EP3920661A1 - Induction hob and method for operating an induction hob - Google Patents

Abstract

- wherein the induction hob (100) has: - an inverter (1) which is supplied from a supply voltage (US), - at least one capacitor (2, 3), and - an induction heating coil (4), - the at least one capacitor (2, 3) and the induction heating coil (4) are connected in such a way that they form an oscillating circuit (5), and the inverter (1) is designed to convert the supply voltage (US) into a pulse-width-modulated excitation voltage (UA) for the The oscillating circuit (5), the method comprising the steps of: a) generating the pulse-width-modulated excitation voltage (UA) with a predetermined voltage curve, b) measuring a resulting oscillating circuit current (iS), in particular through the induction heating coil (4), c) Determination of electrical resonant circuit parameters as a function of the voltage curve of the pulse-width modulated excitation voltage (UA) and the measured resonant circuit current (iS), d) repeating steps a) to c) n times at a m changed voltage curve of the excitation voltage (UA) to determine electrical voltage curve-dependent oscillating circuit parameters, and e) determining operating variables of the induction hob (100) from the voltage curve-dependent electrical oscillating circuit parameters.

Description

The invention relates to a method for operating an induction hob and an induction hob.

The invention is based on the object of providing a method for operating an induction hob and an induction hob which enable the most reliable possible determination of operating parameters of the induction hob.

The method is used to operate an induction hob.

The induction hob has at least one conventional inverter that is supplied from a supply voltage. The supply voltage is preferably a direct voltage. The inverter can, for example, have a conventionally connected inverter branch with two semiconductor switching means. In this respect, reference is also made to the relevant specialist literature.

The induction hob also has at least one capacitor.

The induction hob also has at least one induction heating coil or an inductor, which is assigned to a hob and is provided to generate an alternating magnetic field in a pot base to be heated. In this respect, reference is also made to the relevant specialist literature.

The at least one capacitor and the induction heating coil are connected in such a way that they form an oscillating circuit, for example a parallel or series oscillating circuit.

The inverter is provided to generate a pulse-width-modulated excitation voltage for the resonant circuit from the supply voltage. The pulse-width-modulated excitation voltage is typically a square-wave voltage with a constant or variable duty cycle or duty cycle and a constant or variable period or frequency. In this respect, reference is also made to the relevant specialist literature.

The procedure has the following steps.

Step a), namely generating the pulse-width-modulated excitation voltage with a predetermined voltage curve.

Step b), namely measuring a resulting or setting resonant circuit current, in particular through the induction heating coil.

Step c), namely determining electrical resonant circuit parameters, in particular in the form of a resonant circuit impedance, as a function of the voltage profile of the pulse-width modulated excitation voltage and the measured resonant circuit current. The electrical resonant circuit parameters or the resonant circuit impedance can denote, for example, the electrical equivalent parameters R and L of the induction heating coil with the pot in place, or electrical impedances or quantities of the oscillation differential equation that can be derived therefrom, such as quality or damping or natural frequency. For example, the well-known phasor calculation can be used to determine the electrical oscillating circuit parameters, i.e. the amount and phase of the voltage and the amount and phase of the current are related to one another.

Step d), namely repeating steps a) to c) n times with a changed voltage curve of the excitation voltage to determine the voltage curve-dependent resonant circuit parameters. To change the voltage profile of the excitation voltage, only a voltage difference between a low level of the pulse-width-modulated excitation voltage and a high level of the pulse-width-modulated excitation voltage is preferably changed. A frequency and a pulse duty factor of the pulse-width-modulated excitation voltage preferably remain unchanged. The number n is a natural number and lies, for example, in a number range between 1 and 400. The number n can depend, for example, on a period of the pulse width modulation or the number n can be selected such that the steps are repeated for the duration of an entire network half-wave.

Step e), namely determining (measuring) operating variables of the induction hob from the voltage-dependent oscillating circuit parameters.

According to one embodiment, to change the voltage curve of the excitation voltage, in particular exclusively, the supply voltage of the inverter is changed, whereby, for example, the voltage difference between the high level and the low level of the pulse-width modulated excitation voltage is changed accordingly.

According to one embodiment, a duty cycle of the pulse-width-modulated excitation voltage and / or a period duration of the pulse-width-modulated excitation voltage remains constant during steps a) to e).

According to one embodiment, the operating variables to be determined are selected from: Degree of coverage of the induction heating coil by a cooking vessel to be heated, in particular with a ferromagnetic base, material of the cooking vessel covering the induction heating coil or the material of the base of the cooking vessel, and temperature of the base of the cooking vessel covering the induction heating coil Cooking vessel. The degree of coverage can depend, for example, on whether the cooking vessel completely covers, partially covers or does not cover the induction heating coil.

According to one embodiment, the induction hob also has: a rectifier, which is designed to generate the supply voltage from an AC mains voltage, and an intermediate circuit capacitor, which is designed to buffer the supply voltage and filter the inverter feedback. The method then has the following steps: before step a), with a decreasing amount of the mains AC voltage, ie decreasing half-cycle, continuous discharging of the intermediate circuit capacitor down to a voltage value that is in a predetermined voltage range around the amount of the current mains AC voltage in which the inverter is controlled appropriately. The specified voltage range can, for example, be a few volts, for example between 3 V to 10 V above the amount of the instantaneous AC mains voltage. This is carried out until the mains alternating voltage has a zero crossing and / or the supply voltage has a value below 10V, in particular below 5V. Then, subsequent repetition of steps a) to c) with an increasing amount of the mains AC voltage. Steps a) to c) can be repeated, for example, in a voltage range of the mains alternating voltage between approx. 5 V and 80 V, in particular between 10 V and 50 V.

According to one embodiment, the inverter is controlled independently of the heating output setting during steps a) to e) and is controlled depending on the heating output setting before and / or after steps a) to e) by setting, for example, a pulse duty factor of the pulse width modulation and / or a period of the pulse width modulation of the excitation voltage according to the heating output will.

According to one embodiment, the first harmonic and / or higher harmonic of the pulse-width-modulated excitation voltage or a voltage dependent thereon is additionally determined in step a), and additionally the first harmonic and / or higher harmonic in step b) of the measured resonant circuit current is determined, and in step c) the resonant circuit parameters are determined depending on the determined first harmonic and / or the determined higher harmonics of the pulse-width modulated excitation voltage or the voltage dependent thereon and the determined first harmonic and / or the determined higher harmonics of the measured resonant circuit current determined.

According to one embodiment, the first harmonics and / or the higher harmonics are determined by means of low-pass filters and / or by Fourier analysis.

If necessary, the higher harmonic voltages and currents can also be determined by Fourier analysis and the corresponding higher harmonic impedances calculated.

According to one embodiment, a period of the pulse-width-modulated excitation voltage is selected such that it is shorter than a period of a self-resonant oscillation of the resonant circuit. In other words, the frequency of the pulse-width-modulated excitation voltage is higher than the resonance frequency of the oscillating circuit for most of the cookware available on the market. If the period of the selected excitation voltage is too close to natural resonance, the period can be shortened in order to limit the resonant circuit current to a relevant level.

According to one embodiment, the induction hob has further induction heating coils, the further induction heating coils also being fed from the rectified AC mains voltage with the interposition of associated inverters, wherein during steps a) to e) in a time range around the zero crossing of the mains AC voltage, the further induction heating coils are not removed from the rectified AC mains voltage. The time range around the zero crossing can begin, for example, 1 ms before the zero crossing and end 2 ms after the zero crossing.

The induction hob is designed to carry out the method described above and has: at least one inverter which is supplied from a supply voltage, at least one capacitor, an induction heating coil, the at least one capacitor and the induction heating coil being connected in such a way that they form an oscillating circuit, and wherein the inverter is designed to generate a pulse-width-modulated excitation voltage for the resonant circuit from the supply voltage, and a control unit that is designed to control the inverter in such a way that a method described above is carried out.

Conventionally, when a pot is recognized, it is checked whether a suitable pot is placed on a hotplate or not. This means that it is checked whether there is no pot, an unsuitable pot or one that is too small on the induction heating coil.

In the case of surface hobs or to recognize pot size, the coverage of the cooking area is also required as an operating variable, preferably continuously resolved from 0% to 100% coverage of the active surface of the induction heating coil. To determine the coverage, for example, the impedance or the electrical (substitute) parameters R and L of the induction heating coil can be measured with the pot in place, since these change greatly when the coverage changes.

However, these substitute parameters are - in addition to the dependency on the coverage of the hotplate - also dependent on the pot materials used and on the pot base temperature. These parameters are further dependent on the magnetic excitation or the current through the inductor and the excitation frequency, which is why the measurement is conventionally preferably carried out with constant excitation.

According to the invention, the resonant circuit parameters are determined in the case of excitations of different magnitudes, that is to say, for example, in the case of different supply voltages of a half bridge of a series resonant circuit or in the case of different current levels in a parallel resonant circuit.

The resonance circuit parameters can preferably be measured before or after the mains AC voltage has passed through the mains, whereby the rising or falling mains ac voltage can be used for variable excitation and the voltage curve-dependent or input-voltage-dependent difference in the oscillating circuit parameters can be used as additional information for pot detection , which makes it easier to differentiate between pot materials. In particular, in the case of excitations or supply voltage of less than 80 V, different changes in the resonant circuit parameters can be seen depending on the pot material. Above this voltage, the resonant circuit parameters can be switched from the measuring mode to the power output mode, so that the measurement can take place during operation and the power output does not have to be significantly interrupted for the measurement. The voltage limit for switching to heating mode can be set variably, depending on the current through the induction heating element.

With conventional methods, it is only possible to differentiate to a limited extent whether the values of the impedance or the resonant circuit parameters are due to (insufficient) coverage or due to a (lower resistance) conductivity of the pot material is caused, ie some measurement cases are ambiguous, especially since the temperature of the pot that is set up also changes.

fop

bezeichne die Arbeitsfrequenz des Wechselrichters

u(t)

bezeichne den zeitlichen Verlauf der Ansteuerspannung

i(t)

bezeichne den zeitlichen Verlauf des Stroms durch die Induktionsheizspule

According to the invention, the impedance or the resonant circuit parameters are determined with the (at least partially) set up pot for an extended pot detection, in that the electrical resonant circuit consisting of induction heating coil and at least one resonant circuit capacitor is excited in a defined manner and electrical resonant circuit parameters such as impedance Z, effective resistance R , Reactance X, the resonant circuit inductance L or the phase angle between R and X is determined from the measurement of the current through the induction heating coil and the excitation voltage. The basic calculation formulas are given below as an example:

fop

denote the working frequency of the inverter

u (t)

denote the timing of the control voltage

i (t)

denote the time course of the current through the induction heating coil

$$u_1=\widehat{u_1}\cdot \sin \left({\omega }_{\mathit{op}}\cdot t\right)$$

ist die erste Harmonische der Spannung $$I_1=\widehat{I_1}\cdot \sin \left({\omega }_{\mathit{op}}\cdot t+\phi \right)$$

ist die erste Harmonische des Stroms mit $\phi =\sphericalangle \left(u_1,I_1\right)$

It applies $${\omega }_{\mathit{op}}=2\cdot \pi \cdot {\mathrm{<figure-callout\; id="1"\; label="f\; op\; u"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">f</figure-callout>}}_{\mathit{op}}$$

$$u_{}$$$${}_1=\hat{{\mathrm{<figure-callout\; id="1"\; label="u"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">u</figure-callout>}}_1}\cdot \sin \left({\omega }_{\mathit{op}}\cdot t\right)$$

is the first harmonic of tension $${\mathrm{I.}}_1=\widehat{{\mathrm{I.}}_1}\cdot \sin \left({\omega }_{\mathit{op}}\cdot t+\phi \right)$$

is the first harmonic of the current with $\phi =\sphericalangle \left({\mathrm{<figure-callout\; id="1"\; label="u"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">u</figure-callout>}}_1,{\mathrm{I.}}_1\right)$

$$X_1=\frac{\widehat{u_1}}{\widehat{I_1}}\cdot \mathit{sin\; \phi }=X_{L1}+X_{C1}$$

mit $$X_C=-\frac{1}{{\omega }_{\mathit{op}}\cdot C}$$

$$L_1=\frac{X_1-X_{C1}}{{\omega }_{\mathit{op}}}$$

The following applies: $${\mathrm{R.}}_1=\frac{\hat{{\mathrm{<figure-callout\; id="1"\; label="u"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">u</figure-callout>}}_1}}{\widehat{{\mathrm{I.}}_1}}\cdot \mathit{cos\; \phi }$$

$$X_1=\frac{\hat{{\mathrm{<figure-callout\; id="1"\; label="u"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">u</figure-callout>}}_1}}{\widehat{{\mathrm{I.}}_1}}\cdot \mathit{sin\; \phi }=X_{\mathrm{L.}1}+X_{\mathrm{C.}1}$$

With $$X_{\mathrm{C.}}=-\frac{1}{{\omega }_{\mathit{op}}\cdot \mathrm{C.}}$$

$${\mathrm{L.}}_1=\frac{X_1-X_{\mathrm{C.}1}}{{\omega }_{\mathit{op}}}$$

The inverter's stimulating output voltage is preferably measured, but the voltage across the induction heating coil can also be measured directly. Resonant circuit parameters such as quality Q, damping δ, natural frequency fr or period duration Tr can also be determined in order to be able to determine the coverage of the pot on the inductor.

According to the invention, the input variable voltage (or in general current through the induction heating coil) is changed so as to provide at least one additional variable for determining the established load (pot detection), for example the delta of one or more resonant circuit parameters related to a delta of the excitation voltage or the current. The position of a maximum of a resonant circuit parameter in relation to the variable excitation voltage can also be used as a criterion. Alternatively, instead of using the variable excitation voltage, the resonant circuit parameters can also be evaluated as being variable using the current through the induction heating coil.

The changes in the resonant circuit parameters caused by the voltage change provide additional variables that allow an improved differentiation between pot material classes despite the variable pot coverage on the induction heating coil.

The provision of a variable supply voltage or excitation voltage can be complex, which is why, according to the invention, the AC mains voltage can be used as a variable voltage, i.e. the measurement of the resonant circuit parameters can take place in a time interval after or before a zero crossing of the AC mains voltage. The measurement is preferably carried out after the grid zero crossing, whereby it should be ensured that the intermediate circuit is safely discharged beforehand by reducing the impedance to a minimum value by selecting the control parameters for the inverter accordingly (=> control frequency a few kHz above the resonance frequency of the resonant circuit and / or the duty cycle close to the maximum value of 50%).

The first harmonic of the excitation voltage and current is preferably used to calculate the resonant circuit parameters. For this purpose, a low-pass filter can be provided; alternatively, a corresponding Fourier analysis of the measurement data can be carried out using an algorithm.

The measurement is started, for example, shortly after the grid zero crossing, for example at 8 V AC line voltage and ended again at 40-50 V, and the inverter is switched to conventional heating power mode. This voltage range for pot detection makes it possible to distinguish differences resulting from different permeability curves of pot materials, but on the other hand also to switch to heating mode in good time, so that even large powers can be transmitted and the specification of the limitation of the harmonic of the mains current can be adhered to.

Once a pot classification has been made, the voltage curve-dependent determination or the delta determination of the operating parameters can be dispensed with, and even for smaller ones Voltages can be switched to heating mode. In this operating case, only a change in the coverage needs to be determined.

The inventive approach of implementing pot detection with variable excitation voltage makes it possible to generate additional resonant circuit parameters or measured variables for pot detection (with coverage measurement), which are caused in particular by different permeability characteristics of different pot materials, whereby ambiguities can be made distinguishable. The voltage curve-dependent analysis or the delta analysis allows a better differentiation of the pot materials, which then enables a pot material-specific determination of the coverage of the induction heating coil in a second step.

The use of the increasing AC mains voltage after a zero crossing allows a cost-effective implementation of a variable supply voltage of the inverter and avoids the need for power supplies with different output voltages for different excitation of the pot detection.

Switching over the inverter to measure the relevant operating parameters with only small, increasing voltages up to a maximum of 80 V allows regular pot detection to be carried out during ongoing heating operation, whereby the requirement for EMC limitation of the harmonics of the mains current can also be met.

In a first step, for example, a pot class can be determined and in a subsequent step a degree of coverage can then be determined specifically for the pot material of this pot class.

Fig. 1

ein Induktionskochfeld, das zur Durchführung des erfindungsgemäßen Verfahrens ausgebildet ist,

Fig. 2

Spannungsverläufe und Stromverläufe des in Fig. 1 gezeigten Induktionskochfelds über der Zeit,

Fig. 3

eine Schwingkreisinduktivität mit verschiedenen, marktüblichen Topfmaterialien abhängig von der Bedeckung einer Induktionsheizspule gemessen mit dem in Fig. 1 gezeigten Induktionskochfeld, und

Fig. 4

eine Schwingkreisinduktivität mit verschiedenen, marktüblichen Topfmaterialien abhängig vom Strom durch die Induktionsheizspule gemessen mit dem in Fig. 1 gezeigten Induktionskochfeld.

The invention is described in detail below with reference to the drawings. Here shows:

Fig. 1

an induction hob which is designed to carry out the method according to the invention,

Fig. 2

Voltage curves and current curves of the in Fig. 1 induction hob shown over time,

Fig. 3

a resonant circuit inductance with different, commercially available pot materials depending on the coverage of an induction heating coil measured with the in Fig. 1 induction hob shown, and

Fig. 4

a resonant circuit inductance with different, commercially available pot materials depending on the current through the induction heating coil measured with the in Fig. 1 induction hob shown.

Fig. 1 schematically shows a block diagram of an induction hob 100 which is designed to carry out the method according to the invention.

The induction hob 100 has a conventional inverter 1, which is supplied from a supply voltage US. The inverter 1 has two conventional semiconductor switching means 10 and 11 which are looped in series between the supply voltage US. An excitation voltage UA is output at a connection node of the two semiconductor switching means 10 and 11.

The induction hob 100 also has two capacitors 2 and 3, which are looped in series between the supply voltage US.

The induction hob 100 also has an induction heating coil 4 (also referred to as an inductor). The induction heating coil 4 is looped in between a connection node of the two capacitors 2 and 3 and the connection node of the two semiconductor switching means 10 and 11. The two capacitors 2, 3 and the induction heating coil 4 form a series oscillating circuit 5.

The inverter 1 is designed to generate the pulse-width-modulated excitation voltage UA for the resonant circuit 5 from the supply voltage US. The inverter 1 is controlled by a microprocessor-based control unit 9 with a downstream driver unit 12, which is shown with reference to FIG Fig. 2 will be described in detail below.

The induction hob 100 also has a rectifier 7, which is designed to generate the supply voltage US from an AC mains voltage UN, for example with 230 V / AC and 50 Hz.

The induction hob 100 also has an intermediate circuit capacitor 8 which is designed to buffer the supply voltage US.

Fig. 2 shows voltage and current curves of the in Fig. 1 Induction hob 100 shown over time. US denotes the supply voltage, UA denotes the excitation voltage, UN denotes the AC mains voltage and iS denotes the oscillating circuit current.

4 phases P1, P2, P3 and P4 of the method according to the invention are shown in succession.

During phase P1, excitation voltage UA is generated in a pulse-width-modulated manner in such a way that a predetermined, presently low, heating output is established. The AC mains voltage UN decreases sinusoidally. The supply voltage US generated from the AC line voltage UN by means of rectification remains above the AC line voltage UN due to the buffering by means of the intermediate circuit capacitor 8 and the low power output.

Phase P1 is followed by phase P2, during which the intermediate circuit capacitor 8 is discharged by suitable control of the inverter 1 to the amount of the current line voltage UN.

In the time range around the zero crossing of the AC mains voltage UN, the phase P2 ends and the phase P3 begins, i.e. the actual measuring operation, during which the operating parameters of the induction hob 100 to be determined are determined or measured.

The operating variables are a degree of coverage of the induction heating coil 4 by a cooking vessel 6 to be heated, a material or a material class of a pot bottom of the cooking vessel 6 covering the induction heating coil 4 and a temperature of the bottom of the cooking vessel 6 covering the induction heating coil 4.

For this purpose, the pulse-width-modulated excitation voltage UA is generated with a predetermined voltage curve, for example by generating one or two periods of the pulse-width-modulated excitation voltage UA with a predetermined period, a predetermined pulse duty factor and a voltage difference between the low level and high level of the pulse width modulation, which is approximately the corresponds to the current supply voltage US. The instantaneous supply voltage US in turn corresponds approximately to the instantaneous amount of the mains alternating voltage UN.

A resulting resonant circuit current iS is then measured by the induction heating coil 4, the electrical resonant circuit parameters being calculated as a function of the instantaneous value of the supply voltage US, i.e. the instantaneous voltage curve of the excitation voltage US, and the measured resonant circuit current iS.

As the amount of the AC mains voltage UN increases, the supply voltage US increases accordingly, so that the voltage difference between the low level and high level increases accordingly for corresponding periods of the pulse width modulation, i.e. the voltage curve of the pulse width modulated excitation voltage UA changes accordingly. A duty cycle of the pulse-width-modulated excitation voltage UA and a period duration of the pulse-width-modulated excitation voltage UA remain constant.

The resulting resonant circuit currents iS are measured for a number n of temporally successive voltage profiles of the pulse-width modulated excitation voltage UA and the associated electrical voltage profile-dependent resonant circuit parameters are determined or calculated for each voltage profile of the n different voltage profiles, so that n voltage profile-dependent resonant circuit parameters are determined. In other words, n different resonant circuit parameters are determined with n different voltage curves.

Finally, the operating variables of the induction hob 100 are determined from at least two resonant circuit parameters of the n different electrical resonant circuit parameters that are dependent on the voltage curve.

To calculate a respective voltage curve-dependent electrical oscillating circuit parameter, the first harmonic of the respective pulse-width-modulated excitation voltage UA or a voltage dependent on it can be determined, the first harmonic of the respective measured oscillating circuit current iS can be determined, and the respective electrical oscillating circuit parameter can then be determined as a function of the The first harmonic of the pulse-width-modulated excitation voltage or the voltage dependent thereon and the first harmonic of the measured resonant circuit current can be determined. The first harmonics can be determined, for example, by means of low-pass filters and / or by Fourier analysis.

During phase P3, the inverter 1 is activated independently of the heating power setting, a frequency of the pulse width modulation preferably being higher than a natural resonance frequency of the oscillating circuit 5.

The phase P3 extends over a voltage range of the mains alternating voltage UN between approx. 10 V to 50 V.

Phase P3 is followed by phase P4, during which the inverter 1 is again controlled depending on the heating output setting.

The induction hob 100 can have further induction heating coils, the further induction heating coils also being fed from the rectified AC mains voltage UN, the further induction heating coils not being fed from the rectified AC mains voltage UN during phase P3 in order to avoid crosstalk.

Fig. 3 shows, by way of example, a changing resonant circuit inductance or a changing inductance of an induction heating coil depending on the coverage by cooking utensils. The inductance but also the change in the inductance over the covering are different for different materials of the bottom of the cookware.

Commercially available materials for the base plate of induction-compatible cookware are, for example, ferritic stainless steel or steel. A special feature are cookware consisting of an aluminum body in which a ferritic stainless steel is pressed into the base for heating on an induction. According to the invention, multi-layer stainless steel laminations can also be distinguished from single-layer, ferritic stainless steel.

Fig. 4 shows the non-linear inductance curve of commercially available cookware depending on the current through the induction heating coil. The magnetic field strength of the induction heating coil is proportional to the current through the induction heating coil and forms the magnetic control of the pot material.

The inductance increases with small currents due to the increasing permeability of ferritic materials until increasing areas of the pot bottom reach ferritic saturation and the inductance decreases more or less strongly with increasing modulation.

According to the invention, at least two working points of the modulation are measured and set in relation to one another in order to be able to use an additional measured variable to determine the operating variables.

Claims (11)

Method for operating an induction hob (100), - wherein the induction hob (100) has: - an inverter (1) which is supplied from a supply voltage (US), - At least one capacitor (2, 3), and - an induction heating coil (4), - wherein the at least one capacitor (2, 3) and the induction heating coil (4) are connected in such a way that they form an oscillating circuit (5), and - The inverter (1) being designed to generate a pulse-width-modulated excitation voltage (UA) for the resonant circuit (5) from the supply voltage (US), - wherein the method comprises the steps: a) Generation of the pulse-width-modulated excitation voltage (UA) with a specified voltage curve, b) measuring a resulting resonant circuit current (iS), in particular through the induction heating coil (4), c) Determination of electrical resonant circuit parameters as a function of the voltage curve of the pulse-width modulated excitation voltage (UA) and the measured resonant circuit current (iS), d) repeating steps a) to c) n times with a changed voltage curve of the excitation voltage (UA) to determine electrical voltage curve-dependent resonant circuit parameters, and e) Determining operating variables of the induction hob (100) from the electrical voltage curve-dependent resonant circuit parameters. Method according to claim 1, characterized in that - to change the voltage curve of the excitation voltage (UA) the supply voltage (US) of the inverter (1) is changed. Method according to claim 1 or 2, characterized in that - A duty cycle of the pulse-width-modulated excitation voltage (UA) and / or a period duration of the pulse-width-modulated excitation voltage (UA) remains constant during steps a) to e). Method according to one of the preceding claims, characterized in that - the company sizes are selected from: - Degree of coverage of the induction heating coil (4) by a cooking vessel (6) to be heated, - Material of the cooking vessel (6) covering the induction heating coil (4), - Temperature of a base of the induction heating coil (4) covering the cooking vessel (6). Method according to one of the preceding claims, characterized in that - The induction hob (100) also has: - A rectifier (7) which is designed to generate the supply voltage (US) from an AC mains voltage (UN), and - an intermediate circuit capacitor (8) which is designed to buffer the supply voltage (US), - The method having the further steps: - Before step a), with a decreasing amount of the mains alternating voltage (UN), continuous discharging of the intermediate circuit capacitor (8) to a voltage value which is in a predetermined voltage range around the amount of the current mains alternating voltage (UN) by the inverter (1) being suitable is controlled until the mains alternating voltage (UN) has a zero crossing and / or the supply voltage (US) has a value below 10V, in particular below 5V, and - Subsequent repetition of steps a) to c) with an increasing amount of the mains alternating voltage (UN). Method according to one of the preceding claims, characterized in that - During steps a) to e) the inverter (1) is controlled independently of the heating output setting and before and / or after steps a) to e) the inverter (1) is controlled depending on the heating output setting. Method according to one of the preceding claims, characterized in that - In step a) the first harmonic and / or higher harmonic of the pulse-width-modulated excitation voltage (UA) or a voltage dependent thereon is / are determined, - In step b) the first harmonic and / or higher harmonic of the measured resonant circuit current (iS) is / are determined, and - In step c) the resonant circuit parameters as a function of the determined first harmonic and / or the determined higher harmonics of the pulse-width modulated excitation voltage (UA) or the voltage dependent thereon and the determined first Harmonics and / or the determined higher harmonics of the measured resonant circuit current (iS) is / are determined. Method according to claim 7, characterized in that the first harmonics and / or the higher harmonics are determined by means of low-pass filters and / or by Fourier analysis. Method according to one of the preceding claims, characterized in that - A period of the pulse-width-modulated excitation voltage (UA) is selected such that it is shorter than a period of a self-resonant oscillation of the resonant circuit (5). The method according to any one of claims 5 to 9, characterized in that - The induction hob (100) has further induction heating coils, the further induction heating coils also being fed from the rectified mains alternating voltage (UN), the further induction heating coils not during steps a) to e) in a time range around the zero crossing of the mains alternating voltage (UN) fed from the rectified AC mains voltage (UN). Induction hob (100) which is designed to carry out the method according to one of the preceding claims, comprising: - an inverter (1) which is supplied from a supply voltage (US), - at least one capacitor (2, 3), - an induction heating coil (4), - wherein the at least one capacitor (2, 3) and the induction heating coil (4) are connected in such a way that they form an oscillating circuit (5), and - The inverter (1) being designed to generate a pulse-width-modulated excitation voltage (UA) for the resonant circuit (5) from the supply voltage (US), and - A control unit (9) which is designed to control the inverter (1) in such a way that a method according to one of the preceding claims is carried out.

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