The traditional generator is generally a single-phase power frequency generator, which is bulky and cumbersome, and is also inconvenient to move. If a three-phase intermediate frequency generator is used, the volume and weight can be greatly reduced, and it is easy to carry. However, the output of the intermediate frequency generator is a three-phase alternating current whose frequency and amplitude are different from those of the main power. Generally, the electric equipment cannot be directly used, and it needs to be converted into single-phase power frequency sinusoidal alternating current by the inverter power supply.
The generator output three-phase alternating current frequency is 400 ~ 760Hz, the voltage is 3TO ~ 540V, and it is converted into single-phase alternating current with amplitude of 230V and frequency of 50Hz through the inverter power supply. The generator has throttle control, which can adjust the output of the generator from the outside through the throttle; there is also an AC voltage of about 10V, which can be used as the synchronization signal of the inverter power supply and supply power to the auxiliary power supply.
2 system structure and working principle The main circuit structure of the inverter power supply should realize the function of voltage conversion mentioned in the introduction, it should adopt the method of first rectifying into direct current and then inverting into alternating current. The main circuit of the inverter power supply is as shown. It can be seen from the main circuit diagram that the system structure is mainly divided into two parts, the former stage is the rectification part, and the latter stage is the inverter part.
Inverter power supply main circuit diagram The three-phase AC output of the IF generator enters the inverter power supply, and then passes through the phase-controlled rectification of the three-phase half-control bridge, and then the capacitor is filtered to obtain the DC-side voltage, which is reversed by the single-phase bridge type SPWM type. The variable circuit is inverted and filtered by the LC filter circuit to obtain single-phase alternating current. The DC side voltage is controlled as a feedback amount. By adjusting the firing angle of the three-phase half-controlled bridge thyristor and the generator throttle, the DC side voltage is kept constant; the inverter part is controlled by open loop.
Taking the feedback amount of the DC side voltage as the control amount, it is considered that the output variation range of the generator is too large. If the uncontrolled rectification is used, the inverter part will be brought too much pressure, so the rectification of the front stage uses the thyristor phase. Control rectification, using a digital trigger circuit composed of a single-chip microcomputer to control the thyristor firing angle and the generator output, and stabilize the DC-side voltage at a certain value.
The control of the inverter part can be controlled by open loop or closed loop. The shortcomings of using open loop control are: poor output waveform quality, high total harmonic distortion rate; slow dynamic response of the system. The use of closed-loop control can overcome these problems, but compared with open-loop control, closed-loop control also has shortcomings. First of all, due to the introduction of output feedback, the detection component must be used accordingly, and the cost of the system increases. Secondly, if the control system is not well designed, the inverter output voltage oscillation may be unstable due to various factors during the operation. The reliability is reduced. The closed-loop control that is not used in this test is mainly considered from the cost, and the expected performance index can be achieved by the front thyristor firing angle and the control of the generator throttle. Considering comprehensively, the inverter part should adopt an open-loop control method with low cost and simple operation.
3 control of rectification part 3. Digital trigger circuit and digital PID algorithm of single-chip microcomputer use thyristor phase-controlled rectification device, use general discrete or integrated trigger, hardware circuit is complex, component is easy to aging, debugging is difficult, temperature drift and anti-existence Disadvantages such as poor interference capability.
The digital trigger consisting of ―51 series single-chip microcomputer has simple hardware circuit, high precision of real-time control, serial number of output trigger pulse and sampling time. The agricultural N-step G-number Li is fully reliable, symmetrical, strong anti-interference ability, and overcomes the analog type. The shortcomings of triggers. Can fully utilize the high-speed data processing and computing power of C51 single-chip microcomputer, and perform closed-loop control on DC side voltage to obtain high-performance voltage regulating device. 11. PID control is still the most widely used control method at present, when PID control adopts analog form When implemented, its simple structure parameters are easy to adjust. With the development of single-chip microcomputer and DSP technology, the digital PID algorithm is used to replace the analog PID controller, so that the digital PID control is continuously improved and improved, and the application in many aspects has achieved good results.
The PID algorithm calculates the control amount according to the sampling time deviation amount, and its control expression is - the kth sampling time, the controller's output digital quantity - the k-th sampling time control deviation amount - the k-first sampling time control deviation The quantity-proportional gain-integral constant-differential constant is called a positional PID algorithm. It includes the accumulation of errors, which is cumbersome to calculate. After simple derivation, the incremental PID algorithm can be seen as shown in equation (2). The main parameters of the digital PID controller are Kp, Ti and Td. How to determine the controller parameters, so that the whole system has satisfactory steady state and Dynamic characteristics are the key to system design.
3.2 Control circuit hardware structure and working principle The rectifier part control circuit structure is as shown.
It can be seen that the core of the digital trigger circuit is a single chip microcomputer, and the received signals have a synchronization signal, a voltage sampling signal and a detected fault signal, and the generated signal has a trigger pulse, a throttle control signal and an activation signal of the inverter portion.
After the system power-on reset, the DC-side voltage is sampled and sent to the internal AD of the MCU through the sampling resistor, converted from AD to digital, and compared with the digital quantity set in the MCU. The obtained error amount is calculated by the digital PID algorithm. The triggering time of the thyristor is adjusted to adjust the triggering time of the thyristor to ensure the stability of the DC side voltage. If the DC side voltage cannot be stabilized by the adjustment of the trigger timing of the thyristor, the generator throttle can be controlled by the single chip microcomputer to regulate the power generation. The size of the machine output finally reaches the stability of the DC side voltage.
The synchronizing signal is provided by the generator. After being taken out from the generator, it is isolated by optocoupler and enters the external interrupt of INT1 of the MCU. Each synchronization signal cycle, the interrupt responds once, and the interrupt processing program of the MCU performs synchronization signal cycle calculation, AD conversion, PID calculation. And output the trigger pulse of the three-phase thyristor. The trigger pulse is isolated by the optocoupler, and the transistor is sent to the gate of the thyristor.
Fault detection includes overcurrent and overvoltage detection. When the loop overcurrent occurs, the MCU responds immediately, turning off the trigger signal of the inverter part to stop the inverter part; when detecting the DC side overvoltage, the trigger signal of the thyristor is turned off, so that the rectification part stops working. After the fault is removed, the MCU sends a signal to turn on the inverter trigger pulse to make the system start normal operation.
4 The control of the inverter part is based on the area equivalent principle, so that the width of the PWM waveform pulse changes according to the sine law, which is equivalent to the sine wave, which we call the SPWM waveform.
There are two ways to generate SPWM: natural sampling and regular sampling. Natural sampling method requires complex transcendental equations, which takes a lot of time when using microcomputer control technology. Regular sampling method is a widely used engineering practical method, and its effect is close to natural sampling method, but the calculation amount is much smaller. 2. This experiment uses the rule sampling method.
The data obtained by the regular sampling method is stored in the memory, and the data is sent out by the memory at a certain time interval; by such a method, the complexity of the hardware circuit can be reduced, and the development cycle can be shortened. The inverter part control circuit uses a crystal oscillator, a counter (4040), memory data processing and calculation capability for storing SPWM waveform data, and performs closed-loop control on the DC side voltage. The oscillator provides a clock pulse to the counter. The EPROM outputs a trigger signal according to the address output by the counter, and is sent to the power device via the driving chip. After one cycle is delivered, the counter is reset and the next cycle is started, so that it is repeated.
If /cuxx is the counter clock frequency and N is the number of SPWM waveform data stored in the EPROM, the fundamental frequency can be calculated as follows: 3 409.6 kHz. The carrier frequency is taken as 10 kHz, and the carrier ratio is 200. 5 Experimental results using the above structure and control The method developed an inverter power supply for intermediate frequency generators. The rated output of the power supply is 230V, 50Hz, 1000W. The specific device parameters are as follows: thyristor rectifier bridge uses IXYS three-phase half-control bridge module VVZ1624i1, DC side capacitor 470 rush / 450V, inverter full bridge uses single-tube IGBT-SGH40N60UFD, The LC filter circuit parameters are selected as L=3mH. The C single-chip microcomputer selects the P87LPC767 of the Philips single-chip small package series, and has a 4-channel 8-bit AD converter.
The output voltage rating is 230V, the stable value of the DC side voltage is 390V, and the modulation ratio is 0.9. The maximum output sinusoidal voltage is about 390X0. 9=351V, the effective value is about = the tube drop of the IGBT is removed and the switch is dead. The voltage loss caused by the area influence is about 18V, so that the effective value of the output voltage is about 5, which is the sinusoidal voltage waveform output when the device is no-load and full load. The effective value of the voltage at no load is 242V, and the voltage is valid at full load. The value is 215V, and the output frequency is 50Hz. 6 Conclusion The phase-controlled rectification of thyristor can conveniently control the DC side voltage and reduce the voltage level of the power device on the inverter side. The digital trigger circuit composed of single-chip microcomputer combines the digital trigger device with the PID adjustment technology to fully utilize the sinusoidal waveform outputted by the C51 single-chip high-speed inverter power supply at full load. The inverter power supply for the intermediate frequency generator has been experimentally completed. And will soon be put into mass production. The test prototype and the intermediate frequency generator operate reliably and stably.
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