JPS6171583A - Induction heating cooking device - 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] [Technical Field of the Invention] This invention generates a high-frequency magnetic field from a heating coil and applies it to steel, which is a load, to generate an eddy current in the pot, thereby reducing eddy current loss. This invention relates to an induction heating cooker that performs cooking using self-heating of a pot based on the above.

[Technical background of the invention and its problems]

Conventionally, this type of induction heating cooker uses a heating coil for pots made of materials with high magnetic permeability such as iron, or pots with low magnetic permeability but high resistance such as 18-8 stainless steel. The input resistance is high and heating is possible. However, it has been impossible to heat pots made of materials such as aluminum and 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.

′, resistivity ρ (Ωm) and relative permeability μ of various metals
S (-magnetic permeability) is shown in the table below.

Therefore, when the frequency of the high-frequency magnetic field emitted from the heating coil is f, the skin thickness S of the pot can be determined from the resistivity ρ (0 m) and relative permeability μS.

Note that k is a constant.

In other words, in the case of a pot made of high permeability material such as iron,
Since the relative magnetic permeability μS is large, the skin thickness S is small, causing a large skin effect and the steel itself having high resistance. Therefore,
The input resistance of the heating coil will be high. In addition, the material is 18-
In the case of 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 acts more effectively.
Due to its high resistivity ρ, the pot itself has a high resistance. 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 high.

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 18k.
H2 or higher, so in the case of an iron pot it costs 18k.
If an 8-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 MH2 must be emitted, resulting in increased loss in the heating coil and increased loss in the inverter circuit.

Therefore, we focused on the fact that the input resistance of a heating coil is proportional to the square of the number of turns of the heating coil.If the material of the pot is aluminum or copper, we increase the number of turns of the heating coil, thereby reducing the input resistance of the heating coil. Induction heating cookers have appeared that can heat aluminum and copper pots to the same level as iron and 18-8 stainless steel.

However, in such an induction heating cooker, as the number of turns of the heating coil increases, the leakage magnetic flux of the heating coil increases due to the fact that the material of the pot has low magnetic permeability, so the input inductance of the heating coil increases compared to iron. This increases the resonant frequency of the resonant circuit formed by the heating coil and the resonant capacitor, and the generated high-frequency magnetic field increases, which also causes loss and makes heating difficult in practice.

Here, the formula below represents the resonant frequency f 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 increases.

To deal with this, in the above-mentioned induction heating cooker, the number of turns of the heating coil is made variable, and the capacity of the resonance capacitor is made variable.If the material of the pot is aluminum or copper, the number of turns of the heating coil is made variable. At the same time, the capacitance of the resonance capacitor is reduced, thereby suppressing the resonance frequency fO from becoming too high, and making it possible to heat an aluminum or copper pot without causing loss.

However, when the number of turns of the heating coil increases, the input inductance of the heating coil increases as described above, and an excessive voltage of 1 kV or more occurs at both ends of the heating coil. When such an excessive voltage occurs, a leakage current is induced from the heating coil to the pot through the capacitance between the heating coil and the steel, which is very dangerous if the pot is touched. Moreover,
Since the number of turns of the heating coil and the capacitance of the resonance capacitor are changed using a changeover switch,
When such an excessive voltage as described above is applied to this changeover switch, sparks may be generated there, and the changeover switch may be welded.

[Purpose of the invention]

This invention was made in view of the above circumstances,
The purpose of this is to enable proper and efficient cooking regardless of the material of the load, to prevent the induction of leakage current to the load, and to reduce the number of turns of the heating coil and the capacity of the resonance capacitor. It is an object of the present invention to provide an induction heating cooker that can prevent application of excessive voltage to a switching device for switching and has excellent product quality and safety.

[Summary of the invention]

This invention provides a heating coil consisting of a first coil and a second coil, a resonant capacitor whose capacity can be switched to form a resonant circuit together with the heating coil, and a material of the load having high magnetic permeability or high resistance. Sometimes, a resonant circuit is formed by the first coil and a large-capacity resonant capacitor, and when the material of the load has low transmittance and low resistance, the first coil is
．． A switching device is provided for forming a resonant circuit with the series body of the second coil and a small capacitance resonance capacitor, and an inverter circuit is further provided to excite the resonant circuit formed by the switching device, and the first coil of the heating coil is connected to the first coil of the heating coil. The number of turns is small, and the first coil is arranged in the upper stage and the second coil in the lower stage, and the first coil is arranged in the second stage.
It is connected closer to the inverter circuit than the coil.

[Embodiments of the invention]

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

In Fig. 2, 1 is a top plate provided on the top surface of the main body, and a heating coil 2 is provided on the back side of this top plate 1.
are arranged facing each other at a distance.

This heating coil 2 includes a first coil 2a and a second coil 2b.
and the first coil 2a in the upper stage.
The number of turns is reduced, and the number of turns can be changed by using the coil 2a alone or by connecting the coils 2a and 2b in series.

A pot 3 serving as a load is placed on the top plate 1.

FIG. 1 shows the control circuit. 10 is a commercial AC power supply, and this 1iil! A rectifier circuit consisting of a diode bridge 11 and a smoothing capacitor 12 is connected to 10. At the output end of the rectifier circuit is a switching element called N.
The collector and emitter of the PN transistor 13 and the collector and emitter of the NPN transistor 14 are connected in series. And transistor 14
One end of the coil 2a is connected to the collector of the coil 2a, and the other end of the coil 2a is connected to the selector switch 1, which is a switching device.
It is connected to the movable terminal 15c of No. 5. Changeover switch 1
One end of a large-capacitance resonance capacitor 16 is connected to the fixed terminal 15a of No. 5, and the other end of this common photography capacitor 16 is connected to the negative output end of the rectifier circuit. moreover,
One end of a coil 2b is connected to the fixed terminal 15c of the changeover switch 15, and the other end of this coil 2b is connected to one end of a second co-carrying capacitor 17 having a small capacity. Second
The other end of the shared photography capacitor 17 is connected to one end of the first resonance capacitor 16. In this case, by viewing the resonance capacitors 16 and 17 as one resonance capacitor, the capacitance can be switched. Further, the switch 15 is adapted to respond to the detection result of a load detection circuit 21, which will be described later. Therefore, when the switch 15 is switched to the fixed terminal 15a side, a series resonance circuit is formed by the heating coil (coil 2a) with a small number of turns and the resonance capacitor 16 with a large capacity. When the selector switch 15 is switched to the fixed terminal 15b side, a heating coil with a large number of turns (a series body of coils 2a and 2b) and a military capacitor with a small capacity (a series body of resonance capacitors 16 and 17) are connected. A series resonant circuit is formed. That is, an inverter circuit that excites the resonant circuit is constituted by a rectifier circuit, transistors 13, 14, and the like.

Thus, the voltage of the resonance capacitor 16 is supplied to the load detection circuit 21 and the phase detection circuit 22. The load detection circuit 21 detects the magnitude of the high-frequency current flowing through the resonance circuit based on the voltage of the resonance capacitor 16, determines the input impedance of the heating coil 2, and selects the material of the pot 3 based on the magnitude of the input impedance. The changeover switch 15 is controlled according to the detection result. The phase detection circuit 22 is
The voltage of the resonance capacitor 16 is used to detect the phase of the high frequency current flowing through the resonance circuit. The detection result of this phase detection circuit 22 is supplied to an inverter drive circuit 23.

The inverter drive circuit 23 alternately turns on the transistors 13 and 14 according to the detection result of the phase detection circuit 22.
OFF, thereby exciting the inverter circuit.

Next, the operation in the above configuration will be explained.

The pot 3 is placed on the top plate 1, and the power source 10 is turned on. Then, the transistors 13 and 14 are turned on and off alternately by the inverter drive circuit 23, and current flows through the heating coil 2. At this time, the load detection circuit 21 detects the input impedance of the heating coil 2 based on the voltage of the resonance capacitor 16, that is, the current flowing through the heating coil 2,
The material of the chain 3 is determined based on this detection result. in this case,
If the input impedance of heating coil 2 is high, IJI
It is determined that the material No. 3 is iron, which has high magnetic permeability and high resistance, or 18-8 stainless steel, which has low magnetic permeability but high resistance. Further, if the input impedance of the heating coil 2 is low, it is determined that the material of w43 is aluminum or copper, which has low magnetic permeability and low resistance.

When the load detection circuit 21 determines that the material of the WA3 is iron or 18-8 stainless steel, it switches the changeover switch 15 to the fixed terminal 15a side. Then, the coil 2a with a small number of turns and the resonance capacitor 1 with a large capacity
6 forms a resonant circuit, and thereafter, the resonant circuit is excited by turning on and off the transistors 13 and 14.

On the other hand, if it is determined that the material of the steel 3 is aluminum or copper, the changeover switch 15 is switched to the fixed terminal 15b side. Then, a resonant circuit is formed by the coils 2a and 2b in series, that is, the heating coil with a large number of turns, and the resonance capacitor 16, 17 in series, that is, the resonance capacitor with a small capacitance. ．．
14 is excited by turning on and off. In this way, a high frequency current flows through the heating coil 2, and the pot 3 is heated by induction by the high frequency magnetic field emitted from the heating coil 2. At this time, the phase detection circuit 23 detects the phase of the high frequency current flowing through the heating coil 2 based on the voltage of the resonance capacitor 16, and the transistor 13.
The on/off timing of 14 is taken into consideration, and stable oscillation of the resonant circuit is performed.

In this way, when the material of the pot 3 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, and when the material of the steel 3 has low magnetic permeability and low resistance, By increasing the number of turns of the heating coil 2 by 14 and reducing the capacitance of the resonance capacitor, it can be used not only for iron and 18-8 stainless steel pots, but also for aluminum and copper steel without causing loss and is highly efficient. Can be heated and cooked.

By the way, when looking at the connection relationship between the inverter circuit and the heating coil 2, excessive voltage is likely to occur in the coil on the inverter side (collector of the transistor 14), that is, on the power supply side. In other words, excessive voltage is likely to occur in the coil 2a,
If an excessive voltage were to occur in the coil 2a, it would be applied between the movable terminal 15c and the fixed terminal 15a of the changeover switch 15. In this case, there is a risk that sparks will occur in the changeover switch 15 and the contacts will weld there.

However, the number of turns of the coil 2a is reduced, so that no excessive voltage is generated in the coil 2a. Moreover, since the resonance capacitor 16 connected to the fixed terminal 15a of the changeover switch 15 has a small capacity,
An excessive voltage is never applied between the terminals of the change-over switch 15, and sparks of the change-over switch 15 and contact welding caused by the sparks can be prevented.

Furthermore, when cooking aluminum or copper steel, the coils 2a and 2b are connected in series and the number of turns of the heating coil 2 increases, so an excessive voltage is generated in the heating coil 2 as a whole. However, heating coil 2 is replaced by coil 2a,
The coil 2a is divided into two coils 2b and arranged in upper and lower stages, and since no excessive voltage is generated in the coil 2a located close to the pot, that is, in the upper stage, as described above, induction of leakage current to the pot 3 can be prevented. In other words, it is safe even if the human body touches the pot 3.

Further, the voltage of the resonance capacitor 16 is used as a feedback signal from the resonance circuit to the load detection circuit 21 and the phase detection circuit 22, but since the capacitance of the resonance capacitor 16 is small, the voltage generated there will not become excessive. Without,
In other words, an appropriate voltage can be obtained, and reliable load and phase detection can be performed.

Roughly speaking, the heating coil 2 is divided into coils 2a and 2b and configured in stages to cope with switching the number of turns, so that the outer diameter and inner diameter of the heating coil 2 change depending on the material of f13. In other words, the outer diameter and inner diameter of the heating coil 2 can be made to match the size and bottom area of the pot 3, so that uniform heating is always possible.

It should be noted that the present invention is not limited to the above-mentioned embodiment, and may be implemented as a control circuit as shown in FIG. 3, for example. Third
In the figure, the collector of the transistor 14 is connected to the coil 2a.
One end of the coil 2a is connected to the coil 2a, and the other end of the coil 2a is connected to one end of the resonance capacitor 16. The other end of the resonance capacitor 16 is connected to a fixed terminal 32 of the changeover switch 32.
It is connected to one end of the negative side output of the rectifier circuit via between the movable terminal 32c and the movable terminal 32c. Roughly speaking, one end of a coil 2b is connected to the other end of the coil 2a, and the other end of the coil 2b is connected to one end of a resonance capacitor 31 having a small capacity. The other end of the resonance capacitor 31 is connected to the negative output end of the rectifier circuit via a fixed terminal 32b and a movable terminal 32c of the changeover switch 32. Further, a current transformer 33 is provided on the movable terminal line of the changeover switch 32, and the output of the current transformer 33 is fed back to the load detection circuit 21 and the phase detection circuit 22. Here, the changeover switch 32 responds to the detection result of the load detection circuit 21, and
When the material of tA3 is iron or 18-8 stainless steel, it is switched to the fixed terminal 32a side, and when the material of tA3 is aluminum or copper, it is switched to the fixed terminal 32b side.

Therefore, in this case, if the material of 13 is iron or 18-8 stainless steel, the selector switch 32 is switched to the fixed terminal 32a side, and a resonant circuit is formed by the coil 2a with a small number of turns and the resonant capacitor 16 with a large capacity. be done. Also,
If the material of the pot 3 is aluminum or copper, the changeover switch 32 switches to the fixed terminal 32b side, and the coils 2a, 2b
A resonant circuit is formed by the series body and a resonant capacitor 3 with a small capacitance. In this case, since the number of turns of the coil 2a is small and the capacitance of the resonance capacitor 16 is large, it can be used as the fixed terminal 32a of the changeover switch 32! tl
An excessive voltage is not applied between the terminal and the terminal 32c.

Also,! No leakage current is induced in lI3.

〔Effect of the invention〕

As described above, according to the present invention, the first coil and the second coil
A heating coil consisting of a coil is provided, and a resonant capacitor capable of forming a resonant circuit with the heating coil is provided, and when the material of the load has high magnetic permeability or high resistance, a heating coil with a large capacitance is provided. When a common circuit is formed with a common capacitor and the material of the load has low magnetic permeability and low resistance, the first. A switching device is provided for forming a resonant circuit with the series body of the second coil and a small capacitance resonance capacitor, and an inverter circuit is further provided to excite the resonant circuit formed by the switching device, and the first coil of the heating coil is connected to the first coil of the heating coil. The number of turns is small, and the first coil is arranged in the upper stage and the second coil in the lower stage, and the first coil is connected closer to the inverter circuit than the second coil, so it is suitable regardless of the material of the load. It also enables efficient cooking, prevents the induction of leakage current to the load, and furthermore prevents the application of excessive voltage to the switching device for switching the number of turns of the heating coil and the capacity of the resonance capacitor. It is possible to provide an induction heating cooker with excellent product quality and safety.

[Brief explanation of drawings]

FIG. 1 is a configuration diagram of a control circuit showing an example of an actual flow of the present invention;
FIG. 2 is a configuration diagram of a heating coil and its surroundings in the same embodiment, and FIG. 3 is a configuration diagram of a control circuit showing another embodiment of the present invention. 2... Heating coil, 2a... First coil, 2b...
- Second coil, 3... Pot (load), 15... Changeover switch (switching device), 16.17... Resonance capacitor.

Claims (1)

[Claims] A heating coil with a switchable number of turns consisting of a first coil and a second coil; a resonance capacitor with a switchable capacitance that can form a resonance circuit together with the heating coil; A resonant circuit is formed by the first coil and a large capacity resonance capacitor, and when the material of the load has low magnetic permeability and low resistance, resonance occurs between the series body of the first and second coils and the small capacity resonance capacitor. A switching device for forming a circuit, and an inverter circuit for exciting a resonant circuit formed by the switching device, the first coil of the heating coil having a small number of turns, and An induction heating cooker characterized in that the coils are arranged in a lower stage, and the first coil is connected closer to the inverter circuit than the second coil.