Intermediate frequency chopping parallel inverter multi-induction coil heating system and method

Abstract

The application relates to a kind of middle frequency chopper parallel inverter multi-induction coil heating system and method.The system part mainly includes control module, public power module, several heating branches;The public power module includes sequentially connected voltage conversion rectifier unit, chopper voltage regulation unit and parallel inverter unit, and the output end of parallel inverter unit is connected with resonance capacitor in parallel;The heating branch includes gating switch and induction heating unit;Control module is used to detect the zero-crossing signal of middle frequency alternating current, and controls each gating switch to carry out time-sharing gating switching at the zero-crossing moment of middle frequency alternating current, so that only the heating branch being gated is connected into parallel resonance circuit and works at the same time;Control module compensates and adjusts the output voltage of chopper voltage regulation unit according to the number of currently connected heating branch when heating branch switches, to maintain the stable working state of parallel resonance circuit.The application achieves the effect of eliminating the coupling interference between multiple induction coils and realizing the temperature collaborative precise control of each heating area.

Description

Technical Field

[0001] This application relates to the field of induction heating technology, and in particular to a medium-frequency chopper parallel inverter multi-induction coil heating system and method. Background Technology

[0002] In the field of induction heating technology, multi-zone induction heating is widely used in industrial applications such as metal heat treatment, pipe welding preheating, and uniform heating of large workpieces. To achieve zoned heating of different areas of the workpiece, existing technologies typically employ a separate induction heating power supply system for each heating zone. Each independent induction heating power supply system includes its own rectification, inversion, and control unit, and each system operates independently.

[0003] However, because the inverter frequencies and phases of each independent power supply system are difficult to keep strictly consistent, when the heating areas of multiple induction coils are adjacent or spatially coupled, the alternating magnetic fields of different frequencies and phases will interfere with each other, resulting in local overheating or underheating at the boundaries of the heating areas, which seriously affects the heating uniformity of the workpiece. At the same time, the lack of a unified power coordination mechanism between the independent systems makes it impossible to perform overall coordinated control based on the real-time temperature status of each area, further exacerbating the problem of uneven temperature distribution in multi-zone heating.

[0004] Based on the above, the technical problem to be solved in this field is how to eliminate the inter-regional coupling interference caused by the inconsistency of power frequency and phase of each heating zone in a multi-induction coil heating system, and how to achieve coordinated and precise temperature control of each heating zone. Summary of the Invention

[0005] To address the aforementioned deficiencies or improvement needs of existing technologies, this application provides a medium-frequency chopper parallel inverter multi-induction coil heating system and method. This system employs a common power supply module to provide synchronous and in-phase medium-frequency AC power to multiple parallel heating branches. It also utilizes thyristor selection switches to perform time-division switching of each heating branch at the zero-crossing moment of the synchronous AC power, and adaptively allocates the conduction time ratio of each branch based on temperature feedback from each heating region. This approach achieves the effect of eliminating coupling interference between multiple induction coils and realizing precise and coordinated temperature control of each heating region.

[0006] The embodiments of this application adopt the following technical solutions: In a first aspect, this application provides a medium-frequency chopper parallel inverter multi-induction coil heating system, including a control module, a common power supply module, and several heating branches connected in parallel to the common power supply module; The common power supply module includes a transformer-rectifier unit, a chopper-regulated voltage unit, and a parallel inverter unit connected in sequence. The transformer-rectifier unit is used to convert AC power from the power grid into DC power. The chopper-regulated voltage unit is used to regulate the voltage of the DC power. The parallel inverter unit is used to invert the regulated DC power into medium-frequency AC power. A resonant capacitor is connected in parallel to the output terminal of the parallel inverter unit. The resonant capacitor and the inductive reactance of the selected heating branch form a parallel resonant circuit. Each of the heating branches includes a selector switch and an induction heating unit. The input terminal of the selector switch is connected to the output terminal of the parallel inverter unit, and the output terminal of the selector switch is connected to the induction heating unit. The control module is communicatively connected to each of the selector switches and the chopper voltage regulation unit. The control module is used to detect the zero-crossing signal of the intermediate frequency AC power and control each of the selector switches to perform time-division switching at the zero-crossing moment of the intermediate frequency AC power, so that only the selected heating branch is connected to the parallel resonant circuit at the same time. When the heating branch is switched, the control module compensates and adjusts the output voltage of the chopper voltage regulation unit according to the number of heating branches currently connected, so as to maintain the stable working state of the parallel resonant circuit.

[0007] The above technical solution utilizes a common power supply module to provide all heating branches with medium-frequency AC power of the same frequency and phase, fundamentally eliminating the inter-zone coupling interference caused by the inconsistency of power frequency and phase in each zone; by performing gating and switching at the zero-crossing moment of the medium-frequency AC power, the risk of current surge and arcing during the switching process is avoided; by synchronously compensating the output voltage of the chopper voltage regulation unit during branch switching, the problem of resonant operating point drift caused by the load impedance change of the parallel resonant circuit due to branch switching is solved, ensuring the stable and reliable operation of the system in the multi-branch time-sharing working mode.

[0008] In some implementations, the control module is also used to receive temperature feedback signals from the heating areas corresponding to each heating branch, calculate the temperature deviation between the current temperature and the target temperature of each heating area, and allocate the conduction time ratio of each heating branch according to the temperature deviation of each heating area within a time-sharing control cycle.

[0009] By combining closed-loop temperature feedback with time-sharing conduction time allocation, areas with large temperature deviations automatically receive longer conduction heating times, while areas with small temperature deviations have correspondingly shorter conduction times. This achieves adaptive and coordinated temperature control of each heating zone, ensuring the uniformity of the entire workpiece temperature field.

[0010] In some implementations, the control module allocates the conduction time percentage of each heating branch in the following ways: The temperature deviation of each heating zone is normalized to obtain the deviation weight of each heating zone. The total allocatable conduction time of a time-sharing control cycle is proportionally allocated according to the deviation weight of each heating zone to obtain the conduction time of each heating branch in the time-sharing control cycle. Specifically, when the current temperature of a certain heating zone reaches or exceeds the target temperature, the heating branch corresponding to that heating zone will not be selected in the current time-sharing control cycle.

[0011] By adopting the above technical solution and using a conduction time ratio allocation method based on temperature deviation normalization weight, the allocation of heating power directly corresponds to the actual heating demand of each area. When the temperature of a certain area has reached the standard, the area is automatically skipped to avoid overheating, which further improves the temperature control accuracy and energy utilization efficiency.

[0012] In some embodiments, the control module compensates for and adjusts the output voltage of the chopper voltage regulation unit when the heating branch switches, including the following methods: The control module pre-stores the load impedance parameters corresponding to each heating branch and its combination. When the heating branch is switched, the control module adjusts the chopper duty cycle of the chopper voltage regulation unit according to the load impedance parameters corresponding to the currently connected heating branch after the switch, so that the voltage at the output of the parallel inverter unit matches the current load impedance and maintains the parallel resonant circuit working near the resonance state.

[0013] By using the above technical solution, feedforward compensation is performed based on the pre-stored load impedance parameters of each branch. The chopper duty cycle can be quickly adjusted at the moment of branch switching, avoiding resonant frequency shift and output power fluctuation caused by sudden load changes, and ensuring the continuity and stability of power output during the heating process.

[0014] In some implementations, during the heating period of each of the heating branches, the control module collects the intermediate frequency voltage signal at the output of the parallel inverter unit and the intermediate frequency current signal of the currently conducting heating branch in real time, and calculates the actual load impedance of the current heating area online; the control module dynamically updates and overwrites the pre-stored load impedance parameters corresponding to the heating branches with the calculated actual load impedance, so as to be used for voltage compensation adjustment in the next time-sharing control cycle.

[0015] By employing the above technical solution and utilizing online calculation and dynamic update mechanisms, the problem of impedance dynamic drift caused by sudden changes in permeability and resistivity during the heating process of metal workpieces due to rapid temperature changes, especially when crossing the Curie point, is solved. This ensures that the voltage compensation parameters used by the control module when it next polls the branch are always based on the latest real-time operating conditions, guaranteeing accurate voltage matching throughout the entire temperature range and process.

[0016] In some implementations, after removing the trigger signal of the selector switch of the current heating branch, the control module sets a forced commutation dead time; during the forced commutation dead time, the control module implements pulse blocking on the parallel inverter unit, causing the parallel inverter unit to stop inverter output, in order to wait for the selector switch of the current heating branch to fully restore its forward blocking capability.

[0017] By setting a forced commutation dead time during the branch switching interval and simultaneously implementing inverter pulse blocking, the system effectively prevents short-circuit faults in the preceding and following branches due to insufficient turn-off recovery time of the selector switch. On the other hand, it avoids the risk of rapid voltage surge and breakdown of the resonant capacitor caused by the inverter continuing to inject energy into the unloaded resonant circuit during the dead time, thus greatly improving the safety and reliability of the system commutation process.

[0018] In some implementations, after the forced commutation dead time ends, the control module sends a trigger signal to the selector switch of the next heating branch to turn it on, while simultaneously releasing the pulse blockade of the parallel inverter unit and controlling the output voltage of the chopper voltage regulating unit to smoothly rise from zero according to a preset slope until it reaches a compensation voltage that matches the load impedance of the newly connected heating branch.

[0019] Through the above technical solution, after the dead time ends, the combination of inverter pulse unblocking and chopper voltage ramp soft start mechanism effectively eliminates the huge surge current impact caused by voltage change applied to the induction coil at the moment the new branch is connected, ensuring a smooth transition of energy injection in the resonant circuit and extending the service life of power devices and induction coil.

[0020] In some implementations, the control module is equipped with a digital phase-locked loop (PLL). When the heating branch is switched and the parallel inverter unit is released from pulse blocking, the PLL quickly tracks and locks the new actual resonant frequency of the current parallel resonant circuit due to the change in load inductance based on the phase difference between the intermediate frequency voltage signal and the intermediate frequency current signal of the newly connected heating branch, and drives the switching device of the parallel inverter unit with the locked new actual resonant frequency.

[0021] The above technical solution addresses the issue of resonant frequency shift caused by differences in equivalent inductance between different heating branches. By using a digital phase-locked loop to quickly re-lock phase during soft start-up, the problem of sudden resonant frequency changes caused by branch switching is solved. This ensures that the parallel inverter unit can always operate accurately at the optimal power factor state under any branch, i.e., the current and voltage are in phase, thus avoiding the fatal defect of inverter loss of lock-in and shutdown.

[0022] Secondly, this application provides a medium-frequency chopper parallel inverter multi-induction coil heating method, applied to the medium-frequency chopper parallel inverter multi-induction coil heating system described in the first aspect, the method comprising: S1. The AC power from the grid is sequentially processed by transformer rectification, chopper voltage regulation and parallel inverter through the common power module to generate medium frequency AC power; S2. Detect the zero-crossing signal of the intermediate frequency AC power. At the zero-crossing moment of the intermediate frequency AC power, send a selection signal to the selection switch of the target heating branch according to the control timing to turn on the target heating branch. At the same time, compensate and adjust the output voltage of the chopper voltage regulation unit according to the load impedance parameters after the connection. S3. Obtain the temperature feedback signal of each heating zone and calculate the temperature deviation between the current temperature and the target temperature of each heating zone; S4. Based on the temperature deviation of each heating zone, the conduction time ratio of each heating branch is redistributed proportionally in the next time-sharing control cycle, and the process returns to step S2.

[0023] Through the above technical solutions, a complete closed-loop control process is realized, including zero-crossing safety switching, real-time compensation for load changes, and adaptive conduction time allocation based on temperature deviation. This enables the multi-induction coil heating system to achieve coordinated and precise temperature control of each heating zone while eliminating inter-zone coupling interference.

[0024] In some implementations, in step S2, before switching to the next heating branch after the conduction time of the current heating branch ends, the control module waits for the intermediate frequency AC power to reach the next zero-crossing moment, removes the trigger signal of the selection switch of the current heating branch at the next zero-crossing moment, and simultaneously sends a trigger signal to the selection switch of the next heating branch to complete the shockless switching of the heating branch.

[0025] By employing the above technical solution, the zero-crossing switching sequence is strictly followed during branch switching, ensuring that the current branch's selector switch is naturally turned off before the next branch is turned on. This avoids the short-circuit risk and current surge caused by two branches being turned on simultaneously, thus guaranteeing the safety and reliability of the time-sharing switching process.

[0026] In summary, this application includes at least the following beneficial technical effects: 1. By providing medium-frequency AC power of the same frequency and phase to all parallel heating branches through the common power supply module, and performing time-division switching of each heating branch at the zero crossing of the medium-frequency AC power, and synchronously compensating the output voltage of the chopper voltage regulation unit during branch switching, the interval coupling interference caused by the inconsistency of frequency and phase between multiple induction coils is fundamentally eliminated, while ensuring the stable working state of the parallel resonant circuit during the time-division switching process of multiple branches; 2. By receiving temperature feedback signals from each heating zone and adaptively allocating the conduction time ratio of each heating branch according to the temperature deviation, closed-loop coordinated precise control of the temperature of each heating zone is achieved, ensuring the uniformity of the temperature field of the entire workpiece and meeting the requirements of high-precision process. 3. An online impedance acquisition and parameter dynamic overlay update mechanism is introduced. The pre-stored impedance table is corrected online using real-time acquired voltage and current, which solves the impedance mismatch problem caused by workpiece temperature drift and achieves accurate voltage matching in the entire temperature range. 4. A forced dead zone + pulse blocking + ramp soft start mechanism is introduced to block the inverter pulse within the switching dead zone. After the dead zone ends, the voltage recovers according to the ramp, eliminating the risk of shoot-through short circuit and no-load pump rise overvoltage within the switching dead zone, and realizing a safe switching transition without impact. 5. A digital phase-locked loop re-phase-locked mechanism is introduced, which automatically and quickly captures the actual resonant frequency of the new branch after each pulse block is released, ensuring the inverter's frequency adaptive tracking during inductor abrupt changes, and greatly improving the dynamic stability and engineering anti-disturbance capability of time-sharing multiplexing. Attached Figure Description

[0027] To more clearly illustrate the technical solutions of the embodiments of this application, the drawings used in the embodiments of this application will be briefly described below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0028] Figure 1 A schematic diagram of the architecture of a medium-frequency chopper parallel inverter multi-induction coil heating system provided in this application embodiment; Figure 2 This is a schematic diagram of the time-division strobe control timing provided in an embodiment of this application; Figure 3 This is a schematic diagram of temperature deviation adaptive conduction time allocation provided in an embodiment of this application; Figure 4 The timing diagram for branch switching commutation dead time and soft start control provided in the embodiments of this application; Figure 5 A flowchart of a medium-frequency chopper parallel inverter multi-induction coil heating method provided in an embodiment of this application.

[0029] In the diagram: 100, Common power supply module; 110, Transformer and rectifier unit; 111, 24-pulse rectifier transformer; 112, Diode rectifier bridge; 120, Chopper voltage regulation unit; 130, Parallel inverter unit; 140, Resonant capacitor; 200, Heating branch; 210, Selector switch; 220, Induction heating unit; 221, Medium frequency step-down transformer; 222, Induction coil. Detailed Implementation

[0030] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application. Furthermore, the technical features involved in the various embodiments described below can be combined with each other as long as they do not conflict with each other. The application will now be described in detail with reference to the accompanying drawings and embodiments. Example 1

[0031] It should be noted that existing multi-zone induction heating uses multiple independent power supply systems. The inverter frequency and phase of each system are difficult to be strictly consistent. When the heating areas of multiple induction coils are adjacent, the alternating magnetic fields with different frequencies and phases generate coupling interference, resulting in uneven heating. At the same time, each independent system lacks a unified power coordination mechanism, making it impossible to perform overall coordinated temperature control.

[0032] To address the aforementioned issues, this embodiment provides a medium-frequency chopper parallel inverter multi-induction coil heating system. A common power supply module provides in-phase and synchronous medium-frequency AC power to all parallel heating branches. A thyristor-based selection switch performs time-sharing switching of each heating branch at the zero-crossing point of the medium-frequency AC power. During branch switching, the output voltage of the chopper voltage regulation unit is synchronously compensated to maintain the stable operation of the parallel resonant circuit. This solution fundamentally eliminates the inter-coupling interference caused by frequency and phase inconsistencies between the multiple induction coils, ensuring the stable operation of the parallel resonant circuit during branch switching and achieving safe and reliable time-sharing heating of the multiple induction coils.

[0033] refer to Figure 1 As shown, the medium-frequency chopper parallel inverter multi-induction coil heating system provided in this embodiment includes a common power supply module 100, several heating branches 200 and a control module.

[0034] In some embodiments, the common power module 100 includes a transformer rectifier unit 110, a chopper voltage regulation unit 120, and a parallel inverter unit 130 connected in sequence. A resonant capacitor 140 is connected in parallel to the output terminal of the parallel inverter unit 130, and the resonant capacitor 140 and the load inductive reactance of the induction heating unit 220 in each heating branch 200 together form a parallel resonant circuit.

[0035] In some embodiments, the transformer-rectifier unit 110 includes a 24-pulse rectifier transformer 111 and a diode rectifier bridge 112. The input terminal of the 24-pulse rectifier transformer 111 is connected to the power grid. It employs a special winding structure and connection method to convert the three-phase AC power from the power grid into four sets of AC outputs with specific phase shift relationships. Specifically, the primary side of the 24-pulse rectifier transformer 111 is connected in a delta (or star) configuration, and the secondary side is divided into four independent windings. These four windings are connected in star, delta, and zigzag star configurations (i.e., delta) with different phase shift angles, respectively. By setting the turns ratio of each winding, the AC outputs from these four secondary windings are sequentially phase-differentiated by 15 electrical degrees (e.g., -15°, 0°, +15°, and +30°), thereby achieving the specific phase shift relationship of the 24-pulse wave. The output terminal of the 24-pulse rectifier transformer 111 is connected to the input terminal of the diode rectifier bridge 112. The diode rectifier bridge 112, composed of multiple diodes connected in a bridge rectifier circuit configuration, converts the multiple AC outputs from the 24-pulse rectifier transformer 111 into pulsating DC. Due to the use of a 24-pulse rectification method, the rectified DC ripple is smaller, and the harmonic current content on the grid side is significantly reduced, effectively reducing grid pollution and improving power quality and electromagnetic compatibility. The output of the diode rectifier bridge 112 supplies DC to the chopper voltage regulation unit 120.

[0036] In other embodiments, the 24-pulse rectifier transformer 111 can also be replaced by a 12-pulse rectifier transformer or other multi-pulse rectifier transformers with low harmonic characteristics, but the 24-pulse rectifier transformer is more effective in reducing harmonics and is more suitable for high-power induction heating applications.

[0037] In some embodiments, the chopper voltage regulation unit 120 is a chopper DC voltage regulation circuit, whose input terminal is connected to the output terminal of the diode rectifier bridge 112 to receive the rectified DC power. The chopper voltage regulation unit 120 includes power switching devices (such as IGBTs), freewheeling diodes, filter inductors, and filter capacitors. Under the control of the control module, the power switching devices periodically turn on and off according to a set chopper duty cycle. By adjusting the chopper duty cycle, the average voltage value of the output DC power is changed, thereby achieving smooth and continuous regulation of the DC voltage. The chopper voltage regulation circuit has the characteristics of high regulation efficiency and fast response speed, and can quickly adjust the output voltage according to system requirements, providing a precisely adjustable DC power supply for the subsequent parallel inverter unit 130.

[0038] It is important to note that the communication connection between the control module and the chopper voltage regulation unit 120 is the key channel for load compensation during branch switching in this system. When the heating branch 200 switches, the load impedance of the parallel resonant circuit changes accordingly. If the output voltage of the chopper voltage regulation unit 120 is not adjusted accordingly, the operating point of the resonant circuit will shift, affecting the inverter efficiency and heating effect. Therefore, while issuing the branch selection switching command, the control module simultaneously adjusts the chopper duty cycle of the chopper voltage regulation unit 120 to match the voltage at the output of the parallel inverter unit 130 with the current load impedance, maintaining the parallel resonant circuit operating stably near the resonance state.

[0039] In some embodiments, the parallel inverter unit 130 is a parallel inverter circuit, with its input terminal connected to the output terminal of the chopper voltage regulation unit 120 to receive the regulated DC power. The parallel inverter unit 130 consists of multiple power switching transistors (such as thyristors or IGBTs) and corresponding drive circuits, converting the DC power into medium-frequency AC power. A resonant capacitor 140 is connected in parallel to the output terminal of the parallel inverter unit 130. The resonant capacitor 140 and the equivalent load inductance of the induction heating unit 220 in the heating branch 200 currently connected to the parallel resonant circuit form a parallel resonant circuit. When the inverter frequency is close to the resonant frequency of this parallel resonant circuit, the circuit impedance is purely resistive and reaches its maximum value. At this time, the inverter's power factor is the highest, the switching loss is the lowest, the inverter efficiency is the highest, and the output medium-frequency current waveform is close to a sine wave, which is beneficial to improving the induction heating effect.

[0040] The intermediate frequency AC power output by the parallel inverter unit 130 provides a unified, same-frequency, and same-phase power supply for all heating branches 200, which is the fundamental guarantee for eliminating inter-coil coupling interference between multiple induction coils. Since all heating branches 200 share the same intermediate frequency AC power supply, the frequency and phase of the alternating magnetic field generated by each induction coil 222 are completely consistent. Even if the heating areas are adjacent or there is spatial coupling, there will be no interference due to frequency and phase differences.

[0041] In some embodiments, the input terminals of several heating branches 200 are connected in parallel to the output terminal of the parallel inverter unit 130. Each heating branch 200 includes a selector switch 210 and an induction heating unit 220. The input terminal of the selector switch 210 is connected to the output terminal of the parallel inverter unit 130, and the output terminal of the selector switch 210 is connected to the induction heating unit 220. Specifically, the output terminal of the parallel inverter unit 130 can be connected to the AC bus after connecting a resonant capacitor 140 in parallel, and the input terminal of the selector switch 210 can also be connected to the AC bus.

[0042] In some implementations, the selector switch 210 is a thyristor switch. Thyristors have the advantages of high current handling capacity, high voltage withstand capability, and low on-state voltage drop, making them particularly suitable for medium-frequency, high-power switching applications. The key reason for choosing a thyristor switch as the selector switch 210 in this system lies in the natural commutation characteristics of the thyristor: when the current flowing through the thyristor naturally crosses zero, the thyristor automatically turns off. This characteristic eliminates the need for forced current interruption during the switching operation of the heating branch in the medium-frequency AC circuit, avoiding voltage spikes and switching losses caused by forced turn-off, and improving the reliability and service life of the selector switch 210.

[0043] In some implementations, the control module detects the zero-crossing signal of the intermediate frequency alternating current (IF AC) and sends a trigger pulse to the thyristor switch of the target heating branch 200 at the moment the IF AC current crosses zero to turn it on, or removes the trigger pulse from the thyristor switch of the currently conducting branch to allow it to turn off naturally at the next current zero-crossing moment. (Refer to...) Figure 2 As shown. This switching operation, performed at the zero-crossing moment, enables shock-free and safe switching of each heating branch 200.

[0044] In other embodiments, the selector switch 210 may also be other types of power switching devices such as IGBT switches, but in medium-frequency, high-power induction heating applications, thyristors are more advantageous due to their natural commutation characteristics and high reliability.

[0045] In some embodiments, the induction heating unit 220 includes an intermediate frequency step-down transformer 221 and an induction coil 222. The primary side of the intermediate frequency step-down transformer 221 is connected to the output terminal of the selector switch 210, and the secondary side is connected to the induction coil 222.

[0046] The intermediate frequency step-down transformer 221 is used to step down the higher voltage intermediate frequency AC power output from the parallel inverter unit 130 and perform impedance transformation to match the low voltage and high current operating requirements of the induction coil 222. The intermediate frequency step-down transformer 221 typically uses core materials suitable for intermediate frequency operation, such as ferrite cores or laminated silicon steel cores, to reduce core losses. By selecting an appropriate turns ratio, the intermediate frequency step-down transformer 221 converts the higher voltage and lower current on the primary side to the lower voltage and higher current on the secondary side, allowing electrical energy to be transferred more effectively to the induction coil 222 and the heated workpiece, thus improving the energy transfer efficiency of induction heating. Simultaneously, the intermediate frequency step-down transformer 221 also provides electrical isolation, isolating the induction coil 222 side from the common power module 100 side to ensure operational safety.

[0047] In some implementations, the induction coil 222 is a single-turn pluggable induction coil. A single-turn induction coil has a small inductance, resulting in high heating efficiency in medium-frequency induction heating. The pluggable design means that the induction coil 222 is detachably connected to the secondary side of the medium-frequency step-down transformer 221 via a pluggable connection interface. This pluggable connection interface can be in the form of quick-connect plugs, making the installation, removal, and replacement of the induction coil 222 extremely convenient. In industrial scenarios where the heating area is fixed but frequent replacement of induction coils of different specifications is required or maintenance is necessary, the pluggable design can significantly reduce equipment downtime, improve production efficiency, and enhance equipment versatility.

[0048] The induction coil 222 can be selected in different shapes and sizes according to the shape, size and heating requirements of the workpiece being heated, such as circular, rectangular or irregularly shaped coils. The induction coil 222 in each heating branch 200 corresponds to different heating areas of the workpiece being heated. By time-divisionally selecting each heating branch 200, the heating of each area of ​​the workpiece can be achieved.

[0049] In some implementations, the control module is communicatively connected to each selector switch 210 and the chopper voltage regulation unit 120. The control module generally consists of a microprocessor, timer, analog-to-digital converter, communication interface, and corresponding drive circuits, and implements the control logic through software programs.

[0050] The core functions of the control module include the following three aspects: (1) Zero-crossing detection and time-division gate control. The control module detects the zero-crossing signal of the intermediate frequency AC power output from the parallel inverter unit 130. The zero-crossing signal can be obtained by sampling the waveform of the intermediate frequency AC power through a current transformer or voltage transformer and processing it through a zero-crossing comparator circuit. (Reference) Figure 2 As shown, the control module controls each selector switch 210 to perform time-division switching at the zero-crossing moment of the intermediate frequency AC power, so that only the selected heating branch 200 is connected to the parallel resonant circuit at any given time. The purpose of switching at the zero-crossing moment is to utilize the natural commutation characteristics of the thyristor to achieve safe switching without current surge.

[0051] Specifically, refer to Figure 2 As shown, within a time-sharing control cycle T, the control module sequentially selects each heating branch 200. When the conduction time t1 of branch 1 ends, the control module waits for the intermediate frequency AC power to reach the next zero-crossing moment. At this zero-crossing moment, the trigger signal of the thyristor switch in branch 1 is removed, causing it to turn off naturally. Simultaneously, a trigger signal is sent to the thyristor switch in branch 2 to turn it on, completing the shockless switching of the heating branch. This process is repeated sequentially to complete the polling selection of all heating branches 200.

[0052] (2) Chopper voltage regulation compensation during branch switching. When the heating branch 200 is switched, the equivalent load impedance of the induction heating unit 220 in different heating branches 200 may be different (for example, the equivalent impedance of the induction coil 222 may be different due to the different shapes, materials and temperature states of the workpieces in different heating areas). The total load impedance of the parallel resonant circuit changes before and after the switch. If the input voltage of the parallel inverter unit 130 (i.e. the output voltage of the chopper voltage regulation unit 120) remains unchanged, the change in load impedance will cause the resonant circuit to deviate from the optimal resonant operating point, resulting in output power fluctuations and a decrease in inverter efficiency.

[0053] To address this issue, the control module pre-stores the load impedance parameters corresponding to each heating branch 200 and its combinations. These parameters can be obtained through actual measurements during system commissioning, or calculated based on the design parameters of each induction coil 222 and the intermediate frequency step-down transformer 221. When a heating branch 200 switches, the control module instantly adjusts the chopping duty cycle of the chopper voltage regulation unit 120 according to the load impedance parameters corresponding to the currently connected heating branch 200. The specific adjustment calculation steps are as follows: First, the control module obtains the effective value I of the target heating current set by the current system. ref And the pre-stored load impedance magnitude |Z1| of the heating branch after switching; secondly, calculate the required target compensation DC voltage U according to Ohm's law. dc =I ref ×∣Z1∣; Then, the control module acquires the current DC bus voltage U at the input terminal of the chopper voltage regulation unit 120 in real time. in (i.e., the rectified voltage); finally, the control module uses the buck chopper formula D=U dc / U in The target chopper duty cycle D is calculated, and this duty cycle signal is output to the power switching device drive circuit of the chopper voltage regulation unit 120. Through the above calculation steps, the voltage at the output terminal of the parallel inverter unit 130 is matched with the current load impedance, maintaining the parallel resonant circuit operating near the resonant state.

[0054] refer to Figure 2 As shown in the diagram at the bottom of the time-sharing control timing diagram, the chopper-regulated output voltage is adjusted to compensate for branch switching: when branch 1 is activated, the chopper-regulated output voltage is adjusted to V1 to match the load impedance of branch 1; when switching to branch 2, the output voltage is adjusted to V2; and when switching to branch N, the output voltage is adjusted to VN. This feedforward compensation method based on pre-stored load impedance parameters allows for rapid voltage adjustment at the instant of branch switching, avoiding resonant frequency shifts and output power fluctuations caused by sudden load changes.

[0055] (3) Temperature feedback and closed-loop coordinated control. The control module receives temperature feedback signals from the corresponding heating areas of each heating branch 200. The temperature feedback signals are acquired by temperature sensors (such as thermocouples or infrared thermometers) installed in each heating area. The temperature sensors measure the current temperature of each heating area in real time and convert the temperature information into electrical signals, which are then transmitted to the control module.

[0056] The control module generates selection control signals based on a unified clock signal to ensure that each heating branch 200 is selected within the same frequency and phase cycle of the medium-frequency AC power. Simultaneously, the control module calculates the temperature deviation between the current temperature and the preset target temperature of each heating zone based on the temperature feedback signals from each heating zone. It then dynamically adjusts the conduction time ratio of the corresponding heating branch 200 within the time-sharing control cycle according to the temperature deviation of each heating zone, achieving zoned online temperature control and multi-zone coordinated temperature control. Example 2

[0057] It should be noted that during the multi-induction coil time-sharing heating process, the heat load, heat dissipation conditions, and initial workpiece temperature of each heating zone are different, resulting in differences in the heating rate and temperature state of each zone. If a fixed equal-time polling time-sharing method is used, it is impossible to allocate power differently according to the actual heating needs of each zone, making it difficult to achieve uniform and precise temperature control of each zone.

[0058] Based on this, the control module in this embodiment normalizes the temperature deviation of each heating zone to obtain a deviation weight. According to the deviation weight, the conduction time of each heating branch is proportionally allocated within the time-sharing control cycle. Furthermore, when the temperature of a certain zone reaches the target, that zone is no longer selected. Through this method, adaptive power allocation based on temperature deviation is achieved. Zones with large temperature deviations automatically receive more heating time, while zones that have reached the target temperature automatically stop heating, ensuring the uniformity of the temperature field across the entire workpiece and improving temperature control accuracy and energy utilization efficiency.

[0059] refer to Figure 3 As shown in the figure, this embodiment details how the control module adaptively allocates the conduction time ratio of each heating branch 200 according to the temperature deviation.

[0060] Assume the system contains N heating branches 200, each corresponding to one of the N heating zones. Before the start of each time-sharing control cycle T, the control module performs the following steps: Step 1: Obtain the temperature deviation. The control module obtains the current temperature T_current_i and the preset target temperature T_target_i of each heating zone i (i=1,2,...,N) through the temperature feedback signal, and calculates the temperature deviation of each heating zone: ΔT_i=T_target_i-T_current_i.

[0061] Step 2: Determine whether each area needs heating. For each heating area i, if ΔT_i≤0, that is, the current temperature of the area has reached or exceeded the target temperature, then the area is marked as "not selected". In the current time-sharing control cycle, no selection signal is sent to its corresponding heating branch 200 to avoid overheating.

[0062] Step 3: Normalization. For all regions requiring heating (i.e., regions where ΔT_i > 0), normalize the temperature deviation to obtain the deviation weight w_i for each region: w_i=ΔT_i / Σ(ΔT_j), where the summation iterates through all regions j where ΔT_j>0.

[0063] Step 4: Allocate conduction time. The total allocable conduction time of the time-sharing control cycle T is proportionally allocated according to the deviation weight w_i of each region: t_i = w_i × T; where t_i is the conduction time of the heating branch 200 corresponding to heating region i in the current time-sharing control cycle.

[0064] refer to Figure 3 As shown, taking three heating zones as an example: Zone 1 has a large temperature deviation ΔT1, Zone 2 has a medium temperature deviation ΔT2, and Zone 3 has a small temperature deviation ΔT3. After normalization, Zone 1 obtains approximately 50% of the conduction time, Zone 2 obtains approximately 33% of the conduction time, and Zone 3 obtains approximately 17% of the conduction time. When the temperature of Zone 3 reaches the target temperature (ΔT3≤0), Zone 3 is no longer selected in subsequent time-sharing control cycles. The total conduction time is redistributed between Zone 1 and Zone 2 according to the new deviation weights, for example, Zone 1 obtains 60% and Zone 2 obtains 40% of the conduction time.

[0065] Through the aforementioned adaptive conduction time allocation method, areas with large temperature deviations (i.e., underheated areas) automatically receive longer heating times and more heating power input, while areas with small temperature deviations (i.e., areas close to the target temperature) automatically reduce their heating times, and areas that have reached the target temperature automatically stop heating. As the heating process continues, the temperature of each area gradually approaches its respective target temperature, achieving uniform and precise control of the entire workpiece temperature field. Example 3

[0066] It should be noted that in practical engineering applications, the time-sharing multi-zone heating system also faces the following drawbacks: First, during the heating process of metal workpieces (especially when crossing the Curie point temperature of magnetic metals), their equivalent impedance will undergo a drastic change, resulting in a severe mismatch in the chopper compensation voltage given by the system based on the pre-stored static parameters; Second, the thyristor selection switch needs a certain recovery time after the current crosses zero before it can regain its blocking capability. If the next branch is switched seamlessly, it is very easy to cause a short circuit between the preceding and following branches. If a dead time is forcibly left, the voltage on the resonant capacitor will be pumped up instantly, causing the inverter to crash; Third, the equivalent inductance of different inductors or under different operating conditions is different. The instantaneous switching of branches will cause a sudden jump in the natural frequency of the parallel resonant circuit. If this is not handled, it will cause the inverter to lose phase lock and shut down.

[0067] To avoid the aforementioned problems, this embodiment introduces an online impedance acquisition and dynamic parameter overlay update mechanism, utilizing real-time acquired voltage and current to correct the pre-stored impedance table online; it also introduces a forced dead zone + pulse blocking + ramp soft start mechanism, blocking the inverter pulse during the commutation dead zone, and restoring the voltage according to the slope after the dead zone ends; and it introduces a digital phase-locked loop (PLL) re-locking mechanism, automatically and quickly capturing the actual resonant frequency of the new branch after each pulse blocking is released. Through these methods, the impedance mismatch problem caused by workpiece temperature drift is solved, achieving accurate voltage matching across the entire temperature range; the risks of shoot-through short circuits and no-load pump-up overvoltage during the switching dead zone are eliminated, achieving a shock-free and safe commutation transition; and the inverter's frequency adaptive tracking during inductor abrupt changes is guaranteed, greatly improving the dynamic stability and engineering disturbance immunity of the time-sharing multiplexing system.

[0068] refer to Figure 4 As shown, this embodiment details the underlying control mechanism of the control module during branch switching: I. Online Impedance Identification and Dynamic Update: During the heating period of the current heating branch (e.g., branch 1), the control module uses voltage and current sensors to sample the intermediate frequency voltage signal at the output of the parallel inverter unit 130 and the intermediate frequency current signal of the currently conducting branch in real time at high speed. The control module uses the sampled data to calculate the current actual load impedance (including equivalent resistance and equivalent inductance) corresponding to branch 1 online. The specific online calculation logic is as follows: The control module extracts the effective value U of the intermediate frequency voltage signal and the effective value I of the intermediate frequency current signal sampled within one power frequency cycle, and calculates the impedance modulus |Z2| of the current actual load using the formula |Z2| = U / I. Simultaneously, the control module captures the time difference Δt between the zero-crossing points of the intermediate frequency voltage and current signals, and calculates the phase difference between the voltage and current, i.e., the impedance angle θ = ω × Δt, by combining this with the current operating angular frequency ω (ω = 2πf, where f is the actual inverter frequency). Then, it calculates the equivalent resistance using the formula R = |Z2| × cosθ, and the equivalent inductance using the formula L = (|Z2| × sinθ) / ω. Due to the impedance change caused by the rise in workpiece temperature, the control module dynamically overwrites the pre-stored load impedance parameters of branch 1 in the internal memory with the latest calculated actual load impedance data. When a time-sharing control cycle ends, and the next time branch 1 is polled, the control module will directly call the updated impedance parameters to calculate the compensation voltage required by the chopper voltage regulation unit 120. This ensures that the chopper output voltage always closely follows the dynamic characteristics of the workpiece as its temperature changes.

[0069] II. Forced Dead Zone and Pulse Blockade Control: Reference Figure 4 In the timing process, when the conduction time t1 of branch 1 ends and the intermediate frequency AC current reaches the zero-crossing point, the control module removes the trigger signal for the thyristor switch of branch 1. To wait for the charge carriers inside the thyristor of branch 1 to fully recombine and restore forward blocking capability, the control module forcibly sets a forced commutation dead time of tens to hundreds of microseconds. The specific logic for setting this dead time is as follows: the dead time T... dead From formula T dead =(t q +t delay )×K is determined. Where, t q The circuit turn-off time is the nominal value specified in the datasheet of the selected thyristor selector switch (typically 10 microseconds to 50 microseconds for intermediate frequency thyristors); t delayThe dead time is a fixed delay time for hardware sampling and signal transmission in the control module; K is a set safety margin coefficient, typically ranging from 1.5 to 3.0. The dead time calculated based on this logic usually falls within the range of tens to hundreds of microseconds, ensuring that the thyristor can be reliably turned off even under extreme operating conditions, preventing direct short circuits between the upstream and downstream branches. During this dead time, no branch is conducting, and the resonant circuit is open. To prevent the parallel inverter unit 130 from continuing to output energy and causing a rapid voltage surge across the resonant capacitor 140, the control module implements pulse blocking on the parallel inverter unit 130 during this dead time (i.e., stops sending drive pulses to the inverter switching transistor), and simultaneously reduces the output voltage command of the chopper voltage regulation unit 120 to zero. The system is in a safe energy sleep state.

[0070] III. Soft Start Recovery and PLL Fast Phase Locking: After the forced commutation dead time ends and branch 1 is completely turned off, the control module sends a trigger signal to the thyristor of branch 2 to turn it on and connect it to the system. Subsequently, the control module releases the pulse blockade of the parallel inverter unit 130. In the initial stage of unblocking, to avoid inrush current, the control module controls the output voltage of the chopper voltage regulation unit 120 to rise smoothly from zero according to a preset safety slope (i.e., soft start) until the target compensation voltage V2 calculated based on the updated impedance parameters of branch 2 is reached. At the same time, due to the new equivalent inductance parameters brought about by the connection of branch 2, the natural frequency of the resonant circuit changes. During the soft start stage of pulse unblocking, the digital phase-locked loop (PLL) built into the control module quickly adjusts the voltage-controlled oscillation frequency by comparing the phase difference between the voltage and current of the newly connected branch, rapidly tracks and locks the current new actual resonant frequency, and drives the parallel inverter unit 130 with this new frequency, ensuring that the inverter always operates in the optimal micro-inductive resonant state and avoiding the risk of unlocked commutation failure. Example 4

[0071] Based on the medium-frequency chopper parallel inverter multi-induction coil heating system provided in the aforementioned embodiments, this embodiment provides a medium-frequency chopper parallel inverter multi-induction coil heating method. By sequentially executing a cyclic control process of synchronous AC power generation, time-division switching at zero crossing time and synchronous compensation of chopper voltage regulation, temperature information acquisition and temperature deviation calculation, and conduction time reallocation based on temperature deviation, a complete closed-loop control process of zero-crossing safe switching, real-time compensation for load changes, and adaptive conduction time allocation based on temperature deviation is realized. This enables the system to continuously optimize the heating power distribution of each area during operation, achieving the effect of eliminating inter-area coupling interference and precise temperature control of each area.

[0072] refer to Figure 5 The method includes the following steps: S1. The AC power from the grid is sequentially rectified, chopped, and inverted by the common power module 100 to generate medium-frequency AC power. Specifically, the three-phase AC power from the grid is first processed by a 24-pulse rectifier transformer 111 to produce four sets of AC power with specific phase shift relationships; then it is rectified by a diode rectifier bridge 112 to convert it into DC power with low ripple; the DC power is regulated by a chopper voltage regulation unit 120 to obtain the required voltage value by controlling the chopper duty cycle; finally, it is inverted into medium-frequency AC power by a parallel inverter unit 130, and resonant filtered by a parallel resonant circuit composed of a resonant capacitor 140 and the load inductive reactance to output medium-frequency AC power with a near-sinusoidal waveform.

[0073] S2. Detect the zero-crossing signal of the intermediate frequency AC power. At the zero-crossing moment of the intermediate frequency AC power, send a selection signal to the selection switch 210 of the target heating branch 200 according to the control timing to turn on the target heating branch 200. At the same time, compensate and adjust the output voltage of the chopper voltage regulation unit 120 according to the load impedance parameters after the connection. The specific execution process is as follows: The control module continuously monitors the zero-crossing signal of the intermediate frequency AC power. Within a time-sharing control cycle T, each heating branch 200 is sequentially selected according to a predetermined timing and allocated conduction time. When it is necessary to switch heating from the current branch to the next branch, the control module waits for the intermediate frequency AC power to reach the next zero-crossing moment. At this zero-crossing moment, the control module removes the trigger signal of the thyristor switch in the current heating branch 200, allowing it to naturally turn off when the current crosses zero using its natural commutation characteristics; simultaneously, it sends a trigger signal to the thyristor switch in the next heating branch 200 to turn it on. While completing the branch switching, the control module adjusts the chopper duty cycle of the chopper voltage regulation unit 120 according to the pre-stored load impedance parameters corresponding to the newly connected heating branch 200, so that the output voltage of the parallel inverter unit 130 matches the current load impedance, maintaining the parallel resonant circuit operating stably near the resonance state.

[0074] The induction heating unit 220 in the selected heating branch 200 receives medium-frequency alternating current. The medium-frequency step-down transformer 221 steps down the medium-frequency alternating current and performs impedance transformation before outputting it to the induction coil 222. The induction coil 222 generates an alternating magnetic field to perform induction heating on the corresponding heating area of ​​the workpiece being heated.

[0075] The system continues heating until the conduction time of the current branch ends. When the conduction time ends, the zero-crossing switching process described above is repeated to switch to the next heating branch 200. After all branches requiring heating have been polled within a time-sharing control cycle T, the process proceeds to step S3.

[0076] S3. Obtain the temperature feedback signal of each heating zone and calculate the temperature deviation between the current temperature and the target temperature of each heating zone. Specifically, after completing a time-sharing control cycle T of polling heating (or continuing during the heating process), the control module obtains the temperature feedback signal through the temperature sensors installed in each heating zone. The temperature sensors can be thermocouples, infrared thermometers, or other suitable temperature measuring devices, installed at representative locations in each heating zone to measure the current temperature of each zone in real time. The control module receives the temperature signals fed back by each temperature sensor, converts them from analog to digital to obtain the current temperature T_current_i of each heating zone i, compares it with the preset target temperature T_target_i, and calculates the temperature deviation ΔT_i = T_target_i - T_current_i.

[0077] S4. Based on the temperature deviation of each heating zone, the conduction time ratio of each heating branch 200 is reassigned proportionally in the next time-sharing control cycle, and the process returns to step S2. Specifically, the control module processes the system according to the temperature deviation adaptive conduction time allocation method described in Example 2: for regions with temperature deviation ΔT_i>0, deviation normalization is performed to obtain the deviation weight w_i, and the conduction time t_i of each branch in the next time-sharing control cycle is allocated according to the deviation weight ratio; for regions with temperature deviation ΔT_i≤0 (temperature has reached the standard), the system is not selected in the next time-sharing control cycle.

[0078] After the conduction time is redistributed, the control module returns to step S2 according to the new timing and conduction time to start the polling heating of the next time-sharing control cycle.

[0079] The aforementioned S2-S3-S4-S2 cyclic control process continues until the temperature of all heating zones reaches their respective target temperatures, or until an external stop command is received. Through this closed-loop control method, the system continuously optimizes the heating power distribution strategy based on the real-time temperature status of each zone during operation, adaptively adjusting the heating intensity of each zone, ultimately achieving uniform and precise control of the entire workpiece temperature field.

[0080] Optionally, the method steps of this embodiment can also incorporate the relevant functions of the control module in Embodiment 3. For example, in step S2, the determination of the load impedance parameters can be based on the online impedance acquisition and parameter dynamic coverage update mechanism in Embodiment 3; when switching branches, the forced dead zone + pulse blockade + ramp soft start mechanism and the digital phase-locked loop re-phase-locking mechanism in Embodiment 3 can be used. These mechanisms have been specifically described in Embodiment 3 and will not be repeated here.

[0081] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A medium-frequency chopper parallel inverter multi-induction coil heating system, characterized in that, It includes a control module, a common power supply module, and several heating branches connected in parallel to the common power supply module; The common power supply module includes a transformer-rectifier unit, a chopper-regulated voltage unit, and a parallel inverter unit connected in sequence. The transformer-rectifier unit is used to convert AC power from the power grid into DC power. The chopper-regulated voltage unit is used to regulate the voltage of the DC power. The parallel inverter unit is used to invert the regulated DC power into medium-frequency AC power. A resonant capacitor is connected in parallel to the output terminal of the parallel inverter unit. The resonant capacitor and the inductive reactance of the selected heating branch form a parallel resonant circuit. Each of the heating branches includes a selector switch and an induction heating unit. The input terminal of the selector switch is connected to the output terminal of the parallel inverter unit, and the output terminal of the selector switch is connected to the induction heating unit. The control module is communicatively connected to each of the selector switches and the chopper voltage regulation unit. The control module is used to detect the zero-crossing signal of the intermediate frequency AC power and control each of the selector switches to perform time-division switching at the zero-crossing moment of the intermediate frequency AC power, so that only the selected heating branch is connected to the parallel resonant circuit at the same time. When the heating branch is switched, the control module compensates and adjusts the output voltage of the chopper voltage regulation unit according to the number of heating branches currently connected, so as to maintain the stable working state of the parallel resonant circuit.

2. The medium-frequency chopper parallel inverter multi-induction coil heating system according to claim 1, characterized in that, The control module is also used to receive temperature feedback signals from the heating areas corresponding to each heating branch, calculate the temperature deviation between the current temperature and the target temperature of each heating area, and allocate the conduction time ratio of each heating branch according to the temperature deviation of each heating area within a time-sharing control cycle.

3. The medium-frequency chopper parallel inverter multi-induction coil heating system according to claim 2, characterized in that, The control module allocates the conduction time percentage of each heating branch in the following ways: The temperature deviation of each heating zone is normalized to obtain the deviation weight of each heating zone. The total allocatable conduction time of a time-sharing control cycle is proportionally allocated according to the deviation weight of each heating zone to obtain the conduction time of each heating branch in the time-sharing control cycle. Specifically, when the current temperature of a certain heating zone reaches or exceeds the target temperature, the heating branch corresponding to that heating zone will not be selected in the current time-sharing control cycle.

4. The medium-frequency chopper parallel inverter multi-induction coil heating system according to claim 1, characterized in that, The control module compensates for and adjusts the output voltage of the chopper voltage regulation unit when the heating branch switches in the following ways: The control module pre-stores the load impedance parameters corresponding to each heating branch and its combination. When the heating branch is switched, the control module adjusts the chopper duty cycle of the chopper voltage regulation unit according to the load impedance parameters corresponding to the currently connected heating branch after the switch, so that the voltage at the output of the parallel inverter unit matches the current load impedance and maintains the parallel resonant circuit working near the resonance state. The specific calculation steps for adjusting the chopper duty cycle of the chopper voltage regulating unit include: the control module obtaining the effective value I of the target heating current currently set by the system. ref And the pre-stored load impedance magnitude |Z1| of the heating branch after switching, calculate the target compensation DC voltage U. dc =I ref ×∣Z1∣;The control module collects the current DC bus voltage U at the input terminal of the chopper voltage regulation unit in real time. in And according to the formula D=U dc / U in The target chopper duty cycle D is calculated and then output to the chopper voltage regulation unit.

5. The medium-frequency chopper parallel inverter multi-induction coil heating system according to claim 4, characterized in that, During the heating period of each heating branch, the control module collects the intermediate frequency voltage signal at the output of the parallel inverter unit and the intermediate frequency current signal of the currently conducting heating branch in real time, and calculates the actual load impedance of the current heating area online; the control module dynamically updates and overwrites the pre-stored load impedance parameters corresponding to the heating branch with the calculated actual load impedance, so as to use for voltage compensation adjustment in the next time-sharing control cycle. The specific calculation logic for calculating the actual load impedance of the current heating area online includes: the control module extracts the effective value U of the sampled intermediate frequency voltage signal and the effective value I of the intermediate frequency current signal, and calculates the impedance modulus |Z2| of the current actual load using the formula |Z2|=U / I; the control module captures the time difference Δt between the zero-crossing points of the intermediate frequency voltage signal and the intermediate frequency current signal, and calculates the impedance angle θ=ω×Δt by combining it with the current operating angular frequency ω; the equivalent resistance is calculated using the formula R=|Z2|×cosθ, and the equivalent inductance is calculated using the formula L=(|Z2|×sinθ) / ω.

6. The medium-frequency chopper parallel inverter multi-induction coil heating system according to claim 1, characterized in that, After removing the trigger signal of the selector switch of the current heating branch, the control module sets a forced commutation dead time. During the forced commutation dead time, the control module implements pulse blocking on the parallel inverter unit, causing the parallel inverter unit to stop inverter output, in order to wait for the selector switch of the current heating branch to fully restore its forward blocking capability. The forced commutation dead time T dead From formula T dead =(t q +t delay )×K is determined; where t q t is the nominal circuit turn-off time of the selected selector switch. delay K is the fixed delay time for hardware sampling and signal transmission of the control module, and K is the set safety margin coefficient.

7. The medium-frequency chopper parallel inverter multi-induction coil heating system according to claim 6, characterized in that, After the forced commutation dead time ends, the control module sends a trigger signal to the selector switch of the next heating branch to turn it on, while simultaneously releasing the pulse blockade of the parallel inverter unit and controlling the output voltage of the chopper voltage regulating unit to smoothly rise from zero according to a preset slope until it reaches a compensation voltage that matches the load impedance of the newly connected heating branch.

8. The medium-frequency chopper parallel inverter multi-induction coil heating system according to claim 7, characterized in that, The control module is equipped with a digital phase-locked loop. When the heating branch is switched and the parallel inverter unit is released from pulse blocking, the digital phase-locked loop quickly tracks and locks the new actual resonant frequency of the current parallel resonant circuit due to the change in load inductance based on the phase difference between the intermediate frequency voltage signal and the intermediate frequency current signal of the newly connected heating branch, and drives the switching device of the parallel inverter unit with the locked new actual resonant frequency.

9. A method for heating multiple induction coils using a medium-frequency chopper parallel inverter, characterized in that, The method, applied to the medium-frequency chopper parallel inverter multi-induction coil heating system according to any one of claims 1 to 8, comprises: S1. The AC power from the grid is sequentially processed by transformer rectification, chopper voltage regulation and parallel inverter through the common power module to generate medium frequency AC power; S2. Detect the zero-crossing signal of the intermediate frequency AC power. At the zero-crossing moment of the intermediate frequency AC power, send a selection signal to the selection switch of the target heating branch according to the control timing to turn on the target heating branch. At the same time, compensate and adjust the output voltage of the chopper voltage regulation unit according to the load impedance parameters after the connection. S3. Obtain the temperature feedback signal of each heating zone and calculate the temperature deviation between the current temperature and the target temperature of each heating zone; S4. Based on the temperature deviation of each heating zone, the conduction time ratio of each heating branch is redistributed proportionally in the next time-sharing control cycle, and the process returns to step S2.

10. The method for heating multiple induction coils via medium-frequency chopper parallel inverter according to claim 9, characterized in that, In step S2, before switching to the next heating branch after the conduction time of the current heating branch ends, the control module waits for the intermediate frequency AC power to reach the next zero-crossing moment, removes the trigger signal of the selection switch of the current heating branch at the next zero-crossing moment, and simultaneously sends a trigger signal to the selection switch of the next heating branch to complete the shockless switching of the heating branch.

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