is on the order of 1 to 4 mW.

This pot detection by checking the possible decrease in power (damping) means that a very reliable, short-term access and low-energy measurement instead. Instead of the current measurement in the resonant circuit, a voltage measurement on the resonant circuit capacitor can also be used, for example, in order to carry out the test by measuring the decay of the voltage amplitude with the limit values determined on the basis of the initial measurement.

In any case, the test only works with one half of the converter, so the resonant circuit does not start up during the test phase. If during the two successive measurements (second and third activation of the PLL) the values stored in the memory 75 both show "damping sufficient" (limit values not exceeded), the oscillation circuit is switched on in the controller 72 by the PLL switching circuit 60 by alternately switching on the power switches 60, 61 started at full power. The power release itself then takes place in accordance with the power scheme explained with reference to FIGS. 8 and 9 until either the hotplate is switched off via the setting element 43 or self-protection is accessed by removing the pot and the power is switched off so that it goes back into the test phase .

Claims (11)

Inductive hotplate heating with a high-frequency fed induction coil, characterized in that the thickness of the individual conductors (d) is less than

$\text{D = 1 / √}\overline{\text{π * k * f * µ,}}$

where d and D in meters [m], the frequency in 1 / sec, the electrical conductivity k of the single-conductor material in $\text{A / V * m}$

and its permeability µ in $\text{V * sec / A * m}$

is to be used. Hob heating according to claim 1, characterized in that the thickness d is between 1/4 and 3/4 of the base value D, in particular in the case of individual conductors made of copper wire and for a frequency between 20 and 30 kHz below 0.4, preferably between 0.1 and 0 , Is 2 mm. Cooking area heater according to one of the preceding claims, characterized in that individual conductors (39) are stranded to form a strand (38), preferably a strand with 5 to 9 carding wires, each with 5 to 9 individual conductors isolated from each other (39). Cooking area heating according to the preamble of claim 1, characterized in that the induction coil (30) is associated with thermal insulation (32). Cooking area heater according to claim 4, characterized in that the thermal insulation (32) is arranged between the induction coil (30) and a plate (12) which is designed for mounting cooking vessels (13) and / or has electrical insulating properties and preferably from one Disc is made of silica airgel. Cooking area heater according to one of the preceding claims, characterized in that the induction coil (30) is on its side facing away from the plate (12), possibly via magnetic yoke means (31), such as a ferrite disc, with a heat sink (15) in heat transfer connection. Inductive hotplate heating, in particular according to one of the preceding claims, characterized by an optical measuring device (36, 41) for measuring the temperature of a plate (12), under which the heating is arranged, which preferably works without contact and one in the area of the magnetic field of an induction generating means (14) , preferably in the middle thereof, has an effective sensor (36), which may be arranged outside of this area, and in particular has a certain spectral sensitivity range, which is preferably in the range of infrared radiation, the measuring device for protecting the plate being preferred (12) is provided against overheating and acts on the inductive heating to reduce or switch off power, and is provided with a restart lock. Hob heating according to the preamble of claim 1, characterized by a shield (33) of the induction coil (39) against electrical fields, which is preferably grounded. Cooking area heater according to claim 8, characterized in that the shield (33) is arranged between the induction coil (30) and a plate (12) which is designed for mounting cooking vessels (13) and / or consists of line elements (45, 46), which are low in eddy currents and preferably elongated and distributed over the surface of the hotplate (10), and preferably contain electrically non-closed line structures, in particular with radial branches. Cooking area heater according to claim 9, characterized in that the line elements of the shield (33) consist of a layer of resistance material. Cooking area heater according to one of the preceding claims, characterized in that the thickness and / or width dimensions of line elements of a shield (33) are substantially smaller than the base value D.

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