Analysis of common faults of medium frequency furnace (II)

May 16, 2022

1. The startup equipment cannot start normally

1.1 fault phenomenon: when starting, the DC current is large, the DC voltage and intermediate frequency voltage are low, the equipment sound is dull, and the overcurrent protection.

Analysis and treatment: the thyristor with one arm of the inverter bridge may be short circuited or open circuit, resulting in the operation of the three arm bridge of the inverter bridge. Observe the voltage drop waveform of the thyristor on the four bridge arms of the inverter bridge with an oscilloscope. If the voltage drop waveform of the thyristor on one bridge arm is one line, the thyristor has been penetrated; If it is a sine wave, the thyristor is not conductive. Replace the thyristor that has been penetrated and find out the reason why the thyristor is not conductive.

1.2 fault phenomenon

When starting, the DC current is large, the DC voltage is low, and the intermediate frequency voltage cannot be established normally.

Analysis and treatment: short circuit of compensation capacitor Disconnect the capacitor, find the short-circuit capacitor and replace the short-circuit capacitor.

Fault phenomenon

During the start-up of heavy-duty cold furnace, all electrical parameters and sound are normal, but the power can not rise, so overcurrent protection is required.

Analysis and processing:

(1) The inverse commutation angle is too small. Watch the commutation angle of the inverter thyristor with an oscilloscope and adjust the commutation angle to an appropriate value;

(2) If the insulation resistance of the furnace body is low or short circuited, use a megger to detect the resistance of the furnace body. Eliminate the short-circuit point of the furnace body

(3) The resistance value of the charge steel relative to the induction coil is low, and the resistance value of the charge relative to the induction coil is detected with a megohmmeter; If the resistance value is low, rebuild the furnace.

1.4 fault phenomenon: the zero voltage sweep frequency starting circuit is difficult to start,

Analysis and processing:

(1) If the current negative feedback is not adjusted properly, check the same name end of the current transformer:

(2) Whether the signal line is too long and thin;

(3) If the intermediate frequency transformer and isolation transformer are damaged, pay special attention to the inter turn short circuit of the transformer, readjust the negative current feedback, and replace the damaged parts.

1.5 fault phenomenon zero voltage excitation sweep frequency starting circuit is difficult to start.

Analysis and processing:

(1) If the starting frequency of frequency sweep is not selected properly, re select the starting frequency;

(2) If there is a fault in the frequency sweeping circuit, observe the waveform and frequency of the frequency sweeping circuit with an oscilloscope to eliminate the fault of the frequency sweeping circuit.

1.6 fault phenomenon: all electrical parameters and sound are normal when starting, the current suddenly does not exist when power is increased, and the voltage reaches the rated value for overvoltage and overcurrent protection.

Analysis and treatment: check the load copper bar connector and water cooling cable for open circuit of load.

2. The equipment can be started but the working state is wrong

2.1 fault phenomenon: the equipment can start without load, but the DC voltage cannot reach the rated value, and the DC smoothing reactor has impact sound and jitter.

Analysis and treatment: turn off the inverter control power supply, connect the dummy load on the output end of the rectifier bridge, observe the output waveform of the rectifier bridge with an oscilloscope, and you can see the output phase loss waveform of the rectifier bridge. The causes of phase loss may be:

(1) Loss of rectifier trigger pulse;

(2) The amplitude of trigger pulse is not enough and the width is too narrow, resulting in insufficient trigger power, resulting in thyristor on and off;

(3) The pulse timing of the double pulse trigger circuit is wrong or the pulse is lost;

(4) Open circuit / short circuit / poor contact of the control electrode of the thyristor.

2.2 fault phenomenon:

The equipment can start normally and smoothly. When the power rises to a certain value, overvoltage or overcurrent protection is provided.

Analysis and treatment: find out the fault cause in two steps:

(1) First, operate the equipment without load and observe whether the voltage can rise to the rated value; If the voltage cannot rise to the rated value and overcurrent protection is performed near a certain value of voltage for many times, it may be caused by insufficient withstand voltage of compensation capacitor or thyristor, but it is not ruled out that it is caused by ignition of some part of the circuit,,

(2) If the voltage can rise to the rated value, turn the equipment into heavy load operation and observe whether the current value can reach the rated value; If the current cannot rise to the rated value and overcurrent protection is provided near a certain value of the current for many times, it may be large current interference. Pay special attention to the interference of electromagnetic field with medium frequency and large current to the control part and signal line.

3. Faults prone to occur during normal operation of the equipment

3.1 fault phenomenon: the equipment operates normally, but several KP thyristors and fast melting are burned during normal overcurrent protection action.

Analysis and processing:

During overcurrent protection, in order to release the energy of smoothing reactor to the power grid, the rectifier bridge changes from rectifier state to inverter state. At this time, if α> 120 degrees;, It is possible to overturn the active inverter, burn down multiple thyristors and fast melting, trip the switch, and be accompanied by a huge current short-circuit explosion sound, which will produce a large current and electromagnetic force impact on the transformer, which will damage the transformer in serious cases.

3.2 fault phenomenon:

The equipment operates normally, but the operation of the equipment is unstable near a certain point in the high voltage area, the DC voltmeter shakes, and the equipment is accompanied by squeaking sound. This situation is very easy to cause the inverter bridge to overturn and burn the thyristor.

Analysis and treatment: this kind of fault is difficult to eliminate, which mostly occurs in the high-voltage ignition of a part of the equipment:

(1) Loose screws connecting copper bar joints cause ignition;

(2) The oxidation of the main connector of the circuit breaker leads to ignition;

(3) The screw of the connecting pile of the compensation capacitor is loose, causing ignition, and the internal discharge resistance capacitance of the compensation capacitor absorbs electric ignition;

(4) The insulating part of the water-cooled radiator is too dirty or carbonized, causing fire to the ground;,

(5) The furnace body induction coil ignites the furnace shell / furnace bottom plate. The turn spacing of the furnace body induction coil is too close, and the turn to turn ignition or arcing occurs. The insulating column of the induction coil of the fixed furnace body ignites due to high-temperature carbonization discharge,

(6) Internal ignition of thyristor.

3.3 fault phenomenon: the equipment operates normally, but sharp beeps can be heard from time to time, and the DC voltmeter swings slightly.

Analysis and processing:

Observe the voltage waveform at both ends of the DC of the inverter bridge with an oscilloscope. One cycle fails or an indefinite period fails briefly. The short failure of the parallel resonant inverter circuit can recover itself. The periodic failure is generally caused by the interference of the rectifier pulse in the inverter control part. The aperiodic short failure is generally caused by the poor turn to turn insulation of the IF transformer.

3.4 fault phenomenon: after the equipment operates normally for a period of time, there is abnormal sound, the meter reading shakes, and the equipment works unstable.

Analysis and processing:

The abnormal sound and unstable operation of the equipment after working for a period of time are mainly due to the poor thermal characteristics of the electrical components of the equipment. The electrical part of the equipment can be divided into weak current and strong current for testing respectively. Detect the control part first to prevent damage to the power device of the main circuit. When it is not suitable for the main power switch, only turn on the power of the control part. After the control part works for a period of time, use an oscilloscope to detect the trigger pulse of the control board to see whether the trigger pulse is normal.

On the premise of confirming that there is no problem in the control part, turn on the equipment. After the abnormal phenomenon occurs, observe the voltage drop waveform of each thyristor with an oscilloscope to find out the thyristor with poor thermal characteristics; If the voltage drop waveform of thyristor is normal, pay attention to whether there are problems in other electrical components, especially circuit breaker, capacitor, reactor, copper bar contact and main transformer,

3.5 fault phenomenon: the equipment works normally, but the power cannot go up.

Analysis and processing:

The normal operation of the equipment can only indicate that all parts of the equipment are in good condition and the power can not go up, indicating that the adjustment of various parameters of the equipment is inappropriate. The main reasons affecting the power of the equipment are:

(1) The rectifier part is not adjusted properly, the rectifier tube is not fully connected, and the DC voltage does not reach the rated value, which affects the power output;

(2) If voltage value is adjusted too high / too low to affect power output;

(3) The power output is low due to improper adjustment of the cut-off value;

(4) The furnace body is not matched with the power supply, which seriously affects the power output;

(5) If the compensation capacitor is configured too much or too little, the power output with the best electrical efficiency and thermal efficiency can not be obtained, that is, the best economic power output can not be obtained;

(6) The distributed inductance of the output circuit and the additional inductance of the resonance circuit are too large, which also affects the maximum power output.

3.6 fault phenomenon: the equipment operates normally, but when the power is increased or decreased in a certain power section, the equipment has abnormal sound jitter, and the indication of the electrical instrument swings.

Analysis and treatment: this kind of fault usually occurs on the power given potentiometer. A certain section of the power given potentiometer does not jump smoothly, resulting in unstable operation of the equipment. In serious cases, the inverter overturns and burns the thyristor.

3.7 fault phenomenon: the equipment operates normally, but the bypass reactor is heated and burned.

Analysis and treatment: the main causes of heating and burning of bypass reactor are:,

(1) The quality of bypass reactor is poor;

(2) The asymmetric operation of the inverter circuit exists, and the main reason for the asymmetric operation of the inverter circuit comes from the signal circuit.

3.8 fault phenomenon: the equipment operates normally and often breaks through the compensation capacitor.

Analyze and deal with the causes of faults:

(1) If voltage and operating frequency are too high,

(2) Insufficient capacitor configuration;

(3) In the capacitor boost circuit, the capacity difference between series capacitor and parallel capacitor is too large, resulting in uneven voltage breakdown of capacitor;

(3) But it's not good to break through the capacitor.

3.9 fault phenomenon: the equipment operates normally but has frequent overcurrent.

Analysis and processing:

When the equipment is running, the waveform and sound of all electrical parameters are normal, that is, frequent overcurrent. When such a fault occurs, pay attention to whether it is due to improper wiring that electromagnetic interference and parasitic parameter coupling interference between lines are generated, such as strong line and weak line, power frequency line and intermediate frequency line, signal line and strong line and intermediate frequency line bus are intertwined, etc.

4. DC smoothing reactor

Fault phenomenon: unstable operation of equipment, fluctuation of electrical parameters, abnormal sound of equipment, frequent overcurrent protection and burning of fast thyristor.

Analysis and treatment: in the maintenance of medium frequency power supply, the fault of DC smoothing reactor is a fault that is difficult to judge and deal with. The faults of DC smoothing reactor are:

(1) The user adjusts the air gap and coil turns of the reactor at will, changes the inductance of the reactor, affects the filtering function of the reactor, and makes the output DC current intermittent, resulting in unstable operation of the inverter bridge, inverter failure and burning of the inverter thyristor. Arbitrarily adjust the air gap and coil turns of the reactor, which will reduce the ability of the reactor to block the rise of current and burn the thyristor in case of direct short circuit of the inverter bridge Arbitrarily changing the inductance of the reactor will also affect the starting performance of the equipment;

(2) The reactor coil is loose. If the coil of the reactor is loose, the electromagnetic force makes the coil shake and the inductance changes suddenly during the operation of the equipment, which is easy to cause inverter failure during light load starting and low current operation;

(3) The insulation of the coil is not good. Short circuit to ground or turn to turn short circuit, ignition and discharge cause sudden inductance jump and strong electromagnetic interference of the reactor, making the equipment unstable. The abnormal sound is generated frequently, and the overcurrent burns the thyristor, resulting in poor insulation of the coil insulation layer The causes of short circuit are: a poor cooling and high temperature lead to poor insulation of insulation layer, ignition and carbonization; b. The reactor coil is loose, and the insulation layer is damaged due to relative motion friction between the coil insulation layer and the coil insulation layer, and between the coil insulation layer and the iron core; c. When dealing with the scale of reactor coil, the acid is penetrated into the coil, which corrodes the copper pipe and generates copper salt to destroy the insulation layer.

5. Thyristor

5.1 fault phenomenon: after replacing the thyristor, the thyristor will be burned as soon as it is started.

Analysis and processing:

If the equipment breaks down and burns the thyristor, do not start it immediately after replacing the new thyristor. First, conduct systematic inspection on the equipment to eliminate the fault, and then start it again after confirming that the equipment has no fault, otherwise it will burn the thyristor as soon as it is started. When pressing a new thyristor, be sure to pay attention to pressure balance, otherwise it will cause mechanical damage to the chip inside the thyristor, resulting in a sharp drop in the voltage withstand value of the thyristor, and the phenomenon of burning the thyristor as soon as it is started,

5.2 fault phenomenon: the power on is normal after replacing the new thyristor, but the thyristor is burned after working for a period of time.

Analysis and treatment: the causes of such faults are:

(1) The thermal characteristics of electrical components in the control part are not good;

(2) The installation of thyristor and radiator is misplaced;

(3) The radiator has been used for many times or pressed with a small table thyristor, resulting in the concave center of the radiator table, resulting in poor contact between the radiator table and the thyristor table and burning the thyristor;

(4) The scale in the water chamber of the heat exchanger is too thick and the heat conduction is not good, resulting in overheating and burning of the components;

(5) The temperature of the fast thyristor rises due to poor heat dissipation, and the turn-off time of the thyristor increases with the increase of temperature, resulting in the failure of the element to turn off, resulting in inverter subversion and burning the thyristor;

(6) The operating temperature of the thyristor is too high, the gate parameters are reduced, and the anti-interference ability is reduced, which is easy to cause false triggering and damage the thyristor and equipment;

(7) Check whether the resistance capacitance absorption circuit is in good condition (special attention: the inverter absorption capacitance is charged with a 2500V insulated megger, and then compare the discharge condition with the wire to find out the capacity failure and replace it. The bad one may not be found by measuring with a multimeter).