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    Electric furnace repair

    Fourteen medium frequency induction melting furnace fault attached maintenance method

    Preparation before maintenance  

    1, the tools required for maintenance are: multimeter, 20 megabytes or more double trace oscilloscope, electric iron, screwdriver, wrench and so on.

    2, the information required for maintenance are: equipment related to electrical drawings, instructions and other technical information.

    3. Before repairing, you should first understand the fault phenomenon of the equipment, what happened when the fault occurred, and check the records of the equipment.

    4. Prepare some vulnerable and commonly used components.

    Fourteen fault repairs  

    1. The medium frequency induction melting furnace equipment cannot be started. Only the DC current meter has an indication when starting, and there is no indication of the DC voltage and the intermediate frequency voltmeter. Fault Analysis and Processing: This is one of the most common failure phenomena, which may be caused by:

    1. Inverter trigger pulse has pulse phenomenon - check the inverter pulse with oscilloscope (preferably check on the thyristor AK). If there is a missing pulse, check if the connection has poor contact or open circuit. Is there a pulse output?

    2, inverter thyristor breakdown - replace the thyristor, and check the cause of thyristor damage (for the cause of thyristor damage, see the analysis of the cause of thyristor damage).

    3. Capacitor breakdown - remove the damaged capacitor pole.

    4, the load has a short circuit, grounding phenomenon - eliminate the short circuit point and grounding point.

    5. The IF signal sampling loop has open circuit or short circuit phenomenon!use the 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 not supplied, and find the open circuit point or short circuit point.

    Second, the startup is more difficult. After starting, the intermediate frequency voltage is twice as high as the DC voltage, and the DC current is too large.   Fault Analysis and Handling: The causes of this failure are:

    1. The inverter circuit has a thyristor damage. When the inverter circuit has a thyristor damaged, the device can sometimes be started, but the above failure phenomenon will occur after startup, replace the damaged thyristor, and check for damage. the reason.

    2. The intermediate frequency signal sampling loop has an open circuit or a polarity error phenomenon. This is mostly caused by the intersection angle method. When the intermediate frequency voltage signal is open or the polarity of the intermediate frequency voltage signal is reversed during maintenance of other faults, it will cause This malfunction phenomenon.

    3, the inverter leads to the front angle shifting circuit failure - the load of the intermediate frequency power supply is capacitive, that is: the current leads the voltage. In the sampling control circuit, a phase shifting circuit is designed, which may be caused if the phase shifting circuit fails.

    Third, the difficulty of starting, after the start up, the DC voltage can only rise to 400V, and the reactor vibration is large, the sound is dull. Fault analysis and processing: This fault is a three-phase full-controlled rectifier bridge fault, the main reasons are:

    1. Rectifier thyristor open circuit, breakdown, soft breakdown or electrical parameter performance degradation - Observe the tube voltage drop waveform of each rectifier thyristor with an oscilloscope, and find the damaged thyristor after replacement. When the damaged thyristor breaks down, the tube voltage drop waveform is a straight line; when the voltage is softened, the voltage rises to a certain line, and the waveform changes when the voltage rises to a certain value when the electrical parameter drops. If the above phenomenon occurs, the DC current will be interrupted, causing the reactor to vibrate.

    2, the lack of a set of rectification trigger pulse - use the oscilloscope to check each trigger pulse (preferably on the thyristor), check the loop without a pulse, use the back push method to determine the fault location, replace the damaged device. When this phenomenon occurs, the output wave head of the DC voltage lacks a wave head, causing current interruption, which causes this malfunction.

    Fourth, it can be started, but it will stop immediately after starting, and the equipment is constantly restarting. Fault analysis and processing: This fault is a device fault that belongs to the sweeping start mode. The reason is:

    1. The front angle of the inverter is too small, and the restart is caused by the failure of the commutation after starting up. By using the oscilloscope to observe the intermediate frequency voltage waveform, the front angle of the inverter is appropriately adjusted.

    2. The load oscillating frequency signal is at the edge of its excitation sweep frequency signal range - re-adjusting the scan range of its excitation sweep frequency.

    5. After the medium frequency induction melting furnace equipment is started, when the power rises to a certain value, the equipment overcurrent protection action will sometimes burn out the thyristor components and restart, the phenomenon remains the same.   Fault analysis and processing:

    This type of failure is generally caused by two reasons:

    1. Inverter thyristor water-cooled jacket has a reduced water or heat dissipation effect - replace the water-cooled jacket. Sometimes it is sufficient to observe the water output and pressure of the water jacket, but often due to water quality problems, a layer of scale is attached to the wall of the water jacket. Since the scale is an object with poor thermal conductivity, although there is enough water flow, However, because the scale is isolated, its heat dissipation effect is greatly reduced. The method of judging is: running the power at a power lower than the overcurrent value for about ten minutes, quickly stopping the machine, and quickly touching the core of the thyristor element by hand after stopping, if the hand feels hot, the fault is caused by Caused by this.

    2. Poor contact and disconnection of the connecting line of the tank road - check the connecting line of the tank, and handle it according to the actual situation. When the connection line of the tank circuit has poor contact or disconnection, the fire will rise after a certain value rises, which will affect the normal operation of the equipment and cause the equipment to protect. Sometimes a transient overvoltage occurs across the thyristor due to a fire. If the overvoltage protection action is too late, the thyristor component will burn out. This phenomenon often occurs when overvoltage and overcurrent occur simultaneously.

    6. There is no reaction when the equipment starts. After observation, the phase loss indicator on the control circuit board is on. Fault analysis and processing:

    This kind of failure phenomenon is more obvious and is caused by the following reasons:

      1. Fast-blow fuse--General fast-blow fuses have blown indications. You can observe whether the fuses are blown by observing the indications, but sometimes the fast-acting fuses are used for too long or quality reasons, and the indications are not indicated or the indications are not clear. It must be measured with a multimeter after power off. The treatment method is: replace the fast fuse and analyze the cause of the blow. There are two reasons for the blown fuses: The equipment runs under the conditions of long time, high power and high current, which causes the fuses to heat up and fuse the fuses. The rectifier control circuit fault causes an instantaneous large current shock wave strike. The rectifier circuit should be checked. The rectified load or the intermediate frequency load is short-circuited, causing an instantaneous large current surge, burning the fast fuse and checking its load circuit.

      2. The contact of the main switch is burned out or the front-end power supply system has a phase loss fault. Use the multimeter's AC voltage file to measure the line voltage of each stage to determine the fault location.

    7. When the equipment is running, the DC current reaches the rated value, but the DC voltage and the intermediate frequency voltage are low. The waveform of the intermediate frequency is observed with an oscilloscope. The waveform is normal and the front angle of the inverter is normal. Fault analysis and processing: The fault phenomenon does not belong to the intermediate frequency power supply fault, but is caused by the low impedance of the load, and the load impedance must be readjusted.

    1. In the circuit of boost load, it is removed due to the damage of the series compensation capacitor, and there is no replacement, or the most demanding high-power and uncontrolled increase of the compensation capacitor, so that the compensation amount of the load is over compensated, this will cause this. Failure phenomenon. The solution is to readjust the compensation amount of the compensation capacitor so that the device can operate at rated power.

    2. The sensor has a short circuit between the turns - if the sensor has a short circuit between the turns, the impedance of the load will also decrease, causing this malfunction. There are two possibilities for short-circuit between turns: The copper tube of the sensor is directly short-circuited. The fixed bakelite column of the sensor is severely carbonized. Due to the conductive property of the carbon, the carbonized bakelite between the inductors causes the direct connection between the turns to cause short circuit between the turns of the inductor. The solution is to eliminate the short circuit between turns.

    8. When the equipment is running, the DC voltage and the intermediate frequency voltage are all up to the rated value, but the DC current is small and the power is low.   Fault Analysis and Processing: This fault phenomenon is contrary to the cause of the "8.2.7" fault phenomenon due to the high load impedance.

    1. The compensation amount of the load compensation capacitor is insufficient - increase the compensation capacitor.

    2. The contact resistance of the connecting line of the tank circuit (load LC oscillation circuit) is too large. Due to the long-term use of the equipment, the connection of the copper strip of the tank is affected by dust, which increases the contact resistance and causes the load. The impedance increases and this failure occurs.

    Nine, the equipment is operating normally, the DC current indication is too high. If the current temple is at the rated value, the voltage is too low, and the indication value of the power meter does not match the DC voltage DC current and the product, which was consistent with the previous one.   Fault analysis and processing:

    This kind of fault phenomenon is somewhat similar to the "seven" fault phenomenon, but it is different. The cause of the fault is because the indication of the DC ammeter is inaccurate, and it creates an illusion that the current is large. The cause of this failure is more subtle and difficult to find. If you analyze it carefully, you can find that the indicated value of power does not match the product of voltage and current, indicating that the displayed value of the meter may be incorrect. The voltage value can be checked by the multimeter's DC voltage file. The current value can be corrected by measuring the incoming current with a clamp ammeter and multiplying by 0.816. If it does not match, the ammeter indication is not accurate. The value of the DC ammeter is taken from the 75mV voltage signal generated by the shunt. Under the condition of long service life and harsh operating environment, there is dirt or oxidation between the wiring and the shunt on the shunt, and the contact resistance increases. , the voltage generated on the shunt is increased, greater than 75mV, causing the indication of the DC ammeter to be too large. The treatment is to treat the dirt and oxide layer between the shunt and its wiring.

    10. The medium frequency induction melting furnace equipment is operating normally, but there is no reaction after starting the machine, and there is no protection indication. Fault analysis and processing: There are two possibilities for this type of fault:

    1. The IF start switch is broken. The IF start switch is grounded at the intermediate frequency stop position (connected to the closed point of the switch). If the switch is broken, the ground state cannot be turned on and the device is in the protection state, so there is no response. Solution: Replace the IF start switch.

    2, protection circuit failure - such as the reference "control circuit schematic", integrated circuit IC4 heat during operation will cause this failure. Processing method: heat the IC4 integrated circuit or add a heat sink. 3. In a given circuit, a given signal is interrupted!in a given circuit, an open circuit somewhere during the signal given process, which makes it impossible to phase shift the rectified pulse, which also causes this malfunction. Treatment method: The reverse circuit is used to check the given circuit.

    11. Frequent burnout of the thyristor components. Immediately after replacing the new thyristor, it will burn out. Fault analysis and processing: This is a fault phenomenon that makes people more troublesome and difficult to repair. SCRs are expensive, and burnt thyristors are hard to guard against, so be careful when repairing such faults. When we analyzed the fault "2.5", we introduced a reason for burning the thyristor. In addition to this, there are the following reasons:

      1. When the thyristor is in anti-correlation, the instantaneous glitch voltage withstand reverse voltage is too high - in the main circuit of the intermediate frequency power supply, the instantaneous reverse glitch voltage is absorbed by the RC absorption. If the resistors and capacitors in the snubber circuit are open, the instantaneous reverse glitch voltage will be too high to burn out the thyristor. In the case of power failure, use the Wanxiu meter to measure the resistance of the absorption resistor and the capacity of the absorption capacitor to determine whether the resistance-capacitance absorption circuit has failed.

      2. Load-to-ground insulation is reduced - the insulation of the load circuit is reduced, causing the load to ignite between the ground, disturbing the triggering time of the pulse or forming a high voltage across the thyristor, and burning the thyristor component.

      3, pulse trigger loop fault - if the trigger pulse is suddenly lost when the device is running, it will cause the inverter to open, the high voltage output of the intermediate frequency power supply generates high voltage, and the thyristor component is burned out. This type of fault is generally caused by inverter pulse formation and output circuit failure. It can be inspected by an oscilloscope. It may also be that the inverter pulse lead is in poor contact. You can shake the wire connector by hand to find the fault location.

      4. The device is open when the device is running. When the device is running at high power, if the load is suddenly open, a high voltage burnout thyristor component will be formed at the output.

      5. The device is short-circuited during operation. When the device is running at high power, if the load is suddenly in a short-circuit state, it will have a large short-circuit current impact on the thyristor. If the over-current protection action is not in protection, it will burn. Bad thyristor components.

      6, protection system failure (protection failure) - thyristor can be safe, mainly to protect the system to ensure that if the protection system fails, the equipment is a little bit work is not normal, the crisis will be thyristor safety. Therefore, inspection of the protection system is essential when the thyristor is burned out.

      7. Thyristor cooling system failure - The thyristor generates a lot of heat during operation, and it needs to be cooled to ensure normal operation. Generally, there are two ways to cool the thyristor: one is water cooling, the other is The air is cold. Water cooling is widely used, and air cooling is generally only used for power equipment below 100KW. Water-cooled medium-frequency equipment is usually equipped with a water pressure protection circuit, but it is basically the protection of the total influent. If a water block is blocked, it cannot be protected.

      8. Reactor failure - The internal ignition of the reactor causes the current on the inverter side to be interrupted, and a high voltage burnout thyristor is also generated on the inverter input side. In addition, if the reactor is replaced during maintenance, and the inductance and core area of the reactor are less than the required value, the reactor will burn out the thyristor due to saturation loss of current limit during high current operation.

      12. When the device is started, when the IF start switch is turned on, the main circuit switch protects trip or overcurrent protection.   Fault analysis and processing:

      1. The power adjustment knob is at the highest position - except for the quenching load, the other equipment requires the power adjustment knob to be placed at the minimum position at startup. If the position is no longer the minimum position, the current surge is too large and the overcurrent protection or main circuit The switch protects from tripping.

    2, current regulator failure - when the current regulator circuit failure, especially when the current transformer is damaged or the wiring is open, start the no-current feedback suppression, the DC voltage will directly hit the maximum (Q angle 20 degrees), DC current will Direct impact to the maximum, causing overcurrent protection or main circuit switch trip. Treatment method: Check whether the current transformer is damaged; whether the current transformer to the circuit board is disconnected; whether the current regulator part has component damage or open circuit.

    13. The intermediate frequency transformer is burnt out. After the replacement, the starting equipment still burns out the intermediate frequency transformer.   Fault analysis and processing:

    This type of fault is common on equipment that uses a boost load, mainly because of the open drain. In the boost load, the voltage across the series capacitor bank and the shunt capacitor bank cannot be absolutely identical. When the two sets of compensation capacitors are discharged, the discharge time is different because the terminal voltage is inconsistent, and the discharge time is high. Slow, and this group of capacitors has not completed the charging process again, and the charging process is started. The DC charge is accumulated on the capacitor bank. These DC charges are released by the discharge feeling. If the discharge is open, the DC current accumulated on the capacitor The load will be released by the intermediate frequency transformer. Because the capacity of the intermediate frequency transformer is very small, it cannot withstand such a large current flow, causing the intermediate frequency transformer to burn out.

    14. In the boosting load, the venting discharge is heated or even burned out.   Fault analysis and treatment: There are three reasons for causing venting and venting fever:

      1. In the above example failure analysis, if the capacity of the series-parallel group capacitors is very different, the current of the DC charge release will increase, and if the capacity of the discharge and discharge is small, heat will be generated.

      2, the inverter pulse is asymmetrical - the inverter's requirement for the inverter pulse is the difference between the two groups of pulses   180<, if the inverter pulse difference is not 180<, the positive and negative half cycles of the inverter output voltage are also inconsistent, resulting in inconsistent compensation capacitors charging twice in one cycle, then the capacitors are charged for half a week. When not finished, the capacitor is charged at a short half-cycle to accumulate a certain amount of charge on the capacitor. The greater the time difference between the positive and negative half cycles of the inverter voltage, the higher the DC charge, and the greater the current flowing through the discharge and discharge. When the current reaches a certain level, the discharge feeling will cause heat or even burn. Therefore, when the venting and discharging sensation is hot, the symmetry of the inverter pulse must be carefully checked. If the asymmetry is concerned, the cause should be analyzed, and the inverter pulse forming circuit should be checked to solve the asymmetry phenomenon of the inverter pulse. In the inverter pulse forming circuit, the two pulse forming circuits are symmetrical. If there is an asymmetry of the inverter pulse, it may be caused by a change in the capacity of the capacitor or the resistance of the resistor, or it may be caused by a change in the internal parameters of the integrated circuit.

      3, the inverter thyristor has a burnout - when an inverter thyristor burns out, the equipment can often start. If you do not pay attention to observe the operating state of the equipment, let the equipment work with disease, the intermediate frequency output voltage waveform Is a distorted waveform. Through the above analysis, we can see that the current flowing through the discharge and discharge is very large, causing the venting and discharging feeling to be hot or burnt.   3, maintenance skills In the equipment maintenance, we can often find a lot of experience and time in the maintenance, often in the final reason is very simple: connection virtual connection, screw is not tightened, water, a device burned and other reasons . In order to shorten the repair time, we need to constantly sum up experience during maintenance, diligent observation, but also have some maintenance skills. As with the doctor to see the patient, in the maintenance of the intermediate frequency power supply, you need to look, smell, ask, and cut the fault location. Hope: The so-called hope is to observe the appearance of the instrument's parameters during the operation, whether there is heat, redness, looseness, etc. in the equipment. Here we mainly introduce the relationship between the instrument parameters and the operating state of the equipment. During the operation of the intermediate frequency power supply, there is a close relationship between the three frequency meters, DC voltage and DC current. We can judge whether the intermediate frequency power supply is running normally by observing the parameters of these three meters. We know that the product of DC voltage and DC current is DC power, and the ratio of DC voltage to DC current can reflect the impedance matching state of the load. For example, for 250KW intermediate frequency equipment, the highest average DC voltage is 513V, and the highest average DC current is 500A. If the DC voltage reaches 500V during operation, and the DC current value is 500A, the impedance is the best match. value. If the DC current is less than 500A, the impedance value is high. If the DC current is greater than 500A, the impedance value is low. That is to say: we can see the matching of the load impedance by observing the values of DC current and DC voltage. The ratio of DC voltage to intermediate frequency voltage can reflect the operating state of the inverter. For example, the DC voltage is 510V, the intermediate frequency voltage is 700V, and the lead angle of the inverter is 36<. We use 700V‖510V=1.37. Generally, the ratio of intermediate frequency voltage to DC voltage is between 1.2 and 1.5. We all think that the inverter works normally. If the ratio is less than 1.2, the lead angle is too small, and the inverter is difficult to change phase; if it is 1.5 times larger, then If the lead angle is too large, the device may be faulty; if it is more than twice, the device is faulty.Smell: The so-called smell means that the device listens to its sound while it is running. When it is heard, there is no noise in the middle whistle, whether the sound is continuous, whether there is dull or no vibration of the reactor; the second is to listen to the sound of the fire, etc. The sound is different. Q: The so-called question refers to understanding the condition of the device in the event of a fault. It should be as detailed as possible, and also understand the operating conditions before the device fails.




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