EP4210435B1 - Method and apparatus for measuring a power on an induction heating coil - Google Patents

Description

The invention relates to a method for measuring a power at an induction heating coil in an induction hob and to a device which is designed to carry out this method, in particular a device which is part of an induction hob.

When operating an induction heating coil on an induction hob, it is not only important that an operator can specify a desired power level, which is then roughly maintained with larger deviations or less accuracy. Particularly when several induction heating coils lying next to each other, which are covered by a common cooking vessel, are coordinated, a very precise setting of the power on an induction heating coil can be desired. The power actually generated by the induction heating coil in interaction with the heated cooking vessel is important, so this should be recorded as precisely as possible. Controls can then run as precisely as possible using automatic cooking programs.

From the US 2021/204367 A1 , the US 2021/352772 A1 , the EP2506665A2 and the EP2731403A1 Methods are known for controlling an induction heating coil in an induction hob. Bridge circuits are used, particularly half-bridge circuits or full-bridge circuits. By changing the duty cycle, the power of an induction heating coil can be adjusted, which can be used to inductively heat a cooking vessel.

From the DE 102018214485 A1 is a variation of a duty cycle for a hob with an IR light source as a heater.

Task and solution

The invention is based on the object of creating a method as mentioned above and a device for carrying out the method as mentioned above, with which problems of the prior art can be solved and in particular it is possible to measure a power generated by an induction heating coil as accurately as possible and preferably with as little effort as possible.

This object is achieved by a method having the features of claim 1 and by a device having the features of claim 15. Advantageous and preferred embodiments of the invention are the subject of the further claims and are explained in more detail below. Some of the features are explained only for the method or only for the device. However, they should be able to apply independently and independently of one another to both such a method and such a device.

It is intended that the induction heating coil or a transmitter coil of a device for wirelessly transmitting power to a consumer is controlled by a converter, in particular a converter of a power section or the like of an induction hob. The converter is operated in a half-bridge or full-bridge circuit. Furthermore, the converter is operated with a frequency and a duty cycle, whereby both the frequency and the duty cycle can be varied to adjust the power delivered by the induction heating coil. Furthermore, the current through the induction heating coil is measured, which is relatively simple and accurate in terms of measurement technology.

At least in one operating mode of the induction heating coil control, there is a change between control with a 1st duty cycle and control with a 2nd duty cycle. This change can occur with different frequencies, which is explained in more detail below. The 2nd duty cycle is determined by subtracting the 1st duty cycle from 1, or the duration of the 2nd duty cycle is determined by subtracting a duration of the 1st duty cycle from a total cycle time. Furthermore, the voltage across the half bridge or across the full bridge with which the induction heating coil is controlled is measured or determined. It is therefore possible that by changing the duty cycle, errors when measuring the voltage and thus when measuring the power can be compensated for or calculated out. This is particularly advantageous if the duty cycle is determined as previously defined or the 1st duty cycle and the 2nd duty cycle are determined accordingly, i.e. the duty cycles are inverted, so to speak. The actual power that the induction heating coil actually generates can then be easily determined by multiplying the measured current and the voltage measured or determined as described.

In an embodiment of the invention, it can be provided that in each case in a zero crossing of a voltage for supplying the converter, in particular a mains voltage with which the converter or the entire induction hob is supplied, between the 1st duty cycle and the 2nd duty cycle. This switching at zero crossing brings significant advantages in the switching behavior in the converter as well as in the interference behavior and with regard to network interference. It can be provided that, in order to measure the power at the induction heating coil, an average is taken over two directly consecutive half-waves of the specified voltage or mains voltage or supply voltage. The advantage of this is that external changes to a piece of cookware or a device to be operated, such as its temperature or installation position, can be assumed to be constant within 20msec, i.e. the same load state is measured in the first half-wave and the second half-wave and the deviation in the measurement corresponds to the measurement error of the control. Averaging is advantageously carried out by directly determining an average value of the two measured powers, i.e. adding them together and dividing them by two. This is a very simple and precisely feasible computational process.

In a further embodiment of the invention, within a half-wave of a voltage or mains voltage, in particular the voltage described above with which the converter is supplied, switching between the 1st duty cycle and the 2nd duty cycle can take place as an alternative to the aforementioned switching in a zero crossing. It is possible for an integrated or calculated total portion of the control with the 1st duty cycle and the control with the 2nd duty cycle to be equivalent, in particular over several switching processes within a half-wave. This makes it possible for the same amount to be measured with the 1st duty cycle as with the 2nd duty cycle.

It can be provided that for each half-wave of the aforementioned voltage or mains voltage, a change is made between control with the 1st duty cycle and control with the 2nd duty cycle. In this way, the aforementioned compensation or averaging can be carried out very finely. The resulting high switching frequency for the converter or the power switches contained therein, in particular IGBTs, does not pose a problem and can be easily handled by them. The power at the induction heating coil can be measured permanently, so to speak. In particular, after two half-waves, a power measurement is always immediately correct, since it was controlled and measured once with the 1st duty cycle and once with the 2nd duty cycle. A power measurement is therefore always completely up to date, so to speak.

As long as the operating point does not change, the power measurement is correct even after each half-wave, since two consecutive half-waves can always be used for the calculation, so it always overlaps, so to speak. For example, with the mains half-waves 1, 2, 3, 4, 5 are always measured alternately with DC and (1-DC). After mains half-wave 1, the power measurement is initially incorrect. After mains half-wave 2, the measurement can be averaged over mains half-waves 1+2. After mains half-wave 3, the measurement can be averaged over mains half-waves 2+3, after mains half-wave 4, the measurement can be averaged over mains half-waves 3+4, and so on. It is therefore not necessary to use the measurement over mains half-waves 1+2 and then over mains half-waves 3+4.

Alternatively, control with the 1st duty cycle can be used for more than two half-waves of the aforementioned voltage or mains voltage, and then control with the 2nd duty cycle can be used for the same number of half-waves of the voltage or mains voltage. This is a way of switching between the duty cycles less frequently and thus causing fewer switching operations, which may make the method easier to carry out. It is possible for control with each of the duty cycles to be carried out for an even number of half-waves. In a possible further embodiment of the invention, it can be provided that this number of half-waves of the voltage or mains voltage is used for control with the 1st duty cycle and with the 2nd duty cycle until a predetermined power for the induction heating coil changes, i.e. until the operating point and thus the switching behavior of the converter changes. As long as the measured power does not correspond to the target power or is below it, it makes sense to keep the switching interval short in order to obtain a measured value more quickly. Once the target power is reached, the measurement interval can be extended. Switching between DC and (1-DC) does not change the output power, but merely reduces the measurement error.

In yet another embodiment of the invention, it is possible for a measuring interval, during which a power is determined at the induction heating coil and which is also completely required for this determination, to consist in equal parts of the control with the 1st duty cycle and the control with the 2nd duty cycle. Thus, the power at the induction heating coil is only finally determined after the control with the two duty cycles has been fully carried out, whereby of course current and voltage are measured in the manner described above during the entire time.

In an embodiment of the invention, a measuring interval for determining the power at the induction heating coil can consist of two intervals of half-waves of a voltage or mains voltage with which the converter is supplied. It can advantageously be provided that in In a 1st interval the inverter is operated with the 1st duty cycle and in a 2nd interval the inverter is operated with the 2nd duty cycle. The 1st interval and the 2nd interval are the same length, so that equal parts are used for each of the two duty cycles to correctly determine the power at the induction heating coil.

As an alternative to the previously described possible determination of the power, a measurement interval can comprise a few half-waves, namely exactly two half-waves, of the aforementioned voltage for supplying the converter. These are advantageously two consecutive half-waves, so that the measurement is taken during a whole full wave, so to speak. It can be provided that the 1st duty cycle is used during the first half-wave and the 2nd duty cycle is used during the other or immediately following half-wave.

In a further development of the invention, in order to determine a power at the induction heating coil, the integral of the product of voltage and current over time can be calculated over the intervals of control with the 1st duty cycle and control with the 2nd duty cycle. This involves the current currently measured and the voltage currently measured at the induction heating coil.

In an advantageous development of the invention, it is possible for the voltage across the induction heating coil to be estimated from the control of power switches or semiconductor switches of the converter. This may make it easier to determine the voltage. It may be advantageous to take into account a switching delay of the switches. This allows a voltage to be estimated as accurately as possible.

In one embodiment of the invention, the same duty cycle can be used during several consecutive half-waves of the voltage or mains voltage without changing it. Only then is the other duty cycle used. The number of half-waves per duty cycle is advantageously the same.

For the power calculation, it is advantageous for at least the number of half-waves over which the average is calculated to consist of equal parts of DC and (1-DC). A different number of 1st and 2nd duty cycles can be calculated out by appropriate unequal weighting.

In a preferred embodiment of the invention, the 1st duty cycle is between 0 and 0.5. It is therefore smaller than the 2nd duty cycle. In a half-bridge, for example, the upper switching means is switched on for less than half the period for duty cycles less than 0.5, while the lower switching means in this example is switched on for less than half the period for duty cycles greater than 0.5. is switched on. Both the 1st and 2nd duty cycle result in the same output AC voltage of the inverter.

Preferably, if the operation does not change when the induction heating coil is controlled with regard to the power specified for it and a cooking vessel heated with the induction heating coil, which is placed above it on a hob plate and at least covers this induction heating coil to a significant extent, is not moved, there can no longer be a change in the duty cycle of the control. It is therefore possible for the control to take place permanently with the same duty cycle. For this special case, it is then assumed that the power generated by the induction heating coil does not change and therefore no current or updated power measurement is required. This makes it possible to simplify the control without the measurement of the power becoming inaccurate. In a further embodiment of the invention, it can be provided that the temperature should not change when the method is carried out because a change in temperature shifts the power at a constant operating point. The induction heating coil then simply generates a different power. However, this change occurs relatively slowly, preferably as a kind of drift, so that even an occasional re-measurement would be sufficient to eliminate the resulting error, for example at least every 10 seconds, 60 seconds or 120 seconds.

In a particularly preferred embodiment of the invention, at least two induction heating coils are each operated on a half-bridge of the converter on the same intermediate circuit; alternatively, they are operated on a full bridge of the converter. These two induction heating coils are each operated synchronously with any specified power, which can therefore also differ by at least 30%, in particular by at least 60%. In addition to or as an alternative to the power, a duty cycle can be provided or specified as desired. Here, too, a difference can be at least 30% or at least 60%. The power and duty cycle can be set as desired. Advantageously, the duty cycle for all induction heating coils on this converter can be changed synchronously, so that the interaction with at least the second converter is also neutralized by the switching.

The current of the inverter through the induction coil leads to a ripple with twice the operating frequency of the intermediate circuit voltage in the inverter, because switching the inverter from the upper branch of the inverter to the lower branch of the inverter and back again causes a reversal of the direction of the current in the intermediate circuit capacitor. caused. When controlled with a duty cycle other than 0.5, in addition to the fundamental oscillation, harmonics of the current are also excited by the induction coil. The second harmonic of the current, which has its maximum excitation at a duty cycle of 0.25 or 0.75, together with the double frequency of the ripple of the intermediate circuit voltage, means that the output power of the inverter can supply or remove additional power from the intermediate circuit capacitor in addition to the power in the induction heating coil. Determining this additional power with the intermediate circuit, which is superimposed on the power at the induction coil from the inverter's point of view, is difficult. In addition to the amplitudes of the voltage ripple and the second current harmonic, the phase position between the two is also required. By switching the duty cycle to inverse control, which is advantageous according to the invention, the phase positions are inverted, so that on average the correct power at the induction heating coil can again be measured.

A device designed according to the invention mentioned at the outset has a converter, in particular a conventional and known converter for induction heating coils, a control and at least two half bridges or at least one full bridge for the converter. The control is designed to carry out the method described above and to measure the power currently generated by the induction heating coil as accurately as possible in different ways. A control can be provided either just for the converter or on the converter. Alternatively, the control can be a controller, in particular a microcontroller, for the entire device. An operator can also enter a specification for the power to be generated by the at least one induction heating coil.

The aforementioned converter advantageously has an intermediate circuit, whereby this intermediate circuit is connected to exactly one half-bridge, to exactly two half-bridges or to at least one full bridge. In this way, a simple and practical structure can be achieved.

The device is preferably built into an induction hob or is part of an induction hob. This has one or two such converters, each converter being advantageously fed from its own phase of a three-phase connection in a household or house that is intended exclusively for it. A large number of such induction heating devices are arranged under the hob plate, for example eight to twenty-four.

Short description of the drawings

Fig. 1

eine vereinfachte schematische Schnittdarstellung durch ein erfindungsgemäßes Induktionskochfeld,

Fig. 2

eine Darstellung einer Schaltung, um eine der Induktionsheizspulen des Induktionskochfelds aus Fig. 1 anzusteuern,

Fig. 3 bis 8

Darstellungen des Verlaufs von Spannung und Strom in der Halbbrückenschaltung der Fig. 2.

Embodiments of the invention are shown schematically in the drawings and are explained below. In the drawings:

Fig.1

a simplified schematic sectional view through an induction hob according to the invention,

Fig.2

a representation of a circuit to switch one of the induction heating coils of the induction hob from Fig.1 head for,

Fig. 3 to 8

Representations of the voltage and current curve in the half-bridge circuit of the Fig.2 .

Detailed description of the implementation examples

In the Fig.1 an induction hob 11 is shown with a hob plate 13, on the underside of which two induction heating coils 15a and 15b are arranged. The induction hob 11 has a hob control 16 and separate power electronics 17, which is designed in particular as a converter. The power electronics 17 controls the induction heating coils 15a and 15b. It receives its commands, in particular with regard to the level of power to be generated, from the hob control 16. The hob control 16 contains an operating device with operating elements including a display for an operator, as is usual. A cooking vessel 20 is placed above the left induction heating coil 15a, which is to be heated with it.

The power electronics 17 essentially contains a previously described converter 18. A section of this is shown in the Fig.2 The power switches Z1 and Z2 are designed as IGBTs in the usual way, as are the diodes D1 and D2 as freewheeling diodes with the parallel resonant circuit capacitances C3 and C4. An intermediate circuit capacitor C5 is present.

The diodes D3 to D6 form a rectifier. The induction heating coil 15a is controlled by the converter 18 and heats the cooking vessel 20 placed above it, which corresponds to or is represented by the R shown here. In addition to the cooking vessel 20, the resistance R also represents the internal resistance (= losses) of the induction heating coil 15a. The converter 18 or the rectifier is fed on the left side by a supply voltage, preferably this is the mains voltage in a household. It can In the case of a two- or three-phase connection of the induction hob 11, it can be a single phase.

As previously described, the control of the converter 18 of the induction heating coil 15a for heating the cooking vessel 20 changes between control with a first duty cycle DC and control with the second duty cycle, which is determined by (1-DC). This can be done with the provisions explained above, for example by changing between the two duty cycles either within a half-wave of a supply voltage to which the converter 18 is connected, or each time the supply voltage passes through zero. For example, the change can also take place each time a half-wave of the supply voltage passes through, or alternatively at an integer multiple of a zero crossing. In any case, the duration of the control with the first duty cycle should correspond to the duration of the control with the second duty cycle.

The intermediate circuit voltage is applied, for example, to the intermediate circuit capacitor C5.

In the Fig.3 a curve of the intermediate circuit voltage at C5 is shown. The global curve of the half-waves of the supply voltage with a frequency of 50 Hz can be seen. At the same time, the significantly higher frequency curve of the operating frequency for the converter 18 can be seen.

The Fig.4 shows the course of the inductor current I, which flows through the induction heating coil 15a. Here, too, the same applies as previously stated for the intermediate circuit voltage with regard to the frequencies. In Fig.4 It is also shown that the control with (1-DC) leads to a mirroring of the peak currents along the x-axis. This means that in the first mains half-wave the negative currents are larger in amplitude than the positive currents, while in the second mains half-wave, i.e. when controlled with the second duty cycle, the positive amplitudes are larger than the negative amplitudes.

In the Fig.5 the course of the inductor current is shown in detail, namely when operating with the first duty cycle DC. This duty cycle DC is selected according to the desired power for the induction heating coil 15a, which has been entered by an operator on the control device 16. This is therefore due to the Fig.4 related to the left half-wave in the area around 5 msec. In the Fig.6 the inductor current is shown with the second duty cycle (1-DC), but shifted by the duration of half a half-wave with 10 msec, i.e. around 15 msec. From the comparison of the two representations it can be seen that the two Gradients approximately correspond to each other when reflected along a horizontal mirror axis.

In the Fig.7 is, similar to the representations of the Fig. 5 and 6 , the voltage curve at the first duty cycle DC is shown in magnification. The Fig.8 again shows the voltage curve at the second duty cycle (1-DC). These two curves are each related to the time periods of the inductor current curves according to the Fig. 5 and 6 based.

Thus, with the invention it is easily possible to form the power at the induction heating coil 15a as a product of inductor current and voltage, and then to integrate this over time. This can be carried out mathematically either in the power electronics 17 or in the hob control 16.

Furthermore, it is also possible to estimate the voltage across the induction heating coil 15a. For this purpose, a switching delay of the power switches S1 and S2 can be taken into account, which can be easily measured.

In a further advantageous embodiment of the invention, starting from the Fig.2 another induction heating coil 15b could be operated on the half-bridge of the converter 18. These two induction heating coils 15a and 15b can be operated synchronously, i.e. with the same duty cycle DC or (1-DC). Advantageously, the duty cycle can also be specified as desired for the two induction heating coils, as can the power. Only the duty cycle is then changed synchronously and simultaneously for these two induction heating coils. According to the invention, it is done synchronously and simultaneously for all induction heating coils operated on the converter 18. Changing the duty cycle can advantageously also be carried out to set a desired power, which is specified in particular by the hob control 16 through an operator input.

Claims (15)

1. Method for measuring the power of an induction heating coil (15a, 15b) that is controlled by means of a converter (18), wherein

   - the converter (18) is operated in a circuit of a half-bridge or a full-bridge,

   - the converter (18) is operated at a frequency and with a duty cycle,

   - the current (I) through the induction heating coil (15a, 15b) is measured,

   - at least in one operating mode of the control of the induction heating coil (15a, 15b), a change is made between a control with a first duty cycle and a control with a second duty cycle,

   - the 2nd duty cycle is determined by subtracting the 1st duty cycle from 1,

   - the voltage (V) across the half-bridge or across the full bridge is measured.
2. Method according to claim 1, characterized in that the switchover between the 1st duty cycle and the 2nd duty cycle is carried out in each case in a zero crossing of a voltage (V) with which the converter (18) is supplied, the power being averaged over two directly successive half-waves of the voltage (V) preferably for a power measurement.
3. Method according to claim 1 or 2, characterized in that within a half-wave of a voltage (V) with which the converter (18) is supplied, switching is performed between the 1st duty cycle and the 2nd duty cycle, with an integrated total proportion of the control with the 1st duty cycle and the control with the 2nd duty cycle preferably being equivalent.
4. Method according to claim 1 or 2, characterized in that a control with the first duty cycle is used for more than two half-waves of the voltage (V) and then a control with the second duty cycle is used for the same number of half-waves of the voltage (V).
5. Method according to one of the preceding claims, characterized in that a measurement interval for determining a power at the induction heating coil (15a, 15b) consists of equal proportions of the triggering with the first duty cycle and the triggering with the second duty cycle.
6. Method according to one of the preceding claims, characterized in that a measurement interval for determining the power at the induction heating coil (15a, 15b) consists of two intervals of half-waves of a voltage (V) for supplying the converter (18), wherein in a first interval the converter (18) is operated with the first duty cycle and in a second interval, which is as long as the first interval, the converter (18) is operated with the second duty cycle.
7. Method according to one of claims 1 to 5, characterized in that a measuring interval comprises exactly two half-waves of a voltage (V) for supplying the converter (18), in particular two successive half-waves, the first half-wave being used for the first duty cycle and the second half-wave being used for the second duty cycle.
8. Method according to one of the preceding claims, characterized in that, to determine a power at the induction heating coil (15a, 15b), the integral of the product of voltage (V) and current (I) over time is calculated over the intervals of activation with the first duty cycle and activation with the second duty cycle.
9. Method according to one of the preceding claims, characterized in that the voltage (V) across the induction heating coil (15a, 15b) is estimated from the control of semiconductor switches (Z1, Z2) of the converter (18), a switching delay of the two semiconductor switches (Z1, Z2) being taken into account.
10. Method according to one of the preceding claims, characterized in that the same duty cycle is used without change during several successive half-waves of the voltage (V) before the other duty cycle is used.
11. Method according to claim 10, characterized in that the same number of half-waves is used for the control with the first duty cycle and for the control with the second duty cycle.
12. Method according to one of the preceding claims, characterized in that the first duty cycle is between 0 and 0.5, preferably greater than 0.
13. Method according to one of the preceding claims, characterized in that, in the event that the operation does not change when the induction heating coil (15a, 15b) is actuated with respect to a predetermined power and a cooking vessel (20) heated by the induction heating coil (1 5a, 15b) is not moved, the changeover between activation with the first duty cycle and activation with the second duty cycle is no longer performed in such a way that activation is performed permanently with the same duty cycle.
14. Method according to one of the preceding claims, characterized in that at least two induction heating coils (15a, 15b) are operated on a respective half-bridge of the converter (18) on the same intermediate circuit or on a full bridge of the converter (18), the two induction heating coils (15a, 15b) being operated synchronously with any pre-set power and/or any pre-set duty cycle, preferably the power and the duty cycle being adjustable as desired for each induction heating coil (15a, 15b), preferably the duty cycle for all induction heating coils (15a, 15b) being changed synchronously at the converter (18).
15. Device (11) for carrying out the method according to one of the preceding claims, characterized in that it has a converter (18), a control and at least two half bridges or a full bridge for the converter (18), the control being designed to carry out the method according to one of the preceding claims.

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