EP2574145B1 - Method for preparing food by means of an induction heating device and induction heating device - Google Patents

Description

The invention relates to a process for preparing food cooked in a liquid contained in a cooking vessel by means of an induction heating device and an induction heating device for carrying out the process.

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 course of the temperature of the cooking vessel bottom when cooking water-floating foods, such as rice, behaves differently than when boiling with pure water. The fact that the bottom of the pot is not completely covered with water but to a large extent with the food, the convection is hindered in the water. This makes the boiling point detection considerably more difficult

The DE 102 31 122 A1 shows a method for measuring the temperature of a metallic cooking vessel, wherein at a boiling point Oscillating circuit parameters can be detected, which is used as the setpoint for a control.

The DE 102 53 198 A1 shows a method for thermally monitoring an inductively heatable cooking vessel by evaluating a frequency of an inductive heating causing AC, such boiling is detected.

The EP 2 312 909 A1 shows a method in which the supply of power to a liquid in a cooking vessel is controlled.

The invention has for its object to provide a method for preparing food, which are cooked in a liquid contained in a cooking vessel, by means of an induction heater and an induction heater for performing the method to provide, in particular based on in the DE 10 2009 047 185 A1 disclosed measuring principle, a reliable temperature-controlled or temperature-controlled cooking allow, in particular by reliably determining a boiling point.

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 to prepare foods, such as rice, which are cooked in a liquid contained in a cooking vessel, such as water, broth, etc., by means of an induction heating device comprising a resonant circuit with an induction heating coil.

In the method, a parameter value of the resonant circuit is continuously or periodically determined, in particular a self-resonant frequency of the resonant circuit or a self-resonant frequency associated period. The parameter is dependent on a temperature of the cooking vessel, in particular the cooking vessel bottom.

During a heating phase, the resonant circuit is subjected to a high-frequency square-wave voltage. This serves to feed the heating power into the cooking vessel, in particular into the cooking vessel bottom, with a predefinable heating power setpoint.

The heating power setpoint is changed periodically. Within a respective period of the Heizleistungssollwertveränderung the Heizleistungssollwert is set during a first period, for example about 48 seconds, to a first value. During a remaining duration, ie period duration of the heating power setpoint change minus first duration, for example 12 seconds, the heating power setpoint is set to a second, smaller value.

The period of the Heizleistungssollwertveränderung can be for example 60 seconds. The period can be constant or variable.

A change in the parameter value is determined within the period of the Heizleistungssollwertveränderung, in particular during the remaining duration within the period of the Heizleistungssollwertveränderung at a smaller setpoint.

The determined change in the parameter value is evaluated to determine the boiling point of the liquid.

The heating phase is ended when the boiling point has been determined.

In a further development, a boiling point is determined when evaluating the determined change in the parameter value if the change in the parameter value falls below a predefined level.

In a development, after the heating phase has ended, a continuing cooking phase is carried out, with the following steps: applying a heating power setpoint, which corresponds in particular to 5% to 50%, preferably 10% to 20%, of a maximum heating power setpoint to the oscillating circuit with the high-frequency square-wave voltage. whether the parameter value changes by more than a predefined amount within a monitoring period, and if the parameter value changes by more than the specified amount within the monitoring period, ending the cooking phase.

In a further development, after determining the boiling point, in particular immediately after determining the boiling point, an instantaneous parameter value is stored and a warming-up phase is carried out after the end of the continuing cooking phase, with the steps of: regulating the parameter value to a parameter setpoint which depends on the stored parameter value is determined, for example, by subtracting a predetermined offset value.

In a further development, after ascertaining the boiling point, in particular immediately after determining the boiling point, an instantaneous parameter value is stored, and after the end of the heating phase, a continuing cooking phase is carried out. The cooking phase comprises the steps of: controlling the parameter value to a parameter setpoint determined as a function of the stored parameter value, monitoring a heat output to be applied to the control, and if the heating power to be applied is below a predetermined level, stopping the boiling phase. The warming up phase can be followed by a warming up phase.

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 eines Heizleistungssollwerts der in Fig. 1 gezeigten Induktionsheizvorrichtung 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

time profiles of a heating power setpoint of the Fig. 1 shown induction heating device 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 UR to feed heating power in a ferromagnetic bottom of a cooking vessel 5, wherein in the cooking vessel water. 6 is in the rice 10 in a ratio of 2: 1 is provided.

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 directed.

Fig. 2 shows time curves of a heating power setpoint SW in 0.5% of a nominal heating capacity of Fig. 1 shown induction heater 9 and the period Tp of a self-resonant oscillation of the resonant circuit. 4

The control device 8 continuously or periodically determines 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, which forms a heating-up phase or a heating-up phase, the resonant circuit 4 is supplied with a high-frequency square-wave voltage UR for heating-power feed into the cooking-vessel bottom, the associated heating-output setpoint SW changing periodically. During a first duration, for example 48 seconds, within a respective period P, a first value, for example corresponding to 100% of the nominal heating power, is set and during a remaining duration, for example 12 seconds, a second, smaller value is set, for example corresponding to 10 % of nominal heating capacity.

The control device 8 determines within the period P a change in the period Tp, in particular during which the smaller set value is set, and determines a boiling point when the change of the period Tp falls below a predetermined level.

This is the case at the end of the warm-up time interval I, wherein after the completion of the warm-up time interval I, a continued cooking phase II is carried out. During the boiling phase II, the heating power setpoint is approx. 10% to 20% of a maximum heating power setpoint. It is monitored whether the period Tp changes within a monitoring period, for example 10 seconds, by more than a predetermined amount, which may be caused, for example, by the fact that after the water 6 has been taken up or evaporated from the rice 10, the soil temperature is relatively high rising rapidly.

The cooking phase II is then terminated and it follows a holding phase III, during which the period Tp is controlled to a desired value, which is determined in response to a set immediately after the determination of the boiling point period Tp by a predetermined value of this value Offset value is subtracted.

Instead of the illustrated cooking and holding phase II or III can be proceeded to continue cooking or keeping warm as follows. Immediately after determining the boiling point, a period Tp is stored as a setpoint. The period Tp is then regulated to this setpoint. The heat output to be applied to the control system is monitored and if the heat output to be used falls below a predetermined level, the cooking phase is ended. The warming up phase can be followed by a warming up phase. Rice 10 can be prepared by the so-called swelling method. For this purpose, a lot of rice 10 with an amount of water 6, for example in a ratio of 1: 2, brought to a boil and cooked until the water 6 is completely absorbed or evaporated by the rice 10. The cooking performance is adjusted so that very little water evaporates. With the cooking system 9 described above, this process is very easy to automate.

The process can be divided into 3 phases: heating, cooking and recognizing cooking. A cooking program that depicts the three phases requires the functions boiling with boiling point detection, cooking with temperature monitoring and cooking detection.

The course of the soil temperature when cooking with rice or other foods floating in water behaves differently than when boiling with pure water. The fact that the bottom of the pot is not completely covered with water but to a large extent with the food, the convection is hindered in the water.

For boiling point detection, the heating power is periodically, for example every minute, for example, reduced for 12 seconds and measured the temperature profile or the course of the representative period Tp on the bottom of the pot. The amplitude of the temperature change by the power variation decreases with increasing water temperature to assume a constant value after reaching the boiling point. This property can be used to detect the boiling state.

After detecting the boiling state, the power is reduced to continued cooking power, for example 10% to 20% of the nominal power, and the temperature is constantly monitored. After the water has been absorbed or evaporated by the rice, the soil temperature rises relatively quickly. This increase is detected and a ready signal can be given to a user.

At the same time can be switched to keep warm with a regulated temperature below the boiling point. Since the boiling point is known as the reference temperature or its equivalent in the form of the period Tp from the previous cooking process, the setpoint temperature can be set to a suitable holding temperature, for example 80-90 ° C, and controlled by means of a negative offset.

It is understood that instead of the parameter value of the resonant circuit in the form of the period Tp other / additional parameter values can be used, 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)

1. Method for preparing foodstuffs (10), which are cooked in 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) having an induction heating coil (1), the method comprising the steps:

   - continuous determination of a parameter value of the resonant circuit, the parameter value depending on a temperature of the cooking vessel, and

   - during a heating-up phase (I):

   - applying a high-frequency rectangular voltage (UR) to the resonant circuit in order to supply heating power having a heating power setpoint (SW) to the cooking vessel, wherein the heating power setpoint is periodically varied, wherein within a period (P) the heating power setpoint is set to a first value during a first interval, and during a remaining interval is set to a second, smaller value,

   - determining a change in the parameter value within the period,

   - evaluating the determined change in the parameter value in order to determine the boiling point of the liquid and

   - ending the heating-up phase when the boiling point has been determined.
2. Method according to Claim 1, characterized in that, during evaluating the determined change in the parameter value, a boiling point is determined in case that the change in the parameter value is less than a specified amount.
3. Method according to Claim 1 or 2, characterized in that, on completion of the heating-up phase a simmering phase (II) is carried out, comprising the steps:

   - applying the high-frequency rectangular voltage to the resonant circuit with a heating power setpoint,

   - monitoring whether the parameter value changes by more than a specified amount within a monitoring period, and

   - ending the simmering phase when the parameter value changes by more than the specified amount within the monitoring period.
4. Method according to Claim 3, characterized in that

   - the heating power setpoint corresponds to 5% to 50% of a maximum heating power setpoint.
5. Method according to Claim 3 or 4, characterized in that

   - after determining the boiling point, an instantaneous parameter value is stored and

   - on completion of the simmering phase a keep-warm phase (III) is carried out, comprising the steps:

   - controlling of the parameter value to a parameter setpoint which is determined depending on the stored parameter value.
6. Method according to Claim 1 or 2, characterized in that,

   - after determining the boiling point, an instantaneous parameter value is stored and

   - on completion of the heating-up phase a simmering phase is carried out, comprising the steps:

   - controlling of the parameter value to a parameter setpoint which is determined depending on the stored parameter value,

   - monitoring of a heating power to be expended for control purposes and

   - ending the simmering phase when the heating power to be expended is less than a specified amount.
7. Method according to any one of the preceding claims, characterized in that,

   - parameter value of the resonant circuit is a period duration (Tp) of a natural resonant oscillation of the resonant circuit.
8. Induction heating device (9) having

   - a resonant circuit (4) with an induction heating coil (1), and

   - a control device (8) which is designed to carry out the method according to any one of Claims 1 to 7.

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