JPH031793B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a high-frequency induction heating control device that performs power control using an inverter that can adjust the output frequency in response to a frequency command signal.

[Prior art] This type of conventional high-frequency induction heating control device is
Shown in Figure 2. The circuit shown in FIG. 2 includes a series resonant circuit in which a matching capacitor C is arranged in series with the inductance L of the induction furnace or heater 1;
By using an inverter 2 and controlling the output frequency of the inverter 2 according to the frequency command signal (output command signal, KW command signal) from the output command device (KW command device) 3, the power supplied to the load can be controlled. It's in control.

In this case, the relationship between the load impedance Z seen from the inverter 2 and the output frequency f of the inverter 2 is shown in FIG. Now, the load inductance L,
If the natural resonant frequency f ^* is determined by the resistor R and the matching capacitor C, then the inverter 2
In the left half region of the curve in the figure (leading power factor), the output frequency f of inverter 2 is controlled to change the load impedance Z seen from inverter 2 (see f ₀ , Z ₀ ), and the output of inverter 2 is The load power (I ² R) is controlled by changing the current. When the load current exceeds a certain level, a limit is applied at that point to prevent the frequency from increasing any further, that is, from lowering the impedance any further.

In addition, reference numeral 4 in FIG. 2 is a deviation device that outputs the deviation between the output command (KW command) and the output feedback signal (KW feedback signal), and 5 is a deviation device that outputs the deviation between the output command (KW command) and the output feedback signal (KW feedback signal), and 5 is a PI (proportional, 6 is a deviation device that outputs the deviation between the signal from the PI gain setting device 5 and the current feedback signal; 7 is a deviation device that receives the deviation signal from the deviation device 6 and outputs the PI signal. 8 is a high frequency phase shifter that outputs a control signal to the inverter 2 based on the signal from the PI gain setter 7, 9 is a rectifier (DC power supply), and 10 is a filter.

[Problems to be Solved by the Invention] However, in such a conventional high-frequency induction heating control device, when the KW command is narrowed down or when the natural resonance frequency tends to decrease, the output frequency of the inverter 2 becomes lower than that of the device. The frequency may drop too much compared to the rated frequency. In this case, the allowable operating frequency range of parts and equipment connected to the high-frequency output side must be widened; in particular, if the applied frequency becomes too low, there is a risk of magnetic saturation and burnout of transformers, etc. Therefore, it is necessary to select equipment with more margin than necessary, and accordingly,
The problem was that it became large, expensive, and uneconomical.

The present invention was made to solve these problems, and is a high-frequency induction heating control device that performs power control using an inverter that can adjust the output frequency in response to a frequency command signal. The purpose of the present invention is to provide a control device for high-frequency induction heating that can set a lower limit frequency within a certain range, make it possible to economically select high-frequency equipment, and prevent oversaturation of matching transformers, etc. shall be.

[Means for solving the problem] For this reason, the high frequency induction heating device control device of the present invention sets a lower limit frequency below the rated frequency that provides a guideline for selecting the output side connected equipment, and a comparison means for comparing the output frequency with the lower limit frequency; and a limit circuit that applies a correction signal in the addition direction to the frequency command signal when the comparison means detects that the output frequency has become smaller than the lower limit frequency. It has been established.

[Operation] In the present invention, the inverter output frequency is compared with a preset lower limit frequency by the comparison means, and the comparison means detects that the output frequency has become smaller than the lower limit frequency. Then, the limit circuit adds a correction signal to the frequency command in the addition direction (increasing the frequency).

[Embodiments of the Invention] The present invention will be specifically described below with reference to illustrated embodiments. FIG. 1 is a schematic configuration diagram showing a high-frequency induction heating control device as an embodiment of the present invention.
An inverter 2 is used in which a matching capacitor C1, C2 and a high frequency transformer T are arranged in series with an inductance L of By controlling the power supply to the load, the power supplied to the load is controlled.

Also in this case, the load impedance Z seen from the inverter 2 and the output frequency f of the inverter 2
The relationship between inverter 2 is as shown in Figure 3, and the output frequency ^f of inverter 2 is
The power (I ² R) of the load is controlled by controlling the load impedance Z seen from the inverter 2 and changing the output current of the inverter 2. When the load current exceeds a certain level, a limit is applied at that point to prevent the frequency from increasing any further, that is, from lowering the impedance any further. In other words, KW command unit 3
The signal from the inverter 2 passes through the high frequency phase shifter 8 while being corrected by the output feedback signal (KW feedback signal) and current feedback signal.
This is the same as the conventional one.

In FIG. 1, the same reference numerals as in FIG. 3 indicate substantially the same parts.

By the way, this high-frequency induction heating control device differs from conventional ones in the following points.

First, there is a comparison means that compares the output frequency f of the inverter 2 with the lower limit frequency fmin (if the rated frequency is, for example, 500 to 1 KHz, the above fmin is approximately 1/2 of the frequency) that provides a guideline for selecting output side equipment. It is provided. That is, an F/V converter 12 that converts the output frequency information of the inverter 2 into voltage, and a reference voltage generator 13 that outputs a voltage signal corresponding to the lower limit frequency information are provided. Signals from the comparator 12 and the reference voltage generator 13 are input to the comparator 11.
This comparator 11 compares the signal from the F/V converter 12 and the signal from the reference voltage generator 13, and the output frequency f becomes smaller than the lower limit frequency fmin. That is, when the F/V converter output becomes smaller than the reference voltage generator output, a signal to that effect is output.

Then, when the comparator 11 detects that the output frequency f has become smaller than the lower limit frequency fmin, the signal from the comparator 11 is input to a limit circuit that adds a correction signal in the addition direction to the frequency command signal. It is becoming more and more common. That is, comparator 11
A sign inverting circuit 14 is provided which receives a signal from the sign inverting circuit 14 and inverts its sign, and a PI gain setting device 15 which applies a PI gain to the signal from the sign inverting circuit 14 and outputs a correction signal is further provided. There is. Then, the correction signal output from the PI gain setter 15 is added to the KW command signal from the KW command device 3 in an adder 16 and output to the deviation device 4. Note that this sign inversion circuit 14 is connected to the comparator 1.
When the sign of the output from 1 is positive, no signal is output.

As a result, when the sign of the comparator 11 becomes negative, the sign from the sign inversion circuit 14 becomes positive, and only in this case, the PI output adds a signal in the addition direction to the KW command signal.

In this way, the output frequency f of the inverter 2 and the predetermined lower limit frequency fmin are compared by the comparison means, and when the comparison means detects that the output frequency f has become smaller than the lower limit frequency fmin, Since the limit circuit adds a correction signal in the addition direction to the frequency command signal (KW command), the following effects or advantages can be obtained.

First, since the operating frequency of the inverter 2 is determined within a certain range relative to the rated frequency, the frequency range of the connected devices can be limited, and the device can be configured with just the right amount of equipment. As a result, it is advantageous for reducing the cost and size of the equipment.

Furthermore, even when the high frequency transformer T is used for output matching as in this embodiment, there is no need to unnecessarily lower the magnetic flux density of the transformer T to prevent magnetic saturation.

Note that the present invention can also be applied to a device that does not use the frequency transformer T for output matching.

Moreover, even if a correction signal of a constant value is added to the KW command when the output frequency f becomes lower than the lower limit frequency fmin, the correction signal of a value corresponding to the difference between the output frequency f and the lower limit frequency fmin may be added to the KW command.

[Effects of the Invention] As detailed above, according to the high-frequency induction heating control device of the present invention, the high-frequency induction heating control device performs power control using an inverter that can adjust the output frequency in response to a frequency command signal. In the device,
Since it is possible to set a lower limit frequency limit within a certain range for the rated frequency to prevent the frequency from decreasing further, it is possible to limit the frequency range of the connected equipment, and the equipment can be operated with just the right amount of equipment. As a result, it is advantageous to reduce the cost and size of equipment, and even when a high-frequency transformer is used for output matching, it is necessary to lower the magnetic flux density of the transformer than necessary to prevent magnetic saturation. There is an advantage that there is no

[Brief explanation of the drawing]

FIG. 1 is a schematic configuration diagram showing a high-frequency induction heating control device as an embodiment of the present invention.
FIG. 3 shows a conventional high-frequency induction heating control device, FIG. 2 is a schematic configuration diagram thereof, and FIG. 3 is a graph showing the relationship between its output frequency and output impedance. In the figure, 1...induction furnace or heater, 2...
...Inverter, 3...Output command device (KW command device),
4...deviator, 5...PI gain setting device, 6...
Deviation device, 7... PI gain setting device, 8... High frequency phase shifter, 9... Rectifier, 10... Filter, 11
... Comparator, 12 ... F/V converter, 13 ... Reference voltage generator, 14 ... Sign inversion circuit, 15 ...
PI gain setter, 16... Adder, T... High frequency transformer. In addition, in the figures, the same reference numerals indicate the same or equivalent parts.

Claims (1)

[Claims] 1. In a high-frequency induction heating control device that performs power control using an inverter that can adjust the output frequency in response to a frequency command signal, a lower limit frequency is set below the rated frequency that provides a guideline for selecting output-side connected equipment, and the above-mentioned Comparison means for comparing the output frequency of the inverter with the lower limit frequency, and when the comparison means detects that the output frequency has become smaller than the lower limit frequency, the frequency command signal is corrected in the direction of addition. A control device for high frequency induction heating, characterized in that it is provided with a limit circuit for applying a signal.