Induction heating semiconductor frequency converter

Abstract

This invention discloses an induction heating semiconductor frequency converter, which relates to the field of semiconductor frequency converter technology. It includes a body, display window, cabinet door, operating handle, control panel, control cabinet, and analysis and calculation components. The analysis and calculation components include a frequency converter data acquisition module, a calculation and management module, and a control execution module. The calculation and management module includes a real-time quality unit, a frequency increment unit, and a joint control unit. The real-time quality unit of this invention enables the system to evaluate the load matching status in real time. The frequency increment unit, based on a reference resonant frequency, achieves steady-state fine-tuning and rapid response to sudden changes. The joint control unit outputs a safe power command by progressively correcting the target power through three derating factors. These three factors are based on the quality factor, phase difference, and switching stress coupling effect, respectively, ensuring smooth and gradual power derating. Together with the frequency offset, they achieve resonant tracking and safe power control.

Description

Technical Field

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

[0002] Induction heating technology is based on Faraday's law of electromagnetic induction and the Joule heating effect. It generates an alternating magnetic field in an induction coil by alternating current. Eddy currents are generated inside the conductive workpiece placed in the magnetic field, thereby generating Joule heat inside the workpiece to achieve heating. This technology has significant advantages such as fast heating speed, high efficiency, easy local heating, no open flame, and environmental friendliness. It has been widely used in industrial fields such as metal heat treatment, forging, welding, smelting, quenching, and brazing.

[0003] The core of an induction heating system is a frequency converter. Early induction heating power supplies used thyristors as switching devices, with low operating frequencies, usually not exceeding 10kHz. They were mainly suitable for through-heating and melting of large workpieces. With the development of power electronics technology, semiconductor switching devices, represented by insulated gate bipolar transistors (IGBTs) and metal oxide semiconductor field-effect transistors (MOSFETs), have gradually replaced thyristors, which has greatly increased the operating frequency of induction heating power supplies. The operating frequency of IGBT inverters is usually in the range of 20kHz to 100kHz. The increase in frequency has enabled induction heating devices to make great progress in miniaturization, weight reduction, and heating efficiency.

[0004] Induction heating frequency conversion systems typically employ series resonant or parallel resonant topologies. Series resonant inverters have become the mainstream solution for small and medium power induction heating power supplies due to their simple structure, ease of achieving zero-voltage switching, and convenient output power adjustment. In a series resonant system, the AC voltage output by the inverter is applied to a series resonant circuit consisting of a resonant inductor, a resonant capacitor, and an equivalent load resistance. When the inverter's operating frequency equals the circuit's inherent resonant frequency, the circuit impedance is at its minimum, the current is at its maximum, and the active power reaches its peak. When the operating frequency deviates from the resonant frequency, the circuit impedance increases, the current decreases, the active power decreases, and the reactive power increases.

[0005] In practical industrial applications, the load conditions of induction heating systems are not constant. The material, shape, size, initial temperature, and temperature changes during the heating process of the workpiece will significantly affect the equivalent parameters of the resonant circuit.

[0006] Specifically, the resistivity of the workpiece increases with increasing temperature, leading to an increase in the equivalent series resistance. When the workpiece temperature reaches the Curie point, the ferromagnetic workpiece loses its ferromagnetism, causing the equivalent inductance to drop sharply. In addition, the geometry of the workpiece and its relative position to the induction coil also affect the coupling coefficient, thereby changing the equivalent inductance and equivalent resistance. These factors together cause the inherent resonant frequency of the resonant circuit to drift continuously during operation.

[0007] To address the resonant frequency drift problem, existing induction heating frequency conversion systems generally employ frequency tracking control technology, with the most common approach being phase-locked loop (PLL) based closed-loop control.

[0008] By detecting the phase difference between the inverter's output voltage and output current, a proportional-integral (PI) controller is used to generate a frequency correction, ensuring that the operating frequency always tracks the resonant frequency and keeps the system operating in the weakly inductive region. However, as industrial applications increasingly demand higher precision in heating processes and greater reliability from equipment, the limitations of existing frequency tracking control schemes are becoming increasingly apparent. When the load changes drastically, such as when a workpiece rapidly enters or leaves the induction coil, when switching between workpieces of different materials, or when the workpiece temperature rapidly crosses the Curie point, the drift speed and amplitude of the resonant frequency will increase significantly. A fixed-parameter PI controller cannot simultaneously meet the requirements of steady-state stability and dynamic response speed. When the gain setting is too high, frequency jitter is easily caused by sampling noise in steady state. When the gain setting is too low, the frequency tracking speed cannot keep up with the drift speed of the resonant frequency during dynamic processes, causing the system to be in a detuned state for a long time.

[0009] Furthermore, the power control of existing induction heating frequency conversion systems is usually independent of frequency control. After the operator sets the target power, the system achieves power output by adjusting the DC bus voltage or the inverter output duty cycle. However, the power adjustment process does not take into account the current load matching status and device safety margin. In the case of poor load matching or severe system detuning, it still operates according to the target power output command, which will lead to excessive reactive power in the inverter output current, increase the conduction loss and switching loss of IGBTs, and accelerate device aging or even trigger overcurrent protection shutdown in the long run. Summary of the Invention

[0010] The purpose of this invention is to provide an induction heating semiconductor frequency converter, which solves the problems mentioned in the background art.

[0011] To achieve the above objectives, the present invention provides an induction heating semiconductor frequency converter, comprising:

[0012] The upper part of the machine body is equipped with a display instrument for displaying data during the operation of the induction heating semiconductor frequency converter in real time.

[0013] A display window is located on the cabinet door of the machine body;

[0014] An operating handle is provided on the cabinet door;

[0015] Control panel, which is mounted on the cabinet;

[0016] A control cabinet is installed on the machine body. The control cabinet is used to control and manage the operation of the induction heating semiconductor frequency converter. An analysis and calculation component is installed in the control cabinet. Through the operation of the induction heating semiconductor frequency converter, the analysis and calculation component acquires, processes and analyzes the operation data of the device to output real-time quality factor, adaptive frequency increment and joint control index.

[0017] The analysis and calculation component analyzes the quality factor of the induction heating semiconductor frequency converter based on design parameters and real-time operating status, and performs dynamic sensing and frequency tracking based on adaptive frequency increment, combined with the joint control index to perform multi-dimensional safety derating of power commands.

[0018] Optionally, the analysis and calculation component includes:

[0019] The variable frequency drive data acquisition module collects hardware and operational data from the induction heating semiconductor variable frequency drive to obtain quality-related data, frequency-related data, and control-related data. It also preprocesses the acquired data to save it to the database.

[0020] The calculation management module retrieves the quality-related data, frequency-related data, and control-related data from the database. The calculation management module is based on:

[0021] The current inverter angular frequency, series equivalent inductance, series equivalent resistance, weighting factor 1, current reactive power, and current active power in the quality-related data are used to output the real-time quality factor.

[0022] The reference resonant frequency in the frequency correlation data, the proportional gain adaptively modulated by the phase difference change rate, and the phase difference between voltage and current, combined with the processing of real-time quality factor and saturation limiting function, are used to output an adaptive frequency increment, wherein the phase difference change rate is used to automatically improve the frequency regulation response speed when the load changes dynamically.

[0023] The control index is output by adjusting the target heating power, rated quality factor, maximum allowable phase difference, minimum power ratio, current switching frequency and current DC bus voltage in the related data, and combining the real-time quality factor and the phase difference between voltage and current. The index is then adjusted step by step by the first derating factor based on the load matching degree, the second derating factor based on the degree of detuning and the third derating factor based on the coupling effect of switching loss.

[0024] The real-time quality factor is used as a collaborative state variable and is simultaneously passed to the adaptive frequency increment calculation and the joint control index calculation. The adaptive frequency increment is based on the collaborative state variable for dynamic sensing and frequency tracking, and the joint control index is based on the collaborative state variable for multi-dimensional safety derating of power commands. The real-time quality factor, adaptive frequency increment, and joint control index constitute a three-level collaborative progressive control link, enabling frequency tracking and power control to generate synergistic effects under the drive of shared state variables.

[0025] Control execution module: Receives real-time quality factor, adaptive frequency increment and joint control index output from the calculation and management module to adjust the PWM frequency and PWM duty cycle.

[0026] Optionally, the analysis and calculation component includes a real-time quality unit, a frequency increment unit, and a joint control unit, and the control execution module includes a frequency control unit and a duty cycle control unit.

[0027] Optionally, the processing flow of the real-time quality unit is as follows:

[0028] A1. By acquiring the carrier frequency of the PWM module in real time and converting it into angular frequency, the current inverter angular frequency is obtained and used as the reference frequency for the operating point of the resonant circuit.

[0029] A2. Obtain the room temperature calibration value of the series equivalent inductance through offline calibration, and obtain the correction coefficient by looking up the table through the preset temperature-inductance curve in combination with the real-time temperature of the workpiece. Calculate the series equivalent inductance to reflect the energy storage capacity and load coupling state of the resonant circuit. By sampling the effective voltage and current values ​​at the inverter output in real time, and combining the phase difference between voltage and current, calculate the series equivalent resistance according to the impedance relationship to characterize the workpiece's ability to absorb active power and the strength of system damping.

[0030] A3. Based on the effective values ​​of voltage and current at the inverter output and the phase difference between voltage and current, the current reactive power and current active power are calculated respectively to obtain the proportional relationship between the oscillation intensity of energy in the resonant circuit and the effective transfer to the load. The weighting coefficient is determined through experiments during the system commissioning phase to adjust the sensitivity of the real-time quality factor to the detuning state, thereby finally outputting the real-time quality factor. The real-time quality factor is used as a load state sensing signal and transmitted to the frequency increment unit and the joint control unit for determining the adjustment range of frequency tracking and making safe derating decisions for power commands, respectively.

[0031] Optionally, the processing flow of the frequency increment unit is as follows:

[0032] B1. Determine the reference resonant frequency through the parameters of the LC resonant circuit to serve as the reference point for the inverter switching frequency;

[0033] B2. Determine the reference proportional gain through step response experiments during the system debugging phase, so as to set the frequency adjustment sensitivity of the system under steady-state conditions;

[0034] B3. During the system debugging phase, the adaptive gain coefficient is determined through response experiments under typical dynamic conditions to control the modulation intensity of the phase difference change rate on the proportional gain. The phase difference change rate is obtained by sampling the phase difference between voltage and current in real time and calculating its first derivative with time. This serves as a precursor signal for dynamic load changes. The reference proportional gain is combined with the adaptive gain coefficient and the phase difference change rate to calculate and output the adaptive proportional gain, so that the frequency regulation response speed is automatically adjusted according to the severity of load changes.

[0035] B4. Introduce the real-time quality factor and automatically compress the frequency adjustment range based on the load condition. Then, multiply the adaptive proportional gain by the phase difference and process it through the saturation limiting function to obtain the frequency offset. Finally, add the frequency offset to the reference resonant frequency to output the adaptive frequency increment.

[0036] B5. Use the adaptive frequency increment as the frequency command of the PWM module to update the inverter switching frequency, so that the system always tracks the drift of the resonant circuit and operates in a state close to resonance.

[0037] Optionally, the processing flow of the joint control unit is as follows:

[0038] C1. Obtain the target heating power set by the operator or the host computer as the reference value for the power command;

[0039] C2. During the system's factory commissioning phase, the rated quality factor is measured and recorded under standard load conditions to serve as a normalized benchmark for load matching. The maximum allowable phase difference is set during the system design phase to limit the system's operating boundary in the weakly inductive region. The minimum power ratio is also set during the system design phase to prevent the derating factor from reaching zero and causing a complete system shutdown. The current switching frequency is directly read through the PWM module, and the current DC bus voltage is obtained by sampling across the DC bus capacitor. Based on the rated switching frequency of the IGBT module and the maximum allowable DC bus voltage in the system design, the normalized product of the switching frequency and the bus voltage is calculated to characterize the switching stress level of the IGBT.

[0040] C3. The ratio of the real-time quality factor to the rated quality factor is subjected to an upper limit limiting process to obtain the first derating factor based on the load matching degree.

[0041] The result of subtracting the ratio of the absolute value of the phase difference to the maximum allowable phase difference from 1 is subjected to a lower limit protection process to obtain a second de-rating factor based on the degree of detuning;

[0042] The result of subtracting the normalized product of the switching frequency and the bus voltage from 1 is processed by the lower limit protection to obtain the third derating factor based on the coupling effect of switching loss.

[0043] C4. Multiply the target heating power by the minimum value among the first derating factor, the second derating factor, and the third derating factor to output a joint control index. This joint control index serves as a safety power command that comprehensively considers the load matching state, the degree of detuning, and the safety margin of the devices. The joint control index is used as the input of the power regulation module to control the output power of the inverter, ensuring that the output power of the system is within the safe range under any operating condition.

[0044] Optionally, the frequency control unit specifically comprises:

[0045] The updated frequency value is calculated based on the reference resonant frequency and the adaptive frequency increment. The updated frequency value is then converted into the period register value of the PWM timer and written into the corresponding register. The PWM module automatically switches to the new frequency in the next carrier cycle.

[0046] Optionally, the duty cycle control unit specifically comprises:

[0047] Based on the relationship between output power and duty cycle, the joint control index is converted into PWM duty cycle, and the conversion relationship depends on the inverter topology.

[0048] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0049] I. The real-time quality unit of this invention realizes online calculation of the quality factor by dividing the product of angular frequency and equivalent inductance by the equivalent resistance. The angular frequency is directly read by the pulse width modulation module, the equivalent inductance is obtained through offline calibration and temperature lookup table compensation, and the equivalent resistance is indirectly calculated by cosine operation using real-time sampled inverter output voltage, output current, and phase difference. This allows the system to reassess its own quality status based on the current circuit parameters in each control cycle. On this basis, this unit introduces a correction factor, which corrects the basic quality factor by adding the ratio of reactive power to active power after adjustment by a weighting coefficient to a constant and then taking the reciprocal. The reactive power is calculated from the voltage, current, and the sine value of the phase difference, and the active power is calculated from the voltage, current, and the cosine value of the phase difference. When the system deviates from the resonance point, the proportion of reactive power increases, the denominator of the correction factor increases, and the effective quality factor decreases, accurately reflecting the impact of the operating state on the effective quality of the system. The weighting coefficient is determined during the debugging phase and is used to control the correction intensity. The effective quality factor output by this unit is a comprehensive index that integrates circuit parameters and operating state, providing a basis for judging the load matching state for subsequent control.

[0050] II. The frequency increment unit of this invention calculates the frequency offset based on the reference resonant frequency by multiplying the quality factor-related term and the phase closed-loop term. The quality factor-related term ensures that the complete frequency adjustment range is maintained when the quality factor is high, and actively compresses the range to prevent over-adjustment when the quality factor is low. The phase closed-loop term outputs the product of the proportional gain and the current phase difference after saturation limiting. The saturation limiting prevents the frequency command from exceeding the inverter's allowable range. The proportional gain of this unit is not a fixed value, but is composed of the reference proportional gain multiplied by a constant, the adaptive gain coefficient, and the phase difference change rate. The phase difference change rate is calculated by dividing the phase difference difference between adjacent control cycles by the sampling period. In steady state, the phase difference change rate is close to zero, the gain maintains the reference value, and the system makes smooth fine adjustments. When the load changes abruptly, the phase difference change rate increases, the gain automatically increases, and the system response speed is accelerated. The tracking capability is improved before the phase deviation increases significantly. The frequency offset output by this unit is directly used as a correction command for the inverter's operating frequency, making the system approach the resonant point in each control cycle.

[0051] III. The joint control unit of this invention, based on the user-set target heating power, outputs a safe power command after being progressively corrected by three derating factors. The first factor is the smaller of the ratio of the effective quality factor to the rated quality factor, which actively reduces power when the load matching condition deteriorates. The second factor is the larger of the ratio of a constant minus the ratio of the absolute value of the phase difference to the maximum allowable phase difference, and the ratio of this result to the minimum power, which significantly reduces power when the system is severely detuned to prevent excessive reactive current circulation. The third inventive factor is the product of the ratio of the current switching frequency to the rated switching frequency and the ratio of the current DC bus voltage to the maximum allowable bus voltage, and then... This result is combined with the larger value of the minimum power ratio. The switching frequency is read by the pulse width modulation module, and the bus voltage is obtained by sampling across the DC bus capacitor through a voltage Hall sensor. When the normalized product of frequency and voltage increases, it indicates that the switching stress increases and the power command decreases. This factor captures the coupling effect of switching losses through a product form, solving the problem that independent thresholds cannot identify high-risk operating conditions under moderate parameter combinations. All three factors are continuous variables, making the power derating process smooth and gradual. The safe power command output by this unit works in conjunction with the frequency offset. The frequency offset is responsible for tracking the resonant point, and the safe power command is responsible for determining the safe power level at that operating point.

[0052] Fourth, the three-level collaborative progressive control architecture of the present invention uses the real-time quality factor (QL) as a shared collaborative state variable to achieve joint optimization of frequency tracking and power regulation. The three levels form a progressive coupling relationship of perception, decision-making and execution: the real-time quality factor provides a unified load state benchmark for the latter two levels, the frequency increment unit dynamically adjusts the tracking strategy accordingly, and the joint regulation unit dynamically adjusts the power target accordingly. When the load conditions deteriorate, the three levels automatically coordinate to reduce the derating, and when the load conditions recover, the three levels automatically coordinate to restore the power. The robustness and adaptability of the overall system are significantly better than the simple combination of modules working independently. Attached Figure Description

[0053] Figure 1 This is a front view of the structure of the present invention;

[0054] Figure 2 This is a schematic diagram of the structure of the data analysis component of the present invention;

[0055] Figure 3 This is a structural block diagram of the computing management module of the present invention;

[0056] Figure 4 This is a schematic diagram illustrating the operation of the computing management module of the present invention.

[0057] In the diagram: 1. Main body; 2. Display window; 3. Cabinet door; 4. Operating handle; 5. Control panel; 6. Control cabinet. Detailed Implementation

[0058] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0059] Please see Figures 1 to 4 This embodiment provides an induction heating semiconductor frequency converter, including:

[0060] The upper part of the main body 1 is equipped with a display instrument for real-time display of data during the operation of the induction heating semiconductor frequency converter;

[0061] Display window 2 is located on cabinet door 3 of body 1;

[0062] Operating handle 4 is installed on cabinet door 3;

[0063] Control panel 5 is located on cabinet 1.

[0064] Control cabinet 6 is installed on the body 1. Control cabinet 6 is used to control and manage the operation of the induction heating semiconductor frequency converter. The control cabinet 6 is equipped with an analysis and calculation component. Through the operation of the induction heating semiconductor frequency converter, the analysis and calculation component acquires, processes and analyzes the operation data of the device to output real-time quality factor, adaptive frequency increment and joint control index.

[0065] The analysis and calculation component analyzes the quality factor of the induction heating semiconductor frequency converter based on design parameters and real-time operating status. It also performs dynamic sensing and frequency tracking based on adaptive frequency increments and combines the joint control index to perform multi-dimensional safety derating of power commands.

[0066] Furthermore, the analytical computing components include:

[0067] Variable frequency drive data acquisition module:

[0068] The hardware and operational data of the induction heating semiconductor frequency converter are collected to obtain quality-related data, frequency-related data, and control-related data. The acquired data is then preprocessed and stored in the database.

[0069] Calculation Management Module:

[0070] The quality-related data, frequency-related data, and control-related data are retrieved from the database, and the calculation management module is based on:

[0071] Firstly, the current inverter angular frequency, series equivalent inductance, series equivalent resistance, weighting coefficient 1, current reactive power, and current active power in the quality-related data are used to output the real-time quality factor.

[0072] Secondly, the reference resonant frequency, adaptive proportional gain, and phase difference between voltage and current in the frequency correlation data are combined with the processing of real-time quality factor and saturation limiting function to output adaptive frequency increment;

[0073] Thirdly: The target heating power, rated quality factor, maximum allowable phase difference, minimum power ratio, current switching frequency, and current DC bus voltage in the control-related data are combined with the real-time quality factor and the phase difference between voltage and current to output a joint control index;

[0074] Control execution module: Receives real-time quality factor, adaptive frequency increment, and joint control index output from the calculation and management module to adjust the PWM frequency and PWM duty cycle;

[0075] The analysis and calculation components include a real-time quality unit, a frequency increment unit, and a joint control unit.

[0076] The frequency control unit is specifically as follows:

[0077] The updated frequency value is calculated based on the reference resonant frequency and the adaptive frequency increment. The updated frequency value is then converted into the period register value of the PWM timer and written into the corresponding register. The PWM module automatically switches to the new frequency in the next carrier cycle.

[0078] The control execution module includes a frequency control unit and a duty cycle control unit;

[0079] The duty cycle control unit is specifically as follows:

[0080] Based on the relationship between output power and duty cycle, the joint control index is converted into PWM duty cycle. The conversion relationship depends on the inverter topology.

[0081] Converting the combined control index PC to PWM duty cycle, the conversion relationship depends on the inverter topology:

[0082] For a full-bridge inverter, the relationship between output power and duty cycle is P_output ∝ duty cycle. 2 ,therefore:

[0083] ;

[0084] Where Pmax represents the maximum power that can be output under the current DC bus voltage Pvd;

[0085] For a phase-shifted full-bridge inverter, the power is controlled by the phase shift angle, and the conversion relationship is P_output ∝ cos_phase_shift angle;

[0086] More specifically, in this embodiment:

[0087] The above content is explained as follows: The real-time quality unit extracts the comprehensive state quantity real-time quality factor QL from the original sampled data, which includes information on load matching degree and operating status. The frequency increment unit takes the real-time quality factor QL and the real-time phase signal as inputs to calculate the adaptive frequency increment DF, which directly controls the operating frequency of the inverter. The joint control unit takes the real-time quality factor QL, the phase signal and the hardware state quantity as inputs to calculate the safe power command joint control index PC, which controls the output power of the inverter.

[0088] The real-time quality factor QL of this invention is a shared cooperative state variable of the entire control system. Its core feature is that the real-time quality factor QL simultaneously carries out three missions:

[0089] As a reference for compressing the adjustment range of the frequency increment unit, it enables adaptive tracking of the resonance peak;

[0090] As the first derating factor normalization benchmark of the joint control unit, it realizes load matching degree sensing power derating;

[0091] Serving as a bridge for information coupling between the two units, frequency tracking and power regulation work collaboratively under a unified state awareness. This shared state variable design ensures that when the load state changes, the frequency and power control dimensions can respond synchronously and coordinate automatically, avoiding the drawbacks of frequency control and power control operating independently and conflicting with each other in traditional solutions.

[0092] The characteristics of this link are: each level receives calculations from the previous level, but each level has its own independent creative factor. The real-time quality unit introduces the reactive / active ratio, the frequency increment unit introduces adaptive gain, and the joint control unit introduces product derating. The three factors each perform their own functions, do not overlap with each other, and jointly improve the overall performance of the system.

[0093] refer to Figure 1 , Figure 2 , Figure 3 and Figure 4 ;

[0094] The calculation formula for the real-time quality unit is as follows:

[0095] ;

[0096] in:

[0097] QL stands for real-time quality factor;

[0098] Q1 refers to the current inverter angular frequency, Q1 = 2πf, where f refers to the time frequency in Hertz. The combination of the two is to convert the number of vibrations per second into the number of radians per second. This is a well-known technique to those skilled in the art and will not be elaborated here. This parameter is directly read by the PWM (Pulse Width Modulation) module of the DSP (Digital Signal Processor) / MCU (Microcontroller). In the digital control system, the PWM carrier frequency is the inverter's switching frequency. This value is directly determined by the timer register value in the program. Therefore, Q1 is a known control quantity and does not require additional sensors to obtain it.

[0099] Angular frequency is the core output of the frequency conversion system, which determines the operating point of the resonant circuit. In induction heating, only when the inverter output frequency is close to the inherent resonant frequency of the LC resonant circuit can the maximum alternating magnetic field be generated in the load coil, thereby inducing the maximum eddy current in the workpiece. Q1 is the control output of the system and also the reference frequency for all subsequent calculations.

[0100] QLS refers to the series equivalent inductance, which is the series equivalent inductance seen from the inverter output. It includes the inductive reactance of the resonant inductor itself and the mutual inductance effect after coupling with the workpiece. The unit is H. QLS is a slowly changing variable and does not require real-time online measurement. It can be obtained by offline calibration + temperature lookup table compensation.

[0101] Offline calibration: During the system's factory commissioning phase, the equivalent inductance values ​​under no-load and standard load conditions are measured using an LCR bridge at room temperature to establish a basic database;

[0102] Compensation calculation: During operation, the correction coefficient QLSkl is obtained by looking up a table through the preset temperature-inductance curve based on the real-time temperature of the workpiece (obtained by an infrared thermometer or thermocouple). The correction coefficient QLSkl is then calculated using the formula QLS = QLSA × QLSkl, where QLSA is the room temperature calibration value and QLSkl is obtained by fitting experimental data (QLSkl will decrease significantly when the workpiece reaches the Curie point temperature).

[0103] The series equivalent inductance QLS determines the energy storage capacity and resonant frequency reference value of the resonant circuit. In the induction heating system, the electromagnetic coupling between the coil and the workpiece will change the equivalent inductance. When the workpiece approaches the coil, the mutual inductance increases and the equivalent inductance increases. When the workpiece temperature rises to the Curie point, the workpiece loses its ferromagnetism and the equivalent inductance drops sharply. As the numerator, the change of the series equivalent inductance QLS directly affects the system's judgment of the load state.

[0104] QRS refers to the series equivalent resistance, which is the series equivalent resistance seen from the inverter output. It includes coil copper loss, workpiece eddy current loss, hysteresis loss, and AC resistance caused by skin effect and proximity effect. The unit is ohms. It can be calculated indirectly without directly measuring the resistance. It is calculated by sampling the inverter output voltage, current, and phase difference in real time, according to the following formula:

[0105] QRS=(Vin÷Lin)×cosθ;

[0106] In the above formula, Vin refers to the effective value of the inverter output voltage, which is directly sampled at the inverter output terminal by a voltage Hall sensor, converted by an ADC (analog-to-digital converter), and then calculated by a DSP (digital signal processor).

[0107] In the above formula, Lin refers to the effective value of the inverter output current, which is directly sampled at the inverter output terminal by a current Hall sensor and the RMS value is calculated by the DSP.

[0108] In the above formula, θ refers to the phase difference between voltage and current, which is obtained directly by the phase detector circuit (comparing the time difference between the zero crossing points of voltage and current) or the capture unit of the DSP. The principle of the phase detector is: the voltage and current signals are converted into square waves by the zero-crossing comparator and sent to the capture pin of the DSP. The time difference ∆ between the rising edges of the two square waves is calculated, then θ=2πf×∆.

[0109] The series equivalent resistance QRS directly reflects the workpiece's ability to absorb active power. When the workpiece temperature rises, its resistivity increases, eddy current losses increase, and the series equivalent resistance QRS increases accordingly. When the workpiece approaches the Curie point, the rate of change of the series equivalent resistance QRS will change significantly. The series equivalent resistance QRS is the denominator term, and its magnitude directly determines the strength of the system damping. The larger the series equivalent resistance QRS, the lower the real-time quality factor QL value, the flatter the resonance peak, and the lower the difficulty of frequency tracking.

[0110] Q2 refers to the first weighting factor, i.e., the reactive / active ratio weighting factor. This parameter does not need to be collected in real time. It is a calibration parameter determined during the system design phase and is determined experimentally during system debugging. Under known load conditions, the difference in system response with and without the factor is compared, and a Q2 value is selected that makes the real-time quality factor QL decrease moderately when the system is detuned (usually controlled within 10%-30% of the base value). The typical value range is 0.1-0.5. Q2 determines the influence weight of the creativity factor on the real-time quality factor QL. If Q2 is too large, the real-time quality factor QL will be overly sensitive to detuning, and the system will change its behavior significantly under small disturbances. If Q2 is too small, the creativity factor is ineffective. It acts as a knob for adjusting the creativity intensity in the system.

[0111] QPreac refers to the current reactive power, that is, the reactive power at the inverter output, which is the energy stored in the resonant inductor and capacitor, oscillating back and forth between the load and the resonant network without doing work. The unit is W (watt, actually should be var, but the units can be consistent in numerical calculations). It can be calculated based on the sampled voltage, current and phase difference:

[0112] QPreac = Vin × Lin × sinθ;

[0113] Reactive power reflects the intensity of energy oscillation in a resonant circuit. At full resonance, θ = 0, and the reactive power is zero. All energy is used to heat the workpiece. When the frequency deviates from the resonant point, the reactive power increases, and the energy idles between the coil and the capacitor without doing effective work. QPreac, as a component of the creative factor, is used to sense the degree to which the current system deviates from resonance. The greater the deviation, the higher the proportion of reactive power.

[0114] QPact refers to the current active power, that is, the active power at the inverter output, which is the actual power transferred to the load (workpiece) for heating, measured in watts (W). It is also calculated based on the sampled voltage, current, and phase difference.

[0115] QPact = Vin × Lin × cosθ;

[0116] Active power is a direct output indicator of the heating system, representing the energy actually absorbed by the workpiece. The ratio of QPreac to QPact is a core indicator for measuring the energy utilization efficiency of the system. The lower the ratio, the higher the proportion of energy effectively utilized. This ratio is incorporated into the calculation of this unit, so that the real-time quality factor QL is no longer just a fixed value determined by circuit parameters, but incorporates the current real-time operating information.

[0117] The above content needs clarification:

[0118] In existing induction heating semiconductor frequency converters, the quality factor is usually regarded as a fixed value determined by hardware circuit parameters. After the coil is wound, the real-time quality factor QL value is determined. Engineers will calculate it during the design phase, but will no longer pay attention to it during operation. However, in actual working conditions, changes in load (differences in workpiece material, temperature changes, and changes in coupling distance) will significantly change the series equivalent inductance and series equivalent resistance, causing the real-time quality factor QL value to deviate from the design value.

[0119] This unit solves this problem through the integration of two levels:

[0120] First floor, The system achieves real-time calculation of the quality factor (QL) instead of relying on the design value. This requires the system to obtain the series equivalent inductance and series equivalent resistance online. Although the series equivalent inductance is obtained by looking up a table instead of being measured in real time, the series equivalent resistance is calculated by three real-time sampling quantities: voltage, current, and phase difference. Therefore, the basic items already contain the main information about load changes.

[0121] The second floor, Introducing the reactive / active ratio into the real-time quality factor (QL) correction means that even if the series equivalent inductance and series equivalent resistance remain unchanged (hardware parameters remain unchanged), the real-time quality factor (QL) will automatically decrease simply because the frequency deviates from the resonant point (θ≠0). This is physically reasonable, that is, when deviating from the resonant point, the energy oscillates more intensely between the coil and the capacitor, the proportion of energy effectively transferred to the load decreases, and the effective quality of the system does indeed decrease.

[0122] The calculation result of the real-time quality factor (QL) serves as the input for the subsequent two formulas, thus acting as a load state sensor.

[0123] When the real-time quality factor QL is high, the signal transmitted to the frequency increment unit and the joint control unit is: the system is currently operating well, the loss is small, the resonance peak is sharp, and there is sufficient adjustment space. At this time, the frequency increment unit will provide a larger frequency adjustment range, and the joint control unit will allow higher power output.

[0124] When the real-time quality factor QL is low, the signal passed to the subsequent stage is: the system is currently under heavy load or detuned, with high losses and a flat resonant peak. At this time, the frequency increment unit will compress the frequency adjustment amplitude (because the resonant peak is flat under a low real-time quality factor QL, and there is no need for large frequency adjustment). The joint control unit will reduce the power command to protect the device. The role of the real-time quality factor QL is not to directly control any actuator, but to serve as the first-level judgment of the entire three-layer progressive system, providing a basis for subsequent decisions.

[0125] The three units of this invention are related:

[0126] Explanation of the three-level collaborative progressive mechanism: The core innovation of this invention lies not only in the independent functions of the three units, but also in the fact that they form a three-level collaborative progressive control architecture through the shared state variable of the real-time quality factor QL.

[0127] The first-level sensing layer: the real-time quality unit outputs QL, which serves as the load status sensor for the entire system. It integrates circuit parameters such as angular frequency, inductance, resistance, and operating status such as reactive / active ratio to generate a scalar signal that comprehensively reflects the load matching degree.

[0128] The second-level decision-making layer: the frequency increment unit takes QL as input and dynamically compresses or releases the frequency adjustment range. When QL is high, the complete range is retained, and when QL is low, it is actively compressed. At the same time, the phase difference change rate is used to adaptively modulate the proportional gain to achieve frequency tracking that is fast when it needs to be fast and stable when it needs to be stable.

[0129] The third-level execution layer: The joint control unit uses QL combined with phase difference and switching stress, and corrects the power command step by step through three derating factors to ensure that the power output is within the safe range under any operating conditions;

[0130] The three levels form a positive synergy through QL: the accurate sensing of QL ensures the accuracy of frequency tracking, and the accuracy of frequency tracking ensures the rationality of power derating. At the same time, it forms a reverse constraint: when the load deteriorates and causes QL to decrease, the frequency tracking range automatically narrows and the power command is automatically drated to avoid the system from running under adverse conditions. This two-way synergy mechanism enables the control effect of the present invention to go beyond the simple superposition of the functions of the three units and achieves the excellent effect of synergistic efficiency.

[0131] refer to Figure 1 , Figure 2 , Figure 3 and Figure 4 ;

[0132] The calculation formula for the frequency increment unit is as follows:

[0133] ;

[0134] in:

[0135] DF stands for adaptive frequency increment, i.e. frequency offset, which is the offset of the inverter's current switching frequency relative to the reference frequency DF0, in Hz. A positive value indicates frequency increase, and a negative value indicates frequency decrease. After this parameter is calculated, it is directly sent to the PWM module to update the carrier frequency, fnew = DF0 + DF.

[0136] The introduction of the real-time quality factor (QL) reflects the effective quality factor that comprehensively considers circuit parameters and real-time operating status under the current operating conditions. It is dimensionless and is used in this unit. The higher the QL, the closer this term is to 1, and the greater the adjustment range of the frequency converter. The lower the QL, the closer this term is to 0.5, and the frequency converter will actively compress the frequency adjustment range. This is because a low real-time quality factor QL means that the system damping is large and the resonance peak is flat, so it can remain in a reasonable operating range without large frequency adjustments.

[0137] This calculation result is the core output command of the entire frequency conversion system, which directly determines the operating frequency of the inverter. Its function is to track the drift of the resonant circuit in real time, so that the system always works in a state close to resonance. The magnitude and direction of the adaptive frequency increment DF are jointly determined by the phase deviation and the real-time quality factor QL value compensation. The more severe the detuning, the higher the real-time quality factor QL value (the sharper the resonance peak), and the greater the required frequency conversion offset.

[0138] DF0 refers to the reference resonant frequency, which is determined by the parameters of the LC resonant circuit and is measured in Hz. During the offline commissioning phase of the system, it is determined by a frequency sweep method. Specifically, the inverter output power is fixed, and the frequency is scanned over a wide frequency range while monitoring the output current. The frequency at which the current reaches its maximum value is the resonant frequency. This value is fixed in the program as a basic parameter of the system and is finely adjusted during operation based on the temperature compensation of the series equivalent inductance QLS. DF0 is the frequency reference point of the entire frequency conversion system, and all frequency deviations are referenced to it. It is determined by hardware parameters (resonant inductance and capacitance), and the approximate range (usually 20-100kHz) is determined during the system design phase. DF0 itself does not change drastically with load changes, but the actual resonant point will drift due to changes in the series equivalent inductance QLS and series equivalent resistance QRS. This is the reason why the adaptive frequency increment DF needs to be compensated.

[0139] sat refers to the saturation limiting function, which is a function that limits the input value to a specified range. sat(x) = max(min(x,xmax), -xmax). In this embodiment, xmax is set during system design and is usually taken as 5%-10% of DF0. Saturation limiting is a key guarantee for engineering safety. It ensures that the adaptive frequency increment DF will not exceed the allowable switching frequency range of the inverter and prevents frequency command overflow caused by calculation abnormalities or extreme operating conditions. This is a standard safety measure in control system design.

[0140] DFK refers to the adaptive proportional gain, which is the proportional gain coefficient of the phase closed loop, measured in Hz / rad. It is time-varying, automatically adjusting according to the phase change rate. Traditional phase closed loops use a fixed gain, which can only strike a balance between steady-state stability and rapid dynamic operation. The adaptive proportional gain DFK allows the proportional gain to adjust adaptively with the phase change rate. In steady state, DFK is very small, DFK≈DFKA (reference proportional gain), allowing for smooth fine-tuning of the system. When the load changes abruptly, DFK suddenly increases, automatically increasing the frequency conversion speed and enabling the system to quickly catch up to the new resonant point. This adaptive mechanism does not require additional sensors, utilizing only the first derivative of the existing phase signal. The calculation formula is as follows:

[0141] ;

[0142] In the above formula, DFKA is the reference proportional gain, i.e., the reference proportional gain coefficient of the phase closed loop under steady-state conditions, in Hz / rad. It is determined through step response experiments during the system commissioning phase. Specifically, a known frequency disturbance is applied under steady-state conditions, the response curve of θ is observed, and the DFKA value that makes the system stable and has a moderate response is calculated using the pole placement method. In this embodiment, the typical order of magnitude is 100-500 Hz / rad, which is the reference value for adaptive gain and determines the frequency adjustment sensitivity of the system under steady-state conditions. The larger the DFKA, the stronger the system's response to phase deviation, but too large a value can cause oscillations. The smaller the DFKA, the more stable the system, but the slower the response. It is a parameter that needs to be weighed in the design of control systems.

[0143] In the above formula, D1 refers to the adaptive gain coefficient, used to control... The modulation intensity coefficient of the adaptive proportional gain DFK, measured in seconds (s), is determined during the system debugging phase and does not require real-time acquisition. The system response is observed under typical dynamic conditions (such as the workpiece rapidly entering the coil), and a D1 value is selected that allows the adaptive proportional gain DFK to increase to 1.5-3 times its steady-state value during dynamic operation without causing oscillation. In this embodiment, the typical value range is 0.01-0.1s. D1 determines the sensitivity of the adaptive gain. The larger D1 is, the greater the increase of the adaptive proportional gain DFK when the phase changes rapidly, and the faster the system response. However, it is also more prone to overreaction due to noise interference. The smaller D1 is, the less obvious the adaptive effect is. D1 is the intensity adjustment knob of the creative factor adaptive proportional gain.

[0144] In the above formula, This represents the rate of change of phase difference, i.e., the first derivative of the phase difference between voltage and current with respect to time, in rad / s. It is a precursor signal to the dynamic changes in the system. When the load suddenly changes (workpiece enters the coil, temperature changes abruptly, Curie point crossing), the resonant point begins to drift, and θ will rapidly deviate from zero. It will be much larger than the steady-state value, reflecting the dynamic process the system is undergoing much earlier than θ itself. Therefore, the modulation signal as adaptive gain is forward-looking; the system has already detected the dynamic process before the phase deviation becomes large. Sensing the changing trend, the frequency conversion speed was accelerated in advance;

[0145] θ refers to the phase difference between voltage and current, and this parameter is the most important feedback signal in the frequency conversion control system. In a series resonant system, when θ = 0, the system is in a fully resonant state with maximum active power. When θ > 0 (inductive), the system operates in the weakly inductive region. The direction and magnitude of θ directly reflect the degree to which the system deviates from the resonant point and are the core basis for frequency regulation.

[0146] The above content needs clarification:

[0147] The frequency tracking of existing induction heating frequency converters usually adopts a fixed parameter PI regulator and performs closed-loop control with θ→0 as the target. The inherent contradiction of this scheme is that the fixed gain cannot simultaneously meet the requirements of steady-state stability and dynamic speed. If the gain is set too high, frequency jitter is easily generated by noise disturbance in steady state. If the gain is set too low, it cannot keep up with the speed of resonant point drift when the load changes abruptly.

[0148] The frequency increment unit solves this problem through two mechanisms:

[0149] The first mechanism, The term enables adaptive frequency adjustment range based on real-time quality factor (QL). When the real-time quality factor (QL) is high, the resonance peak is sharp and requires precise tracking. This term is close to 1 and does not compress the frequency adjustment range. When the real-time quality factor (QL) is low, the resonance peak is flat and does not require large-scale frequency adjustment. This term is reduced to about 0.5 and actively compresses the frequency adjustment range. This prevents excessive frequency adjustment under low real-time quality factor (QL) conditions from causing system oscillation.

[0150] The second mechanism, It achieves adaptive gain adjustment. It is the rate of change of the phase difference, which reflects the dynamic changes of the system earlier than θ itself. Even before θ becomes very large, dθ / dt already senses the trend of change. As the value increases, the adaptive proportional gain DFK automatically increases, accelerating the system's response to phase deviation and enabling it to catch up to the new resonant point more quickly. When the adaptive proportional gain DFK is reduced, it returns to the reference value, and the system resumes smooth fine-tuning.

[0151] The combination of these two mechanisms makes the frequency conversion system stable in steady state and decisive in dynamic state, without the need for manual trade-offs between response speed and steady-state stability. The calculation result of the adaptive frequency increment DF is directly used as the frequency command of the PWM module, which is the final execution quantity of the frequency conversion system. Its function is to adjust the operating frequency of the inverter in real time so that the system always works in a state close to resonance.

[0152] Specifically, the adaptive frequency increment DF contains two information dimensions: direction and magnitude. The direction is determined by the sign of θ (when θ > 0, the system is in the inductive region and needs to be down-frequency; when θ < 0, the system is in the capacitive region and needs to be up-frequency). The magnitude is determined by the amplitude of θ and the real-time quality factor QL (the larger the deviation and the higher the real-time quality factor QL, the larger the adjustment amplitude needs to be).

[0153] The adaptive frequency increment DF is added to the current frequency to obtain a new frequency fnew = DF0 + DF, and then the PWM carrier frequency is updated. This process is performed once in each control cycle to form a closed-loop frequency tracking.

[0154] When the adaptive frequency increment DF is large, it means that the system is in one of the following situations: large phase deviation (severe detuning), or If the resonant point is large (the resonant point is drifting rapidly) or the real-time quality factor QL is high (the resonant peak is sharp and requires a large adjustment to return to the resonant point), the system is undergoing a significant dynamic process and needs a rapid and large frequency adjustment to catch up with the resonant point. The large adaptive frequency increment DF is not a problem in itself; it is the system's normal response to the dynamic process.

[0155] When the adaptive frequency increment DF is small, it means that the system is in or close to a steady state: θ is close to zero and the resonant point is basically stable. At this time, only a small frequency adjustment is needed to maintain the resonant state.

[0156] Key points to note: The size of the adaptive frequency increment DF is constrained by the limiting function and will not increase indefinitely. When the adaptive frequency increment DF remains near the limiting value, it indicates that the system may have encountered a problem that cannot be solved by frequency conversion (such as severely improper load matching). In this case, it is necessary to combine the power reduction of the joint control unit to protect the system.

[0157] refer to Figure 1 , Figure 2 , Figure 3 and Figure 4 ;

[0158] The calculation formula for the joint control unit is as follows:

[0159] ;

[0160] in:

[0161] PC stands for Joint Control Index, which is the maximum power command allowed by the system under current operating conditions, measured in watts (W). It is the final power target value after being progressively corrected by three derating factors. This calculation result represents the final execution command of the entire system. It is not simply a matter of providing the exact power requested by the user, but rather a safe and reasonable power target that comprehensively considers load matching, detuning, and device safety margins. Its core function is to output power that meets process requirements as much as possible while ensuring that the IGBT (Insulated Gate Bipolar Transistor) does not exceed its limits.

[0162] Pref refers to the target heating power, which is the target heating power required by the process. It is set by the operator or the host computer and is in W. It can be input by the user through the human-machine interface or sent from the host computer through the communication bus. The target heating power Pref is the target value of the system, which represents the heating intensity that the user wants to achieve. It serves as the benchmark for the joint control index PC, and all derating corrections are scaled based on this benchmark.

[0163] Qref refers to the rated quality factor, which is the calibrated value of the quality factor of the system under rated operating conditions (standard load, rated power, room temperature). It is dimensionless and is measured and recorded under standard load conditions during the system's factory commissioning phase. It is then fixed in the program as a system parameter. The ratio of QL to Qref reflects the degree of deviation of the current load state from the rated state. The significance of introducing Qref is that the absolute range of the real-time quality factor QL value of different specifications of induction heating systems may vary greatly. By dividing by their respective Qref for normalization, the derating logic becomes universal for systems of different specifications.

[0164] This indicates the upper limit. When QL > Qref (light load condition), if no limit is applied, QL ÷ Qref > 1, which will cause PC > Pref, meaning the power command exceeds the user-set target value. This is unreasonable. Under light load, the workpiece itself has limited absorption capacity. Increasing the power command will only increase reactive current circulation and will not improve the actual heating effect. Limiting ensures that this factor only plays a derating role in the range of [0,1] and will not gain any benefit.

[0165] This represents the absolute value of the phase difference, that is, the absolute value of the voltage-current phase difference. The significance of taking the absolute value is that, regardless of whether the system is in the inductive region (θ > 0) or the capacitive region (θ < 0), the degree of deviation from the resonant point is measured by the absolute value. In this unit, The larger, The smaller the value, the lower the power command, reflecting the correct physical relationship that the further away from resonance the power command is, the less active power there is, and the power command should be actively reduced.

[0166] θmax refers to the maximum allowable phase difference, i.e., the maximum allowable phase difference limit of the system. In this embodiment, it is preset to 15°-30° to ensure that the system always operates in the weak inductance region. θmax is the reference value of the second derating factor. When approaching θmax, ≈0, the power command is significantly reduced, which serves two purposes: first, to prevent the system from continuing to output high power under severe detuning (avoiding reactive circulating current damage to IGBTs); second, to trigger protective power reduction in extreme cases (such as sudden load disconnection).

[0167] ϵ refers to the minimum power ratio, the lower limit protection value of the derating factor, which is dimensionless. In this embodiment, it is preset to 0.05-0.1. The existence of ϵ prevents the derating factor from dropping to 0, which would cause the joint control index PC=0 and the system to shut down completely. In engineering practice, restarting after a complete shutdown requires an additional soft start process, and the workpiece cannot be heated during the shutdown. Retaining a minimum power (such as 5% of the rated power) allows the system to maintain a minimum operation under extreme conditions and automatically return to normal power output after the operating conditions are restored.

[0168] Pfsw refers to the current switching frequency, which is the current switching frequency of the inverter IGBT, in Hz. It can be directly read by the PWM module of the DSP / MCU. In the loss analysis of IGBT, the switching loss is proportional to the switching frequency. The higher the frequency, the greater the energy loss of each switching action of the IGBT. The product of Pfsw and Pvd reflects the current switching stress level of the IGBT.

[0169] Prate refers to the rated switching frequency of the IGBT, that is, the rated switching frequency of the selected IGBT module, in Hz;

[0170] Pvd refers to the current DC bus voltage, i.e., the DC bus voltage of the inverter, which is the DC voltage after rectification and filtering, and the unit is V. It can be directly sampled across the DC bus capacitor by a voltage Hall sensor or a resistor divider circuit, and then read by the DSP after ADC conversion. Pvd is the basis of the inverter's output power. Under the same duty cycle, the higher the Pvd, the greater the output power. At the same time, in the loss analysis of IGBT, the switching loss is also proportional to the bus voltage. The higher the voltage, the greater the voltage stress for each switching action.

[0171] Pvdc refers to the maximum allowable DC bus voltage, which is the upper limit of the DC bus voltage allowed in the system design. The unit is V. It can be determined by combining the IGBT withstand voltage rating and the electrolytic capacitor withstand voltage rating, taking the smaller value between the two and leaving a safety margin (usually derating 10%-20%). Pvdc is the normalized reference of Pvd and also the hardware protection threshold. When Pvd exceeds this value, the hardware protection circuit will directly trigger the overvoltage protection action.

[0172] This represents the normalized product of the switching frequency and the bus voltage, and is dimensionless. Traditional protection logic sets independent thresholds for frequency and voltage (alarm when the switching frequency exceeds the maximum value, alarm when the bus voltage exceeds the maximum value). However, the actual switching loss is the product of the two. The loss generated by 0.8Prate × 0.8Pvdc may be greater than that generated by 0.95Prate × 0.5Pvdc. Independent thresholds cannot capture this coupling risk. Multiplying the normalized values ​​of the two results in a switching stress index. The higher the index, the smaller the product of 1 and the greater the reduction in power command, thus realizing true loss-aware power dispatch.

[0173] To clarify the above: In existing induction heating frequency converters, power protection and device protection typically employ threshold triggering. When a parameter (current, temperature, frequency, etc.) exceeds a set threshold, the protection circuit is directly triggered, causing the system to shut down or reduce power. The problem with this approach is that the protection action is discrete and abrupt; the system either operates at full power or suddenly reduces power or shuts down, lacking a smooth transition.

[0174] The joint control unit achieves smooth and gradual derating of power commands through continuous adjustment of three derating factors:

[0175] First depreciation factor It implements derating based on load matching. When the load matching is good, the adaptive frequency increment DF is close to or higher than the rated quality factor Qref. This factor is 1 and there is no power reduction. When the load matching deteriorates, the adaptive frequency increment DF decreases and the factor decreases accordingly. The power command is smoothly reduced. The role of this factor is to actively reduce the power when the load condition deteriorates, rather than waiting for the overcurrent protection to be triggered.

[0176] The second depreciation factor Derating based on the degree of detuning is achieved when the system approaches resonance. When the system is severely detuned, the factor approaches 0, and when the system is severely detuned, As the factor approaches θmax, it drops to ϵ, resulting in a significant reduction in power command. The purpose of this factor is to prevent the continued output of high power in a detuned state. During detuning, the reactive current is large, and the current stress on the IGBT is high. Therefore, the active power must be reduced to ensure device safety.

[0177] The third depreciation factor A coupling derating based on switching losses is achieved. It captures the product effect of IGBT switching losses, which are proportional to the product of frequency and voltage. The combination of two moderately high parameters may be more dangerous than a single extreme parameter. Traditional independent thresholds cannot capture this coupling risk, but the product form of the derating factor can. When the current switching frequency Pfsw and the current DC bus voltage Pvd are both at moderately high levels, the product increases, the factor decreases, and the power command is smoothly reduced.

[0178] The combined effect of these three factors makes the joint control index PC output by the joint control unit a safe power command that integrates load status, operating status and device safety margin.

[0179] The joint control index PC is directly used as the input of the power regulation module to control the output power of the inverter (by adjusting the PWM duty cycle or DC bus voltage). Its function is to ensure that the output power of the system is within a safe range under any operating condition.

[0180] The joint control index PC and the adaptive frequency increment DF work together: the adaptive frequency increment DF is responsible for adjusting the frequency to a position close to the resonant point, while the joint control index PC is responsible for determining the safe power level at this operating point. One controls the frequency position and the other controls the power magnitude, together forming a complete frequency conversion-power regulation joint control.

[0181] When the combined control index PC is close to the target heating power Pref, it means that the three derating factors are close to 1, the load is well matched, the system is close to resonance, and the device switching stress is low. At this time, the system is in the best working state and can output full power according to the target power set by the user.

[0182] When the combined control index PC is significantly lower than the target heating power Pref, it means that at least one derating factor is in effect. It is necessary to combine the values ​​of the three factors to determine which factor is dominating the derating:

[0183] in the case of The dominant signal indicates a problem with load matching, which may be due to improper workpiece coupling or being in a special temperature range.

[0184] in the case of The dominant signal indicates severe system detuning, which may be due to the frequency converter speed not keeping up with load changes. It is necessary to check whether the calculation of the adaptive frequency increment DF is normal.

[0185] If the third factor is dominant, it indicates that the switching stress of the IGBT is high, possibly because the combination of frequency and voltage is in a dangerous zone.

[0186] When the combined control index PC drops to around ϵ×Pref, it indicates that at least two derating factors are in a state of deep derating, and the system is in a critical condition. At this time, although the system is still running (it will not completely shut down), the actual heating power is very low, and the process effect is greatly reduced. Operators need to pay attention and investigate the cause.

[0187] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. An induction heating semiconductor frequency converter, characterized in that, include: The upper part of the body (1) is provided with a display instrument for displaying data of the induction heating semiconductor frequency converter in real time. Control cabinet (6), the control cabinet (6) is installed on the body (1), the control cabinet (6) is used to control and manage the operation of the induction heating semiconductor frequency converter, the control cabinet (6) is equipped with an analysis and calculation component, through the operation of the induction heating semiconductor frequency converter, the analysis and calculation component acquires, processes and analyzes the operation data of the device, so as to output the real-time quality factor, adaptive frequency increment and joint control index; The analysis and calculation component analyzes the quality factor of the induction heating semiconductor frequency converter based on design parameters and real-time operating status, and performs dynamic sensing and frequency tracking based on adaptive frequency increment, combined with the joint control index to perform multi-dimensional safety derating of power commands.

2. The induction heating semiconductor frequency converter according to claim 1, characterized in that: The analysis and calculation components include: The variable frequency drive data acquisition module collects hardware and operational data from the induction heating semiconductor variable frequency drive to obtain quality-related data, frequency-related data, and control-related data. It also preprocesses the acquired data to save it to the database. The calculation management module retrieves the quality-related data, frequency-related data, and control-related data from the database. The calculation management module is based on: The current inverter angular frequency, series equivalent inductance, series equivalent resistance, weighting factor 1, current reactive power, and current active power in the quality-related data are used to output the real-time quality factor. The reference resonant frequency in the frequency correlation data, the proportional gain adaptively modulated by the phase difference change rate, and the phase difference between voltage and current, combined with the processing of real-time quality factor and saturation limiting function, are used to output an adaptive frequency increment, wherein the phase difference change rate is used to automatically improve the frequency regulation response speed when the load changes dynamically. The control index is output by adjusting the target heating power, rated quality factor, maximum allowable phase difference, minimum power ratio, current switching frequency and current DC bus voltage in the related data, and combining the real-time quality factor and the phase difference between voltage and current. The index is then adjusted step by step by the first derating factor based on the load matching degree, the second derating factor based on the degree of detuning and the third derating factor based on the coupling effect of switching loss. The real-time quality factor is used as a collaborative state variable and is simultaneously passed to the adaptive frequency increment calculation and the joint control index calculation. The adaptive frequency increment is based on the collaborative state variable for dynamic sensing and frequency tracking, and the joint control index is based on the collaborative state variable for multi-dimensional safety derating of power commands. The real-time quality factor, adaptive frequency increment, and joint control index constitute a three-level collaborative progressive control link, enabling frequency tracking and power control to generate synergistic effects under the drive of shared state variables. Control execution module: Receives real-time quality factor, adaptive frequency increment and joint control index output from the calculation and management module to adjust the PWM frequency and PWM duty cycle.

3. The induction heating semiconductor frequency converter according to claim 2, characterized in that: The analysis and calculation components include a real-time quality unit, a frequency increment unit, and a joint control unit, and the control execution module includes a frequency control unit and a duty cycle control unit.

4. The induction heating semiconductor frequency converter according to claim 3, characterized in that: The processing flow of the real-time quality unit is as follows: A1. By acquiring the carrier frequency of the PWM module in real time and converting it into angular frequency, the current inverter angular frequency is obtained and used as the reference frequency for the operating point of the resonant circuit. A2. Obtain the room temperature calibration value of the series equivalent inductance through offline calibration, and obtain the correction coefficient by looking up the table through the preset temperature-inductance curve in combination with the real-time temperature of the workpiece. Calculate the series equivalent inductance to reflect the energy storage capacity and load coupling state of the resonant circuit. By sampling the effective voltage and current values ​​at the inverter output in real time, and combining the phase difference between voltage and current, calculate the series equivalent resistance according to the impedance relationship to characterize the workpiece's ability to absorb active power and the strength of system damping. A3. Based on the effective values ​​of voltage and current at the inverter output and the phase difference between voltage and current, the current reactive power and current active power are calculated respectively to obtain the proportional relationship between the oscillation intensity of energy in the resonant circuit and the effective transfer to the load. The weighting coefficient is determined through experiments during the system commissioning phase to adjust the sensitivity of the real-time quality factor to the detuning state, thereby finally outputting the real-time quality factor. The real-time quality factor is used as a load state sensing signal and transmitted to the frequency increment unit and the joint control unit for determining the adjustment range of frequency tracking and making safe derating decisions for power commands, respectively.

5. The induction heating semiconductor frequency converter according to claim 4, characterized in that: The processing flow of the frequency increment unit is as follows: B1. Determine the reference resonant frequency through the parameters of the LC resonant circuit to serve as the reference point for the inverter switching frequency; B2. Determine the reference proportional gain through step response experiments during the system debugging phase, so as to set the frequency adjustment sensitivity of the system under steady-state conditions; B3. During the system debugging phase, the adaptive gain coefficient is determined through response experiments under typical dynamic conditions to control the modulation intensity of the phase difference change rate on the proportional gain. The phase difference change rate is obtained by sampling the phase difference between voltage and current in real time and calculating its first derivative with time. This serves as a precursor signal for dynamic load changes. The reference proportional gain is combined with the adaptive gain coefficient and the phase difference change rate to calculate and output the adaptive proportional gain, so that the frequency regulation response speed is automatically adjusted according to the severity of load changes. B4. Introduce the real-time quality factor and automatically compress the frequency adjustment range based on the load condition. Then, multiply the adaptive proportional gain by the phase difference and process it through the saturation limiting function to obtain the frequency offset. Finally, add the frequency offset to the reference resonant frequency to output the adaptive frequency increment. B5. Use the adaptive frequency increment as the frequency command of the PWM module to update the inverter switching frequency, so that the system always tracks the drift of the resonant circuit and operates in a state close to resonance.

6. The induction heating semiconductor frequency converter according to claim 5, characterized in that: The processing flow of the joint control unit is as follows: C1. Obtain the target heating power set by the operator or the host computer as the reference value for the power command; C2. During the system's factory commissioning phase, the rated quality factor is measured and recorded under standard load conditions to serve as a normalized benchmark for load matching. The maximum allowable phase difference is set during the system design phase to limit the system's operating boundary in the weakly inductive region. The minimum power ratio is also set during the system design phase to prevent the derating factor from reaching zero and causing a complete system shutdown. The current switching frequency is directly read through the PWM module, and the current DC bus voltage is obtained by sampling across the DC bus capacitor. Based on the rated switching frequency of the IGBT module and the maximum allowable DC bus voltage in the system design, the normalized product of the switching frequency and the bus voltage is calculated to characterize the switching stress level of the IGBT. C3. The ratio of the real-time quality factor to the rated quality factor is subjected to an upper limit limiting process to obtain the first derating factor based on the load matching degree. The result of subtracting the ratio of the absolute value of the phase difference to the maximum allowable phase difference from 1 is subjected to a lower limit protection process to obtain a second de-rating factor based on the degree of detuning; The result of subtracting the normalized product of the switching frequency and the bus voltage from 1 is processed by the lower limit protection to obtain the third derating factor based on the coupling effect of switching loss. C4. Multiply the target heating power by the minimum value among the first derating factor, the second derating factor, and the third derating factor to output a joint control index. This joint control index serves as a safety power command that comprehensively considers the load matching state, the degree of detuning, and the safety margin of the devices. The joint control index is used as the input of the power regulation module to control the output power of the inverter, ensuring that the output power of the system is within the safe range under any operating condition.

7. The induction heating semiconductor frequency converter according to claim 3, characterized in that: The frequency control unit is specifically: The updated frequency value is calculated based on the reference resonant frequency and the adaptive frequency increment. The updated frequency value is then converted into the period register value of the PWM timer and written into the corresponding register. The PWM module automatically switches to the new frequency in the next carrier cycle.

8. The induction heating semiconductor frequency converter according to claim 3, characterized in that: The duty cycle control unit is specifically: Based on the relationship between output power and duty cycle, the joint control index is converted into PWM duty cycle, and the conversion relationship depends on the inverter topology.

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