EP0471171A2 - Device for regulating and limiting the power of a heating plate of ceramic or similar material - Google Patents

Abstract

A method is described for regulating and limiting the power of a heating plate made of ceramic or similar material, in particular a glass-ceramic cooking plate. In a heating plate in which the individual heating zones are in each case heated by a plurality of heating elements which can be switched and controlled independently of one another, it is provided according to the invention that the individual heating elements are switched and regulated independently of one another, by means of a plurality of temperature sensors which are independent of one another and are arranged in the region of the heating zone such that all the points of the regions which are significant for one load case, particularly local hotspots, are detected, in such a manner that the power distribution in the heating zone region is largely matched to the locally different heat extraction. This has the advantage that the heating system can be used optimally, while the thermal stress of the heating plate material is, however, reduced. <IMAGE>

Description

The invention relates to a method for power control and limitation in a heating surface made of glass ceramic or a comparable material, in particular a glass ceramic cooking surface, the individual heating zones of the heating surface being heated in a manner known per se with heating devices with a plurality of independently switchable and controllable heating elements. The invention further relates to a preferred device for carrying out the method in a hob with a glass ceramic hob.

Heating surfaces made of glass ceramic or a comparable material are also used, for example, as wall or ceiling radiators, heat exchangers or other large-area heating devices which can be heated in any way.

Of particular interest nowadays are electric or gas-heated hobs or individual hobs, the heating surface of which is made of glass ceramic. Hobs of this type are generally known and have been described many times in the patent literature. The heating zones of these cooktops are heated (without limitation of generality, the heating zones for cooktops are also referred to below as cooking zones) by means of heating devices arranged underneath the glass ceramic cooktop, e.g. electrically operated contact heating elements, radiant heating elements or gas burners. Induction hobs are also known.

In the known domestic cooktops, the heating power for the heating devices is set permanently by the user or electronically, electromechanically or, in the case of gas stoves, by means of a selectable time program via valves, purely mechanically controlled. Corresponding controls are described for example in the patent specification DE-PS 3 639 186 A1.

It is known to heat heating zones of a glass ceramic cooktop which have a larger diameter, for example in order to heat pots with a larger diameter and / or non-circular, for example oval, bottom surface, with heating elements with a plurality of heating circuits. It is also known to use so-called add-on heating elements in addition to the permanent heating elements which are in operation and which are only subjected to power in the heating phase in order to achieve accelerated heating of the cooking zone. The geometric arrangement of the heating elements or heating circuits below a heating zone is usually adapted to the geometry of the cookware.

For example, DE-OS 33 14 501 A 1 describes a heating plate with two concentric heating circuits, in which the outer heating circuit is designed as an auxiliary heating circuit.

DE-PS 34 06 604 relates to a heating device in which the heating zone is heated by means of several high and normal temperature radiation heating elements. The heating elements are arranged in such a way that the heating point is divided into two concentric zones, the inner zone being heated only by the high-temperature radiation heating elements which can preferably be used as auxiliary heating elements in the boiling phase and the outer zone by the normal-temperature radiation heating elements. A comparable arrangement of several radiant heating elements in the area of a cooking zone can also be found in US Pat. No. 4,639,579.

A heating device with a gas burner which has two burner chambers which can be acted upon independently of one another and which, for example, Can limit concentric zones in the cooking zone area is described in US Pat. No. 4,083,355.

With the commonly used glass ceramics, the maximum operating temperatures must be limited to 700 ° C. To avoid overheating of the glass ceramic heating surface, so-called Protective temperature limiter, for example between the heating elements and the glass ceramic surface mostly along a diameter arranged rod expansion switches, which usually switch off the heating device completely or reduce its performance when a certain limit temperature is exceeded. After running through a hysteresis, the full heating output is switched on again. From DE-OS 3 314 501, for example, a rod expansion switch with two different switching points is known, which switches accordingly at two different temperatures.

From the German patent DE-PS 21 39 828 it is known that glass, glass ceramics or similar materials have a temperature-dependent electrical resistance, so that from this by applying conductor tracks, e.g. from precious metals, temperature measuring resistors with a steep resistance-temperature characteristic, similar to that of the known NTC resistors, can be produced.

This type of temperature sensors are used in DE-OS 37 44 372 in connection with appropriate wiring to the above. Protective temperature limiter to be replaced completely. For this purpose, two mutually parallel conductor tracks, each of which delimit a strip-shaped glass ceramic resistor, are applied to the glass ceramic cooking surface along a half diameter in each cooking zone.

Practice has shown that abnormal thermal stress conditions in glass ceramic cooktops are mostly caused by the use of poor cookware or incorrect operation.

For example, in the case of cookware with an uneven support surface, there is a different local extraction of heat in the cooking zone. Carelessly empty dishes can cause even higher temperature / time loads for the glass ceramic. Other extreme loads cause pots with diameters that are too small and that they are accidentally offset, ie pots that are not centered. In these cases, the cooking zone is overheated in the areas not covered by the pot. In such cases, the surface temperature of the glass ceramic can be considerably higher than that when idling, ie without pot, measured temperatures. Temperature increases of up to 200 K above the surface temperature when idling are possible.

These abnormal thermal loads in the area of the cooking zones can add up to high temperature / time loads over time and result in the destruction of the cooking surfaces. Extremely high temperatures can damage the attached cookware and the glass ceramic cooktop. Pot enamel can melt, for example, if steel enamel dishes accidentally empty. Empty aluminum dishes can also damage the glass ceramic surface due to melting aluminum.

Since in practice both bad or unsuitable cookware is used as well as the above Operating errors occur, the maximum surface temperature must be limited when idling. For the same reason, the specific power density of the heating devices, based on the area of the heated zone, is currently limited to approximately 7 watts / cm².

The abnormal loading conditions described above can lead to damage to the glass ceramic cooktop on the one hand and on the other hand considerably impair the efficiency of the cooking system.

It is known that in the case of poor cookware, the average power offered by the heating device can be increased if the idle adjustment of the heating device is increased. This usually leads to a shorter cooking time. However, if this tableware is used continuously, the increase in the idle adjustment means that the temperature / time load limit can be exceeded and thus the possible destruction of the glass ceramic cooktop cannot be ruled out.

If good cookware is used, this method cannot achieve an increase in the average output and, as a result, the cooking time can be reduced. Good cookware draws so much heat from the glass ceramic that the protective temperature limiter rarely or not at all responds during the cooking process. As a rule, in the parboiling process Combined with good cookware, the full rated output of the heating device is always available. The efficiency can only be increased here by increasing the heating power and simultaneously increasing the idle adjustment of the protective temperature limiter with the disadvantages already described.

The object of the invention is to provide an improved method for power control and limitation in the case of a heating surface made of glass ceramic or a comparable material, in particular in the case of a glass ceramic cooking surface, which enables the cooking system to be used optimally even when poor cookware is used, but also the thermal load to keep the heating surface small.

Another object of the invention is to provide a suitable device for carrying out the method in a hob with a glass ceramic hob.

The object is achieved by a method having the features of claim 1. A suitable device is described in claim 5.

According to the invention, the temperature distribution in the heating zone, in particular local overheating, is recorded with a plurality of independent temperature sensors arranged in the area of a heating zone, which can be integrated into the cooking zone area, for example, and the temperature signals obtained therefrom are used to detect the temperature distribution in the heating zone Switching and controlling the heating elements or heating circuits assigned to the heating zone independently of one another in such a way that the power distribution and thus the surface load of the heating zone is adapted to the locally different heat flow, which, for example, depends on the geometry of the support surface of the pots placed on the hob.

In places where energy is extracted the most, heating takes place, for example, even when the pot quality is poor, with optimal heating output, while in places where there is little heat extraction, the heating output is prevented by overheating by reducing the number of cycles, for example.

The conversion of the temperature measurement signals into control signals for the power supply of the heating elements is carried out with the help of control devices known per se.

In the simplest case, when a predetermined threshold temperature is exceeded, the power supply for the heating elements is interrupted until the temperature in the assigned overheated cooking zone area is again below the threshold temperature. The full heating output is then switched on again.

In the case of hobs, however, shorter cooking times are achieved if the power supply for the heating elements is reduced continuously or stepwise at intervals, for example to a level which is reduced by at least 10%, until the heating power of the heating elements is optimally matched to the maximum possible heat removal in the assigned area of the heating zone is adapted.

The gradual power reduction at different switching temperatures can take place in a manner known per se in such a way that for each switching temperature there is a separate temperature sensor in the area of the heating zone assigned to the respective heating element. However, it is advantageous to use only a single temperature sensor for this purpose, which is followed by a switching and control element, which switches back in succession to different power levels at different temperatures.

Mutually independent temperature sensors in the sense of this invention can be, for example, electromechanical temperature sensors with a plurality of mutually independent switching contacts, such as the known rod expansion switches, for example in the form of capillaries filled with molten salt, with several, but at least two, mutually independent switching contacts. The switching contact, which limits the maximum surface temperature, should advantageously respond at a temperature which is at least 10 K above the switching temperatures of the other switching contacts, with the aid of which the power is reduced.

Thermally conductive rods or sheets or the like can also be used as temperature sensors, to which the actual temperature sensor is coupled outside the radiator or the heated zone.

In cooktops with cooking zones with essentially circular geometries, most of the known anomalous load cases, namely those that lead to a radially symmetrical temperature distribution in the cooking zone area, can be completely detected with the aid of rod expansion switches which are arranged along a half or diameter of the cooking zone. However, local temperature peaks cannot be detected. In addition, temperature monitoring is only possible indirectly because the rod expansion switch has no direct contact with the underside of the glass ceramic, since it is only arranged in the space between the heating source and the underside of the glass ceramic.

Comprehensive temperature monitoring can be achieved, for example, with the aid of temperature sensors which consist of thermocouples or other suitable temperature sensors arranged in the form of a grid in the area of the heating zone. In order to ensure sufficient thermal contact with the heating surface, these must be pressed onto the heating surface. Such temperature sensors can also be integrated into the heating surface. For example, thermocouples can be embedded or rolled into the heating surface.

The temperature sensors known from DE-PS 21 39 828 and integrated into the heating surfaces are preferably used. For this purpose, two parallel conductor tracks are applied to the heating surface in the area of the heating zones in a manner known per se, for example by means of screen printing or vapor deposition or other methods, and then baked. The very strong temperature-dependent electrical resistance of the glass ceramic delimited between the conductor tracks represents the actual temperature sensor.

With this method, large-scale temperature sensors of any shape can be implemented in a simple manner, which permit area-wide temperature monitoring. This can also be used, for example Monitor and control large-area heat radiators and heat exchangers with heating surfaces made of glass ceramic, glass or similar materials.

The geometrical arrangement of the conductor tracks in the region of a heating zone is expediently adapted to the geometrical arrangement of the heating elements and to the expected temperature distribution in the case of known anomalous thermal load cases.

The temperature sensors advantageously detect all essential parts of the heated areas of the heating zone assigned to the heating elements, so that local overheating is also detected. For example, heating coil loops or in the area of flame tips, e.g. with gas heating, higher temperatures occur at these points than neighboring points. These temperature peaks must be recorded, otherwise the heating surface can be damaged at these points.

Figur 1

in einer schematischen Darstellung eine Vorrichtung zur Durchführung des erfindungsgemäßen Verfahrens bei einem Haushaltskochfeld mit Glaskeramikkochfläche, wobei zwei ringförmige, zueinander konzentrisch angeordnete Temperatursensoren entsprechend der Anordnung der Heizkreise eines Zweikreisheizelements den Mitten- und den Randbereich einer Kochzone überwachen;

Figur 2

die Vorrichtung aus Figur 1 in einer Längsschrittdarstellung;

Figuren 3a und 3b

eine Sensoranordnung für nichtrunde Mehrkreisheizelemente

Figur 4

zur Verdeutlichung der Funktionsweise eines Glaskeramik-Temperaturmeßwiderstandes in einer schematischen Darstellung einen vergrößerten Ausschnitt aus einer Anordnung aus zwei parallel verlaufenden Leiterbahnen mit dazwischenliegendem Glaskeramikwiderstand;

Figur 5a

in einer schematischen Darstellung eine bekannte Schaltungsanordnung für die Sensoranordnung aus Figur 1 zur Einstellung des Temperaturbereichs mit größter Meßempfindlichkeit;

Figur 5b

in einer schematischen Darstellung eine bekannte Schaltungsanordnung für die Sensoranordnung aus Figur 1 zur Umwandlung der Temperaturmeßsignale in Steuersignale für die Leistungsversorgung der Heizkreise.

Figur 6

für eine mit einem Zweikreisheizelement beheizte Kochzone für vier verschiedene Belastungsfälle den Verlauf der Sensorsignale mit der Zeit bei einer Leistungssteuerung und -begrenzung gemäß der Erfindung.

The invention is explained in more detail below with reference to the figures:
Show it:

Figure 1

in a schematic representation of an apparatus for performing the method according to the invention in a domestic hob with a glass ceramic hob, two ring-shaped, concentrically arranged temperature sensors to monitor the center and the edge area of a cooking zone in accordance with the arrangement of the heating circuits of a dual-circuit heating element;

Figure 2

the device of Figure 1 in a longitudinal step representation;

Figures 3a and 3b

a sensor arrangement for non-circular multi-circuit heating elements

Figure 4

to illustrate the operation of a glass ceramic temperature measuring resistor in a schematic representation an enlarged section of an arrangement of two parallel conductor tracks with glass ceramic resistor in between;

Figure 5a

a schematic representation of a known circuit arrangement for the sensor arrangement from Figure 1 for setting the temperature range with the greatest sensitivity;

Figure 5b

a schematic representation of a known circuit arrangement for the sensor arrangement of Figure 1 for converting the temperature measurement signals into control signals for the power supply of the heating circuits.

Figure 6

for a cooking zone heated with a two-circuit heating element for four different load cases, the course of the sensor signals over time with a power control and limitation according to the invention.

FIGS. 1 and 2 show an example of a device which is particularly suitable for carrying out the method according to the invention in a hob with a glass ceramic hob.

In the present arrangement, gold conductor tracks (2) are arranged within the cooking zone (1) of a glass ceramic hob on the glass ceramic underside. The conductor track is selected such that the outer circle (3a) and the inner circle (3b) of a two-circuit heating element (4) are each covered with ring-shaped conductor tracks. The connection areas (5) lie outside the cooking zone (1) for protection against thermal loads.

Figure 2 shows the arrangement consisting of the glass ceramic plate (6), the two-circuit heating element (4) with the heating coils (4a) and the printed conductors (2) printed on the underside of the glass ceramic and the connection areas (5) in section.

The invention is in no way limited to the use of the two-circuit heating elements shown in FIGS. 1 and 2. In principle, any heating device can be used that is composed of several heating elements that can be switched and controlled independently of one another in the area of a cooking zone. The invention can e.g. can also be used with gas burners, e.g. also in the gas burner known from US Pat. No. 4,083,355 with two burner chambers which can be acted upon independently of one another by fuel.

The heating elements can e.g. be arranged in a grid below the cooking zone. However, the geometric arrangement of the heating elements is advantageously adapted to the geometry of the cookware or to the temperature distribution in the cooking zone area in the case of known anomalous thermal loads, so that effective control of the power distribution to the locally different heat extraction is possible.

In the event of incorrect operation and / or inadequacy of the cookware in cooktops with a glass ceramic cooktop, i.a. a different heat extraction in the edge and middle area of the cooking zone. The use of multi-circuit heating elements (with and without an insulation barrier between the heating circuits) - in particular two-circuit heating elements with two concentric heating circuits - which permit separate heating of the edge and center areas is therefore particularly advantageous for the use of the method according to the invention. Depending on the application, it may be useful to design the individual heating circuits for different surface loads. With the aid of a ring-shaped arrangement of the conductor tracks above the heating circuits, not only is it possible to effectively monitor the areas of the cooking zone assigned to the individual heating circuits, it also detects all points in the area of the cooking zone that are relevant to a load.

The conductor tracks (2) cover only a small part of the cooking zone. Track widths of <3 mm are preferred. In the present case, the conductor tracks are 1-2 mm wide, so that the total area of the conductor tracks is small in relation to the area of the heated zone. Influencing the This minimizes the total heat flow. The surface resistance of these interconnect layers is ≦ 50 mΩ / ^□ with layer thicknesses below 1 µm.

In this way, two independent temperature sensors are obtained, which monitor the two heating circuits (3a) and 3b) separately. Analogously to the arrangement described above, conductor track arrangements are selected for other, non-circular heating elements, which are adapted to the respective contours or geometries and with which the individual cooking zone areas are monitored separately. FIGS. 3a and 3b show corresponding arrangements for square and oval multi-circuit heating elements.

R ist der spezifische Widerstand der Glaskeramik in Ohm*cm bei der absoluten Temperatur T in Kelvin.
a und b sind von der Geometrie der Leiterbahnen und von der Glaskeramik abhängige Konstanten (a in Ohm*cm und b in K).The parallel conductor tracks (2) within the cooking zone (1) delimit narrow circular or linear temperature measuring zones in which the glass ceramic volume delimited by the conductor tracks serves as a temperature-dependent resistor. As with glasses, the electrical conduction of the glass ceramic is based on the ion conduction. The dependency is described by the law of Rasch and Hinrichsen:

$\text{R = a * exp (b / T) (Eq. 1)}$

R is the specific resistance of the glass ceramic in Ohm * cm at the absolute temperature T in Kelvin.
a and b are constants dependent on the geometry of the conductor tracks and on the glass ceramic (a in ohm * cm and b in K).

The temperature coefficient of these measuring resistors is negative. It is strongly temperature-dependent and is e.g. for glass ceramics of the SiO₂-Al₂O₃-Li₂O system at 300 ° C 3.3% / ° C.

Der temperaturabhängige Widerstand jedes differentiellen Widerstands R_i(T) läßt sich durch die nachfolgende Gleichung ausdrücken

${\text{R}}_{\text{i}}{\text{(T}}_{\text{i}}{\text{) = l}}_{\text{i}}{\text{/A}}_{\text{i}}{\text{* a * exp (b/T}}_{\text{i}}\text{) (Gl. 3)}$

worin l_i die Länge in cm und A_i die Querschnittsfläche in cm² eines jeden differentiellen Glaskeramik-Widerstands darstellen. Die Konstanten a und b sind von der Geometrie der Leiterbahnen und von der Glaskeramik abhängige Konstanten (a in Ohm*cm und b in Kelvin). T_i ist die absolute Temperatur eines jeden differentiellen Widerstands in Kelvin.The total electrical resistance of such an arrangement is composed of any number of differential resistors connected in parallel with a negative temperature coefficient and can be expressed by the following equation:

${\text{1 / R = 1 / R₁ (T) + 1 / R₂ (T) + ... + 1 / R}}_{\text{i}}{\text{(T) + ... 1 / R}}_{\text{n}}\text{(T) (Eq. 2)}$

The temperature-dependent resistance of each differential resistor R _i (T) can be expressed by the following equation

${\text{R}}_{\text{i}}{\text{(T}}_{\text{i}}{\text{) = l}}_{\text{i}}{\text{/ A}}_{\text{i}}{\text{* a * exp (b / d}}_{\text{i}}\text{) (Eq. 3)}$

where l _{i represents} the length in cm and A _{i represents} the cross-sectional area in cm² of each differential glass ceramic resistor. The constants a and b are constants dependent on the geometry of the conductor tracks and on the glass ceramic (a in Ohm * cm and b in Kelvin). T _i is the absolute temperature of each differential resistor in Kelvin.

The total electrical resistance is determined by the smallest resistance at the point of the highest temperature of the sensor zones, which results in an automatic display of the maximum temperature in the respective sensor zone. Locally occurring high temperatures cause one or more differential resistors to become low-ohmic in relation to the other differential resistors that are in colder zones, so that the total resistance of a sensor according to Eq. 2 becomes very small.

FIG. 4 schematically shows a section of the opposite conductor tracks (2) for clarification. The glass ceramic defined between them can be understood as a parallel connection of many temperature-dependent differential resistors.

At low temperatures this arrangement according to Eq. 2 and 3 have a very high resistance. At higher temperatures, for example the typical temperatures measured when idling, the resistance decreases by several orders of magnitude. The resistance also decreases considerably if high temperatures only occur in a small area of the glass ceramic, for example in the offset pot. A temperature compensation between neighboring zones, which have different temperatures, hardly takes place due to the low heat conduction with glass, glass ceramic or similar material with a λ of typically less than 2 W / mK.

The conversion of the temperature-dependent change in conductivity of the glass ceramic into a measurement signal can be implemented in a voltage divider supplied with AC voltage, in which a resistance is formed by the temperature-dependent resistance of the sensor surfaces. Depending on the sensor geometry, the fixed resistors of the voltage divider must be selected so that at temperatures that exceed the permissible temperature / time load, sufficient signal changes can be picked up on the voltage divider for further processing. The temperature range in which the largest signal swing occurs can be changed by adjusting the fixed resistors. The fixed resistors also serve to limit the current.

The AC voltage is necessary to avoid polarization effects of the glass ceramic and the associated electrochemical decomposition due to the ion migration. Frequencies in the range between 50 Hz and 1000 Hz are preferred for the applied AC voltage.

FIG. 5a schematically shows the circuit arrangement according to the invention, a voltage divider (7) for each temperature sensor being shown in each case. Both voltage dividers are supplied by an AC voltage source (8), shown here as a transformer. This ensures that the glass ceramic, shown here as a temperature-dependent resistor (9), is not flowed through by direct current. The two fixed resistors (10a) and (10b) were chosen so that a large signal change in the range from 500 to 600 ° C occurs. This temperature range is characteristic of the surface temperatures that occur in practice within the cooking zones (1) of glass ceramic hobs.

The AC voltage signal present at the voltage divider is rectified via a rectifier circuit and fed to a suitable electronic circuit. These can be operational amplifiers, which are connected as comparators, or other circuits and components known from electronics, such as μ-processors or the like.

The signals supplied by the sensors are processed in these circuits in such a way that a signal is available at their output with which the individual heating circuits can be controlled via relays or power semiconductor components such as triac's or MOS-FET's. The power control can take place, for example, by means of phase control, half or full wave packet control with different duty cycles, so that constant temperature controls are also possible. The output signal of the control electronics can also be supplied to the semiconductor components described above via optocouplers or other circuits which serve for the electrical isolation between control electronics and power section. So-called zero voltage switches can also be implemented, which only switch the individual heating circuits of the heating elements at zero voltage crossing.

In the existing arrangement (FIG. 5b), the signal tapped at the voltage divider (7) is fed via a rectifier circuit (11) to the one input of an operational amplifier (12) connected as a comparator. The comparator has the task of comparing the temperature-dependent signal originating from the sensor arrangement with a fixed voltage value, the threshold voltage Us in FIG. 5b. If the voltage from the sensor is above the threshold voltage, which would be the case in the present arrangement at relatively low temperatures, e.g. when using good dishes, the output of the comparator is switched through. This signal is fed via a diode (13) and an optocoupler (14) to a semiconductor AC switch (triac) with an integrated zero voltage switch (15) which controls the heating coil (4a) of a heating circuit. It is particularly important that the present arrangement provides galvanic isolation between the electronic measuring circuit and the power section.

If the temperature falls below the threshold voltage, the output of the comparator (12) switches to negative potential. The diode (13) blocks, so that the triac (15) also blocks. The corresponding heating circuit is switched off. As a result, the temperature of the glass ceramic decreases again, as a result of which the electrical resistance of the sensors increases again. This increases the voltage at the output of the voltage divider back to. As soon as the rectified voltage U _i or U _{a is} again above the threshold voltage U _s , the output of the comparator (12) switches back to positive potential, as a result of which the triac (15), which is now conductive again, ignites at zero crossing and thus the corresponding one Heating coil is switched on. With this arrangement, regulation is thus possible, separately for each heating circuit.

In practice, this has the following effects:
When using good dishes, the surface temperature of the glass ceramic remains below a temperature corresponding to the threshold voltage both in the outer circle (3a) and in the inner circle (3b). The outputs of the two comparators have a positive potential, so that both heating circuits are switched on and can therefore deliver their full nominal output. FIG. 6a shows the voltage curve over time for U _i (inner circle) and U _a (outer circle).

In the case of cookware with a retracted base, the glass ceramic under the base of the pan heats up considerably more than in the outside of the cooking zone (1) due to the poor heat removal in the area of the inner circle, since the glass ceramic is in contact with the bottom of the pan in the outside. The consequence for the inner circle is that the threshold voltage is not reached due to the higher temperature. The performance of the inner circle is therefore reduced so far on average that it is impossible for the glass ceramic to exceed the temperature / time exposure limit. Figure 6b shows the typical course for U _i and U _a . The clocking when the threshold voltage U _{s is} reached can be clearly seen for the inner circle. The hysteresis can be set by a suitable connection of the comparator (12). In the case of a pan with a base that curves outwards, the conditions are similar, only the output for the inner, but for the outer heating circuit is reduced, depending on the position of the overheated zone in the outer area of the cooking zone.

In the case of the "offset pan" or "too small pan", which is also possible, the outer area of the cooking zone heats up more than that Interior, so that the average power in the external heating circuit is reduced accordingly, Figure 6c.

In the event that a saucepan boils empty, the temperature of the glass ceramic inside and outside rises sharply. In this case, the power is reduced for both heating circuits, Figure 6d.

With the arrangement described above it is achieved that the power supplied to the pot is optimally adapted to its quality. Because of the good heat extraction, pots of good quality are provided with the full nominal output, which, based on the area of the cooking zone, can be considerably higher than that of the heating elements previously used in glass ceramic cooktops. This significantly increases the performance of the cooking system.

If poor pot quality is used or if the cookware is incorrectly positioned, the power distribution is changed so that the temperature / time load on the glass ceramic is reduced under the pan base. In the areas of the cooking zone in which the pot stands up and good heat extraction takes place, an increased power density compared to conventional heating systems is maintained, while in areas with poor thermal contact the power is reduced accordingly. Overall, the cooking time is reduced due to the higher average power offered for parboiling with poor dishes.

Claims (9)

Method for power control and limitation in the case of a heating surface made of glass ceramic or a comparable material, in particular a glass ceramic cooking surface, which heating surface has a plurality of heating zones, the heating zones of the heating surface being heated in a manner known per se with heating devices having a plurality of heating elements which can be switched and controlled independently of one another,
characterized,
that by means of a plurality of mutually independent temperature sensors which are arranged in the area of the heating zone, all points of the areas which are essential for a load case are recorded,
and that the individual heating elements arranged in the area of the heating zone are switched and controlled independently of one another on the basis of the values determined by the temperature sensors,
that the power distribution in the heating zone area is largely adapted to the locally different heat extraction. Method according to claim 1,
characterized,
that the power supplied to the individual heating elements is gradually or continuously adapted to the maximum possible heat removal in the areas of the heating zone assigned to the heating elements. Method according to claim 1 or 2,
characterized,
that the edge and center areas of the cooking zone are heated and monitored independently of one another in a glass ceramic cooktop. Method according to at least one of claims 1 to 3,
characterized,
that the temperature-dependent, electrical resistance of the heating surface material is measured for temperature monitoring and control of the heating surface. Device for carrying out the method according to at least one of claims 1-4, in a hob with a glass ceramic hob and heating devices with a plurality of heating elements which can be switched and controlled independently of one another in the region of a cooking zone,
marked by
several independent temperature sensors in the cooking zone,
which are arranged in such a way that all points essential for a load case can be recorded,
as well as suitable control and regulating devices in operative connection with the sensors for controlling and limiting the power supply for the heating elements. Device according to claim 5,
characterized,
that the heating devices are known multi-circuit heating elements. Apparatus according to claim 6,
characterized,
that the heating devices are known double-circuit heating elements. Device according to claim 6 or 7,
characterized,
that the individual heating circuits are each designed for different surface loads. Device according to at least one of claims 5 to 8,
characterized,
that the temperature sensors are known, strip-shaped glass ceramic temperature measuring resistors which are delimited and contacted in the heating surface by parallel conductor tracks.

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