The working principle of the 400Hz power supply involves using a three-phase bridge full-control rectifier circuit to convert AC power into DC power. After passing through a reactor to smooth the current, it becomes a constant DC current source. Then, through a single-phase inverter bridge, the DC current is inverted into a single-phase intermediate frequency current at a certain frequency (typically between 1000 to 8000 Hz). The load consists of an inductive coil and a compensating capacitor, connected in a parallel resonance circuit.
Generally, the failure of a 400Hz intermediate frequency power supply can be categorized into two main types based on the fault symptoms: complete inability to start and inability to operate normally after starting. As a general rule, when a fault occurs, the entire system should be inspected after the power is turned off.
The water-cooled cable connects a 400Hz intermediate-frequency power supply to an induction coil, and it is made by twisting copper wires with a diameter of Φ0.6–Φ0.8. For a 500kg electric furnace, the cable cross-sectional area is 480 square millimeters, while for a 250kg electric furnace, the area is between 300 to 400 square millimeters. The outer sheath of the water-cooled cable uses a pressure-resistant rubber tube capable of withstanding 5 kg of pressure, through which cooling water flows. It is part of the load circuit, subjected to tensile and torsional forces during operation. As the furnace tilts, the cable bends, making it prone to断裂 at the flexible joints over time. The cable's breakage process usually involves first snapping off the majority, then during high-power operation, the remaining small section is quickly burnt through, causing the 400Hz intermediate-frequency power supply to generate high overvoltage. If the overvoltage protection is unreliable, it can damage the thyristors. Once the water-cooled cable breaks, the 400Hz intermediate-frequency power supply cannot be started. If the cause is not identified, it is likely to burn out the intermediate-frequency voltage transformer. To check for faults, an oscilloscope can be used; attach the probe to both ends of the load and observe for any attenuation in the waveform when the start button is pressed. To determine if the cable has a broken core, first disconnect the water-cooled cable from the output copper bar of the electric heating capacitor, then use a multimeter in the resistance mode (200Ω range) to measure the cable's resistance. Normally, the resistance should be zero, and it should be infinite when the cable is broken. When measuring with a multimeter, the furnace body should be tilted to ensure the water-cooled cable is fully elevated, allowing for a complete disconnection of the break for accurate core breakage assessment.
