Constant Power Intermediate Frequency Power Supply Control Panel is the sixth generationIntermediate frequency power control board.
1. Principle of Control Circuit
The entire control circuit is made into a printed circuit board structure except for the inverter final stage trigger circuit board, which is functionally divided into a rectifier trigger part, a regulator part, an inverter part, and a starting calculation part.
(1) Working principle of rectification trigger
This part of the circuit includes three-phase synchronization, digital trigger, final stage drive and other circuits. The trigger part adopts digital trigger, which has the characteristics of high reliability, high precision, and easy debugging. The characteristic of the digital flip-flop is to use the method of counting (clock pulse) to realize the phase shift. The clock pulse oscillator of the digital flip-flop is a voltage controlled oscillator, and the output pulse frequency is controlled by the α phase shift control voltage Vk. When Vk decreases, the oscillation frequency increases, and the count number of the counter is fixed (256). The high pulse frequency of the counter means that the time required to count a certain number of pulses is short, that is, the delay time is short, the angle α is small, and vice versa. The angle is big. The time when the counter starts counting is also controlled by the synchronization signal, and starts counting when α=0 degrees. Now suppose that at a certain Vk value, according to the relationship between the control voltage and frequency of the voltage controlled oscillator, the output oscillation frequency is determined to be 25kHz, and the time required to count to 256 pulses is (1/25000)〜256=10.2( mS), which is equivalent to about 180＜ electrical angle. The count reset pulse of this trigger is at 30＜ of the synchronous voltage (line voltage), which is equivalent to the β=30＜ position of the three-phase fully controlled bridge rectifier circuit. From the clear pulse, the output trigger pulse generated by the delay of 10.2mS is close to the position of a phase of the thyristor of the three-phase bridge rectifier circuit α=150 degrees. If you need to get an accurate α=150 degree trigger pulse, you can adjust it slightly Click the potentiometer W4. Obviously, there are three sets of the same trigger circuit, and the voltage-controlled oscillator and the Vk control voltage are common, so that 6 trigger pulses with a phase difference of 60 degrees are generated in one cycle.
The advantage of digital flip-flops is stable operation, especially with HTL or CMOS digital integrated circuits, which can have a strong anti-interference ability.
IC16A and its surrounding circuits constitute a voltage-frequency converter, and the period of its output signal changes linearly with the output voltage Vk of the regulator. Here W4 trimming potentiometer is the lowest output frequency adjustment (equivalent to analog circuit sawtooth amplitude adjustment).
The three-phase synchronization signal is directly obtained from the three-phase incoming line of the main circuit by the gate leads K4, K6, and K2 of the thyristor, filtered and phase shifted by R23, C1, R63, C40, R102, and C63, and then passed through 6 photoelectric couplings The device is electrically isolated to obtain the output of 6 rectangular wave synchronization signals (such as IC2C, IC2D) with a phase difference of 60 degrees and a duty cycle slightly less than 50%.
IC3, IC8, IC12 (14536 counter) constitute a three-way digital delayer. After the three-phase synchronization signal resets the counter, it outputs a delay pulse for every 256 pulses of the output pulse of the voltage-frequency converter. Because the frequency of the count pulse is controlled by Vk, in other words, Vk is controlled. Delay pulse.
The pulse output by the counter is isolated and differentiated into a narrow pulse and sent to the subsequent LM556. It has both a synchronous frequency divider function and a fixed output pulse width function. The output narrow pulse is synthesized into a double narrow pulse by resistance, and then amplified by a transistor to drive the pulse transformer to output. The specific timing diagram is shown in the attached drawings.
(2) How the regulator works
There are four regulators in the regulator part: intermediate frequency voltage regulator, current regulator, impedance regulator, and inverter angle regulator.
Among them, the voltage regulator and the current regulator form a conventional current and voltage double closed-loop system. During the whole stage of start-up and operation, the current loop is always involved in the work, while the voltage loop only works in the running stage; the other impedance regulator, from the input From the above point of view, it is completely connected in parallel with the input of the current regulator LT2. The only difference is that the negative feedback coefficient of the impedance regulator is slightly larger than that of the current regulator, and the output of the current regulator controls the output of the rectifier bridge. DC voltage, and the output of the impedance regulator controls the proportional relationship between the intermediate frequency voltage and the DC voltage, that is, the inverter power factor angle.
The working process of the regulator circuit can be divided into two situations: one is when the DC voltage does not reach the maximum value, because the feedback coefficient of the impedance regulator is slightly larger, and the impedance regulator's given value is less than the feedback, the impedance regulator will work In the limiting state, it corresponds to the minimum inverter θ angle. At this time, it can be considered that the impedance regulator is working in the limiting state, and the corresponding is the minimum inverter θ angle. At this time, it can be considered that the impedance regulator does not work and the system is completely It is a standard voltage and current double closed-loop system; another situation is that the DC voltage has reached the maximum value, the current regulator starts to limit the amplitude, and no longer works, the output of the voltage regulator increases, but the feedback current does not change, which affects the impedance. Regarding the regulator, when the feedback current signal is slightly smaller than the given current, the impedance regulator exits the limiter and starts to work, adjusting the given value of the θ angle of the inverter angle regulator, so that the output intermediate frequency voltage increases, and the DC current is also Increase with it, reaching a new balance. At this time, only the voltage regulator and the impedance regulator work. If the load equivalent resistance RH continues to increase, the inverter θ angle will increase accordingly until the maximum inverter θ angle.
The inverter angle regulator is used to make the inverter bridge work stably at a certain angle θ.
After the intermediate frequency voltage signal from the intermediate frequency voltage transformer is input by CON2-1 and CON2-2, it is divided into two channels, one is sent to the inverter part, and the other is divided into three channels after rectification by D7゛D10, and sent to the other one. Voltage regulator; one way is sent to over-voltage protection; one way is used for voltage closed loop automatic input.
The voltage PI regulator is composed of IC13A, and its output signal is clamped and limited by IC13D. IC13C and IC21C form a closed-loop voltage automatic input circuit, and the DIP-3 switch is used for voltage open-loop debugging. The inner loop uses a current PI regulator for automatic current adjustment, and the control accuracy is above 1%. The current signal obtained by the main loop AC transformer is input from CON2-3, CON2-4, and CON2-5, and is three-phase rectified by diodes. Bridge (D11゛
D15) After rectification, it is divided into three ways. One is the current protection signal, the other is the feedback signal of the current regulator, and the other is the feedback signal of the impedance regulator. The current PI regulator is formed by IC17B, and then isolated by IC17A, which controls the voltage-frequency converter of the trigger circuit.
IC17C constitutes an impedance regulator, which is in parallel with the current regulator and is used to control the leading angle of the inverter bridge. Its function can indirectly achieve constant power output, or can improve the input power factor of the rectifier bridge. DIP-1 can turn off this regulator.
IC19B constitutes an inverter angle regulator, and its output is clamped and limited by IC19C.
(3) Working principle of the inverter part
The inverter trigger part of this circuit adopts sweep frequency zero-voltage soft start. Due to the need of automatic frequency modulation, although the inverter circuit adopts self-excitation mode, the control signal is also taken from the load side, but there is no need for additional on the main circuit The starting circuit does not require pre-magnetization or pre-charge starting process, therefore, the main circuit can be simplified, but the accompanying problem is that the control circuit is more complicated.
The starting process is roughly like this. Before the inverter circuit starts, an external excitation signal higher than the tank circuit resonance frequency is used to trigger the inverter thyristor. When the circuit detects the DC current of the main circuit, it controls the frequency of the external excitation signal. Sweep from high to low. When the frequency of the external excitation signal drops to close to the tank circuit resonance frequency, the intermediate frequency voltage is established and fed back to the automatic frequency modulation circuit. Once the automatic frequency modulation circuit is put into operation, it stops the frequency scanning of the external excitation signal and turns The inverter lead angle is controlled by the automatic frequency modulation circuit, so that the equipment enters a steady state operation.
If a start is unsuccessful, that is, the automatic frequency modulation circuit does not grasp the intermediate frequency voltage feedback signal, at this time, the excitation signal will scan to the lowest frequency. Once the repeated start circuit detects that the excitation signal enters Start again at the lowest frequency band, push the excitation signal to the highest frequency again, and scan again until the start is successful. The cycle of repeated start is 0.5 seconds, and the time from completion of a start to full power operation does not exceed 1 second.
The intermediate frequency voltage signal input by CON2-1 and CON2-2 is isolated by transformer and sent to ZPMK (intermediate frequency starter module), ZPMK pin 3, pin 4 output signal is differentiated by IC18B and IC20B into narrow pulse output, Drives the last-stage MOS transistor of the inverter. IC20A constitutes a frequency-to-voltage converter and is used to drive a frequency meter. W7 is used to set the reading of the frequency meter. IC18A constitutes an over-voltage protection oscillator. When an over-voltage occurs in the inverter bridge, the oscillator starts to oscillate, so that all 4 thyristors of the inverter bridge are turned on.
IC19D is a start failure detector, and its output controls the repeated start circuit. IC19A is a successful start detector, and its output controls the output limiting level of the intermediate frequency voltage regulator, that is, the direct current of the main circuit.
W6 sets the potentiometer for the highest frequency of the inverter's excitation signal.
(4) The working principle of starting calculation
After the overcurrent protection signal is reversed by IC13B, it is sent to the overcurrent cut-off trigger composed of IC15A to block the trigger pulse (or pull the inverter); drive the "overcurrent" indicator light and drive the alarm relay. After the over-current trigger is activated, it can run again only after the reset signal or by turning off and then turning on the machine to perform a "power-on reset". The over-current level can be set by the W2 trimming potentiometer.
When the three-phase AC input lacks phase, the control panel can protect and indicate the power supply. The principle is: the cathodes (K) of 4#, 6#, 2# thyristors take the three-phase voltage signals of A, B, and C (through the gate lead), and then send them to IC14 and IC16 for detection through the isolation of the photocoupler. Sum discrimination, once a "phase loss" fault occurs, in addition to blocking the trigger pulse, it also drives the "phase loss" indicator light and alarm relay.
In order to enable the control circuit to operate more reliably and accurately, a start timer and control power supply undervoltage detection protection are also set on the control circuit. At the moment of starting, the operation of the control circuit is unstable. Set a timer of about 3 seconds. After the timing is set, the trigger pulse is allowed to be output. This part of the circuit is composed of C11, R20 and other components. If the DC supply voltage on the control board is too low due to some reason, the voltage stabilizer cannot stabilize the voltage, which will also cause control errors. Set up an undervoltage detection circuit (consisting of DW4, IC9B, etc.) to block the trigger pulse when the VCC voltage is lower than 12.5V to prevent incorrect triggering.
The automatic repeat start circuit is composed of IC9A. The DIP-2 switch is used to close the automatic repeat start circuit.
IC5B constitutes an over-voltage cut-off trigger, which blocks the trigger pulse of the rectifier bridge (or pulls the inverter); drives the "over-voltage" indicator light and drives the alarm relay; Q9 enables the over-voltage protection oscillator IC18A to oscillate. After the over-voltage trigger is activated, like an over-current trigger, it can only be operated again by performing a "power-on reset" through a reset signal or by turning off the machine and then turning it on again. Adjust the W1 trimming potentiometer to set the overvoltage level.
The water pressure of Q7 and surrounding circuits is too low, and the delay time of the delay protection circuit is about 8 seconds.
The reset switch signal is input by CON2-6 and CON2-7, and the closed state is reset pause.
(1) Commissioning of the rectifier part
Before debugging, the inverter bridge should be disabled. For example: disconnect one end of the smoothing reactor or disconnect the input line of the last stage of the inverter, so that the thyristor of the inverter bridge has no trigger pulse. Then connect a 1~ to the rectifier bridge port
Resistive load of 2kW. Turn the If trimming potentiometer W2 on the circuit board clockwise to the highest sensitivity end (when a short circuit occurs during the debugging process, it can provide overcurrent protection). The DIP switches on the main control board are all set to the ON position; prepare to measure the DC voltage waveform output by the rectifier bridge with an oscilloscope; turn the "given" potentiometer on the panel counterclockwise to the minimum.
Send three-phase power supply (no phase sequence can be divided), check whether there is a lack of phase alarm indication, if so, check whether the incoming fast fuse is damaged.
Turn the "given" potentiometer on the panel clockwise to large, the DC voltage waveform should be almost fully released, and then turn the "given" potentiometer on the panel counterclockwise to the minimum, adjust the W4 fine-tuning potential on the rectifier control panel The DC voltage waveform is completely closed, and the phase shift angle is about 120 degrees. The output DC waveform should be continuous and smooth in the entire phase shift range.
Connect the inverter bridge to turn on the inverter trigger pulse. Turn the Vf trimming potentiometer W1 on the circuit board clockwise to the highest sensitive end (overvoltage protection can be provided when inverter overvoltage occurs during debugging). Turn the "given" potentiometer on the panel clockwise slightly to increase, and the inverter bridge will work at this time. When there is a through phenomenon, continue to turn the "given" potentiometer on the panel clockwise to half. At this time, the DC ammeter should indicate about 25% of the rated current. If the indication of the ammeter is not 25% of the rated current, it can be adjusted. The W2 current feedback fine-tuning potentiometer on the control board makes the DC ammeter indicate about 25% of the rated output current. Once the inverter starts to vibrate, the DC current can be close to the rated current value, and the precise rated current setting can only be carried out when it is running at full load.
If the "given" potentiometer on the panel is turned slightly clockwise, the inverter will start to vibrate and there will be no through phenomenon. The phase of the intermediate frequency voltage transformer can be adjusted, that is, the output line of the intermediate frequency voltage transformer 20V winding can be adjusted. Once, it won't rise again.
In this way, the debugging of the rectifier bridge is basically completed, and the inverter bridge can be debugged.
(2) Debugging of the inverter part
1) The frequency meter should be calibrated first. Use an oscilloscope to measure the other excitation frequency of the inverter trigger pulse (the other excitation frequency can be adjusted by W6), and adjust the W7 trimming potentiometer to make the reading of the frequency meter consistent with that measured by the oscilloscope.
2) Start the inverter. Adjust the W6 trimming potentiometer on the control board to make it slightly higher than the resonant frequency of the tank circuit, and turn the W3 and W5 trimming potentiometers in the middle position. Turn the "given" potentiometer on the panel clockwise slightly to increase, then its excitation frequency starts to scan, and the inverter bridge enters the working state. When the start is successful, the "P.P" indicator on the control panel will go out. The "given" potentiometer on the panel can be turned up and down to operate repeatedly, so that the excitation signal also repeatedly performs frequency sweeping actions. If there is no vibration, you can adjust the phase of the intermediate frequency voltage transformer, that is, reverse the output line of the 20V winding of the intermediate frequency voltage transformer. The debugging of this step can also make the DIP-2 and DIP-3 switches in the OFF position. At this time, the repeated start function is added, and the voltage loop is also put into operation.
3) After the inverter starts to vibrate, you can do the work of setting the front angle of the inverter. Turn the DIP-1 switch to the OFF position, adjust the W5 trimming potentiometer, so that the ratio of the intermediate frequency output voltage to the DC voltage is about 1.2 (if the If the overlap angle is large, the ratio value can be increased appropriately); then turn the DIP-1 switch to the ON position, adjust the W3 trimming potentiometer, so that the ratio of the intermediate frequency output voltage to the DC voltage is about 1.5 (or higher). This debugging work can be carried out at a lower intermediate frequency output voltage. Note that the 1.2 times relationship must be adjusted first, and then 1.5 times the relationship, otherwise the order will be reversed, and there will be problems with each other.
4) In the next step, the voltage outer loop can be adjusted under light load conditions. Turn the DIP-3 switch on the main control board to the OFF position, and turn the W1 fine-tuning potentiometer clockwise to the maximum. Turn the "given" potentiometer on the panel clockwise to a large amount, and the inverter bridge will work. Continue to turn the "given" potentiometer on the panel clockwise to the maximum, and adjust the W1 trimming potentiometer counterclockwise to make the output intermediate frequency voltage reach the rated value. In this debugging, it can be seen that the impedance regulator is working, that is, the DC voltage is not rising, but the intermediate frequency output voltage can continue to rise with the "given" potentiometer's spin.
(3) Overvoltage protection
The over-voltage protection level has been fixed at 1.2 times the rated output voltage on the control circuit. When the rated voltage is set, the over-voltage protection is automatically set. If 1.2 times is not appropriate, you can change the resistance value of R13 on the control board and increase R13 to increase the overvoltage protection level; otherwise, decrease it.
(4) Overcurrent protection
The over-current protection level has been fixed at 1.5 times the rated DC current on the control circuit. When the rated current is set, the over-current protection is automatically set. If 1.5 times is not appropriate, you can change the resistance value of R43 on the control board and increase R43 to increase the overcurrent protection level; otherwise, decrease it.
(5) Rated current setting
Under full load, adjust the W2 current feedback trimming potentiometer on the control board to make the DC current expressed to the rated value.
3. Matters needing attention
(1) Tools needed for debugging
For a 20M oscilloscope, if the power cord of the oscilloscope is a three-core plug, be careful not to connect the ＾ground wire￣. The oscilloscope housing needs to be insulated from the ground. Only one trace probe is used. Both the X-axis and Y-axis of the oscilloscope must be aligned. , The probe needs to be compensated under the test signal.
If there is no high-voltage oscilloscope probe, use resistors as a voltage divider to adapt to the 600V voltage measurement.
A resistive load ＋500Ω, −500W.
(2) Problems in the debugging of the rectifier part
If during the debugging of the rectifier part, it is found that 6 rectifier wave heads cannot be found, check whether the serial numbers of the 6 rectifier thyristors are connected correctly and whether the gate lines of the thyristors are connected reversely or short-circuited.
During the debugging process of the rectifier part, it was also indirectly checked whether the "given" potentiometer on the panel was connected reversely. If it was connected reversely, the DC voltage was almost the maximum. Only when the "given" potentiometer was turned clockwise to the end, the DC The voltage will decrease.
(3) Matters needing attention in setting the rated output current
After powering on for a few seconds, turn the "given" potentiometer on the panel slowly clockwise, and the inverter bridge will appear in two working states, one is the inverter bridge oscillating, the other is the reverse The change bridge is directly connected. What is needed at this time is the inverter bridge direct connection. If the inverter bridge is in the oscillating state, the phase of the intermediate frequency voltage transformer can be adjusted in the state of power failure, that is, the output line of the intermediate frequency voltage transformer 20V winding can be adjusted. It will vibrate. In the operation of slowly turning the "given" potentiometer on the large panel, pay close attention to the response of the ammeter. If the indication of the ammeter increases rapidly, you should quickly turn the "given" potentiometer down counterclockwise to indicate the current There is a problem with the sampling circuit and the system is in the current open loop state. Check whether the current transformer is connected correctly, especially whether the primary and secondary sides of the 5A:0.1A current transformer are connected reversely, and whether the 68Ω resistor on the 0.1A winding is connected. The normal performance is that as the "given" potentiometer slowly increases, the indication of the ammeter also increases. When the "given" potentiometer stops rotating, the indication of the ammeter can stably stop at a certain scale.
When there is a through phenomenon, continue to turn the "given" potentiometer on the panel clockwise to make the DC ammeter indicate about 20% of the rated current, and use an oscilloscope to observe the normal waveform of D15 on the main control board, that is, the current sampling waveform , (The ground wire of the oscilloscope probe is clamped to the jumper of the main control board), the normal current sampling waveform should be the same level of the 6 negative polarity wave heads, if the wave heads are too different, it means the same name end of the current transformer If there is no connection, it must be changed, otherwise it will affect the normal operation of the current regulator.
It needs to be pointed out that when the DC resistance of the smoothing reactor is small, when the rated current is set in the through state, the DC current will oscillate, which can be solved by adding a resistor in the DC circuit. In addition, the water cooling device must be cooled by water during this debugging.
When the power supply of the debugging site cannot supply the rated current of the device, the setting of the rated current can be carried out when the site is running at full load. This is the same as the current setting of a general intermediate frequency power supply. However, you should first determine whether the current sampling loop is working properly under the condition of small current.
(4) Calibration frequency table
If the intermediate frequency power supply uses a special intermediate frequency frequency meter, this step of debugging can be omitted. However, it is recommended to use a frequency meter modified from a DC milliampere meter. On the one hand, the highest external excitation frequency can be measured. On the other hand, it is cheap.
(5) Simple inspection of inverter pulse
In order to judge whether the gate line of the inverter thyristor is connected correctly, first check whether the brightness of the LED on the last stage of the inverter is normal. If it is not bright, it means that the E and C terminals of the last stage of the inverter are connected reversely; then connect the main control board Disconnect the external connection of CON3-5 and see if the last stage of the LED inverter that is extinguished is in the diagonal position of the inverter bridge.
(6) Inverter does not vibrate
If the output line of the 20V winding of the intermediate frequency voltage transformer cannot be started after the adjustment, check whether the resonant frequency of the tank circuit is correct at this time. You can use a capacitance/inductance meter to measure the capacitance of the heating capacitor and the inductance of the inductor Calculate the resonant frequency of the tank circuit. When the resonant frequency of the tank circuit is in the range of 0.6 to 0.9 of the highest external excitation frequency, it should be easy to start. Then check whether the inverter thyristor is damaged.
(7) Problems in setting the front angle of inverter
After the inverter starts to vibrate, you can do the work of setting the front angle of the inverter, turn the DIP switches in the OFF position, observe the waveform of the voltage transformer 100V winding with an oscilloscope, and adjust the W5 fine-tuning potentiometer on the main control board to reverse the phase The lead angle is about 22 degrees. 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, the ratio can be increased appropriately). The setting of this step is the minimum inverter lead angle, generally It is hoped that it is as small as possible. Of course, a too small inverse conversion phase lead angle will cause the inverse conversion phase to fail, which is manifested by repeated starting when the intermediate frequency voltage rises.
Turn the DIP-1 switch to the ON position, adjust the W3 trimming potentiometer on the main control board, and set the maximum reverse conversion phase lead angle. According to the requirements of different intermediate frequency output voltages, the maximum inverse conversion phase lead angle is also different. For example, when the three-phase input voltage of the intermediate frequency device is 380V and the rated intermediate frequency output voltage is 750V, the maximum reverse conversion phase lead angle is required to be about 42 degrees. 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 is still 380V, and the rated intermediate frequency output voltage is 1000V, the maximum inverse conversion phase lead angle is required to be about 56 degrees. , The ratio of the intermediate frequency output voltage to the DC voltage is 2.0. It is generally hoped that it is as large as possible, which can still ensure that the intermediate frequency output voltage reaches the rated value when the input voltage of the system is low. When the input voltage of the system is high, due to the function of the voltage regulator, the intermediate frequency output will still not appear over-voltage.
This debugging work can be carried out 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 the problem of mutual involvement will occur. Sometimes the voltmeter is inaccurate, which brings wrong conclusions to debugging, so the lead angle measured by the oscilloscope should prevail.
If there is a phenomenon that the front angle of the inverter is not small during debugging, after eliminating the reason for the low resonance frequency of the tank circuit, check whether the inverter thyristors are all working. When only three thyristors are working, the reverse will occur. The phenomenon that the rake angle is too large.
(8) Setting of rated output voltage
Set the rated output voltage under light load conditions. In this debugging, we can see the phenomenon 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 "given" potentiometer.
When setting the rated output voltage, it should be done under the condition that the DC current is lower than the rated current, otherwise the intermediate frequency output voltage cannot be adjusted due to the function of the current regulator.
(9) Other excitation frequency
The excitation frequency must be higher than the maximum possible resonance frequency of the tank circuit, otherwise, the system cannot operate normally due to the "pulling" of the excitation frequency. It is appropriate that its excitation frequency is 1.2 times higher than the maximum possible resonance frequency of the tank circuit.
(10) Constant power output
For the melting load, constant power output is very important. To make the range of the constant power zone large, the range of the inverter lead angle from the smallest to the largest is only as large as possible At the same time, the matching of load impedance is also very important. Even if it is not a smelting load, doing so will help improve the power factor of the rectifier.