Characteristics and classification of medium frequency induction heating power supplies

Medium frequency induction heating furnace equipment, also known as solid-state medium frequency induction heating power supply, is a static variable frequency power supply composed of power semiconductor components. It has two methods: AC-to-AC frequency conversion and AC-to-AC frequency conversion. This section only covers the former type, which is the most commonly used medium-frequency heating power supply for furnaces.

Compared with the traditional medium frequency heating power generator set, the medium frequency heating power supply has the advantages of higher efficiency, variable frequency, low noise, small size, light weight, easy installation, easy operation and maintenance.

Compared with power frequency induction furnaces, medium frequency induction furnaces have significant advantages in operational flexibility and economy. Mainly manifested in the following aspects.

①High power density, high melting rate and thermal efficiency.

② The electric furnace can be emptied without leaving any molten liquid, making it easy to change metal types.

③ Wet or oily furnace materials do not need to be dried and can be directly put into the furnace for melting.

④ It can automatically convert frequency to adapt to changes in furnace material parameters. There is no need to use a contactor to switch capacitors to compensate for power factor like a power frequency induction furnace.

⑤At the same productivity, the size of the medium frequency induction furnace body is small, so the heat loss and thermal stress of the furnace lining are also small, and the service life of the furnace lining is extended.

⑥ There is no need for a voltage regulating switch that is prone to failure, reducing the maintenance workload.

⑦No need for three-phase balancing device.

⑧It occupies a small area and requires less civil engineering and installation costs.

Medium frequency heating power supplies also have certain shortcomings. The harmonics and noise generated by high-power medium frequency power supplies will interfere with the power grid and the environment, and measures must be taken to solve them.

According to the different filters used, intermediate frequency power supplies can be divided into current type and voltage type. The current type uses DC smoothing reactor filtering to obtain a relatively flat DC current, the load current is a rectangular wave, and the load voltage is approximately a sine wave; the voltage type uses capacitor filtering to obtain a relatively flat DC voltage, and the voltage at both ends of the load It is a rectangular wave, and the load current is approximately a sine wave.

According to the load resonance mode, the intermediate frequency power supply can be divided into parallel resonance type and series resonance type. According to the load resonance mode, the current type of the intermediate frequency power supply is often used in parallel and series-parallel resonance inverter circuits, while the voltage type is mostly used in series resonance inverter circuits. .

Parallel resonant medium frequency heating power supply

Parallel resonant intermediate frequency power supply is an earlier applied intermediate frequency power supply in China. It is a frequency conversion device that converts three-phase power frequency AC power into single-phase intermediate frequency power. The advantage is that it has strong load adaptability and can be used as a power supply for induction diathermy, smelting, quenching and other induction heating equipment.

The main circuit principle of parallel resonant intermediate frequency power supply mainly consists of isolation switch (DK), AC contactor (or electric circuit breaker KM), incoming line inductor (Ll-L3), fast fuse (FU), rectifier (VTl-VT6), It consists of smoothing reactor (LF), inverter (VT7-VT10) and parallel load (L, C). The rectifier converts three-phase power frequency alternating current into direct current. The smoothing reactor is used to filter out the rectified current ripples and isolate different ripple voltages between the rectifier and the inverter. The inverter converts the direct current into a single-phase intermediate frequency. Alternating current, a parallel resonant load composed of an inductor and a compensation capacitor, can better adapt to changes in load properties during the heating process.

(1) Three-phase bridge type fully controlled rectifier circuit. The rectifier circuit of the parallel resonant intermediate frequency power supply adopts a three-phase bridge type fully controlled rectifier circuit. Since the inductance L of the smoothing reactor is large, the load is an inductive load. Therefore, the load current of the rectified output is continuous and a straight line. When a≤60°, the output waveform of the rectifier circuit is the same as that of the resistive load, and the conduction law is also the same as that of the resistive load. When a>60°, due to the effect of inductance L, the thyristor still conducts after the power supply voltage crosses zero until the next thyristor is triggered and is turned on. In this way, a negative area appears in the waveform of the rectifier output voltage, but the rectifier output current Still a horizontal line. When the control angle increases to 90°. When , the positive and negative areas in the output voltage waveform are equal, then the average value of the output voltage Ud=0. When a>90. When , the rectifier circuit works in the active inverter working state. The phase shift range of the rectifier circuit of the intermediate frequency power supply is 0°~150°.

(2) Inverter circuit. Parallel inverter circuit schematic diagram The capacitor C in the load circuit is connected in parallel with the inductor coil L. The commutation is based on the principle of parallel resonance, which is called a parallel resonant inverter circuit. The DC voltage Ud provided by the thyristor fully controlled rectifier circuit is continuously adjustable, and the parallel inverter circuit inverts the DC power into intermediate frequency AC power to supply the load. There is a large filter inductor LF on the DC side, so it is a current-type inverter. Because the operating frequency is relatively high, the thyristors on the four bridge walls of the inverter circuit are fast thyristors. L7~LlO is the commutation inductance of the inverter thyristor, which is used to limit the current rise rate di/dt during commutation of the thyristor.

(3) High-power double rectifier power supply. When the power of the smelting furnace is large, in order to eliminate harmonic interference, a double rectifier, that is, a 12-pulse rectifier circuit, is often used. The working principles of its rectifier circuit and inverter circuit are exactly the same as those of the parallel resonance type. The main problem it needs to solve is the balance of the two rectified currents. At the same time, it is also required that the phases of the two secondary windings of the rectifier transformer should be staggered by 30°.