Principle and maintenance of medium frequency melting furnace
The working principle of the medium frequency power supply is: a three-phase bridge-type fully-controlled rectifier circuit is used to rectify the alternating current into a direct current. After being smoothed by a reactor, it becomes a constant direct current source, and then the direct current is inverted through a single-phase inverter bridge. A single-phase medium frequency current with a certain frequency (generally 1000 to 8000 Hz). The load is composed of an induction coil and a compensation capacitor, which are connected into a parallel resonant circuit.
Under normal circumstances, the fault of the medium frequency power supply can be divided into two categories: completely unable to start and unable to work normally after starting. As a general principle, when a fault occurs, the entire system should be inspected completely in the event of a power failure, which includes the following aspects:
(1) Power supply: Use a multimeter to check whether there is electricity behind the main circuit switch (contactor) and control fuse. This will rule out the possibility of disconnection of these components.
(2) Rectifier: The rectifier uses a three-phase fully controlled bridge rectifier circuit, which includes six fast fuses, six thyristors, six pulse transformers and a freewheeling diode. There is a red indicator on the quick-acting fuse. Normally, the indicator is retracted inside the shell. When the quick-acting blows, it will pop up. Some quick-acting indicators are tight. When the quick-acting blows, it will get stuck inside. , So for the sake of reliability, you can use a multimeter to test the fast-blow on/off gear to determine whether it is blown.
The simple way to measure the thyristor is to measure its cathode-anode and gate-cathode resistance with a multimeter electrical barrier (200Ω block), and the thyristor does not need to be removed during the measurement. Under normal circumstances, the anode-cathode resistance should be infinite, and the gate-cathode resistance should be between 10-50Ω. Too large or too small indicates that the gate of this thyristor fails, and it cannot be triggered to conduct.
The secondary side of the pulse transformer is connected to the thyristor, the primary side is connected to the main control board, and the primary resistance is about 50Ω measured with a multimeter. The freewheeling diode is generally not prone to failure. Use a multimeter diode to measure its two ends during inspection. The multimeter shows that the junction voltage drop is about 500mV in the forward direction, and the reverse direction is blocked.
(3) Inverter: The inverter includes four fast thyristors and four pulse transformers, which can be inspected according to the above methods.
(4) Transformer: Each winding of each transformer should be connected. Generally, the resistance of the primary side is about tens of ohms, and the secondary resistance is a few ohms. It should be noted that the primary side of the medium frequency voltage transformer is connected in parallel with the load, so its resistance value is zero.
(5) Capacitors: Capacitors connected in parallel with the load may be broken down. Capacitors are generally installed in groups on the capacitor rack. When checking, determine the group where the broken capacitor is located. Disconnect the connection point between the bus bar of each group of capacitors and the main bus bar, and measure the resistance between the two bus bars of each group of capacitors. Normally, it should be infinite. After confirming the bad group, disconnect the soft copper skin that each capacitor leads to the bus bar, and check each one to find the capacitor that has broken down. Each capacitor is composed of four cores. The shell is one pole, and the other pole is led to the end cap through four insulators. Generally, only one core will be broken down. The capacitor can continue to be used, and its capacity is 3/4 of the original. Another fault of the capacitor is oil leakage, which generally does not affect the use, but pay attention to fire prevention.
The angle steel where the capacitor is installed is insulated from the capacitor frame. If the insulation breakdown will ground the main circuit, measure the resistance between the capacitor shell lead and the capacitor frame to determine the insulation status of this part.
(6) Water-cooled cable: The function of the water-cooled cable is to connect the medium frequency power supply and the induction coil. It is formed by twisting each diameter Φ0.6-Ф0.8 copper wire. For a 500 kg electric furnace, the cable cross-sectional area is 480 square millimeters, and for a 250 kg electric furnace, the cable cross-sectional area is 300 to 400 square millimeters. The outer hose of the water-cooled cable adopts a pressure rubber hose with a pressure resistance of 5 kg, which is filled with cooling water. It is a part of the load circuit. It is subjected to tensile and torsion forces during operation. The connection is broken. The breaking process of the water-cooled cable generally involves cutting off most of the parts first, and then quickly breaking the unbroken part during high-power operation. At this time, the medium frequency power supply will generate a very high overvoltage. If the overvoltage protection is unreliable, Will burn out the thyristor. After the water-cooled cable is disconnected, the medium frequency power supply cannot start to work. If you restart repeatedly without finding out the cause, the medium frequency voltage transformer is likely to be burned out. When checking the fault, use an oscilloscope to clamp the oscilloscope probe at both ends of the load and observe whether there is an attenuation waveform when the start button is pressed. When confirming that the cable is broken, first disconnect the water-cooled cable from the capacitor output copper bar, and measure the resistance of the cable with a multimeter (200Ω block). The resistance value is zero when it is normal, and it is infinite when it is disconnected. When measuring with a multimeter, the furnace body should be turned to the dumping position to make the water-cooled cable fall off, so that the broken part can be completely separated, so that it can be correctly judged whether the core is broken.
Through the above inspections, most of the causes of failures can generally be found, and then the control power supply can be turned on for further inspections. The main circuit of the medium frequency power supply can be closed manually and automatically. For automatic closing systems, the power cord should be temporarily disconnected to ensure that the main circuit will not be closed. After turning on the control power, you can check the following aspects.
