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Medium-frequency electric furnace Fault classification

1. Rectifier section failure:

The rectifier trip also triggers a high-voltage trip. Seventy percent of such failures are caused by coil issues, while the second most common cause is the breakdown of the filter capacitor. This issue can lead to overcurrent breakdown of the rectifying thyristor and simultaneously cause the trigger board to burn out the diodes in the pulse-triggering circuit.

A: 1. If the issue is caused by damp coils, then during production, after applying the insulating clay to the coils, you should first use an electric heating element or infrared lamps to heat and dry the coils thoroughly before proceeding with furnace lining installation. Before firing up the newly lined furnace, it’s essential to externally heat-dry the furnace lining as well. This is because the refractory materials used for furnace construction can absorb moisture. Avoid immediately initiating the sintering process through the control system, as this could lead to thyristor damage due to coil dampness, resulting in costly repair expenses. On the day the rectifier thyristors were burned out, we measured the insulation resistance between the furnace mold and the furnace body at 50 MΩ. After drying the steel mold with a masonry dryer for two days, we re-measured the insulation resistance between the steel mold and the furnace body, which now stood at 200 MΩ. The higher the insulation resistance, the more reliable the furnace lining sintering process will be. In this particular failure, the root cause was at least partly due to our omission of the step where we would have used an electric heating element to dry the insulating clay—instead, we proceeded directly to furnace lining installation right after applying the clay to the induction coils. In the past, we used to let the clay air-dry for several days before starting the lining process. Additionally, when drying components generally, it’s sufficient to keep the furnace cooling water flow at about half capacity, which allows for efficient furnace drying.

Typically, after melting 3 to 10 batches, the moisture in a new furnace lining will start to evaporate. At this point, the moisture is driven into the induction coil. The furnace lining itself also contains moisture. Generally, the drier a material is, the more readily it will absorb water and moisture, which can lead to diode burnout.

2: Problems caused by damage to coil insulation—such coil insulation issues are typically triggered by furnace leaks. As long as the furnace lining is regularly inspected and attention is paid to the number of times it has been used, furnace leaks can generally be avoided.

In summary: The safe approach to starting a new furnace and avoiding malfunctions is as follows: 1. First, use an electric heating element to dry the coil insulation paste, or let it air-dry for a period of time before lining the furnace—this is a safer method. 2. Next, line the furnace and then carry out external heating and drying. 3. For small-power melting, use currents below 1,000 amperes; after running the furnace for about 10 batches, proceed with high-power melting.

B: After the furnace is shut down, cooling pipes are prone to leaking. Therefore, before powering on the furnace, it’s essential to inspect the cooling pipes for leaks. Any cooling pipes that are aged or cracked must be replaced. Use a clamp screwdriver to tighten the clamps and secure the water cuffs. Also, adjust the tightness of the pipe joints due to thermal expansion and contraction. Furthermore, dry any areas of the furnace body where there’s leakage using a hot air blower, so as to prevent moisture from being transferred to the furnace coil.

Tool for tightening clamp bolts: clamp wrench, size 7 or 8. Currently, both the 8-ton West Furnace and the East Furnace have aging cooling water pipes with cracks. We recommend inspecting and replacing these pipes. Before putting the East Furnace into operation, it would be best to replace the cooling water pipes with new ones to ensure there are no leaks.

2. Inverter fault:

A: Inverter tripping

a. During furnace operation, the inverter capacitor may break down, and at the same time, the inverter thyristor may burn out. The typical cause is that the capacitor’s voltage rating is insufficient, leading to breakdown. This issue is also related to the capacitor’s service life. b. If, immediately after finishing pouring out the molten material from one batch, there is very little molten iron left in the furnace and the second batch has not yet been fully loaded, then drawing power at this stage can result in high voltage and low current, which may cause the capacitor to break down.

B: The inverter can't increase power.

Individual capacitors on the inverter trigger board overheated and burned out, causing damage to the circuit board and burning out the insulation between the windings of the pulse transformer.

C: The inverter cannot be turned on.

There’s a printing circuit board error on the inverter motherboard base, which prevents the oven from being turned on. When the oven is not in use, the DC busbar remains connected to the oven, causing severe heating of the DC busbar.