First, the development process of induction melting technology
The application of induction furnace in domestic foundry enterprises can be traced back to the 1970s. At that time, the power frequency induction furnace began to be gradually promoted. At the peak, there were 10t 2.8MW and 20t 4MW specifications of power frequency induction furnaces. Although there are many defects, and in the end, they have not been widely promoted, but pioneered the use of electric power and electromagnetic induction of cast iron melting, indicating the development direction for more advanced melting methods.
In the same period, with the advent of power semiconductor devices, IF induction furnaces were invented and gradually improved abroad (excluding intermediate frequency generator sets), and then introduced to China. Since the late 1970s, there have been research and experimental properties in China. Application, but not large. Until the 1990s, under the vigorous promotion of some companies in Europe and America, the medium frequency induction furnace entered the large-scale practical application stage in China's foundry industry. At that time, the main equipments were produced from abroad, including medium-frequency induction furnaces with parallel resonance technology represented by old-fashioned traditional companies, and one-to-two inverter induction furnaces using series resonance technology represented by some emerging companies. Xi'an Institute of Mechanical and Electrical Engineering also began research and promotion of medium frequency induction furnace technology at that time, and in the early 21st century, it launched a one-to-two frequency induction induction furnace based on IGBT devices, and created the application of domestic high-power one-to-two frequency induction induction furnace. precedent.
At present, China's medium-frequency electric furnace industry is developing very rapidly. Technically, the level of domestically produced equipment is close to the highest level in the world. For the intermediate frequency power supply, the main circuit adopts the AC-DC-AC commutation method, that is, the commutator is used to rectify the power frequency mains into a pulsating DC power, and then the filter is used to turn it into a smooth DC power. The inverter is inverted into a single-phase intermediate frequency. According to the difference between the compensation capacitor and the load connection form, it can be divided into two types: series resonant intermediate frequency power supply and parallel resonant intermediate frequency power supply.
Parallel resonant power supply Because the compensation capacitor and the induction coil of the electric furnace are self-oscillated, the current flowing through the inverter power semiconductor device is usually 1/10 to 1/6 (melting load) of the load current, which makes the parallel resonant intermediate frequency power supply the same. The inverter power semiconductor device is easy to achieve greater power under the conditions.
The application of series resonant intermediate frequency power supply is relatively late, and it has only been promoted and applied in China since the 1990s. Because of its unique power adjustment method, it can realize one-to-two output and continuous production of molten iron, which is favored by users. Generally, the power adjustment of the intermediate frequency power supply is performed on the frontmost rectifier, and the output voltage is controlled by controlling the conduction angle of the rectifier thyristor to achieve the purpose of adjusting the output power. The series resonant intermediate frequency power supply adopts the method of changing the operating frequency of the inverter to adjust the output power. When the working frequency is close to the resonant frequency, the output power increases, and when far away, the output power decreases, and the output voltage of the rectifier is constant, which is also the series resonant intermediate frequency. The furnace is called the reason of the inverter furnace. Because of this characteristic, multiple inverters can be mounted on one rectifier. With proper power interlock control, as long as the sum of the output power of all inverters does not exceed the load capacity of the rectifier, each inverter Each unit is capable of controlling its own output power to meet the demand. The typical application is a one-to-two frequency conversion furnace. One rectifier has two inverters to drive two electric furnaces of the same specification, so that the power of one variable frequency power supply can be flexibly distributed to two furnaces, that is, one The high power of the power supply is distributed to the melting furnace, and the remaining small power is distributed to the holding furnace, thereby realizing "one melting and one heat preservation", so that the electric furnace system has the ability to continuously produce molten iron.
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