Application of Induction Furnace

At present, the application of induction furnace has been widely used in the industrial field, and it is necessary to further strengthen its research. However, according to the current situation, there are still some problems in its application process, so effective measures need to be taken to control it.

1. Composition of Induction Furnace

(1) System Composition

The induction furnace is mainly composed of electric control, furnace body, insulated electrode shaft, vacuum hydraulic system, circulating water system, etc.

(2) Working Principle

The induction furnace is an electric heating equipment that converts three-phase power frequency AC power into DC through the rectification part, and then into medium frequency AC power through the inverter part, and then enters the induction coil after the capacitor bank is boosted. The magnetic line of force is generated in the coil, and the non-ferrous metal in the induction coil is cut through the magnetic line of force, and the colored metal generates the changing heat, thus heating and melting the non-ferrous metal.

2. Application of Induction Furnace in the Industry

(1) Selection of IF Power Frequency

With the increase of the frequency of the medium frequency power supply, the speed of the magnetic wire cutting the charge will increase. In the closed circuit formed by the charge, the eddy current value will increase, so the heating effect is significant. Some studies have shown that the higher the frequency of the power supply, the higher the electrical efficiency. However, when the electrical efficiency is close to 90%, the effect of increasing the frequency of the power supply on the electrical efficiency is not significant. At the same time, due to the skin effect, the depth of the direct heating layer of the furnace charge is inversely proportional to the frequency of the power supply. By conducting for a long time, the surface and core of the workpiece can obtain a close temperature, but the thermal efficiency will be reduced. The reason for the reduction of thermal efficiency is that the heat is lost in the surrounding medium due to the long heat transfer time. Considering that the size of the furnace charge affects the electrical and thermal efficiency, the frequency of the medium frequency power supply should be reasonably selected, that is, neither too large nor too small.

(2) Matching of the Power Transformer and Intermediate Frequency Power Supply

A reasonable match should be that the capacity of the power transformer is slightly greater than the rated power of the medium-frequency power supply. If the capacity of the power transformer is larger than the rated capacity of the medium frequency power supply, it will cause a waste of resources, but if the capacity of the power transformer is equal to or even less than the rated capacity of the medium frequency power supply itself, due to the relatively small capacity of the power transformer, the current limit value of the medium frequency power source is forced to be appropriately lowered, and the electrical efficiency and thermal efficiency of the system are lost. If the current limit value of the intermediate frequency power supply is not changed, the power transformer is in the overload operation state under the condition of full load of the furnace charge, and the voltage drop can not guarantee the incoming line voltage of the intermediate frequency power supply seriously, and the grid voltage may even be lower than 360V. The serious reduction of thermal efficiency will make the total efficiency of the system seriously decrease. The reason for this result is that the incoming voltage of the IF power supply is too low. Under the same load, the power is proportional to the square of the voltage, the output power is seriously insufficient, and the electrical efficiency and thermal efficiency are reduced.

(3) Optimal Design of Induction Winding and Furnace Lining

Different winding turns and the distance between turns will affect the smelting efficiency of the electric furnace. In addition, the main parameter that determines the characteristics of the furnace is the thickness of the furnace lining. The furnace lining should be as thin as possible to have higher electrical efficiency and greater input power to meet the electrical characteristics of the furnace. In order to maintain better strength and prolong the service life of the furnace lining, the furnace lining is required to be thicker in consideration of the erosion of the furnace lining caused by mechanical erosion of the molten metal. Therefore, it is necessary to select a suitable lining thickness, which should not only take into account the electrical characteristics of the furnace, but also ensure the service life of the furnace lining.

(4) Perform Correct Smelting Operation

In order to accelerate the melting of the furnace charge, the charging method should be noted. The furnace space needs to be fully utilized and loaded with large and small pieces of furnace materials. Because the distribution of the magnetic flux of the furnace charge in the induction furnace is uneven, the closer the magnetic flux density is to the crucible wall, the greater the magnetic flux density is to the center line of the crucible, the smaller the magnetic flux density is to the center line of the crucible, and the greater the electromotive force and current generated in the outer layer than in the inner layer, that is, "skin effect". Generally speaking, the charge gap is filled with small pieces of charge. Large pieces of charge are installed near the crucible wall, and small pieces of charge are installed at the bottom and middle part of the furnace. The medium-frequency power supply generally has the necessary voltage and current limiting protection. When charging, the furnace body can be filled with about 2/3 of the furnace charge first, and the temperature will rapidly exceed the Curie point, and then the furnace charge impedance will increase, and the current is within the current limit value, which can ensure that the furnace operates at rated power, and then add another 1/3 of the furnace charge, which will shorten the current limiting time. During the whole melting process, the unnecessary pressure limiting time must be reduced. The furnace drill rod is used to poke the material to avoid "overlapping" due to the different sizes and lengths of the furnace material, which causes the furnace to have high intermediate frequency voltage and voltage limiting, and the melting speed will slow down if the power does not rise.

3. Strengthen Maintenance

3.1 Routine Maintenance

(1) Furnace: first measure the furnace depth and the maximum diameter of the furnace and make records before opening, carefully check whether the furnace lining is cracked or damaged, repair the damaged furnace mouth and nozzle in time, and detect the furnace leakage alarm device after the first furnace is melted.

(2) Electrical: comprehensively check the control main board, pulse main board, thyristor, resistance, capacitance, reactor, copper bar, commutation inductance, etc. to ensure that there is no open circuit, short circuit, looseness, faulty soldering, liquid leakage, etc.

(3) Dust: the periphery of the circuit breaker, thyristor, reactor, capacitor and induction coil shall be thoroughly purged with air.

(4) Circulating water: the valve switch is accurate, the water pipe has no leakage, the water pressure is 0.1~0.2MPa, the water level is 1/3~1/2, the inlet temperature is not more than 35 ℃, and the outlet temperature is not more than 65 ℃. The conductivity of pure water shall not exceed 600 μ Ω/cm。

3.2 Regular Maintenance

(1) Element measurement: each time the rectifier element is installed, the thyristor control electrode and the cathode resistance should be measured (15~50 Ω is normal). The series with close resistance values should be installed together. The resistance between the cathode and the cathode should be tens of megohm, which is intact. The resistance value is infinite or more than zero. This element is considered to be damaged.

(2) System calibration: comprehensively calibrate the DC voltage, DC current, IF voltage, commutation angle, overcurrent setting and overvoltage setting of the system every three months.

(3) Dust cleaning: wipe the dust on the surface of the thyristor, resistor, control panel and capacitor insulator with a brush every month.

(4) Bolt fastening: fully tighten the connections of the reactor, resistor, reactor, copper bar, water-cooled cable, etc.

(5) Pipeline cleaning: backwash the soft water circulating water pipes and components once every two months, and reinstall the new pure water; Backwash the external circulating water pipe, induction coil and magnetic yoke once every six months. All the aging rubber hoses shall be replaced, and the seriously rusted valves shall be updated in time.

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