Failure analysis of intermediate frequency power supply

The working principle of the intermediate frequency power supply is: a three-phase bridge type fully controlled rectifier circuit is used to rectify the alternating current into direct current. After smoothing by the reactor, it becomes a constant direct current source, and then the single-phase inverter bridge inverts the direct current. into a single-phase medium frequency current of a certain frequency (generally 1000 to 8000Hz). The load consists of an induction coil and a compensation capacitor, which are connected into a parallel resonant circuit. Under normal circumstances, the faults of the intermediate frequency power supply can be divided into two categories according to the fault phenomenon: complete inability to start and inability to work normally after starting. As a general principle, when a fault occurs, a comprehensive inspection of the entire system should be carried out under the condition of power outage, which includes the following aspects:

1. Water-cooled cable: The role of the water-cooled cable is to connect the intermediate frequency power supply and the induction coil. It is made of twisted copper wires with a diameter of Φ0.6–Φ0.8 each. 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 rubber tube of the water-cooled cable is a pressure rubber tube with a pressure resistance of 5 kg, with cooling water flowing inside. It is part of the load circuit and is subject to tension and torsion during operation. It tilts together with the furnace body and twists, so it is easy to become flexible after a long time. The connection is broken. The process of water-cooled cable rupture is usually to break most of it first, and then quickly burn out the small unbroken part during high-power operation. At this time, the intermediate frequency power supply will generate a very high overvoltage. If the overvoltage protection is unreliable, it will Will burn out the thyristor. After the water-cooling cable is disconnected, the intermediate frequency power supply cannot start working. If the cause is not checked and started repeatedly, the medium frequency voltage transformer is likely to be burned out. When checking for faults, you can use an oscilloscope. Clamp the oscilloscope probes at both ends of the load and observe whether there is attenuation of the waveform when the start button is pressed. When confirming that the cable core is broken, first disconnect the water-cooled cable from the output copper bar of the electrothermal capacitor, and use a multimeter to measure the resistance value of the cable. The resistance value is zero under normal conditions and infinite when disconnected. When measuring with a multimeter, the furnace body should be turned over to the dumping position to let the water-cooled cable fall up, so that the broken point is completely separated, so that it can be correctly judged whether the core is broken.

2. Power supply: Use a multimeter to test whether there is power behind the main circuit switch (contactor) and control fuse. This will rule out the possibility of circuit breakage in these components.

3. 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-blow fuse. Normally, the indicator is retracted inside the shell. When the quick-blow blows out, it will pop out. Some quick-blow indicators are tighter. When the quick-blow blows out, it will get stuck inside. , so for the sake of reliability, you can use a multimeter to test the fast fuse to determine whether it is burned out. The simple way to measure the thyristor is to use a multimeter to measure the electrical resistance (200Ω block) of its cathode-anode and gate-cathode resistance. There is no need to remove the thyristor during measurement. Under normal circumstances, the resistance between the anode and the cathode should be infinite, and the resistance between the gate and the cathode should be between 10-50Ω. If it is too large or too small, it indicates that the gate of this thyristor has failed and it will not be triggered to conduct. The secondary side of the pulse transformer is connected to the thyristor, and the primary side is connected to the main control board. Use a multimeter to measure the primary side resistance to be about 50Ω. Freewheeling diodes are generally not prone to failure. During inspection, use a multimeter to measure the two ends of the diode. In the forward direction, the multimeter shows that the junction voltage drop is about 500mV, but in the reverse direction, there is no connection.

4. Inverter: The inverter includes four fast thyristors and four pulse transformers, which can be checked according to the above method.

5. Capacitor: The electrothermal capacitor connected in parallel with the load may be broken down. Capacitors are generally installed in groups on the capacitor rack. During inspection, the group where the broken down capacitor is located 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 electric heating capacitor to the busbar, and check each unit one by one to find the broken capacitor. Each electric heating capacitor is composed of four cores. The shell is one pole, and the other pole is led to the end cover through four insulators. Generally, only one core will be broken down, and the lead on this insulator will be jumped. The capacitor can continue to be used, and its capacity is 3/4 of its original capacity. Another fault of the capacitor is oil leakage, which generally does not affect use, but attention should be paid to fire prevention. The angle steel on which the capacitor is installed is insulated from the capacitor frame. If the insulation breaks down, the main circuit will be grounded. Measuring the resistance between the capacitor shell lead and the capacitor frame can determine the insulation condition of this part.

6. Transformer: Each winding of each transformer should be connected. Generally, the primary side resistance is about dozens of ohms and the secondary pole resistance is several 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.