Intermediate frequency furnace control panel - Fault analysis and overhaul

November 9, 2019

The thyristor medium frequency induction heating power source uses a thyristor to convert three-phase power frequency AC power into single-phase AC power of several hundred or several kilohertz. It has the characteristics of convenient control, high efficiency, reliable operation and low labor intensity. It is widely used in the smelting of cast steel, stainless steel or alloy steel, vacuum smelting, heating of forgings and bending of steel pipes, extrusion molding, preheating of workpieces, steel. Surface quenching, annealing heat treatment, welding of metal parts, powder metallurgy, pipe heating for transporting high-temperature working fluids, crystal growth, etc.


The working principle of the intermediate frequency power supply is: using a three-phase bridge type full control rectifier circuit to rectify the alternating current into direct current, after the reactor is flat wave, it becomes a constant direct current source, and then the direct current is inverted by the single-phase inverter bridge. A single-phase intermediate frequency current at a certain frequency (typically 1000 to 8000 Hz). The load consists of an induction coil and a compensation capacitor connected in a parallel resonant circuit.


Under normal circumstances, the fault of the intermediate frequency power supply can be divided into two categories according to the fault phenomenon, which cannot be started completely and cannot be operated normally after starting. As a general principle, when a fault occurs, the entire system should be thoroughly checked in the event of a power outage, which includes the following aspects:


(1) Power supply: Use a multimeter to measure whether there is power behind the main circuit switch (contactor) and the control fuse. This will eliminate the possibility of disconnection of these components.

(2) Rectifier: The rectifier adopts a three-phase fully controlled bridge rectifier circuit, which includes six fast fuses, six thyristors, six pulse transformers and one freewheeling diode. There is a red indicator on the quick fuse. When the indicator is normal, it will shrink inside the housing. When the fuse is blown, it will pop out. Some fast-melting indicators are tight. When the fuse is blown, it will get stuck inside. Therefore, for the sake of reliability, you can use the multimeter on/off to measure the fast-melting to determine if it is blown.

A simple way to measure a thyristor is to measure its cathode-anode, gate-cathode resistance with a multimeter electrical barrier (200Ω block). The thyristor does not need to be removed during measurement. Under normal circumstances, the anode-cathode resistance should be infinite, and the gate-cathode resistance should be between 10 and 50 Ω. Too large or too small indicates that the thyristor gate fails and it cannot be triggered to conduct.

The pulse transformer is connected to the thyristor on the secondary side, and the primary side is connected to the main control board. The primary side resistance is about 50Ω measured by a multimeter. The freewheeling diode is generally not prone to failure. When checking, the multimeter diode is used to block the two ends. In the forward direction, the multimeter shows that the junction voltage drop is about 500mV, and the reverse is not possible.

(3) Inverter: The inverter includes four fast thyristors and four pulse transformers, which can be inspected as described above.

(4) Transformer: Each winding of each transformer should be open. Generally, the primary side resistance is about several tens of ohms, and the second pole is a few ohms. It should be noted that the primary side of the IF voltage transformer is connected in parallel with the load, so its resistance is zero.

(5) Capacitors: The electric heating capacitors connected in parallel with the load may be broken down. The capacitors are generally grouped and mounted on the capacitor holder. The group in which the capacitors are broken down should be determined first. Disconnect the connection point between the busbar of each group of capacitors and the main busbar, and measure the resistance between the two busbars of each group of capacitors. Normally, it should be infinite. After confirming the bad group, disconnect the soft copper from each electric capacitor to the busbar, and check the breakdown capacitors one by one. Each electric heating capacitor is composed of four cores, the outer casing is one pole, and the other pole is respectively led to the end cover through four insulators. Generally, only one core is broken down, and the lead wire on the insulator is tripped. The capacitor can continue to be used, and its capacity is 3/4 of the original. Another fault of the capacitor is oil leakage, which generally does not affect the use, but pay attention to fire prevention.

The angle of the capacitor is insulated from the capacitor holder. If the insulation breakdown will ground the main circuit and measure the resistance between the capacitor housing lead and the capacitor holder, the insulation condition of this part can be judged.

(6) Water-cooled cable: The function of the water-cooled cable is to connect the intermediate frequency power supply and the induction coil. It is formed by twisting each diameter Φ0.6–Ф0.8 copper wire. For a 500 kg electric furnace, the cable cross-sectional area is 480 mm 2 , and for a 250 kg electric furnace, the cable cross-sectional area is 300 to 400 mm 2 . The outer tube of the water-cooled cable is made of pressure rubber tube with a pressure of 5 kg. The inside is connected with cooling water. It is part of the load circuit. It is subjected to tension and torsion when working. It is twisted and twisted together with the furnace body, so it is easy to be flexible after a long time. The joint breaks open. The water-cooled cable break process generally breaks off most of the time, and the uninterrupted small part is quickly blown off during high-power operation. At this time, the intermediate frequency power supply will generate a high overvoltage. If the overvoltage protection is unreliable, The thyristor will burn out. After the water-cooled cable is disconnected, the IF power supply cannot start working. If you repeatedly start without checking the cause, it is likely to burn out the IF voltage transformer. When checking the fault, use the oscilloscope, clip the oscilloscope probe to both ends of the load, and observe whether there is any attenuation waveform when pressing the start button. When determining the core breakage of the cable, first disconnect the water-cooled cable from the output copper discharge of the electric heating capacitor, and measure the resistance value of the cable with a multimeter electric barrier (200Ω block). When normal, the resistance value is zero, and when disconnected, it is infinite. When measuring with a multimeter, the furnace body should be turned to the dumping position, so that the water-cooled cable is dropped, so that the broken part can be completely separated, and the core can be correctly judged.

Through the inspection of the above aspects, it is generally possible to find out most of the causes of the failure, and then the control power can be turned on for further inspection. The main circuit of the intermediate frequency power supply is closed manually or automatically. For systems that are automatically closed, the power cord should be temporarily disconnected to ensure that the main circuit does not close. After the control power is turned on, the following aspects can be checked.


1. Connect the oscilloscope probe to the gate and cathode of the rectifier thyristor. The oscilloscope is placed in the power supply synchronization. After pressing the start button, the trigger pulse waveform can be seen. It should be double pulse and the amplitude should be greater than 2V. Press the stop button and the pulse will disappear immediately. Repeat six times, look at each thyristor. If there is no pulse at the gate, you can move the probe of the oscilloscope to the original side of the pulse transformer. If there is a pulse on the primary side and the secondary side does not, the pulse transformer is damaged, otherwise the problem It may be on the transmission line or on the main control board.

2. Connect the oscilloscope probe to the gate and cathode of the inverter thyristor. The oscilloscope is placed in the internal synchronization. After the control power is turned on, the inverter trigger pulse can be seen. It is a series of sharp pulses, and the amplitude should be greater than 2V. The pulse period is read out and the trigger pulse frequency is calculated. When normal, it should be about 20% higher than the nominal frequency of the power cabinet. This frequency is called the starting frequency. When the start button is pressed, the pulse spacing increases and the frequency becomes lower. Normally, it should be about 40% lower than the nominal frequency of the power cabinet. Press the stop button and the pulse frequency immediately jumps back to the starting frequency.

Through the above checks, it is basically possible to eliminate faults that cannot be started at all. After starting, the work is not normal, generally in the following aspects:

1. Rectifier phase loss: The fault is that the sound is not normal when working, the maximum output voltage rises below the rated value, and the power cabinet blame becomes louder. At this time, the output voltage can be lowered at about 200V, and the output voltage waveform of the rectifier is observed with an oscilloscope. (The oscilloscope should be placed in the power supply synchronization). When the input voltage waveform is normal, there are six waveforms per cycle. When there is no phase, there will be two missing, as shown in Figure 2. This fault is generally caused by a thyristor of the rectifier that does not have a trigger pulse or a non-conduction of the trigger. In this case, first look at the gate pulse of the six rectifier thyristors with an oscilloscope, and if so, use a multimeter 200Ω file after shutdown. With each gate resistance, you can replace the thyristor that is not connected or has a particularly large gate resistance.

2. Inverter three-legged arm work: The fault is characterized by a particularly large output current, the same for the empty furnace, and the sound of the power cabinet is very heavy. After starting, the power knob is adjusted to the minimum position, and the intermediate frequency output voltage is higher than normal. . The voltage waveform between the anode and cathode of the four inverter thyristors is sequentially observed by an oscilloscope, and each waveform of the normal one is as shown in FIG. If the three-bridge arm works, it can be seen that the waveforms of two adjacent thyristors in the inverter are normal, and the other two adjacent ones have no waveform and the other is a sine wave, as shown in Figure 4, KK2 triggers. No, the waveform between the anode and cathode is a sine wave; at the same time, the non-conduction of KK2 will cause KK1 to be unable to turn off, so there is no waveform at the two ends of KK1.

3. Induction coil failure: The induction coil is the load of the intermediate frequency power supply. It is made of square copper tube with a wall thickness of 3 to 5 mm. Its common faults are as follows:

The induction coil leaks water, which may cause the coil to ignite between the turns, and must be repaired in time to run.

The molten steel sticks to the induction coil, and the steel slag is hot and red, which will cause the copper tube to burn through and must be cleaned up in time.

Induction coils are short-circuited between turns, and such faults are particularly prone to occur in small-sized medium-frequency induction furnaces. Because the furnace is small, it is deformed by thermal stress during operation, resulting in a short circuit between turns, and the fault is characterized by a large current and a higher operating frequency than usual. .

In summary, in order to use the correct method for fault repair of intermediate frequency power supply, it is necessary to be familiar with the characteristics and causes of common faults of intermediate frequency power supply, in order to reduce detours, save time, eliminate faults as soon as possible, and restore normal operation of intermediate frequency power supply. In order to ensure the smooth progress of production.


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