JPH04269488A - Electromagnetic inductive heating cooker - Google Patents

Abstract

PURPOSE:To precisely judge the presence, material and position of a metal vessel by comparing three data of the current value, phase angle and driving frequency of a heating coil. CONSTITUTION:The current sent to a heating coil 2 is detected by a current transfer 8, the voltage applied to the coil 2 from a high frequency inverter driving circuit 4 is detected, and their phase angle is detected by a phase detecting circuit 9. The driving frequency of an high frequency inverter circuit 3 is swept by a main control part 11 so that the phase angle is a fixed value, and the driving frequency at the time of reaching the target phase angle is detected by a frequency detecting circuit 10. The current value sent to the coil 2 at this time is also detected by a coil current detecting circuit 13. The detected driving frequency and current value are judged by matrix, whereby the shape difference and position to put of a metal vessel can precisely be judged.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electromagnetic induction heating cooker, and relates to determining the presence or absence of a metal container used therein, the position thereof, and the material of the metal container.

0002

2. Description of the Related Art FIG. 6 is a block diagram showing detection means for determining the presence or absence of a metal container and the material of the container in a conventional electromagnetic induction cooking device. In the figure, an alternating current from a commercial power supply 6 is rectified and smoothed by a smoothing circuit (not shown), and converts the direct current into a single-ended voltage, for example.
It is supplied to a push-pull type high frequency inverter circuit 3. An input variable circuit 5 is provided in the middle, which will be explained later. This high frequency inverter circuit 3 includes a pair of transistors 14a and 14b and flywheel diodes 15a and 15b. The inverter circuit 3 is driven by an inverter drive circuit 4,
A high frequency current is passed through the heating coil 2 to generate a high frequency alternating magnetic field. As a result, an eddy current is generated in the metal container 1 placed on the heating coil 2, and the metal container 1 is heated by this eddy current. The capacitor 7 constitutes a series resonant circuit together with the heating coil 2. The main control unit 11 is the input variable circuit 5
, are connected to the inverter drive circuit 4. Further, the inverter drive circuit 4 is connected to the main control section 11 via a frequency detection circuit 10 and a phase detection circuit 9 parallel thereto. The output of a current transformer 8 provided in a part of the circuit to the heating coil 2 is supplied to a phase detection circuit 9.

[0003] In the conventional configuration described above, the presence or absence of the metal container 1, the material, etc. are determined in the following manner. first,
In the phase detection circuit 9, the current flowing through the heating coil 2 is detected by a current transformer 8, etc., and the output voltage of the high frequency inverter circuit 3, that is, the inverter drive circuit 4 is detected.
By detecting the drive signal, the phase angle of the voltage and current applied to the heating coil 2 is detected. The main control unit 11 sweeps the drive frequency of the high frequency inverter circuit 3 from a high frequency to a low frequency until the phase angle becomes 0, and reaches the frequency when the phase angle becomes 0, that is, the resonance frequency. Stop the sweep at a point. The frequency at this time is measured by the frequency detection circuit 10.

As shown in FIG. 7, the resonance frequency varies depending on the presence or absence of the metal container 1 and the material.
By measuring this resonance frequency, the presence or absence of a metal container, the material, etc. were determined. In FIG. 7, the vertical axis represents the heating coil current, and the horizontal axis represents the frequency. f0 is the resonant frequency when no load is applied, f1 is the resonant frequency for magnetic materials such as iron, and f2 is the resonant frequency for non-magnetic materials such as aluminum.

FIG. 9a shows an equivalent circuit of the heating coil 2 and the metal container 1 driven by a high frequency power source. In the figure, R1 is the resistance of the heating coil 2, L1 is the inductance of the heating coil 2, R2 is the skin resistance of the metal container 1, and L2 is the resistance of the heating coil 2.
is the inductance of the metal container 1, M is the mutual inductance between the heating coil 2 and the metal container 1, and E is the high frequency power source.

When FIG. 9a is converted to FIG. 9b, the equivalent inductance L3 of the coupling circuit between the heating coil 2 and the metal container 1 is:

[0007]

[Math 1]

[0008] Here, L3 is a function of the time constant τ of the metal container 1, and the resonant frequency f0 of the series resonant circuit composed of this L3 and the resonant capacitor C1 is:

[0009]

[Math 2]

[0010] For example, when the metal container 1 is in an unloaded state where it is not placed on the heating coil 2, L3 =
Since L1 and L3 become larger, the resonant frequency f
0 is low. Furthermore, the value of L3 changes depending on whether the metal container 1 is made of a magnetic material such as iron or a non-magnetic material such as aluminum, so the resonance frequency f0 changes. For this reason, in the past, the difference in resonance frequency was used to
The presence or absence of the material and the material were determined.

[0011]

The conventional method for determining the presence or absence of the metal container 1 and its material as described above involves the following problems because the determination is based on the difference in resonance frequency.

First, since the heating coil 2 and the resonance capacitor 7 shown in FIG. 6 are connected in series to form a series resonant circuit, the impedance becomes minimum at the resonance point, and therefore a large current flows through the heating coil 2. It was a flow problem. To solve this problem, it was necessary to lower the voltage applied to the heating coil 2, and a variable input circuit 5 such as a chopper type inverter or a thyristor was required.

Furthermore, as shown in FIG. 8a, as the shape of the metal container 1 becomes smaller, it approaches the no-load state, and the resonant frequency f2 of the non-magnetic metal container becomes f2', and the resonance frequency f2 of the non-magnetic metal container becomes f2', The resonant frequency f1 of the magnetic metal container becomes f1', which approaches the resonant frequency f0 of the no-load state, making it difficult to distinguish. Conversely, as shown in Fig. 8b, when the shape of the metal container becomes larger, the resonant frequency becomes higher, that is, f1
The magnetic and non-magnetic metal containers could no longer be distinguished from each other because they moved closer to f1'' and f2 to f2''.

Similarly, depending on whether the metal container 1 is placed at the center of the heating coil 2 or off-center, the difference in the size of the metal container 1 shown in FIG. , b, the resonance frequency changed, making it difficult to distinguish.

Furthermore, as shown in FIG. 8c, in the case of a material that is intermediate between a non-magnetic material and a magnetic material, such as stainless steel, its resonance frequency f3 is close to either of the two, making it difficult to distinguish between magnetic and magnetic materials. If it is determined that it is a body, no input will be received,
There was a problem in that too much input was received if the material was determined to be non-magnetic.

[0016]

[Means for Solving the Problems] The present invention has been made to eliminate the above-mentioned drawbacks, and does not determine the presence or absence of a metal container and the material of the container using only the resonant frequency by setting the phase angle to 0 as in the past. Furthermore, current detection means for detecting the value of the current flowing through the heating coil is provided, and the determination is made by comparing this current value, the phase angle, and the drive frequency.

[0017]

[Operation] By comparing the three data of the current value of the heating coil, the phase angle, and the drive frequency, the present invention can more accurately determine the presence or absence of a metal container, its material, position, etc.

[0018]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a block diagram showing an embodiment of the present invention. The difference from the conventional example shown in FIG. 6 is that a coil current detection circuit 13 is provided between the current transformer 8 and the main control section 11, and that the variable input circuit 5 is omitted. Further, some functions of the main control section 11 are added. Other heating-related operations are the same as in FIG. 6.

Next, a method for determining the presence or absence of the metal container 1 and its material will be explained. First, a case will be described in which the determination is made based on the drive frequency and current value at a certain predetermined phase angle.

The current flowing through the heating coil 2 is detected by a current transformer 8 or the like, the voltage applied to the heating coil 2 is detected by the high frequency inverter drive circuit 4, and their phase angles are detected by a phase detection circuit 9. The driving frequency of the high frequency inverter circuit 3 is swept by the main controller 11 so that the phase angle between the current and the voltage becomes a certain constant value. Then, the frequency detection circuit 10 detects the driving frequency when the target phase angle is reached. At this time, the value of the current flowing through the heating coil 2 is also
Detected by 3.

By detecting the driving frequency while keeping the phase angle constant, the resonance point can be removed and the value of the current flowing through the heating coil 2 can also be lowered.

From the equivalent circuit of FIG. 9b, the impedance Z3 seen from the power supply side is:

[0023]

[Math 3]

[0024] L3 is given by formula (1), and R3 is given by the formula below,

[0025]

[Math 4]

##EQU1## Z3 is also a function of the coupling coefficient K between the heating coil and the metal container and the time constant τ of the metal container. From this, the current IL flowing through the heating coil is:

0027
]

[Math 5]

[0028] Here, phase angle θ and driving frequency f
From the equivalent circuit of FIG. 9b, the relationship is as shown below.

[0029]

[Math 6]

However, ω=2πf. As mentioned above, L3 and R3 change depending on the presence or absence of the metal container 1, the material, etc., so if a certain phase angle θ is determined from the above equation, the driving frequency f is also uniquely determined.

By distinguishing the two values of the detected driving frequency and current value using a matrix, even if the frequencies are close to each other due to difficult-to-distinguish materials such as stainless steel, differences in the shape of the metal container, placement position, etc. These can be accurately determined by determining the difference in current values.

FIG. 2 is a graph of this relationship. In the figure, f0, f1, f2, f3, respectively, represent the resonance frequencies in the case of no load, iron, aluminum, and stainless steel at a phase angle of 0, and F0, F1
, F2 and F3 represent respective drive frequencies when fixed at a certain phase angle θa. As shown in the figure, the resonant frequency f2 of aluminum and the resonant frequency f3 of stainless steel are similar, but if the phase angle is properly set, the driving frequencies at that time can be clarified as F2 and F3.

Furthermore, since it cannot be completely clarified by setting the phase angle alone, the drive can be determined by taking advantage of the fact that the current value flowing through the heating coil 2 also changes depending on the presence or absence of the metal container 1 and its material, as shown in equation (5). The frequency and current value are determined using a matrix as shown in FIG.

Line a indicating the characteristics is for non-magnetic materials such as aluminum, line b is between non-magnetic materials such as stainless steel and magnetic materials, and line c is for magnetic materials such as iron. It is. This makes it possible to accurately identify materials that are intermediate between magnetic and non-magnetic materials, such as stainless steel.

The above-described determination is performed by the main control section 11 and can be displayed by an appropriate display means.

Furthermore, the boundaries on both sides of lines a, b, and c shown in the figure represent the range of variation when the shape of the metal container is changed or when the metal container is shifted from the center of the heating coil. . For example, when the shape of the metal container becomes smaller or when it is placed at a position shifted from the center, the drive frequency is moved to the higher side.

[0037] As described above, it is no longer difficult to distinguish the material, and there is no possibility of misjudgment due to shape or position.

Furthermore, in the above explanation, discrimination was made using a binary matrix of drive frequency and current value when the phase angle was kept constant, but as shown in FIGS. 4 and 5, when the frequency is kept at a certain constant value Fa. It can be determined in the same way by fixing it and determining it using a matrix based on the phase angle and current value for each material at that time. 4 corresponds to FIG. 2, and FIG. 5 corresponds to FIG. 3. The characteristic line a' is for a non-magnetic material, and the line b' is for an intermediate point between a non-magnetic material and a magnetic material.
c' is for magnetic material.

The accuracy can be further improved by switching between the methods shown in FIGS. 2 and 3 and the methods shown in FIGS. 4 and 5, or by using both of them.

[0040]

As described above, according to the discrimination method of the present invention, discrimination does not become difficult even if the shape of the metal container becomes larger or smaller;
Even if the position of the metal container is off the center of the heating coil,
It is possible to accurately determine the no-load state or the material of the metal container. Furthermore, it is also possible to discriminate materials between magnetic and non-magnetic materials, such as stainless steel, which was difficult to distinguish using conventional discrimination methods.

[0041] Therefore, it is possible to accurately inform those using the cooker of the material of the metal container and the unloaded state.
Furthermore, since the material can be determined in more detail than in the past, optimal heating and cooking can be performed for the metal container made of each material.

Furthermore, the variable input circuit can be omitted.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing one embodiment of the present invention.

FIG. 2 is a graph showing the relationship between phase angle and drive frequency.

FIG. 3 is a matrix diagram of driving frequency and current value when the phase angle is kept constant.

FIG. 4 is a graph showing the relationship between phase angle and drive frequency.

FIG. 5 is a matrix diagram of phase angles and current values when the driving frequency is kept constant.

FIG. 6 is a block diagram showing a conventional example.

FIG. 7 is a graph showing the difference in resonance frequency depending on the material.

FIGS. 8A, 8B, and 8C are graphs showing changes in resonance frequency when the shape, position, and material of the metal container change, respectively.

FIGS. 9a and 9b are equivalent circuit diagrams of a heating coil and a metal container, respectively.

[Explanation of symbols]

1 Metal container 2 Heating coil 3 High frequency inverter circuit 4 Inverter drive circuit 7 Capacitor 8 Current transformer 9 Phase detection circuit 10 Frequency detection circuit 11 Main control section 13 Coil current detection circuit

Claims (1)

[Claims] 1. A heating coil, a high-frequency inverter that supplies a high-frequency current to the heating coil, current detection means for detecting a current value flowing through the coil, and frequency detection means for detecting a driving frequency of the high-frequency inverter. A phase detection means detects the phase angle of the current and voltage flowing through the heating coil, and the data detected by the detection means is compared to determine the presence, material, position, etc. of a metal container that causes eddy currents to be generated by the heating coil. An electromagnetic induction heating cooker having a determining means.