METHOD FOR OPERATING AN INDUCTION COOKTOP AND INDUCTION COOKTOP

Abstract

In order to operate an induction cooktop with a cooktop plate, at least two induction heating coils thereunder, a cooktop controller, and a power unit for supplying power to the induction heating coils, the two induction heating coils are jointly supplied with power and operated with in each case one power density spectrum with precisely one maximum. In a first operating mode, the power density spectra are measured and it is established how their respective maxima are located relative one another, and the sum thereof is formed. The difference in power density between a local minimum of the sum, which is located between the two maxima of the sum, and the two maxima of the sum is then reduced. To this end, the switch-on time and/or the switch-off time of at least one of the circuit-breakers is varied in order to actively modify the power density spectrum of the power supply.

Claims

1. A method for operating an induction cooktop, wherein said induction cooktop comprises: a cooktop plate, at least two induction heating coils under said cooktop plate, a cooktop controller, a power unit for a power supply for said induction heating coils, wherein said power unit: is triggered by said cooktop controller, has a plurality of circuit-breakers which can be triggered by way of parameters as switch-on time and/or as switch-off time, is configured to generate from a line voltage a higher frequency triggering for said power supply for said induction heating coils, wherein power density spectra of said power supply of said two induction heating coils are estimated or measured, wherein each said power density spectrum has a maximum at a frequency, in a first operating mode, said switch-on time and/or said switch-off time of at least one of said circuit-breakers is varied in order to actively modify said power density spectrum of said power supply in such a manner that said two power density spectra of said power supply for said two induction heating coils overlap more or that a frequency difference between said two maxima is reduced and/or that a resultant sum of said two power density spectra is formed and a difference in power density between a local minimum of said resultant sum and said maxima of said resultant sum is reduced, wherein said local minimum is located between said two maxima of said resultant sum.

2. The method of claim 1, wherein said resultant sum of said two power density spectra is used instead of individual power density spectra.

3. The method of claim 1, wherein a resultant sum of said two different power density spectra of said power supply is formed with a local minimum of said sum, which local minimum is located between said two maxima of said sum, and wherein at least one of said power density spectra of said power supply is actively modified in such a manner that a difference between said local minimum and said maxima amounts to at most 40 dB or less.

4. The method of claim 1, wherein, in said first operating mode, said power density spectra of said power supply are actively modified in such a manner that, as a result, no pronounced local minimum is present between said two maxima of said power density spectra.

5. The method of claim 4, wherein said two maxima of said power density spectra differ by at most 10%.

6. The method of claim 4, wherein said two maxima of said power density spectra are of identical size or are smaller than a difference from said local minimum.

7. The method of claim 1, wherein said two induction heating coils are simultaneously supplied with power and operated with in each case one power density spectrum with precisely one maximum of said power density, wherein said power density spectra for said two induction heating coils are measured or estimated and it is established whether said two power density spectra in each case overlap or how said two power density spectra are each located with regard to a frequency of their respective maximum, wherein, in a first case, in which said two power density spectra are located such that said two maxima are more than 5 kHz apart, said power supply of said induction heating coils is not modified, wherein, in a second case, in which said two power density spectra are located such that said two maxima are no more than 5 kHz apart and wherein, in a resultant sum of said two power density spectra, a pronounced local minimum arises between said two maxima, said power supply of said induction heating coils is modified and a wobble is generated in said power supply of at least one induction heating coil and said parameter switch-on time and/or said parameter switch-off time of at least one of said circuit-breakers is modified such that a sum of said power density spectra with said maxima changes such that said local minimum between said two maxima is increased or a difference of said power density at said local minimum from said power densities of said two maxima becomes smaller.

8. The method of claim 7, wherein said first case is still considered to prevail if said frequency difference between said two maxima amounts to more than 2 kHz to 4 kHz.

9. The method of claim 7, wherein an active modification of said power supply of said induction heating coils in said second case is a modification of said power density spectrum of said induction heating coil operated with said higher frequency triggering from higher frequency components toward lower frequency components such that said local minimum relative to said two maxima of said resultant sum is reduced and/or eliminated.

10. The method of claim 7, wherein a predetermined power setpoint for at least one of said induction heating coils is modified in order to enable overlapping in said second case, if this is otherwise not possible without modifying said instantaneous power by more than 2%.

11. The method of claim 7, wherein said power setpoint of said induction heating coil with said higher power setpoint is modified such that said frequency difference between said two maxima corresponds to said first case.

12. The method of claim 1, wherein said at least two induction heating coils are arranged adjacent one another without a further induction heating coil therebetween, wherein said at least two induction heating coils are of rectangular or polygonal configuration and extend with at least one side or longitudinal side adjacent one another and approximately parallel to one another.

13. The method of claim 1, wherein three induction heating coils which are arranged adjacent one another without further induction heating coils therebetween are operated therewith, wherein said parameters of their circuit-breakers are appropriately varied such that said maxima of said three power density spectra of said power supply of said three induction heating coils are no more than 5 kHz apart with in each case precisely one said local minimum between in each case two said maxima of said three maxima.

14. The method of claim 1, wherein said power density spectra are determined by measuring said voltage of a capacitor connected in parallel to said induction heating coil or by measuring a current through said induction heating coil.

15. An induction cooktop with: a cooktop plate, at least two induction heating coils under said cooktop plate, a cooktop controller, a power unit for a power supply to said induction heating coils which is triggered by said cooktop controller, wherein said power unit has a plurality of circuit-breakers which can be triggered by way of said parameters switch-on time and/or switch-off time, and is connected to a line voltage and is configured to generate from said line voltage a higher frequency triggering for supplying said induction heating coils with power, wherein said power unit and said cooktop controller are configured for carrying out said method of claim 1.

16. The induction cooktop of claim 15, wherein said power unit has an antiresonant circuit with at least one circuit-breaker per said induction heating coil.

17. The induction cooktop of claim 16, wherein said power unit is configured for operation of said at least one circuit-breaker as a quasi-resonant inverter.

18. The induction cooktop of claim 15, wherein said power unit has a rectifier for connection to a line voltage, wherein two identical circuit branches, each of which has an LC member, are connected to said rectifier, wherein an induction heating coil, a resonant circuit capacitor and a circuit-breaker are connected thereto.

19. The induction cooktop of claim 18, wherein said circuit-breaker is a power semiconductor switch.

20. The induction cooktop of claim 15, wherein said circuit branches are in each case isolated from said rectifier by way of an inductor or filter choke.

21. The induction cooktop of claim 20, wherein precisely one dedicated said inductor is provided between said rectifier and each said circuit branch.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Exemplary embodiments of the invention are explained in more detail in the below and are shown schematically in the drawings, in which:

[0030] FIG. 1 shows an induction cooktop according to the invention with a plurality of induction heating coils,

[0031] FIG. 2 shows a power density spectrum 1 of the power supply for an induction heating coil 1 with a maximum at f1,

[0032] FIG. 3 shows a power density spectrum 2 of the power supply for an induction heating coil 2 with a maximum at f2,

[0033] FIG. 4 shows the power density spectra 1 and 2 and the sum of the power density spectra together,

[0034] FIG. 5 shows the curve of the sum of the power density spectra alone,

[0035] FIG. 6 shows a flow chart describing the method according to the invention,

[0036] FIG. 7 shows the curve of the sum of power density spectra 1 and 2′, wherein power density spectrum 2 has been shifted onto power density spectrum 1, and

[0037] FIG. 8 shows the curve of the sum of power density spectra 1 and 2′, wherein power density spectrum 2 has been not entirely shifted onto power density spectrum 1.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

[0038] FIG. 1 shows an induction cooktop 11 according to the invention with a cooktop plate 12 and a plurality of induction heating means 14 thereunder which are configured as conventional induction heating coils. Eight induction heating means 14, all of which are the same size or of identical configuration, are here arranged in a regular pattern. The induction cooktop 11 has a cooktop controller 16, a power supply 18 and an operating means 20 which also has display functions, as is generally known. The power unit 18 is configured as explained above and substantially as known from the prior art, i.e. with circuit-breakers, for example IGBTs. The power unit 18 is of conventional configuration. It may have a plurality of bridge circuits of the above-stated type and is connected to each of the induction heating means 14 to supply them with power.

[0039] A pan T1 is placed on the two left front induction heating means 14, wherein the coverage on the front induction heating means 14 is somewhat greater than on the middle induction heating means. The pan T1 is to be heated jointly by the two heating means 14. An operator has to this end set a power level P1 via the operating means 20. Due to the different coverages, different working frequencies are established despite the similar power setpoint, and unpleasant interference noise may arise between the two heating means 14. This should, if possible, be reduced. It is, however, immaterial for the purposes of the invention whether the frequency differences between the heating means 14 are caused by differences in coverage of one single pan T1 with the same power level P1 or by a plurality of pans T1, T2 with different power levels P1, P2 and/or coverages.

[0040] FIG. 2 shows the power density spectrum 1 of the power supply for the front left induction heating means 14 under pan T1. The power density spectrum 1 has a maximum of 40 dB at frequency f1 = 43 kHz.

[0041] Similarly to FIG. 2, FIG. 3 shows the power density spectrum 2 of the power supply for the middle and also for the rear induction heating means 14. A maximum of approximately 48 dB is here present at a frequency f2 = 46 kHz. Purely with regard to shape, the two power density spectra 1 and 2 of FIGS. 2 and 3 are similar in appearance but are not entirely identical. Furthermore they are also not mirror-symmetrical to the vertical axis at the frequency of the maximum. These curves of FIG. 2 and FIG. 3 are either the envelopes of spectra actually obtained by FFT or alternatively, the stated spectra can also be smoothed.

[0042] For clarity, FIG. 4 once again shows power density spectra 1 and 2 on one diagram together with the curve obtained therefrom of the sum of the power density spectra. To provide a better illustration, FIG. 5 shows only the sum of the power density spectra. It has two relative maxima at f1 and f2 with the frequency difference here amounting to approximately 3 kHz. A local minimum in the sum at fmin of approximately 44 kHz is located between f1 and f2. At approximately -10 dB, it is located approximately 50 dB below the maximum at f1 and approximately 58 dB below the maximum at f2. The two maxima f1 and f2 are located approximately 3 kHz apart.

[0043] According to the flow chart of FIG. 6, the method for operating the induction cooktop 11 starts with the induction heating means 14. Two setpoints of power levels P1 and P2 for heating pans T1 and T2 are input via the operating means 20 as has previously been explained. The power supply 18 and the cooktop controller 16 then ascertain the frequency spectra 1 and 2 of the power densities, i.e. the power density spectra 1 and 2, for the power density at the two operated induction heating means 14. This corresponds respectively to FIGS. 2 and 3.

[0044] In the next step, a difference between the maxima f1 and f2 is ascertained, see FIG. 4. In the present case, this is 3 kHz, and the absolute value of 3 is thus less than the 5 stated above in the condition as an amount of 5 kHz difference between the maxima. If the difference in amount were greater than 5, the two maxima would be so far part that noise generation would in any event be perceived as relatively slight. No intervention would then have to be made, such that the power supply remains the same and in particular the switch-on times and switch-off times and the power density spectra remain unchanged.

[0045] However, as has previously been explained, since the difference in amount of 3 is smaller than a value of 5, the method is continued by determining the entire power density or the sum corresponding to FIG. 5 is determined. It is then verified as the next condition whether there is a local minimum located between the two maxima f1 and f2, i.e. fmin, of more than 40 dB below f1 and/or f2. Such is discernibly the case in FIG. 5, indeed the minimum is located even more than 50 dB therebelow. If this were not the case, there would likewise be no need to modify the power supply. However, since such is the case here, the switch-on times and switch-off times of the circuit-breakers (not shown) in the power unit 18 are actively changed. In so doing, the attempt is made to keep power P1 and power P2 approximately constant or not to modify it too much. The procedure used here was to modify the power density spectrum 2 for the induction heating coils 14 under pan T2, namely the frequencies were reduced or the second maximum f2 was brought together with the first maximum f1 or alternatively shifted to 43 kHz. As a result, there is thus no longer any difference or at least no measurable difference. While the first condition from the flow chart of FIG. 6 is indeed still met, there is then however quite clearly no longer any appreciable local minimum of the combined sum, in particular no difference of at least 40 dB relative to the maximum. Operation can thus continue at this power setpoint which has been modified according to the invention. Noise reduction has been successfully achieved. This is shown in FIG. 7 with the power density spectra 1 and 2′, wherein power density spectrum 2′ has been shifted leftward up to power density spectrum 1. They are thus congruent. In practice, this would indeed be achievable only rarely or with difficulty. It does, however, serve to illustrate the general possibility of shifting a power density spectrum in order to reduce the frequency difference between the local minimum and the two adjacent maxima. In this case, the sum does not any longer even have a local minimum.

[0046] FIG. 8 shows a further possibility for shifting a power density spectrum. In this case, power density spectrum 2 has been shifted by approximately 1.5 kHz leftward. The original power density spectrum 2 is shown as a dashed line, the shifted power density spectrum 2′ is actually adjacent to the left. The sum of power density spectra 1 and 2′ is shown as a continuous line. It is clearly apparent here that there is still a local minimum between the two maxima at frequencies f1 and f2′, but the difference relative to the two maxima is considerably less than in the sum according to FIG. 5. It amounts to approximately 18 dB relative to the maximum of power density spectrum f1 and approximately 23 dB to that of power density spectrum f2′. The two maxima at frequencies f1 and f2′ are located approximately 1.8 kHz apart, i.e. less than 2 kHz. This case of FIG. 8 is considerably more realistic than that of FIG. 7, see the above explanation.

[0047] The focus on the difference between the maxima at f1 and f2 and the determination of the presence and size of the local minimum therebetween are very straightforward to achieve in metrological and computational terms. As a result, the method is advantageous and practical to implement.

[0048] The term spectral power density can generally also be used instead of power density spectrum. As an alternative to evaluating a power density spectrum or a spectral power density, it is accordingly also possible to evaluate a signal spectrum. The signals are namely effective value measurements (root mean square of the measured signal) of voltage at the induction heating coil or of current through the induction heating coil. Where signal spectra are used instead of power density spectra, the stated dB limit values should be halved in accordance with the known logarithmic rule for dB signal versus dB power.

[0049] A power density spectrum represents the distribution of the power components of a signal over frequency and can be ascertained by FFT over a time interval, preferably a periodic time interval. This time interval may be a whole, half or multiple of a line voltage period.