At present, induction heating technology has the highest heating efficiency, the fastest speed, low consumption and environmental protection. It has been widely used in hot processing, heat treatment, hot assembly, welding, smelting and other processes of metal materials in all walks of life. It can not only heat the workpiece as a whole, but also heat the workpiece locally; It can realize the deep heat penetration of the workpiece, or only the surface and surface can be heated centrally; It can not only directly heat metal materials, but also indirectly heat non-metallic materials. wait. Therefore, induction heating technology will be more and more widely used.
Basic principle:
Put the workpiece into the inductor (coil). When a certain frequency of alternating current is connected to the inductor, an alternating magnetic field will be generated around it. The electromagnetic induction of alternating magnetic field produces a closed induced current eddy current in the workpiece. The induced current is unevenly distributed on the section of the workpiece. The current density on the surface of the workpiece is very high and gradually decreases inward. This phenomenon is called skin effect. The electric energy of the high-density current on the surface of the workpiece is transformed into heat energy, which increases the temperature of the surface, that is, the surface heating is realized. The higher the current frequency, the greater the current density difference between the surface and the interior of the workpiece, and the thinner the heating layer. Surface quenching can be achieved by cooling rapidly after the temperature of the heating layer exceeds the critical point temperature of the steel.
Recent development of induction heating technology:
The industrial application of induction heating in China started in the 1950s and was first applied in machine tool manufacturing, textile machine manufacturing, automobile, tractor industry and other departments. At that time, most of the induction heating technology came from the former Soviet Union and a few from Czech, Belgium and other countries. In the late 1950s, China has made its own electronic tube type high-frequency power supply and mechanical medium frequency generator. Various equipment and processes such as induction melting, induction diathermy, quenching and medium heating have been applied in industry one after another. After the 1960s, inspired by the spirit of self-reliance, various departments and enterprises developed thyristor medium frequency power supply, improved electronic tube high frequency power supply, and designed and manufactured various types of quenching machine tools. Their typical structures have been compiled into the quenching machine Atlas of the Sixth Design Institute of the former Ministry of machinery, with a total of 55 kinds. Since the opening to the outside world, through various channels such as overseas investigation, import of equipment and introduction of technology, the modern induction heating technology of industrial developed countries has gradually entered all sectors of China's industry, making more effective use of induction heating, which is energy-saving, efficient, automatic, high reproducibility and environmental protection.
At present, the induction heating power supply mainly adopts thyristor in the intermediate frequency band and IGBT in the ultra audio frequency band. However, in the high frequency band, due to the lack of heat such as high conduction loss of sit, the power supply usually has high flying power and hot air rate, and has many special requirements for power devices, passive devices, cables, wiring, grounding and shielding. Therefore, the development of induction heating power supply and high-frequency local heating power supply is realized.
Considering the large capacity of induction heating power supply from the perspective of circuit, the large capacity quantization technology can be divided into two categories: one is the series and parallel of devices; The other is the series and parallel connection of multiple bridges or multiple power supplies. In the series and parallel mode of devices, the voltage sharing problem of series devices and the current sharing problem of parallel devices must be carefully handled. Due to the discreteness of device manufacturing process and parameters, the number of series and parallel devices is limited, and the more the number of series and parallel devices, the worse the reliability of the device. The series and parallel technology of multiple power supplies is an effective means to further re capacity based on the device series and parallel technology. With the help of reliable power series and parallel technology, under the condition of appropriate single machine capacity, large capacity devices can be simply obtained through series and parallel operation mode, and each single machine is only a unit of the device.
Induction heating power inverter mainly includes parallel inverter and series inverter. The output of series inverter can be equivalent to a low impedance voltage source. When the two voltage sources are connected in parallel, the difference or fluctuation of amplitude, phase and frequency will lead to great circulating current and serious imbalance of inverter current. Therefore, it is difficult to expand the capacity of series inverter in parallel; Parallel inverter: the large DC reactor at the input end of the inverter can act as the current buffer link between the parallel inverters, so that the Ag / DG or dg / DG link at the input end has enough time to correct the deviation of DC current and achieve multi machine parallel capacity expansion. Transistorized ultra audio and high-frequency currents mostly adopt parallel inverter structure, and the parallel inverter is easy to be modular Large volume quantization is one of the main reasons.
The load objects of induction heating power supply are various, and the power inverter and load are an organic whole. Generally, matching transformers are used to connect the power supply and load inductors. The matching transformers for high frequency and ultra audio power supply are further optimized and improved from magnetic materials to winding structure. At the same time, three passive components can be used to replace two passive components in circuit topology, To cancel the transformer and realize efficient and low-cost matching.