EP3177107B1 - Method for operating an induction cooking hob - Google Patents

Description

Area of application and state of the art

The invention relates to a method for operating an induction hob, whereby a temperature adjustment is to be effected or a specific cooking vessel temperature is to be achieved or set as the target temperature and kept constant. A special feature of the process is that no temperature measuring devices are used that record the absolute cooking vessel temperature. The cooking vessel temperature is only determined indirectly via other properties of the cooking vessel, such as temperature-dependent changes in permeability. Only a relative temperature change can be recorded, but not an absolute temperature. The measuring method is known from EP 2330866 A2 .

It's from the EP 2574144 A2 known to be able to maintain a constant temperature for frying, which is usually slightly above 200 °C. A target temperature that has been reached must be confirmed, so to speak.

WO 2010/139598 A1 discloses a method for operating an induction hob according to the preamble of claim 1.

Task and solution

The invention is based on the object of creating a method mentioned at the beginning, with which problems of the prior art can be avoided and in particular it is possible for a predetermined or entered target temperature for an induction hob to be set in an advantageous manner Cooking vessel can be controlled and held automatically, so to speak.

This problem is solved by a method with the features of claim 1. Advantageous and preferred embodiments of the invention are the subject of the further claims and are explained in more detail below. The wording of the claims is incorporated into the content of the description by express reference.

An induction hob has a control and a hotplate with at least one induction heating coil. A connection is advantageously stored in the control between a cooking vessel temperature and a heating output of the induction heating coil as area power or area power density, which sets or results in the specified and desired specific cooking vessel temperature in the steady state or stable state or in continuous operation.

It is intended that in the process of operating this induction hob, a cooking vessel is placed on the hob and is inductively heated by the induction heating coil. Before the cooking vessel is heated up, a target temperature for the cooking vessel or an application that implies a specific target temperature is entered into the control of the induction hob, for example as "roast steak". At the beginning of the heating process, the cooking vessel is heated for a first heating time with a first relatively large heating output as an area output, in order above all to bring about the fastest possible rise in temperature in order to quickly get close to the target temperature.

After the first heating-up time, the heating output of the induction heating coil is reduced to a first relatively small heating output, which would lead to the target temperature in the long term. This can correspond to the aforementioned relationship between cooking vessel temperature and heating output, if this is saved. This first small heating output is significantly smaller than the aforementioned large heating output, preferably only about 1% to 20% or only up to 10%. Then it is checked, advantageously after a short checking time of one second to thirty seconds, whether the cooking vessel temperature remains constant, rises or falls at the first relatively small heating output. The process used for this will be explained in more detail below.

In a first case, when the cooking vessel is heated for the short check time with the first relatively small heating power, the cooking vessel temperature remains constant and corresponds to the target temperature, advantageously at least after the aforementioned short check time of a few seconds. The target temperature is then considered to have been reached and is preferably maintained, for example the actual frying process can then begin. To maintain the frying temperature, it is advantageous to use a continuous control or a two-point control, as is the state of the art. In general, the temperature can be kept approximately constant, possibly with a slight increase in heating power due to the food to be fried.

In a further case that, due to the setting of the first, relatively small heating power, the cooking vessel temperature does not reach the target temperature or does not reach a constant temperature within the short checking time or after the short checking time, the relatively small heating power is set by the control in adapted or changed to their size. You can try to find a different heating output that leads to a constant temperature during the short check time. This different heating output is advantageously also a relatively small heating output. This can also be used to get a temperature value at all to determine the temperature that is currently present in order to be able to approach the target temperature more specifically or quickly.

After finding the corresponding correlation between heating power and cooking vessel temperature with sufficient accuracy, the control preferably considers the heating process to be complete and cooking or frying or cooking continues. This is advantageously signaled to an operator, and further procedural steps may also be initiated.

In another embodiment of the invention, in a second case, when the cooking vessel is heated with the first relatively small heating power, the cooking vessel temperature continues to rise after the short check time. Under certain circumstances, there may first be a brief drop in the signal used to determine the temperature, but this is not a problem here. Then the cooking vessel is heated again more strongly or further for an intermediate heating time with an intermediate heating power, since the cooking vessel temperature is still below the target temperature, so that its temperature increases again. The intermediate heating output is advantageously greater than the first, relatively small heating output, but can also be the same size. Then, after an intermediate heating time, it is checked by resetting the relatively low heating power to see whether the cooking vessel temperature increases or remains constant during a short checking time, possibly after a short checking time of one second to half a minute or a minute. If the cooking vessel temperature then remains constant, not only is a constant temperature set, but the first case applies, namely that the target temperature is considered to have been reached.

Advantageously, in the event that the cooking vessel temperature increases further after the intermediate heating time and after the short checking time when heating with the first relatively low heating power, a cooking vessel temperature that is below the target temperature can be determined again. The cooking vessel can then be heated more strongly again with an intermediate heating output for an intermediate heating time. After the intermediate heating time, it can then be checked again by setting the relatively low heating output for a short checking time to see whether the cooking vessel temperature still increases or remains constant after this short checking time, with the cooking vessel temperature remaining constant being the first case of reaching the target temperature applies.

In a third case, if the cooking vessel temperature continues to fall when the cooking vessel is heated with the first relatively small heating output even after the check time has elapsed, a cooking vessel temperature above the target temperature is determined. The target temperature can then be achieved in different ways, which will be explained in more detail becomes. The simplest way is to simply continue heating with the relatively low heat output and after some time or a few minutes the target temperature will have been reached. Alternatively, the heating operation can be suspended for a short time, for example 5 seconds to 30 seconds or a minute.

Thus, with the invention, in particular also in its aforementioned optional embodiments, the knowledge can be implemented that in a practically applied method a certain heating output as an area output leads to a certain final temperature or permanently maintained cooking vessel temperature, largely regardless of what is used for a cooking vessel. This mainly applies in the range between 150°C and 250°C, especially 200°C to 250°C, which is advantageous for frying processes. It should be noted that the aforementioned relationship between the cooking vessel temperature and heating power as area power, so to speak, requires the information as to what power the induction heating coil or several induction heating coils connected together in a cooking area generate, i.e. is introduced into the cooking vessel. Furthermore, the approximate area of the cooking vessel or the bottom of the cooking vessel is required so that the area performance can be determined. However, since hotplates are usually designed for certain sizes of cooking vessels, and this is indicated in particular by a marking on the top of a hob plate, an approximate range of cooking vessel size to be expected for a defined hotplate is known. Furthermore, it is also possible, in particular, to determine whether the induction heating coil is covered by the cooking vessel by monitoring operating parameters of the induction heating coil, in particular an efficiency of the induction heating coil. If the size of the induction heating coil is known, the approximate area of the cooking vessel or the bottom of the cooking vessel can then be determined. This is already known to the person skilled in the art from another context. The method requires that there is no food in the dishes during the heating process and the determination of the cooking vessel temperature according to the invention. This would distort the temperature setting process described above. However, the falsification would be so significant that the control can detect this case and display it to an operator.

The target temperature can be entered into the control either by an operator using controls. Alternatively, the input can be through an automatic cooking program which takes place in the control itself. What is important is that a target temperature is given.

The first heating time mentioned can be relatively short. In particular, since relatively high target temperatures are to be approached, an attempt is made to select the first relatively large heating output as very large, advantageously as large as possible. So it can be 3 W/cm ² to 12 or even 14 W/cm ² , in particular 6 W/cm ² to 10 W/cm ² . Then this initial heating time can be between one minute and five minutes or even eight minutes. It can also be specified for a specific hotplate or induction heating coil depending on its size and thus an expected cooking vessel size from empirical values stored in a table in the control, for example two minutes for small induction heating coils, five minutes for medium-sized induction heating coils and eight minutes for large induction heating coils . These empirical values are based on the fact that when a cooking vessel, in particular a pan, of the appropriate size is set up, this time elapses until a temperature between 200 ° C and 250 ° C is reached at the first relatively high heating output. Alternatively, the heating time can also be theoretically calculated using the heat capacity of the dishes, surface power density and the desired temperature increase in the control.

The first relatively small heating output can be significantly lower than the first large heating output.

According to the invention it is between 0.3 W/cm ² and 2 W/cm ^2. Particularly advantageously it is between 0.6 W/cm ² and 0.8 W/cm ² . In the context of the invention, it has been found that with such relatively low heating powers, cooking vessel temperatures between 200 ° C and 250 ° C can be maintained over the long term. Of course, such cooking vessel temperatures could also be achieved simply by setting such a relatively small heating output as an area output, but this would then predictably take a very long time.

The first relatively small heating output is advantageously set or introduced into the cooking vessel for at least one second to 30 seconds or even a minute, i.e. the aforementioned short time as a check time before the cooking vessel temperature is expected to remain constant. The temperature compensation processes usually take a few seconds, especially in the aforementioned first or second case, until the first small heating output defines the energy input. The check time is advantageously 5 seconds to 20 seconds. An aforementioned intermediate heating time can be in a range similar to the checking time, for example between 5 seconds and 60 seconds, preferably between 10 seconds and 20 seconds. The intermediate heating output should advantageously be relatively larger than the first Small heating output, can also be significantly larger, but it doesn't have to be. The advantage of choosing a slightly larger intermediate heating output is that if the cooking vessel temperature is obviously still below the target temperature, the target temperature can be reached more quickly. According to the invention, the intermediate heating power is between 1 W/cm ² and 12 W/cm ² , in particular between 1.5 W/cm ² and 8 W/cm ² , or it can be 5% to 100% larger than the first relatively small heating output.

In an advantageous embodiment of the invention it can be provided that in the third case the cooking vessel is simply heated with an intermediate heating power as described above after the cooking vessel temperature has been determined to be too high. Then, when the cooking vessel temperature becomes constant, it corresponds to the target temperature. However, this results in a somewhat slower drop in the cooking vessel temperature, which means that the determination of the specific cooking vessel temperature as the actual frying temperature can only take place later, in particular after several minutes, and therefore the operator can only start the frying process with a time delay.

Alternatively and more quickly, heating can be carried out with a second intermediate heating output, which can then be slightly above the first, relatively small heating output, advantageously between 105% and 200% of it. It is waited until this second intermediate heating output leads to a constant cooking vessel temperature. The cooking vessel temperature could then be determined from the relationship between the cooking vessel temperature and heating output stored in the control. This means that the control system can not only detect that the cooking vessel temperature is above the target temperature, but also how much it is above it. In this case, the cooking vessel temperature is not at the target temperature, but above it, but the control can again determine its absolute value based on the second intermediate heating power at a constant cooking vessel temperature. The heating output can then be reduced again. Either it can be switched off for a short time to cause a faster temperature drop towards the target temperature. Since the cooking vessel temperature and the target temperature are known, the control can estimate this based on stored empirical values. Then the first relatively small heating output can be set, which leads to the target temperature. Alternatively, the operator can also be given the signal to start the frying process. The inserted food will then cool the cooking vessel to the target temperature relatively quickly. The control can then use the actually desired target temperature for the temperature control already described, even if this was not explicitly set beforehand.

Checking the cooking vessel temperature or checking whether the cooking vessel temperature changes or whether it remains constant is advantageously carried out using a sensorless method or without a specially provided temperature sensor. During heating operation, the vibration response on at least one induction heating coil is used to determine whether the temperature of the cooking vessel or the bottom of the cooking vessel above this induction heating coil changes or whether this temperature increases. A temperature gradient of the cooking vessel can thus be detected by the induction heating coil, which is preferably done according to a method as described in EP 2330866 A2 is described. If this determination of the vibration response only takes place periodically, it should advantageously be every 0.01 milliseconds to 1 second, advantageously up to 1 millisecond. In general, the vibration response of an induction heating coil can be understood as the evaluation of the change in oscillating circuit parameters due to temperature changes of the cooking vessel or cooking vessel bottom, in particular the changing permeability. Preferably, the vibration response can be recorded when operating several induction heating coils on the hotplate or, for this cooking vessel, on each induction heating coil.

This method advantageously comprises the steps: generating an intermediate circuit voltage at least temporarily depending on a single-phase or multi-phase, in particular three-phase, alternating network voltage; Generating a high-frequency control voltage or a control current from the intermediate circuit voltage, for example with a frequency in a range of 20kHz to 70kHz; and applying the control voltage or the control current to a resonant circuit comprising the induction heating coil. In this way, the cooking vessel is conventionally heated inductively. To measure the temperature, the following steps are then carried out: Generating the intermediate circuit voltage during predetermined time periods, in particular periodically, with a constant voltage level, the intermediate circuit voltage preferably being generated independently of the AC mains voltage during the time periods; Generating the control voltage during the predetermined time periods such that the resonant circuit oscillates at its natural resonance frequency in a substantially undamped manner; Measuring at least one vibration parameter of the vibration during the predetermined time periods; and evaluating the at least one measured vibration parameter to determine the temperature. Since the intermediate circuit voltage is kept constant during the temperature measurement, signal influences due to a changing intermediate circuit voltage can be eliminated, thereby enabling a reliable and interference-free temperature determination or determination of a temperature change.

In a further development, the method includes the steps: determining zero crossings of the AC mains voltage and selecting the time periods in the area of the zero crossings. In the area of zero crossings with single-phase AC mains voltage, the intermediate circuit voltage usually decreases sharply. The constant voltage level is preferably chosen such that it is greater than the voltage level that usually occurs in the area of zero crossings, so that the intermediate circuit voltage is clamped to the constant voltage level in the area of zero crossings. There are then constant voltage conditions in the area of zero crossings, which enable reliable temperature measurement. So no additional temperature sensors are needed here, even if they could be present.

In an embodiment of the invention, it is possible for not just a single induction heating coil to be provided at the cooking point for the cooking vessel, but several. In principle, the same applies here, in which case the stated performance values are based on all induction heating coils that are present on the hob and are used to heat the cooking vessel. Your output or area output or heating output is then considered together as described above for temperature measurement.

In an advantageous embodiment of the invention, it is possible to record and monitor the amount of energy introduced or the heating output of the induction heating coil over time. Estimates can also be made about the temperatures reached. Based on this, the control can vary the heating output slightly or, above all, set the first heat-up time, the check time, the intermediate heating time or off times. The aforementioned check times in the various cases can be the same or similar, but do not have to be. They can also differ by a factor of 1 to 5.

Brief description of the drawings

Fig. 1

für mehrere verschiedene Kochgefäße ein Verlauf der stabil auf Dauer gehaltenen Kochgefäßtemperatur abhängig von einer Flächenleistung,

Fig. 2

eine Seitenansicht eines Induktionskochfelds mit einer Induktionsheizspule und aufgesetztem Kochgefäß,

Fig. 3 bis 6

verschiedene Verläufe der Kochgefäßtemperatur und der Flächenleistung über der Zeit in verschiedenen Fällen der Ansteuerung für leere Kochgefäße, also ohne Zugabe eines Lebensmittels.

Embodiments of the invention are shown schematically in the drawings and are explained in more detail below. Shown in the drawings:

Fig. 1

for several different cooking vessels, a course of the cooking vessel temperature that is kept stable over the long term depending on an area output,

Fig. 2

a side view of an induction hob with an induction heating coil and attached cooking vessel,

Fig. 3 to 6

different curves of the cooking vessel temperature and the area output over time in different cases of control for empty cooking vessels, i.e. without adding any food.

Detailed description of the exemplary embodiments

In the Fig. 1 It is recorded how empirically determined values for four different cooking vessels indicate the relationship between how the long-term reached or set cooking vessel temperature depends on the corresponding area performance. This shows that, on the one hand, the relationship is somewhat linear and is therefore very easy to determine mathematically. On the other hand, the temperatures are only a maximum of 30°C to 35°C apart for a certain area performance. It is therefore possible to determine relatively precisely at a certain area output Q*/A which cooking vessel temperature will be reached on a cooking vessel after a certain longer period of operation, for example 10 minutes to 30 minutes.

In the Fig. 2 is an induction hob 11 shown with a hob plate 12, on which a hotplate 13 is formed. An induction heating coil 15 is arranged under the hob plate 12, which defines and also heats the hotplate 13. This could also consist of several induction heating coils, which plays no role for the invention. The induction heating coil 15 is supplied with power and controlled by a controller 17, whereby the controller 17 can monitor the power fed into the induction heating coil 15. Furthermore, the controller 17 has a memory, not shown, in which, so to speak Fig. 1 A connection is stored between the cooking vessel temperature and the area output. Either the mathematical relationships can be saved when the temperature curves are out Fig. 1 can be viewed as a straight line. Alternatively, temperature values for gradually increasing area performance can be stored with sufficiently good resolution.

In an expanded embodiment of the invention, it is possible for this to be stored in the control 17 for several cooking vessels, so that the control 17 knows exactly which of the four or more curves from the Fig. 1 should be used in the respective case. Alternatively, certain parameters could also be entered into the control 17 by an operator or programmed from the outside, which, detached from the specific cooking vessel present, inform the control 17 which cooking vessel is now being used or which of the stored curves applies. Under certain circumstances, the controller 17 can then also recognize the size range in which a cooking vessel placed above the hotplate 13 is located.

Of course, the area of the induction heating coil 15 is known. Advantageously, the area output mentioned is not based on the area of the induction heating coil 15, but rather on the area of the cooking vessel 19. Matching the cooking area 13, the surface or the bottom surface of the cooking vessel 19 will move in a relatively narrow range, since suitable cooking vessels within certain diameter classes usually only have a diameter variation of up to 3 cm. Cooking vessels that are significantly too large or significantly too small are rarely put on; this could also be recognized by the control 17 and signaled to an operator as an error.

In the Fig. 3 is shown how heating takes place at time t=0 with a large heating output, here 7 W/cm ² , which is constant. This heating lasts until time t1 as the heating time, which can be predefined.

Beforehand, a target person or an automatic control or the like. a target temperature of 200°C has been entered. This temperature should be maintained permanently on the cooking vessel 19, which here is a pan. This temperature applies advantageously to the top of the bottom of the cooking vessel, i.e. where food to be cooked, for example a steak to be fried, comes into contact with the cooking vessel 19. The top curve from the applies to the cooking vessel 19 Fig. 1 .

After the heating time t1 has elapsed, the heating output is greatly reduced and set to 0.68 W/cm ² . This corresponds to the Fig. 1 the top curve or at this area performance the temperature of 200°C is permanently maintained.

From the Fig. 3 According to the first case, it can be seen that the temperature T only drops slightly and then becomes constant relatively quickly, for example in 5 seconds to 20 or 30 seconds as the adaptation time. Both the small temperature drop and the constant Temperature can be achieved by a method mentioned above or in accordance with the EP 2330866 A2 or the EP 2574144 A2 be recognized.

Since the cooking vessel temperature remains permanently constant at an area output of 0.68 W/cm ² , this is determined according to the Fig. 1 set at 200°C and can therefore be maintained permanently.

In the next case according to the Fig. 4 is heated up to time t1' as a heating time with a large area output of 7 W/cm ² , whereby the temperature T increases again. At time t1' the power is reduced to 0.68 W/cm ² corresponding to a desired target temperature of 200°C here too. The control 17 or the temperature detection can now determine that with this area output now set, the cooking vessel temperature is still rising, although this is probably weaker than before. This means that the cooking vessel temperature at time t2` is still below the target temperature of 200°C. The time between t1' and t2` is the aforementioned check time. For this reason, at time t2', which is, for example, a few seconds to one or two minutes after time t1', a significantly larger power and in particular the previously set large power of 7 W/cm ² is set again. Then the temperature T increases more strongly again. After a certain time as an intermediate heating time between t2` and t3`, for example a few seconds to one minute to three minutes, the output is reduced again to the target temperature, i.e. back to the first small heating output of 0.68 W/cm ² . Now the temperature detection recognizes that the cooking vessel temperature T initially decreases slightly and then relatively quickly, for example within a minute or even just a few seconds as an adaptation time, only shows a small drop or becomes constant. This means that it is again the case that a constant cooking vessel temperature is achieved with an area output of 0.68 W/cm ² . This must then be the target temperature of 200°C Fig. 1 or as before Fig. 3 described. In this case, reheating with the higher heating output was necessary because the cooking vessel requires more energy to reach the specific temperature than the control system assumed. The heat capacity of the cooking vessel therefore deviated from the value stored in the control.

The second time or intermediate heating time with high heating output Fig. 4 between t2` and t3` could also be done with a different area performance than the heating time up to time t1'. However, the heating processes should take place relatively quickly, so that at least a high area performance close to the maximum area performance should be selected. The case of overheating during the heating time is in the Fig. 5 shown. Here, even at a desired target temperature of 200 ° C for the heating time up to a time t1 ", heating is carried out with the high power of 7 W / cm ² , whereupon the temperature T increases. Then from time t1" for a check -Heated for a period of time with the low area output of 0.68 W/cm ² , i.e. for a few seconds to half a minute, to see whether the cooking vessel temperature becomes constant relatively quickly, which would be considered as reaching the target temperature. However, the controller 17 determines via the aforementioned temperature monitoring that the cooking vessel temperature falls permanently even after the check time has elapsed, even after one or two minutes as an adjustment time. This means that the cooking vessel temperature is well above the target temperature. Now either the power can be switched off completely for a short time, for example for 10 seconds to 30 seconds, in order to achieve rapid cooling to the target temperature or close to it. Then operation could start again with the low heating output of 0.68 W/cm ² and experience has shown that the temperature would then have to become constant relatively quickly and then be the target temperature of 200°C.

Or, according to another possibility, an attempt is made to roughly determine the prevailing temperature. For this reason, a slightly larger heating output is fed into the induction heating coil 15 as an intermediate heating output for the intermediate heating time between t2" and t3", namely 0.8 W/cm ² here. A constant temperature is established relatively quickly, which according to the Fig. 1 is around 230°C. The controller 17 therefore knows that the temperature is still approx. 30°C too high. Then, as described above, you can either switch off the induction heating coil 15 completely for a short time for a slightly faster cooling, for example for 10 seconds to 30 seconds, in which case the low heating output is then set again to reach and maintain the target temperature. Alternatively, the area output of 0.68 W/cm ² corresponding to the target temperature can be set from time t3", so that the cooking vessel temperature T falls somewhat more slowly to the target temperature, which is then ultimately reached and maintained. Faster cooling can also be achieved can be achieved by inserting the food to be cooked. Advantageously, for temperature control following the addition of food, the measured value that corresponds to 200 ° C is used as the setpoint and not the measured value that corresponds to 230 ° C.

Fig. 6 shows a further advantageous embodiment of the method for achieving a specific cooking vessel temperature in a defined manner. If the constant steady-state temperature is not reached after a short period of time, regardless of whether the signal falls or rises, no discrete power levels are subsequently approached between t2‴ and t3‴. Rather, one will Setpoint T _S of the temperature signal is determined after a set time, here at t2‴ with 230°C. The control then regulates to this setpoint T _S , for example using a proportional controller, which can also have integral or derivative components. This means that a constant temperature is reached relatively quickly at t3‴, faster than would be possible with discrete temperature levels. According to Fig. 1 Corresponds to a cooking vessel temperature of 230°C and an area output of 0.8 W/cm ² . That is why the cooking vessel temperature is kept at 230°C at this surface power density. In this way, the corresponding correlation between power and constant temperature is found, with the power being known, which enables temperature determination and thus temperature adjustment. Now, based on known relationships, the specific cooking vessel temperature of 200 ° C can be reached by reducing the power starting from the known temperature, for example with the simultaneous insertion of the food to be cooked.

Thus, with the invention it is possible to regulate the temperature or start and hold a specific one without absolute temperature measurement and only by relative temperature measurement, i.e. monitoring whether a temperature rises, falls or is constant, and a known relationship between temperature and permanently set area power density To allow temperature on a cooking vessel.

Furthermore, the invention is advantageous in that in a steady state, i.e. a permanently prevailing state, a thermal conduction resistor is connected in series to a parallel connection as a radiant heat resistance and a convection heat resistance. This results in the in Fig. 1 connection to be recognized.

The invention therefore uses an energy balance to solve the problem set out at the beginning. By finding a steady state, i.e. a state without a change in the cooking vessel temperature, the internal energy of the cooking vessel is kept constant. As a result, it is known that the energy introduced into the cooking vessel by the heating is completely released again, be it through convection, heat radiation or heat conduction into the hob surface. However, the energy introduced can be measured by the heating. Because the connection Fig. 1 is known, the absolute temperature can be determined by measuring energy per time or power, under certain general conditions.

Claims (11)

1. Method for operating an induction hob (11) for defined attainment of a specific cooking vessel temperature, the induction hob having a control (17) and a cooking point (13) with at least one induction heating coil (15), having the steps:

   - a cooking vessel (19) is placed on the cooking point (13) and is inductively heated by the induction heating coil (15),

   - prior to a heating process of a cooking vessel (19), a target temperature for the cooking vessel or a use case implying a certain target temperature is entered into the control (17) of the induction hob (11),

   - at the beginning of the heating process, the cooking vessel (19) is heated for a first heating time (t1) with a first relatively large heating power as area power,

   - after the first heating time (t1, t1', t1"), the heating power of the induction heating coil (15) is reduced to such an extent to a first relatively small heating power that would lead to the target temperature in the long run during operation with this first relatively small heating power,

   - it is checked whether, after a short checking time (t2'-t1', t2"-t1") at the first relatively small heating power, the cooking vessel temperature remains constant, rises or falls,

   - wherein in a first case, when the cooking vessel (19) is heated with the first relatively small heating power after the short review time, the cooking vessel temperature remains constant and corresponds to the target temperature, the target temperature is considered to be reached,

   characterized in that the first relatively small heating power is from 0.3 W/cm² to 2 W/cm²,

   in that, in a second case, when the cooking vessel (19) is heated with the first relatively small heating power, the cooking vessel temperature continues to rise after the short checking time (t2'-t1'), a cooking vessel temperature below the target temperature is detected and the cooking vessel (19) is heated more strongly once again with an intermediate heating power of between 1 W/cm² and 12 W/cm² for an intermediate heating time (t3'-t2') and then after the intermediate heating time it is checked again by setting the first relatively small heating power whether the cooking vessel temperature still rises after the short checking time or remains constant, wherein with the cooking vessel temperature remaining constant the first case of reaching the target temperature applies,

   and in that in a third case, that by the setting of the first, relatively small heating power the cooking vessel temperature has not reached the target temperature after the short checking time (t2"-t1"), but drops, a cooking vessel temperature above the target temperature is detected and the first relatively small heating power is increased by the control (17) for an intermediate heating time to an intermediate heating power, which is 10% to 100% greater than the first relatively small heating power, so that a constant temperature is achieved, whereupon heating is resumed at the first relatively small heating power until the temperature has dropped to the target temperature and is maintained there.
2. Method according to claim 1, characterized in that the target temperature is between 200°C and 250°C.
3. Method according to claim 1 or 2, characterized in that in the event that the cooking vessel temperature continues to rise after the intermediate heating time and after the short checking time (t2'-t1'), a cooking vessel temperature below the target temperature is again detected and the cooking vessel (19) is once again heated more strongly with the intermediate heating power for the intermediate heating time, and then, after the intermediate heating time, it is checked again by setting the first relatively small heating power for the short checking time whether the cooking vessel temperature still rises after the short checking time or remains constant, the first case of reaching the target temperature applying if the cooking vessel temperature remains constant.
4. Method according to one of the preceding claims, characterized in that the short checking time (t2'-t1', t2"-t1") is 1 second to 30 seconds, preferably 5 seconds to 20 seconds.
5. Method according to of the preceding claims, characterized in that the intermediate heating time (t3'-t2') is 5 seconds to 60 seconds, preferably 10 seconds to 30 seconds.
6. Method according to one of the preceding claims, characterized in that the cooking vessel (19) is operated at a cooking point (13) with one or more induction heating coils (15) and the power of the induction heating coils is jointly considered as area power or heating power.
7. Method according to one of the preceding claims, characterized in that the amount of energy introduced or the heating power of the induction heating coil (15) is monitored over time.
8. Method according to one of the preceding claims, characterized in that the first relatively large heating power is 3 W/cm² to 12 W/cm², in particular 6 W/cm² to 10 W/cm².
9. Method according to one of the preceding claims, characterized in that the first relatively small heating power is 0.6 W/cm² to 0.8 W/cm².
10. Method according to one of the preceding claims, characterized in that the intermediate heating power is 1.5 W/cm² to 8 W/cm².
11. Method according to one of the preceding claims, characterized in that a cooking vessel size is determined by considering the efficiency of the induction heating device by overlapping the induction heating coil (15) with the placed cooking vessel (19).

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