JPH01684A - Control device for high frequency induction heating - 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 [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] A conventional control device for high frequency induction heating of this type is shown in FIG. 2. In the device shown in FIG. A series strong vibration circuit is arranged.

Using an inverter 2, the output frequency of the inverter 2 is determined by a frequency command signal (
The power supplied to the load is controlled by controlling according to the output command signal and the KW command signal.

In this case, load impedance 2 seen from inverter 2
The relationship between the output frequency f of the inverter 2 and the inductance L of the load is shown in FIG.

Assuming that the natural resonance frequency f is determined by the resistor R and the matching capacitor C, the inverter 2 controls the output frequency f of the inverter 2 in the left half region (leading power factor) of the curve in Figure 3. Then, the load impedance Z seen from the inverter 2 is changed (see fe, Zo), and the inverter 2
By changing the output current of the load (I”
R) is controlled. When the load current exceeds a certain level, a limit is applied to prevent the frequency from increasing beyond that point, that is, from lowering the impedance 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 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, and 8 is a control device for the inverter 2 based on the signal from the deviation device 7. A high frequency phase shifter outputs a signal, 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 is on the verge of decreasing, the output frequency of the inverter 2 is 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, and furthermore, if the applied frequency becomes too low, transformers etc. will cause magnetic saturation and burn out. There is a risk.

The present invention has been 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. It is an object of the present invention to provide a high-frequency induction heating control device that sets a lower limit frequency in the range of 1 to 1 to prevent the frequency from decreasing further.

[Means for Solving the Problems] Therefore, the high-frequency induction heating control device of the present invention includes a comparison means for comparing the output frequency of the inverter and a predetermined lowest frequency, and a comparison means for comparing the output frequency with the predetermined lowest frequency. The present invention is characterized by the provision of a limit circuit that adds a correction signal in the addition direction to the frequency command signal when it is detected that the frequency has become lower than the lowest frequency.

[Function] In the high-frequency induction heating control device of the present invention described above, the output frequency of the inverter is compared with a predetermined minimum frequency by the comparison means, and the comparison means makes the output frequency smaller than the minimum frequency. When this is detected, a correction signal is added to the frequency command signal in the direction of addition (increasing the frequency) using the limit circuit. The invention will be specifically explained. FIG. 1 is a schematic configuration diagram showing a high-frequency induction heating control device as an embodiment of the present invention. In this embodiment, a matching capacitor C is connected in series with the inductance L of the induction furnace or heater 1. 1. Using an inverter 2 in which C2 and a high frequency transformer T are arranged, this inverter 2
The power supplied to the load is controlled by controlling the output frequency of the motor according to a frequency command signal from an output command device (KW command device) 3.

Also 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 the third
As shown in the figure, inverter 2 controls the output frequency f of Inno 5-2 in the left half region (leading power factor) of the natural resonance frequency f* shown in Figure 3, and By changing the load impedance Z and changing the output current of the inverter 2, the load power (1”R
) is controlled. 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, to prevent the impedance from decreasing any further.In other words, the signal from the KW command unit 3 The high frequency phase shifter 8 is corrected by the output feedback signal (KW feedback signal) and current feedback signal.
This is the same as in the conventional system.

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

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

First, the output frequency f of the inverter 2 and a predetermined minimum frequency fmin (this minimum frequency is, for example, 5 of the rated frequency)
Comparison means is provided for comparing the percentage (set to about 0%). That is, an F/V converter 12 that converts output frequency information of the inverter 2 into voltage, and a reference voltage generator 13 that outputs a voltage signal corresponding to the lowest frequency information are provided, and these F/V conversions Vessel 12. A signal from the reference voltage generator 13 is 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 is smaller than the lowest frequency fmin, that is, the F/V converter output is When the voltage becomes smaller than the reference voltage generator output, a signal to that effect is output.

The signal from the comparator 11 is sent to a limit circuit that applies a correction signal in the addition direction to the frequency command signal when the comparator 11 detects that the output frequency f has become smaller than the lowest frequency f min. It is designed to be input to . That is, a sign inversion circuit 14 that receives a signal from the comparator 11 and inverts its sign is provided, and a PI gain setter 15 that applies a PI gain to the signal from the sign inversion circuit 14 and outputs a correction signal. is provided. 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 the sign inverting circuit 14 does not output a signal when the sign of the output from the comparator 11 is positive.

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 minimum frequency fmin are compared by the comparison means, and the output frequency f is determined to be the minimum frequency fmin by the comparison means.
When it is detected that the frequency has become smaller, the limit circuit adds a correction signal in the addition direction to the frequency command signal (KW command), so that a first-order effect or advantage 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 high frequency transformer T for output matching.

Furthermore, even if a fixed value of a correction signal is added to the KW command when the output frequency f becomes lower than the lowest frequency fmin, the value will be corrected according to the difference between the output frequency f and the lowest frequency fmin. The signal may be added to the KW command.

〔Effect 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.

It is possible to set a lower frequency limit within a certain range for the rated frequency to prevent the frequency from decreasing further, making it possible to limit the frequency range of the connected equipment.
This device can be configured with just the right amount of equipment, which is advantageous in terms of equipment cost and downsizing, and even when a high-frequency transformer is used for output matching, the magnetic flux density of the transformer can be reduced to more than necessary. It has the advantage that it is not necessary to prevent magnetic saturation by lowering the magnetic field.

[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, and Fig. 2-3 shows a conventional high-frequency induction heating control device. The schematic configuration diagram, FIG. 3, is a graph showing the relationship between the 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-P
I gain setter, 6-deviator, 7-PI gain setter, 8-high frequency phase shifter, 9-rectifier, 10-filter, 1
1-Comparator, 12-F/V converter, 13-Reference voltage generator, 14-Sign inversion circuit, 15-PI gain setter, 1
6.-adder, T-high frequency transformer. In addition, in the figures, the same reference numerals indicate the same or equivalent parts.

Claims (1)

[Claims] 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 comparison means for comparing the output frequency of the inverter with a predetermined minimum frequency; Therefore, a limit circuit for applying a correction signal in an addition direction to the frequency command signal when it is detected that the output frequency is lower than the minimum frequency is provided. Control device.