Analysis of common faults of intermediate frequency power supply

Jun 20, 2022

In general, the faults of intermediate frequency power supply can be divided into two categories: completely unable to start and unable to work normally after starting. As a general principle, in case of failure, the whole system shall be comprehensively inspected in case of power failure, which includes the following aspects:

(1) Power supply: use a multimeter to measure whether there is power behind the main circuit switch (contactor) and control fuse, which will eliminate the possibility of open circuit of these components.

(2) Rectifier: the rectifier adopts 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. Normally, the indicator is retracted in the shell. When the quick fuse is burnt out, it will pop up. Some quick fuse indicators are tight. When the quick fuse is burnt out, it will get stuck in it. Therefore, for the sake of reliability, the quick fuse can be measured with the on-off gear of the multimeter to judge whether it is burnt out.

The simple method to measure the thyristor is to use the multimeter resistance block (200 Ω block) to measure the cathode anode and gate cathode resistance. The thyristor does not need to be removed during measurement. Under normal conditions, the resistance between anode and cathode shall be infinite, and the resistance between gate and cathode shall be 10-50 Ω. Too large or too small indicates that this thyristor gate is invalid, 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. The resistance of the primary side measured with a multimeter is about 50 Ω. Freewheeling diode is generally not prone to failure. During inspection, the diode of multimeter 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 direction is blocked.

(3) Inverter: the inverter includes four fast thyristors and four pulse transformers, which can be checked according to the above methods.

(4) Transformer: each winding of each transformer should be connected. Generally, the primary side resistance is about tens of ohms, and the secondary pole resistance is several ohms. It should be noted that the primary side of the intermediate frequency voltage transformer is connected in parallel with the load, so its resistance value is zero.

(5) Capacitor: the electric heating capacitor connected in parallel with the load may be broken down. The capacitor is generally installed on the capacitor rack in groups. During inspection, the group of the broken down capacitor shall 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 busbar of each group of capacitors, which shall be infinite under normal conditions. After confirming the bad bank, disconnect the soft copper sheet from each electric heating capacitor to the busbar, and check 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 disconnected. This 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 attention should be paid to fire prevention.

The angle steel on which the capacitor is installed is insulated from the capacitor frame. If insulation breakdown occurs, the main circuit will be grounded. The insulation condition of this part can be judged by measuring the resistance between the lead of the capacitor shell and the capacitor frame.

(6) Water cooling cable: the function of water cooling cable is to connect intermediate frequency power supply and induction coil. It uses each diameter Φ 0.6– Ф 0.8 stranded copper wire. For 500 kg electric furnace, the cable cross-sectional area is 480 mm2, and for 250 kg electric furnace, the cable cross-sectional area is 300 to 400 mm2. The outer rubber tube of the water-cooled cable is a 5kg pressure rubber tube, which is filled with cooling water. It is part of the load circuit. It is subject to tension and torsion during operation and tilts with the furnace body, resulting in twists and turns. Therefore, it is easy to break at the flexible connection after a long time. During the breaking process of water-cooled cables, most of them are cut off first, and then the small part that is not cut off is burned off quickly during high-power operation. At this time, the medium frequency power supply will generate high overvoltage. If the overvoltage protection is not reliable, the thyristor will be burned out. After the water-cooling cable is disconnected, the intermediate frequency power supply cannot be started. If it is started repeatedly without checking the cause, it is likely to burn out the intermediate frequency voltage transformer. Use an oscilloscope to check the fault, clamp the oscilloscope probe at both ends of the load, and observe whether there is attenuation waveform when pressing the start button. Disconnect the water-cooled cable from the output copper bar of the electric heating capacitor when determining the cable core breaking, and measure the resistance value of the cable with the multimeter resistance block (200 Ω block). The resistance value is zero in normal condition and infinite in case of breaking. When measuring with a multimeter, turn the furnace body over to the dumping position to make the water-cooling cable fall off. In this way, the broken part is completely separated, so as to correctly judge whether the core is broken.