Failure analysis and treatment of induction melting furnace during startup

1. The induction melting furnace cannot be started

When starting, only the DC ammeter has instructions, and neither the DC voltmeter nor the intermediate frequency voltmeter has any instructions. This is one of the most common failure phenomena, and the causes are as follows.

There is a lack of pulse phenomenon in the inverter trigger pulse. Use an oscilloscope to check the inverter pulse (preferably on the G K of the thyristor). If there is a lack of pulse, check whether the connection is poor or open, and whether there is a pulse output in the previous stage.

Inverter thyristor breakdown. Use a multimeter to measure the resistance between A and K. In the absence of cooling water, the value between A and K should be greater than 10kC, and the resistance is equal to 10kC. Time is broken. If two of them are damaged during the measurement, you can remove one of the connecting copper bars, and then judge whether one or two are damaged. Replace the thyristor and check the cause of damage to the thyristor (for the cause of damage to the thyristor, please refer to the following analysis of the cause of damage to the thyristor). Capacitor breakdown. Use the RXlk block of a multimeter to measure whether each terminal of the capacitor is charged or discharged to the common terminal. If there is no indication that the terminal is damaged, remove the damaged capacitor pole. The load is short-circuited and grounded. A 1000V insulation resistance meter (shaking meter) can be used to measure the resistance of the coil to ground (when there is no cooling water), and it should be greater than 1MH, otherwise the short-circuit point and grounding point should be excluded. The sampling circuit of the intermediate frequency signal has an open circuit or a short circuit. Use an oscilloscope to observe the waveform of each signal sampling point, or use a multimeter to measure the resistance value of each signal sampling loop when the power is off, and find the open or short circuit point. Focus on checking the intermediate frequency feedback transformer to see if the primary side is open (caused by the virtual connection of the leakage sense).

2. It is difficult to start

After starting, the intermediate frequency voltage is more than one time higher than the DC voltage, and the DC current is too large. The reasons for this failure are as follows.

One thyristor in the inverter circuit is damaged. When a thyristor is damaged in the inverter circuit, the induction melting furnace can sometimes be started, but the above-mentioned failure phenomenon will occur after the start. Replace the damaged thyristor and check the cause of the damage. One of the inverter thyristors is non-conducting, that is, "three legs" work. It may be that the gate of the thyristor is open, or the wire connected to it is loose or has poor contact. There is an open circuit or wrong polarity in the sampling loop of the intermediate frequency signal. This kind of reason is mostly in the line that adopts the angle method. Open circuit of the intermediate frequency voltage signal or reverse polarity of the intermediate frequency voltage signal when repairing other faults will cause this fault phenomenon. The front angle phase shift circuit of the inverter has failed. The load of the intermediate frequency power supply is capacitive, that is, the current leads the voltage. In the sampling control circuit, a phase shift circuit is designed. If the phase shift circuit fails, it will also cause this malfunction.

3. Difficulty in starting

After starting, the maximum DC voltage can only be raised to 400V, and the reactor vibrates loudly and the sound is dull. This kind of failure is a three-phase fully-controlled rectifier bridge failure, and the main reasons are as follows.

The rectifier thyristor has open circuit, breakdown, soft breakdown or performance degradation of electrical parameters. Use an oscilloscope to observe the tube voltage drop waveform of each rectifying thyristor, find the damaged thyristor and replace it. When the damaged thyristor breaks down, its tube voltage drop waveform is a straight line; in soft breakdown, when the voltage rises to a certain value, it becomes a straight line. When the electrical parameter drops, the waveform changes when the voltage rises to a certain value. If the above phenomenon occurs, the DC current will be cut off, causing the reactor to vibrate. A set of rectified trigger pulses is missing. Use an oscilloscope to check each trigger pulse separately (it is better to check on the thyristor). When checking the circuit without pulse, use the backward push method to determine the fault location and replace the damaged component. When this phenomenon occurs, the output wave head of the DC voltage will lack a wave head, causing the current to cut off, resulting in this failure phenomenon. The gate of the rectifier thyristor is open or short-circuited, causing the thyristor not to be triggered. Generally, the resistance value between G-K is about 10~30Q.

4. Stop immediately after starting

It can be started, but it stops immediately after starting, and the induction melting furnace is in a state of repeated starting. This failure is a failure of an induction melting furnace with a sweep-frequency start mode, and the reasons are as follows.

The lead angle is too small, and repeated start is caused by the failure of commutation after starting. By observing the intermediate frequency voltage waveform with an oscilloscope, increase the inverter lead angle appropriately.

The load oscillation frequency signal is at the edge position of the external excitation scanning frequency signal range. Re-adjust the scanning range of the other excitation scanning frequency.

5. Overcurrent trip after starting

After the induction melting furnace is started, when the power rises to a certain value, the induction melting furnace is prone to overcurrent protection action, and sometimes the thyristor will be burned out and restarted, the phenomenon remains the same. This failure phenomenon is generally caused by the following reasons.

If overcurrent is likely to occur under low voltage just after starting, it is caused by the fact that the front angle of the inverter is too small and the inverter thyristor cannot be reliably turned off.

Water is cut off or the heat dissipation effect is reduced in the water cooling jacket of the inverter thyristor. Replace the water cooling jacket. Sometimes it is enough to observe the water output and pressure of the water cooling jacket, but often due to water quality problems, a layer of scale is attached to the wall of the water cooling jacket. Because scale is a substance with extremely poor thermal conductivity, although there is sufficient water flow However, the heat dissipation effect is greatly reduced due to the isolation of scale. The judgment method is: run the power at a power lower than the over-current value for about 10 minutes, and quickly shut down, and quickly touch the core of the thyristor with your hand after the shutdown. If you feel hot, the fault is caused by this reason.

The connection wires of the tank circuit have poor contact and disconnection. Check the connection wires of the tank circuit and deal with it according to the actual situation. When the connecting wire of the tank circuit has poor contact or disconnection, the power will rise to a certain value, it will cause ignition, which will affect the normal operation of the induction melting furnace, which will lead to the protection of the induction melting furnace. Sometimes due to sparking, instantaneous overvoltage will be generated at both ends of the thyristor. If the overvoltage protection action is too late, the thyristor components will be burnt out. This phenomenon often causes simultaneous actions of overvoltage and overcurrent.

6. No response at startup

When the induction melting furnace starts, there is no response. After observation, the lack of phase indicator light on the control circuit board is on. This failure is caused by the following reasons: the fast fuse blown. Generally fast fuse has a fusing indication, you can judge whether the fuse is burned out by observing the indication, but sometimes due to the long use time of fast fuse or quality reasons, the indication is not clear or the indication is not clear, you need to cut off the power or use a multimeter to measure . The treatment method is: replace the fast fuse and analyze the cause of the blow. The general reasons for blowing fast fuse are as follows. The induction melting furnace runs under the conditions of high power and high current for a long time, causing the fast fuse to generate heat, which causes the fuse core to melt. The rectifier load or the intermediate frequency load is short-circuited, causing an instantaneous high current impact and burning the fast fuse. The load circuit should be checked. The failure of the rectifier control circuit caused an instantaneous high current impact. The rectifier circuit should be checked.

The contact of the main switch is burned out or the front-level power supply system has a phase failure. Use the AC voltage block of a multimeter to measure the line voltage of each level to determine the location of the fault.