A 500kW induction melting furnace can be started smoothly, and the intermediate frequency voltage can also rise with the adjustment of the power potentiometer, and the ammeter, voltmeter, power meter, and frequency meter indication are normal, and the workpiece can be heated. However, when the intermediate frequency voltage rises to 500V, there is a loud bang, and there are no indications. After investigation, three KK inverter thyristors have been burned. At this point, referring to Figure 1, the induction melting furnace can be started smoothly, indicating that the main control board has no problem. The fault range can be defined in the main rectifier circuit, the inverter circuit and the output load circuit. First, the inverter trigger pulse on the main control board is turned off, only the rectifier circuit works, and then the power is turned on. After the DC voltage is applied, the output can still reach 550V, which is in accordance with the nominal value, and it can be determined that the rectifier circuit has no fault. Then check the inverter circuit and the output load circuit, and perform the withstand voltage test on these two components one by one by the method described above, but no problem is found. Another two intermediate frequency output cables are connected to the load circuit from another induction melting furnace, which can also work normally. The overhaul was in a deadlock. Looking back at the circuit, it was found that there is still a transformer B in the loop that was not detected. Disassemble and use a multimeter and a megohmmeter to check the resistance between the windings and the windings in turn. When the L1 to L4 is found, the withstand voltage is only about 500V. The transformer that is considered to be the least likely to have a problem has failed. Replace this transformer and try the machine again. Experience in repairing induction melting furnaces with megohmmeters.