METHOD FOR CONTROLLING AN INDUCTION COIL AND INDUCTION COIL DEVICE

Description

SCOPE OF APPLICATION AND STATE OF THE ART

[0001] The invention relates to a method for controlling an induction coil, particularly on an induction cooktop, to adjust the primary power output of the induction coil. The invention also relates to a combination of an induction coil device with an electrical load. The induction cooktop has control means for power generation, wherein the induction coil forms a primary-side resonant circuit with a capacitor connected in series. The control means of the induction coil have at least two control variables by which they change the generated primary power: firstly, changing the operating frequency of the primary-side resonant circuit, and secondly, changing the voltage effectively applied to the primary-side resonant circuit.

[0002] The induction coil can transfer power to an electrical device mounted on a cover above it. This device has a receiver coil and a connected electrical load. The induction coil and receiver coil are then transformer-coupled such that a current in the induction coil induces a voltage in the receiver coil, which in turn causes a current to flow, thereby generating secondary power in the load of the electrical device. The control mechanism of the induction coil is provided with information about the desired secondary power output at the load, for example, via a control knob on top of the electrical device.

[0003] Market studies over the last ten years have explored solutions for cordless appliances that can simply be placed and operated on a dining table or kitchen countertop, particularly an induction cooktop. Eliminating a power cord significantly increases user flexibility, as the same space can be used for food preparation, cooking, and operating a variety of electric kitchen gadgets and appliances, as well as for other uses beyond food preparation. This advantage is particularly anticipated for very small apartments with compact kitchens or kitchenettes.

[0004] Work is underway on an open standard for the inductive power supply of kitchen appliances. This standard, in addition to the aforementioned tabletop application, also envisions power supply to special induction cooktops via their induction coil. These cooktops would then be compatible with both conventional cookware and wireless kitchen appliances. These wireless kitchen appliances are generally equipped with a receiver coil and control intelligence, as their controls are located within the appliance itself. Power can then be requested via a wireless connection or similar method. One advantage of an integrated wireless connection is that the wireless kitchen appliance registers with the cooktop and is identified before power generation can begin.In a use case on a normal tabletop, power generation may only start after prior identification, since conventional cooking pots must not be heated there under any circumstances due to the heat development on their underside, and only suitable wireless devices may be operated.

[0005] When an induction cooktop is used to house the induction coil and its control elements, the same induction coil should be able to heat both an induction-compatible cooking pot and supply power or energy to a wireless device. In both cases, the induction coil and the cooking pot or receiver coil are coupled via a transformer; that is, a current in the induction coil induces a voltage in the receiver coil, which in turn causes a current to flow in the load of the electrical device, corresponding to a secondary power output. This, in turn, leads to a reverse voltage in the induction coil.

[0006] Problematic are load changes or load jumps at the electrical consumer, for example if it is suddenly switched off or moved relative to the induction coil, for example by a few cm or even completely removed.

[0007] From WO 2013 / 098227 A1, it is known to operate a kitchen appliance on an induction cooktop, which is inductively supplied with electrical energy by an induction heating coil of the cooktop. The kitchen appliance has a receiver coil and an electrical load connected to it, with the receiver coil and the induction heating coil being coupled by a transformer. To adjust the power transfer, the number of resonant circuit capacitors in the kitchen appliance can be varied. This allows the characteristics of the resonant circuit, together with the receiver coil, to be adjusted. Increasing the power is the exact opposite of decreasing it.

[0008] From EP 2 112 861 A1, a general method is known for how, on the one hand, passive cooking vessels, such as a frying pan, can be inductively heated on an induction cooktop. On the other hand, active kitchen appliances can be operated, with the energy supply being inductive. Such a kitchen appliance could be, for example, a toaster.

[0009] From US patent 2014 / 125147 A1, it is known to inductively charge a mobile phone on a flat base using one of several charging stations provided on the base. For this purpose, each charging station on the base has a primary coil in a resonant circuit, which is inductively coupled to a receiver coil in the mobile device. The requested electrical power is adjusted for transmission.

[0010] US Patent 2014 / 158680 A1 discloses yet another inductive energy transfer system, specifically integrated into an induction cooktop. A kitchen appliance, similarly designed to the one described above, can be placed above an induction heating coil of the cooktop. This could be, for example, a kitchen mixer. The energy required to operate the electric motor of the kitchen mixer is inductively transferred from the induction heating coil to a receiver coil in the mixer. An alternating voltage for the motor can also be rectified in the process.

[0011] From JP 2016 134355 A, a further induction cooktop is known that has several induction heating coils. A kitchen appliance can be placed on a cooktop surface above one of these coils and is inductively supplied with electrical energy for its operation. Rectification may also be provided for an electrical load, such as a motor. TASK AND SOLUTION

[0012] The invention is based on the objective of creating a method mentioned above and a combination of an induction coil device with an electrical consumer suitable for carrying it out, with which problems of the prior art can be solved and it is particularly possible to operate an electrical consumer with receiver coil and electrical load connected to it advantageously above the induction coil, preferably also in the event of sudden load changes or very rapid movement of the consumer.

[0013] This problem is solved by a method with the features of claim 1 and by a combination of an induction coil device with an electrical consumer with the features of claim 13. Advantageous and preferred embodiments of the invention are the subject of the further claims and are explained in more detail below.

[0014] Some of the features are described only for the method or only for the combination of an induction coil device with an electrical load. However, they should be able to apply independently and separately to both the method and to such a combination of an induction coil device with an electrical load. The wording of the claims is made explicit by reference to the content of the description.

[0015] The induction coil is designed to transmit power to an electrical appliance placed on a cover above the coil. This appliance includes a receiver coil and a connected electrical load, as described above. The corresponding induction coil device is advantageously designed as an induction cooktop, allowing for cooking. However, it can also be installed or integrated into any suitable piece of furniture, such as a table, cabinet, or chest of drawers.

[0016] Control means for power generation are provided, advantageously including power switches, for example, semiconductor power switches. The induction coil forms a primary-side resonant circuit with a series-connected capacitor, as is customary. The primary resonant circuit can advantageously be supplied by a half-bridge or a full-bridge. During operation, the induction coil and the receiver coil are transformer-coupled such that, due to the generated primary power, a current in the induction coil induces a voltage in the receiver coil. This voltage, in turn, causes a current flow and thus the generation of secondary power in the load of the electrical consumer, which again leads to a back EMF in the induction coil. It is advantageous to also include a control circuit for the induction coil, configured to carry out the method according to one of the preceding claims.It may be associated with the control means or provide this information.

[0017] The control elements of the induction coil are provided with information regarding a desired secondary power at the load of the consumer, and these control elements have at least two manipulated variables. They can use these two manipulated variables to change the generated primary power. The first manipulated variable is the changing of the operating frequency for the primary-side resonant circuit. The second manipulated variable is the changing of the effective voltage applied to the primary-side resonant circuit. The control elements operate with a transfer function P(f) for the primary power, which has at least one local peak, or possibly only a single peak, in the vicinity of which an operating procedure is advantageous. With good coupling, the transfer function can have two peaks with a minimum point in between. From the aforementioned at least one local peak, the function, or rather its curve, flattens out on both sides.With this transfer function P(f), to the right of at least one local peak, a decrease in the operating frequency leads to higher primary power, and an increase in the operating frequency leads to lower primary power. This is the so-called inductive operating region. To the left of at least one local peak, a decrease in the operating frequency leads to lower primary power, and an increase in the operating frequency leads to higher primary power. This is the so-called capacitive operating region. Preferably, an operating frequency is used for the induction coil that is higher than the operating frequency at this local peak of the transfer function, so that the induction coil operates in the inductive operating region. The first derivative of the transfer function is negative here, as it decreases at this point.The local peak is advantageously located in a range of the operating frequency between 15 kHz and 50 kHz, particularly advantageously between 20 kHz and 36 kHz, and in this range it is the only peak of the transfer function P(f) for the primary power.

[0018] Moving the pot while maintaining a constant operating frequency alters the coupling between the induction coil and the receiver coil, potentially shifting the operation from the preferred inductive range to the capacitive range of the transfer function. In such cases, an attempt can be made to return to the preferred inductive range.

[0019] According to the invention, in a first operating mode, the control means always attempt to maintain the desired secondary power at a steady state by maximizing the voltage effectively applied to the primary-side resonant circuit, which constitutes the aforementioned second control variable. This applies in the following cases: • when the desired secondary power changes or • when there is a deviation between a measured secondary power and the desired secondary power. This can be caused by a change in the transformer coupling between the induction coil and the receiver coil and / or by a change in the electrical load, which may be caused by integrated switching devices in the load.

[0020] To increase the primary power, especially because a higher secondary power is required according to one of the two aforementioned cases, the control devices reduce the operating frequency as the first control variable.

[0021] To reduce the primary power, particularly when lower secondary power is required in one of the two aforementioned cases, the control system first reduces the voltage effectively applied to the primary-side resonant circuit as the second control variable. Then, in a second, subsequent step, the operating frequency is increased as the first control variable. If necessary, these two steps must be repeated several times to maintain the current or power within a desired operating range. In this way, one can again arrive at an operating point on a curve of a transfer function P(f), but on a different curve, one that corresponds to the new physical conditions. In particular, a change in the transformer coupling between the induction coil and the receiver coil due to a shift obviously results in a new curve.

[0022] The induction coil and receiver coil are advantageously interoperable, or matched to each other, as is the goal of the aforementioned open standard. This means that the induction coil and receiver coil are matched in terms of diameter and inductance for the preferred application in an induction cooktop or for installation in furniture, such as a table or chest of drawers, and with regard to power class. This allows the respective defined power levels to be transmitted.

[0023] In a further embodiment of the method, if the load is moved relative to the induction coil, thereby reducing the transformer coupling and dedamping the primary-side resonant circuit within a short time, preferably less than 0.5 seconds or even less than 0.2 seconds, the control means, in a first step, maintain the primary-side current at a constant operating frequency with a fluctuation range of ±5%, at an initial primary-side current level of I_1 prior to the load being moved. For this purpose, the control means reduce the voltage effectively applied to the primary-side resonant circuit as a second control variable. This generates a primary power that corresponds to a secondary power that is lower than the desired secondary power.The resulting operating point is robust against the various possible causes of damping loss, especially if a change towards it is such that the resulting operating point should lie within the aforementioned capacitive operating range of the transfer function, where a reduction in frequency leads to a current increase that can even destroy a converter. A disadvantage of this first step is that the resulting operating point typically exhibits increased losses. At a higher operating frequency with maximum RMS voltage across the induction coil, the losses would be lower, i.e., the efficiency would be higher. Therefore, subsequent steps should be taken to achieve an operating point with lower losses.

[0024] In a second step, the operating frequency is increased as the first control variable until the current in the induction coil has changed by a maximum of ±10%, or until the primary power deviates by a maximum of ±10% from the primary power value at which the actual secondary power corresponds to the desired secondary power. Measuring the secondary power to determine whether the desired secondary power, requested by the consumer from the induction coil, is present at the load would take too long for the control process and is therefore technically pointless. Subsequently, the voltage effectively applied to the primary-side resonant circuit is again adjusted as the second control variable until the primary-side current returns to its initial level of I_1 before any movement of the consumer or any change in the desired secondary power or load, or until the desired secondary power is established at the load.

[0025] The first and second steps mentioned above are then performed alternately until the secondary power measured at the load is less than or equal to the desired secondary power, or until the maximum possible voltage effectively applied to the primary-side resonant circuit is reached as the second control variable. In the latter case, a higher secondary power cannot be achieved; the load must then be operated with a lower secondary power.

[0026] In an advantageous embodiment of the invention, in the aforementioned second step, when increasing the operating frequency, the threshold for a change in the current in the induction coil and / or the threshold for an upward change in primary power (i.e., an increase) is smaller than the threshold for a downward change (i.e., a decrease). This can preferably be a maximum of +5% upward and a maximum of -10% downward, and particularly preferably a maximum of +2% upward and a maximum of -5% downward. This allows for a greater decrease than an increase, which noticeably helps to protect the circuit components.

[0027] Advantageously, information regarding the desired secondary power output at the load of the consumer is provided to the control devices; particularly advantageously, the electrical consumer sends the target value for power generation to the power generation unit. In addition to a transmitter and a control unit or a certain level of intelligence, it can also have an operating device where an operator can adjust the operation of the consumer and thus the load. For example, the consumer could be a mixer that can operate at different speed levels. Therefore, there is a varying power requirement at the electric motor of the mixer, depending on the speed level set by the operator on the control device, for example, using a knob or rotary switch. The control unit evaluates this set speed level and calculates a desired secondary power output, which the motor or the consumer, if necessary, then supplies.In addition to the power required for the basic operation of the control unit, this desired secondary power is then sent by the consumer to the control device of the induction coil, for example in an induction cooktop with a corresponding receiver.

[0028] The desired secondary power can be an absolute value, for example, "1500 W". This allows a consumer without integrated secondary power measurement to specify a desired power that, based on experience, corresponds to the desired operating point. Alternatively, if the consumer lacks a device for measuring secondary power at the load, the receiver can send a deviation between the measured secondary power and the desired secondary power to the control circuitry of the induction coil. This is because primary and secondary power differ due to transmission losses, and this efficiency can vary. The induction coil can only regulate against its own values, so the specified secondary power serves to determine a desired primary power. The induction coil is then operated with this primary power.

[0029] Changing the voltage effectively applied to the primary-side resonant circuit as a second control variable is preferably achieved by changing the ratio of the on-time to the off-time of the switching elements in a half-bridge power control circuit, which is also known as the duty cycle. If a full bridge, formed jointly from two half-bridges, is used in the power control circuit, the phase angle between the control signals of the two half-bridges is changed, which, with respect to changing the effective voltage applied, corresponds to the previously described method.

[0030] For a DC-controlled half-bridge, an advantageous duty cycle of 50% corresponds to the operating point with the maximum effective voltage at a given operating frequency. However, there are also alternative control methods where a 50% duty cycle corresponds to a phase angle of 180° for a full bridge. Reducing the duty cycle preferably keeps the operating frequency constant.

[0031] Advantageously, power transmission can be achieved continuously at the maximum possible effective voltage applied to the primary-side resonant circuit as a second control variable. At the resulting operating frequency, the effective voltage across the induction coil is at its maximum.

[0032] It can be provided that changing the operating frequency, particularly to change the primary power, is stopped as soon as a resulting change in the primary power reaches a threshold of + / -10%. The aforementioned threshold values ​​can apply, preferably being lower than the upper limit. These thresholds can also be +5% and -10%, and in particular +2% and -5%.

[0033] Preferably, the induction coil can be operated continuously to supply the electrical load at an operating frequency higher than the operating frequency at which the transfer function P(f) curve has its aforementioned at least local peak of maximum primary power, or at which the transfer function P(f) curve has a negative first derivative, i.e., it decreases. In this case, as previously mentioned, the transfer function P(f) is in the so-called inductive region. Here, losses can be kept lower.

[0034] In a method according to the preamble of the claim, the current through the induction coil is dynamically limited to protect both the device and the load, with the limit being set at +10%, and in particular at +5%, of the current value. This dynamic limit restricts the current and thus the magnetic field, which, as a current source within the load, could damage or even destroy it. To protect the primary-side device, it is sufficient to limit the current there to an absolute value at which it is guaranteed not to be destroyed. This limit must not be exceeded for a maximum duration of 1 second, preferably a maximum of 0.1 seconds, or even a maximum of 0.02 seconds.Otherwise, power generation at the induction coil is switched off or the current is reduced to a value at least 25% or even 50% below the current value, whereby the voltage effectively applied to the primary-side resonant circuit is reduced as a second control variable to reduce the current.

[0035] The aforementioned limit can be adjusted regularly, preferably every 8 ms to 500 ms. In particular, it is adjusted every 16.6 ms to 20 ms, so that every second half-wave can be checked. This can serve to provide continuous protection against load changes, especially load shedding, of the installed load during transition processes between operating points.

[0036] Advantageously, the current through the induction coil can be measured using comparators that directly drive a power semiconductor driver, which acts as the control element for the induction coil. This achieves a design that is as error-free as possible.

[0037] In one embodiment of the invention, when power is generated using a half-bridge circuit for the induction coil, a current peak can be measured as the current generated by exactly one power switch of the half-bridge circuit. This power switch has a shorter on-time compared to the other power switch.

[0038] Preferably, several induction coils in an induction cooktop are controlled using this method as a possible induction coil device, particularly preferably in a special operating mode for surface-mounted appliances with an electrical load. These induction coils can then still be used in another normal operating mode for the usual inductive heating of pots. The operating frequency for increasing the primary power of one of the induction coils is not continuously reduced, but rather reduced in steps or stages. Preferably, the change in the operating frequency of one induction coil is synchronized with those of other induction coils that are currently operating.

[0039] The present invention relates, in a first instance, to handling changes in the desired secondary power of a connected device, particularly in the case of significant changes in power demand. It also relates to changes in the transformer coupling between the induction coil and the receiver coil, for example, when the coils are repositioned. The transformer coupling between the induction coil and the receiver coil decreases when the receiver coil is not positioned centrally above the induction coil. Even with a repositioned receiver coil, it is still possible to transmit the maximum intended power, provided the permissible currents and voltages in the induction coil and receiver coil are not exceeded. However, a reduction in coupling usually requires more current in the induction coil for transmission.Inducing the same secondary power is required, but current and power depend particularly strongly on the operating frequency f of the induction coil, since the impedance of the entire setup also changes significantly with frequency. If a receiver coil is moved down from the induction coil, thus reducing the coupling, the point at which maximum power is transferred shifts to a higher operating frequency. This point of maximum power transfer corresponds to a resonance point with at least one local minimum in impedance. Due to the lower impedance at a constant operating frequency, the current I in the induction coil and the voltage U_load in the receiver coil increase. If the operating frequency of the power generation for the induction coil is now increased, there is a risk that the current in the induction coil or the voltage U_load in the receiver coil will be too high.The voltage in the receiving coil will increase if the power generation cannot raise the operating frequency faster than the coupling decreases or the load is shifted. If this causes limits in the receiving coil and the induction coil to be exceeded, it can lead to damage or even failure of the induction coil and / or the receiving coil or their respective drive systems. This risk is particularly high when the operating point, due to the weaker coupling, transitions from the aforementioned preferred inductive region of the transfer function, where increasing the frequency leads to higher impedance (i.e., current or voltage is reduced), to a capacitive region of the transfer function, where increasing the frequency leads to lower impedance (i.e., current or voltage increases with frequency). Therefore, it is disadvantageous to limit the current in the induction coil or...The operating frequency is increased by increasing the voltage in the receiver coil.

[0040] Advantageously, when generating power for the induction coil with a half-bridge, the duty cycle is reduced according to the shorter on-time of the circuit breaker over the entire cycle period. Conversely, when generating power for the induction coil with a full-bridge, the phase angle between the activation of the two bridges is reduced. On the other hand, an operating point with a maximum duty cycle of close to 50%, or operation with a 180° phase angle, results in the lowest losses in a power generation converter, so frequency control leads to higher efficiency.

[0041] In a second instance, the present invention specifically addresses the handling of a further, specific risk, namely when the appliance has means that can disconnect its load from the receiver coil. This is because the appliance could be destroyed if placed on a conventional induction cooktop whose pot detection cannot distinguish the appliance from an ordinary, induction-compatible pot. Advantageously, relays are used as these means. Due to the lack of communication between the appliance and the conventional induction cooktop, i.e., in this case, the induction hob, the appliance could exceed its permissible limits when the corresponding induction coil is in operation, potentially damaging or even destroying it, and possibly even causing a fire.It is also conceivable that a consumer has multiple loads, for example, one or more individually switchable heating elements as well as motor-based loads such as fans or stirrers. These must be able to be operated individually.

[0042] Normally, the user should first initiate a system-integrated communication device, preferably wireless, to cause the induction coil to switch off the transmitted power or at least reduce it to a level that is not critical for them before disconnecting or switching off a load. If a load step occurs during operation, the impedance changes abruptly, which in most cases leads to a sudden increase in the current in the induction coil and consequently also in the voltage U_load in the receiver coil. The induction coil can protect itself from damage by limiting the current to its maximum permissible value, provided that sufficiently fast detection of the load step and a current reduction device are in place. Advantageously, the induction coil can also reduce the duty cycle in the DC regulator or the phase angle.As long as current flows in the primary side of the induction coil, it acts like a current source for the consumer; that is, even if the heating circuits have supposedly been switched off, current will flow in the consumer as long as it finds any path, potentially damaging the consumer in the process.

[0043] It is possible to start the operation of the induction coil preferably at the highest possible operating frequency of an inverter in the power generation stage, since a higher impedance can usually be expected there, which can have a current-limiting effect. Since such appliances as the aforementioned mixer, toaster, food processors, etc., exhibit resonant circuits on both the primary and secondary sides, two local resonance points can occur, particularly in strongly coupled systems. One of these can also be located at the highest operating frequency of the inverter. Therefore, the operation of the induction coil should always be started with the smallest possible duty cycle or phase angle.

[0044] If relatively small amounts of power, for example 2 W to 200 W, are to be transferred to the consumer—for instance, if only its control unit or switching device for heating circuits within the consumer is to be switched on and operated—then this is usually less power than can be transferred with the highest operating frequency and the smallest permissible duty cycle in a half-bridge power generation system. The phase angle in a full-bridge power generation system can theoretically be reduced to almost zero, thus enabling even the smallest power levels to be achieved. The common method used by induction cooktops, which involves the on-duration of mains half-waves in a short cycle of usually 1 to 10 seconds, is unsuitable for power transmission in this case because it can lead to both gaps in the consumer's power supply and impermissible power spikes within the consumer.Therefore, this operating mode is implemented either by clipping each mains half-wave, preferably symmetrically in the ascending and descending parts of the mains half-wave. Phase-angle control is permissible here because the maximum short-term power, thanks to a suitable choice of high operating frequency and low duty cycle, already acts as a significant power limiter. Alternatively, the RF control can be operated in pulse frequency mode, in which the RF on-time is kept constant at the smallest permissible value, corresponding to a pulse from one or both switches of the half-bridge. This corresponds to a minimum duty cycle at the highest operating frequency. Simultaneously, the operating frequency is reduced, meaning the oscillation occurs less frequently. A comparable result can be obtained by activating a pot detection pulse, as is common in induction cooktops, at a suitable frequency.If the power is adjusted according to one of the described methods, the resulting gaps in the calculation of the effective power must be assessed and factored out in order to avoid large tolerances in the power control. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Exemplary embodiments of the invention are shown schematically in the drawings and are explained in more detail below. The drawings show: Fig. 1 a schematic representation of a combination according to the invention of an induction coil device with a mixer mounted as an electrical load, Figs. 2 to 4 different flow diagrams for power generation of the induction coil and Fig. 5 a transfer function of the power as a function of the frequency P(f) with control shown. DETAILED DESCRIPTION OF THE EXECUTION EXAMPLES

[0046] In the Fig. 1 An induction coil device according to the invention is shown as an induction cooktop 11, comprising a cooktop plate 12 and an induction coil 14 on its underside. A conventional power generation unit 16 is provided for the induction coil 14, as is also found in standard induction cooktops according to the prior art, namely with a converter. The power generation unit 16 has a first primary-side resonant circuit capacitor 18 and power switches 19, which can be configured as a half-bridge or a full bridge. A heat sink 21 is also provided. The power generation unit 16 can supply or control several induction coils of the induction cooktop 11, which are not shown here.

[0047] A control unit 23 of the induction cooktop 11 controls the power generation 16. It has an antenna 24 for receiving and, if necessary, transmitting information wirelessly. The control unit 23 is connected to a display and control unit 26, which can also be of a conventional design. Advantageously, it has display elements, preferably LEDs, and control elements, preferably touch switches. A combination of these can also be a touchscreen. This allows the induction cooktop to be operated with its conventional functions as usual. Furthermore, this also allows part of the invention to be applied.

[0048] An electrical appliance 30, a mixer, is positioned on the cooktop 12 above the induction coil 14. The mixer has a mixing bowl 32 containing a stirring blade 33 and an electric motor 35 to drive it. The important components for the power supply are contained in a housing 37, namely a receiver coil 39 and its associated switching elements 41. The receiver coil 39 is transformer-coupled to the induction coil 14, as explained earlier, which is readily apparent and visualized here. A current in the induction coil 14 corresponding to the generated primary power transformer-coupled into a voltage in the receiver coil 39, which causes a current flow and thus the generation of secondary power in the load of the electrical appliance 30, namely in the electric motor 35. The switching elements 41 are advantageously designed as fuses or similar devices and not as conventional switches for controlling the electric motor 35.This is achieved by controlling the power supply via the induction coil 14. A fuse is provided only for emergencies, for example, a standard fuse. A power measurement device 42 is also provided, which can record exactly how much power arrives at the receiver coil 39 or, alternatively, at the electric motor 35. The power measurement device 42 is designed according to prior art.

[0049] The housing 39 is provided on the right with a control unit 43, which includes a controller 45 with antenna 46 and operating elements 48, advantageously also a display for an operator. The controller 45 and the controller 23 can communicate with each other or exchange information via the antennas 46 and 24; in particular, the controller 45 can send a desired power as a desired secondary power, which an operator has entered at the operating elements 48, to the controller 23.

[0050] In the method according to the invention, an operator can, for example, input a power level 3 as high power for the mixer using the control elements 48, which is considered the desired secondary power and is therefore sent to the control unit 23. For this purpose, a power supply in the form of a battery can generally be provided in the control unit 43. The control unit 23 then converts the desired secondary power into a primary power to be generated by means of the power generation unit 16, for example, based on empirical values ​​or calculations. The induction coil 14 is thus operated with a specific primary power, which should in itself lead to the desired secondary power, at least approximately, and transforms power to the receiver coil 39. This transferred power is used to operate the control unit 43 and, above all, primarily to operate the drive motor 35 to drive the mixer.The received secondary power is measured by means of the power meter 42, and if it does not yet correspond to the desired secondary power according to the set power level 3 because it is too low, the controller 45 sends a message to the controller 23 indicating that more primary power must be generated. The aforementioned magnetic coupling between the induction coil 14 and the receiver coil 39 can also play a role here, as it degrades the power transmission if it is not optimal, especially if the induction coil 14 and receiver coil 39 are not exactly concentric or similar.

[0051] Based on the information sent by consumer 30, the control unit 16 adjusts the primary power output until it receives information that the actual secondary power output matches the desired secondary power output. This primary power output is then continued to be generated, and consumer 30, or mixer 30, operates with it.

[0052] This initial adjustment at the start of consumer 30 is shown in the flowchart of the Fig. 2 The diagram shows that P_target corresponds to the desired secondary power that should be present at the mixer during continuous operation. Power generation 16 with the inverter starts with an initial frequency f_0 and an initial voltage U_0 effectively applied to the primary circuit of the induction coil 14. These are advantageously chosen such that the actual secondary power is very likely to be below the desired secondary power P_target. The voltage U for the induction coil 14 is then increased to increase the primary power. It is then measured whether the resulting measured secondary power P_actual already corresponds to the desired secondary power P_target. If this is the case, the steady-state or continuous operating point has already been reached. If this is not yet the case, it is checked whether the applied voltage U corresponds to a maximum permissible voltage U_max. If this is still not the case, the voltage U is increased again, repeating the process as described above.Once the maximum permissible voltage U_max has been reached, but not yet the desired secondary power P_soll, the operating frequency f is reduced.

[0053] If an initial reduction of the operating frequency f results in the measured secondary power P_actual matching the desired secondary power P_target, then the aforementioned steady-state operating point is reached. If this is not yet the case, the operating frequency is reduced until the desired condition is met.

[0054] In the flowchart of the Fig. 3 This section illustrates how the steady-state operating point is set and regulated, taking into account the previously described dynamic current limiting. Starting from a steady-state operating point like the one described earlier, the actual current I_act in the induction coil 14 suddenly increases. If it increases by less than 5% compared to the previously flowing current I_1, or if it does not change at all, it is checked whether the measured secondary power P_actual still corresponds to the desired secondary power P_set. The desired power for the load 30 may have changed due to small changes in electrical parameters, for example, due to temperature effects. If the measured secondary power P_actual still corresponds to the desired secondary power P_set, the operating point remains the same as before.

[0055] If the measured secondary power P_actual no longer corresponds to the desired secondary power P_set, a single control variable is used to adjust the system until the measured secondary power P_actual again matches the desired secondary power P_set. At that point, the operating point is restored.

[0056] However, if the actual current I_is in the induction coil 14 is at least 5% higher than the previously flowing current I_1, then a load change in the consumer 30 is assumed, for example, because an operator has selected a different power level or operating level for the mixer, and this information may not have reached the controller 23. In this case, the inverter or power generator 16 is immediately switched off to cut off the current as quickly as possible so that no additional energy is supplied to the consumer. The system then waits for a new desired secondary power input P_set. Once this input has been received, the system can then proceed according to... Fig. 2 the operating point is controlled.

[0057] In Fig. 4 is related to Fig. 5 The diagram illustrates how a transition from a previous operating point to a new operating point occurs. It shows a [condition / situation] according to [reason / reason]. Fig. 2 A steady-state operating point is set with a current I_ist through the induction coil 14, corresponding to an existing current I_1. The power P_p #1 is 2200 W, and the coupling factor k is 0.75. This is intended to correspond to a relatively good coupling between the induction coil 14 and the load 30 or receiver coil 39, advantageously as a concentric arrangement. Then, either the power requirement of the desired secondary power can change, for example, significantly, or the transformer coupling can change as described above due to a displacement of the load 30 relative to the induction coil 14, for example, by 3 cm to 5 cm.

[0058] First, it is checked whether the measured primary power P_actual is lower than the desired power P_target, which corresponds to the desired secondary power, i.e., whether the power at the consumer 30 should be increased, for example, because the desired power there has been increased by an operator. If this is the case, the operating frequency of the power generation 16 for the induction coil 14 is reduced. Based on the Fig. 5 It can be seen how the primary power P increases on curve P #1 because the operating point shifts to the left on the transfer function P(f). This is checked and the operating frequency reduced until the measured secondary power P_actual corresponds to the desired power P_setpoint. Then a steady-state operating point is found with precisely this power, whereby the actually measured current I_actual flows through the induction coil 14 and produces a corresponding primary power there.

[0059] If the measured secondary power P_actual is not less than the desired power P_target, but rather greater, then the search for a new operating point begins, but according to the jagged curve in Fig. 5 First, the effective voltage across the induction coil 14 is reduced until the current I_actual through the induction coil 14 is at most as large as the previously flowing current I_1, or proportionally even smaller due to the target power reduction. Then, the effective voltage in the power generation unit 16, which is applied to the induction coil 14, is reduced again. The goal is to reduce the actual power P_target; therefore, the transfer function P(f) of the Fig. 5 to the right. Reducing the voltage changes the power output at a constant operating frequency; it becomes lower.

[0060] If the current I_actual through the induction coil 14 is less than or equal to the previously flowing current I_1, the operating frequency f is increased as explained earlier, for example, between 0.2 kHz and 1 kHz or 2 kHz. If a maximum voltage U_max is reached, or if the measured power P_actual corresponds to the desired power P_set, the flow diagram moves to the left, resulting in the new steady-state operating point described earlier. If this condition is not yet met, it is checked whether the ratio of measured secondary power P_actual to desired power P_set lies within a certain bandwidth, here specified as +5% / -10%. If this is not the case, a significant change has occurred, and the previously described step of increasing the operating frequency must be performed again.

[0061] If the condition is met, the loop returns to the top to reduce the effective voltage. This results in a zigzag line from left to right. While a target power P_target of 2200 W was originally desired to operate load 30 at an operating frequency of approximately 24.2 kHz, the new operating point with this secondary power lies at an operating frequency of approximately 28.6 kHz.

[0062] Joint Fig. 5Shifting the frequency reduces the transformer coupling, with the coupling factor k becoming only 0.43. The transfer function now changes from that of P#1 to that of P#2, and is therefore significantly different, with a local peak at a different and slightly higher operating frequency. Initially, with the operating frequency f remaining constant, the current and primary power at the induction coil 14 would increase sharply in order to generate the desired power. This is prevented during power generation because it could otherwise cause damage, as described previously. The voltage is then significantly reduced until the current and power return to their previous values, and the desired power would actually be present according to the previous transfer function P#1. Then the operating frequency is increased slightly, for example by 0.2 kHz. This initially causes the primary power to increase again, since the current is now being applied to the left or right channel.The capacitive branch of the new transfer function P#2 is used. Increasing the operating frequency primarily serves to improve efficiency. During the second increase in the operating frequency, the local peak is passed, and the system transitions into the right inductive region. Therefore, increasing the operating frequency causes a decrease in primary power and thus in the measured power. This zigzag curve is repeated until, after each increase in voltage (shown with long dashed lines) and an increase in operating frequency f_op, the primary power P is generated at which the desired power P_setpoint of 2200 W is delivered to load 30. According to transfer function P#2, this occurs at the specified primary power at an operating frequency of approximately 28.6 kHz.

[0063] The procedures and processes would be similar if, instead of transformer coupling, the desired secondary power at the consumer changes by shifting, for example because an operator sets a different power level for the mixer.

Claims

1. Method for controlling an induction coil (14) for adjusting power generation (16) for a primary power at the induction coil (14), wherein: - the induction coil (14) is designed for power transmission to an electrical consumer (30) placed on a cover (12) above the induction coil, which has a receiver coil (39) and an electrical load (35) connected to it, - control means are provided for the power generation (16), - the induction coil (14) forms a primary-side resonant circuit with a capacitance (18) connected in series, - the induction coil (14) and the receiver coil (39) are coupled in a transformer-like manner such that a current in the induction coil induces a voltage in the receiver coil, which causes a current to flow and thus generates a secondary power in the load (35) of the electrical consumer (30), - the control means of the induction coil (14) are provided with information about a desired secondary power at the load (35) of the consumer (30), - the control means of the induction coil (14) have at least two control variables via which they change the generated primary power, namely changing a working frequency (f) for the primary-side resonant circuit as a first control variable and changing a voltage (U) effectively applied to the primary-side resonant circuit as a second control variable, - the control means of the induction coil (14) operate with a transfer function (P(f)) for the primary power, which has at least one local peak and which, at least locally, is such that a reduction in the operating frequency (f) leads to a higher primary power and an increase in the operating frequency leads to a lower primary power, wherein - in a first operating mode, the control means always attempt to regulate the desired secondary power in a steady state with maximum modulation of the voltage (U) effectively applied to the primary-side oscillating circuit as the second control variable in the following cases: ∘ when the desired secondary power changes, or ∘ in the event of a deviation between a measured secondary power and the desired secondary power caused by a change in the transformer coupling between the induction coil (14) and the receiver coil (39) and / or by a change in the electrical load (35) caused by integrated switching means (41) in the consumer (30), characterized in that - to increase the primary power, the control means reduce the operating frequency (f) as the first control variable, - to reduce the primary power, in a first step, the voltage (U) effectively applied to the primary-side oscillating circuit is reduced as a second control variable before, in a second following step, the operating frequency (f) is increased as the first control variable.

2. Method according to claim 1, characterized in that: - in the event that the consumer (30) is shifted and, as a result, the transformer coupling is reduced and the primary-side resonant circuit is damped in a short period of time, the control means, in a first step, keep the primary-side current at an output level of the primary-side current (I_1) before shifting the consumer (30) at a constant operating frequency (f) with a fluctuation range of + -5% at an output level of the primary-side current (I_1) before the load (30) is shifted, whereby the voltage (U) effectively applied to the primary-side oscillating circuit is reduced as a second control variable, thereby generating a secondary power that is lower than the desired secondary power, - then, in a second step, the operating frequency (f) is increased as the first control variable until the current in the induction coil (14) has changed by a maximum of +-10% or the primary power deviates by a maximum of + -10% from the value for the primary power at which the secondary power corresponds to the desired secondary power, - then the voltage (U) effectively applied to the primary-side oscillating circuit is changed again as the second control variable in such a way that the primary-side current (I_1) has returned to its initial level before the consumer (30) was moved or the desired secondary power has been set, - then the first aforementioned step and the second aforementioned step are performed alternately until the measured secondary power is less than or equal to the desired secondary power or until the maximum possible voltage (U) effectively applied to the primary-side oscillating circuit is reached as the second control variable.

3. Method according to claim 1 or 2, characterized in that, in the second step, when increasing the operating frequency (f), the threshold for a change in the current in the induction coil (14) or the threshold for a change in the primary power is smaller upwards than downwards as the first control variable.

4. Method according to one of the preceding claims, characterized in that the control means are provided with information on a desired secondary power at the load (35) of the consumer (30), wherein the electrical consumer sends the specified value for power generation (16) to the power generation.

5. Method according to one of the preceding claims, characterized in that the power transmission is permanently carried out at the maximum possible effective voltage (U) applied to the primary-side oscillating circuit as a second control variable, wherein at the operating frequency (f) that sets in, an effective voltage that is maximally large is applied to the induction coil (14) as the first control variable.

6. Method according to one of the preceding claims, characterized in that the increase in the operating frequency (f) is stopped as soon as a resulting change in the primary power reaches a threshold of + / -10%.

7. Method according to claim 2, characterized in that continuous operation of the induction coil (14) for supplying the electrical consumer (30) takes place at a working frequency (f) at which the curve of the transfer function P(f) has a negative first derivative over the working frequency.

8. Method according to the preamble of claim 1, characterized in that a dynamic limitation of the current is performed by the induction coil (14), wherein a limitation is at +10%, in particular +5%, of the current value, wherein this limitation must not be exceeded for a duration of more than 1 second, and otherwise the power generation (16) at the induction coil (14) is switched off or the current is reduced to a value at least 50% below the current value, wherein, in order to reduce the current, the voltage (U) effectively applied to the primary-side oscillating circuit is reduced as a second control variable.

9. Method according to claim 8, characterized in that this limitation is regularly updated every 8 ms to 500 ms.

10. Method according to claim 8 or 9, characterized in that the current through the induction coil (14) is measured by comparators which act directly on a driver for power semiconductors as a control means for the induction coil.

11. Method according to one of claims 8 to 10, characterized in that, in the case of power generation (16) with a half-bridge circuit for the induction coil (14), a current peak is measured as current, which is generated by exactly one power switch (19) of the half-bridge circuit, wherein this power switch has the shorter on-time in comparison to the other power switch.

12. Method according to one of the preceding claims, characterized in that several induction coils (14) in an induction hob (11) are controlled using this method, whereby a working frequency (f) for increasing the primary power of one of the induction coils is not reduced continuously, but is reduced in steps or stages.

13. Combination of an induction coil device (11) with an electrical consumer (30), wherein the induction coil device (11) has at least one induction coil (14) which is designed for power transmission to the electrical consumer (30), wherein the electrical consumer (30) has a receiver coil (39) and an electrical load (35) connected thereto, wherein the electrical consumer (30) is placed on a cover (12) above the induction coil (14), wherein the induction coil device (11) comprises: - control means for power generation (16) for the primary power at the induction coil (14), - a capacitance (18) connected in series with the induction coil (14) and forming a primary-side resonant circuit, wherein: - the induction coil (14) and the receiver coil (39) are coupled in a transformer manner such that a current in the induction coil induces a voltage in the receiver coil, which causes a current to flow and thus generates the secondary power in the load (35) of the electrical consumer (30), characterized in that: - the induction coil device (11) has a control (23) which is designed to carry out the method according to one of the preceding claims, - the control means of the induction coil (14) have at least two control variables for changing the generated primary power, namely changing a working frequency (f) for the primary-side resonant circuit as a first control variable and changing a voltage (U) effectively applied to the primary-side resonant circuit as a second control variable, - the control means of the induction coil (14) are designed to operate with a transfer function (P(f)) for the primary power, which has at least one local peak and which is at least locally such that a reduction in the operating frequency (f) leads to a higher primary power and an increase in the operating frequency leads to a lower primary power.

14. Combination of an induction coil device (11) with an electrical consumer (30) according to claim 13, characterized in that an operating frequency (f) is used which is above the operating frequency at which the transfer function (P(f)) has its local peak.