JPS63198283A - Induction heating cooker - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial Application Field) The present invention relates to an induction heating cooker that heats a heated object based on eddy current loss.

(Prior Art) Conventionally, in an induction heating cooker, an induction heating coil is disposed below a top plate on which an object to be heated, such as a pot, is placed, and high-frequency power is applied by an inverter to a resonant circuit consisting of the induction heating coil and a resonant capacitor. By supplying
A high-frequency magnetic field is applied to the pot, which generates eddy currents in the pot and generates heat.

In such an induction heating cooker, the electric current flowing through the induction heating coil and the eddy current generated in the pot, which is the object to be heated, are substantially in opposite phases, so that the pot is subjected to a repulsive action. Here, if the pot is made of iron and isomagnetic material, the pot will be attracted by the magnetic force because of its high relative magnetic permeability, and the repulsive force acting on the pot will eventually be small.

However, if the pot is made of a non-magnetic material such as aluminum, the relative magnetic permeability is low and the skin resistance is small, so the attraction force due to magnetic force is small. In order to make the pot equal to The pot will lift off the top plate and the pot will move on the top plate, which is very dangerous.

Incidentally, FIG. 5 shows the relationship between the output [W] of the inverter and the repulsive force [g] against the pot. In this case, the pot is made of aluminum, the number of turns of the induction heating coil is 80T (turns), and the frequency is 50KHz. As can be seen from FIG. 5, the repulsive force increases in proportion to the magnitude of the output of the inverter, and the greater the output, the stronger the tendency for the pot to rise above the top plate.

Therefore, by utilizing the phenomenon shown in FIG. 5, it is possible to detect the floating of the pot by the following method. In other words, when the inverter's output is changed from low to high and the pot floats up, the gap between the induction heating coil and the pot increases, the equivalent inductance increases by △, and the resonant frequency of the resonant circuit increases. Decrease. In this case, assuming that the capacitance of the resonant capacitor of the resonant circuit is C, the decrease width Δf is Δf=1/(2πJ]knee r -). In particular, the equivalent inductance of a pan made of non-magnetic material, where the floating phenomenon is noticeable, changes more than that of a pan made of magnetic material, so the range of change in the resonant frequency is also large, and therefore the inverter's low output By sampling the resonant frequency at high output to obtain a reference value and detecting the change in the resonant frequency at high output with respect to this value, even the slightest lifting of the pot can be detected with high accuracy.

(Problem to be solved by the invention) However, when the temperature of the pot increases by 1 due to heating, the resistivity and relative permeability of the pot gradually change and the equivalent inductance decreases, so the resonant frequency of the resonant circuit decreases. A phenomenon occurs in which the value gradually increases. Therefore, even if, for example, water in the pot evaporates while the pot is being heated by the high output of the inverter, the total weight of the pot and the food to be heated becomes lighter and the pot floats up, the resonance frequency will drop due to floating. There is a problem in that the increase in the resonant frequency due to the rise in temperature causes an error, and the sensitivity in detecting the rise of the pot deteriorates.

In order to solve this problem, while the inverter is heating up due to its high output, the output of the inverter is temporarily changed to low at intervals of several seconds, and the resonant frequency at that low output is sampled to determine the reference value. It is also possible to update it.

However, in this case, the inverter frequently repeats high output and low output, which causes a new problem in that the heating capacity decreases and adversely affects the power supply, such as causing one fluorescent lamp to flicker.

The present invention has been made in view of the above circumstances, and its purpose is to be able to reliably detect when a heated object floats up, to control the output of an inverter, and to prevent a decrease in heating capacity. To provide an induction heating cooker which can prevent the above-mentioned problems and does not adversely affect the power supply.

[Structure of the Invention] (Means for Solving the Problems) The induction heating cooker of the present invention is provided with an inverter output control means for changing the inverter from high output to short-time low output at a predetermined cycle, and The resonant frequency of the resonant circuit consisting of an induction heating coil and a resonant capacitor that supplies high-frequency output is sampled when the inverter's output is low, and thereafter the reference value is obtained by sampling at a shorter period than the above-mentioned period. The present invention is characterized by a structure in which frequency change detection means is provided to control the output of the inverter when the resonant frequency becomes below this reference value.

(Function) According to the induction heating cooker of the present invention, when the inverter output control means changes the inverter's output from high output to low output for a short period of time, the frequency change detection means detects when the inverter's output is low. Of course, even at subsequent high outputs, the reference value is obtained by sampling the resonant frequency of the resonant circuit at a cycle shorter than the above-mentioned cycle, so the reference value obtained by sampling is based on the resonance according to the temperature rise of the heated object. It is changed according to the change in frequency, and therefore, even when the heated object is being heated with high output, it is possible to detect the floating of the heated object.

(Example) An embodiment of the present invention will be described below with reference to the drawings.

First, the overall electrical configuration will be described according to FIG. 1 is a DC power supply circuit, which is composed of a rectifier circuit 6 having thyristors 2, 3 and diodes 4, 5, and a smoothing circuit 9 having a smoothing reactor 7 and a capacitor 8. Smoothing is performed by a rectifying and smoothing circuit 9. Reference numeral 11 denotes a resonant circuit consisting of an induction heating coil 12 and a resonance capacitor 13 for heating an object to be heated. A high-frequency magnetic field is applied to the pot 14, which is a heating element, and the pot 14 is heated by the resulting eddy current loss.

Reference numerals 15 and 16 designate switching transistors as switching elements, which are connected to the resonant circuit 11 as shown in the figure to constitute an inverter 17, and the inverter 17 is connected to the resonant circuit 11 from the DC power supply circuit 1. Power is supplied. The inverter 17 is driven by an inverter drive circuit 18 to be described later, and the induction heating coil 12
supplies high-frequency power to the Reference numeral 19 denotes an inverter control circuit, which includes a phase comparator circuit 20, a voltage controller φ oscillator (V.0.C) 21, and an inverter drive circuit 18. The phase comparison circuit 20 uses the collector voltage of the switching transistor 16 and the capacitor 13.
It outputs a control voltage Vf with a phase difference of 90 degrees by inputting the terminal voltage of
The oscillator 21 receives this control voltage Vf, oscillates at a frequency corresponding to the voltage, and provides the oscillation output signal to the inverter drive circuit 18, which in turn controls the transistors 15 and 16 in accordance with the oscillation output signal. supplying base current to turn them on and off alternately,
Thus, the output frequency of the inverter 17 is feedback-controlled so that the resonant circuit 11 has a resonant frequency with respect to the pot 14, which is the object to be heated. 22 is an inverter output control circuit which is an inverter output control means for controlling the output of the inverter 17, which changes the output voltage of the DC power supply circuit 1 by controlling the phase of the Cyrisk 2.3,
Thus, the output of the inverter 17 is controlled.

This inverter output control circuit 22 receives a timing signal St from a timing circuit 23 as described later, and when the timing signal St is at a high level, the output of the inverter 17 is set to a low level, and when it is at a low level, the output of the inverter 17 is set to a low level. The phase of the thyristor 2.3 is controlled so that the output is high. 2
Reference numeral 4 denotes a frequency change detection circuit serving as a frequency change detection means, which detects whether the pot 14 floats or not.

The configurations of the timing circuit 23 and frequency change detection circuit 24 will be described below with reference to FIG.

First, in the timing circuit 23, 25 is an oscillator that outputs a high-level oscillation pulse Pa (for example, pulse width 500 ms) with a short period T2 (for example, 2 seconds), and the oscillation pulse Pa is connected to the clock terminal of the counter circuit 26. It is now given to CK. This counter circuit 26 outputs an oscillation pulse Pa to the clock terminal CK.
It performs a counting operation every time a signal is given, and outputs the count contents to 4-bit output terminals Ql, Q2 . It is designed to output high level (rlJ) and low level (rOJ') binary encoded signals to Q3 and Q4. 27 is an AND circuit, which has two non-inverting input terminals and two inverting input terminals, and the two non-inverting input terminals receive the oscillation pulse Pa of the oscillator 25 and the signal of the output terminal Q1 of the counter circuit 26. is provided, and signals from output terminals Q2 and Q3 of the counter circuit 26 are provided to the two inverting input terminals. The signal from the output terminal Q4 is applied to the reset terminal R of the counter circuit 26 via the OR circuit 28. Now, the frequency change detection circuit 24 will be described. 30 is a buffer 31. This is a hold circuit including an analog switch 32 and a capacitor 33, and the buffer 31 includes the phase comparison circuit 2.
The control voltage Vf output from 0 is manually input, and the voltage level of this control voltage Vf is determined by the resonant circuit 11.
is proportional to the resonant frequency of The analog switch 32 is turned on only during the period when the high-level oscillation pulse Pa from the oscillator 25 of the timing circuit 23 is applied. Therefore, when the analog switch 32 is turned on, the buffer 31
The sampling voltage Vs output from the phase comparator circuit 2
In other words, the control voltage Vf from 0 has a voltage level corresponding to the resonant frequency of the resonant circuit 11, and this sampling voltage Vs is held by the capacitor 33 as the sampling frequency. The sampling voltage Vs is applied to a detection reference value setting circuit 35 via a buffer 34.

This detection reference value setting circuit 35 includes a diode 36. resistance 3
The reference voltage vh, which is the detection reference value of this detection reference value setting circuit 35, is Vh-Vs-Vdf. Here, Vdf-Vf fist (R36/ (R3B+R3
9) IVf...Forward voltage R38 of diode 36,
R39: Each resistance value of resistor 38.39. The voltage Vdf is a predetermined value corresponding to the range of change in the resonance frequency based on the sampling voltage Vs. 40 is a comparator, the non-inverting input terminal (+) of which is supplied with the reference voltage vh, and the inverting input terminal (-) of which is supplied with the control voltage Vf from the phase comparison circuit 2o. Therefore, this comparator 40 detects that the control voltage Vf is detected by the detection reference value setting circuit 3.
In other words, when the reference voltage vh from 5 becomes lower than the reference voltage vh, the change width (Vs-Vf') of the resonance frequency reaches the predetermined value (Vdf).
When this happens, it is detected that the pot 14 has floated, and a high-level floatation detection signal Se is output. The floating detection signal Se is applied to an inverter output stop circuit 41 serving as a stop means and the inverter output control circuit 22. When this inverter output stop circuit 41 receives the floating detection signal Se, it outputs an output stop signal to the inverter drive circuit 18 and outputs an output stop signal to the inverter drive circuit 18.
The high-level output stop signal from this inverter output stop circuit 41 is sent via the OR circuit 28. It is also applied to the reset terminal R of the counter circuit 26. Further, when the inverter output control circuit 22 receives the high-level floating detection signal Se, it stops supplying gate signals to the thyristors 2 and 3, thereby stopping the rectification operation of the rectification circuit 6, and also stops the rectification operation of the rectification circuit 6 (not shown). forms a discharge circuit for the smoothing capacitor 8.

Next, the operation of this embodiment will be explained with reference to FIGS. 3 and 4.

When the cooking operation is started by turning on the power, first, the oscillator 25 of the timing circuit 23 generates an oscillation pulse Pa [Fig.
C) and FIG. 4(c)], and the count content of the counter circuit 26 becomes "1" by the first oscillation pulse Pa, and the output terminal Q4. Q3+
Q2. Ql becomes "0001". As a result, the timing signal St from the AND circuit 27, which is the output signal of the timing circuit 23, becomes a high level [see FIGS. 3(d) and 4(d)], and in response, the inverter output control circuit 22 The output of the inverter 17 is low [Figure 3 (
A) and (b) Phase control of the thyristors 2 and 3 is performed so as to become the reference phase.

(1) When the pot 14 is made of a magnetic material such as iron or the pot 14 itself is heavy (see FIG. 3). In this case, the first oscillation pulse Pa from the oscillator 25 is also given to the analog switch 32. , the sampling voltage Vs corresponding to the resonant frequency fo of the resonant circuit 11 at that time, that is, when the inverter 17 is at low output, is held in the capacitor 33. And this sampling voltage Vs
is the reference voltage Vh (frequency reference value f) by subtracting the voltage Vdf (frequency fs) which is a predetermined value for the frequency change width.
oo) as the non-inverting input terminal (+) of the comparator 40
is input. In this low output state, the comparator 40
The current control voltage Vf is at the inverting input terminal (-) of
(resonant frequency fO) is input, the floating detection signal Se from the comparator 40 is at a low level. Next, when the first oscillation pulse Pa from the oscillator 25 of the timing circuit 23 disappears, the timing signal St of the AND circuit 27 becomes low level, and the inverter output control circuit 22 controls the phase of the thyristors 2 and 3 to output the inverter 17. is set to a predetermined high output [see FIG. 3(a)], and the analog switch 32 is turned off as the oscillation pulse Pa disappears. Here, since the pot 14 placed on the top plate is made of magnetic material,
A relatively large suction force acts on the pot 14, and as a result, the repulsive force that the pot 14 receives is small and the pot 14 does not float. Moreover, since the change in equivalent inductance of the pot 14 at this time is small, the change width (Vs - Vf) of the resonant frequency f in the resonant circuit 11 is a predetermined value (Vd
f) less than. Therefore, the control voltage Vf at this high output never becomes lower than the reference voltage vh, and the comparator 40 keeps its output signal, the floating detection signal Se, at a low level. Therefore, the output of the inverter 17 remains at the predetermined high output, and induction heating continues. After that, when the oscillator 25 outputs the second oscillation pulse Pa, the count content of the counter circuit 26 becomes "2" and the output terminal Q41 Q3. Q+! , Ql is “0
010'', and the timing signal St remains at the low level. Also, since the second oscillation pulse Pa is applied to the analog switch 32 and turns it on, the capacitor 33 samples and holds the control voltage Vf at that time, and therefore the reference voltage vh is updated. It turns out. Similarly, the reference voltage vh is updated every time the oscillator 25 outputs the oscillation pulse Pa, and as shown in FIG. Resonant frequency f. Even if the frequency becomes larger gradually, the frequency reference value foO follows this step by step and becomes larger. After that, when the oscillator 25 outputs the eighth oscillation pulse Pa and the count content of the counter circuit 26 becomes "8", the output terminal Q of the counter circuit 26
a.Q3+Ql! +Ql becomes rloooJ, and the high level signal at its output terminal Q4 is applied to the reset terminal R via the OR circuit 28. As a result, the counter circuit 26 is reset so that the count becomes "0". Then, the oscillator 25 next generates one oscillation pulse Pa
, the count content of the counter circuit 26 becomes "1" again, and the output terminals Qa, Q3 . Q2. Since Qt becomes ro OOIJ, the timing signal St from the AND circuit 28 becomes high level, and the output of the inverter 17 becomes low again. The subsequent operation is the same as described above, and the inverter output control circuit 22 changes the output of the inverter 17 from high output to low output for a short time (500 m5) at a period of 16 seconds, which is the predetermined period Tl, and at the same time changes the output from the inverter 17 to a low output for a short period of time (500 m5). Cycle period T is 2s
A frequency reference value f is obtained by sampling the resonance frequency fO (control voltage Vf) at a period of ec (including sampling at low output, of course).

6 (reference voltage vh) will be updated.

It goes without saying that even if the pot is made of a non-magnetic material and is heavy, the pot 14 will not float up, and the same operation will occur.

(2) When the pot 14 is made of non-magnetic material such as aluminum and the weight of the pot I4 itself is small (see Fig. 4) In this case, as described above, the first oscillation pulse Pa from the oscillator 25 is Since it is also applied to the analog switch 32, the capacitor 33 is supplied with
7, the resonant frequency f of the resonant circuit 11 at low output. ′
The sampling voltage Vs corresponding to the current value is held. Then, this sampling voltage Vs is inputted to the non-inverting input terminal (+) of the comparator 40 as a reference voltage vh (frequency reference plane foo-:1) after subtracting the voltage Vdf (frequency fs) which is a predetermined value for the frequency change width. In this low output state, the current control voltage Vf (
Since the resonant frequency fo'') is input -18=, the floating detection signal Se from the comparator 40 is at a low level. Next, when the first oscillation pulse Pa disappears and the timing signal St becomes low level, the inverter output control circuit 22
Let the output be a predetermined high output [see FIG. 4(a)]. Here, even if the weight of the pot 14 itself is small, if the total weight including the food to be heated is large, the pot 14 will not float even if it receives a repulsive force. Therefore, the control voltage Vf at this high output never becomes lower than the reference voltage vh, and the floating detection signal Se remains at a low level. Thereafter, when the oscillator 25 sequentially outputs the oscillation pulse Pa, the capacitor 33 sequentially outputs the control voltage Vf at that time as described above.
Now it is possible to sample and hold the
As shown in b), the frequency reference value f follows stepwise the change in the resonance frequency f, - as the temperature of the pot 14 rises.

0'' will also change.Thus, such a pot 1
During the heating with the high output of 4, as shown in FIG. 4(b), for example, the reference voltage vh (frequency reference value f.o-) is set based on the 6-19 = th oscillation pulse Pa from the oscillator 25. Immediately after updating, the total weight including the pot 14 decreases due to evaporation of the water in the pot 14, and the pot floats up, then the control voltage Vf (resonant frequency fo-)
becomes small rapidly.

In this case, the reference voltage Vh (frequency reference value foo ″
) is the control voltage V as the temperature rises due to heating of the pot 14.
Since the control voltage Vf, which has become smaller due to the floating of the pot 14, becomes lower than the reference voltage vh, the comparator 40 detects the floating of the pot 14 because it is updated to increase following the change in f (resonant frequency f.'). The signal Se is set to high level. As a result, the inverter output stop circuit 41 outputs an output stop signal to the inverter drive circuit 18 to instantly stop the output of the inverter 17, and the inverter output control circuit 22 stops the rectification operation of the rectifier circuit 6. let Of course, at this time, the capacitor 8 is also discharged. Then, the inverter output stop circuit 41 also applies a high-level output stop signal to the reset terminal R of the counter circuit 26 via the OR circuit 28, so the counter circuit 26 is reset. Note that when the output of the inverter 17 is stopped in this way, the oscillator 25 also stops its oscillation operation. This causes the cooking operation to be interrupted. Therefore, if the total weight is increased by, for example, replenishing water in the pot 14, and the operation is restarted after a predetermined time,
The oscillator 25 starts oscillating again, and the same operation as described above is performed.

As described above, according to this embodiment, the output of the inverter 17 is changed from high output to low output for a short time (for example, 500 m5) at a predetermined period T1 (for example, 16 seconds), and at a period T2 ( For example, 2 seconds
), the resonant frequency f of the resonant circuit 11 including the low output time
A frequency reference value f is obtained by sampling the control voltage Vf corresponding to O+fO-. A reference voltage vh corresponding to 0+foo- is obtained, and when the control voltage Vf becomes equal to or less than the reference voltage vh, that is, the resonance frequency f. , fo − is the frequency reference value fO
Since the output of the inverter 17 is stopped when the temperature becomes O+ fo O- or less, even if the pot 14 floats to the surface when the inverter 17 has a high output, this can be reliably detected regardless of whether the temperature of the pot 14 has risen. Therefore, even if the cycle T1 for changing the output of the inverter 17 to a low output is made large, the floating of the pot 14 can be detected reliably, so that it is possible to prevent the heating ability from decreasing. At the same time, it is possible to prevent adverse effects on the power supply, such as flickering of fluorescent lamps.

In the above embodiment, the output of the inverter 17 is stopped when the floating of the pot 14 is detected, but instead, the output of the inverter 17 is changed from high output to slightly lower than this via the inverter output control circuit 22. The operation may be continued by controlling the output so that the output is maintained.

In addition, the present invention is not limited to the embodiments described above and shown in the drawings; for example, various types of inverters can be adopted, and each circuit can be implemented entirely or partially by a microcomputer. It goes without saying that the present invention may be modified and implemented as appropriate without departing from the scope of the invention, such as by being configured as follows.

[Effects of the Invention] As explained above, in the induction heating cooker of the present invention, the cycle for sampling the resonant frequency and obtaining the reference value is smaller than the cycle for changing the output of the inverter from high output to low output. Even if the object to be heated floats up during heating by the high output of the inverter, this can be reliably detected and the output of the inverter can be controlled, and furthermore, it is possible to prevent the heating capacity from decreasing, and also to reduce the power supply. It has an excellent effect with no adverse effects.

[Brief explanation of the drawing]

The drawings show one embodiment of the present invention, and FIG. 1 is an overall electrical configuration diagram, FIG. 2 is a specific electrical configuration diagram of a timing circuit and a frequency change detection circuit, and FIGS. FIG. 5 is a signal waveform diagram of each part for explaining the operation, and is a diagram showing the relationship between the output of the inverter and the repulsive force. In the drawing, 1 is a DC power supply circuit, 11 is a resonance circuit, 12 is an induction heating coil, 13 is a resonance capacitor, 14 is a pot (heated object), 17 is an inverter, 19 is an inverter control circuit, and 22 is an inverter output control circuit. (Inverter output control means), 23 is a timing circuit, 24 is a frequency change detection circuit (frequency change detection means), and 41 is an inverter output stop circuit.

Claims (1)

[Claims] 1. An inverter that supplies high-frequency power to a resonant circuit consisting of an induction heating coil and a resonant capacitor for heating the object to be heated, and an inverter output control that changes this inverter from high output to short-term low output at a predetermined cycle. and sampling the resonant frequency of the resonant circuit when the output of the inverter is low and thereafter sampling at a shorter period than the period to obtain a reference value so that the resonant frequency of the resonant circuit becomes equal to or lower than the reference value. and frequency change detection means for controlling the output of the inverter when the inverter is turned on.