What are the common faults of medium frequency induction melting furnace

Medium frequency electric furnaces are widely used in related fields such as steel, casting, special steel, etc. Different application fields have different requirements for medium frequency power supplies. Only on the basis of mastering the basic working principles of these circuits and the basic characteristics of power devices, can we quickly and accurately analyze and judge the cause of the fault and take effective measures to eliminate the fault.

1. Turn on the machine, the equipment cannot start normally

Fault phenomenon: The DC current is large during startup, the DC voltage and medium frequency voltage are low, and the equipment sound is dull.

Analysis and processing: The thyristor in one arm of the inverter bridge may be short-circuited or open-circuited, causing the inverter bridge to operate in a three-arm bridge. Use an oscilloscope to observe the thyristor tube voltage drop waveform on the four arms of the inverter bridge. If the tube voltage drop waveform of the thyristor on one arm is a line, the thyristor has been through; if it is a sine wave, the thyristor is not turned on. Replace the thyristor that has been worn; find out the reason why the thyristor is not turned on.

2. The equipment can start, but the working state is not right

Fault phenomenon: The equipment can start normally and smoothly. When the power rises to a certain value, overvoltage or overcurrent protection occurs.

Analysis and processing: Find the cause of the fault in two steps:

1) First run the equipment without load to observe whether the voltage can rise to the rated value. If the voltage cannot rise to the rated value, and overcurrent protection occurs near a certain voltage value many times. This may be caused by insufficient pressure of the compensation capacitor or thyristor, but it is not ruled out that it is caused by sparking in a certain part of the circuit;

2) If the voltage can rise to the rated value, the equipment can be switched to heavy load operation to observe whether the current value can reach the rated value. If the current cannot rise to the rated value, and overcurrent protection occurs near a certain current value many times, this may be caused by large current interference. Pay special attention to the interference of the electromagnetic field of medium-frequency large current on the control part and signal line.

3. Faults that are easy to occur when the equipment is running normally

Fault phenomenon: The equipment runs normally, but when the normal overcurrent protection is activated, multiple KP thyristors and fast fuses are burned.

Analysis and treatment: During overcurrent protection, in order to release the energy of the smoothing reactor to the power grid, the rectifier bridge switches from the rectifier state to the inverter state. At this time, if а>1500, it may cause active inverter subversion, burn multiple thyristors and fast fuses, and trip the switch, accompanied by a huge current short-circuit explosion. A large current and electromagnetic force impact is generated on the transformer, which may damage the transformer in severe cases.