1 Introduction

The intermediate frequency power supply technology for induction heating is a technology that converts the power frequency (50 Hz) into an intermediate frequency (400 Hz to 200 kHz) through a thyristor or a power semiconductor device such as a MOSFET or an IGBT. Since it has a flexible control method, the output power is large, and the efficiency is higher than that of the unit. The advantages of changing the operating frequency are convenient, so it has been widely used in building materials, metallurgy, national defense, railway, petroleum and other industries. This paper wants to pursue the development history of this field in China, introduce its development status, and then explore its development trend.

2. Development history of medium and high frequency power supply technology for induction heating

2.1 Development research period in which many units participated in the 1970s

Throughout the history of the development of medium frequency power supply for induction heating in China, we can generalize its development as the development research period of the 1970s, the mature application period of the 1980s, the large-scale promotion period of the 1990s, and the performance improvement period of the late 20th century. .

China's application of power semiconductor devices to develop medium and high frequency power supplies for induction heating can be traced back to the 1970s. With the advent of the first thyristor in China in 1963, China developed a fast thyristor around 1970. The unit has started research on thyristor intermediate frequency power supply. It can be said that the development period of many units involved in the 1970s set off the first intermediate frequency heat in China. The mid-frequency heat of this period is mainly represented by many units engaged in research and development in this field. The core device used in this period is the fast thyristor, and its control circuit is a plug-in box structure composed of a plurality of control boards composed of many discrete components. At the same time, due to the limitation of thyristor fabrication technology, it is determined that the main circuit structure is not high enough because of the blocking voltage of the fast thyristor, but the two thyristors or three thyristors are connected in series to form the inverter bridge arm. The number of applied fast thyristors is 8 Or 12, so it is inevitable to accompany the voltage doubling network of the fast thyristor. At the same time, it should be seen that because the turn-off time of the thyristor cannot be too short, it is determined that the output frequency of the intermediate frequency power supply cannot be high; The dynamic parameters dv/dt and di/dt of the fast thyristor are not very high, which leads to a large and complicated network limiting dv/dt and di/dt in the system. Third, the reliability of the whole thyristor is still not ideal at this stage ( At that time, the domestic play was called "terrible silicon". It was decided that the intermediate frequency power supply at this stage was mostly laboratory products, and there were few applications in the industry.

2.2 Mature application period in the 1980s

After 1980, due to the rapid progress in the manufacturing process of domestic thyristors, and the reliability of the introduction of thyristors in China due to the reform and opening up technology, great progress has been made. Therefore, the gradual induction heating IF power supply has bid farewell to the laboratory and entered the industrial production. During this period, the thyristor IF power inverter bridge has gradually transitioned from multiple fast thyristors in series to a single thyristor, but the output operating frequency is still not very high, mostly below 2.5 kHz. To obtain an output frequency of 4 kHz or 8 kHz, the frequency doubling has to be used. And other complex control techniques. It should be seen that the starting scheme of the thyristor intermediate frequency power supply in this period is mostly a percussive starting scheme with a special charging link, and the control panel is a plug-in box structure composed of a plurality of small control panels. Generally, the entire control system consists of twelve control boards (six rectifier trigger boards, two inverter pulse boards, one positive power board, one negative power board, one protection board, one adjustment board), and this period is fast. The thyristor domestic horizontal turn-off time is about 35μs, and the blocking voltage is up to 1600V, and the on-state average current is up to 500A, which determines that the IF power supply for induction heating with a power capacity exceeding 350kW has to use multiple fast thyristors. Parallel solution.

2.3 Wide-scale promotion and application period in the 1990s

After the above two periods, it can be said that China's thyristor IF power supply technology has been relatively mature. After entering 1990, due to the use of neutron irradiation technology in the manufacturing process of domestic fast thyristors, the turn-off time was further shortened, and the capacity of domestic fast thyristors was further improved. The control technology has percussive start, zero-voltage start, internal and external bridge conversion. The start-up and other programs, coupled with the large demand for steel in the domestic construction industry, prompted the second medium-frequency heat in the country from 1991 to 1993. In the past few years, the domestically added tens of thousands of intermediate-frequency power supplies have been put into operation, so many users Withdrawal of goods, the induction heating medium frequency power supply has been widely promoted in China, and its power capacity has increased from tens of kilowatts to 500 kW. Even the manufacturing level of 1000 kW fast thyristor has been reduced from 35 μs to 25 μs, even Below 20μs, the blocking voltage has increased from 1600V to 2000V, and the single-tube capacity has been increased from 500A to 1000A. The main circuit scheme in this phase is divided into two types in China, one is the parallel scheme represented by Zhejiang University, and the other is a parallel scheme represented by Zhejiang University. One is a series solution represented by Xiangtan Motor Factory.

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