JPH0475633B2 - - Google Patents

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 a load on a top plate, that is, a pot, thereby producing an eddy current in the pot. The present invention relates to an induction heating cooker that performs cooking using self-heating of a pot based on heat loss.

[Technical background of the invention and its problems]

Recently, induction heating cookers that can heat not only iron and 18-8 stainless steel pots but also aluminum and copper pots have been developed and are already in practical use.

By the way, when cooking in an induction heating cooker,
The current flowing through the pot will be in the opposite direction to the current flowing through the heating coil. For this reason, there is a problem in that a repulsive force acts between the pot and the heating coil, and if the pot (including food) is light, the pot may float off the top plate.

However, if the material of the pot is iron with high magnetic permeability,
The magnetic force between the pot and the heating coil is large, and therefore a strong attractive force acts there, so even if the above-mentioned repulsive force occurs, the pot will not float due to it.

However, when the material of the pot is low permeability and low resistance, such as aluminum or copper, a large current is passed through the pot in order to increase the input resistance of the heating coil to the same level as that of an iron pot. The repulsive force between the two becomes extremely large. Furthermore, since aluminum and copper are specific magnetic materials, there is no magnetic attraction between the pot and the heating coil.
Therefore, in the case of a light pot made of aluminum or copper, the pot may float off the top plate, and if the top plate is even slightly tilted, the pot may move on the top plate, which is dangerous. . Furthermore, if the pot on the top plate deviates from its correct setting position, proper heating will not be possible and the quality of the cooking will be adversely affected.

[Purpose of the invention]

This invention was made in view of the above circumstances, and its purpose is to accurately detect mechanical movement of a load, which tends to occur in the case of loads with low resistance and low magnetic permeability. Moreover, it is an object of the present invention to provide a highly reliable induction heating cooker that can immediately stop cooking to ensure safety when the load moves, and thereby always enable only proper heating.

[Summary of the invention]

In this invention, when the positional relationship between the heating coil and the load changes, the inductance of the heating coil changes.
This method focuses on the fact that the frequency of the high-frequency current flowing through the heating coil changes as a result, and it is a detection means that detects the resonant frequency of the resonant circuit after a load with low resistance and low magnetic permeability is placed on the top plate. and determining means for determining that the load has moved due to the repulsive force generated between the heating coil and the heating coil when the resonance frequency detected by the detecting means changes, and the determining means determines that the load has moved. When this is determined, cooking can be stopped immediately to ensure safety.

[Embodiments of the invention]

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

In FIG. 1, reference numeral 1 denotes a commercial AC power source, and a rectifier circuit 5 consisting of a diode bridge 3 and a smoothing capacitor 4 is connected to this power source 1 via a control switch (relay contact) 2b. Rectifier circuit 5
At the output end of is a switching element.
A series circuit between the collector and emitter of the NPN transistor 6 and the collector and emitter of the NPN transistor 7 is connected. One end of a heating coil 8 is connected to the collector of the transistor 7, and the other end of the heating coil 8 is connected to one end of a resonance capacitor 9. The other end of the resonance capacitor 9 is connected to the negative output end of the rectifier circuit 5. That is, the heating coil 8 and the resonance capacitor 9 constitute a series resonance circuit. Furthermore, a half-bridge type inverter circuit is constructed mainly of a rectifier circuit 5 and transistors 6 and 7 for exciting the above-mentioned resonant circuit. The heating coil 8 is provided on the back side of a top plate (not shown), facing away from it, and a pot 10 serving as a load is set at a predetermined position on the top plate.

Thus, the resonant circuit is provided with a current transformer 20 for detecting high frequency current, and the output end of this current transformer 20 is connected to a phase detection circuit 21 and a frequency change detection circuit 30. The phase detection circuit 21 detects the phase of the high frequency current flowing through the heating coil 8 based on the output voltage of the current transformer 20, and the detection result is supplied to the inverter drive circuit 22. This inverter drive circuit 22 drives the transistors 6 and 7 alternately on and off at timings according to the detection results of the phase detection circuit 21. moreover,
The frequency change detection circuit 30 first detects the frequency of the high frequency current flowing through the heating coil 8 based on the output voltage of the current transformer 20, and then detects a change in the detected frequency. The detection result of the frequency change detection circuit 30 is supplied to the safety circuit 90. The safety circuit 90 turns off the control switch 2b in response to detecting even a slight change in frequency by the frequency change detection circuit 30, thereby stopping the operation of the inverter circuit.

Here, a specific example of the frequency change detection circuit 30 and the safety circuit 90 is shown in FIG.

First, the frequency change detection circuit 30 will be explained. The output voltage of the current transformer 20 is applied to a resistor 31, and the voltage generated across the resistor 31 is inverted and shaped into a rectangular wave by a waveform shaping circuit 32 to form a frequency-voltage conversion circuit (hereinafter abbreviated as F-V conversion circuit). ) 40. This F-V conversion circuit 4
0 consists of a monostable multivibrator circuit 41 which is sequentially triggered by the logic "1" output of the waveform shaping circuit 32, and an average value circuit 42 which is responsive to the inverted output of this monostable multivibrator. The average value circuit 42 connects NPN via a resistor 43.
A constant DC voltage is applied between the collector and emitter of a type transistor 44, a capacitor 46 is connected to the collector of the transistor 44 via a resistor 45, and the base of the transistor 44 is connected to the inverting output terminal of the monostable multivibrator. When the inverted output of the monostable multivibrator 41 is logic "0", it forms a charging path for the capacitor 46, and when it is logic "1", it forms a discharging path for the capacitor 46. Thus, the output of the F-V conversion circuit 40 is supplied to a sample hold circuit 50. This sample and hold circuit 50 consists of a voltage follower (gain is "1") 51, an analog switch 52, and a capacitor 53.
2 is turned on, the output voltage of the F-V conversion circuit 40 is held in the capacitor 53.

Reference numeral 60 denotes an RC oscillation circuit, which includes an operational amplifier 61, a capacitor 62, resistors 63, 64, 65, 66, and a diode 67, and generates a rectangular wave signal of a predetermined frequency. In this case, the resistor 65,
The “1” and “0” duty of the oscillation output is determined by the resistance value of the resistor 66, and by making the resistance value of the resistor 65 lower than the resistance value of the resistor 66, the “1” and “0” duty of the oscillation output is determined.
The “0” period is longer than the period. Thus, the output of the RC oscillation circuit 60 is supplied to the analog switch 52 in the sample hold circuit 50. That is, when the output of the RC oscillation circuit 60 is logic "1", the analog switch 52 is turned on,
Sample hold is now in place.

The output of the sample and hold circuit 50 is applied to a series circuit of resistors 71 and 72 via a voltage follower 70, and the voltage generated at the interconnection point P of the resistors 71 and 72 is applied to the non-inverting input terminal of the comparator 80 ( +). Furthermore, the output voltage of the F-V conversion circuit 40 is supplied to the inverting input terminal (-) of the comparator 80.

On the other hand, in the safety circuit 90, 2 is a relay having the control switch 2a;
A DC voltage is applied between the collector and emitter of the NPN transistor 91. and,
The base of transistor 91 is connected to R through resistor 92.
-S is connected to the output terminal Q of the flip-flop 93, and the input terminal S of the flip-flop 93 is connected to the output terminal of the comparator 80 in the frequency change detection circuit 30. Note that 94 is a diode for preventing back electromotive force.

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

First, the imaginary center frequency of the resonant circuit consisting of the heating coil 8 and the resonant capacitor 9 is expressed by the following formula.

Note that L is the inductance of the heating coil 8, and C
is the capacitance of the resonance capacitor 9.

That is, when the pot 10 on the top plate is started, the degree of magnetic coupling between the heating coil 8 and the pot 10 changes, and the inductance L of the heating coil 8 changes. When the inductance L changes, as is clear from the above equation, the resonance frequency changes, which appears as a change in the frequency of the high-frequency current flowing through the heating coil 8.

Then, the pot 10 is placed on the top plate and the power source 1 is turned on. Then, the inverter drive circuit 22 operates, the resonant circuit is excited, and a high frequency current flows through the heating coil 8. In this way, a high frequency magnetic field is emitted from the heating coil 8, which is transmitted to the pot 10.
The pot 10 generates heat by itself.

At this time, the high frequency current flowing through the heating coil is detected by the current transformer 20, and the output of the current transformer 20 is supplied to the phase detection circuit 21, whereby stable oscillation is performed. Furthermore, the output of current transformer 20 is supplied to frequency change detection circuit 30, and the following operations are performed in frequency change detection circuit 30 and safety circuit 90.

The current transformer 20 outputs a voltage corresponding to a high frequency current, and the voltage is inverted and shaped into a rectangular wave by the waveform shaping circuit 32. That is, the waveform shaping circuit 32 outputs a rectangular wave signal corresponding to the frequency of the high frequency current. Then, a voltage at a level corresponding to that frequency is output from the F-V conversion circuit 40 and sequentially held in the sample and hold circuit 50 in synchronization with the output of the RC oscillation circuit 60.
The held voltage is applied to the resistors 71 and 72, and the voltage generated at the connection point P is supplied to the comparator 80. Comparator 80 connects the voltage at connection point P and F-
The output voltage of the V conversion circuit 40 is compared.

In this case, if the pot 10 is in the correct set position on the top plate, the voltage generated at the connection point P will be lower than the output voltage of the F-V conversion circuit 40 by the amount divided by the resistors 71 and 72. Comparator 8
The output of 0 becomes logic "0". If the output of comparator 80 is a logic "0", flip-flop 90 remains reset and relay 2 is not activated, thus continuing heating operation.

By the way, if the pot 10 is made of aluminum or copper with low magnetic permeability and low resistance, and is also light, the pot 10
The repulsive force caused by the current generated in the heating coil 8 and the current flowing through the heating coil 8 causes the pot 10 to float off the top plate, and if the top plate is tilted, the pot 10 will move on the top plate. When the pot 10 moves, the inductance L of the heating coil 8 increases, and the frequency of the high-frequency current decreases. When the frequency decreases, the output voltage of the F-V conversion circuit 40 becomes lower than the voltage at the connection point P due to sample and hold, and the output of the comparator 80 becomes logic "1". When the output of comparator 80 becomes logic "1", flip-flop 93 is set, transistor 91 is turned on, and relay 2 is operated. Then, the control switch 2a is turned off, power to the inverter circuit is cut off, and the heating operation is stopped. In this case, since aluminum and copper are non-magnetic materials, the leakage magnetic flux is very large, and therefore the inductance change of the heating coil 8 and the resulting frequency change of the high-frequency current are minute, but it is difficult to capture it reliably. can.

In this way, when the pot 10 on the top plate moves from its correct setting position, it is immediately detected and the heating operation is stopped, so that, for example, if the pot 10 moves to the edge of the top plate and touches the user. There is no burn, and the pot is 10.
It is safe because heating can be prohibited when the top plate is about to fall off the top plate. Moreover, 1 pot
Continuing to cook with 0 deviated from the correct setting position will not allow proper heating and will have a negative effect on the quality of the cooking, but such inconveniences can be prevented and reliability improved. Improvements can be made.

In the above embodiment, the case where the inverter circuit is a half-bridge type has been described, but the present invention can be similarly implemented for other types of inverter circuits. Further, although the high frequency current is directly detected by the current transformer 20, it may be indirectly detected by the voltage of the heating coil 8 or the voltage of the resonance capacitor 9. Furthermore, if a VCO (voltage controlled oscillator) is used in the inverter drive circuit 22, the input voltage to the VCO corresponds to the frequency of the high-frequency current, and the input voltage is applied to the frequency change detection circuit. 30 inputs may also be used. Also,
The safety circuit 90 is designed to cut off power to the inverter circuit when there is a frequency change, but it can also be used to control the drive of the inverter drive circuit 22 to stop the operation of the inverter circuit or reduce the heating output. Good too.

Furthermore, as the frequency change detection circuit 30, F-V
Although the conversion circuit 40 is used to detect frequency changes in an analog manner, it is also possible to detect frequency changes by, for example, counting the output of the waveform shaping circuit 32 for a certain period of time using a counter and successively comparing the counted values. It may also be detected digitally.

〔Effect of the invention〕

As described above, according to the present invention, there is a detecting means for detecting the resonant frequency of the resonant circuit, and when the resonant frequency detected by the detecting means changes, the repulsive force generated between the load and the heating coil is Since the system is equipped with a means to determine whether the load has moved, if the load moves due to repulsive force, cooking can be stopped immediately to ensure safety.This ensures reliability and ensures proper heating at all times. It is possible to provide an induction heating cooker with excellent performance.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the configuration of a control circuit in an embodiment of the present invention, and FIG. 2 is a diagram showing a specific configuration of the main parts in FIG. 1. 2... Relay, 2a... Control switch (relay contact), 8... Heating coil, 9... Resonance capacitor, 20... Current transformer, 30... Frequency change detection circuit, 40... F-V conversion circuit , 50...
Sample hold circuit, 60...RC oscillation circuit,
80... Comparator, 90... Safety circuit.

Claims (1)

[Claims] 1 Induction heating in which a heating coil inserted in a resonant circuit generates a high-frequency magnetic field, and this high-frequency magnetic field is applied to a load with low resistance and low magnetic permeability on the top plate, thereby inductively heating the load. In the cooker, a detection means detects a resonant frequency of the resonant circuit after the low resistance and low magnetic permeability load is placed on the top plate, and the resonant frequency detected by the detection means changes. An induction heating cooking device characterized by comprising: determining means for determining that the load has moved due to a repulsive force generated between the heating coil and the heating coil.