JPH04101382A - Induction heating apparatus - Google Patents

Abstract

PURPOSE:To realize good temperature rising characteristics without an excess of a target heating temperature on the way of heating by constructing a heating coil with multiple section coils provided along the transporting direction of objects to be heated, and controlling independently power supplied to each section coil. CONSTITUTION:Heating coils 14, 17 are constructed by providing multiple section coils 14 and 17 along the transporting direction of objects 7 to be heated, and electric power supplied to at least one of the section coils 14 and 17 is controlled independently from the other according to processing amount of objects 7 to be heated. When the amount is large the two section coils 14, 17 are provided with the same current, when it is low, the section coil 17 on an exit side is provided with relatively larger current compared with the section coil 14 on an entrance side to prevent a temperature decrease due to a power equivalent to thermal loss. Even in the case that processing weight is varied in a wide range, induction heating with optimal temperature rising characteristics is always performed accordingly.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to an induction heating device that inductively heats an object to be heated while continuously conveying it.

[Conventional technology]

Section 6 includes, for example, this type of conventional induction heating device disclosed in the magazine "Industrial Heating JVO 1, 26, No. 6, P. 72-79," published in November 1989, and the paper "Bipolar power transistor and its application examples." In Figure 0, which is a complete circuit diagram, (1) is a power receiving board connected to a commercial frequency three-phase power supply system, (2) is a transformer connected to the output side of the power receiving board (1), and ( 3) is a high frequency inverter connected to the secondary side of the transformer (2), and (2) is a high frequency inverter (3) connected to the secondary side of the transformer (2).
), a heating coil (51) is a capacitor connected in parallel with the heating coil (51), and (6) is a heating coil (2
) is a pinch roller.

Next, the operation will be explained. The voltage of the commercial frequency three-phase power source received by the power receiving board (1) is lowered to the voltage required by the high frequency inverter (3) by the transformer (2). Then, the high frequency inverter (3) once converts this commercial frequency power into direct current, converts it into a high frequency power necessary for induction heating, and supplies it to the heating coil 4.

The capacitor (5) is for power factor correction and is used for high frequency inverter (
The capacitance value is set to resonate with the inductance of the heating coil (2) at the output frequency of (3).

The object to be heated (7) is usually a steel material on a round bar, which is continuously conveyed by pinch rollers (6), and heated by induction by the alternating magnetic flux generated within the heating coil (4) to raise the temperature.
At the exit portion, the temperature reaches a predetermined temperature, for example, about 1250° C., and the material is taken out and sent to the next step, for example, a forging process. In other words, a control device (not shown) is used to ensure that the target value of the weight to be transported and processed at the opening of the heated object within a certain period of time (hereinafter referred to as processing weight) and the final temperature at the exit of the heating coil (A) is satisfied. This is to control the output of the high frequency inverter (3).

[Problem to be solved by the invention]

Since the conventional induction heating apparatus is configured as described above, the temperature increase characteristics of the object to be heated (7), especially when the processing weight of the object to be heated (to) becomes a low level, are as follows. There is a problem that the desired result cannot be obtained, and this point will be explained below with reference to FIG.

Figure 7 shows the temperature rise characteristics of the heated object (to), that is, the heated object (
7) shows the temperature distribution along the transport direction and the distribution of power supplied to the heated object (7) also along the transport direction. First, in the same figure (a), the target heating temperature is 1
These are the characteristics when the processing weight is maximized at 250°C.

In the figure, the shaded area is mainly due to radiant heat (from 1971, the heat loss radiated to the surroundings is converted into electric power, and the temperature of the heated object m is a constant value as shown in As a result, the electric power that contributes to heating the opening of the heated object is transferred to the heating coil (A).
The temperature gradient of the object to be heated (7) decreases rapidly from a position near the exit of the heated object (7), and the temperature gradient of the heated object (7) also decreases rapidly to asymptotically approach the target temperature. On the other hand, the number of turns of the heating coil (2) and the power to be supplied are set so that the temperature distribution of the object to be heated becomes a smooth curve as shown in FIG.

FIG. 7(b) shows the temperature characteristics, etc. when the processing weight becomes considerably low due to, for example, the pre-process or post-process of the induction heating apparatus. In this case too, the target temperature is 1250°C
Since this is the same as in the case shown in FIG. 4(a), it is natural that the power supplied to the heating coil (4) must be reduced accordingly. However, the above-mentioned power equivalent to heat radiation loss mainly generated near the outlet of the heating coil (4) has almost the same characteristics even when the processing weight is low because its value is mainly determined by the temperature at the mouth of the object to be heated. As a result, the heating coil (a)
Near the exit, the power equivalent to heat radiation loss exceeds the supplied power, and the temperature gradient in this area reverses to negative as shown in the figure. Therefore, if the temperature at the final outlet of the heating coil (2) is controlled to reach the target value of 1250°C, the temperature will exceed 1250°C in the middle of heating (point A in the figure), and the object to be heated ( This will cause a problem in which some of the parts (to) will melt.

In this way, in conventional induction heating devices, when trying to keep the heating temperature characteristics of the object to be heated within a certain allowable limit, the lower limit is, for example, about 60% of the processing weight at a high processing level; There has been a problem in that it is not possible to adequately handle cases where operation is required in a wide processing weight range exceeding the limit.

This invention was made to solve the above-mentioned problems, and it provides an induction heating device that can always inductively heat the object with optimal temperature rise characteristics even when the processing weight fluctuates over a wide range. The purpose is to obtain.

[Means and effects for solving the problem] The induction heating device according to the present invention includes a heating coil that includes a plurality of segmented coils arranged along the conveying direction of the object to be heated,
The electric power supplied to at least one of the divisional coils is controlled independently of the electric power supplied to the other divisional coils in accordance with the conveyance processing amount of the object to be heated.

Then, to explain the case where the heating coil is configured with two section coils, for example, when the transfer throughput is high, the current supplied to both section coils is the same, and when the transfer throughput is low, the electric current is supplied to the inlet side section coil. On the other hand, the current supplied to the outlet-side segmented coil is relatively increased to prevent a temperature drop due to power equivalent to heat dissipation loss.

〔Example〕

FIG. 1 is a circuit diagram showing an induction heating device according to an embodiment of the present invention. The major difference from the conventional method is that the heating coil is divided into two, and that the current flowing through each coil can be independently controlled, which will be explained in detail below.

(11) is a forward conversion section of a high frequency inverter that converts the commercial frequency power from the transformer (2) into DC power; (12) is an accelerator for smoothing the output from the forward conversion section (11);
3) is the first inverse conversion section of the high frequency inverter that converts the DC power source into a predetermined high frequency power source necessary for induction heating, and is composed of a bridge of thyristor elements (41) to 44) shown in FIG. . (14) is a heating coil for low temperature range as a divisional coil, (15) is a heating coil for low temperature range (1
4) is a power factor correction capacitor connected in parallel with the power factor correction capacitor.

(+6), (17) and (18) are
) to 15), respectively, are a second inverse conversion section, a heating coil for high temperature range as a section coil, and a capacitor for power factor improvement.

(20) is the thyristor element (
41) as a control device that sends gate signals to etc.
The gate control circuit consists of the following elements.

That is, (21) is a voltage detector that detects the output voltage waveform of the first inverse converter (13), (22) is a waveform shaper, and (2
3) is a pulse generator, and (24) is a gate signal generator.

Further, (30) is a second gate control circuit as a control device that sends a gate signal to the thyristor element (41) etc. of the second inverse conversion section (16), and includes a voltage detector (31), a waveform shaper, etc. (32), a pulse generator (33), a pulse frequency divider (35) and a gate signal generator (34).

Next, the operation will be explained. First, the operation of the first gate control circuit (20) will be mainly explained using the time chart of FIG. Figure <a) shows the output voltage waveform of the first inverse converter (13), which is a sine waveform that matches the parallel resonance frequency determined by the inductance of the low-temperature heating coil (14) and the capacitance of the capacitor (15). It has become. This output voltage waveform is detected by a voltage detector (2]) (see (b) in the same figure), and further shaped by a waveform shaper (22) so that the phase is 180 degrees with respect to the positive and negative polarities of the output voltage.
It is converted into two signals shifted by ° ((c) and () in the same figure).
d)). Next, based on this signal, a pulse generator (23)
A pulse signal is created by ((e) and (f) in the same figure).
)). The phase difference Toff between the signals shown in (c) and (d) in the figure is generally called the reverse voltage time, and is the time required for commutation of the thyristor element. After i&, gate signal generator (
24) is the first inverse transformer (13) based on this pulse signal.
) A gate signal is created to control the firing of the thyristor element (41), etc. ((g) and (h) in the same figure). The signal of <g) in the same figure is the signal of Cyrisk element (41) (4
4), the signal in (h) of the same figure is the thyristor element (42)
(43), whereby each element is alternately turned on and off to supply a predetermined high frequency power to the low temperature region heating coil (14).

The amount of power to be supplied can be determined by adjusting the firing angle of the thyrisk element constituting the forward conversion section (11) and changing its output DC voltage Ed. Adjust by changing the size.

Next, the operation of the second gate control circuit (30) will be explained with reference to the time chart shown in FIG. However, as will be described later, the waveforms in (a) and (b) of the same figure will change slightly, but the operations related to the waveforms shown in (a) to f) of the same figure are the same as in the case of Fig. 3. Avoid duplication by omitting explanations.

Pulse signal from the pulse generator (33) (Fig. 4(e)
(f)) is input to a pulse frequency divider (35), which has a frequency division ratio of 1/J (J is a variable integer, and in the embodiment of FIG. 4, J- In step 2), the frequency of the pulse signal is divided and the signals shown in (g) and (h) of the same figure are output.

The gate signal generator (34) generates the thyristor element (4) of the second inverse conversion section (16) based on this frequency-divided pulse signal.
1) Create a gate signal to control the ignition ((i) and (J) in the same figure).

Then, the signal of 1] (i) is sent to the thyristor element (41)
(44), the signal of (j) in the same figure is transmitted to the thyristor element (42
) (43), but unlike the case in FIG. 3, here there is a rest period (indicated by Y in FIG. 4) in the gate signal.

Therefore, power is supplied from the second inverse converter (16) to the high-temperature heating coil (17) only during the period indicated by X in FIG. Free vibration occurs when the area heating coil (17) and capacitor (18) are opened, and the amplitude of the voltage is attenuated by the resistance of the circuit.

Therefore, by inserting the pulse frequency divider (35) with a frequency division ratio of 1/2, the power supplied to the high-temperature range heating coil (17) is approximately equal to that when the pulse frequency divider (35) is not inserted (in the case of a frequency division ratio of 1). The circumference rate is reduced to 1/J times.

Next, a description will be given of how to perform a heating operation when the processing weight varies over a wide range using the induction heating apparatus as described above. FIG. 5 shows the temperature and power distribution characteristics along the conveyance direction of the object to be heated, and corresponds to the conventional FIG. 7. First, the frequency division ratio of the pulse frequency divider (35) is set to 1/2, and the number of turns of each coil is adjusted so that a good temperature distribution as shown in Fig. 5(a) is obtained during high processing weight. Set.

That is, in this case, the power equivalent to heat radiation loss is mainly generated in the part of the object to be heated (7) located in the heating coil for high temperature range (17), but in the second inverse conversion part (16) and The heating power supplied to the same part of the object to be heated by the area heating coil (17) exceeds the power equivalent to the heat radiation loss, resulting in smooth temperature distribution characteristics.

Next, when the processing weight decreases significantly, the output voltage Ed of the forward conversion section (11) is lowered in accordance with this reduction in the processing weight, and the power supplied to both coils (14) and (17) is reduced overall. At the same time, the frequency division ratio of the pulse frequency divider (35) is changed from 1/2 to 1. As a result, the object to be heated (7
), while the power supplied to the heating coil (17) for high temperature range increases relatively, as shown in the figure (
As shown in h), the phenomenon in which the power equivalent to heat dissipation loss exceeds the supplied power is avoided, and good temperature characteristics similar to those obtained when processing heavy weights are obtained. In other words, the target temperature of 125
This results in a smooth temperature increase curve with no portion exceeding 0°C.

In addition, in the above embodiment, the integer J of the frequency division ratio of the pulse frequency divider (35) is set in two stages, 2 and 1, but it is possible to set it in the range of J = 4 to 1, for example. , J=4
As the processing weight decreases from the high processing weight of
By sequentially changing the settings in multiple stages as shown in 3.2.1, it becomes possible to control the temperature of the wood more precisely, and the lower limit of the possible processing weight can be further expanded.

Further, in the above embodiment, a pulse frequency divider is provided in the second gate control circuit iM (30) that controls the power supply to the heating coil (17) for high temperature range, but the pulse frequency divider is provided to the heating coil (14) for low temperature range. It may also be provided in the first gate control circuit (20) that controls the current. However, in this case, the integer J of the frequency division ratio is set to 1 when the processing weight is high, and as the processing weight decreases, it is changed to J = 2.3... The power supply will be relatively reduced.

Furthermore, a pulse frequency divider can be provided in the recontrol circuits (20) and (30) to perform more fine control.

Further, in the above embodiment, the heating coil is composed of two segmented coils, but it may be composed of three or more. In this case, at least one of the gate control circuits that control the current to each section coil may be provided with a pulse frequency divider to control the current independently of the others.

Furthermore, in the above embodiment, the power supplied to each coil is controlled by adjusting the division ratio of the pulse signal to the gate signal generator, but for example, the output voltage of the second inverse converter (16) is It is also possible to adopt other methods for control, such as a method of adjusting the output voltage, a method of providing a forward converter for each coil, and adjusting the output voltage thereof.

Furthermore, other switching elements such as transistor elements may be used instead of the thyristor elements employed in the first inverse conversion section (13) and the like.

〔Effect of the invention〕

This invention is configured as described above, and by controlling the power supplied to each section coil so that the power supplied to the heated object near the heating coil outlet does not always fall below the power equivalent to heat radiation loss. , good temperature increase characteristics can be obtained without exceeding the target heating temperature during heating.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing an induction heating device according to an embodiment of the present invention, and FIG. 2 is a circuit diagram showing an inverse conversion section thereof.
3 and 4 are time charts for explaining the operations of the first and second gate control circuits, respectively, and FIG.
FIG. 6 is a characteristic diagram showing the distribution of heating temperature and supplied power according to the present invention, FIG. 6 is a circuit configuration diagram showing a conventional induction heating device,
FIG. 7 is a characteristic diagram showing the distribution of heating temperature and supplied power in the conventional case. In the figure, (to) is the object to be heated, (13) and (16)
) are the first and second inverse converters, respectively, (14) and (17) are the heating coil for the low temperature range and the heating coil for the high temperature range, respectively, as segmented coils, (20) and (3
0) are first and second gate control circuits as control devices, respectively, and (35) is a pulse frequency divider. Note that the same reference numerals in each figure indicate the same or corresponding parts. Agent Patent Attorney Masuo Oiwa (Figure 2 PC Buddha) Figure 3 Figure 1 Figure 4 Figure 5

Claims (1)

[Scope of Claims] A device comprising a heating coil and a control device for controlling electric power supplied to the heating coil, wherein an object to be heated is continuously fed into the heating coil, heated by induction, and taken out at a predetermined temperature, The heating coil is configured with a plurality of segmented coils arranged along the conveying direction of the heated object, and electric power is supplied to at least one of the segmented coils according to the amount of conveyance of the heated object. An induction heating device characterized in that the temperature of the object to be heated does not exceed the predetermined temperature during heating by controlling the power independently of the electric power supplied to other section coils.