EP1878309B1 - Method and arrangement for supplying power to several induction coils in an induction apparatus - Google Patents

Abstract

The aim of the invention is to operate induction coils (L1, L2) of an induction cooking hob (11) with as little noise as possible. Said aim is achieved by operating the induction coils (L1, L2) at the same frequency fg (mode of operation a) or at a frequency difference ?f of about 18 kHz (mode of operation c), especially during alternating operation. Changing back and forth between said modes of operation makes it possible to reach predefined values (P1, P2) for the power of the induction coils (L1, L2) at the mean-time value while disturbing noise is prevented from developing.

Description

Field of application and state of the art

The invention relates to a method for supplying power to a plurality of induction coils in an induction device and to an arrangement for carrying out this method.

For induction hobs, there is often the problem that there may be audible noise when operating multiple burners. These are sometimes perceived by an operator as unpleasant, in particular not only because of the noise itself, but also because the assumption is obvious that there is a malfunction of the induction hob. The perception of noise, in turn, is also dependent on the intensity of the sound level as well as the agreement with a human hearing curve, thus depending on the frequency of the sounds.

There are several causes for these noises. On the one hand, ferrites for magnetic field guidance are provided under induction coils. These ferrites are subject to magnetostriction, ie a change in length, depending on the operating frequency of the induction coil. This is at least partially true for used cookware. Although the working frequency of induction coils is usually above the audible range, the noise can be heard through intermodulation with another powered induction coil. In this case, audible mixing products can arise from the frequency difference of the operating frequencies and their harmonic harmonics. Further intermodulation can occur when two frequency converters for the induction coils are connected to a common supply voltage are. In this case, one frequency converter modulates the supply voltage for the other frequency converter.

The ES-A1-2201937 describes a method for powering a plurality of induction coils in an induction device, wherein in each case a frequency converter supplies power to each induction coil. When several induction coils are operated together, cut-off frequencies of 2 kHz and 14 kHz are used, which are set as a frequency difference if the outputs are to differ. With the upper cutoff frequency, an operation of the induction coil can be achieved, which is at the upper end or even outside the human hearing threshold.

The US-A-4542273 describes a similar method for powering a plurality of induction coils in an induction device. In turn, each induction coil has its own power supply. If two or more resonant circuits of the induction coils are operated, they operate at the same frequency or a frequency difference is equal to zero. This avoids unwanted noise.

Task and solution

The invention has for its object to provide an aforementioned method and an arrangement with which the problems of the prior art can be avoided and in particular an advantageous operation of multiple induction coils is possible with the lowest possible noise.

This object is achieved by a method having the features of claim 1 and an arrangement having the features of claim 7. Advantageous and preferred embodiments of the invention are the subject of the other claims and will be described in more detail below explained. The wording of the claims is incorporated herein by express reference.

1. a) Die Frequenzdifferenz beträgt nahezu Null, vorteilhaft ist sie Null.
2. b) Die Frequenzdifferenz beträgt weniger als 1 kHz, ist also vorteilhaft zwar vorhanden, aber relativ gering.
3. c) Die Frequenzdifferenz beträgt zwischen 15 kHz und 25 kHz bzw.
   liegt nicht mehr im hörbaren Bereich.

Each induction coil is powered by its own frequency converter or drive unit. According to the invention, the operating frequencies or the frequencies of the respective frequency converter for the individual induction coils depending on a planned performance or by an operator by means of input for the performance required values with respect to a difference between the frequencies, ie a frequency difference, after the operation of multiple induction coils one of the following possible modes of operation:

1. a) The frequency difference is almost zero, advantageously it is zero.
2. b) The frequency difference is less than 1 kHz, so it is advantageous, but relatively low.
3. c) The frequency difference is between 15 kHz and 25 kHz or
   is no longer in the audible range.

According to the invention, the induction coils at the respective start of operation, ie when several coils are to be operated, with the required values, which have been input by an operator for each induction coil via an operating device, first operated at high frequency or highest operating frequency of the system. The function as pot detection coils is particularly advantageous. This makes it possible to detect whether an appropriate cooking vessel is to be heated via an induction coil. Then the frequencies are lowered with the frequency converters. This takes place until the total power of the induction coils corresponds to the total power of the required values for the individual powers. Since this still takes place at the same frequency, as a rule, ie at different required values for the power P, the one induction coil operated with more power than required and the other with less power.

This true total power is at a common frequency f _g . Subsequently, the induction coil, which is operated at excessive power, set by the frequency difference according to operation mode c) upwards. The other induction coils remain at the existing frequency. If the frequency difference is set as required, then all inductors are then driven down in their operating frequency with fixed frequency difference .DELTA.f down, and that until the total power again corresponds to the required value.

In the first operating mode a) there can be no frequency differences. Thus, no disturbing intermodulation or no audible effects can arise.

In the second operating mode b) the frequencies are very close in operation. Advantageously, the frequency difference is even 500 Hz and less. Although this results in a certain intermodulation of the set frequencies or operating frequencies of the induction coils, but because of the very low frequency differences, they are barely perceptible, since they are in an area of the human hearing curve in which the average human ear is relatively insensitive.

In the third operating mode c) the frequency difference is in a very high range for the human ear or is no longer in the audible range. Here, it has additionally been found in the context of the invention that, with a frequency difference of about 18 kHz as well as 24 kHz, a particularly good suppression of audible noise is possible.

Thus, there are three options available to operate several induction coils together without this being annoying to hear. These three modes of operation are advantageous in that both the average power for each individual inductor and the total average power corresponds to a power level selected by an operator. If this is possible by means of a constant operation with one of the operating modes a) or b), ie with a respectively fixed and unchanged frequency, this is an advantageous mode of operation of a plurality of induction coils.

Advantageously, the method operates exactly two induction coils, as described in the present application. Here, the possible variation of the operating frequencies and setting a certain frequency difference is particularly good and predictable possible.

It is possible, on the one hand, for each induction cooker to have a single induction coil. Alternatively, an induction cooking station may have an induction coil which consists of several partial coils and / or which can be driven by a plurality of power generators or frequency converters. This then corresponds to so-called multi-circuit heaters, as they are known from radiant heaters.

Induction coils, in particular for household use, such as an induction oven or an induction hob, are advantageously operated in a frequency range of about 16 kHz to 100 kHz.

Subsequently, a clocking or alternating operation of the induction coils can take place if the required values do not change. This operation is such that operation takes place either at the common frequency f _g for a certain time t _g , the time t _{g being} calculated as follows: $$t_G=\frac{{\tilde{P}}_1-P_1\left(f_{v1}\right)}{P_1\left(f_G\right)-P_1\left(f_{v1}\right)}=\frac{{\tilde{P}}_2-P_2\left(f_{v2}\right)}{P_2\left(f_G\right)-P_2\left(f_{v2}\right)}$$

Or it is then followed by an operation with the two different frequencies and the frequency difference .DELTA.f, namely for the time t _v , where t _g + t _v = T. Thereafter, the operation switches between these two modes.

Should one of the required values for the power for one of the induction coils change, this method of determining the values for the frequencies and the times is carried out again.

The sum of the powers at common frequency f _g corresponds to the sum of the powers at different frequencies. she is at the same time the required total power for both induction coils.

In such an operation, a flicker-free connection to a supply network is possible. However, if none of the aforesaid operating modes can be used to find a setting which matches the required values, and at which the frequency difference also moves within said range, it may be necessary or necessary to operate with a low level of flicker for a certain period of time. Restrictive constraints here may be e.g. a minimum operating frequency of a frequency converter, a maximum permissible amplitude of the current in the frequency converter, a smallest permissible phase in a resonant circuit in the frequency converter as well as saturation effects in ferrites, which are provided on the induction coils for influencing the generated magnetic field.

Another possibility may be to first try to satisfy the conditions with a first lower frequency difference, for example the said 18 kHz. If this is not the case, or if the intended algorithm does not work for the adjustment, then it can be attempted with a second somewhat higher frequency difference of about 24 kHz.

These and other features will become apparent from the claims and from the description and drawings, wherein the individual features each alone or more in the form of sub-combinations in one embodiment of the invention and in other fields be realized and advantageous and protectable Represent embodiments for which protection is claimed here. The subdivision of the application into individual sections as well as intermediate headings does not restrict the general validity of the statements made thereunder.

Brief description of the drawings

Fig. 1

ein Schaltbild einer Anordnung von zwei Induktionsspulen in einem Induktionskochfeld mit jeweils einem Frequenzumrichter,

Fig. 2

ein Schaubild der Betriebsfrequenz f über der Zeit t und

Fig. 3

ein Schaubild der Leistung P über der Zeit t.

An embodiment of the invention is shown schematically in the drawings and will be explained in more detail below. In the drawings shows:

Fig. 1

a circuit diagram of an arrangement of two induction coils in an induction hob, each with a frequency converter,

Fig. 2

a graph of the operating frequency f over the time t and

Fig. 3

a graph of power P over time t.

Detailed description of the embodiment

In Fig. 1 an induction hob 11 is shown in section. On a hob plate 13, an operating device 15 is arranged with two rotary knobs 16 and 17 for the power setting. The representation of the operating device 15 is to be understood very schematically and of course all other controls can be provided, in particular also so-called touch switch.

The operating device 15 is connected to a controller 18 and, among other things, passes on the control commands by adjusting the rotary knobs 16 and 17 to the controller 18. The controller 18 in turn is connected to a first inverter 19 which is supplied with a frequency f _1, a first inductor L1, and with a second inverter 20 which is supplied with the frequency f _2, a second inductor L2.

The induction coils L1 and L2 are arranged in a known manner below the hob plate 13. On its underside, ferrites 21 are arranged in a likewise known manner for influencing the magnetic field generated by the induction coils L. Above the induction coils L1 and L2 are on the hob plate 13 cookware 22 and 23 attached. The larger cookware 23 illustrates to what extent the coupling of a higher power should or should be done here. This can also be seen in the position of the rotary knob 17, which is further adjusted to the right and thus to a higher power level than the left rotary knob 16. Here, the rotary knob 16 is used to adjust the power for the induction cooker formed by the left induction coil L1 and the right rotary knob 17 for the induction cooker formed by the right induction coil L2.

In FIGS. 2 and 3 For example, as will be described below together, it is shown how the inductors L1 and L2 are supplied with power P at a certain frequency of the supply voltage. The curves for the frequency and the power are shown in dashed lines for the coil L2.

The operation starts with the fact that both induction coils L1 and L2 are operated with common frequency, namely with f _max in the sense of a pan detection function. This is sufficiently known to the person skilled in the art and need not be explained in more detail here. In both inductors L1 and L2 it is found that suitable cookware, namely the cookware 22 and 23, are set up and thus an operation is possible. Accordingly, power is released by the controller 18 and the frequency converters 19 and 20.

Subsequently, the frequency set by the frequency converters 19 and 20 is lowered by an equal value to the value f _g . This value f _g results from the specifications that both induction coils L1 and L2 should be operated at the same frequency, namely f _g , and with the powers P ₁ (f _g ) and P ₂ (f _g ). The powers P ₁ (f _g ) and P ₂ (f _g ) result from the specification with f _g and the set and predetermined value for the total power generated via the rotary knobs 16 and 17.

It can be seen to what extent in the first operation with the common frequency f _g, the induction coil L1 is operated with the power P ₁ (f _g ), which is greater than the average intended power P ₁ . The inductor L2 is operated at a power P ₂ (fg) which is below the mean intended power P ₂ lies. Then, the operated at excessive power inductor L1 is increased in terms of their operating frequency f ₁ by a frequency difference .DELTA.f, which is 18 kHz in the present case. Depending on this, both operating frequencies are lowered with the fixed frequency interval Δf. To the same extent then increase the performance of the inductors L1 and L2 again. The lowering takes place until the frequencies f _v1 and f _{v2 are} reached with the powers P ₁ (f _v ) and P ₂ (f _v ). The sum of P ₁ (f _v) and P ₂ (f _v) the sum of P ₁ (f _g) and P ₂ (f _G).

Subsequently, an operation takes place for a certain time t _v with precisely these values for f _v1 and f _v2 and the resulting frequency difference Δf. This is followed by the operation at the common frequency f _g , in which case the powers P ₁ (f _g ) and P ₂ (f _g ) amount, that is to say again the induction coil L1 is operated with excessive power and the induction coil L2 with lowered power. This period t _{g is} calculated according to the following formula: $$t_G=\frac{{\tilde{P}}_1-P_1\left(f_{v1}\right)}{P_1\left(f_G\right)-P_1\left(f_{v1}\right)}=\frac{{\tilde{P}}_2-P_2\left(f_{v2}\right)}{P_2\left(f_G\right)-P_2\left(f_{v2}\right)}$$

Subsequent to this time t _g , the previously explained operation with the frequencies f _v1 and f _v2 takes place again for a time t _v . In this case, t _g + t _v = T.

Overall, changes from here, as long as the given power values P ₁ and P ₂ for the inductors L1 and L2 by an operator not changed, the operation from. This applies to the calculated times t _g and t _v .

Thus, here is an operation with the aforementioned operating mode a), namely for the time t _g , and operating mode c), namely for the time t _v . During the time t _g , no noise is generated at all, since the same frequency is used and thus no intermodulation can occur at all.

During the time t _v , the above-mentioned frequency difference of 18 kHz during the operating mode c) is present, which means a barely audible noise.

Thus, it is altogether possible by the described and inventive method to avoid or greatly reduce the noise while still producing a required performance, at least in the mean, by the induction heaters.

Kicking while in the FIGS. 2 and 3 shown operation with changing performances changes to the given values for the performance P ₁ and P _{2 of} the induction coil L on, for example, by adjusting the rotary knob 16 or 17, the calculation and adjustment is new. This is followed by one of the above-mentioned operating states.

Claims (9)

1. Method for supplying power to several induction coils (L1, L2) in an induction apparatus (11), each induction coil (L1, L2) being supplied with power by means of its own frequency converter (19, 20), characterized in that during the common operation of several induction coils the frequencies of the given frequency converter (19, 20) are set as a function of the power provided with respect to a frequency difference (Δf) according to one of the following operating modes:

   - a) the frequency difference is equal to zero,

   - b) the frequency difference is less than 1 kHz,

   - c) the frequency difference is between 15 and 25 kHz,

   at the start of operation the induction coils (L1, L2) being operated with the requisite values for the individual power levels and the total power of the induction apparatus (11) initially with a high frequency and subsequently accompanied by the reduction of the frequency and the rise of the power the total power for the induction coils is set at the requisite value, one induction coil (L1) having more power than required (P₁(f_g)) and another induction coil (L2) less power than required (P₂(f_g)), the induction coils having up to then the same common frequency (f_g), and subsequently the induction coil with the increased power is moved upwards in frequency by a frequency difference according to operating mode c), and then the frequencies of the induction coils with the fixed mutual frequency difference (Δf) are lowered to the extent that the total power of the induction coils corresponds to the requisite value ( P ₁ + P ₂) for the total power.
2. Method according to claim 1, characterized in that precisely two induction coils (L1, L2) are operated according to one of the aforementioned operating modes (a) to c).
3. Method according to claim 1 or 2, characterized in that the induction coils (L1, L2) are operated in a frequency range of 16 kHz to 100 kHz.
4. Method according to claim 1, characterized in that subsequently alternating operation of the induction coils (L1, L2) takes place with either the aforementioned first common frequency (f_g) for a first specific time (t_g) or with the aforementioned frequency difference (Δf) for a second specific time (t_v), the first specific time (t_g) being equal to: $$t_g=\frac{{\bar{P}}_1-P_1\left(f_{v1}\right)}{P_1\left(f_g\right)-P_1\left(f_{v1}\right)}=\frac{{\bar{P}}_2-P_2\left(f_{v2}\right)}{P_2\left(f_g\right)-P_2\left(f_{v2}\right)}\mathrm{.}$$

   
5. Method according to any of the preceding claims, characterized in that the frequency difference (Δf) for operating mode b) is max 500 Hz.
6. Method according to any of the preceding claims, characterized in that the frequency difference (Δf) for operating mode c) is 18 kHz.
7. Arrangement for performing the method according to any of the preceding claims, characterized in that it is an induction cooker, particularly an induction hob (11), with at least two induction hotplates.
8. Arrangement according to claim 7, characterized in that each induction hotplate has a single induction coil (L1, L2).
9. Arrangement according to claim 7, characterized in that one induction hotplate has an induction coil comprising several partial coils and/or which is controllable by several power generators or frequency converters.

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