Common Faults in Medium-Frequency Melting Furnaces
2021-05-25
After the medium-frequency melting furnace is started, it fails to operate properly, typically manifesting in the following ways:
1. Rectifier Phase Loss: This fault is characterized by abnormal noises during operation, along with a significant drop in output voltage below the rated level. When the power cabinet emits unusual sounds, the output voltage can drop to around 200V. In such cases, you can use an oscilloscope to observe the rectifier’s output voltage waveform (ensure the oscilloscope is connected directly to the power supply). Under normal input voltage conditions, the waveform should show six distinct peaks per cycle; however, if a phase is missing, two of these peaks will be absent. This issue is typically caused by problems within the rectifier itself—specifically, either the thyristors lack trigger pulses altogether or their triggers are not properly activated. To diagnose the problem, use an oscilloscope to inspect the gate pulses of all six rectifier thyristors. If this is the case, after shutting down the system, use a multimeter to measure the resistance at each gate. A gate resistance below 200Ω indicates that the gate is shorted and needs replacement. Alternatively, if a gate exhibits extremely high resistance, the faulty thyristor simply requires replacement.
2. Operation of the inverter's three bridge arms: The output current is exceptionally high—similar to when the furnace is idle. However, during operation, the power cabinet emits a heavy, noticeable noise, indicating a potential fault. After startup, if you turn the power knob to a lower setting, you’ll observe that the medium-frequency melting furnace’s output voltage rises above its normal level. Using an oscilloscope to examine the voltage waveforms across the anodes and cathodes of the four inverter thyristors, you’ll find that when three bridge arms are functioning properly, two adjacent thyristors within the inverter will exhibit overlapping waveforms.
3. Induction Coil Failure: The induction coil is the load of the medium-frequency power supply, constructed from square copper tubing with a wall thickness of 3 to 5 millimeters. Common faults include the following:
The induction coil is leaking, which could potentially ignite a fire between the coil windings. Therefore, timely repairs are essential. Molten steel has already adhered to the induction coil, causing the steel slag to heat up and turn red—this, in turn, could lead to the copper tubing catching fire. Immediate cleaning is crucial to prevent further issues. Additionally, short circuits in the induction coil are particularly common in small medium-frequency melting furnaces, as these furnaces are compact and tend to heat up significantly during operation.
Deformation caused by mechanical stress leads to short circuits between turns, resulting in higher-than-normal current and increased operating frequency. Therefore, to effectively address medium-frequency power supply failures with the right approach, it’s essential to thoroughly understand the typical symptoms and root causes of these common issues. This knowledge will help avoid unnecessary detours, save time, quickly diagnose and resolve the problem, restore the system to normal operation, and ultimately ensure uninterrupted production.
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