Intermediate frequency furnace principle
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 the direct current is inverted by the single-phase inverter bridge. 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 fault of the intermediate frequency power supply 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 whether there is power behind the main circuit switch (contactor) and the control fuse. This will eliminate the possibility of disconnection of these components.
(2) Rectifier: The rectifier adopts a three-phase fully controlled bridge rectifier circuit, which includes six fast fuses, six thyristors, six pulse transformers and one freewheeling diode. There is a red indicator on the quick fuse. When the indicator is normal, it will shrink inside the housing. When the fuse is blown, it will pop out. Some fast-melting indicators are tight. When the fuse is blown, it will get stuck inside. Therefore, for the sake of reliability, you can use the multimeter on/off to measure the fast-melting to determine if it is blown.
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 fails and it cannot be triggered to conduct.
The pulse transformer is connected to the thyristor on the secondary side, and the primary side is connected to the main control board. The primary side resistance is about 50Ω measured by a multimeter. The freewheeling diode is generally not prone to failure. When checking, the multimeter diode is used to block the two ends. In the forward direction, the multimeter shows that the junction voltage drop is about 500mV, and the reverse is not possible.
(III) Inverter: The inverter includes four fast thyristors and four pulse transformers, which can be inspected as described above.
(4) Transformer: Each winding of each transformer should be open. Generally, the primary side resistance is about several tens of ohms, and the second pole 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.
(5) Capacitors: Capacitors connected in parallel with the load may be broken down. Capacitors are generally grouped and mounted on the capacitor holder. The group in which the capacitor is broken down should be determined first. 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 capacitor to the busbar and check the breakdown capacitors one by one. Each capacitor consists of four cores, one pole is the outer pole, and the other pole is led to the end cap through four insulators. Generally, only one core is broken down, and the lead on the insulator is tripped. Capacitors can continue to be used, with a capacity of 3/4. Another fault 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 and measure the resistance between the capacitor housing lead and the capacitor holder, the insulation condition of this part can be judged.
(6) Water-cooled cable: The function of the water-cooled cable is to connect the intermediate 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 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 off 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. When checking the fault, use the oscilloscope, clip the oscilloscope probe to both ends of the load, and observe whether there is any attenuation waveform when pressing the start button. When determining the cable breakage, first disconnect the water-cooled cable from the capacitor output copper strip, and measure the resistance value of the cable with a multimeter electric block (200Ω block). The resistance value is zero when normal, and is infinite when disconnected. When measuring with a multimeter, the furnace body should be turned to the dumping position, so that the water-cooled cable is dropped, so that the broken part can be completely separated, and the core can be correctly judged.
Through the inspection of the above aspects, it is generally possible to find out most of the causes of the failure, and then the control power can be turned on for further inspection. The main circuit of the intermediate frequency power supply is closed manually or automatically. For systems that are automatically closed, the power cord should be temporarily disconnected to ensure that the main circuit does not close. After the control power is turned on, the following aspects can be checked.