METHOD FOR OPERATING AN INDUCTION HOB AND INDUCTION HOB

Abstract

A method for operating an induction hob (100), wherein the induction hob (100) comprises: an inverter (1), which is supplied with 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 interconnected such that they constitute an oscillating circuit (5), and wherein the inverter (1) is configured to generate a pulse-width modulated excitation voltage (UA) for the oscillating circuit (5) from the supply voltage (US), wherein the method comprises the following steps: a) generation of the pulse-width modulated excitation voltage (UA) having a predefined voltage characteristic, b) measurement of a resulting oscillating circuit current (iS), particularly by means of the induction heating coil (4), c) determination of electrical oscillating circuit parameters, according to the voltage characteristic of the pulse-width modulated excitation voltage (UA) and the measured oscillating circuit current (iS), d) n-times repetition of steps a) to c) using a different voltage characteristic of the excitation voltage (UA) for the determination of electrical voltage characteristic-dependent oscillating circuit parameters, and e) determination of operating variables of the induction hob (100) from voltage characteristic-dependent electrical oscillating circuit parameters.

Claims

1. A method for operating an induction hob (100), wherein the induction hob (100) comprises: an inverter (1), which is supplied with 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 interconnected such that they constitute an oscillating circuit (5), and wherein the inverter (1) is configured to generate a pulse-width modulated excitation voltage (UA) for the oscillating circuit (5) from the supply voltage (US), wherein the method comprises the following steps: a) generation of the pulse-width modulated excitation voltage (UA) having a predefined voltage characteristic, b) measurement of a resulting oscillating circuit current (iS), particularly by means of the induction heating coil (4), c) determination of electrical oscillating circuit parameters, according to the voltage characteristic of the pulse-width modulated excitation voltage (UA) and the measured oscillating circuit current (iS), d) n-times repetition of steps a) to c) using a different voltage characteristic of the excitation voltage (UA) for the determination of electrical voltage characteristic-dependent oscillating circuit parameters, and e) determination of operating variables of the induction hob (100) based on the electrical voltage characteristic-dependent oscillating circuit parameters.

2. The method as claimed in claim 1, wherein for the variation of the voltage characteristic of the excitation voltage (UA), the supply voltage (US) of the inverter (1) is varied.

3. The method as claimed in claim 1, wherein a pulse duty factor of the pulse-width modulated excitation voltage (UA) and/or a period of oscillation of the pulse-width modulated excitation voltage (UA) remain/remains constant during steps a) to e).

4. The method as claimed in claim 1, wherein the operating variables are selected from the following: degree of coverage of the induction heating coil (4) by a cooking vessel (6) which is to be heated, the material of the cooking vessel (6) which covers the induction heating coil (4), the temperature of a base of the cooking vessel (6) which covers the induction heating coil (4).

5. The method as claimed in claim 1, wherein the induction hob (100) further comprises the following: a rectifier (7), which is configured to generate the supply voltage (US) from a mains AC voltage (UN), and an intermediate circuit capacitor (8), which is configured to buffer the supply voltage (US), wherein the method further comprises the following steps: prior to step a), as the magnitude of the mains AC voltage (UN) declines, the intermediate circuit capacitor (8) is progressively discharged to a voltage which lies within a predefined voltage range about the value of the instantaneous mains AC voltage (UN), by actuating the inverter (1) appropriately, until such time as the mains AC voltage (UN) passes through a zero-crossing and/or the supply voltage (US) assumes a value below 10 V, and particularly below 5 V, and the subsequent repetition of steps a) to c), as the value of the mains AC voltage (UN) increases.

6. The method as claimed in claim 1, wherein during steps a) to e), the inverter (1) is actuated in a heating power setting-independent manner, and prior to and/or subsequently to steps a) to e), the inverter (1) is actuated in a heating power setting-dependent manner.

7. The method as claimed in claim 1, wherein in step a), additionally, the first harmonic and/or a higher harmonic of the pulse-width modulated excitation voltage (UA), or of a voltage which is dependent thereupon, is/are determined, in step b), additionally, the first harmonic and/or a higher harmonic of the measured oscillating circuit current (iS) is/are determined, and in step c), oscillating circuit parameters are determined in accordance with the determined first harmonic and/or the determined higher harmonic of the pulse-width modulated excitation voltage (UA), or the voltage which is dependent thereupon, and in accordance with the determined first harmonic and/or the determined higher harmonic of the measured oscillating circuit current (iS).

8. The method as claimed in claim 7, wherein the first harmonics and/or the higher harmonics are determined by means of low-pass filters and/or by means of Fourier analysis.

9. The method as claimed in claim 1, wherein a period of oscillation of the pulse-width modulated excitation voltage (UA) is selected such that it is shorter than a period of oscillation of a self-resonant oscillation of the oscillating circuit (5).

10. The method as claimed in claim 5, wherein the induction hob (100) comprises further induction heating coils, wherein the further induction heating coils are likewise supplied with the rectified mains AC voltage (UN), and wherein, during steps a) to e), in a time interval about the zero-crossing of the mains AC voltage (UN), the further induction heating coils are not supplied with the rectified mains AC voltage (UN).

11. An induction hob (100), which is configured for the execution of the method as claimed in claim 1, comprising: an inverter (1), which is supplied with 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 interconnected such that they constitute an oscillating circuit (5), and wherein the inverter (1) is configured to generate a pulse-width modulated excitation voltage (UA) for the oscillating circuit (5) from the supply voltage (US), and a control unit (9), which is configured to actuate the inverter (1) such that a method as claimed in one of the preceding claims is executed.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0045] The invention is described in detail hereinafter, with reference to the drawings. In the drawings:

[0046] FIG. 1 shows an induction hob, which is configured for the execution of the method according to the invention;

[0047] FIG. 2 shows voltage characteristics and current characteristics over time for the induction hob represented in FIG. 1;

[0048] FIG. 3 shows an oscillating circuit inductance for various conventional proprietary cooking vessel materials, according to the coverage of an induction heating coil measured in the induction hob represented according to FIG. 1, and

[0049] FIG. 4 shows an oscillating circuit inductance for various conventional proprietary cooking vessel materials, according to the current flowing in the induction heating coil, measured in the induction hob represented according to FIG. 1.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

[0050] FIG. 1 shows a schematic representation of a block circuit diagram of an induction hob 100, which is configured for the execution of the method according to the invention.

[0051] The induction hob 100 comprises a conventional inverter 1, which is supplied with a supply voltage US. The inverter 1 comprises two conventional semiconductor switching means 10 and 11, which are looped-in in series between the supply voltage US. At a connecting node point of the two semiconductor switching means 10 and 11, an excitation voltage output UA is delivered.

[0052] The induction hob 100 further comprises two capacitors 2 and 3, which are lopped-in in series between the supply voltage US.

[0053] The induction hob 100 further comprises an induction heating coil 4 (also described as an inductor). The induction heating coil 4 is looped-in between a connecting node point of the two capacitors 2 and 3 and the connecting node point of the two semiconductor switching means 10 and 11. The two capacitors 2, 3 and the induction heating coil 4 constitute a series oscillating circuit 5.

[0054] The inverter 1 is configured to generate the pulse-width modulated excitation voltage UA for the oscillating circuit 5 from the supply voltage US. The inverter 1 is actuated by means of a microprocessor-based control unit 9 having a down-circuit driver unit 12, as described in detail with reference to FIG. 2 hereinafter.

[0055] The induction hob 100 further comprises a rectifier 7, which is configured to generate the supply voltage US from a mains AC voltage UN, for example 230 V/AC and 50 Hz.

[0056] The induction hob 100 further comprises an intermediate circuit capacitor 8, which is configured for the buffering of the supply voltage US.

[0057] FIG. 2 shows voltage characteristics and current characteristics, over time, for the induction hob 100 represented in FIG. 1. US represents the supply voltage, UA represents the excitation voltage, UN represents the mains AC voltage and iS represents the oscillating circuit current.

[0058] Four temporally sequential phases P1, P2, P3 and P4 of the method according to the invention are represented.

[0059] During phase P1, the excitation voltage UA is generated with pulse-width modulation, for the setting of a predefined and, in the present case, low heating capacity. The mains AC voltage UN decreases in a sinusoidal waveform. On the grounds of buffering by means of the intermediate circuit capacitor 8 and the low power output, the supply voltage US which is generated from the mains AC voltage UN by rectification remains above the mains AC voltage UN.

[0060] Phase P1 is followed by phase P2, during which the intermediate circuit capacitor 8, by the appropriate actuation of the inverter 1, is discharged to the magnitude of the instantaneous mains voltage UN.

[0061] In the time interval about the zero-crossing of the mains AC voltage, phase P2 ends and phase P3 commences, i.e. the actual measuring operation, during which the operating variables of the induction hob 100 which are to be established are determined or measured.

[0062] The operating variables comprise a degree of coverage of the induction heating coil 4 by a cooking vessel 6 which is to be heated, a material or material category of a base of the cooking vessel 6 which covers the induction heating coil 4, and a temperature of the base of the cooking vessel 6 which covers the induction heating coil 4.

[0063] To this end, the pulse-width modulated excitation voltage UA with a predefined voltage characteristic is generated, for example wherein one or two periods of the pulse-width modulated excitation voltage UA are generated with a predefined period of oscillation, a predefined pulse duty factor and a voltage difference between the low level and high level of pulse-width modulation which approximately corresponds to the instantaneous supply voltage US. The instantaneous supply voltage US, in turn, approximately corresponds to the instantaneous magnitude of the mains AC voltage UN.

[0064] A resulting oscillating circuit current iS is then measured by the induction heating coil 4, wherein electrical oscillating circuit parameters are calculated according to the instantaneous value of the supply voltage US, i.e. the instantaneous voltage characteristic of the excitation voltage US, and the measured oscillating circuit current iS.

[0065] As the magnitude of the mains AC voltage UN increases, the supply voltage US rises correspondingly such that, for the corresponding periods of pulse-width modulation, the voltage difference between the low level and the high level increases correspondingly, i.e. the voltage characteristic of the pulse-width modulated excitation voltage UA varies accordingly. A pulse duty factor of the pulse-width modulated excitation voltage UA and a period of oscillation of the pulse-width modulated excitation voltage UA remain constant.

[0066] For a number n of temporally sequential voltage characteristics of the pulse-width modulated excitation voltage UA, the resulting oscillating circuit currents iS are measured and, for each voltage characteristic of the n different voltage characteristics, the associated electrical voltage characteristic-dependent oscillating circuit parameters are determined or calculated, such that n voltage characteristic-dependent oscillating circuit parameters are determined. In other words, n different oscillating circuit parameters are determined for n different voltage characteristics.

[0067] Finally, operating variables of the induction hob 100 are determined from at least two oscillating circuit parameters of the n different voltage characteristic-dependent electrical oscillating circuit parameters.

[0068] For the calculation of a respective voltage characteristic-dependent electrical oscillating circuit parameter, the first harmonic of the respective pulse-width modulated excitation voltage UA, or a voltage which is dependent thereupon, can be determined, the first harmonics of the respectively measured oscillating circuit current iS are determined, and the respective electrical oscillating circuit parameter then determined in accordance with the first harmonic of the pulse-width modulated excitation voltage thus determined, or the voltage which is dependent thereupon, and the first harmonic of the measured oscillating circuit current. The first harmonics can be determined, for example, by means of low-pass filters and/or by Fourier analysis.

[0069] During phase P3, the inverter 1 is actuated in a heating power setting-independent manner, wherein a frequency of pulse-width modulation is preferably higher than a natural resonant frequency of the oscillating circuit 5.

[0070] Phase P3 spans a voltage magnitude range of the mains AC voltage UN from approximately 10 V to 50 V.

[0071] Phase P3 is followed by phase P4, during which the inverter 1 is again actuated in a heating power setting-dependent manner.

[0072] The induction hob 100 can comprise further induction heating coils, wherein the further induction heating coils are likewise supplied with the rectified mains AC voltage UN, and wherein the further induction heating coils, during phase P3, are not supplied with the rectified mains AC voltage UN, in order to prevent any crosstalk.

[0073] FIG. 3 shows an exemplary representation of a variable oscillating circuit inductance or a variable inductance of an induction heating coil, according to the coverage thereof by an item of cookware. Not only the inductance, but also the variation in inductance in relation to coverage are different, according to the various materials employed for the base of cookware.

[0074] Conventional proprietary materials for the base of cookware which is suitable for induction heating include, for example, ferritic special steel or steel. A particular case is constituted by cookware which is comprised of an aluminum body having a press-fit ferritic special steel base, for the purposes of induction heating. According to the invention, a distinction can also be drawn between single-layered ferritic special steel and multi-layered special steel laminates.

[0075] FIG. 4 represents the non-linear inductance characteristic of conventional proprietary cookware, according to the current flowing in the induction heating coil. The magnetic field strength of the induction heating coil is proportional to the current flowing in the induction heating coil, and constitutes the magnetic modulation of the cooking vessel material.

[0076] At low currents, the inductance rises in conjunction with the increasing permeability of ferritic materials, until increasing regions of the cooking vessel base achieve a state of ferritic saturation, and the inductance decays, to a varying degree, as modulation increases.

[0077] According to the invention, at least two working points of modulation are measured and considered in relation to one another, such that an additional measuring variable can then be employed for the determination of operating variables.