The end of debugging of medium frequency induction heating power supply

Last time we talked about the debugging of frequency heating power supply, we only talked about the first half. Today we will make up the remaining part. Even if it is the end of the medium frequency induction heating power supply.

10. The inverter cannot vibrate. If the output lines of the 20v winding of the medium frequency voltage transformer are reversed. Still can't start. At this time, you should confirm whether the resonant frequency of the tank circuit is correct. You can use a capacitance/inductance meter to measure the capacitance of the capacitor and the inductance of the sensor to calculate the resonant frequency of the tank circuit. When the resonant frequency of the tank circuit is at its highest, it When the excitation frequency is 0.6~0.9, it should be very easy to start. Then check whether the inverter thyristor has a damaged ring.

11. Problems in setting the inverter rake angle. After the inverter starts oscillating, you can adjust the inverter lead angle. Turn all DIP switches to the OFF position, use an oscilloscope to observe the waveform of the 100V winding of the voltage transformer, and adjust the W5 trimmer potentiometer on the main control board to make the inverter phase The lead angle is about 22°. At this time, the ratio of the intermediate frequency output voltage to the DC voltage is about 1.2 (if the commutation overlap angle is large, this ratio can be increased appropriately). This step is to set the minimum inverter lead angle. Generally, we hope that it is as small as possible. Of course, if the reverse conversion phase lead angle is too small, the reverse conversion phase will fail, which will appear as repeated starts when the intermediate frequency voltage rises.

Then put the DIP-l switch in the ON position, adjust the W3 trimmer potentiometer on the main control board, and set the maximum inverse conversion phase lead front angle. According to the requirements of different intermediate frequency output voltages, the maximum inverse conversion phase lead front angle is also different, such as the intermediate frequency device. When the three-phase input voltage is 380V and the rated intermediate frequency output voltage is 750v, the maximum inverse conversion phase lead angle is required to be about 42°. At this time, the ratio of the intermediate frequency output voltage to the DC voltage is 1.5; if the three-phase input voltage of the intermediate frequency device It is still 380V, and the rated intermediate frequency output voltage is 1000v, which requires the maximum inverse conversion phase rake angle to be around 56°. At this time, the ratio of the intermediate frequency output voltage to the DC voltage is 2.0. It is generally hoped that it will be as large as possible, so that when the system input voltage is low, the intermediate frequency output voltage can still be guaranteed to reach the rated value. When the system input voltage is on the high side, due to the function of the voltage regulator, the IF output will still not experience overvoltage. This debugging work can be performed at a lower intermediate frequency output voltage. Note that the minimum inverter rake angle must be adjusted first, and then the maximum inverter rake angle, otherwise the order will be reversed and problems involving each other will occur. Sometimes the voltmeter is inaccurate, which brings wrong conclusions to the debugging, so the lead angle measured by the oscilloscope should be used as the standard.

If there is a phenomenon that the inverter lead angle is not adjusted too small during debugging, after eliminating the cause of the tank resonance frequency being too low, you should check whether all the inverter thyristors are working. When only three thyristors are working, inversion will occur. The rake angle is too large.

12. Setting of rated output voltage. Set the rated output voltage under light load conditions. In this debugging, it can be seen that the impedance regulator works, that is, the DC voltage no longer rises, but the intermediate frequency output voltage can continue to rise with the rotation of the "given" potentiometer. When the rated output voltage is not adjusted, it should be carried out under the condition that the DC current is lower than the rated current. Otherwise, the intermediate frequency output voltage will not be adjusted due to the action of the current regulator. Adjust Wl so that the intermediate frequency voltage reaches the rated value.

13. It excites frequency. Its excitation frequency must be higher than the possible maximum resonant frequency of the tank. Otherwise, the system will not be able to operate normally due to the "pull" of its excitation frequency. It is appropriate for its excitation frequency to be 1.2 times higher than the possible maximum resonant frequency of the tank. .

14. Constant power output. For the smelting load, constant power output is very important. In order to make the range of the constant power zone large, it is necessary to change the inverter lead angle from the smallest to the largest range as large as possible. At the same time, the load impedance matching should also be Very important, even if it is not a melting load, doing so will help improve the rectifier power factor.

(2) Inspection before powering on. After the intermediate frequency power supply is transferred to the new construction site with the construction site and installed, or after the installation of the newly manufactured intermediate frequency power supply is completed, the following inspections must be carried out before powering on.

① Check the wiring of the main circuit and control circuit. They must be consistent with the drawings. When the line number is unclear and there is doubt, you must use a meter to check the line and confirm it is correct. When the polarity of magnetic components (transformers, pulse transformers) is unclear, polarity experiments must be performed.

② Use a wrench to check and tighten all the connecting screws of the main circuit one by one. For contact surfaces or contactor contacts that have been thermally oxidized or singed, they should be cleaned with sandpaper. Use a screwdriver to check and tighten the crimping screws of the terminal block one by one.

③The indicator bulbs on the panel and buttons are easy to loosen. You can use batteries for power-on inspection. In particular, the obstacle-passing indicator light must be in good condition, otherwise it will affect the judgment of the protection action.

④ In addition to the solder joints of printed circuit boards that should be inspected, the solder joints of power transformers, pulse transformers, pulse boards, etc. should also be inspected. When manufacturing intermediate frequency power supplies, due to negligence in the welding process, desoldering and virtual soldering may occur due to corrosion after a period of use. Some components can even be pulled out with a slight pull and must be carefully inspected. For the found virtual solder joints, they must be cleaned first and then soldered. You should also check whether the plug-in part of the printed circuit board and the plug-in connector of the component are in good contact, and wipe off the flux with alcohol or gasoline.

⑤ Use the R×lK scale of the multimeter to measure the resistance between the anode and cathode of the thyristor in the main circuit to initially judge the quality of the component. After the water system is filled with water, the water resistance is connected across the component, and the forward and reverse resistance of the water-cooled component is about tens of thousands. Europe. When two or more components are used in series, since there is a voltage equalizing resistor in parallel, the positive and negative resistances are generally below lOk.Q, and their resistance values must be equal to ensure static voltage equalization.

⑥Insulation inspection. In order to avoid breakdown and damage to silicon components due to the application of voltage between the cathode and anode of the thyristor component, a megger is generally not used. A multimeter can be used to measure the main circuit with high resistance. What should be the resistance between the control circuit and the case (ground)? Ten thousand euros. The insulation resistance of the intermediate frequency power supply output line to the ground should generally be above loko. If it is too low, further inspection should be carried out.

⑦ If the intermediate frequency power supply uses a forced air cooling system, attention should be paid to checking the tightness of the cooling air duct to prevent air leakage. At the same time, check the air inlet and outlet to avoid being too close to each other to avoid self-circulation and reduce the cooling effect. For intermediate frequency power supplies that use water cooling, in order to avoid water channel scale and sediment accumulation from clogging the pipelines, attention should be paid to the requirements for cooling water quality, and sufficient cooling water volume should be ensured. If the water quality cannot meet the water quality requirements of the intermediate frequency power supply, water softening equipment such as ion exchange water softening equipment or magnetic water purifiers can be used for water treatment to ensure the safe operation of the device. Furthermore, when the moisture content in the air is high in summer, external condensation is likely to occur on the water-cooling components. Care should be taken to adjust the cooling water temperature to prevent condensation. Therefore, corresponding insulation measures must be taken. For equipment that is not in use temporarily, the cooling water system should be left empty. Drain the water and blow it out with compressed air if necessary to avoid freezing and damaging the equipment. The connection of waterways must be carried out according to the principle of equipotentiality, and waterways with the same potential can be connected in series.

⑧ Check the power phase sequence. The thyristor intermediate frequency power supply has strict requirements on the phase sequence of the power supply. The phase sequence of the power supply must be calibrated before powering on, otherwise it will cause the phase of the trigger system to be inconsistent. The phase sequence of the power supply must be aligned before powering on, otherwise it will cause the phase of the triggering system to be inconsistent. confusion, the device will be damaged. For the intermediate frequency power supply of 380V incoming line, since the synchronization signal is taken from the gate of the thyristor, there is no need to check the phase sequence. However, when a synchronous transformer is required for the high-voltage incoming line, the correctness of the phase sequence must be checked. Return to equipment ground. According to safety regulations, the shell of the equipment must be safely grounded, the shield lead of the shielded wire should be reliably grounded, and the shield leads of various small control transformers and pulse transformers should be grounded or not grounded according to the requirements of the manufacturer's instructions. The common neutral line of the weak current control loop is generally connected to zero in a floating manner and not grounded. If the manufacturer's instructions have special requirements, it should be handled as required.

(3) Power on and debug. After completing the above inspections, the following procedures should be followed for live debugging.

① Check the relay contact action. When the main circuit is not charged, conduct a simulated action test on the action program of the relay contact part according to the drawing. When conducting this test, it is generally required to short-circuit the contacts of the hydraulic relay first, follow the procedure, use the operation buttons on the operation panel, and observe whether the action sequence and delay of the relay contactor are correct. Some relay actions can be artificially short-circuited. Instead, you may find:

a. After the contactor is closed, the electromagnetic vibration sound is very loud. Most of the reasons are rust or dirt on the core suction surface, or the grid voltage is low.

b. A certain action fails or is unreliable. You should check whether the relay, especially the mechanical time relay, is flexible in action, whether the contact contact is good, and use a multimeter to check whether the relay contacts are on and off in compliance with the requirements.

②Inspection of magnetic components. For newly installed intermediate frequency power supplies, you should first check whether the primary and secondary sides of the synchronous transformer, the positive and negative synchronous voltage waveforms, phase sequence, phase, etc. are consistent with the drawings.

③Rectifier trigger system inspection

a. Disconnect the inverter output pulse (disconnect from the main control board).

b. Connect a resistive load of about 1~2kW to the DC side of the rectifier bridge. The current trimmer potentiometer W2 on the circuit board is turned clockwise to the most sensitive end (it can provide overcurrent protection when a short circuit occurs during debugging). Use an oscilloscope to prepare to measure the rectified output DC voltage waveform; turn the "given" potentiometer on the panel counterclockwise to the minimum.

Start at medium frequency and supply three-phase power supply. Turn the "given" potentiometer on the panel clockwise. The DC voltage waveform should be almost completely released. The output DC waveform should be a continuous six-wave first wave, or observe the voltmeter indication. It should be about 50ov. When adjusting the power adjustment potentiometer, the output DC waveform should be continuous and smooth in the entire phase shift range. If the six wave heads are uneven, individual sawtooth waves can be slightly adjusted. If a wave head is lost, you can easily find out the component that lost the wave head from the waveform of the thyristor (because there is no straight line during conduction on its waveform, or the length of the straight line does not reach 1200), it may be caused by one of the following reasons:

a. The control electrode of the thyristor component is disconnected or short-circuited

b. The trigger power required by the thyristor is too large, and the output power of the trigger circuit is not enough.

c. Pulse transformer = the secondary polarity is reversed, resulting in a negative pulse, and the thyristor does not conduct.

If there is a skip phenomenon in the DC output waveform, it means that the thyristor is misdirected. The conduction angle of the misdirected thyristor will increase. Therefore, it is easy to use an oscilloscope to detect the misdirected component from the voltage waveform of the thyristor, causing possible damage. Common reasons for misleading silicon control include:

a. The triggering voltage and current of the thyristor are too small and are extremely susceptible to false triggering by interference signals.

b. The forward blocking capability of the thyristor is insufficient, and it is mistakenly turned on when there is no trigger pulse.

c. The resistance-capacitance absorption circuit at both ends of the thyristor is open, and the du/dt applied to the thyristor is too high, causing misleading conduction.

If it is due to poor characteristics of the thyristor itself, the only option is to replace the thyristor.

④Inverter trigger system test

a. The inverter bridge thyristor trigger pulse, its pulse amplitude should be greater than 4V, generally use a strong trigger amplitude or even above iov. At the same time, please note that the pulse front edge should be less than 2P.s, and the pulse width should meet 50---100us.

b. Observe whether the diagonal pulses of the inverter bridge overlap.

c. Observe whether the phase difference between the two adjacent groups of inverter bridge pulses is 180°. In terms of position, if there is a deviation in the pulse position, the pulse forming plate should be replaced.

When there is no pulse when the inverter triggers, or the pulse is abnormal, use an oscilloscope to check step by step.

⑤Inspection of intermediate frequency power supply startup. After the above inspections are completed, the voltage and current values can be adjusted according to the debugging steps of the intermediate frequency power supply. For safety reasons, first set the voltage value without load, then run it for 20 to 30 minutes without load, observe the working condition of the entire system, especially whether there is heating in the furnace body, then put a crucible into the furnace body to set the current value, and finally Build furnaces, bake ovens, and smelt metals.