EP1732357A2 - Heating device for induction cooking devices - Google Patents

Abstract

The device has an inductor heating a heating unit of a cooking container and a control unit. The control unit monitors the heating unit on reaching the cooking temperature. The monitoring takes place by adjusting the frequency of alternating current that flow through the inductor and detection of an electrical parameter based on a time period. The control unit controls an output of inductor based on the parameter. An independent claim is also included for a method for controlling a heating device for an induction cooking device.

Description

The invention relates to a heating device for a Induktionsgargerät according to the preamble of claim 1.

From the EP 1 420 613 A2 a heating device for a Induktionsgargerät is known in which a control unit monitors the temperature of a cooking vessel to reach a cooking temperature. As a result, the cooking of a liquid or a dish in the cooking container can be detected and the heat output of an inductor for heating the cooking vessel can be automatically reduced so that the water in the cooking vessel boils only slightly or the dish in the cooking vessel does not burn.

The object of the invention is in particular to provide a generic heating device for a Induktionsgargerät, which is improved in terms of monitoring to achieve a cooking temperature of the cooking vessel.

This object is achieved by the features of claim 1. Advantageous embodiments and further developments of the invention can be taken from the subclaims.

The invention relates to a heating device for a Induktionsgargerät comprising an inductor for heating a heating element of a cooking vessel and a control unit which is prepared to monitor the heating element to reach a cooking temperature.

It is proposed that the monitoring be done by adjusting a frequency of the alternating current flowing through the inductor and detecting at least one electrical quantity as a function of time. By setting a frequency monitoring can be done very reliable and timely. The detection of the electrical size allows a particularly simple monitoring of the cooking temperature. As a result, the cooking can be detected very quickly and reacted quickly, for example, by reducing the heating power.

The Induktionsgargerät can be a stove or just a hob or oven and the cooking container a pot or a baking oven or the like. The electrical quantity may be a size of the induction cooking appliance, for example the input voltage used by a circuit for generating an alternating current in the inductor. Conveniently, however, the electrical variable is a size of the inductor, for example the current through the inductor and / or the voltage at the inductor. For monitoring, a single electrical quantity may be detected as a function of time, for example a current or voltage value, or a plurality of electrical variables may be detected as a function of time, for example a current and a voltage value. In the following, the designation of the electrical quantity is also used for an ensemble of interrelated and time-dependent electrical quantities.

The electrical quantity is advantageously in a known relationship to the temperature of the heating element. This relationship can be absolute, so that from the electrical size, the temperature of the heating element can be determined directly. However, it is sufficient if the relationship is relative, so that from a change in the electrical quantity on a change in the temperature of the heating element can be concluded. The detection of the electrical variable as a function of time is expediently carried out by a chronological sequence of several measurements of the size and in particular by determining the gradient of the size from the measurements.

In an advantageous embodiment of the invention, the electrical quantity is correlated with the inductance of a system comprising the inductor and the heating element. By a change in the temperature of the heating element typically also changes the inductance of the heating element and thereby the inductance of the system comprising the inductor and the heating element. If the electrical variable with the inductance of this system in indirect or direct connection, it can be concluded in a simple manner from the electrical variable to the temperature or the temperature movement of the heating element. Monitoring the temperature for cooking is possible in a simple manner. Conveniently, the electrical quantity is correlated with the power of the inductor and in particular is in direct proportion to the power, which makes it particularly easy to deduce the temperature of the heating element.

A particularly simple and convenient control of the heating device can be achieved when the control unit for controlling the power of the inductor is prepared based on the electrical size. In this case, the electrical quantity can be evaluated both for the purpose of controlling the electrical power and for monitoring the cooking temperature, whereby the same size - or the same ensemble of sizes - can be used to control two processes. Components can be saved and data processing processes simplified. In particular, the control unit for keeping constant the temperature of the heating element is prepared based on the electrical size.

Advantageously, the control unit is prepared for keeping the frequency constant during the detection as a function of time. As a result, an operating point of the measurements can be kept stable, whereby reliably usable results of the measurement can be achieved.

In particular, the control unit is provided for determining measuring time ranges with at least one intermediate heating time range, wherein the frequency in the measuring time ranges is set at an identical measuring frequency and in the heating time range at a heating frequency different from the measuring frequency. The heating frequency can be varied independently of, for example, a favorable measuring frequency for power control of the inductor without impairing the reliability of the monitoring. By using the identical, ie the same measuring frequency during at least two measuring time ranges, a reliable monitoring result can be achieved.

In a preferred embodiment of the invention, the control unit is provided for detecting an undershoot of the absolute value of the gradient of the electrical variable below a limit value and for a response of the control to the undershooting. A particularly simple monitoring of the achievement of the cooking temperature can be achieved by detecting the undershooting of the absolute value of the gradient of the electrical variable below a limit. In a transition from a heating process to a boiling of water or a dish in the cooking container, the gradient of the electrical quantity, that is, the change of the electrical quantity with time, decreases as the temperature increases after a rise during heating during cooking remains largely constant. This stability can be recognized in a particularly simple way by suitably setting the limit value. By a reaction of the controller to the undershooting, for example, the reduction of the electric heating power, overcooking or burning of the dish can be effectively counteracted.

Conveniently, the reaction includes controlling the power of the inductor to maintain the temperature of the heating element constant. This can prevent burning of the dish. A particularly advantageous possibility of the control is that the temperature is kept constant at the value of the time of falling below. Here, the temperature can be kept constant even at the beginning of cooking, so that a strong cooking or overcooking is prevented.

In an additional or alternative embodiment of the invention, the reaction is a control of the power of the inductor to reduce the temperature of the heating element by a predetermined amount of temperature. As a result, a gentle simmer can be achieved even when the limit for reliable detection of cooking is high. Conveniently, the temperature after the reduction is kept constant, thereby allowing the gentle simmering to be maintained.

A particularly convenient operation of the heater can be achieved if the temperature amount is selectable by an operator. The temperature amount may be a selectable temperature range or a category such as "large", "medium" or "small". The operator can thus choose how much the water or the dish should cook.

A particularly reliable avoidance of overcooking or burning can be achieved if the control unit for determining the limit value is provided as a function of the time profile of the electrical variable. Recognition of cooking takes some time. In addition, the heating element is already slightly hotter than the cooking temperature when cooking the water or the dish. If there is only a small amount of water or a small amount of food in the cooking container, the time delay or temperature difference can lead to overcooking or burning. This can be counteracted if the limit is set higher during rapid heating, cooking is thus "detected" sooner so that the time delay and the temperature difference are somewhat compensated. With a large amount of water or a dish in the cooking container, a little more time is available, so that the limit can be set lower.

High reliability of detection of cooking can be achieved if the control unit is prepared to control a verification phase prior to reaction and after underflow. Random errors can be avoided, for example, by several further measurements that are made after detecting the undershoot. The reaction is then carried out after the further measurements. Conveniently, the duration of the verification phase is dependent on the time profile of the electrical variable, whereby a time delay can be kept low at a rapid boil.

The invention is also directed to a method of controlling a heating device for an induction cooking appliance, in which a heating element of a cooking container is monitored by a control unit for reaching a cooking temperature. It is proposed that the monitoring be done by adjusting a frequency of the alternating current flowing through an inductor and the detection of an electrical quantity as a function of time. It can be achieved a quick and reliable detection of cooking in a simple manner.

Further advantages emerge from the following description of the drawing. In the drawings, embodiments of the invention are shown. The drawing, the description and the claims contain numerous features in combination. The person skilled in the art will expediently also consider the features individually and combine them into meaningful further combinations.

Fig. 1

eine Heizvorrichtung für ein Induktionsgargerät, auf dem ein Topf steht,

Fig. 2

ein Diagramm, in dem die Leistung eines Induktors in Abhängigkeit von der Frequenz des Wechselstroms durch den Induktor und die Temperatur des Topfs dargestellt ist,

Fig. 3

ein Diagramm, auf dem die Temperatur des Topfs und die Leistung des Induktors gegen die Zeit dargestellt ist,

Fig. 4

eine Steuersequenz der Steuereinheit mit Heizzeitbereichen und Messzeitbereichen und

Fig. 5

ein Diagramm wie in Figur 2 mit einem weitere Schritte umfassenden Steuerungsablauf.

Show it:

Fig. 1

a heater for an induction cooker with a pot on it,

Fig. 2

a diagram in which the power of an inductor as a function of the frequency of the alternating current through the inductor and the temperature of the pot is shown,

Fig. 3

a diagram showing the temperature of the pot and the power of the inductor versus time,

Fig. 4

a control sequence of the control unit with Heizzeitbereichen and measuring time ranges and

Fig. 5

a diagram as in Figure 2 with a further steps comprehensive control flow.

1 shows a heating device 2 with a schematically illustrated and designed as a winding inductor 4 under a support plate 6. On the support plate 6, a cooking container 8 is shown in the form of a cut pot, the bottom of the pot comprises a heating element 10 of a ferritic material. Connected to the inductor 4 is a control unit 12 which generates an alternating current in the inductor 4 via a circuit 14. The schematically illustrated circuit 14 comprises a half-bridge circuit of power transistors and rectifiers, which are supplied with an input voltage u _{0 of} a current source 16, for example a power supply network. The control unit 12 serves to control the circuit 14 and comprises measuring elements for measuring the voltage u ₁ , which is applied to the inductor 4, and the current i _i , which flows through the inductor 4. In addition, the input voltage u _{0 is} detected by the control unit 12, which has means for detecting peak values and mean values of the input voltage u ₀ , the voltage u ₁ and the current i ₁ . In addition, the control unit 12 has arithmetic, storage and / or evaluation means which are provided for detecting one or more of these variables as electrical variables as a function of time.

To heat the heating element 10 and thus a dish in the cooking container 8, the circuit 14 is switched by the control unit 12 so that an alternating current with the frequency f flows through the inductor 4. The magnetic field generated by the alternating current generates eddy currents in the heating element 10, which heat the heating element 10. The power P at which the heating element 10 is heated is dependent on the frequency f, as shown in the diagram in FIG. In Figure 2, the power P is above the frequency f of the current i _{1 applied} by the inductor 4. The power P is in units of the maximum power P _max and the frequency f is plotted in units of the resonant frequency f _{r of} a resonant circuit comprising the inductor 4. At the resonant frequency f _r , for example 30 kHz, the power P reaches the maximum power P _max , for example three kW. If the frequency f is switched higher or lower by the control unit 12, the power P is smaller than the maximum power P _max .

The power P is dependent not only on the frequency f but also on the inductance L of the system comprising the inductor 4 and the heating element 10. The inductance L is again dependent on the temperature T of the heating element 10, so that the power P can be expressed as a function which is dependent on the frequency f of the current i ₁ through the inductor 4, the inductance L at standard conditions, for example at room temperature , and the temperature T of the heating element 10: P = f (f, L, T). In FIG. 2, the power P is shown in two curves, namely at a low temperature T ₁ and at a higher temperature T _{2 of} the heating element 10. If the power P is measured, for example, at a frequency f = 1.4 f _r at a first time in which the temperature T _{1 of} the heating element 10 is low, and at the same frequency f at a later time when the temperature T _{2 of} the heating element 10 is higher, the measurement will show that the power P is lower at the second time as at the first time. The measurement of the power P can be carried out by measuring one or more of the above-mentioned electrical variables whose change is correlated with the temperature T of the heating element 10.

By detecting the electrical variable as a function of time, a change in the temperature T of the heating element 10 can thus be determined. This relationship is shown in FIG. FIG. 3 shows an example of a course of the temperature T (t) of the heating element 10 together with the associated curve of the power P (t) of the inductor 4 and a profile of the electrical variable p (t), which is determined or calculated by the control unit 12 , Also shown is the first derivative dp / dt of the time course of the electrical quantity p (t), thus the gradient of the electrical quantity p (t).

At a first time t ₀ , which represents the beginning of the heating of the heating element 10, the power P of the inductor 4 is set to a power value P ₁ . This happens, for example, in that the control unit 12 sets the frequency f to a first Value adjusts the power P of the inductor 4 by measuring and / or calculating the electrical quantity p detected and in a deviation of the power P from the desired power value P ₁ by an iterative process, the frequency f changed until the power P to the desired Power value P _{1 is} set. By connecting the power P ₁ to the inductor 4, the heating element 10 is heated so that its temperature T (t) increases. Due to this, the value of the electrical quantity p (t) slowly decreases. The gradient dp / dt of the electrical quantity p (t) is negative in this case, but increases slowly by a flattening of the time profile of the electrical quantity p (t).

If the temperature T (t) reaches the boiling temperature of the liquid or of the dish in the cooking container 8, the temperature T (t) no longer rises or only very slightly increases. The gradient dp / dt of the electrical quantity p (t) approaches zero or is very low. At this time t ₁ , the gradient dp / dt exceeds a predetermined limit value G or the absolute value of the gradient dp / dt falls below this limit value G. The limit value G can be fixed in advance. In the method according to FIG. 3, however, it is determined from the curvature of the course of the temperature T (t) or the curvature of the course of the electrical variable p (t), the limit value G being greater at a larger curvature and the curvature being greater at a smaller curvature Limit G is kept smaller.

By exceeding or falling below the limit value G, a reaction is initiated by the control unit 12, which comprises a reduction of the power P (t) to a lower power value P ₂ . The recognition of exceeding or falling below the limit value G can be effected by one or more triggers and / or comprise measuring or calculating steps. The power value P ₂ is chosen so that the water or the dish in the cooking container 8 slightly simmer. By correlating the electrical quantity p (t) with the temperature T (t) of the heating element 10, the temperature T (t) can also be kept constant by keeping the electrical quantity p (t) constant. The control of the power P (t) of the inductor 4 is thus based on the electrical variable p (t).

In the further course of the time t, the temperature T (t) is adjusted by constant checking of the electrical quantity p (t) at the constant value, so that burning of the dish is at least largely prevented. If, for example, at a point in time t _x, the dish cakes on the heating element 10, forming an insulating layer and thus only one low heat transfer from the heating element 10 to the court be possible, the power P of the inductor 4 is successively corrected downwards, so that the temperature T (t) of the heating element 10 remains stable at the set value.

FIG. 4 shows a diagram in which the frequency f of the current i ₁ is plotted against the time t by the inductor 4. While a number of measurement time periods .DELTA.t _1, the frequency f stable at a measuring frequency f _M, and held the electric variable p (t) in each case once several times or recorded respectively. Heating time ranges Δt ₂ , in which the frequency f is set to a heating frequency f _H1 or f _H2 , are respectively arranged between the measuring time ranges Δt ₁ . The heating frequencies f _H1 , f _H2 are independent of the measuring frequency f _M and can be varied as desired to control the power P within predetermined limits. In contrast, the measuring frequency f _M - at least over a measuring cycle of several measuring time ranges .DELTA.t ₁ - always the same, so that a stable operating point for detecting the electrical quantity p (t) is achieved.

FIG. 5 shows a diagram analogous to FIG. 3, in which the temperature T (t) of the heating element 10 and the power P (t) of the inductor 4 are plotted against time. For the sake of clarity, the application of the electrical variable p (t) and the gradient dp / dt of the electrical variable p (t) analogously to FIG. 3 has been omitted, the course of these variables being analogous to FIG. As in FIG. 3, the absolute value of the gradient dp / dt falls below the limit value G at the instant t ₁ , the control unit 12 leaving the power P (t) of the inductor 4 initially at the high power value P ₁ . In a subsequent to the time t ₁ verification phase .DELTA.t _V , the behavior of the electrical variable p (t) or its gradient dp / dt is checked. As a result, random or systematic errors in the detection of the electrical quantity p (t) as a function of the time t can be avoided.

In the case of a positive test result, for example if the absolute value of the gradient dp / dt remains below the limit value G, at the time t ₂ at the end of the verification phase Δt _V the power P is reduced to a very low power value P ₃ . As a result, the temperature T (t) decreases from the first temperature T ₁ , which was reached at the time t ₁ , by a predetermined temperature amount ΔT to a lower temperature T ₂ . The temperature amount .DELTA.T was previously set by an operator by a corresponding input to the control unit 12 to "medium", so that the temperature T ₂ ensures a medium strong cooking on a court in the cooking container 8 is. At time t ₃ , the temperature T _{2 is} reached and the power P (t) of the inductor 4 is increased to a somewhat increased power value P ₂ to maintain the temperature T (t) of the heating element 10 at the temperature T ₂ and the desired one Cooking the dish in the cooking tray 8 upright.

reference numeral

2

heater

4

inductor

6

support plate

8th

cooking containers

10

heating element

12

control unit

14

circuit

16

power source

u ₀

input voltage

u ₁

tension

i ₁

electricity

f

frequency

f _r

resonant frequency

f _M

measuring frequency

f _H1

heating frequency

f _H2

heating frequency

P

power

P _max

maximum power

P ₁

power value

P ₂

power value

P ₃

power value

L

inductance

T

temperature

T ₁

temperature

T ₂

temperature

.DELTA.T

temperature amount

p

electrical size

t

Time

t ₀

time

t ₁

time

t ₂

time

t ₃

time

t _x

time

Δt ₁

Measuring the time domain

Δt ₂

Heizzeitbereich

Δt _v

verification phase

G

limit

Claims (12)

A heating appliance (2) for an induction cooking appliance, comprising an inductor (4) for heating a heating element (10) of a cooking vessel (8) and a control unit (12) which is prepared to monitor the heating element (10) for reaching a cooking temperature , characterized in that the monitoring takes place by the adjustment of a frequency (f) of the alternating current flowing through the inductor (4) and the detection of at least one electrical variable (p) as a function of the time (t). Heating device (2) according to claim 1, characterized in that the electrical quantity (p) is correlated with the inductance (L) of a system comprising the inductor (4) and the heating element (10). Heating device (2) according to claim 1 or 2, characterized in that the control unit (12) for controlling the power (P) of the inductor (4) based on the electrical variable (p) is prepared. Heating device (2) according to one of the preceding claims, characterized in that the control unit (12) is prepared for keeping constant the frequency (f) during the detection in dependence on the time (t). Heating device (2) according to claim 4, characterized in that the control unit (12) is provided for determining measuring time ranges (Δt ₁ ) with at least one intermediate heating time range (Δt ₂ ), wherein the frequency (f) in the measuring time ranges (Δt ₁ ) is set at an identical measuring frequency (f _M ) and in the heating time range (Δt ₂ ) at a heating frequency different from the measuring frequency (f _M ) (f _H1 , f _H2 ). Heating device (2) according to one of the preceding claims, characterized in that the control unit (12) is provided for detecting an undershooting of the absolute value of the gradient of the electrical variable (p) below a limit value (G) and to a response of the control to the undershooting , Heating device (2) according to claim 6, characterized in that the reaction comprises a control of the power (P) of the inductor (4) for keeping constant the temperature (T) of the heating element (10). Heating device (2) according to claim 6 or 7, characterized in that the reaction is a control of the power (P) of the inductor (4) for reducing the temperature (T) of the heating element (10) by a predetermined amount of temperature (ΔT). Heating device (2) according to claim 8, characterized in that the temperature amount (ΔT) is selectable by an operator. Heating device (2) according to one of claims 6 to 9, characterized in that the control unit (12) for determining the limit value (G) in dependence on the time course of the electrical variable (p) is provided. Heating device (2) according to one of claims 6 to 10, characterized in that the control unit (12) is prepared for controlling a verification phase (Δt _V ) before the reaction and after the underrun. Method for controlling a heating device (2) for an induction cooking appliance, in which a heating element (10) of a cooking container (8) is monitored by a control unit (12) for reaching a cooking temperature, characterized in that the monitoring is controlled by setting a frequency (f ) of the alternating current flowing through an inductor (4) and the detection of an electrical quantity (p) as a function of time (t).

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