JPS63195988A - 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 capable of heating a heated object made of a material with low skin resistance, such as aluminum or copper.

(Prior art) As is well known, induction heating cookers supply high frequency power from an inverter to an induction heating coil, and reduce the eddy current loss that occurs in iron or stainless steel pots, which are materials with high skin resistance. This is a configuration for heating and cooking. By the way, in recent years, attempts have been made to make induction heating cookers capable of heating not only pots made of iron and stainless steel, but also pots made of materials with low skin resistance such as copper and aluminum. In order to heat a pot made of such a material with 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 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 with low skin resistance. The resistance of the induction heating coil itself is greater than when heating a pot made of high quality material. Therefore, when heating a pot 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 pot becomes small,
The current flowing through the induction heating coil will hardly increase. Therefore, the loss of the induction heating coil increases rapidly and the temperature rises, so there is a risk of damage to the coil, and in reality, there is a problem that it is not possible to heat a pot with a small bottom area. .

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide an induction heating cooker that can heat a relatively small object when heating an object made of a material with low skin resistance.

[Structure of the Invention] (Means for Solving the Problems) The induction heating cooker of the present invention uses high frequency in a resonant circuit of an induction heating coil and a resonant capacitor for heating an object to be heated made of a material with low skin resistance. An inverter for supplying electric power is provided, a frequency detecting means for detecting the resonant frequency of the resonant circuit is provided, and an inverter output adjusting means is provided for adjusting the output of the inverter according to the detected frequency of the frequency detecting means. It has characteristics.

(Function) When the object to be heated is made of a material with low skin resistance such as aluminum or copper, for example, when heating a relatively small object, there is less leakage than when heating a large object. The magnetic flux increases, the equivalent human power impedance of the induction heating coil increases, and the resonant frequency of the resonant circuit decreases. Thereby, it is possible to distinguish whether a small heated object is being heated by the detection frequency from the frequency detection means. Therefore, by adjusting the impark output according to the detection frequency using the inverter output adjustment means, it is possible to prevent the temperature rise of the induction heating coil and further damage, so that it is possible to heat even small objects to be heated, unlike in the past. .

(Example) An example of the present invention will be described below with reference to FIGS. 1 and 2.

1 is a commercial power supply, 2 is a bridge rectifier circuit equipped with a thyristor 3, 4 is a smoothing capacitor, and 5 is an inverter output adjustment means, such as a voltage control circuit, which controls the conduction angle of the thyristor 3 to adjust the smoothing capacitor 4. It is designed to adjust the DC output voltage between both terminals. Here, the voltage control circuit 5 maintains 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 and shifts to a heating operation. 6 is an inverter constructed so that two switching transistors 7 are switched by an inverter drive circuit 8;
High frequency power is supplied to a resonant circuit 13 including a second resonant capacitor 11 and a second resonant capacitor 11, 12. Each of the induction heating coils 9 and 10 is disposed below a top plate (not shown), and applies a high-frequency magnetic field to an object to be heated, such as a pot 14, placed on the top plate, causing an eddy current loss in the pot. 14 is heated.

Reference numeral 15 denotes a changeover switch, and depending on the switching state of this switch, the induction heating coil 9.10 and resonant capacitor 11.12 of the resonant circuit 13 are all activated, and the first induction heating coil 9 and the second induction heating coil 9. It is possible to switch between enabling only the resonant capacitor 12 and having different numbers of turns and fields. Here, the former condition is suitable for heating the pot 14 made of a material with low skin resistance such as aluminum or copper (hereinafter referred to as low skin resistance material heating condition), and the latter condition is suitable for heating the pot 14 made of a material with low skin resistance such as iron or stainless steel. This is suitable for heating the pot 14 made of a material with high resistance (hereinafter referred to as high skin resistance material heating conditions). 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 to, for example, N-15 turns. It is set. 16 is a phase comparison circuit, which compares the potential VZ of the first induction heating coil 9 and the potential VC of the second resonant capacitor 12, and determines that the phase difference between the two potentials Vl and VC is approximately 9
Frequency command voltage signal vf of voltage that brings the state to 0°
is output to a voltage controlled oscillator 17 (hereinafter referred to as VCO 17). The VCO 17 oscillates at a frequency according to the frequency command voltage signal f to control the inverter drive circuit 8, so that the inverter 6 is in a state where both potentials Vt and Vc always have a phase difference of about 90'' (resonant state).
It is controlled to operate at a frequency (resonant frequency) such that That is, the phase comparator circuit 16. The VCO 17 and the inverter drive circuit 8 constitute a resonance control means 18 that performs feedback control of the output frequency of the inverter 6 so that it is in a resonant state when the pot 14 is heated, and also controls the frequency output from the phase comparator circuit 16. The command voltage signal V" has a value proportional to its resonance frequency.

Reference numeral 19 denotes frequency detection means, which receives a frequency command voltage signal Vf proportional to the resonance frequency from the phase comparator circuit 16 and outputs a detection voltage signal Sts corresponding thereto.

Here, the voltage control circuit 5 receives the detection voltage signal S19 from the frequency detection means 19 under the low skin resistance material heating condition, and receives the detection voltage signal S! For example, as the resonance frequency of the resonant circuit 13 becomes lower, the output of the bridge rectifier circuit 2 is adjusted so as to lower the voltage across the smoothing capacitor 4, that is, the input voltage of the inverter 6. It is designed to control thyristor 3. On the other hand, 20 is a current detection means, which includes a current transformer 21 that detects the output current of the inverter 6, receives a detected current from the current transformer 21, and generates a detected voltage signal 5ffiO according to this detected current. It is designed to output . Reference numeral 22 denotes a load state determining means, which receives the detected voltage signal Sts from the frequency detecting means 19 and the detected voltage signal S20 from the current detecting means 20, and determines whether the low Skin resistance material detection signal 5
22a or a high skin resistance material detection signal 522b is output to the constant switching circuit 23. Here, the constant switching circuit 23 switches the changeover switch 15, and changes the changeover switch 15 into a contact state as shown in FIG. (a-b) Open and close, high skin resistance material detection signal 5
When receiving tzb, changeover switch 15 is set to contact (a-
C) Switch to open/closed state.

Next, the operation of the above configuration will be explained with reference to FIG. 2 as well.

First, the relationship between the material of the pot 14, the inverter current that is the output current of the inverter 6, and the resonant frequency of the resonant circuit 13 will be described. FIG. 2 shows the results of actually measuring the inverter current and resonant frequency when the material of the pot 14 is iron, stainless steel, copper, and aluminum and when there is no load. Here, the curves La and Lb representing copper and aluminum, respectively, are curved when the changeover switch 15 is in the open/closed state with the contacts (a-b) under conditions for heating low skin resistance materials, and when the curves La and Lb represent copper and aluminum, respectively. The curves Lc and Ld shown are when the changeover switch 15 is the contact point (a-c)
This is a case where the opening/closing state is in a condition for heating a high skin resistance material. The three points for each material indicate cases where the diameter of the bottom of the pot 14 is D-10φ, D-16φ, and D-22φ, respectively. As is clear from the figure, when the pot 14 is made of a low skin resistance material such as aluminum or copper,
Since the resonant frequency clearly changes depending on the change in the diameter of the pot bottom (in this case, the inverter current does not change much), this means that the size of the pot 14 can be determined by the change in the resonant frequency. The present invention focuses on such points, and next, the operation of this embodiment will be explained using the pot 14 made of each of the above-mentioned materials.
The case of heating will be explained separately.

(1) When heating the pot 14 made of a material with low skin resistance such as copper or aluminum, first, under the low skin resistance material heating condition where the changeover switch 15 is in the open/closed state of the contacts (a-b), the voltage control circuit 5 is heated. When the load state determination operation is performed at a constant low output voltage to the inverter 6, the input impedance of the induction heating coil 9, 10 decreases, and the inverter current increases, since aluminum or the like has a low specific resistance. Therefore, the detection voltage signal S output from the current detection means 20
When the voltage level of 2o becomes high and the diameter of the bottom of the pot 14, such as aluminum, is D-22φ, the resonant frequency increases to 50o as shown in FIG.
Since the frequency is as high as KHz, the voltage level of the detection voltage signal Sts output from the frequency detection means 19 increases accordingly. Therefore, the load condition determining means 2
2 outputs a low skin resistance material detection signal 522a.

In this case, since the changeover switch 15 is already in the contact (a-b) open/closed state 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 to perform normal heating. will be held.

t+ t+ Te, Ml 4t7) Bottom diameter D f) <,
D-10φ.

In each case of D-16φ, as shown in Figure 2, D-22φ
The resonance frequency changes to become lower than in the case of . In such a case, the frequency detection means 19 detects the change in the resonance frequency, and the voltage level of the detection voltage signal 519 becomes low. At this time, the detection voltage signal S
19, the voltage control circuit 5 adjusts the output voltage of the inverter 6 to be lower accordingly. As a result, even if the diameter of the bottom of w414 becomes smaller,
Unlike the conventional method, it is possible to prevent a rise in temperature of the induction heating coils 9, 10 and further damage, and it is possible to heat a relatively small pot 14 with a large amount of leakage magnetic flux.

(2) Pot made of material with high skin resistance such as iron or stainless steel 1
In the case of heating 4, as described above, when the load condition determination operation is performed under the heating condition of low skin resistance material, in this case, since iron etc. have a large specific resistance, the human power impedance of the induction heating coils 9 and 10 will be large. Therefore, the inverter current becomes extremely small.

Therefore, since the voltage level of the detection voltage signal S20 output from the current detection means 20 becomes extremely low, the high skin resistance material detection signal 522b is output from the negative 6I state discriminating circuit 22 that receives this. As a result, the changeover switch 15 is switched to open bottom between the contacts (a-C), and the resonant circuit 13 is set to the high skin resistance material heating condition in which only the first induction heating coil 9 and the second resonant capacitor 12 are activated. Under these conditions, the output voltage of the inverter 6 is increased by the voltage control circuit 5, and the pot 14 is heated. Furthermore, since the inverter 6 is controlled by the resonance control means 18 to always be in a resonant state, if the pot 14 is made of iron, the output frequency of the inverter 6 is relatively low, about 25 KHz (if the pot is made of stainless steel, the output frequency is about 25 KHz). (approximately 33 KHz).

Here, the diameter of the bottom of the pot 14 is D-10φ.

In this case, as shown in Fig. 2, the inverter current and resonance frequency will change significantly compared to the case of the low skin resistance material heating condition. Since there is no heat exchanger, it is possible to heat pots with small diameter bottoms.

As described above, in this embodiment, the frequency detecting means 19 for detecting the resonant frequency of the resonant circuit 13 is provided, and
Detected voltage signal Sl! of this frequency detecting means 19! Since the voltage control circuit 5 is provided to adjust the output of the inverter 6 according to can also be heated.

In the above embodiment, the leakage magnetic flux increases when the diameter of the bottom of the pot 14 changes, but the invention is not limited to this, and the case where the bottom of the pot is uneven or when the pot is It may also be applied to cases where heating is carried out by floating, and the same effects as in the above embodiments can be obtained.

In addition, the present invention is not limited to the embodiments described above and shown in the drawings, and instead of providing a voltage control circuit for controlling the input voltage to the inverter as an inverter output adjustment means, the inverter drive circuit is provided with an inverter output. It is also possible to provide a pulse width night adjustment type output adjustment circuit for adjusting the output voltage, and various modifications can be made without departing from the gist of the invention.

[Effects of the Invention] As is clear from the above description, the present invention includes an inverter that supplies high-frequency power to a resonant circuit of an induction heating coil and a resonant capacitor for heating a heated object made of a material with low skin resistance. In addition, since the structure is provided with a frequency detecting means for detecting the resonant frequency of the resonant circuit and an inverter output adjusting means for adjusting the output of the inverter according to the detected frequency of the frequency detecting means, it is possible to use a material with low skin resistance. When heating an object to be heated, it has an excellent effect of being able to heat even a relatively small object with a large amount of leakage magnetic flux.

[Brief explanation of the drawing]

The drawings show one embodiment of the present invention, and FIG. 1 is an overall block diagram, and FIG. 2 is a characteristic diagram showing the relationship between the material of the heated body, the inverter current, and the resonant frequency. In the drawing, 5 is voltage control means (inverter output adjustment means)
, 6 is an inverter, 9. lO is an induction heating coil, 11.
12 is a resonant capacitor, 13 is a resonant circuit, 14 is a pot (heated object), and 19 is a pigeon wave number detector and a mountain means. Applicant: Toshiba Co., Ltd. Kyousou Shuhakyo □ Figure 2

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 an object to be heated made of a material with low skin resistance; a frequency detection means for detecting the resonant frequency of the resonant circuit; An induction heating cooker comprising: inverter output adjusting means for adjusting the output of the inverter according to the detected frequency of the frequency detecting means.