Intermediate frequency power supply

Introduction

The intermediate frequency power supply is a static frequency conversion device that converts the three-phase power frequency power supply into a single-phase power supply. It has strong adaptability to various loads and wide application range. It is mainly used for melting, heat preservation, sintering, welding, quenching, tempering, diathermy, metal liquid purification, heat treatment, bending, and crystal growth of various metals.

Standard output power series: 30kW~4000kW

The standard configuration melting furnace series is: 5kg (30kW) ~ 5000kg (4000kW)

The standard oscillation frequency series is: 400Hz~10kHz

The medium frequency induction heating technology heats the workpiece by the principle of electromagnetic induction and using eddy current. Because induction heating has a series of advantages such as fast heating speed, high heat generation efficiency inside the material, uniform heating and selectivity, good product quality, almost no environmental pollution, good controllability and easy automation of production, it has been rapidly developed in recent years. cut. At present, induction heating has been widely used in industries such as casting smelting, forging blank heating, metal surface heat treatment, and aluminum electrolysis. Most of the traditional heating methods in these industries are based on coal, oil and gas as energy or box-type electric furnace heating. There are defects such as high energy consumption, poor working conditions, serious environmental pollution, and difficult control of process quality, which seriously restricts equipment manufacturing in China. The development of the industry. Therefore, the comprehensive promotion of induction heating technology is an inevitable trend in transforming China's traditional industries, and the development of this technology is closely related to the level of induction heating power.

Medium frequency power supply structure

The intermediate frequency power supply has evolved from an early intermediate frequency generator set to a thyristor type variable frequency power supply, and has been continuously developed into a new generation of variable frequency power supply unit.

The intermediate frequency power supply mainly includes a rectifier transformer, a thyristor rectifier, a freewheeling diode, an inverter, and a DC reactor that couples the rectifier and the inverter, and a corresponding control loop and a protection loop.

Transformer and rectifier

The thyristor rectifier of the IF power supply can generate a large amount of high-order harmonic current, which can be regarded as a harmonic source. In order to reduce its harmonic hazard, the design of its rectifier device is to increase the number of rectification pulsations as the main measure to suppress harmonics. Normally, 6-pulse rectification is applied to an intermediate frequency power supply device below 1000 kW, and the generated harmonics are mainly 6 k士1 (k is a positive integer) times characteristic harmonic current; and for medium frequency power supply devices above 1000 kw, according to capacity Size can be 12 pulsating or 24 pulsating rectification. For a 12-pulse rectifier circuit, it consists of two sets of 6-pulsed three-phase bridges connected in parallel. The AC sides of the two sets of bridges are respectively connected to the two secondary windings of the three-winding transformer. One winding is a star connection and the other is a delta connection. The line voltage difference between the two is 30. When the two sets of bridges are synchronously controlled, the two sets of rectifier bridges get the same firing angle. After analysis, the 5th and 7th harmonic currents from the two sets of rectifier bridges will cancel each other on the primary side of the transformer. Similarly, the 17th and 19th harmonic currents cancel each other out. At this time, the lowest-order characteristic harmonics on the grid side will be the nth and 13th harmonics, followed by the 23rd and 25th harmonics. The waveform of the primary line current of the transformer is three-stepped, closer to a sine wave.

working principle

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 direct current power source, and then through the single-phase inverter bridge, the direct current is inverted into a certain frequency ( A single phase intermediate frequency current of typically 1000 to 8000 Hz). The load is composed of an induction coil and a compensation capacitor, and is connected into a parallel resonant circuit (which can also be connected in series. In general, the IGBT power supply uses series resonance, of course, the IGBT power supply can also adopt parallel resonance).

Under normal circumstances, the fault of the intermediate frequency power supply can be divided into two categories according to the fault phenomenon that 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 fast fuse. When the indicator is normal, it will shrink inside the casing. When the fuse is blown, it will pop up. 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 is ineffective and will not 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 direction is unreasonable.

(III) 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 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 rack. 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 failure 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, the resistance between the capacitor housing lead and the capacitor holder can be measured to determine the insulation condition of this part.

(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 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. The oscilloscope can be used to check the fault, and the oscilloscope probe is clamped at both ends of the load to observe whether there is any attenuation waveform when the start button is pressed. 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 block (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.

Troubleshooting

Through the inspection of the above aspects, it is generally possible to find out most of the causes of the fault, 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 the gate has no pulse, 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 and the trigger pulse frequency is calculated. Normally, 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. This fault is generally caused by a thyristor of the rectifier that has no trigger pulse or trigger non-conduction. At this time, you should first use the oscilloscope to look at the gate pulse of the six rectifier thyristors. If there is any, after the shutdown, measure the gate resistance with a multimeter 200Ω file, and replace the thyristor that is not connected or has a particularly large gate resistance. can.

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. . Observe the voltage waveform between the anode and cathode of the four inverter thyristors in turn with an oscilloscope. 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.

main feature

The main features of the IF power supply are as follows:

First, the advanced repetitive start function, achieving 100% successful start-up mode adopts its self-excited swept-type zero-voltage soft start mode. During the whole startup process, the frequency adjustment system and current, voltage regulation and shutdown system, Tracking the change of load at all times to achieve the ideal soft start. This kind of starting mode has small impact on the thyristor, which is beneficial to prolonging the service life of the thyristor. At the same time, it has the advantages of easy start of light and heavy load, especially the steel furnace is full. The freezing furnace can be easily started.

Second, the cutting-edge technology, excellent energy-saving effect control circuit adopts the microcomputer constant power processing circuit system, and the inverter automatic angle adjustment circuit is installed, and the voltage, current and frequency changes are automatically monitored at any time during the operation. Therefore, the change of the load is judged, and the matching of the load impedance is automatically adjusted to achieve a constant power output, thereby achieving the purpose of saving time and saving power factor.

Third, the perfect design, the circuit is reliable. Because the control circuit uses CPLD software design, its program input is completed by computer, its pulse precision is high, anti-interference, fast response, convenient debugging, with interception, interception, over Flow, overvoltage, undervoltage, phase loss, water shortage, and many other protection functions, because the circuit components always work within the safe range, thus greatly improving the service life of the equipment.

Fourth, the device can automatically judge the phase sequence of the three-phase incoming line, without the need to distinguish A, B, C phase sequence, debugging is very convenient.

5. The software used in the program is imported from the United States. The production of the circuit board is all automatic soldering with wave peaks. There is no virtual soldering phenomenon. All kinds of adjustment systems adopt non-contact electronic adjustment. There is no fault point, the failure rate is extremely low, and the operation is very convenient. .

Sixth, high-frequency equipment has obvious energy saving and small grid pollution.