Induction heating power supply has experienced a development process of the generator set and vacuum tube generator in the 1920s, thyristor (SCR) generator in the early 1960s, transistor generator in the early 1980s, and modern power transistor generator (IGBT, MOSFET, etc.) generator in the mid-1990s.
Modern induction heating power supply refers to the induction heating power supply with various power transistors, such as MOSFET, IGBT, and so on as power devices, also known as (all) solid-state induction heating power supply, “solid-state” induction heating power supply that is for the old thyristor (SCR) and vacuum tube induction heating power supply.
The frequency range of the inductive heating power supply is very wide. A power supply with a frequency of less than 10kHz is called intermediate frequency inductive heating power supply. A power supply with a frequency between 10~100kHz is called a super audio inductive heating power supply, and a power supply with a frequency higher than 100kHz is called a high-frequency inductive heating power supply. According to the frequency characteristics and power capacity of power devices SCR, MOSFET, and IGBT, SCR is mainly used in intermediate frequency induction heating. According to the current manufacturing level of IGBT induction heating power supply, the international manufacturing level of MOSFET induction heating power supply is up to 1200kW/180kHz, and the domestic one is 10000 kW/50kHz. The international manufacturing level of MOSFET induction heating power supply is 2000kW/400kHz, the domestic one is 10~250kW/50~400kHz, and 1800kW / 150kHz.
Modern induction heating power supply has the following characteristics:
(1) The basic theory of the circuit mentioned in The part is not changed much. Due to the advent of new power devices, their circuit and implementation technology have developed rapidly.
(2) The power rectifier and inverter circuit devices mostly adopt module devices instead of single power devices. In order to expand the output power, series, parallel, or series-parallel power devices are used. A combination of multiple power units.
(3) The devices used in the control circuit and protection circuit are changed from the original simulator such as crystal audion to a large number of digital devices (such as comparator, flip-flop, counter, timer, photoelectric isolator, phase-locked loop, etc.);The use of special-purpose integrated circuit is also another characteristic of modern induction heating power supply, such as 15 ~ 50kW/20 ~ 50kHz MOSFET and IGBT power supply widely used integrated PWM chip SG3525 and digital potentiometer;Phase control and trigger circuit integrated chips TC787, TC788, CMOS-CD4536 and MPU-1016 programmable logic devices for three-phase controllable rectifier;The adoption of integrated chip simplifies the control circuit and improves the reliability;Proportional integral (PI) circuit, digital PLL loop frequency automatic tracking circuit, single-chip computer technology, DSP digital signal processor used to improve the quality of control and power supply device performance, the system to achieve intelligent control.
(4) New circuit elements such as CDE (non-inductive) capacitance module, non-inductive resistance applied to the buffer circuit can greatly improve the absorption effect; Mn-zn power ferrites are used in power output circuits to reduce loss and power volume.
(5) the frequency range is wide, from 0.1 to 400kHz, which covers the range of medium frequency, super audio, and high frequency; The output power ranges from 15kW to 20000kW to meet the requirements of different heat treatment processes.
(6) High conversion efficiency and obvious energy saving. The load power factor of the transistor inverter can be close to 1, which can reduce the input power by 22% ~ 30% and cooling water consumption by 44% ~ 70%.
(7) The whole device is compact in structure, small in size, and saves space. Compared with the vacuum tube power supply, it can save 66% ~ 84%.
(8) Perfect protection circuit and high reliability. The inductive heating power supply can run safely in the case of a workpiece touching the sensor, no load or overload, and other misoperation. The safety measures of the circuit include dc side current overcurrent, AC side current overcurrent, phase loss protection, incoming line voltage overvoltage, and Undervoltage protection, working frequency over the limit, and power over-limit protection, etc. The safety measures of devices include: current imbalance of inverter bridge and through, overheat of power devices, short circuit of slot circuit, over-voltage of the capacitor of slot circuit, over-limit of the voltage of slot circuit, etc. The safety measures of the equipment include cooling water flow and inlet and outlet water temperature detection, interlocking protection of cabinet door and power supply, etc.
(9) There is no high voltage inside or at the output end of the power supply (relative to the vacuum tube power supply), so the working voltage is low and the safety is high. The dc operating voltage of the single-phase ac power transistor induction heating power supply is 220 ~ 250V, the DC operating voltage of the three-phase AC power supply is 510 ~ 560V, and the DC operating voltage of vacuum tube power supply is about 14kV.
It is precise because of the small size of solid-state induction heating power supply, low loss, high conversion efficiency of the inverter, easy control, and good safety, has completely replaced the intermediate frequency generator type power supply, in some fields also replaced the vacuum tube type induction heating power supply. The replacement of vacuum tube power supply is a system engineering, whether to replace it should be taken into full consideration, such as the requirements of induction heating process, productivity, efficiency, safety, cost, reliability, and maintenance, etc. From the perspective of use frequency, especially for applications higher than 1000kHz, the vacuum tube power supply still has a place. In China, the vacuum tube induction heating power supply after two aspects of technology transformation and a considerable number of equipment is still in use.
The modifications in these two aspects are as follows :(1) replace (mercury-filled) thyratron with high-voltage silicon rectifier to realize high-voltage rectifier; (2) adopt three-phase SCR ac voltage regulation to realize positive pressure regulation of vacuum tube, so as to realize smooth regulation of output power. This is a typical thyristor AC voltage regulator – anode transformer booster – high voltage silicon rectifier circuit structure.
