JPH07109794B2 - Induction heating cooker - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention relates to a material having a low skin resistance such as aluminum or copper and a material having a high skin resistance such as iron or stainless steel. The present invention relates to an induction heating cooker that heats an object to be heated.

(Prior art) An induction heating cooker supplies high-frequency power from an inverter to an induction heating coil, as is well known, and heats it by eddy current loss generated in an iron or stainless steel pan that is a material with high skin resistance. It is a configuration for cooking. In this one,
Since the number of turns of the induction heating coil is small and the frequency of the supplied high frequency power is low, the resistance of the induction heating coil itself is small. For this reason, in the unloaded state where the pot is removed or when small objects such as knives and forks are erroneously placed, the wetting magnetic flux increases rapidly and the resistance of the induction heating coil itself is small. The load current flowing through the induction heating coil suddenly increases. Therefore, the load current is used to detect no load or small objects, and the operation of the inverter is stopped.

(Problems to be Solved by the Invention) In recent years, it has been attempted to heat not only iron and stainless steel but also pots made of materials with low skin resistance, such as copper and aluminum, in induction heating cookers. There is. In order to heat a pan made of such a material having a low skin resistance, the number of turns of the induction heating coil may be increased and the output frequency of the inverter may be increased.
In this way, the resistance of the induction heating coil itself increases when the pan with low skin resistance is heated. Therefore, when no load is applied or when small items are placed by mistake, the wetting magnetic flux increases and the effective number of turns of the induction heating coil decreases,
Since the resistance of the induction heating coil itself is large, the load current flowing through the induction heating coil hardly increases. Therefore, when a pan made of a material having a low skin resistance is heated, there is a problem that no load or small objects cannot be detected due to the load current different from the case of a pan made of a material having a high skin resistance.

Therefore, an object of the present invention is to provide an induction heating cooker that can reliably detect a no-load state or a state in which a small object is erroneously placed instead of the object to be heated.

[Structure of the Invention] (Means for Solving the Problems) The present invention applies high-frequency power to a resonance circuit including an induction heating coil capable of switching the number of turns for heating an object to be heated and a resonance capacitor capable of switching the capacity. Equipped with an inverter to supply
In an induction heating cooker in which a material having a low skin resistance and a material to be heated in the skin resistance are respectively heated, the number of turns of the induction heating coil and the capacity of the resonance capacitor are set to the value of a material having a low skin resistance. First load detection means for detecting a load in which the load current of the resonance circuit is larger than a predetermined current value and the resonance frequency of the resonance circuit is smaller than a predetermined frequency value in a state of being switched for body use, and Second load detecting means for detecting a load based on the input voltage of the inverter and the load current when the number of turns of the induction heating coil and the capacitance of the resonance capacitor are switched to those for the object to be heated made of a material having a high skin resistance. Is provided, and the operation of the inverter is controlled by the output signals from the first and second load detecting means.

(Operation) First, when the number of turns of the induction heating coil and the capacity of the resonance capacitor are switched to those for the heated object made of a material having a low skin resistance, when the unloaded state or the state where a small object is placed, the wet magnetic flux Is increased, the load current of the resonance circuit is increased, and the resonance frequency is decreased. As a result, the load current becomes larger than the predetermined current value and the resonance frequency becomes smaller than the predetermined frequency value, so that the first load detecting means detects the no load or the small object as the load. In addition, when the number of turns of the induction heating coil and the capacity of the resonance capacitor are switched to those for the heated object made of a material with high skin resistance, the wetting magnetic flux increases when there is no load or a small object is placed. The load current of the resonant circuit increases according to the input voltage of the inverter. As a result, the second load detecting means can detect a no-load or a small object as a load.

(Example) Hereinafter, one example of the present invention will be described with reference to the drawings.

Reference numeral 1 is a commercial power supply, 2 is a bridge rectifier circuit having a thyristor 3, 4 is a smoothing capacitor, 5 is a voltage control circuit, and this is a DC output between both terminals of the smoothing capacitor 4 by controlling the conduction angle of the thyristor 3. It is designed to adjust the voltage. Here, the voltage control circuit 5 maintains the DC output voltage at a sufficiently low constant value when performing a load state determination operation described later, and then increases the voltage and shifts to the heating operation. Reference numeral 6 is an inverter, which is configured to switch two switching transistors 7 by an inverter drive circuit 8. The inverter 6 includes a resonance circuit 1 including first and second induction heating coils 9 and 10 that are capable of switching the number of turns, and first and second resonance capacitors 11 and 12 that are capable of switching capacitance.
It supplies high frequency power to 3. The induction heating coils 9 and 10 are arranged below a top plate (not shown), and a high-frequency magnetic field is applied to a heated object, for example, a pan 14 placed on the top plate, and the pan is caused by eddy current loss. It is designed to heat 14.

Reference numeral 15 is a relay switch of a changeover switch type in a relay which will be described later, and has a contact (ab) depending on the changeover state thereof.
By closing all the induction heating coils 9 and 10 and the resonance capacitors 11 and 12 of the resonance circuit 13, the contact (a
-C) It is possible to activate only the first induction heating coil 9 and the second resonance capacitor 12 by closing for the second time, and switch to a state in which the number of turns and the capacitance thereof are different. Here, the former state is suitable for heating the pan 14 made of a material having a low skin resistance such as aluminum or copper (hereinafter referred to as a low skin resistance material heating condition), and the latter state is a surface made of iron or stainless steel. It is suitable for heating the pot 14 of high resistance material (hereinafter referred to as high skin resistance heating condition). In this case, the number N of turns of the induction heating coil under the low skin resistance material heating condition is set to, for example, N = 65 turns, and the number of turns N of the induction heating coil under the high skin resistance material heating condition is, for example, N =
It is set for 15 turns. 16 is a phase comparison circuit,
This is a frequency command voltage of a voltage such that the potential Vl of the first induction heating coil 9 and the potential Vc of the second resonance capacitor 12 are compared and the phase difference between the potentials Vl and Vc is approximately 90 °. signal
Outputs Sf to the voltage controlled oscillator 17 (hereinafter referred to as VCO17). The VCO 17 oscillates at a frequency according to the frequency command voltage signal Sf and controls the inverter drive circuit 8, so that the inverter 6 is always in a phase difference state (resonance state) in which both potentials Vl and Vc have a phase difference of approximately 90 °. It is controlled to operate at a different frequency (resonance frequency). That is, the phase comparison circuit 16, VCO 17 and inverter drive circuit 8 are
Resonance control means for feedback-controlling the output frequency of the inverter 6 so as to be in a resonance state when the pan 14 is heated.
The frequency command voltage signal Sf, which constitutes 18 and is output from the phase comparison circuit 16, has a value proportional to its resonance frequency.

Now, 19 is a current detection circuit, which is provided with a current transformer 20 for detecting the load current of the resonance circuit 13, which is the output current of the inverter 6, and receives the detection current from this current transformer 20 to obtain this detection current. A detection signal S ₁₉ having a corresponding voltage is output. Reference numeral 21 denotes a material detection circuit, which receives the detection signal S ₁₉ from the current detection circuit 19 and makes the pot 14 of a material having a low skin resistance or a skin resistance by detecting whether the high material and the relay switch 15 causes the switching operation to drive a relay, not shown, and outputs a high skin resistance material detection signal S ₂₁ when it detects a high skin resistance material Therefore, the detection signal S ₂₁ is given to the voltage control circuit 5. Here, when the material detection circuit 21 detects the initial state when the power is turned on and the material with low skin resistance, the relay switch 15 is closed between the contacts (ab) as shown in FIG. When a high material is detected, the relay switch 15 is contacted (a-
It is adapted to switch to the closed state during c). 22 is a resonance frequency detection circuit, which receives the frequency command voltage signal Sf proportional to the resonance frequency from the phase comparison circuit 16,
The frequency command voltage signal Sf, that is, the detection signal S ₂₂ corresponding to the resonance frequency is output. 23 is a first load detecting means, which receives the detection signal S ₂₂ from the detection signal S ₁₉ and the resonance frequency detection circuit 22 from the current detection circuit 19, the load current is predetermined current value Is of the resonant circuit 13 When the resonance frequency of the resonance circuit 13 is higher than a predetermined frequency value Fs, for example, 35 KHz, a no-load small object detection signal S ₂₃ is output, and this is given to the inverter output stop circuit 24, for example. This inverter output stop circuit 24 uses the unloaded small object detection signal S ₂₃
In response to this, a stop signal S ₂₄ is given to the inverter drive circuit 8 to stop the operation of the inverter 6.

Further, 25 is a voltage detection circuit, which detects the potential Vi of the smoothing capacitor 4, that is, the input voltage Vi of the inverter 6, and outputs a detection signal S ₂₅ corresponding thereto. 26 is an overcurrent detection circuit diagram as a second load detection means. This is a voltage detection circuit 25.
Receiving a detection signal S ₁₉ from the detection signal S ₂₅ and the current detecting circuit 19 from the inverter output stop circuit 2 overcurrent detection signal S ₂₆ detects the overcurrent state based on these
Is supposed to give to 4.

Next, the operation of the above configuration will be described with reference to FIGS. 2 and 3.

First, the relationship between the material of the pan 14 and the load current and resonance frequency of the resonance circuit 13 will be described with reference to FIG. Figure 2 shows
The results of actually measuring the load current and the resonance frequency when the material of the pot 14 is iron, stainless steel, copper, and aluminum and in the unloaded state are shown. here,
Curves 27, 28 and 29 showing copper, aluminum and iron respectively
When the relacing switch 15 is closed between the contacts (ab) and is in a condition for heating a material having a low skin resistance (number of turns N = 65), a curve 30 and a curve 30 indicating iron and stainless steel, respectively.
31 is the case where the relay switch 15 is in the closed state between the contacts (ac) and is in the condition for heating the material with high skin resistance (number of turns N = 15). The three points on each curve indicate the case where the diameter D of the bottom of the pot 14 is D = 10φ, D = 16φ, D = 22φ, respectively. In FIG. 2, a point X is a relatively small item made of, for example, 1 to 2 spoons made of iron, a point Y is a relatively large item made of 4 to 5 spoons made of iron, and a point Z.
Shows the case where small articles made of, for example, copper or aluminum are placed on the top plate, respectively. Next, the operation of this embodiment will be described separately for the case of heating the pot 14 of each of the above materials.

(1) Pan made of material with low skin resistance such as copper or aluminum
When heating 14 First, the material detection operation is performed by the voltage control circuit 5 at a constant low output voltage to the inverter 6 under the low skin resistance material heating condition in which the relay switch 15 is closed between the contacts (ab). When the above is performed, since aluminum or the like has a small specific resistance, the input impedance of the induction heating coils 9 and 10 becomes small and the load current becomes large. Therefore, since the load current becomes larger than the predetermined current value Is, in this case,
In the initial state, the relay switch 15 has the contact (a-
Since the switch 15 is in the closed state during b), the output voltage of the inverter 6 is increased by the voltage control circuit 5 without switching the switch 15, and normal heating is performed.

(2) When detecting no load or small objects X and Z When the material detection operation is performed under low skin resistance material heating conditions, the unloaded pan 14 is removed as in this case, as described above. When the small object X made of one or two spoons made of iron or the small object Z made of aluminum is placed, the input impedance of the induction heating coils 9 and 10 becomes small and the load current becomes large. Therefore, the load current becomes larger than the predetermined current value Is. In this case, since the relay switch 15 is already closed between the contacts (ab) as described above, the switch 15 is switched. There is no such thing. However, at this time, as shown in FIG. 2, since the resonance frequency of the resonance circuit 13 becomes low, the resonance frequency becomes smaller than the predetermined frequency value Fs, which is 35 KHz. Small load detection signal S
₂₃ is provided to the inverter output stop circuit 24. As a result, the stop signal S ₂₄ is given from the inverter output stop circuit 24 to the inverter drive circuit 8, whereby the operation of the inverter 6 is stopped. After the inverter 6 is stopped, initialization is performed.

(3) When detecting a small item Y and when heating the pan 14 made of a material having a high skin resistance such as iron or stainless steel. As described above, when the material detection operation is performed under the low skin resistance material heating condition, In the case of an iron spoon 4 ~
The induction heating coil is used when a relatively large small item Y consisting of 5 pieces or the like is placed and when a pot 14 made of iron or the like is placed.
The input impedance of 9, 10 increases and the load current decreases (see Fig. 2). Therefore, since the load current becomes smaller than the predetermined current value Is, the material detection circuit 21 receiving the detection signal S ₁₉ from the current detection circuit 19 switches the relay switch 15 between the contacts (ac) to close. , Resonant circuit 13
Is the first induction heating coil 9 and the second resonance capacitor 12
Only the high-skin-resistive material heating conditions are enabled. In this case, the relationship between the input voltage of the inverter 6 and the load current is
Depending on the material of the pan 14 or the small items to be placed, it will be as shown in FIG. In FIG. 3, in the straight line, 32 is iron, 33 is a clay pot for induction heating, 34 is stainless steel, and 35 is a spoon (4 ~
5) are shown respectively. As is apparent from this, in the case of a small object such as a spoon, the load current sharply increases as the input voltage increases. Therefore, in the overcurrent detection circuit 26, as the threshold value of the load current, for example, a straight line 36 having a larger inclination than the straight line 34 indicating stainless steel is set, and when it exceeds this, the overcurrent detection signal S ₂₆ is generated. Is output.

When the material detecting circuit 21 supplies the high skin resistance material detecting signal S ₂₁ to the voltage controlling circuit 5, the voltage controlling circuit 5
Controls the bridge rectifier circuit 2 so that a low voltage of, for example, about 20 V is applied to the inverter 6. This allows
The overcurrent detection circuit 26 is supplied with a detection signal S ₂₅ corresponding to the input voltage of the inverter 6 from the voltage detection circuit 25, reads the threshold value of the overcurrent corresponding thereto from the straight line 36, and further detects the current detection circuit. Detection signal corresponding to load current from 19
Compare S ₁₉ with the previously read threshold value. When the spoon (4 to 5 pieces) is placed on the top plate, the load current exceeds the threshold value, so the overcurrent detection circuit 26 outputs the overcurrent detection signal S _26. Are supplied to the inverter output stop circuit 24, and the operation of the inverter 6 is stopped. Further, when the pan 14 made of a material having a high skin resistance such as iron or stainless steel is placed on the top plate, the load current does not exceed the threshold value. It does not output the current detection signal S _26. Therefore, the voltage control circuit 5 appropriately raises the output voltage of the inverter 6 to perform normal heating.

According to this embodiment having such a configuration, the following effects can be obtained. That is, when the load current of the resonance circuit 13 is larger than the predetermined current value and the resonance frequency of the resonance circuit 13 is smaller than the predetermined frequency value under the low skin resistance material heating condition, the first load detecting means 23 does not detect. Small load detection signal
Since outputs the S ₂₃ was configured to stop the operation of the inverter 6, it is possible to detect a relatively small accessories placed incorrectly on the top plate in the low skin resistance material heating condition. Moreover, in this case, since it is not necessary to switch the number of turns and the capacitance of the induction heating coils 9 and 10 and the resonance capacitors 11 and 12, the relay switch 15 does not unnecessarily switch and a switching sound is generated. Can be prevented and the life of the relay switch 15 can be extended. Further, in the state of switching to the high skin resistance material heating condition, when the load current of the resonance circuit 13 exceeds the threshold value, the overcurrent detection circuit 26 outputs the overcurrent detection signal S ₂₆ . Even if a relatively large object such as 4 to 5 iron spoons is erroneously placed on the top plate, the operation of the inverter 6 can be stopped by detecting this.

In the above embodiment, in the overcurrent detection circuit 26, the threshold value is set as shown by the straight line 36, but the value may be appropriately set as necessary, and in this case, the same operational effect can be obtained. Obtainable.

In addition, the present invention is not limited to the embodiments described above and shown in the drawings. For example, various types of inverters may be used, or similar sequence control may be performed by a microcomputer. Various modifications can be made without departing from the spirit of the invention, such as a gate array or the like.

[Effects of the Invention] As is apparent from the above description, the present invention allows the load current of the resonant circuit to be set to a predetermined value in a state where the number of turns of the dielectric heating coil and the capacitance of the resonant capacitor are switched to the heating action of a material having a low skin resistance. First load detection means for detecting a load that is higher than the current value and the resonance frequency of the resonance circuit is lower than a predetermined frequency value is provided, and the number of turns of the induction heating coil and the capacitance of the resonance capacitor are set as skin resistance. Since the second load detecting means for detecting the load based on the input voltage of the inverter and the load current is provided in the state of switching to the object to be heated of high material, instead of the unloaded state or the object to be heated. This has an excellent effect that it is possible to reliably detect a state in which a small object is erroneously placed.

[Brief description of drawings]

The drawings show one embodiment of the present invention. FIG. 1 is a block diagram of the entire electrical configuration, and FIG. 2 is a characteristic diagram showing the relationship between the material of the object to be heated and the load current and resonance frequency. FIG. 3 is a characteristic diagram showing the relationship between the material of the object to be heated and the load current and input voltage. In the drawing, 6 is an inverter, 9 and 10 are induction heating coils, and 11 and 1.
Reference numeral 2 is a resonance capacitor, 13 is a resonance circuit, 14 is a pot (object to be heated), 23 is a first load detection means, and 26 is an overcurrent detection circuit (second load detection means).

Claims (1)

[Claims] 1. A material having a low skin resistance and a skin resistance comprising an inverter for supplying high frequency power to a resonance circuit comprising an induction heating coil capable of switching the number of windings for heating an object to be heated and a resonance capacitor capable of switching the capacity. In an induction heating cooker that heats a heated object made of a high-quality material, the resonance is performed in a state in which the number of turns of the induction heating coil and the capacitance of the resonance capacitor are switched for the heated object made of a material having a low skin resistance. First load detection means for detecting a load in which the load current of the circuit is higher than a predetermined current value and the resonance frequency of the resonance circuit is lower than the predetermined frequency value, the number of turns of the induction heating coil, and the capacitance of the resonance capacitor. Second load detecting means for detecting a load based on the input voltage of the inverter and the load current when the device is switched to a heated object made of a material having a high skin resistance. Provided, the induction heating cooker and controlling the operation of said inverter by the output signals from the first and second load detecting means.