Analysis of signs of abnormal operation of the intermediate frequency power supply after startup

1. The three-arm inverter is working: The fault is manifested by a particularly large output current, even when the furnace is empty, and the power cabinet makes a heavy sound when it is working. After starting, adjust the power knob to the minimum position, and you will find that the intermediate frequency output voltage is higher than normal. . Use an oscilloscope to observe the voltage waveforms between the anode and cathode of the four inverter thyristors in sequence. If the three bridge arms are working, you can see that the waveforms of the two adjacent thyristors in the inverter are normal. In addition, only one of the two adjacent thyristors has no waveform, and the other one is a sine wave. As shown in Figure 4, KK2 triggers If there is no conduction, the waveform between the anode and the cathode is a sine wave; at the same time, the failure of KK2 will cause KK1 to be unable to turn off, so there will be no waveform at both ends of KK1.

2. Induction coil failure: The induction coil is the load of the intermediate frequency power supply. It is made of square copper tubes with a wall thickness of 3 to 5 mm. Its common faults include the following: The induction coil leaks, which may cause sparks between turns of the coil, and must be repaired and welded in time before it can operate. The molten steel sticks to the induction coil and the steel slag becomes hot and red, which will cause the copper pipe to burn through and must be removed in time. Short circuit between turns of the induction coil. This type of fault is particularly likely to occur on small medium frequency induction furnaces. Because the furnace is small, it is deformed by thermal stress during operation, resulting in a short circuit between turns. The fault is manifested by a larger current and a higher operating frequency than usual. .

3. Rectifier phase loss: The fault is manifested as abnormal sound during operation, the maximum output voltage does not rise to the rated value, and the strange noise from the power supply cabinet becomes louder. At this time, the output voltage can be lowered to about 200V, and the output voltage waveform of the rectifier can be observed with an oscilloscope. (The oscilloscope should be set to power synchronization). Normally, the input voltage waveform has six waveforms per cycle. When the phase is missing, there will be two missing. This fault is generally caused by a certain thyristor in the rectifier not triggering a pulse or triggering non-conduction. At this time, you should first use an oscilloscope to check the gate pulses of the six rectifier thyristors. If there are any, use the 200Ω range of a multimeter to measure each gate resistance after shutting down. Just replace the thyristor that is blocked or has a particularly large gate resistance.