EP2574143A2 - Method for heating a liquid contained in a cooking vessel and induction heating device - Google Patents

Abstract

The method involves sequential determination of a parameter value of an oscillating circuit (4), and pressurizing the oscillating circuit with a high frequency voltage wave based on heating power fed into bottom of a cooking vessel (5). A time course of a parameter value is evaluated for determining a boiling point of liquid. A heating power reference value is reduced after a boiling point. A momentary parameter value is determined and stored, and rules of the parameter value are provided based on a reference value. An independent claim is also included for an induction heating device.

Description

The invention relates to a method for heating a liquid contained in a cooking vessel by means of an induction heater and an induction heater for performing the method.

In Induktionsheizeinrichtungen an alternating magnetic field is generated by means of an induction heating, which induces eddy currents in a heated cooking vessel with a bottom of ferromagnetic material and causes loss of magnetization, whereby the cooking vessel is heated.

The induction heating coil is part of a resonant circuit comprising the induction heating coil and one or more capacitors. The induction heating coil is usually designed as a flat, spirally wound coil with associated ferrite cores and arranged, for example, under a glass ceramic surface of an induction hob. The induction heating coil forms an inductive and a resistive part of the resonant circuit in conjunction with the cookware to be heated.

To control or excitation of the resonant circuit, a low-frequency AC mains voltage is first rectified with a mains frequency of, for example, 50 Hz or 60 Hz and then converted by means of semiconductor switches into an excitation or drive signal of higher frequency. The excitation signal or the drive voltage is usually a square-wave voltage with a frequency in a range from 20 kHz to 50 kHz. A circuit for generating the excitation signal is also referred to as a (frequency) converter.

To set a heating power supply in the cooking vessel in response to a set heating power setpoint different methods are known.

In a first method, a frequency of the excitation signal or of the rectangular voltage is changed as a function of the heat output to be delivered or fed in or of the desired power consumption. This method for adjusting the heating power output makes use of the fact that with a stimulation of the resonant circuit with its resonant frequency, a maximum heat output occurs. The greater the difference between the frequency of the excitation signal and the resonant frequency of the resonant circuit, the smaller the output of heating power.

However, if the induction heating device has a plurality of oscillation circuits, for example, if the induction heating device forms an induction hob with different induction cookers and different heating powers are set for the oscillating circuits, beats may be caused by superimposing the different frequencies of the excitation signals, which may lead to disturbing noises.

A method for heating power adjustment, which avoids noise due to such beats, is a pulse width modulation of the excitation signal at constant excitation frequency, in which an effective value of a heating power is adjusted by changing the pulse width of the excitation signal. In such RMS control by changing the pulse width at a constant excitation frequency, however, high on and off currents in the semiconductor switches, whereby a broadband and high-energy interference spectrum is caused.

Often it is desirable to determine a temperature of such inductively heated cooking vessel bottom, for example, to generate specific heating profiles temporal, to determine a boiling point and / or to enable automatic cooking functions.

The DE 10 2009 047 185 A1 discloses a method and an induction heater in which temperature dependent ferromagnetic properties of the cooking vessel bottom are measured at high resolution and evaluated to determine the temperature of the cooking vessel bottom.

The invention has for its object to provide a method for heating a liquid contained in a cooking vessel by means of an induction heater and an induction heater for performing the method, which, in particular based on the in the DE 10 2009 047 185 A1 revealed measuring principle, a temperature-controlled or temperature-controlled cooking allow.

The invention achieves this object by a method having the features of claim 1 and an induction heating device having the features of claim 8.

The method is used for heating and boiling a liquid contained in a cooking vessel, for example water, by means of an induction heating device, wherein the induction heating device comprises a resonant circuit with an induction heating coil. The method comprises the steps: a) continuous or periodic determination of at least one parameter value of the resonant circuit, in particular a natural resonant frequency of the resonant circuit or a self-resonant frequency associated period which depends on a temperature of the cooking vessel, in particular the cooking vessel bottom, in particular as in the DE 10 2009 047 185 A1 b) subjecting the resonant circuit with a drive signal in the form of a high-frequency square wave voltage for heating power in the cooking vessel, in particular in a cooking vessel bottom, with a predetermined heating power setpoint, in particular a maximum heating power setpoint, preferably during the determination of the at least one parameter value, the application of the resonant circuit c) evaluating the time profile of the at least one parameter value or evaluating a time profile of a derived from the parameter value size for determining the boiling point of the liquid, for example by a change of the at least one of the high-frequency square-wave voltage Parameter value or the derived size is determined and the boiling point is determined when the change is below a predetermined value or over d) falls, in particular falls short, d) after the boiling point has been determined, reducing the heating power setpoint by a predetermined amount during a predetermined period of time, e) after, in particular immediately after, the predetermined period has expired, determining and storing a current parameter value and f) regulating of the at least one parameter value to a setpoint value that depends on the stored parameter value, wherein the control variable is conventional heating output-determining Sizes can be used, for example, a frequency of the square wave voltage and / or a pulse width or a duty cycle of the square wave voltage. In accordance with the invention, therefore, such a parameter setpoint is automatically determined, which makes it possible to continue cooking with an optimum continuous cooking level, since the parameter value which is established after the boiling point detection is ideally suited as the basis for the setpoint calculation.

In a further development, the predetermined period of time is in a range between one second and 50 seconds, preferably between three seconds and 20 seconds.

In a further development, the predetermined amount by which the heating power setpoint is reduced during the predetermined period of time is determined as a function of a set cooking level, in particular such that at a higher cooking level the heating power setpoint is reduced by a smaller amount during the predetermined time compared to a lower cooking level is reduced.

In a further development, the setpoint value of the parameter value is equal to the stored parameter value.

In a further development, the heating power setpoint is reduced during the predetermined period of time to 10% to 50% of the maximum heating power setpoint.

In a further development, an offset is subtracted from the stored parameter value for determining the setpoint value of the parameter value, the offset being greater the smaller a set further cooking level is.

In a further development, the following steps are carried out following the above-mentioned steps: Evaluation of the time profile of the parameter value or a variable derived from the parameter value, for example derived by inverse value formation, if the parameter value or the variable derived from the parameter value changes within a monitoring time interval changed more than a maximum amount, for example, due to introduction of food into the liquid: Setting the heating power setpoint to a Nachkochsollwert, and repeating steps c) to g). If the parameter value or the derived variable changes within the monitoring time interval by less than the maximum dimension, it is possible to gently reheat with the previously trapped setpoint value as a reference variable, for example for a PI controller. In this way, the introduction of food, which cools the liquid strongly, optimally taken into account, since a quick re-heating and subsequent cooking can take place immediately.

The induction heater includes: a resonant circuit having an induction heating coil and a controller configured to perform the above-mentioned method.

Fig. 1

eine Induktionsheizeinrichtung mit einem Schwingkreis, der eine Induktionsheizspule aufweist, und einer Steuereinrichtung und

Fig. 2

zeitliche Verläufe einer Temperatur von Wasser in einem Kochgefäß, das mittels der in Fig. 1 dargestellten Induktionsheizeinrichtung erwärmt wird, einer in das Kochgefäß mittels der Induktionsheizeinrichtung eingespeisten Heizleistung und einer Periodendauer einer eigenresonanten Schwingung des Schwingkreises.

The invention will now be described with reference to the drawings which illustrate preferred embodiments of the invention. This shows schematically:

Fig. 1

an induction heater having a resonant circuit comprising an induction heating coil and a control device and

Fig. 2

temporal courses of a temperature of water in a cooking vessel, which by means of in Fig. 1 shown induction heating device is heated, one in the cooking vessel by means of the induction heater fed heating power and a period of self-resonant oscillation of the resonant circuit.

Fig. 1 schematically shows an induction heater 9 with a resonant circuit 4, which has a Induktionsheizspule 1 and capacitors 2 and 3, and a power unit 7, controlled by a control device 8 conventionally rectified a low-frequency mains AC voltage UN with a mains frequency of 50Hz, for example, and then by means not shown semiconductor switches in a square wave voltage UR with a frequency in a range of 20kHz to 50kHz converts, wherein the resonant circuit 4 and the induction heating coil 1 is applied to the square wave voltage UR to feed heating power in a ferromagnetic bottom of a cooking vessel 5.

The capacitors 2 and 3 are conventionally looped in series between poles UZK + and UZK- an intermediate circuit voltage, wherein a connection node of the capacitors 2 and 3 is connected to a terminal of the induction heating coil 1.

The induction heating device 9 has measuring means (not shown in greater detail) which provide a continuous or periodic determination of a parameter value of the oscillating circuit 4 in the form of a period Tp (see FIG Fig. 2 ) allow a self-resonant oscillation of the resonant circuit 4, wherein the period Tp of the temperature of the cooking vessel bottom is dependent, that also increases with increasing temperature, since with increasing temperature of the cooking vessel bottom, the effective inductance increases, so that the resonant frequency decreases and correspondingly increases the period. The period Tp can be determined, for example, by means of a timer of a microcontroller.

For the structure and the basic function of the measuring equipment, the measuring method and the heating power setting is also on the DE 10 2009 047 185 A1 Reference is hereby made to the content of the description to avoid repetition.

Fig. 2 shows time courses of a temperature Θ of water 6 in the cooking vessel or pot 5, by means of in Fig. 1 shown induction heating device 9 is heated, a fed into the cooking vessel 5 by means of the induction heater heating power P (in% of a rated heat output) and the period Tp an intrinsically resonant oscillation of the resonant circuit 4 in carrying out the method for heating and cooking according to the invention.

The control device 8 continuously determines periodically the period Tp of a self-resonant oscillation of the resonant circuit 4, for which purpose the heating power supply is temporarily interrupted and switched over to self-resonant operation of the resonant circuit 4. Due to the low temporal resolution, these phases are in Fig. 2 not shown.

In a time interval I, the resonant circuit 4 is supplied with the high-frequency square-wave voltage UR with a maximum heating power setpoint in order to bring about boiling or boiling of the water 6 as quickly as possible. The maximum heating power setpoint, known as the boost, is approximately 1.6 times the nominal heating power.

The control device 8 evaluates the time profile of the period Tp for determining the boiling point. At the end of the time interval I, the slope of the period Tp decreases below a predetermined minimum value, which suggests boiling of the water 6. A short decrease of the period Tp at the beginning of the time interval I is inherent in the principle and is not evaluated by the controller 8 as boiling.

Thereafter, in a subsequent time interval II, the heating power setpoint is reduced by a predetermined amount for a predetermined time TR of about 20 seconds, the predetermined amount being determined in dependence on a user selected cooking level.

In the time interval II, the water temperature Θ decreases only slightly due to the high heat capacity of water, whereas the pot bottom temperature, represented by the period Tp decreases, to a value PM, which is stored in the control device 8 and corresponds to a desired Fortkochleistung as a reference variable.

In a subsequent time interval III, the period Tp is controlled by suitable Heizleistungseinspeisung on the stored parameter value PM.

After the automatic detection of the boiling point, a more or less strong cooking is usually desired. The intensity of the cooking progress depends on the heat output, which is supplied to the pot 5. Different, so-called "wall degrees" can be achieved by feeding different heat outputs.

For this purpose, the induction heating device 9 provides several selectable Fortkochstufen, for example, 9 different Fortkochstufen.

Levels 1 and 2 are designed for simmering at temperatures between 75 ° C to 95 ° C. Accordingly, here is a temperature control or a control of the period Tp used, which corresponds to a temperature control of the pot bottom. The reference variable PM for the temperature controller is derived from the boiling point. For this purpose, after detecting the boiling point, the power can be reduced to about 10% to 20% of the rated or maximum power, and after about 3 seconds to 20 seconds the current measured value PM of the period duration, unlike in Fig. 2 represented, minus an offset, which corresponds to about 15K at level 1 and about 5K at level 2, are adopted as a reference variable for the temperature controller or period duration controller.

Levels 3-9 are assigned to minimum follow-on services that can not be undercut and are selected by the user depending on the desired degree of walling.

For the continuation of the cooking process, it is useful if the Fortkochzustand is maintained or quickly achieved even after the addition of food without user intervention. This will be like in Fig. 2 ensured thereby ensures that after the detection of the boiling point, the heating power is reduced to a value corresponding to the selected Fort cooking level and after a settling time of a few seconds, about 3 to 20 seconds, the measured value PM of the period Tp is taken as the setpoint. Now, the pot bottom temperature can be controlled to this set point, whereby the minimum heating power can not fall below the value of the desired heating power for the selected wall degree according to the cooking level.

The addition of food usually results in a lowering of the temperature, which can be detected on the bottom of the pot and used for reheating. Depending on the type of court, different reheating strategies can be used. So should be chosen for strong foaming food gentle after-heating, while for non-foaming foods a vigorous reheating can be applied.

The addition of food may result in shifts in the boiling point. These can be detected by a higher or lower temperature than the setpoint temperature (boiling temperature measured at the beginning) during a rated continuous cooking performance. In this case, the setpoint temperature is corrected.

Immediately after the addition of food, the pot bottom temperature decreases due to the heat removal of the food. Depending on the type and amount of food results in a smaller or larger temperature jump. Depending on the height and the speed of the temperature jump, different reheating strategies are used according to the invention. For example, more than 3K in less than 10s to a strong, constant postheating with high power (> 75%) until a boiling is recognized again and the strong reheating is stopped.

Smaller temperature drops lead to gentle reheating with the previously captured setpoint as a reference variable, for example for a PI controller.

It is understood that other / additional parameter values can be used instead of the parameter value of the resonant circuit in the form of the period, for example an amplitude of a resonant circuit voltage, a voltage across the induction heating coil, an amplitude of a resonant circuit current and / or a phase shift between the resonant circuit voltage and the resonant circuit current ,

It is further understood that the invention may also find application in the context of a parallel resonant circuit or a series resonant circuit with full bridge drive.

Claims (8)

Method for heating a liquid (6) contained in a cooking vessel (5) by means of an induction heating device (9), wherein the induction heating device comprises a resonant circuit (4) with an induction heating coil (1), comprising the steps: a) continuously determining a parameter value of the resonant circuit, in particular a period (Tp) of a self-resonant oscillation of the resonant circuit, which is dependent on a temperature of the cooking vessel, in particular the cooking vessel bottom, b) subjecting the resonant circuit with a high-frequency square-wave voltage (UR) for heating power supply in the cooking vessel, in particular in a cooking vessel bottom, with a predetermined heating power setpoint, in particular a maximum heating power setpoint, c) evaluating the time profile of the parameter value for determining the boiling point of the liquid, d) after the boiling point has been determined, reducing the heating power setpoint by a predetermined amount for a predetermined period of time (TR), e) after the predetermined period of time has elapsed, determining and storing a current parameter value (PM) and f) controlling the parameter value to a setpoint that depends on the stored parameter value. A method according to claim 1, characterized in that the predetermined period of time is in a range between one second and 30 seconds. A method according to claim 1 or 2, characterized in that the predetermined amount by which the heating power setpoint during the predetermined time is reduced, is determined in dependence on a set Fortkochstufe. Method according to one of the preceding claims, characterized in that the desired value of the parameter value is equal to the stored parameter value. Method according to one of claims 1 to 3, characterized in that the heating power setpoint during the predetermined period of time is reduced to 10% to 50% of the maximum heating power setpoint. Method according to one of the preceding claims, characterized in that for determining the desired value of the parameter value, an offset is subtracted from the stored parameter value, wherein the smaller the offset is, the larger the offset is. Method according to one of the preceding claims, characterized by the steps: - Evaluation of the time course of the parameter value and - if the parameter value changes by more than a maximum within a monitoring time interval: - Setting the heating power setpoint to a postcooking setpoint and - Repeat steps c) to f). Induction heater (9) with - A resonant circuit (4) with an induction heating coil (1) and - A control device (8) which is adapted to carry out the method according to one of claims 1 to 7.

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