JP2633546B2 - Induction heating cooker - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial application field) The present invention relates to an induction heating cooker capable of heating a material to be heated made of a material having a low skin resistance, for example, aluminum or copper.

(Conventional technology) As is well known, an induction heating cooker supplies high frequency electric power from an inverter to an induction heating coil and generates an overcurrent loss in an iron or a stainless steel pot as a material to be heated having a high skin resistance. This is a configuration for performing heating cooking. By the way, recent induction heating cookers have attempted to be able to heat not only iron and stainless steel but also pans made of a material having low skin resistance such as copper and aluminum. In order to heat such a pot made of a material having a low skin resistance, the number of turns of the induction heating coil is increased, and the output frequency of the inverter is increased.

(Problems to be Solved by the Invention) In the above-described conventional configuration, the number of turns of the induction heating coil is increased and the output frequency of the inverter is increased in order to heat a pot made of a material having a low skin resistance. The resistance of the induction heating coil itself is greater than when a high quality pot is heated. For this reason, when heating a pan with a small bottom area, the leakage flux increases, the effective number of turns of the induction heating coil decreases, and even if the equivalent input resistance of the pan decreases, the current flowing through the induction heating coil almost increases. It will be in a state where it does not. Accordingly, the induction heating coil has a problem that the loss is sharply increased and the temperature rises, so that the coil may be damaged, and it is not possible to actually heat the pan having a small bottom area. .

Therefore, an object of the present invention is to provide an induction heating cooker that can heat a relatively small object to be heated when heating the object to be heated having a low skin resistance.

[Structure of the Invention] (Means for Solving the Problems) An induction heating cooker according to the present invention provides a high-frequency circuit in a resonance circuit of an induction heating coil and a resonance capacitor for heating an object to be heated having a low skin resistance. In addition to providing an inverter for supplying power, a frequency detection unit for detecting a resonance frequency of the resonance circuit is provided, and an inverter output adjustment unit for reducing the output of the inverter when the resonance frequency detected by the frequency detection unit is low is provided. However, it has features.

(Function) When the object to be heated is made of a material having a low skin resistance such as aluminum or copper, for example, when a relatively small object to be heated is heated, leakage is larger than when a large object to be heated is heated. The magnetic flux increases, the equivalent input impedance of the induction heating coil increases, and the resonance frequency of the resonance circuit decreases. This makes it possible to distinguish that a small object to be heated is being heated by the detection frequency from the frequency detection means. Therefore, when the resonance frequency detected by the frequency detecting means by the inverter output adjusting means is low, the output of the inverter is reduced, thereby preventing the temperature of the induction heating coil from being increased, and thus preventing damage. Be able to heat.

(Embodiment) An embodiment of the present invention will be described below with reference to FIG. 1 and FIG.

1 is a commercial power supply, 2 is a bridge rectifier circuit provided with a thyristor 3, 4 is a smoothing capacitor, 5 is an inverter output adjusting means, for example, a voltage control circuit.
The DC output voltage between the two terminals of the smoothing capacitor 4 is adjusted by controlling the conduction angle of. Here, the voltage control circuit 5 keeps the DC output voltage at a sufficiently low constant value when performing a load state determination operation to be described later, and then increases the voltage to shift to the heating operation. Reference numeral 6 denotes an inverter configured so that two switching transistors 7 are switched by an inverter driving circuit 8, which comprises first and second induction heating coils 9, 10 and first and second induction heating coils 9, 10.
A high-frequency power is supplied to the resonance circuit 13 including the resonance capacitors 11 and 12. The induction heating coils 9 and 10 are arranged below a top plate (not shown), and apply a high-frequency magnetic field to the object to be heated, for example, the pot 14 placed on the top plate, and the pot is heated by overcurrent loss.
14 is designed to heat.

Reference numeral 15 denotes a changeover switch, and the induction heating coils 9 and 10 and the resonance capacitors 11 and 1 of the resonance circuit 13 are switched according to the switching state.
2 and the first induction heating coil 9
In addition, only the second resonance capacitor 12 is made effective, and it is possible to switch to a state in which the number of turns and the capacity thereof are different. Here, the former condition is suitable for heating the pot 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 condition is a skin such as iron or stainless steel. Suitable for heating a pot 14 of a material having a high resistance (hereinafter referred to as a high skin resistance material heating condition). In this case, the number of turns N 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 set.
Is set, for example, to N = 15 returns. Reference numeral 16 denotes a phase comparison circuit, which is the potential of the first induction heating coil 9.
Vl and the potential Vc of the second resonance capacitor 12 are compared, and a frequency command voltage signal Vf of a voltage such that the phase difference between the two potentials Vl and Vc is substantially 90 ° is obtained by the voltage controlled oscillator 17 ( (Hereinafter referred to as VCO17). The VOC 17 oscillates at a frequency corresponding to the frequency command voltage signal Vf to control the inverter drive circuit 8, so that the inverter 6 always has a state where both potentials Vl and Vc have a phase difference of approximately 90 ° (resonance state). Is controlled to operate at an appropriate frequency (resonant frequency). That is, the phase comparison circuit 16, the VCO 17, and the inverter drive circuit 8 constitute a resonance control unit 18 that performs feedback control so that the output frequency of the inverter 6 is in a resonance state when the pan 14 is heated. The frequency command voltage signal Vf output from the comparison circuit 16 has a value proportional to the resonance frequency.

Now, reference numeral 19 denotes frequency detection means, which is a frequency command voltage signal proportional to the resonance frequency from the phase comparison circuit 16.
Receiving vf, and outputs a detection voltage signal S ₁₉ corresponding thereto. Here, the voltage control circuit 5 operates under the heating condition of the low skin resistance material, and
Receives the detection voltage signal S ₁₉ from, the detected voltage signal S ₁₉
For example, the thyristor of the bridge rectifier circuit 2 reduces the voltage across the smoothing capacitor 4, that is, the input voltage of the inverter 6, in response to the decrease in the resonance frequency of the resonance circuit 13. 3 is controlled. On the other hand, reference numeral 20 denotes a current detecting means, which includes a current transformer 21 for detecting an output current of the inverter 6, receives a detection current from the current transformer 21, and detects a detection voltage signal S corresponding to the detection current. ₂₀ is output. 22 is a load condition determining means, which, as is apparent from the action described later, the detection voltage signal S ₁₉ from the frequency detector 19, the detected voltage signal S from the current detector 20
In response to _20, and outputs a low skin resistance material detection signal S ₂₂ a or the high skin resistance material detection signal S ₂₂ b to the constant switching circuit 23. Here, the constant switching circuit 23 is intended for switching operation of the change-over switch 15, to indicate the switch 15 in FIG. 1 when having received the initial state and a low skin resistance material detection signal S ₂₂ a power-on to the contact (a-b) between closed and when subjected to high skin resistance material detection signal S ₂₂ b switches the changeover switch 15 to the contact (a-c) between a closed state.

 Next, the operation of the above configuration will be described with reference to FIG.

First, the relationship between the material of the pot 14 and the inverter current, which is the output current of the inverter 6, and the resonance frequency of the resonance circuit 13 will be described. FIG. 2 shows that the material of the pot 14 is iron, stainless steel,
FIG. 9 shows the results of actually measuring the inverter current and the resonance frequency when copper and aluminum are used and when there is no load. Here, curves La and Lb representing copper and aluminum, respectively, indicate that the changeover switch 15 has the contacts (ab)
Curves Lc and Ld, respectively, in the closed state and in the condition for heating the material with low skin resistance, and indicating iron and stainless steel, respectively.
The case where the changeover switch 15 is in the closed state between the contacts (ac) and under the condition for heating the high skin resistance material. The three points of each material are the diameter D of the bottom of the pot 14, D = 10φ, D
= 16φ and D = 22φ. As can be seen from the figure, when the pot 14 is made of a material having a low skin resistance such as aluminum or copper, the resonance frequency clearly changes according to the change in the diameter D of the pot bottom (in this case, the inverter current is Because it does not change much)
14 means that the size can be determined. The present invention focuses on such a point, and the operation of the present embodiment will be described separately for the case of heating the pot 14 of each of the above materials.

(1) Pot made of material with low skin resistance such as copper or aluminum
When Heating 14 First, the load control circuit 5 discriminates a load condition at a constant value where the output voltage to the inverter 6 is low under a heating condition of a low skin resistance material in which the changeover switch 15 is in a closed state between the contacts (ab). When the operation is performed, since the specific resistance of aluminum or the like is small, the input impedance of the induction heating coils 9 and 10 decreases, and the inverter current increases. Therefore, the voltage level of the detected voltage signal S ₂₀ output from the current detecting means 20 is high, when the diameter D of the bottom of the pot 14 of aluminum or the like is heated to be a D = 22Fai the resonant frequency since becomes high frequency of approximately 50KHz as shown in FIG. 2, the voltage level of the detected voltage signal S ₁₉ output from the frequency detecting means 19 is increased accordingly. Therefore, low skin resistance material detection signal S ₂₂ a is outputted from the load state judging means 22. In this case, since the changeover switch 15 is already in the closed state between the contacts (ab) in the initial state, the switch 15 is not switched, and the output voltage of the inverter 6 is increased by the voltage control circuit 5 so that the normal operation is performed. Heating is performed.

Here, when the diameter D of the bottom of the pot 14 is D = 10φ and D = 16φ, the resonance frequency changes so as to be lower than the case of D = 22φ as shown in FIG. . In such a case, a change in the resonant frequency detected frequency detection means 19, the voltage level of the detected voltage signal S ₁₉ Te than decreases. In this case, the voltage control circuit 5 that has received the detection voltage signal S _19, the output voltage of the inverter 6 is adjusted to be lower in response to each. As a result, the diameter D of the bottom of the pan 14
Even when the temperature is reduced, unlike the conventional case, it is possible to prevent the temperature rise of the induction heating coils 9 and 10 and to prevent the damage, and it is possible to heat the relatively small pot 14 having a large amount of leakage magnetic flux.

(2) Pot 14 made of a material with high skin resistance such as iron or stainless steel
As described above, when the load state determination operation is performed under the low skin resistance material heating condition, as described above, the input impedance of the induction heating coils 9 and 10 increases because iron and the like have a large specific resistance. Therefore, the inverter current becomes extremely small. Therefore, the detection voltage signal S ₂₀ output from the current detection means ₂₀
Of from the voltage level becomes very low, so that the received load state judgment circuit 22 from the high skin resistance material detection signal S ₂₂ b is which is output. As a result, the changeover switch 15 is switched to the closed state between the contacts (ac), and the resonance circuit 13 is set to the high skin resistance material heating condition in which only the first induction heating coil 9 and the second resonance capacitor 12 are enabled. Under this condition, the output voltage of the inverter 6 is increased by the voltage control circuit 5 and the pot 14 is heated. Further, since the inverter 6 is controlled by the resonance control means 18 so as to always be in a resonance state, when the pot 14 is made of iron, the output frequency of the inverter 6 is relatively low, about 25 KHz (in the case of stainless steel, About 3
3KHz).

Here, the diameter D of the bottom of the pot 14 is D = 10φ, D = 16φ, D =
Assuming that the inverter current and the resonance frequency change as shown in FIG. 2, the inverter current and the resonance frequency do not greatly change as compared with the case of the low skin resistance material heating condition as shown in FIG. Small diameter pots can also be heated.

As described above, in this embodiment, provided with a frequency detection means 19 for detecting the resonant frequency of the resonant circuit 13, the voltage for adjusting the output of the inverter 6 in response to the detected voltage signal S ₁₉ of the frequency detection means 19 Since the control circuit 5 is provided,
When heating a pot made of a low skin resistance material such as copper or aluminum, it is possible to heat a pot having a small diameter at the bottom, which has been conventionally difficult to heat.

In the above-described embodiment, the case where the diameter of the bottom of the pan 14 changes as the case where the leakage magnetic flux increases is not limited to this. The present invention may be applied to a case where heating is performed while floating, and the same operation and effect as those of the above embodiment can be obtained.

In addition, the present invention is not limited to the embodiment described above and shown in the drawings.
Instead of providing a voltage control circuit for controlling the input voltage to the inverter, an output adjustment circuit of a pulse width modulation method for adjusting the output of the inverter may be provided in the inverter drive circuit. Deformation is possible.

[Effects of the Invention] As is clear from the above description, the present invention provides an inverter that supplies high-frequency power to a resonance circuit including an induction heating coil and a resonance capacitor for heating a material to be heated having a low skin resistance. In addition, a frequency detecting means for detecting the resonance frequency of the resonance circuit is provided, and when the resonance frequency detected by the frequency detecting means is low, an inverter output adjusting means for lowering the output of the inverter is provided. When heating an object to be heated of a low material, an excellent effect that a relatively small object to be heated having a large leakage magnetic flux can be heated can be obtained.

[Brief description of the drawings]

FIG. 1 shows an embodiment of the present invention. FIG. 1 is an overall block diagram, and FIG. 2 is a characteristic diagram showing a relationship between a material to be heated and an inverter current and a resonance frequency. In the drawing, 5 is a voltage control means (inverter output adjusting means), 6 is an inverter, 9 and 10 are induction heating coils, and 11 and 12
Is a resonance capacitor, 13 is a resonance circuit, 14 is a pan (a body to be heated), and 19 is a frequency detecting means.

Claims (1)

(57) [Claims] An inverter for supplying high-frequency power to a resonance circuit including an induction heating coil and a resonance capacitor for heating an object to be heated made of a material having a low skin resistance; and frequency detection means for detecting a resonance frequency of the resonance circuit. And an inverter output adjusting means for reducing the output of the inverter when the resonance frequency detected by the frequency detecting means is low.