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In general, you can... Medium-frequency furnace Faults are categorized, according to their symptoms, into two major types: complete failure to start and failure to operate normally after starting. As a general principle, when a fault occurs, the entire system should be thoroughly inspected while the power is disconnected. This inspection should cover the following aspects:

(1) Power Supply: Use a multimeter to check whether there is voltage after the main circuit switch (contactor) and the control fuse. This will rule out the possibility of an open circuit in these components. (2) Rectifier: The rectifier employs a three-phase fully controlled bridge rectifier circuit, which includes six fast thyristors, six pulse transformers, and one freewheeling diode. There is an indicator on the fast fuse; under normal conditions, the indicator retracts into the housing. However, once the fast fuse blows, the indicator will pop out. In some cases, the indicator may be tightly fitted and remain stuck inside after the fuse has blown. Therefore, for greater reliability, you can use a multimeter to test the fast fuse and confirm whether it has indeed blown.

A simple method for testing a thyristor is to use the resistance setting (200Ω range) on a multimeter to measure the gate-to-cathode resistance. You don't need to remove the thyristor from its circuit during this test. Under normal conditions, the anode-to-cathode resistance should be infinite. The gate-to-cathode resistance should be between 10 and 50Ω; if the resistance is too high or too low, it indicates that the gate of the thyristor has failed and the device will no longer be able to trigger and turn on. The secondary side of the pulse transformer is connected to the thyristor, while the primary side is connected to the main control board. Use a multimeter to measure the resistance of the primary side, which should be around 50Ω. The freewheeling diode generally does not fail easily. To check it, set your multimeter to the diode mode and measure across its two terminals. In the forward direction, the multimeter should display a forward voltage drop of approximately 500mV; in the reverse direction, the diode should be non-conductive.

(3) Inverter: The inverter includes four fast thyristors and four pulse transformers, which can be checked using the method described above.

(4) Transformers: Each winding of every transformer should be conductive. Typically, the primary-side resistance is around several tens of ohms, while the secondary-side resistance is several ohms. It should be noted that the primary side of an intermediate-frequency voltage transformer is connected in parallel with the load, so its resistance value is zero.

(5) Capacitors: Electrothermal capacitors connected in parallel with the load may become punctured. Capacitors are typically installed in groups on capacitor racks; when inspecting, first identify the group containing the punctured capacitor. Disconnect the connection point between the busbar of each capacitor group and the main busbar, then measure the resistance between the two busbars of each group. Under normal conditions, this resistance should be infinite. After confirming the faulty group, disconnect the flexible copper leads connecting each electrothermal capacitor to its respective busbar. By examining each capacitor individually, you can pinpoint the one that has been punctured. Each electrothermal capacitor consists of four cores; the outer casing serves as one electrode, while the other electrode is led out through four insulators to the end cover. Usually, only one core will be punctured. By disconnecting the lead connected to the insulator corresponding to the punctured core, the capacitor can continue to be used, though its capacitance will be reduced to three-quarters of its original value. Another common failure mode for capacitors is oil leakage. Such leakage generally does not affect operation, but it’s important to take fire safety precautions.

The angle steel used for mounting the capacitors is insulated from the capacitor rack. If insulation breakdown occurs, it could cause the main circuit to ground. To assess the insulation condition of this part, measure the resistance between the capacitor housing lead and the capacitor rack. (6) Water-cooled cables: The function of water-cooled cables is to connect the medium-frequency power supply to the induction coil. These cables are made by twisting together individual hydrogen-free copper wires, each with a diameter ranging from 0.15 to 0.10 mm. For a 40-ton medium-frequency furnace, the cable cross-sectional area is 630 square millimeters; for a 10-ton electric furnace, the cable cross-sectional area is 500 square millimeters. The outer rubber hose of the water-cooled cable is made of carbon-free insulating rubber that can withstand a pressure of 0.5 MPa. Cooling water flows through the inner tube, and the cable forms part of the load circuit. During operation, it is subjected to tensile and torsional forces and bends along with the furnace body as it tilts. Consequently, over time, the flexible connection points tend to break easily. Typically, when a water-cooled cable breaks, most of the cable first severs, and then, during high-power operation, the remaining small section quickly burns out. At this point, the medium-frequency power supply generates a very high overvoltage. If the overvoltage protection is unreliable, it can damage the thyristors. Once the water-cooled cable has broken, the medium-frequency power supply cannot be started. If the fault is not identified and the power supply is repeatedly restarted without proper diagnosis, there is a high risk of damaging the medium-frequency voltage transformer. To diagnose the fault, an oscilloscope can be used. Attach the oscilloscope probe across the load terminals and observe whether there is any damped waveform when the start button is pressed. To confirm a cable core break, first disconnect the water-cooled cable from the output copper busbar of the electrothermal capacitor. Use a multimeter set to the resistance range (200 Ω) to measure the cable’s resistance. Under normal conditions, the resistance should read zero; when the cable is broken, the resistance will show infinity. When using the multimeter, the furnace body should be tilted to its tipping position so that the water-cooled cable hangs freely. This ensures that the break point is fully isolated, allowing for an accurate determination of whether the cable core is indeed broken. By performing the checks described above, most common fault causes can usually be identified. Next, the control power supply can be connected for further inspection. The main circuit of the medium-frequency power supply can be switched on either manually or automatically. For systems with automatic switching, the power supply line should first be temporarily disconnected to ensure that the main circuit does not close inadvertently. After connecting the control power supply, the following checks can be performed.

1. Connect the oscilloscope probe to the gate and cathode of the rectifier thyristor. Set the oscilloscope to power-line synchronization. After pressing the start button, you should be able to observe the trigger pulse waveform, which should be a double pulse with an amplitude greater than 2V. Press the stop button, and the pulse will immediately disappear. Repeat this process six times, checking each thyristor carefully. If no pulse is detected at the gate, move the oscilloscope probe to the primary side of the pulse transformer and check there. If there’s a pulse on the primary side but none on the secondary side, it indicates that the pulse transformer is damaged; otherwise, the problem may lie in the transmission line or the main control board.

2. Connect the oscilloscope probe to the gate and cathode of the inverter thyristor. Set the oscilloscope to internal synchronization. After turning on the control power supply, you should be able to observe the inverter trigger pulses—these appear as a series of sharp pulses with an amplitude greater than 2V. Use the oscilloscope’s time base to measure the pulse period and calculate the trigger pulse frequency. Under normal conditions, this frequency should be approximately 20% higher than the rated frequency of the power cabinet; this frequency is referred to as the startup frequency. After pressing the start button, the spacing between pulses widens, and the frequency decreases. Under normal circumstances, the frequency should be about 40% lower than the rated frequency of the power cabinet. When you press the stop button, the pulse frequency immediately returns to the startup frequency.

Through the above-mentioned checks, faults that cause the system to fail to start at all can basically be ruled out. Once the system does start, any abnormal operation typically manifests itself in the following areas:

1. Phase Loss in the Rectifier: The fault manifests as abnormal noise during operation, failure of the maximum output voltage to reach its rated value, and an increase in the unusual humming sound from the power cabinet. In such cases, you can reduce the output voltage to around 200V and use an oscilloscope to observe the output voltage waveform of the rectifier (the oscilloscope should be set to power-line synchronization). Under normal conditions, the input voltage waveform should show six cycles per period; however, with a phase loss, two cycles will be missing. This fault is typically caused by one of the rectifier’s thyristors failing to receive a trigger pulse or failing to turn on properly. To diagnose this issue, first use an oscilloscope to check the gate pulses of all six rectifier thyristors. If gate pulses are present, after shutting down the system, use a multimeter set to the 200Ω range to measure the gate resistance of each thyristor. Simply replace the thyristor that shows no continuity or has an exceptionally high gate resistance.

2. Operation of the inverter’s three bridge arms: The fault manifests as an exceptionally high output current—even when the furnace is empty—and the power cabinet produces a heavy, resonant sound during operation. After startup, if you turn the power knob to its minimum position, you’ll notice that the intermediate-frequency output voltage is higher than normal. Use an oscilloscope to sequentially observe the voltage waveforms across the anode-to-cathode terminals of the four inverter thyristors. Under normal conditions, the waveform of each thyristor should resemble the one shown in Figure 3. However, if the three bridge arms are operating, you’ll find that the waveforms of two adjacent thyristors are normal, while the waveforms of the other two adjacent thyristors show either no waveform at all or a sinusoidal waveform, as illustrated in Figure 4. Specifically, since KK2 fails to trigger properly, the waveform across its anode-to-cathode terminals becomes sinusoidal. At the same time, because KK2 remains non-conducting, KK1 cannot be turned off, resulting in no waveform appearing across the terminals of KK1.

3. Induction Coil Failure: The induction coil is the load of the medium-frequency power supply, and it is made from rectangular electrolytic copper tubing with a wall thickness ranging from 4 to 10 millimeters. Its common failures include the following:

The induction coil is leaking, which could cause arcing between the coil windings. It must be promptly repaired by welding to ensure safe operation.

When molten steel sticks to the induction coil, the steel slag heats up and turns red, which can cause the copper pipe to burn through. Therefore, it’s essential to remove the slag promptly and thoroughly.

Interturn short circuits in induction coils are particularly prone to occur in medium-frequency melting furnaces with high-voltage inlets ranging from 1500 to 2000V. This is because the small spacing between turns causes them to deform under thermal stress during operation, leading to interturn shorts. The fault manifests as higher current and a working frequency that is higher than usual.

To ensure that fault repairs on medium-frequency furnaces are carried out using the correct methods, it is essential to be familiar with the characteristics and causes of common faults in medium-frequency furnaces. This will help avoid unnecessary detours, save time, quickly eliminate faults, restore the furnace to normal operation, and thereby guarantee the smooth continuation of production.