The working frequency of medium frequency induction furnace (hereinafter referred to as medium frequency furnace) is between 50-10hz, which is widely used for smelting non-ferrous metals and ferrous metals. Compared with other casting equipment, medium frequency induction furnace has the advantages of high thermal efficiency, short melting time, less alloy element burning loss, wide melting materials, less environmental pollution, and accurate control of the temperature and composition of molten metal.

1. Working principle of medium frequency furnace due to the above advantages, medium frequency furnace, as the main smelting equipment in the foundry, has been more and more applied. The medium frequency furnace is mainly composed of a power supply, an induction ring and a crucible made of refractory materials in the induction ring. There is a metal charge in the crucible, which is equivalent to the auxiliary winding of the transformer. When the induction coil is connected to the AC power supply, an alternating magnetic field is generated in the induction coil, and its magnetic line of force cuts the metal charge in the crucible, and an induced electromotive force is generated in the charge. Because the charge itself forms a closed loop, the auxiliary winding is characterized by only one turn and is closed. Therefore, induction current is generated in the charge at the same time. When the induction current passes through the charge, it heats the charge to promote its melting.

2 fault analysis of medium frequency furnace

2.1 rectifier fault

(1) The input three-phase current is unbalanced, the smoothing reactor has high vibration and noise, and the power cannot rise. The main reason for this failure is fast melting. After the fast melting is found, check whether the damaged fast melting is hot and hot, and then check whether the rectifier thyristor has breakdown after eliminating the cause of the fast melting itself.

(2) The DC voltage drops unsteadily, the smoothing reactor has high vibration and noise, and the power cannot rise. The main reason for this fault phenomenon is the breakdown of rectifier thyristor. After the rectifier thyristor breakdown is found, check whether there is a problem with the fast melting first, and then replace the rectifier thyristor.

(3) The shock sound of smoothing reactor is loud. The cause of this fault phenomenon is complex. First, check whether the ratio of intermediate frequency voltage to DC voltage is between 1.3 ~ 1.5. If it is within the range of normal ratio, check whether the indicator light of rectifier thyristor trigger pulse amplifier board is normal and whether the insulation of smoothing reactor itself is normal; If it is not within the normal ratio range, check the inverter side.

(4) The rectified DC voltage is low or the voltage jumps. This fault may be caused by insufficient trigger pulse width, poor performance of rectifier thyristor, faulty soldering of rectifier trigger plate, or poor contact of power regulating potentiometer.

(5) Three phase circuit breaker protection action, rectifier thyristor breakdown, serious damage to most of the rectifier circuit components. This fault belongs to the malignant fault of the rectifier circuit. It is suspected that the primary and secondary sides of the pulse transformer are broken down and short circuited. First use the resistance gear R of the multimeter × Measure the six thyristors in the three-phase fully controlled rectifier trigger circuit with a resistance of 0 in the 1 Ω range. Do not rush to power on after replacing the thyristor. Check the pulse transformer to see whether it plays the role of voltage transformation and isolation. Replace it.

2.2 inverter fault

2.2.1 if there is no voltage and the medium frequency current is large, the fault is inverter through. First check whether the inverter thyristor has breakdown. If abnormal, replace the inverter thyristor; If it is normal, it is usually the breakdown of intermediate frequency capacitors, and the capacitors should be separated from the circuit one by one until the power supply is normal.

2.2.2 the inverter thyristor is frequently burnt out and irregular during normal smelting. The reasons are as follows

(1) The heating of the thyristor itself increases the turn-off time and cannot be turned off, resulting in inverter overturning. The reason for heating may be that the cooling water is not unobstructed.

(2) Check whether the external load has a virtual contact. Mainly check the connecting screws (including the connecting plate screws of the capacitor bank) and the secondary connection of the current transformer with capacitive current is loose, which seems to be connected but not connected. The above two situations have occurred.

(3) Poor contact at the connection of the main circuit, especially the contact parts of the load circuit, the main contactor and the low-voltage distribution cabinet, caused the sudden disconnection of the circuit under high current.

2.3 capacitor failure

(1) There is a spark inside the capacitor. When the machine is running, there is a noise inside. It is insulation breakdown. Replace it with a new one. (2) The capacitor is ignited externally. When the machine is running, the capacitor shell is poorly insulated from the ground. The insulation resistance of capacitor is usually measured by two or more levels of insulation resistance to the shell. Discharge before telemetering. Take 750V as an example, use a 1000V megger. When telemetering, shake the megger to the specified speed first. After the pointer is stable, connect the megger to two levels of capacitor, continue the megger, start charging, and the pointer drops. Later, it slowly rose to stability, generally 1000V is not less than 1 megohm. After reading the number, remove the meter and discharge the capacitor to avoid electric shock. (3) Heating and insulation damage caused by blocked cooling water should be handled in time. The main circuit of intermediate frequency power supply adopts thyristor devices, which have poor ability to withstand overvoltage and overcurrent. The operation of intermediate frequency power supply is also complex, with strong load changes, and the probability of short circuit, open circuit, overvoltage and overcurrent is high. Protective measures must be taken properly to ensure the safe operation of the device. Protective devices can be installed in appropriate places, such as installing RC absorption circuit at the incoming side of power frequency power supply, or connecting fast fuses in series on thyristors to protect current. This method is commonly used in practice.

3 maintenance of medium frequency furnace

3.1 for dry baking, the steel mold in the oven is not taken out. When baking the oven, the principle of adding small materials and filling must be adopted to avoid local ignition and melting of the liner, so that the dry materials collapse. Therefore, it is required to accumulate some small materials in front of the oven as much as possible. At the same time, the steel formwork shall be strictly observed during the heating process. When there is serious arcing, the current shall be reduced. If the current is too high, the steel formwork will be broken down by the arc and lead to the collapse of refractory materials. 3.2 metal charge

Generally, when adding metal charge, the size of lump should be appropriate, generally not more than 1/2 of the inner diameter of the furnace, and the charge should be added in small batches and many times; When adding large materials, it is necessary to add some small materials at the bottom first, which can not only ensure that there is no jamming, but also reduce the impact of furnace materials on furnace wall and bottom.

3.3 remelting of molten iron after solidification

When the medium frequency furnace is shut down due to an emergency and the molten iron in the furnace solidifies, attention should be paid to:

(1) If the medium frequency furnace cannot be restarted in a short time, 3-5 steel pipes (with a diameter of about 20mm) should be inserted into the furnace to facilitate the emission of gas during the remelting process.

(2) Before the top of the solidified molten iron in the furnace melts, the furnace body should be shaken up, tilted 15-30, and powered with a insulation power lower than that of the medium frequency furnace.

(3) The power shall not be increased before the solidified metal in the furnace is softened. If the temperature rises too fast, the expansion of the metal will exceed the expansion of the furnace lining, which can crack the furnace lining and lead to furnace penetration in serious cases; After softening the solidified metal, the power can be increased to about the insulation power until the metal shell above the condensed metal is melted through by the liquid metal below, then the furnace body can be leveled, and the temperature can be heated and melted according to the normal power.

3.4 discharging

When the molten iron is discharged from the furnace, the remaining molten iron in the furnace can be appropriately more without affecting the material and feeding without splashing, which can not only accelerate the melting speed of the metal furnace charge, but also avoid the furnace charge directly hitting the furnace bottom. After a package of molten iron is discharged from the furnace, the furnace body should be shaken back to reduce the baking of the molten iron in the furnace to the rear wall of the medium frequency furnace.

3.5 continuous furnace opening

In order to reduce the cracking of furnace lining caused by repeated sudden heating and cooling, continuous blowing in is very important to prolong the service life of furnace lining, which has attracted enough attention of many users.

If the medium frequency furnace cannot be used continuously, after one use, add metal charge into the furnace and cover the furnace cover, which can not only preheat the metal charge, but also prevent cracks in the furnace lining caused by rapid cooling.

3.6 reheating

(1) When the medium frequency furnace is opened to about 20 heats, all the furnace mouths should be removed in order to check whether the furnace lining is drilled and whether the dry materials of the furnace lining are loose and have gaps. When repairing the furnace mouth, it is best to use metal furnace mouth rings with different heights, and pay special attention to the cleaning of the old and new interfaces. When carrying out the next furnace repair operation, it should be removed to the bottom of the last furnace repair interface, and so on.

(2) The furnace mouth and mouth are places that are often slagged, corroded and impacted by furnace charge, which need regular maintenance and repair to ensure the normal maintenance and use of the medium frequency furnace.

epilogue

In a word, there may be many faults and problems in the use of medium frequency furnace, which is worth our continuous exploration and learning. We should constantly summarize them to improve the maintenance speed and quality.