JPH0632288B2 - Induction heating cooker - Google Patents

Description

Description: TECHNICAL FIELD OF THE INVENTION The present invention generates a high-frequency magnetic field from an induction heating coil and applies it to a pot that is a load to generate an eddy current in the pot, resulting in an eddy current loss. The present invention relates to an induction heating cooker for heating and cooking by self-heating of a pot.

[Technical background of the invention and its problems] In this type of induction heating cooker, an induction heating coil is arranged below a top plate on which a pot serving as a load is placed, and an induction heating coil is used to drive the high frequency current of 20 K to 30 KHz. By applying a high-frequency magnetic field to the pan, thereby causing an eddy current to flow in the pan to generate heat.

In this case, the eddy current is concentrated on the induction heating coil side of the pan bottom due to the skin effect, and is therefore determined by the skin resistance R _s of the pan. The skin resistance R _s is also determined by the skin thickness δ and the specific resistance ρ of the material. Here, R _s and ρ are expressed by the following equations. In the following equation, f is frequency and μS is relative magnetic permeability.

δ = (2 · ρ / ω · μ) ^1/2 = {1 / (4π ² × 10 ⁻⁷ )} ^1/2 × (ρ / f · μS) (1) Rs = ρ / δ = ( ω ・ μ ・ ρ) ^1/2 = (4π ² × 10 ^-7 ) ^1/2 × (ρ ・ f ・ μS) ^1/2・ ・ ・ (2) The material of the pan is made of iron or magnetic stainless steel and has a relative permeability. In the case of a ferromagnetic material having a large magnetic susceptibility or a non-magnetic stainless steel having a large specific resistance, the skin resistance is high, so that there is no problem in realizing induction heating.

On the other hand, in the case of a pan made of aluminum or copper whose relative magnetic permeability and specific resistance are both extremely small, there is a problem that a large current such as a short-circuit current flows in the induction heating coil because the skin resistance is extremely small.

To solve this problem, the frequency of the high frequency current may be further increased, but it is necessary to increase it to about several MHz. However, it is impossible to raise the frequency to such an extent due to the operating speed characteristics of the switching elements that make up the inverter, and even if it is possible, the frequency will be extremely high and the skin effect will be applied to the induction heating coil. However, there is a drawback that the actual resistance increases suddenly, resulting in extremely low efficiency.

[Object of the Invention] Therefore, an object of the present invention is to make a pan made of a material having a small relative permeability and a specific resistance such as aluminum or copper from a material having a large relative permeability or a specific resistance such as iron or non-magnetic stainless steel. Another object of the present invention is to provide an induction heating cooker that can perform induction heating as in the case of the Nari pot.

[Outline of the Invention] In order to prevent the input power of the induction heating coil from largely changing depending on the material of the pan, the present invention determines the magnitude of the high frequency voltage applied to the induction heating coil and the input impedance of the induction heating coil. By adopting a configuration that can be switched according to the material, even a pan with a small skin resistance can be induction-heated without any trouble, like a pan with a large skin resistance.

[Embodiment of the Invention] First, description will be given of each embodiment of the present invention. First, as a variable impedance means, a first embodiment in which both the number of turns and the frequency of the induction heating coil are switched at the same time will be described with reference to FIGS. Will be described. Reference numeral 1 denotes a variable voltage type DC power supply device, which comprises a rectifier circuit 3 for receiving AC power from a commercial power supply 2 and converting it into DC power, a smoothing capacitor 4 and a choke coil 5. And the rectifier circuit 3 has 2
It is composed of a diode 6 and two thyristors 7 forming a voltage switching means. By controlling the phase of the thyristor 7 by a phase control circuit 8, an input voltage between the busbars 9a and 9b of the inverter 9 as a high frequency power supply unit. ei is made variable. On the other hand, as shown in FIG. 2, the induction heating coil 12 for inductively heating the pan 11 placed on the top plate 10 can switch the number of windings so as to also serve as impedance varying means for switching the input impedance of the pan 11 depending on the material of the pan 11. To this end, the first coil 13 and the second coil 14 are provided, and these are arranged below the top plate 10 in two layers. The number of turns of the first coil 13 is N ₁ , and the number of turns of the second coil 14 in series with the first coil 13 is N ₂ (where N ₁ <N ₂ ).
Is set to be.

In addition, the induction heating coil 12 includes the two switching transistors 15 and 16 and the first resonance capacitor 1 which also serves as input impedance varying means and thus frequency varying means.
The current resonance push-pull inverter 9 is configured by the seventh and second resonance capacitors 18. Reference numeral 19 is a load detection circuit having an output contact 20. The output contact 20 connects the first and second induction heating coils 13 and 14 in series when the contact terminals (20a to 20b) are turned on, and the coils 13 and 14 and the first and second resonance capacitors are connected. 17 and 18 are connected in series between the emitter-collector common connection point P _{1 of} both switching transistors 15 and 16 and the bus bar 9b to form a low skin resistance operating state, and contact terminals (20a-20c). ) Is on, the first induction heating coil 13 and the first resonance capacitor 17 are interposed in series between the common connection point P1 and the bus bar 9b to form a connection formed in a high skin resistance operating state. Has become. The phase detection circuit 21 outputs the inverter output voltage e
₀ , that is, the phase between the voltage at the common connection point P ₁ of both switching transistors 15 and 16 and the current passing through the induction heating coil 13 (or both coils 13 and 14) is detected, and the phase is always 90 °. The inverter drive circuit 22 alternately turns on and off the switching transistors 15 and 16 at such timing. This prevents the resonance frequency of the inverter 9 from varying due to subtle differences in the material of the pot 11 and variations in the mounting state. On the other hand, the current path of the first resonance capacitor 17 is provided with a current transformer 23, the output of which is given to the load detection circuit 19, so that the material of the pot 11 placed on the top plate 10, that is, the skin resistance The size is judged.

Next, the operation of the above configuration will be described. First, when the pot 11 which is a load is made of a material having a high skin resistance such as iron, the current value detected by the current transformer 23 is small, so that the load detection circuit 19 has the contact terminals (20a-20b) of the output contact 20.
The circuit is closed and a voltage switching signal is given to the phase control circuit 8. As a result, the induction heating cork 12 has the number of turns N ₁
The first resonance capacitor 1 is switched to a low input impedance because the first coil 13 is selected.
Since 7 is selected, the output frequency of the inverter 9 is switched to the lower value f ₁ and the thyristor 7 of the rectifier circuit 3 is phase-controlled to the higher value e ₁ of the input voltage ei of the inverter 9. Can be switched. When the first switching transistor 15 and the second switching transistor 16 are alternately turned on and off, the first coil 13 and the first switching transistor 15 are turned on when the switching transistor 15 is turned on.
A voltage is applied to the resonance capacitor 17 of the inverter, energy is exchanged in the LC circuit when the switching transistor 16 is turned on, and feedback control is applied by the phase detection circuit 21 to cause the inverter 9 to operate at the frequency f ₁
Driven by, a high-frequency current of f ₁ flows through the _first coil 13, and the pan 11 is induction-heated.

On the other hand, the contact terminal of the output contact 20 (20a-20
When the aluminum pot 11 is placed on the top plate 10 when the space between c) is closed, the skin resistance of the aluminum pot 11 is extremely small, so that the output of the current transformer 23 becomes large, which causes the load detection circuit 19 to automatically operate. to the output contact 20 of the inverter input voltage ei by the phase control circuit 8 with Kirikaeraru as between contact terminals (20a-20b) is closed from _{e 1 e 2} (out _e 1> _{e 2)} to viz low voltage Switch to. At the same time, the induction heating coil 12 is switched to the state of the number of turns N ₂ in which the first coil 13 and the second coil 14 are connected in series to be in the state of high input impedance, and the first and second resonance capacitors 17 are also provided. , 18 is selected, the frequency f _{2 of the} inverter 9 (however, f ₁ <f ₂ )
Will be increased.

In this way, the material of the pot 11 is changed from a material having a high skin resistance to a material having a low skin resistance, and at the same time, the output voltage of the inverter is switched to a low value, and at the same time, the output contact 20 makes the contact terminals (20a ...
Since the input impedance of the induction heating coil 12 is increased by an increase in the number of turns and an increase in the inverter output frequency by switching from the closed circuit between 20c) to (20a to 20b), the input power of the induction heating coil 12 is increased. It is possible to suppress an increase due to switching from a material having a high skin resistance to a material having a low skin resistance to an increase that does not pose a practical problem.

By the way, the relationship between input power and voltage is as follows.
That is, as an explanation thereof, FIG. 3 shows waveforms of the input voltage ei and the output voltage e ₀ of the inverter 9 (in this example, it is assumed that there is no voltage drop between the input and output of the inverter) and the load current i _L. Has been done. The input power Pai per cycle of the inverter 9 is expressed as follows.

Where _{i L (t) = i p} sinωt i P = 1 / R L · 2 / π · e R L: Load resistance because viewed from the input terminal of the induction heating coil 12, Input power Pb _i when the voltage e _i is not sufficiently smoothed
Is expressed by the following equation because the voltage includes the ripple of the commercial power frequency.

Here, T is the cycle of the commercial power supply voltage, and E _i is the effective value of the inverter input voltage e _i . As is clear from the equation (5), it is understood that the range of the variation of the input power due to the difference in the skin resistance of the pan 11 can be reduced by switching the input voltage.
However, if you try to equalize the input power only by switching the voltage, you have to set a large voltage difference between the actual product made of iron and aluminum, and the maximum voltage that can actually be obtained is made iron. If they are combined, the voltage required for aluminum is too low, which hinders the driving of the transistors 15 and 16. Therefore, in the present invention, as described above, the input impedance of the induction heating coil 12 is switched to be higher in the case of aluminum than in the case of iron, and the voltage difference is reduced. As a result, the control range of the voltage can be narrowed, and the configuration of the power supply unit can be simplified.

By the way, in the configuration in which the number of turns is used together as in the above embodiment, the voltage ratio for making the input power the same without changing the number of turns between the iron pot and the aluminum pot is about 4: 1. On the other hand, if the number of turns is switched at a ratio of 1: 2, the voltage ratio will be 2: 1. Therefore, according to the above device, it is not necessary to raise the frequency extremely unlike the frequency-only switching system, and there is no instability of the transistor as in the voltage-only switching system. Become.

Next, a second embodiment of the present invention will be described with reference to FIG. 4 showing the same parts as those in FIG. That is, in this embodiment, the DC power supply device 24 has two capacitors 25,
26 and two thyristors 27 and 28 for rectifying and varying the voltage (which form a voltage switching means) form a voltage doubler rectifying circuit configuration, thereby widening the voltage switching range and switching the number of turns of the induction heating coil 29. Without capacitor 1
The inverter output frequency is switched only by switching the total capacity of 7 and 18.

In other words, in the embodiment shown in FIG. 1, the input impedance of the heating induction coil is controlled by both the number of turns switching and the frequency switching, whereas the second embodiment shown in FIG.
In the embodiment, the input impedance is switched by frequency switching, and as the switching ratio of the input impedance becomes smaller, the DC power supply unit 24 is made a double voltage type and the shortage is compensated by expanding the voltage ratio. I am trying.

Needless to say, the present invention is not limited to the above-described embodiment. In particular, in the above-mentioned embodiment, the induction heating coil and the resonance capacitor are components of the inverter. It is needless to say that the variable frequency type may be formed completely independently of the capacitor for use.

〔The invention's effect〕

As described above, the present invention can suppress the difference in input power due to the difference in the skin resistance of the pan to a small value by switching the voltage applied to the induction heating coil and its input impedance according to the size of the skin resistance of the pan. It is possible to provide an induction heating cooker having a simpler configuration than a method of switching only voltage or only switching frequency.

[Brief description of drawings]

FIG. 1 is a connection diagram showing a first embodiment of the present invention, FIG. 2 is a schematic side view showing a positional relationship of essential parts, FIG. 3 is a voltage / current waveform diagram, and FIG. 4 is a diagram showing the present invention. It is a connection diagram which shows 2 Example. In the figure, 1 and 24 are DC power supply devices, 7, 27 and 28 are thyristors (voltage switching means), 8 is a phase control circuit, 9 is an inverter (high frequency power supply unit), 11 is a pot, 12 and 29.
Is an induction heating coil, 13 and 14 are first and second coils (input impedance varying means), 15 and 16 are switching transistors, 17 and 18 are first and second resonance capacitors (input impedance varying means), 19
Is a load detection circuit, 20 is an output contact, 21 is a phase detection circuit, and 22 is an inverter drive circuit.

Claims (4)

[Claims] 1. An induction heating coil for inductively heating a pan, a high frequency power supply unit for supplying a high frequency current to the induction heating coil, and an output voltage of the high frequency power supply unit depending on the material of the pan. An induction heating cooker comprising voltage switching means for switching and impedance varying means for switching the input impedance of the induction heating coil according to the material of the pot. 2. The induction heating cooker according to claim 1, wherein the impedance varying means switches the output frequency of the high frequency power source. 3. The induction heating cooker according to claim 1, wherein the impedance varying means switches the number of turns of the induction heating coil. 4. The induction heating cooker according to claim 1, wherein the impedance varying means simultaneously switches the output frequency of the high frequency power source and the number of turns of the induction heating coil.