The medium frequency power supply control cabinet fails to start and an alarm is triggered.

For this fault phenomenon, first check the power switch and control switch to confirm whether they are closed; if an alarm is triggered after the control switch is turned on, perform the following treatment according to the alarm information.

(1) Phase loss alarm (OP) indicator is on. Check whether the fuse of the main circuit is blown; if it works normally, check the three-phase incoming line, and use the 500V AC voltage gear of a multimeter to detect whether there is phase loss. If it is still normal, check whether the cathode (K) wiring of the rectifier thyristor is open-circuited.

(2) Water shortage alarm (WPL) indicator is on. Check whether the water pump, water pipeline and electric contact pressure gauge work normally.

(3) Undervoltage alarm (LV) indicator on the main control board is on. Check whether the control power transformer works normally; if it is normal, check whether the filter capacitor on the main control board has leakage or failure; if it is still normal, use the 50V AC voltage gear of a multimeter to check whether the secondary voltage (17V) of the control power transformer is normal.

(4) Overvoltage (OV) indicator is on. Check whether there is sparking between the smelting workpiece and the induction coil. If sparking occurs, adjust the gap between the induction coil and the smelting workpiece. Check whether the induction coil sparks with oxide scale; if sparking occurs, clean up the oxide scale. Check whether the screws of each connecting copper bar of the induction coil, compensation capacitor and main circuit are loose and cause sparking; if the screws are loose and cause sparking, tighten the screws.

If all the above checks are normal, then check whether the inverter thyristor (KK) is damaged. Use the R×1 gear of a multimeter for detection. If the measured resistance between the cathode and anode of the thyristor is zero, the thyristor is damaged and needs to be replaced. If one (two) inverter thyristors of the medium frequency power supply are damaged, the inverter leading angle is greater than or equal to 2, and the ratio of DC current to DC voltage is much larger than the normal value, the inverter leading angle should be readjusted to 1.5.

(5) Overcurrent (OC) indicator is on. The possible fault causes for the lighting of the OC (overcurrent) indicator are: there are short-circuit points on the induction coil, compensation capacitor and their connecting copper bar; two (four) inverter thyristors are damaged; inverter thyristors have poor performance. If OV and OC indicators are both on, the overhaul shall focus on overvoltage. Use the R×1k gear of a multimeter to detect whether the compensation capacitor is broken down. If the capacitor cannot be charged and discharged normally or its resistance is zero, the compensation capacitor is faulty. If the above checks are normal, it is preliminarily judged that the water-cooled cable is faulty. Use the R×1 gear of a multimeter to detect the resistance of the water-cooled cable. If there is a resistance reading that deviates from the normal, the water-cooled cable is faulty. The detection of water-cooled cable is relatively difficult, and the most direct and effective method is the replacement method, that is, temporarily use a power cable to replace the water-cooled cable. If the water-cooled cable is normal, check whether there is metal debris between turns of the induction coil that causes inter-turn short circuit; if it is normal, check whether the intermediate frequency transformer is faulty, and replace it with a new intermediate frequency transformer if there is a fault.