Power Electronics Devices Of Induction Heating
Induction heating is mainly used for metal smelting, heat preservation, sintering, welding, quenching, tempering, diathermy, liquid metal purification, heat treatment, pipe bending, and crystal growth. The induction power supply is consisted of rectifier circuit, inverter circuit, load circuit, control and protection circuit.
The medium frequency power supply technology for induction heating is a technology that rectifies the alternating current power frequency (50Hz) to direct power then converts to medium frequency (400Hz~200kHz) through power semiconductor devices such as thyristor, MOSFET or IGBT. The technology features in flexible control methods, large output power, and higher efficiency than the unit, and convenient to change frequency according to heating requirement.
The rectifier of the small and medium power supply equipment adopts three-phase thyristor rectification. For high-power power supply equipment, 12-pulse thyristor rectification will be applied to improve the power level of the power supply and reduce the grid-side harmonic current. The inverter power unit is composed of high-voltage high-current fast switch thyristor parallel then series connected to realize the high power output.
Inverter and resonant circuit can be divided into two types according to structural properties: 1) parallel resonant type, 2) series resonant type.
Parallel resonant type: high-voltage high-current water-cooled thyristor (SCR) is used to form a current-type inverter power unit, and high power output is realized by superposition of thyristors. The resonant circuit generally uses a complete parallel resonance structure, also choose double-voltage or transformer mode to increase the voltage on inductor according to different requirement, mainly applied in heating treatment process.
Series resonant type: high-voltage high-current water-cooled thyristor (SCR) and fast diode are used to form a voltage-type inverter power unit, and high power output is realized by superposition of thyristors. The resonance circuit uses a series resonance structure, and the transformer is adopted to match the load requirement. In addition to the advantages of high power factor at Grid-side, wide power adjustment range, high heating efficiency and high start-up success rate, it’s become more and more widely used in current years and mainly applied in melting process.
After the improving of manufacturing process, Runau manufactured fast switch thyristor uses neutron radiation and other processes to further shorten the turn-off time and the power capacity is improved consequently.
The induction heating medium frequency power supply adopts thyristor as the main power device has covered all fields with operating frequency below 8kHz. The output power capacity is divided into 50, 160, 250, 500, 1000, 2000, 2500, 3000kW, 5000KW, 10000KW the operating frequency is 200Hz, 400Hz , 1kHz, 2.5kHz, 4kHz, 8kHz. 10 tons, 12 tons, 20 tons for steel melting and thermal reservation, the main power equipment is medium frequency power supply. Now the maximum output power capacity comes to 20000KW of 40Ton. And the thyristor is the key power conversion & inversion component to be applied.