At present, the medium frequency induction furnace with thyristor frequency conversion has been widely used. With the continuous improvement of thyristor capacity and quality, and the continuous improvement of intermediate frequency technology, induction heating and melting medium frequency furnace has made great progress in use and maintenance. In 1997, Ningbo Shenguang Electric Furnace Co., Ltd. purchased two 500kg intermediate frequency furnaces produced by Zhejiang University power electronics factory, which are mainly used for melting cast iron. Through several years of production and maintenance practice, it is considered that it is necessary to make good use of and repair the intermediate frequency furnace, and be familiar with the common electrical faults and treatment methods of the intermediate frequency induction furnace. Summarizing the experience in the maintenance process is very helpful to guide the future work.
Characteristics of medium frequency furnace and its power supply
1.1 for the 500kg medium frequency furnace used in the plant, 380V three-phase power supply is adopted for the incoming line of medium frequency power supply device, and the rated output power is 250KW. The medium frequency voltage is 750V and the intermediate frequency current is 550A. There are phase sequence indication circuit and display, rectifier control voltmeter, rectifier pulse ammeter, inverter control voltmeter, inverter pulse ammeter, and working? Check transfer switch. In addition to the power supply board, there are also a rectifier board, an inverter board and a protection board. Self excited pre magnetization impact starting is adopted. Its over-current protection is not to use rectifier pull inverter, but to turn off the bridge protection mode, that is, when the main circuit has over-current or over-voltage, it sends a signal to make the control power supply short-circuit instantaneously and block the rectifier pulse. At the same time, the freewheeling diode makes the energy in the filter reactor consumed through the inverter bridge. In addition, each control board adopts kc04, kc41 chips and some operational amplifiers.
1.2 the main circuit circuit circuit and start-up control circuit are shown in Fig. 1 and 2.
2 common electrical fault analysis
The medium frequency induction furnace can be divided into two parts: the control part and the main circuit, that is, the resonance circuit including compensation capacitor and inductor, water cooling cable and busbar. In terms of fault types, there are mainly over-current, over-voltage and low output intermediate frequency power. The causes of these failures are various, which will be analyzed one by one.
2.1 after the control power is turned on and the start button is pressed, the intermediate frequency power supply device does not respond. The main reasons for this kind of fault are as follows:
(1) The circulating cooling water is not opened or the water pressure is not enough. As a result, the normally open contact in the water pressure gauge of the electric contact is not connected, and the rectifier power board in the intermediate frequency cabinet has no power, that is, there is no rectifier voltage output. Therefore, the rectifier trigger board and inverter trigger board have no trigger pulse. Of course, the intermediate frequency power supply device does not respond. Generally, the rectifier pulse voltmeter and ammeter in the cabinet have no display.
(2) The 1kt normally open delay closing contact of the time relay of starting control circuit is damaged or the starting delay time is too long or too short.
The normal delay time is 3-5 S. if the delay time is too short, the rectifier bridge on the main circuit can not timely supplement the energy consumed by the load circuit and reactor (the supplementary energy is generated by the impact during startup), then the attenuation wave formed by the impact will soon tend to zero, so the startup fails. If the delay time is too long, the starting resistance will be seriously heated. Moreover, the current of the main circuit may increase too fast and increase too much, so that the commutation time will be delayed too long, so as to exceed the current exchange capacity of the system at this stage, and the start-up may not be successful.
In addition, if the time relay is normally open, the delay closing contact is damaged, and the control pole has no voltage, then the starting thyristor vt15 cannot be connected. Therefore, the starting main circuit thyristor vt11 can not be connected because there is no trigger pulse, so there is no impact attenuation wave and it is impossible to start successfully.
(3) The starting transformer T4 is burnt out, or the fuse of the primary coil of T4 is burnt out, and the contact of the starting contactor km 2 is not closed or the contact is poor. The start-up transformer T4 is designed for short-time operation, and the normal working time is only a few seconds. If the coil is started repeatedly, the coil will be heated continuously. Therefore, when the main contact of km 2 is stuck or stuck, the coil of T 4 is easily burnt out. In addition, when the fuse 9fu of T4 primary coil
In case of fusing and poor contact of km 2 main contact, the starting trigger circuit will not be electrified, so the intermediate frequency cabinet will not react during starting.
(4) The pre magnetizing resistance r623 of the main circuit is burnt out or the resistance value becomes larger. The main function of the pre magnetizing resistor R6 is to deliver the pre magnetizing current I d before starting, I d ≈ U D? R6 (U D is the DC voltage output by the rectifier bridge when the control angle is a at the start-up; R6 is the pre magnetization resistance at the start-up). It makes the reactor pre magnetized and stores a certain amount of energy in the magnetic field. After the inverter is started, because the equivalent resistance of the load circuit is smaller than that of R6, the current in R6 (i.e. the pre magnetizing current) will transfer to the load and supply energy to the load circuit in time. If the pre magnetizing resistance is burnt out or the resistance value becomes larger, the energy added to the load circuit will disappear or decrease, and the attenuation wave of the starting circuit will fade quickly, so the start-up fails. R6 is composed of four 108200w winding resistors in two series and two parallels, and the resistance value is still 108. The purpose of series connection is to increase the wattage.

(5) the start-up precharge voltage is less than 500V, so that the start-up thyristor vt11 cannot be opened, so the energy of the start-up circuit can not be obtained. Usually, the deterioration or leakage of the precharge capacitor, the damage of the rectifier diode vd10 of the precharge circuit, and the desoldering or faulty soldering of a certain point in the circuit can all cause this kind of fault.

2.2 after pressing the start button, if the sound is heard, then the inverter fails. Over current or over-voltage, over-current and over-voltage fault are the most common faults of intermediate frequency furnace. The fault phenomena are various and the causes are different, among which the over-current fault is more than the over-voltage fault.
(1) when the leading angle B is adjusted too small, the commutation time TC of the inverter thyristor increases, and the corresponding reserve time TB decreases. Therefore, it is difficult for the inverter to exchange current, resulting in over-current and inverter failure. We know that td = TC + TB (where TD is the trigger time before the lead, TC is the commutation time, TB is the reserve time). Only when TB > TQ (TQ is the turn off time), the forward blocking characteristic of the thyristor can be ensured and the commutation can be successful. When TB decreases due to the increase of TC, the thyristor which should be turned off during commutation can not restore the forward blocking characteristic, which leads to the direct short circuit and finally leads to the inverter failure.

3. Inspection methods and steps of intermediate frequency furnace failure
(1) First, observe whether the indication value of the four small meters in the intermediate frequency cabinet is normal. Among them, the rectifier control voltmeter is 30V, the rectifier pulse ammeter is 130 ~ 150mA, the inverter control voltmeter is 12V, and the inverter pulse ammeter is 100 ~ 120mA. If the value is in the normal range, it proves that there is no problem with the power supply part.
(2) Check the anode and cathode resistance of rectifier and inverter thyristor and the resistance value of control pole and cathode with digital multimeter 2008 (it is unnecessary to remove it from the cabinet for measurement, and the pipe radiator still has cooling water). The positive and negative resistance values of anode and cathode are ∞, and the resistance values of control electrode and cathode are 10-508. In addition, the fuse should be checked for fusing.
(3) Put the transfer switch SA in the check gear, check the waveform of rectifier and inverter trigger pulse with oscilloscope, and check whether the amplitude and time interval are normal. Among them, the rectifier trigger pulse is double pulse, and the time interval is 3.33M
s. The inverter trigger pulse is a continuous pulse train, and the amplitude is generally 4-6V. The pulse should be neat and without burr. The inspection sequence is from thyristor control pole to pulse transformer, then to rectifier board and inverter board.
(4) Check whether the rectifier board is normal. The inverter board can be pulled out, and the change-over switch is placed in the check gear. Press the start button and turn the power adjusting potentiometer to see if the DC voltage can be adjusted to about 500V. If the power (4) is used, check whether the rectifier board is normal. The inverter board can be pulled out, and the change-over switch is placed in the check gear. Press the start button and turn the power adjusting potentiometer to see whether the DC voltage can be adjusted to about 500V. If the voltage can be adjusted to 500V, it is proved that the rectifier board is normal.
(5) Check the capacitor charging circuit in the starting circuit. Still pull out the inverter board and connect the check file. After pressing the start button, use a multimeter to measure the voltage at both ends of the capacitor cf. if the voltage reaches about 500V, it is proved that the charging circuit of the starting capacitor is normal.
(6) Check whether the pre magnetization resistance R6 is burnt out and whether the low-pass filter is broken.
(7) If the above inspection is normal, it can be considered that the fault is basically the load part of the main circuit. At this time, check whether the capacitor has obvious burn out trace or serious oil leakage, whether the insulation of capacitor bracket to ground is about 2m8, whether the water-cooled cable is burnt out, and whether the induction coil has short circuit to ground and turn to turn (generally caused by molten iron leakage in furnace lining). When the induction coil is connected with water, its resistance to ground should be more than 5k8, and the insulation resistance of induction coil to yoke should be about 2m8 (when the yoke is not grounded).
(8) Through the inspection, if it is considered that the medium frequency power supply cabinet is normal, the capacitor is also normal, the induction coil and yoke have passed the intermediate repair, the insulation meets the requirements, and the furnace lining is newly built, but there is still over-current phenomenon in power transmission, it can be considered that the thermal characteristics of some inverter thyristor are not good, that is, the characteristic data are normal under the condition of no power transmission, But after power transmission, there is a forced positive turn due to heating, resulting in over-current. At this time, the inverter tube should be replaced one by one to see if it is still over-current.
(2) The failure of automatic frequency modulation circuit and inverter trigger circuit leads to the disorder of trigger pulse and waveform distortion. During inverter current conversion, the on pipe is not connected, and the turn off is continuous, resulting in short circuit and over-current. Under normal conditions, the diagonal pulse of inverter bridge should overlap, and the waveform should be clear and neat, and the amplitude and width should meet the requirements. The phase difference between two adjacent groups of inverter bridge should be 180 ℃. The automatic frequency modulation loop of the device adopts the timing principle, that is, the frequency modulation signal is obtained from the synthesis of intermediate frequency voltage and current signals. The synthesized signal is capacitor capacitor and IC is capacitor current, which is sent to inverter trigger circuit and then to inverter thyristor through pulse transformer. If the voltage signal or current signal is distorted or disordered due to line fault or interference, the synthetic signal is bound to be abnormal. If the inverter trigger circuit fails, the trigger pulse will be abnormal (in phase, amplitude, waveform). As a result, the inverter tube is short circuited, resulting in over-current.
(3) The characteristics of an inverter tube on the left or right arm changes, but when it should not be turned on, it will cause a straight through short circuit in the left or right arm of the inverter circuit, resulting in over-current. This mainly refers to that the thermal characteristics of an inverter thyristor are not good, and all characteristic parameters are normal in the cold state. However, once the power is transmitted, when the intermediate frequency voltage reaches a certain value (about 300V), the positive turning occurs, resulting in over-current due to the through.
(4) The induction coil is short circuited to ground or inter turn. When the induction coil is insulated to the ground, the resistance value should be more than 2m 8. If water is connected and measured with a multimeter, it is above 5k8 (because the water is connected with the ground, but there is a certain resistance). The short circuit to ground or turn to turn short circuit of induction coil is mostly caused by the leakage of molten iron in furnace lining. After the molten iron is cooled, the induction ring is connected with the ground and inter turn connected, resulting in short circuit.
(5) The capacitor is short circuited between phases or to ground. The medium frequency capacitor is an electrothermal capacitor with cooling water inside. When there is scale or foreign matter in the water circuit, the water flow will be greatly reduced or even cut off, which will aggravate the heating of the capacitor and burn out, resulting in phase to phase short circuit. If the capacitor oil leakage is serious, the electric capacity will be greatly reduced, at the same time, the capacitor heating will surge and the phase to phase short circuit will occur.
The intermediate frequency capacitor shell is a pole, which is directly placed on a bracket welded by angle steel. The capacitor bracket is separated from the ground of the electrical network by a bakelite base. If the insulation of the bakelite seat to the ground is greatly reduced due to moisture and oil pollution, it is easy to break through the ground and short circuit. Both of the above short circuits will cause over current.
(6) When starting, due to the fault of the rectifier trigger circuit, the rectifier trigger pulse is abnormal, which makes the angle a larger, then the output DC voltage is too low, and the energy of oscillation attenuation wave generated by impact is too small during startup, which makes the startup fail and forms over-current.
Generally speaking, when the intermediate frequency power supply is started, under a certain load, the minimum and maximum value of the DC voltage U D have a certain range, that is to say, the starting range is generally 30-80v. Beyond this range, either the supplementary energy is too small, or the main circuit current growth rate is too fast, too large, resulting in difficult commutation.
(7) the circuit impedance of the low-pass filter (DL) becomes larger, which makes the thyristor working asymmetrically at the inverter end and the static charge accumulated by the carrier accumulation effect can not be released in time, so the turn-on and turn-off characteristics become worse. Poor turn-off results in inverter failure, resulting in over-current. The low-pass filter is similar to a small reactor, which is made of 6 M2 enameled wire, connected to the load part, connected in parallel with the voltage multiplier capacitor and connected with the induction coil of the furnace. As a way to discharge the static charge accumulated by the thyristor when the inverter works asymmetrically. The cross-section of the connecting wire between it and the load circuit should be copper wire of more than 10 m2. The shorter the length, the better, so as to reduce the impedance of the discharge circuit, ensure that the electrostatic charge is discharged at a certain rate and ensure the success of inverter. Sometimes the current formed by electrostatic charge is very large, which will make the low-pass filter burn out seriously.
(8) When the characteristic of the thyristor in the over-current protection board deteriorates, its sensitivity will increase. When the starting current does not reach the over-current setting value, it will trigger the conduction and send the over-current signal to block the control power supply and make the inverter fail.
(9) One of the inverter tubes of diagonal bridge arm is not connected because there is no trigger pulse. In this way, the open pipe will be closed continuously. At this time, the power factor is very low and the starting current is very large, that is, the over-current protection action is generated.
(10) The poor contact of the power regulating potentiometer causes the DC voltage to fluctuate from large to small, from one to another, and the energy supplement to the load circuit is very unstable when starting, which is also easy to cause starting failure and cause over-current. The main reasons for over-voltage are as follows: (1) the FM signal circuit is disconnected due to wire burning or loose joint, which makes the leading angle B + C2 larger (where B is the phase angle from T2 to T3; C is the phase angle from T1 to T2), that is, the U angle becomes larger and COSU becomes smaller (COSU is the load power factor), then the output intermediate frequency voltage UA increases (because u a = 1.1u DCOS U), Therefore, it is easy to cause overpressure. (2) There is a fuse burning in the inverter bridge, which can cause over-current and over-voltage at the inverter end. (3) If the load circuit fails, such as the induction coil and the water-cooling cable are burnt out due to the leakage of molten iron or liquid breaking in the furnace lining, it is easy to generate high voltage instantly. This is because after the induction coil or water-cooled cable is disconnected, there is no oscillation in the slot circuit, and only the capacitor itself has been charged with a certain voltage in the load circuit. If the energy in the filter reactor is charged to the capacitor again, it is bound to generate high voltage and cause overvoltage action. (4) The characteristics of the thyristor in the over-voltage protection board are deteriorated and the sensitivity is increased. When the voltage does not reach the over-voltage setting value, the thyristor will trigger the conduction and generate the over-voltage protection action. (5) When the medium frequency power supply is powered on, the charge is added. One end of the iron block or return charge at the furnace mouth is connected with the molten iron, and the other end collides with the metal shell of the furnace (such as platform or furnace mouth iron ring) to ignite. The interference signal generated affects the control circuit and causes overvoltage action.
(6) The insulation of medium frequency voltage transformer coil to ground is decreased, and there is incomplete short circuit. When the medium frequency voltage rises to more than 300V, it will be completely short circuited to ground. The voltage signal generated in the short circuit makes the overvoltage protection board act.
2.3 the output power is too low to reach the rated value
500kg medium frequency furnace, its rated output power is 250KW. However, due to various reasons, sometimes the output power is far below the rated rate, even only 40-60kw. The common reasons are as follows:
(1) Low rectifier voltage results in low output power. The main causes of low rectifier voltage are as follows:
a. The rectifier circuit lacks phase. At this time, the oscillograph is used to observe the waveform of the rectifier main circuit, and the main circuit filter reactor emits dull and irregular vibration sound. If the resistance value of a phase thyristor measured with a multimeter is infinite, it is proved that the thyristor has been broken after damage, or a fuse has been burnt out. At this time, if the current is still working under the rated current, the current flowing through other bridge arms is too large, which will shorten the life of thyristor. Generally, the output power can only reach 60kW after phase loss.
b. There is an incomplete short circuit between induction coils or to ground. With the increase of medium frequency voltage and power, it becomes a complete short circuit, so the over-current occurs in a certain intermediate frequency voltage due to short circuit, so the power can not go up.
c. When the current and voltage cut-off circuit fails, it will output without reaching the setting value, which limits the rise of rectifier voltage.
d. The trigger delay angle of phase shift pulse cannot be adjusted to a = 0.
e. A bridge arm thyristor trigger circuit failure, or the performance of the thyristor deteriorated can not trigger conduction.
(2) If the phase angle of inverter voltage and current (leading phase angle) is too small, the intermediate frequency voltage UA is lower than the rated value, which seriously affects the improvement of intermediate frequency output power. This is generally caused by the damage of capacitors in parallel on the channel, so it is appropriate to make u a? UD ≈ 1.4.
(3) When the furnace wall thickens, the furnace becomes smaller, that is, the equivalent resistance R of the furnace increases and the intermediate frequency power decreases.
2.4 the instability of DC voltage and intermediate frequency voltage may lead to the instability of DC voltage: ① the trigger circuit has false soldering, sometimes there is no trigger pulse, which makes the thyristor turn on and off, at this time, it can be seen that the rectifier trigger ammeter also swings. ② If interference causes the rectifier thyristor to misdirect, so the DC voltage output is irregular and fluctuates. If the filter reactor collides with the line, its inductance value will decrease, and the isolation effect of intermediate frequency power supply will be reduced. If voltage will invade the rectifier circuit, the rectifier thyristor will turn forward under the action of intermediate frequency voltage, resulting in unstable load voltage. ③ When the rectifier voltage is out of phase and unbalanced, the reactor LD will vibrate and make a great noise, which will cause DC voltage instability and DC voltmeter swing back and forth. The causes of voltage and current instability may be as follows:
(1) The quality of the inverter thyristor is poor, and the thermal performance is unstable, such as the opening time and reverse recovery time are inconsistent, and the temperature characteristics are not good.
(2) The asymmetrical trigger pulse of inverter makes the time of opening and closing of the tube different, which leads to the fluctuation of medium frequency voltage and current.
(3) The DC voltage fluctuation of rectifier bridge causes the fluctuation of intermediate frequency voltage.
(4) The setting value of the potentiometer on the inverter board changes and some components such as triode are unstable.