Generally, faults of medium-frequency power supply can be divided into two categories by fault phenomena: complete failure to start and abnormal operation after startup. As a general rule, when a fault occurs, a comprehensive inspection of the whole system shall be conducted under power-off condition, which includes the following aspects:
1. Power Supply: Use a multimeter to test whether there is power behind the main circuit switch (contactor) and control fuse, which will rule out the possibility of open circuit of these components.
2. Rectifier: The rectifier adopts a three-phase fully-controlled bridge rectifier circuit, which includes six quick fuses, six thyristors, six pulse transformers and a freewheeling diode. There is a red indicator on the quick fuse: the indicator retracts inside the shell under normal condition, and pops out when the quick fuse blows. For some quick fuses, the indicator is relatively tight and will get stuck inside after blowing. Therefore, for reliability, you can use the continuity gear of the multimeter to test the quick fuse to judge whether it is blown.
A simple method for measuring thyristor is to use the resistance gear (200Ω range) of the multimeter to measure the resistance between cathode-anode and gate-cathode, and the thyristor does not need to be removed during measurement. Under normal conditions, the resistance between anode and cathode should be infinite, and the resistance between gate and cathode should be between 10Ω and 50Ω. Too large or too small resistance indicates that the gate of the thyristor is failed and cannot 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 primary side resistance measured by multimeter is about 50Ω. Freewheeling diodes are not prone to faults. When inspecting, use the diode gear of the multimeter to test its two ends: the multimeter displays a junction voltage drop of about 500mV in forward direction, and no conduction in reverse direction.
3. Inverter: The inverter includes four fast thyristors and four pulse transformers, which can be inspected according to the method above.
4. Transformer: Each winding of each transformer should be conductive. Generally, the resistance of the primary winding is about tens of ohms, and that of the secondary winding is several ohms. It should be noted that the primary side of the medium-frequency voltage transformer is parallel to the load, so its resistance value is zero.
5. Capacitor: The heating capacitor parallel to the load may break down. Capacitors are usually installed in groups on the capacitor frame, and the group where the broken-down capacitor is located should be determined first during inspection. Disconnect the connection point between the busbar of each capacitor group and the main busbar, measure the resistance between the two busbars of each capacitor group, and the resistance should be infinite under normal condition. After confirming the faulty group, disconnect the flexible copper strip leading each heating capacitor to the busbar, and check one by one to find the broken-down capacitor. Each heating capacitor consists of four cores, the shell is one pole, and the other pole is led to the end cover through four insulators respectively. Usually only one core will break down. After removing the lead on this insulator, the capacitor can still be used, and its capacity is 3/4 of the original. Another fault of capacitors is oil leakage, which generally does not affect use, but fire prevention should be noted.
The angle steel for installing capacitors 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 judge the insulation status of this part.
(6) Water-cooled Cable: The function of a water-cooled cable is to connect the medium-frequency power supply and the induction coil, which is twisted from red copper wires with a diameter of Φ0.6–Ф0.8 mm each. For a 500kg electric furnace, the cross-sectional area of the cable is 480 square millimeters, and for a 250kg electric furnace, the cross-sectional area of the cable is 300 to 400 square millimeters. The outer rubber sleeve of the water-cooled cable adopts a pressure-resistant rubber hose rated at 5kg pressure, with cooling water flowing through the interior. It is a part of the load circuit, which is subject to tensile force and torsion during operation, and bends when tilting together with the furnace body. Therefore, it is easy to break at the flexible connection after a long time of use. The fracture process of a water-cooled cable is generally that after most of the cable core is broken, the remaining small unbroken part will be burned out quickly during high-power operation, and the medium-frequency power supply will generate a very high overvoltage at this time. If the overvoltage protection is unreliable, the thyristor will be burned. After the water-cooled cable is broken, the medium-frequency power supply cannot be started. If you repeatedly start the power supply without finding out the cause, it is very likely to burn out the medium-frequency voltage transformer. An oscilloscope can be used when checking the fault: clamp the oscilloscope probe on both ends of the load, and observe whether there is an attenuated waveform when pressing the start button. After confirming the broken core of the cable, first disconnect the water-cooled cable from the output copper busbar of the electric heating capacitor, use the resistance range (200Ω range) of a multimeter to measure the resistance of the cable. The resistance is zero under normal conditions and infinite when the cable is broken. When measuring with a multimeter, the furnace body should be turned to the tilted position to make the water-cooled cable hang, so that the broken part is completely separated, and you can correctly judge whether the core is broken.
