JPH0319192Y2 - - Google Patents

Description

[Detailed explanation of the idea]

[Technical field of the invention] This invention generates a high-frequency magnetic field from a heating coil and applies it to the pot, which is a load, to generate an eddy current in the pot. The present invention relates to an induction heating cooker that performs heating cooking. [Technical background of the invention and its problems] Conventionally, this type of induction heating cooker has been made of materials with high magnetic permeability, such as iron, or with low magnetic permeability, such as 18-8 stainless steel. For high-resistance pots, the input resistance of the heating coil becomes high and heating is possible. However, it has been impossible to heat pots made of materials such as aluminum or copper that have low magnetic permeability and low resistance because the input resistance of the heating coil is low. Here, the relationship between the material of the pot and the input resistance of the heating coil will be explained. First, the resistivity ρ (Ωm) and relative magnetic permeability μs (=magnetic permeability) of various metals are shown in the table below.

[Table] Therefore, if the frequency of the high-frequency magnetic field emitted from the heating coil is f, then the above resistivity ρ (Ωm)
The skin thickness s can be determined from the relative magnetic permeability μs. Note that k is a constant. That is, in the case of a pot made of a material with high magnetic permeability such as iron, the relative magnetic permeability μs is large, so the skin thickness s is small, producing a large skin effect and the pot itself having a high resistance. Therefore, the input resistance of the heating coil becomes high. In addition, in the case of a pot made of 18-8 stainless steel, the relative magnetic permeability μs is as small as 1, so the skin thickness s is calculated to be large, but the actual thickness of the pot is not that thick, so the resistivity ρ is greater. acts effectively, and the pot itself has high resistance due to its high resistivity ρ.
Therefore, the input resistance of the heating coil becomes high. On the other hand, when the material is aluminum or copper, the relative magnetic permeability μs is as small as 1, so the skin thickness s is large and the skin effect is difficult to occur, and the resistivity ρ itself is small, so the pot itself has a low resistance. Therefore, the input resistance of the heating coil becomes low. However, in the case of aluminum or copper pots, if the frequency f of the high-frequency magnetic field is increased, the input resistance of the heating coil can, in principle, be made almost the same as that for iron pots, which makes heating possible. Become. However, in this case, the frequency f of the high-frequency magnetic field must be several hundred times that of an iron pot, which is difficult to achieve. In other words, the frequency of the high-frequency magnetic field used in induction heating cookers is the audio frequency, or 18kHz.
For this reason, it costs 18k for an iron pot.
If a high-frequency magnetic field of Hz is to be emitted, in the case of an aluminum or copper pot, a high-frequency magnetic field of several MHz must be emitted, leading to increased loss in the heating coil and increased loss in the inverter circuit. Therefore, the number of turns of the heating coil is made variable, and when the material of the pot is aluminum or copper, the number of turns of the heating coil is increased.This increases the input resistance of the heating coil to the same level as that of iron or 18-8 stainless steel.
Induction heating cookers have appeared that can even heat aluminum and copper pots. However, in such an induction heating cooker,
Because the diameter of the heating coil cannot be made larger than the diameter of the pot, and because reducing the cross-sectional area of the heating coil winding increases the resistance of the heating coil itself, it is possible to divide the heating coil into multiple coils. The number of turns can be increased by arranging them in multiple stages above and below. In other words, if the material of the pot is iron or 18-8 stainless steel, some coils of the heating coil are energized, and if the material of the pot is aluminum or copper, the series of each coil of the heating coil is energized. By doing so, the number of turns can be increased. However, when dealing with an increase in the number of turns using a multi-stage configuration, the following problems arise. That is, when the number of turns of the heating coil is increased, the inductance of the heating coil increases, including an increase in leakage magnetic flux, and the voltage across the heating coil reaches 1 kV or more. In this case, an excessive voltage will be generated between the upper and lower coils, and dielectric breakdown may occur between the coils. [Purpose of the invention] This invention was made in view of the above-mentioned circumstances, and its purpose is to provide a heating coil that has a plurality of coils arranged vertically in multi-stages to enable an increase in the number of turns. In an induction heating cooker that changes the number of turns of the heating coil depending on the material of the load,
It is an object of the present invention to provide an induction heating cooker which is excellent in safety and reliability and can prevent dielectric breakdown between each coil of a heating coil. [Summary of the invention] In order to achieve the above object, in the induction heating cooker according to the invention, a plurality of spirally wound coils are stacked with an insulator interposed between each coil, and the coils are stacked adjacently in the stacking direction. A heating coil formed by sequentially connecting the inner and outer ends of the coils and connecting each coil in series, and a plurality of series-connected coils constituting this heating coil, depending on the material of the cooking pot. and means for generating a high frequency magnetic field for heating the cooking pot by selecting a number of coils that provide a corresponding number of turns. [Embodiment of the invention] An embodiment of the invention will be described below with reference to the drawings. In FIGS. 1 and 2, reference numeral 1 denotes a top plate on the upper surface of the main body, and a heating coil 2 is disposed on the back surface of the top plate 1 in a spaced-apart manner.
This heating coil 2 is constructed by stacking a plurality of spirally wound coils 2a and 2b vertically, and
The inner end of the coil 2b and the outer end of the coil 2b are interconnected, making it possible to increase the number of turns. and,
Connect the outer end of coil 2a to terminal A, and
The inner end of coil 2a and the outer end of coil 2b, which are interconnected, are connected to terminal B. Also, coil 2
An insulator such as a sheet-like glass cloth 3 is provided between a and 2b, and the glass cloth 3 is fixed to the coils 2a and 2b with silicone adhesive. FIG. 3 shows the control circuit. Reference numeral 10 denotes a commercial AC power source, and a rectifier circuit consisting of a diode bridge 11 and a smoothing capacitor is connected to this power source 10. A series circuit of switching elements such as the collector-emitter of an NPN transistor 13 and the collector-emitter of an NPN transistor 14 is connected to the output end of the rectifier circuit. The heating coil 2 is connected to the collector of the transistor 14.
One end of the heating coil, that is, terminal A, is connected, and the other end of the heating coil, that is, terminal C, is connected to one end of the resonance capacitor 15a. The other end of the resonance capacitor 15a is connected to the emitter of the transistor 14 via a switch 16. Furthermore, one end of a resonant capacitor 15b is connected to a terminal B serving as an intermediate tap of the heating coil 2, and the other end of this resonant capacitor 15b is connected to the emitter of a transistor 14 via a switch 17. In this case, the resonance capacitor 15a has a small capacity, and the resonance capacitor 15b has a large capacity, and the resonance capacitors 15a,
By viewing 15b as one resonance capacitor, its capacitance can be switched.
Further, the switches 16 and 17 are adapted to respond to the detection results of a load detection circuit 21, which will be described later. Therefore, when the switch 16 is turned on,
An energizing path is formed for the series body of the coils 2a and 2b, and the number of turns of the heating coil 2 increases,
The capacitance of the resonance capacitor is reduced (capacitor 15
a) A series resonant circuit is constituted by this heating coil with a large number of turns and a resonant capacitor with a small capacity. Furthermore, when the switch 17 is turned on, an energizing path is formed for some of the coils 2a, and the number of turns of the heating coil 2 is reduced, and the capacitance of the resonance capacitor (capacitor 15b) is increased, and this heating coil with a small number of turns is A series resonant circuit is constructed with a large-capacitance resonant capacitor. In other words, the rectifier circuit and transistors 13,1
4 constitutes an inverter circuit that excites the resonant circuit. Further, a current transformer 18 is provided in the resonant circuit, and the output of the current transformer 18 is supplied to a load detection circuit 21 and a phase detection circuit 22. The load detection circuit 21 detects the magnitude of the high frequency current flowing through the heating coil 2 based on the output of the current transformer 18, determines the input resistance of the heating coil 2 from this, and adjusts the pot depending on the magnitude of the input resistance. The material of 4 is detected, and the switch 1 is turned on according to the detection result.
6 and 17 are turned on and off. The phase detection circuit 22 detects the phase of the high frequency current flowing through the heating coil 2 based on the output of the current transformer 18. The detection result of this phase detection circuit 22 is then supplied to the inverter drive circuit 23. This inverter drive circuit 23 detects the transistors 13 and 13 according to the detection result of the phase detection circuit 22.
14 is turned on and off alternately, thereby exciting the inverter circuit. Next, the operation in the above configuration will be explained. First, the input resistance of a heating coil is proportional to the square of the number of turns of the heating coil. Therefore, when the material of the pot is aluminum or copper, by increasing the number of turns of the heating coil, cooking can be performed in the same way as in the case of a pot made of iron or 18-8 stainless steel. However, in reality, in the case of aluminum or copper pots, the leakage flux of the heating coil is large due to the low magnetic permeability, and the number of turns is increased as described above, so when the input inductance of the heating coil is iron, As a result, heating becomes difficult. Here, the formula below represents the resonant frequency f ₀ of the resonant circuit. That is, Lc is the input inductance of the heating coil, Cr is the capacitance of the resonance capacitor, and as the input inductance Lc of the heating coil increases, the resonance frequency f ₀ decreases, making heating difficult. To deal with this, by increasing the number of turns of the heating coil and at the same time decreasing the capacitance of the resonant capacitor, it is possible to suppress the drop in the resonant frequency _f0 . -8 It becomes possible to heat it like a stainless steel pot. Therefore, pot 4 is placed on top plate 1 and electric current 10 is applied. Then, the transistors 13 and 14 are turned on and off alternately by the inverter drive circuit 23, and a high frequency current flows through the heating coil 2. At this time, the load detection circuit 21 detects the input resistance of the heating coil 2 based on the output of the current transformer 18, and determines the material of the pot 4 based on this detection result. In this case, if the input resistance of the heating coil 2 is high, it is determined that the material of the pot 4 is iron, which has high magnetic permeability and high resistance, or 18-8 stainless steel, which has low magnetic permeability but high resistance. In addition, heating coil 2
If the input resistance is low, it is determined that the material of the pot 4 is aluminum or copper, which has low magnetic permeability and low resistance. When the load detection circuit 21 determines that the material of the pot 4 is iron or 18-8 stainless steel, it turns off the switch 16 and turns on the switch 17. When switch 16 is turned off and switch 17 is turned on, heating coil 2 with a small number of turns (coil 2
A) and the large-capacitance resonance capacitor 15b form a resonant circuit, and thereafter, the resonant circuit is excited by turning the transistors 13 and 14 on and off. Further, if it is determined that the material of the pot 4 is aluminum or copper, the switch 16 is turned on and the switch 17 is turned off. When the switch 16 is turned on and the switch 17 is turned off, a resonant circuit is formed by the heating coil 2 with a large number of turns (coil 2a + coil 2b) and the resonant capacitor 15a with a small capacity.
Thereafter, the resonant circuit is excited by turning the transistors 13 and 14 on and off. In this way, a high frequency current flows through the heating coil 2, and the heating coil 2
The pot 4 is induction heated by the high frequency magnetic field emitted from the pot. At this time, the high frequency current flowing through the heating coil 2 is detected by the current transformer 18, and the phase of the high frequency current is detected by the phase detection circuit 22. Then, the on/off timing of the transistors 13 and 14 is controlled according to the detection result, and stable oscillation of the resonant circuit is performed. By the way, when the number of turns of the heating coil 2 is increased to heat an aluminum or copper pot, that is, when the coils 2a and 2b are used together for cooking, the inductance of the heating coil 2 increases. A high voltage of 1kV or more is generated at both ends. Let this voltage across both ends be Vc. Therefore, when the inner end of the coil 2a and the outer end of the coil 2b are interconnected as in this embodiment (FIG. 2), if the number of turns of the coils 2a and 2b is the same, As shown in FIG. 4, a voltage _V1 approximately half of the voltage Vc across the coils 2a and 2b is generated between the coils 2a and 2b. on the other hand,
When the inner end of the coil 2a and the inner end of the coil 2b are interconnected as shown in FIG.
The voltage V ₂ between the inside of the coil 2a and the inside of the coil 2b is almost zero, but the voltage V 2 between the outside of the coil 2a and the inside of the coil 2b is almost zero.
A voltage _V3 , which is the same as the voltage Vc, is generated between the outside of b and the outside. (If the outer ends of the coil 2a and the outer ends of the coil 2b are interconnected, the voltage between the outside of the coil 2a and the outside of the coil 2b is almost zero, but the voltage between the inside of the coil 2a and the outside of the coil 2b is almost zero. The same voltage as Vc is generated between the inner side and the inside.) Therefore, the above-mentioned excessive voltage V ₃ (=Vc)
occurs between the coils 2a and 2b, the coil 2
Although there is a risk of dielectric breakdown between a and 2b, in this embodiment, the inner end of coil 2a and the outer end of coil 2b are interconnected, and the voltage generated between both coils is reduced to Vc, which is half of the voltage Vc. ₁ (=V ₃ /2), that is, to prevent excessive voltage V ₃ from occurring, and a glass cloth 3, which is an insulator, is provided between the coils 2a and 2b, thereby preventing dielectric breakdown. can be prevented from occurring. In this way, when the material of the pot 4 has high magnetic permeability or high resistance, the number of turns of the heating coil 2 is reduced and the capacity of the resonance capacitor is increased; By increasing the number of turns of the heating coil 2 and reducing the capacity of the resonant capacitor, it can be used not only for iron and 18-8 stainless steel pots, but also for aluminum and copper pots, with no loss and high efficiency. Can be heated and cooked. In particular, the coil 2a of the heating coil 2 configured in multiple stages
The inner end of the coil 2b and the outer end of the coil 2b are interconnected to suppress the voltage generated between the two coils, that is, to prevent excessive voltage from occurring between the coils 2a and 2b, and to prevent the voltage between the coils 2a and 2b. Since the glass cloth 3 is provided, it is possible to prevent dielectric breakdown between the coils 2a and 2b when increasing the number of turns of the heating coil 2 when heating an aluminum or copper pot, thereby improving safety and reliability. I can figure it out. In the above embodiment, the glass cloth 3 was used as an insulator between the coils 2a and 2b.
Any material with high withstand voltage and good workability is fine.
For example, an inorganic material such as mylar or mica may be used. In addition, as shown in FIG. 6, the coil 2
The coils 2a and 2b may be insulated by integrally molding them with an inorganic material 30. In addition, this invention is not limited to the above-mentioned embodiments, and it goes without saying that various modifications can be made without changing the gist. [Effects of the invention] As described above, this invention is equipped with a heating coil that can increase the number of turns by stacking a plurality of spirally wound coils, and the number of turns of this heating coil is determined by the material of the load. In those that switch according to
Since the inner and outer ends of each coil of the heating coil are interconnected, and an insulator is provided between each coil,
It is possible to provide an induction heating cooker with excellent safety and reliability that can prevent dielectric breakdown between the heating coils.

[Brief explanation of the drawing]

Fig. 1 is a block diagram showing the main parts of an embodiment of this invention, Fig. 2 is a diagram showing the connection state of each heating coil in Fig. 1, and Fig. 3 is a configuration of a control circuit in the same embodiment. 4 is a diagram for explaining the inter-coil voltage in the same embodiment, and FIG. 5 is a diagram for explaining the inter-coil voltage when the coil connection is different from that in the same embodiment. FIG. 6 is a diagram showing a modification of the insulator. 1...Top plate, 2...Heating coil, 2
a, 2b...Coil, 3...Glass cloth (insulator), 4...Pot (load).

Claims (1)

[Scope of utility model registration request] Heating that consists of stacking a plurality of spirally wound coils with insulators interposed between each coil, and sequentially connecting the inner and outer ends of adjacent coils in the stacking direction to connect each coil in series. a coil, and a high-frequency magnetic field for heating the cooking pot by selecting a number of coils that provide a number of turns corresponding to the material of the cooking pot from among the plurality of series-connected coils constituting the heating coil. An induction heating cooker comprising: means for generating.