An electrician equipped with an intermediate frequency furnace manufacturer wants to be able to quickly and accurately solve the failures in the operation of the induction melting furnace and ensure the continuity of production. How can there be an maintenance manual for the melting furnace? I have not been exposed to the induction melting furnace before, and the manufacturer's manual has only a few descriptions. It doesn't matter if you follow these steps, I believe you will be able to find out where the problem is.
The thyristor medium frequency induction heating power source uses a thyristor to convert three-phase power frequency AC power into a single-phase AC power of several hundred or several kilohertz. It has the characteristics of convenient control, high efficiency, reliable operation and low labor intensity. It is widely used in the smelting of cast steel, stainless steel or alloy steel, vacuum smelting, heating of forgings and bending of steel pipes, extrusion molding, preheating of workpieces, steel. Surface quenching, annealing heat treatment, welding of metal parts, powder metallurgy, pipe heating for transporting high temperature working fluids, crystal growth, etc.
The working principle of the intermediate frequency power supply is: using a three-phase bridge type full control rectifier circuit to rectify the alternating current into direct current, after the reactor is flat wave, it becomes a constant direct current source, and then through the single-phase inverter bridge, the direct current is inverted. A single-phase intermediate frequency current at a certain frequency (typically 1000 to 8000 Hz). The load consists of an induction coil and a compensation capacitor connected in a parallel resonant circuit.
Under normal circumstances, the faults of the intermediate frequency furnace can be divided into two categories according to the fault phenomenon, which cannot be started completely and cannot be operated normally after starting. As a general principle, when a fault occurs, the entire system should be thoroughly checked in the event of a power outage, which includes the following aspects:
1、power supply
Use a multimeter to measure the main circuit switch (contactor) and control the fuse to eliminate the presence or absence of these components.
2、Rectifier:
The rectifier uses a three-phase fully controlled bridge rectifier circuit that includes six fast fuses, six thyristors, six pulse transformers, and a freewheeling diode.
A simple way to measure a thyristor is to measure its cathode-anode, gate-cathode resistance with a multimeter electrical barrier (200Ω block). The thyristor does not need to be removed during measurement. Under normal circumstances, the anode-cathode resistance should be infinite, and the gate-cathode resistance should be between 10 and 50 Ω. Too large or too small indicates that the thyristor gate is ineffective and will not be triggered to conduct.
3、Inverter:
The inverter consists of four fast thyristors and four pulse transformers, which can be inspected as described above.
Each winding of each transformer should be open. Generally, the primary side resistance is about tens of ohms and the secondary is a few ohms. It should be noted that the primary side of the IF voltage transformer is connected in parallel with the load, so its resistance is zero.
The electrothermal capacitors in parallel with the load may be broken down. The capacitors are generally grouped and mounted on the capacitor holder. The inspection should be performed to determine the group in which the capacitor is broken. Disconnect the connection point between the busbar of each group of capacitors and the main busbar, and measure the resistance between the two busbars of each group of capacitors. Normally, it should be infinite. After confirming the bad group, disconnect the soft copper from each electric capacitor to the busbar, and check the breakdown capacitors one by one. Each electric heating capacitor is composed of four cores, the outer casing is one pole, and the other pole is respectively led to the end cover through four insulators. Generally, only one core is broken down, and the lead wire on the insulator is tripped. The capacitor can continue to be used, and its capacity is 3/4 of the original. Another failure of the capacitor is oil leakage, which generally does not affect the use, but pay attention to fire prevention.
The angle of the capacitor is insulated from the capacitor holder. If the insulation breakdown will ground the main circuit, the resistance between the capacitor housing lead and the capacitor holder can be measured to determine the insulation condition of this part.
The function of the water-cooled cable is to connect the intermediate frequency power supply and the induction coil. It is twisted with each diameter Φ0.6–Ф0.8 copper wire. For a 500 kg electric furnace, the cable cross-sectional area is 480 mm 2 , and for a 250 kg electric furnace, the cable cross-sectional area is 300 to 400 mm 2 . The outer tube of the water-cooled cable is made of pressure rubber tube with a pressure of 5 kg. The inside is connected with cooling water. It is part of the load circuit. It is subjected to tension and torsion when working. It is twisted and twisted together with the furnace body, so it is easy to be flexible after a long time. The joint breaks open.
The water-cooled cable break process generally breaks off most of the time, and the uninterrupted small part is quickly blown during high-power operation. At this time, the intermediate frequency power supply will generate a high overvoltage. If the overvoltage protection is unreliable, The thyristor will burn out. After the water-cooled cable is disconnected, the IF power supply cannot start working. If you repeatedly start without checking the cause, it is likely to burn out the IF voltage transformer. The oscilloscope can be used to check the fault, and the oscilloscope probe is clamped at both ends of the load to observe whether there is any attenuation waveform when the start button is pressed. When determining the core breakage of the cable, first disconnect the water-cooled cable from the output copper discharge of the electric heating capacitor, and measure the resistance value of the cable with a multimeter electric block (200Ω block). When normal, the resistance value is zero, and when disconnected, it is infinite. When measuring with a multimeter, the furnace body should be turned to the dumping position to lift the water-cooled cable, so that the broken part can be completely separated, and the core can be correctly judged.
Article Source：www.firstfurnace.com
