Induction heating cluster device

Abstract

This invention provides an induction heating cluster device, relating to the field of induction heating technology, including an induction heating power supply, an induction heating device group, and a smart terminal. The induction heating power supply includes a DC power supply and a current inverter circuit. The current inverter circuit includes an inverter inductor L1, a current compensation capacitor C1, and a freewheeling diode Zk1. The inverter inductor L1 and the current compensation capacitor C1 are connected in series, then in parallel with the freewheeling diode Zk1, and finally connected to the output terminal of the DC power supply. The two poles of the current compensation capacitor C1 are the output terminals of the induction heating power supply. The induction heating device group includes 1 to M induction heating devices, which are connected in parallel to the output terminal of the induction heating power supply. Each induction heating device includes an induction heating main circuit and a control circuit. The control circuit is connected to the induction heating main circuit, and the smart terminal drives the control circuit to control the power of the induction heating main circuit, thereby achieving the goals of cost reduction, energy saving, and safe production.

Description

Technical Field

[0001] This invention relates to the field of induction heating technology, specifically to an induction heating cluster device. Background Technology

[0002] Induction heating, as an electric heating technology, has been used for over a century. With the development of semiconductor technology, induction heating power supplies have evolved from early AC power to contemporary DC power, leading to explosive growth in induction heating. In different application areas, the power of a single induction heating device ranges from 1 watt to tens of megawatts. For example, in the service industry, a restaurant uses dozens or more induction cookers; in the equipment manufacturing industry, a forging plant uses several induction heating through-heating furnaces, a foundry uses several induction heating melting furnaces, a heat treatment plant uses several induction heating surface hardening or overall hardening equipment, and a large piece of equipment requires several or even more heating zones (using several induction heating devices). These are collectively referred to in this invention as application scenarios for induction heating cluster devices.

[0003] With the development of technology, the power source for induction heating has evolved from early AC power to modern DC power. However, people still use the induction heating theory based on AC power and have invented many practical induction heating devices. Figure 1 This is a commonly used series resonant induction heating main circuit diagram. Figure 2 It is the inductor voltage (Reference: Li Shuxin, Zhu Xingfa, eds. Energy-saving design method and debugging of series resonant inverter power supply, Beijing: Machinery Industry Press, August 2016, 1st edition, p39, p128). Figure 3 This is a circuit diagram of a CCLC induction heating power supply disclosed in patent application number CN 117812769 B. The main drawbacks of the prior art can be summarized in the following six points: (1) Using thyristor rectification generates severe harmonics. To mitigate these harmonics, a linear inductor needs to be added to the circuit (see Figure 1 This not only increases costs but also increases power consumption in induction heating. (2) Due to cost considerations, the capacitance value of filter capacitors is generally small, resulting in a small filtering effect on the current. The current of the induction heating power supply contains a large amount of medium frequency components, resulting in large line losses and large heat generation of reactors and transformers. (3) Connecting a freewheeling diode in parallel with the inverter switch increases cost, reduces power factor, increases line loss, is not used at the resonant point, and the inductor also generates harmonics; (4) Parallel resonant capacitor (see Figure 3 C in P When disconnected from the induction heating power supply, it is connected to the induction heating coil L ( Figure 3 L) forms an oscillating circuit, which is achieved by releasing capacitor C. P CS1 C S2 (C S1 C S2 Essentially, they can be placed in the same location, equivalent to the energy of a capacitor, to achieve heating. When the induction heating power supply is turned on, C P It is a single load, and, during switching, C P A sudden change in voltage across the terminals will result in a huge instantaneous current, which can easily damage the semiconductor switch or shorten its lifespan. Figure 3 The freewheeling diode in the circuit is merely decorative and does not allow any current to flow through it. (5) There are two existing technical solutions for adjusting load power: (1) Using thyristor rectification to adjust power through DC voltage. The disadvantage is that it generates harmonics. Under the premise of the same power, the disadvantage of adjusting power by reducing voltage is that it increases the current and increases line loss; (2) Working in non-resonant state, it is necessary to add freewheeling diodes Zk1~Zk4 (see Figure 1 The disadvantages are increased cost, reduced power factor, increased losses, and the generation of harmonics.

[0004] (6) In order to achieve the rated power of the induction heating device, the rated voltage of the induction heating power supply needs to match the rated power of the induction heating device. Therefore, the existing technology can only use a common induction heating power supply for the same load, such as the "one-to-two" or "one-to-three" technology of medium frequency melting furnace (one induction heating power supply supplies two or three medium frequency melting furnaces). If the number of medium frequency melting furnaces increases or the capacity changes, multiple transformers are required, which is costly and the transformer capacity expansion fee is also high.

[0005] When the capacitor and the induction heating coil L are connected in series, the voltage across the coil L is related to the amount of material being heated. The voltage across the coil L increases rapidly as the amount of material decreases, reaching several to tens of times the voltage of the induction heating power supply. Improper handling can cause serious safety hazards, such as the induction heating coil L breaking down. Controlling the operation of the medium-frequency melting furnace within a safe voltage range is also a technical challenge. Summary of the Invention

[0006] To address the aforementioned technical problems, this invention provides an induction heating cluster device. Based on the theory of induction heating using direct current as a power source, and according to the principles of topology, an induction heating main circuit is constructed. Using a simple digital control circuit and combined with the application of a smart terminal, a brand-new induction heating ecosystem is realized, achieving the goals of cost reduction, energy saving, and safe production.

[0007] To achieve the above technical objectives, the adopted technical solution is as follows: an induction heating cluster device, comprising an induction heating power supply, an induction heating device group, and an intelligent terminal; the induction heating power supply includes a DC power supply and a current inverter circuit, the current inverter circuit includes an inverter inductor L1, a current compensation capacitor C1, and a freewheeling diode Zk1, the inverter inductor L1 and the current compensation capacitor C1 are connected in series and then in parallel with the freewheeling diode Zk1, and then connected to the output terminal of the DC power supply, the two poles of the current compensation capacitor C1 are the output terminals of the induction heating power supply; the induction heating device group includes 1 to M induction heating devices, 1 to M induction heating devices are connected in parallel to the output terminal of the induction heating power supply, the induction heating device includes an induction heating main circuit and a control circuit; the control circuit is connected to the induction heating main circuit, and the intelligent terminal drives the control circuit to control the power of the induction heating main circuit.

[0008] The DC power supply is a constant voltage DC energy storage power supply, or a power supply that converts AC power into DC power through diode rectification.

[0009] The inverter inductor L1 is an inductor with a magnetic core.

[0010] The current compensation capacitor C1 is a capacitor bank consisting of 1 to N electrolytic capacitors connected in parallel, or a capacitor unit consisting of at least two capacitor banks connected in series.

[0011] The induction heating main circuit includes a turn-off semiconductor switch, a freewheeling diode D, and an induction heating coil L. The turn-off semiconductor switch and the induction heating coil L are connected in series to the output terminal of the induction heating power supply to form an active heating circuit. The freewheeling diode Zk1 is connected in parallel with the induction heating coil L to form a passive heating circuit.

[0012] The induction heating main circuit includes three thyristors kk1, kk2, and kk5, one induction heating coil L, and one energy storage and boosting capacitor C. One terminal of the energy storage and boosting capacitor C is connected to point A, and the other terminal is connected to one end of the induction heating coil L. The other end of the induction heating coil L is connected to point B. The anode of kk1 is connected to the positive terminal of the induction heating power supply, and the cathode of kk1 is connected to point A. The anode of kk2 is connected to point B, and the cathode of kk2 is connected to the negative terminal of the induction heating power supply, forming an active heating circuit I1. The anode of kk5 is connected to point A, and the cathode of kk5 is connected to point B, forming a passive heating circuit I3.

[0013] The induction heating main circuit also includes a thyristor kk6, with the anode of kk6 connected to point B and the cathode of kk6 connected to point A, forming a passive heating circuit I4.

[0014] The induction heating main circuit also includes a thyristor kk 3、The anodes of kk4 and kk3 are connected to the positive terminal of the induction heating power supply, the cathode of kk3 is connected to point B, the anode of kk4 is connected to point A, and the cathode of kk4 is connected to the negative terminal of the induction heating power supply, forming an active heating circuit I2.

[0015] The induction heating main circuit includes four thyristors kk1~kk4, one induction heating coil L, and one energy storage and boosting capacitor C. One pole of the energy storage and boosting capacitor C is connected to point A, and the other pole is connected to one end of the induction heating coil L. The other end of the induction heating coil L is connected to point B. The anode of kk1 is connected to the positive terminal of the induction heating power supply, and the cathode of kk1 is connected to point A. The anode of kk2 is connected to point B, and the cathode of kk2 is connected to the negative terminal of the induction heating power supply, forming an active heating circuit I1. The anode of kk3 is connected to the positive terminal of the induction heating power supply, and the cathode of kk3 is connected to point B. The anode of kk4 is connected to point A, and the cathode of kk4 is connected to the negative terminal of the induction heating power supply, forming an active heating circuit I2.

[0016] The induction heating main circuit also includes a thyristor kk7, with the anode of kk7 connected to point A and the cathode of kk7 connected to the positive terminal of the induction heating power supply, forming a passive heating circuit I5.

[0017] The induction heating main circuit also includes a thyristor kk 5、 The anodes of kk6 and kk5 are connected to point A, and the cathode of kk5 is connected to point B, forming a passive heating circuit I3; the anode of kk6 is connected to point B, and the cathode of kk6 is connected to point A, forming a passive heating circuit I4.

[0018] The induction heating main circuit also includes a thyristor kk7, with the anode of kk7 connected to point A and the cathode of kk7 connected to the positive terminal of the induction heating power supply, forming a passive heating circuit I5.

[0019] The induction heating main circuit also includes a thyristor kk8, with the anode of kk8 connected to point B and the cathode of kk8 connected to the positive terminal of the induction heating power supply, forming a passive heating circuit I6.

[0020] The beneficial effects of this invention are: (1) A novel induction heating ecosystem has been constructed through the integrated application of clustering, digitalization, networking and intelligent terminal technologies; (2) Through theoretical innovation in induction heating, and by applying the principle of topology, 10 types of induction heating main circuits, including oscillating and non-oscillating ones, were developed. For the same induction heating power supply and the same load (induction heating coil L), 10 circuit structures with rated power were created simply by changing the switch to meet different application scenarios.

[0021] (3) Due to the innovation of circuit structure and control method, the induction heating device no longer generates high-order harmonics, and at the same time ensures that the power factor is always 1, reducing the loss of induction heating power supply and power supply line. (4) Due to the technological innovation of the passive heating circuit, the maximum voltage of the induction heating coil L is effectively controlled. Even under no-load (no material being heated) conditions, it can be controlled within the voltage of the induction heating power supply, which improves the safety and flexibility of the induction heating device. (5) Due to the innovative design of minimizing the circuit structure, the production cost of the induction heating device is reduced, resources are saved, and market competitiveness is improved; (6) Due to the adoption of centralized power supply technology, the installed capacity of induction heating power supply has been reduced, saving users the cost of capacity expansion. Attached Figure Description

[0022] Figure 1 This is a diagram of an existing series resonant circuit; Figure 2 This is a measured waveform of the sensor voltage during non-resonant operation using existing technology; Figure 3 This is a circuit diagram of an existing CCLC. Figure 4 This is a block diagram of the structure of an induction heating cluster device according to the present invention; Figure 5 This is a DC power supply circuit diagram of the present invention; Figure 6 This is a current inverter circuit diagram of an induction heating cluster device according to the present invention; Figure 7 This is a schematic diagram of the connection of the filter capacitor bank of an induction heating cluster device according to the present invention; Figure 8 This is a schematic diagram of the capacitor unit connection of an induction heating cluster device according to the present invention; Figure 9 This is a schematic diagram of a non-oscillating induction heating main circuit and a corresponding current waveform of the present invention; Figure 10 This is a schematic diagram of an induction heating main circuit containing three thyristors according to the present invention; Figure 11 This is a schematic diagram of an induction heating main circuit containing four thyristors according to the present invention; Figure 12 This is a schematic diagram of an induction heating main circuit containing 5 thyristors according to the present invention; Figure 13 This is a schematic diagram of an induction heating main circuit containing 6 thyristors according to the present invention; Figure 14This is a schematic diagram of an induction heating main circuit containing four thyristors and a corresponding current waveform diagram of the present invention. Figure 15 This is a schematic diagram of an induction heating main circuit containing 5 thyristors according to the present invention; Figure 16 This is a schematic diagram of an induction heating main circuit containing 6 thyristors according to the present invention; Figure 17 This is a schematic diagram of an induction heating main circuit containing 7 thyristors according to the present invention; Figure 18 This is a schematic diagram of an induction heating main circuit containing eight thyristors according to the present invention. Detailed Implementation

[0023] The present invention will be further described in detail below with reference to the embodiments, but this should not be construed as limiting the invention in any way.

[0024] Figure 4 This is a block diagram of an induction heating cluster device according to the present invention, including an induction heating power supply, an induction heating device group, and a smart terminal; the induction heating power supply includes a DC power supply and a current inverter circuit; the induction heating device group includes 1 to M induction heating devices, which are connected in parallel to the output terminal of the induction heating power supply; each induction heating device includes an induction heating main circuit and a control circuit; the control circuit is connected to the induction heating main circuit, and the smart terminal drives the control circuit to control the power of the induction heating main circuit.

[0025] The following is a more detailed explanation in the order of induction heating power supply, induction heating device, and smart terminal.

[0026] 1. Induction heating power supply The induction heating power supply is the power source for the induction heating device, including a DC power supply and a current inverter circuit.

[0027] 1.1 DC Power Supply Furthermore, the DC power supply is a constant-voltage DC energy storage power supply. When using green energy—photovoltaic power generation—or storing electricity during off-peak hours and using it during peak hours, its beneficial effects are obvious.

[0028] Furthermore, the DC power supply is an AC power supply that is rectified into DC power by a diode, such as... Figure 5As shown. In existing technologies, rectifier transformers and thyristor rectification are commonly used. This invention replaces the rectifier transformer with a common transformer, reducing transformer costs by about 30%, resulting in significant economic benefits. The advantage of using thyristor rectification is that it can control the DC voltage, thereby controlling the power of the induction heating device. Its disadvantage is the generation of high-order harmonics. Therefore, mitigating these harmonics requires additional costs—adding absorbing capacitors or even reactors, making the power supply structure more complex, increasing both cost and induction heating power losses. This invention uses diode rectification, which does not generate high-order harmonics, thus eliminating the need for harmonic mitigation. Therefore, compared with existing technologies (see...), this invention... Figure 1 Compared to the previous version, this reduces the number of linear inductors Ls by 6. CR The induction heating device of this invention also does not generate high-order harmonics, which simplifies the circuit structure and reduces the cost of controlling high-order harmonics, thus lowering costs. Of course, a problem with diodes is that the power of the induction heating device cannot be controlled by controlling the DC voltage. This characteristic perfectly meets the technical requirements of this invention—because according to the technical solution of this invention, changing the DC voltage would change the power of all induction heating devices, which is obviously unacceptable. The technical solution for controlling the power of this invention will be described in the induction heating device section.

[0029] 1.2 Current Inverter Circuit The current inverter circuit includes an inverter inductor L1, a current compensation capacitor C1, and a freewheeling diode Zk1. The inverter inductor L1 and the current compensation capacitor C1 are connected in series, then connected in parallel with the freewheeling diode Zk1, and finally connected to the output terminal of the DC power supply. The two terminals of the current compensation capacitor are the output terminals of the induction heating power supply. Figure 6 As shown.

[0030] The function of a current inverter circuit is to convert the DC current with a small ripple current output from the DC power supply into a pulse current required by the load at the output terminal of the inverter circuit. Therefore, it is called a current inverter circuit.

[0031] Under normal circumstances, the freewheeling diode has no function. It only comes into play when there is a sudden change in the current through the inverter inductor L1 (the load is suddenly disconnected). At this time, L1, C1, and Zk1 form a unidirectional loop, releasing the energy in L1 into C1 and preventing high voltage from being generated across L1. Zk1 does not need cooling, and there is no need to choose a large rectified current.

[0032] Furthermore, the inverter inductor L1 is an inductor with a magnetic core. To achieve good inverter performance, a reactor with a large inductance value is required. Compared with a reactor without a magnetic core, a reactor with a magnetic core and the same inductance value has a larger quality factor, resulting in lower power consumption, lower heat generation, smaller size, and easier installation.

[0033] Furthermore, the current compensation capacitor C1 is a capacitor bank composed of 1 to N electrolytic capacitors connected in parallel, or a capacitor unit composed of two or more capacitor banks connected in series, such as... Figure 7 and Figure 8 As shown. To achieve good inverter performance, large-capacitance capacitors are required. Electrolytic capacitors have large capacitance, but due to their structure, they allow relatively small currents. This is addressed by connecting them in parallel to increase the current carrying capacity, which also increases the capacitance. Electrolytic capacitors have relatively low rated voltages; when this is insufficient, the rated voltage of current-compensating capacitors is increased by connecting them in series. Figure 7 This is a schematic diagram of 4 × 10 = 40 electrolytic capacitors connected in parallel. If the rated voltage of each electrolytic capacitor is 450VDC and its nominal capacitance is... Then the nominal capacitance of the electrolytic capacitors connected in parallel is The rated voltage remains 450VDC. Figure 8 This diagram shows 4 × 10 = 40 electrolytic capacitors connected in parallel, and then the two groups connected in series. If each electrolytic capacitor has a rated voltage of 450VDC and a nominal capacitance of... Then the rated capacity of the electrolytic capacitors connected in parallel is After being connected in series, the nominal capacity is The rated voltage is 900V.

[0034] exist Figure 6 The diagram also shows the voltage and current waveforms at the input and output terminals of the current inverter circuit. The current inverter circuit has two functions.

[0035] (1) It has a filtering effect on voltage, which is the function of a filter circuit. After passing through the filter circuit, the voltage ripple is reduced, such as Figure 6 The diagram illustrates the voltage-time relationship of an AC rectified power supply. However, when the power supply is a constant voltage source, Figure 6 The energy storage power supply in the middle will actually produce a small ripple voltage at the output end.

[0036] (2) Current inversion function. The so-called current inversion function refers to the DC current with a small ripple current output by the DC power supply. After passing through the current inversion circuit, due to the effect of the current compensation capacitor, the DC current with any waveform required by the load can be output at the output end of the inverter circuit.

[0037] The current inverter circuit described in this invention corresponds to the technical term "filter circuit" in the prior art. The function of a filter circuit is to reduce the ripple voltage of a DC power supply, which is necessary in applications such as constant voltage power supplies. However, as a power source for induction heating, a constant or stable voltage is not required, since alternating current can also be used for induction heating. Therefore, from the perspective of voltage stability, a filter circuit is unnecessary. However, induction heating is a special type of load. When the energy storage and boosting capacitor is connected in series with the induction heating coil L, the current at its input terminal is approximately a half-wave of a sine wave, and its frequency is much higher than the power frequency. This type of current, when passing through lines, diodes, transformer windings, and power supply lines, will generate losses much greater than those of DC current, not only wasting electrical energy but also generating a large amount of heat, causing the temperature to rise, leading to numerous problems.

[0038] A DC power supply plus a current inverter circuit constitutes the induction heating power supply, which is the power source for each induction heating device. As a power source, the most important technical parameters of the induction heating power supply are the voltage and current values ​​it can provide; these two parameters describe the power supply capability of the induction heating power supply. Therefore, the induction heating power supply described in this invention must first meet the power supply requirements of the induction heating cluster device, that is, meet the voltage requirements and current or power requirements of the induction heating devices. If the induction heating cluster device contains M induction heating devices, the sum of the rated power or maximum power of each induction heating device is P. M The maximum power required in actual use is less than P. M This is because only fewer than (or equal to) M induction heating devices are used simultaneously, and even when used simultaneously, they will not all operate at maximum power at the same time. Therefore, the rated power of the induction heating power supply is less than P. M This reduces the cost of the induction heating power supply, which is one of the beneficial effects of this invention. Taking an industrial casting melting furnace as an example, suppose a casting workshop is equipped with six 3-ton melting furnaces, each with a rated power of 2000 kW. No more than four furnaces are used simultaneously (the other two are backups). Two furnaces are used at their rated power (heating and melting stage), while the other two are in the heat preservation stage (requiring approximately 1 / 4 of their rated power) or the pouring / loading stage (not requiring heating). A 5000 kW induction heating power supply would suffice. If each melting furnace is equipped with its own power supply, six 2000 kW power supplies would be needed, totaling 12000 kW. Even with a "one-to-two" configuration, three 3000 kW power supplies would be required, totaling 9000 kW. Therefore, the technical solution proposed in this invention reduces the total transformer capacity, thus lowering both equipment costs and capacity expansion fees.

[0039] 2. Induction heating device The induction heating device group comprises 1 to M induction heating devices. The induction heating device is characterized by including 2.1 an induction heating main circuit and 2.2 a control circuit. The control circuit is characterized by including a controller, a data acquisition module, a drive module, and a communication module, etc.

[0040] 2.1 Induction Heating Main Circuit When using a DC power supply for induction heating, the main circuit for induction must include two heating loops. These two loops must periodically connect and disconnect to achieve induction heating—this is a necessary condition that the topology of the induction heating main circuit must meet when using a DC power source. The functions of existing technical terms such as "inverter," "inverter bridge," "half-bridge inverter," and "full-bridge inverter" are essentially the same as those described in this invention, consisting of "semiconductor switches" that allow for the periodic connection and disconnection of two circuits. By breaking through the traditional concept of "inverter," various oscillating induction heating main circuits can be topologically constructed.

[0041] Furthermore, a (non-oscillating) induction heating main circuit includes a turn-off semiconductor switch and a freewheeling diode D. The turn-off semiconductor switch includes a MOSFET, an IGBT, or a GTO / Gate Turn-Off Thyristor. The circuit connection is as follows: the turn-off semiconductor switch is connected in series with the induction heating coil L and then connected to the output terminal of the induction heating power supply, forming an active heating circuit; the freewheeling diode is connected in parallel with the induction heating coil L, forming a passive heating circuit. Figure 9 As shown. The turn-off semiconductor switch periodically turns on and off with a period of T and a duty cycle of T1 / T. When the turn-off semiconductor switch is on, the current through the active heating circuit is i, and the conduction time is T1. As the conduction time increases, the current increases, as shown... Figure 9 The solid line in the diagram shows that when the switchable semiconductor switch is closed, the passive heating circuit is turned on, and the current i flowing through the freewheeling diode D and the induction heating coil L is... D Decrease over time, such as Figure 9 As shown by the dashed line in the diagram. This load circuit has no capacitor, is a non-oscillating circuit, has no capacitive reactance, and the voltage across the induction heating coil L is equal to the power supply voltage U, making it safer and suitable for high-power applications.

[0042] The power of a non-oscillating induction heating main circuit is related to the induction heating power supply voltage U, the inductance L of the induction heating coil L, the equivalent resistance R, the duty cycle T1 / T, and the period T. The induction heating power increases with increasing U and duty cycle. R decreases with increasing T, therefore the induction heating power increases with increasing T. The advantages of a non-oscillating main circuit are: simple structure, fewer switches, low power loss, and low cost. The disadvantages are: too high power, requiring the power supply voltage U to match the rated power of the induction heating device, making it more suitable for high-power and ultra-high-power applications (tens of megawatts) and unsuitable for medium- and low-power (kilowatt-level) applications.

[0043] To achieve better rated power matching, an oscillating induction heating main circuit is required. With a fixed power supply voltage U, the oscillating induction heating main circuit can change the rated power of the induction heating through two techniques: (1) changing the capacitance value of the energy storage booster capacitor C. Decreasing the capacitance value increases the oscillation frequency and the equivalent resistance, thereby reducing the rated power; (2) employing different circuit topologies and control methods. Different circuit topologies have different rated powers. With a fixed rated power, the power can be further reduced by increasing the time interval between the switching of the two heating circuits.

[0044] Furthermore, an induction heating main circuit includes three thyristors kk1, kk2, and kk5, one induction heating coil L, and one energy storage and boosting capacitor C. The circuit connection is as follows: one terminal of the energy storage and boosting capacitor C is connected to point A, and the other terminal is connected to one end of the induction heating coil L. The other end of the induction heating coil L is connected to point B. The anode of kk1 is connected to the positive terminal of the induction heating power supply, and the cathode of kk1 is connected to point A. The anode of kk2 is connected to point B, and the cathode of kk2 is connected to the negative terminal of the induction heating power supply, forming an active heating circuit I1. The anode of kk5 is connected to point A, and the cathode of kk5 is connected to point B, forming a passive heating circuit I3. Figure 10 As shown.

[0045] Furthermore, an induction heating main circuit includes four thyristors kk1, kk2, kk5, and kk6, one induction heating coil L, and one energy storage and boosting capacitor C. The circuit connections are as follows: one terminal of the energy storage and boosting capacitor C is connected to point A, and the other terminal is connected to one end of the induction heating coil L. The other end of the induction heating coil L is connected to point B. The anode of kk1 is connected to the positive terminal of the induction heating power supply, and the cathode of kk1 is connected to point A. The anode of kk2 is connected to point B, and the cathode of kk2 is connected to the negative terminal of the induction heating power supply, forming an active heating circuit I1. The anode of kk5 is connected to point A, and the cathode of kk5 is connected to point B, forming a passive heating circuit I3. The anode of kk6 is connected to point B, and the cathode of kk6 is connected to point A, forming a passive heating circuit I4. Figure 11 As shown.

[0046] Furthermore, an induction heating main circuit includes five thyristors kk1~kk5, one induction heating coil L, and one energy storage and boosting capacitor C. The circuit connection is as follows: one terminal of the energy storage and boosting capacitor C is connected to point A, and the other terminal is connected to one end of the induction heating coil L. The other end of the induction heating coil L is connected to point B. The anode of kk1 is connected to the positive terminal of the induction heating power supply, and the cathode of kk1 is connected to point A. The anode of kk2 is connected to point B, and the cathode of kk2 is connected to the negative terminal of the induction heating power supply, forming an active heating circuit I1. The anode of kk3 is connected to the positive terminal of the induction heating power supply, and the cathode of kk3 is connected to point B. The anode of kk4 is connected to point A, and the cathode of kk4 is connected to the negative terminal of the induction heating power supply, forming an active heating circuit I2. The anode of kk5 is connected to point A, and the cathode of kk5 is connected to point B, forming a passive heating circuit I3. Figure 12 As shown.

[0047] Furthermore, an induction heating main circuit includes six thyristors kk1~kk5, kk7, one induction heating coil L, and one energy storage and boosting capacitor C. The circuit connection is as follows: one terminal of the energy storage and boosting capacitor C is connected to point A, and the other terminal is connected to one end of the induction heating coil L. The other end of the induction heating coil L is connected to point B. The anode of kk1 is connected to the positive terminal of the induction heating power supply, and the cathode of kk1 is connected to point A. The anode of kk2 is connected to point B, and the cathode of kk2 is connected to the negative terminal of the induction heating power supply, forming an active heating circuit I1. The anode of kk3 is connected to the positive terminal of the induction heating power supply, and the cathode of kk3 is connected to point B. The anode of kk4 is connected to point A, and the cathode of kk4 is connected to the negative terminal of the induction heating power supply, forming an active heating circuit I2. The anode of kk5 is connected to point A, and the cathode of kk5 is connected to point B, forming a passive heating circuit I3. The anode of kk7 is connected to point A, and the cathode of kk7 is connected to the positive terminal of the induction heating power supply, forming a passive heating circuit I5. Figure 13 As shown.

[0048] Furthermore, an induction heating main circuit includes four thyristors kk1~kk4, one induction heating coil L, and one energy storage and boosting capacitor C. The circuit connection is as follows: one terminal of the energy storage and boosting capacitor C is connected to point A, and the other terminal is connected to one end of the induction heating coil L. The other end of the induction heating coil L is connected to point B. The anode of kk1 is connected to the positive terminal of the induction heating power supply, and the cathode of kk1 is connected to point A. The anode of kk2 is connected to point B, and the cathode of kk2 is connected to the negative terminal of the induction heating power supply, forming an active heating circuit I1. The anode of kk3 is connected to the positive terminal of the induction heating power supply, and the cathode of kk3 is connected to point B. The anode of kk4 is connected to point A, and the cathode of kk4 is connected to the negative terminal of the induction heating power supply, forming an active heating circuit I2. Figure 14 As shown. This is a main circuit diagram for an induction heating system with the highest rated power, very similar to commonly used main circuit diagrams (such as...). Figure 1 ).

[0049] exist Figure 14 In Chinese, the symbols for current and voltage are defined as follows: U -- power supply voltage (V), which can be considered a constant.

[0050] i -- Output current of the induction heating power supply (A), I m Let L be a constant, L be the inductance of the induction heating coil L (H), C be the capacitance of the energy storage and boosting capacitor C (F), and R be the equivalent resistance ( ). ), its direction is as follows Figure 14 As indicated by the arrow.

[0051] T1 -- On-time (s) of the active or passive heating circuit .

[0052] The time interval (s) between the reversal of the two heating circuits.

[0053] T -- the commutation cycle (s) of the two heating circuits. .

[0054] I1 -- The current (A) through the active heating circuit I1, i.e., the current when kk1 and kk2 are conducting, with the direction as follows: Figure 14 As shown in the figure, when kk1 and kk2 are on, I1=i=I L The conduction time is T1 each time. When kk1 and kk2 are closed, I1=0.

[0055] I2 -- The current (A) flowing through the active heating circuit I2, i.e., the current when kk3 and kk4 are conducting, with the direction as follows: Figure 14 As shown in the figure, when kk3 and kk4 are on, I2=i=-I L The conduction time is T1 each time. When kk3 and kk4 are closed, I2=0.

[0056] I L --The current flowing through the induction heating coil L is in the following direction. Figure 14 A positive value indicates the direction of the arrow, while a negative value indicates the direction of the arrow opposite to the direction of the middle arrow. The waveform of the current is unrelated to the semiconductor switch; it only serves to commutate the current.

[0057] U C --The voltage across the energy storage booster capacitor C. The arrow in the diagram points from the high potential to the low potential of the energy storage booster capacitor. When the voltage across the energy storage booster capacitor is in the same direction as shown in the diagram, U... C For positive, U is the opposite direction. C Negative, UC0 It is the voltage across the energy storage booster capacitor at time t=0.

[0058] U L --The voltage across the induction heating coil L. The arrow in the diagram points from the high potential to the low potential of the capacitor. .

[0059] like Figure 14 As shown, I1 and I2 cannot both be non-zero, because both being non-zero would mean a short circuit. Therefore, .when At that time, the power of induction heating is at its maximum, using P m If it means that, then when At that time, its power P is: It can be increased The method is to reduce power.

[0060] when At that time, that is In existing technical terminology, this is called "resonance," which has nothing to do with the power supply frequency because direct current (DC) has no frequency concept. Here, T is the commutation period of the switch, which is completely different from the power supply frequency in alternating current (AC). When using the concept of resonance, a diode needs to be added to the switch, such as... Figure 1 As shown, this leads to the existing technology of reducing power through non-resonance (T>2T1), and in this solution, I... L The sudden change in voltage across the induction heating coil L causes a sudden change in voltage, such as... Figure 2 As shown, harmonics and discontinuities in the first derivative of voltage occur. The technique used in this invention always involves I... L A power factor of 0 triggers the thyristor to turn on or turns it off automatically, preventing high-order harmonics and ensuring the power factor is always 1.

[0061] During each heating process, the energy of the energy storage and boosting capacitor increases, therefore the voltage U across the boosting capacitor increases. C As the capacitor grows larger, the energy stored in it also increases; hence, it is called an "energy storage booster" capacitor. After a certain number of cycles, U... C It will stabilize at a maximum value U cm U cm and Yes, there is a relationship; the smaller R is, the more U... cm The larger U cm It can reach several to tens of times the power supply voltage U, possibly exceeding the withstand voltage of the energy storage booster capacitor or the induction heating coil L (where the highest voltage across its terminals is U). cm The withstand voltage value of +U), therefore, U cmRestrictions must be implemented through technical means. Restricting U cm The technical approach is that when U C ≥U cm (put U cm When the set threshold is reached, a passive heating circuit is activated to release part or all of the energy stored in the energy storage booster capacitor. This requires adding a passive heating circuit.

[0062] Furthermore, an induction heating main circuit includes five thyristors kk1~kk4, kk7, one induction heating coil L, and one energy storage and boosting capacitor C. The circuit connection is as follows: one terminal of the energy storage and boosting capacitor C is connected to point A, and the other terminal is connected to one end of the induction heating coil L. The other end of the induction heating coil L is connected to point B. The anode of kk1 is connected to the positive terminal of the induction heating power supply, and the cathode of kk1 is connected to point A. The anode of kk2 is connected to point B, and the cathode of kk2 is connected to the negative terminal of the induction heating power supply, forming an active heating circuit I1. The anode of kk3 is connected to the positive terminal of the induction heating power supply, and the cathode of kk3 is connected to point B. The anode of kk4 is connected to point A, and the cathode of kk4 is connected to the negative terminal of the induction heating power supply, forming an active heating circuit I2. The anode of kk7 is connected to point A, and the cathode of kk7 is connected to the positive terminal of the induction heating power supply, forming a passive heating circuit I5. Figure 15 As shown.

[0063] The following is a circuit that can fully release the energy of the energy storage booster capacitor. Furthermore, an induction heating main circuit includes six thyristors kk1~kk6, one induction heating coil L, and one energy storage and boosting capacitor C. The circuit connection is as follows: one terminal of the energy storage and boosting capacitor C is connected to point A, and the other terminal is connected to one end of the induction heating coil L. The other end of the induction heating coil L is connected to point B. The anode of kk1 is connected to the positive terminal of the induction heating power supply, and the cathode of kk1 is connected to point A. The anode of kk2 is connected to point B, and the cathode of kk2 is connected to the negative terminal of the induction heating power supply, forming an active heating circuit I1. The anode of kk3 is connected to the positive terminal of the induction heating power supply, and the cathode of kk3 is connected to point B. The anode of kk4 is connected to point A, and the cathode of kk4 is connected to the negative terminal of the induction heating power supply, forming an active heating circuit I2. The anode of kk5 is connected to point A, and the cathode of kk5 is connected to point B, forming a passive heating circuit I3. The anode of kk6 is connected to point B, and the cathode of kk6 is connected to point A, forming a passive heating circuit I4. Figure 16 As shown. With Figure 14 Compared to previous models, two passive heating circuits, I3 and I4, are added. These two passive heating circuits alternately conduct, reducing the voltage across the energy storage and boosting capacitor C to 0, thus enabling arbitrary control of the voltage across capacitor C. Because this induction heating main circuit includes two active heating circuits and two passive heating circuits, various heating methods can be combined. (1) Two active heating circuits I1 and I2 are used, with the maximum rated power.

[0064] (2) Using two active heating circuits I1 and I2, when the voltage across the energy storage boost capacitor C exceeds the set threshold, I3 or I4 or both can be used again to reduce the voltage across the energy storage boost capacitor C.

[0065] (3) Using one active heating circuit I1 and one passive heating circuit I3, the rated power is relatively small.

[0066] (4) Using an active heating circuit I1 and a passive heating circuit I3, when the voltage across the energy storage boost capacitor C exceeds the set threshold, I4 can be used again to reduce the voltage across the energy storage boost capacitor C.

[0067] (5) Using an active heating circuit I2 and a passive heating circuit I4 has the same effect as (3).

[0068] (6) Using an active heating circuit I2 and a passive heating circuit I4, when the voltage across the energy storage boost capacitor C exceeds the set threshold, I3 can be used again to reduce the voltage across the energy storage boost capacitor C.

[0069] Furthermore, an induction heating main circuit includes seven thyristors kk1~kk7, one induction heating coil L, and one energy storage and boosting capacitor C. The circuit connection is as follows: one terminal of the energy storage and boosting capacitor C is connected to point A, and the other terminal is connected to one end of the induction heating coil L. The other end of the induction heating coil L is connected to point B. The anode of kk1 is connected to the positive terminal of the induction heating power supply, and the cathode of kk1 is connected to point A. The anode of kk2 is connected to point B, and the cathode of kk2 is connected to the negative terminal of the induction heating power supply, thus forming an active heating circuit. I1; The anode of kk3 is connected to the positive terminal of the induction heating power supply, and the cathode of kk3 is connected to point B. The anode of kk4 is connected to point A, and the cathode of kk4 is connected to the negative terminal of the induction heating power supply, forming an active heating circuit. I2; The anode of kk5 is connected to point A, and the cathode of kk5 is connected to point B, forming a passive heating circuit. I3; The anode of kk6 is connected to point B, and the cathode of kk6 is connected to point A, forming a passive heating circuit. I4; The anode of kk7 is connected to point A, and the cathode of kk7 is connected to the positive terminal of the induction heating power supply, forming a passive heating circuit. I5, as... Figure 17 As shown.

[0070] Furthermore, an induction heating main circuit includes eight thyristors kk1~kk8, one induction heating coil L, and one energy storage and boosting capacitor C. The circuit connection is as follows: one terminal of the energy storage and boosting capacitor C is connected to point A, and the other terminal is connected to one end of the induction heating coil L. The other end of the induction heating coil L is connected to point B. The anode of kk1 is connected to the positive terminal of the induction heating power supply, and the cathode of kk1 is connected to point A. The anode of kk2 is connected to point B, and the cathode of kk2 is connected to the negative terminal of the induction heating power supply, forming an active heating circuit I1. The anode of kk3 is connected to the positive terminal of the induction heating power supply. The cathode of kk3 is connected to point B, the anode of kk4 is connected to point A, and the cathode of kk4 is connected to the negative terminal of the induction heating power supply, forming an active heating circuit I2; the anode of kk5 is connected to point A, and the cathode of kk5 is connected to point B, forming a passive heating circuit I3; the anode of kk6 is connected to point B, and the cathode of kk6 is connected to point A, forming a passive heating circuit I4; the anode of kk7 is connected to point A, and the cathode of kk7 is connected to the positive terminal of the induction heating power supply, forming a passive heating circuit I5; the anode of kk8 is connected to point B, and the cathode of kk8 is connected to the positive terminal of the induction heating power supply, forming a passive heating circuit I6, as shown below. Figure 18 As shown. This induction heating main circuit includes two active heating circuits I1 and I2, and four passive heating circuits I3 to I6. It is suitable for high-power and high-load applications, i.e., applications with a large variation in equivalent resistance R, such as medium-frequency melting furnaces with rated power of several megawatts. It can be used at full load with the furnace charge filled, or it can be used without the furnace charge filled.

[0071] 2.2 Control Circuit The control circuit completes all testing and control functions of one induction heating device. The control circuit includes a controller, a data acquisition module, a drive module, and a communication module. The data acquisition module, drive module, and communication module are connected to the controller, the drive module is connected to the induction heating main circuit, and the communication module is connected to the smart terminal.

[0072] Furthermore, the signal acquisition module varies greatly depending on the application scenario, but at a minimum it includes acquiring: the average value of the input current I. p Input instantaneous current i, input average voltage U P The instantaneous current of the passive heating circuit and the voltage U across the energy storage boosting capacitor. c wait.

[0073] Furthermore, the drive circuit module includes, but is not limited to: triggering the turn-on and turn-off of the turn-off semiconductor switch according to the control signal issued by the controller, and turning on the thyristors kk1~kk8.

[0074] Furthermore, the functions of the controller vary greatly depending on the application scenario, including: reading data from the signal acquisition module, reading parameter settings through the communication module, power calculation, PID calculation, etc., sending on and off signals to trigger the turn-off of the turn-off semiconductor switch, triggering the turn-on of the thyristor, etc. to the drive circuit, and exchanging data with the smart terminal through the communication module.

[0075] Furthermore, the communication module includes wireless communication and wired communication. The wireless communication includes Bluetooth, Wi-Fi, etc., and the wired communication includes, but is not limited to, RS485, RS232, etc.

[0076] 3. Smart terminals The smart terminal can be one or both of fixed terminals and mobile terminals.

[0077] The fixed terminal includes, but is not limited to, a display screen with input control function, a computer, etc., and the mobile terminal includes, but is not limited to, a smartphone, a tablet computer, etc.

[0078] The following section provides further explanation based on different application scenarios.

[0079] Example 1: 220V AC power supply, home application scenario.

[0080] The purpose of induction heating in the home is to heat water, which may be used for boiling water, heating tap water to obtain warm water (in the kitchen, bathing, washing, etc.), and cooking (induction cookers, slow cookers, pressure cookers, etc.). Their common characteristic is that they are used to heat water, but water itself cannot be heated through eddy currents; rather, the water is heated by heating the "container," and the container then heats the water. For this application scenario, the following solutions can be considered.

[0081] 1. DC power supply and current inverter circuit. A 220V AC rectifier and current inverter circuit can be used as the power supply in the household power cabinet. Alternatively, a rectifier and current inverter circuit can be added directly to each induction heating appliance.

[0082] 2. Induction heating device. Due to its low power and fixed load, there is no need to use a controllable voltage-limiting passive circuit. A load circuit consisting of one active heating circuit and one passive heating circuit is selected, such as... Figure 10 As shown.

[0083] 3. Smart Terminal. A smartphone with Bluetooth communication is selected; a Bluetooth communication module is used in the control circuit.

[0084] Example 2: Service industries such as restaurants, hotels, and accommodation.

[0085] These application scenarios are similar to home applications, but due to the significantly increased number of devices, in addition to smartphones, computers can be added for smart terminals, primarily for management. Furthermore, a fixed display screen is preferable for kitchen induction heating equipment to facilitate chef operation. Regarding power supply, choose appropriately based on available power conditions; AC 380V or 220V are both acceptable. Since kitchen induction heating equipment has high power, consider using a load circuit with two active heating loops, such as... Figure 13 , 14 As shown.

[0086] Example 3: Industrial metal rotary drum furnace. A metal rotary drum furnace refers to a furnace using a cylindrical metal drum. During operation, the drum is heated, and its rotation moves the material inside along its length. Induction heating is performed in sections outside the drum. Because the drum itself is heated, a controllable, voltage-limiting passive heating circuit is not required. However, considering the power requirements of different sections, different induction heating main circuits can be used. The main purpose is to match different rated power levels. For intelligent terminals, computing is used for management and recording, while mobile terminals are used for monitoring and operation.

[0087] Example 4: Industrial metal induction heating furnace. Industrial metal induction heating furnaces include metal heat treatment furnaces, forging heating furnaces, etc. Their typical characteristics are: the furnace body is placed horizontally, the metal to be heated enters from one end of the furnace and exits from the other, and the heating is divided into several sections along its length. The power required for each section varies considerably because the size and temperature of the heated metal differ, resulting in significant differences in the equivalent resistance of the eddy currents. Furthermore, there are significant differences between different induction heating furnaces. It is recommended to use active and passive induction heating main circuits, computer management and recording of process parameters, and mobile terminals for monitoring and operation.

[0088] Example 5: Medium-frequency melting furnace used in foundry workshop. The technical difficulties of medium-frequency melting furnace include: (1) large power, the power of a medium-frequency melting furnace ranges from hundreds of kilowatts to tens of thousands of kilowatts, the power of a single unit is large, and the power range is also large; (2) large change in equivalent resistance, different types of smelting metals, different temperatures, especially the magnetic transformation at Curie temperature, and the amount of furnace charge all significantly affect the equivalent resistance of eddy currents; (3) large difference in the power requirements for eddy current heating at different stages. For high power, unless absolutely necessary, it is not recommended to use parallel induction heating coils L, because the equivalent resistance of eddy currents at the upper and lower positions differs greatly during use. For extra-large melting furnaces, if one induction heating coil L cannot meet the power requirements, two induction heating coils L are required, and they must be controlled separately, which is actually two furnaces. From the perspective of power, non-oscillating circuits (such as Figure 9This is because the voltage across the induction heating coil L in this circuit is equal to the power supply voltage, and it has no capacitive reactance. Under the same voltage conditions, its output power is maximized, and the power can be controlled by both frequency control and semiconductor switch on-time. Secondly, two active heating circuits plus a passive heating circuit are used, such as... Figure 17 and Figure 18 It can be configured with various rated power, plus a delay. Power adjustment is necessary to handle complex operating conditions. The water cooling system may require monitoring the temperature at dozens of points, as well as inlet water pressure, to ensure safe equipment operation. If displaying or controlling the furnace charge temperature is required, it also necessitates testing the furnace charge temperature. The intelligent terminal needs to be equipped with a computer to record the equipment's operating status, such as voltage, current, power, temperature at various points, cooling water flow, etc., and also includes a fixed intelligent display screen and a mobile terminal for monitoring and operating the equipment.

Claims

1. An induction heating cluster device, characterized in that: The system includes an induction heating power supply, an induction heating device assembly, and a smart terminal. The induction heating power supply includes a DC power supply and a current inverter circuit. The current inverter circuit includes an inverter inductor L1, a current compensation capacitor C1, and a freewheeling diode Zk1. The inverter inductor L1 and the current compensation capacitor C1 are connected in series, then in parallel with the freewheeling diode Zk1, and finally connected to the output terminal of the DC power supply. The two poles of the current compensation capacitor C1 are the output terminals of the induction heating power supply. The induction heating device assembly includes 1 to M induction heating devices, which are connected in parallel to the output terminal of the induction heating power supply. Each induction heating device includes an induction heating main circuit and a control circuit. The control circuit is connected to the induction heating main circuit, and the smart terminal drives the control circuit to control the power of the induction heating main circuit.

2. The induction heating cluster device as described in claim 1, characterized in that: The DC power supply is a constant voltage DC energy storage power supply, or a power supply that converts AC power into DC power through diode rectification.

3. The induction heating cluster device as described in claim 1, characterized in that: The inverter inductor L1 is an inductor with a magnetic core.

4. The induction heating cluster device as described in claim 1, characterized in that: The current compensation capacitor C1 is a capacitor bank consisting of 1 to N electrolytic capacitors connected in parallel, or a capacitor unit consisting of at least two capacitor banks connected in series.

5. The induction heating cluster device as described in claim 1, characterized in that: The induction heating main circuit includes a turn-off semiconductor switch, a freewheeling diode D, and an induction heating coil L. The turn-off semiconductor switch and the induction heating coil L are connected in series to the output terminal of the induction heating power supply to form an active heating circuit. The freewheeling diode Zk1 is connected in parallel with the induction heating coil L to form a passive heating circuit.

6. The induction heating cluster device as described in claim 1, characterized in that: The induction heating main circuit includes three thyristors kk1, kk2, and kk5, one induction heating coil L, and one energy storage and boosting capacitor C. One terminal of the energy storage and boosting capacitor C is connected to point A, and the other terminal is connected to one end of the induction heating coil L. The other end of the induction heating coil L is connected to point B. The anode of kk1 is connected to the positive terminal of the induction heating power supply, and the cathode of kk1 is connected to point A. The anode of kk2 is connected to point B, and the cathode of kk2 is connected to the negative terminal of the induction heating power supply, forming an active heating circuit I1. The anode of kk5 is connected to point A, and the cathode of kk5 is connected to point B, forming a passive heating circuit I3.

7. The induction heating cluster device as described in claim 6, characterized in that: The induction heating main circuit also includes a thyristor kk6, with the anode of kk6 connected to point B and the cathode of kk6 connected to point A, forming a passive heating circuit I4.

8. The induction heating cluster device as described in claim 6, characterized in that: The induction heating main circuit also includes a thyristor kk 3、 The anodes of kk4 and kk3 are connected to the positive terminal of the induction heating power supply, the cathode of kk3 is connected to point B, the anode of kk4 is connected to point A, and the cathode of kk4 is connected to the negative terminal of the induction heating power supply, forming an active heating circuit I2.

9. The induction heating cluster device as described in claim 1, characterized in that: The induction heating main circuit includes four thyristors kk1~kk4, one induction heating coil L, and one energy storage and boosting capacitor C. One pole of the energy storage and boosting capacitor C is connected to point A, and the other pole is connected to one end of the induction heating coil L. The other end of the induction heating coil L is connected to point B. The anode of kk1 is connected to the positive terminal of the induction heating power supply, and the cathode of kk1 is connected to point A. The anode of kk2 is connected to point B, and the cathode of kk2 is connected to the negative terminal of the induction heating power supply, forming an active heating circuit I1. The anode of kk3 is connected to the positive terminal of the induction heating power supply, and the cathode of kk3 is connected to point B. The anode of kk4 is connected to point A, and the cathode of kk4 is connected to the negative terminal of the induction heating power supply, forming an active heating circuit I2.

10. An induction heating cluster device as described in claim 8 or 9, characterized in that: The induction heating main circuit also includes a thyristor kk7, with the anode of kk7 connected to point A and the cathode of kk7 connected to the positive terminal of the induction heating power supply, forming a passive heating circuit I5.

11. The induction heating cluster device as described in claim 9, characterized in that: The induction heating main circuit also includes a thyristor kk 5、 The anodes of kk6 and kk5 are connected to point A, and the cathode of kk5 is connected to point B, forming a passive heating circuit I3; the anode of kk6 is connected to point B, and the cathode of kk6 is connected to point A, forming a passive heating circuit I4.

12. The induction heating cluster device as described in claim 11, characterized in that: The induction heating main circuit also includes a thyristor kk7, with the anode of kk7 connected to point A and the cathode of kk7 connected to the positive terminal of the induction heating power supply, forming a passive heating circuit I5.

13. The induction heating cluster device as described in claim 12, characterized in that: The induction heating main circuit also includes a thyristor kk8, with the anode of kk8 connected to point B and the cathode of kk8 connected to the positive terminal of the induction heating power supply, forming a passive heating circuit I6.

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