Design of agricultural low-power single-phase frequency conversion control system based on SPWM

　Abstract: Aiming at the problem that agricultural low-power three-phase AC motors cannot be widely used due to the difficulty in obtaining three-phase power supply, a three-phase AC frequency conversion system with stepless output voltage amplitude regulation is designed under the condition of agricultural 220 V single-phase AC power supply, which can realize the wide use of small-power three-phase AC motors under the condition of agricultural single-phase power supply.

　　The experimental results show that when 220 V AC power is input, three-phase controllable AC power can be output. The system hardware design is reasonable, which provides technical support for the wide application of frequency conversion control of agricultural small power supply.

　　0 Introduction

　　At present, there are many studies on AC frequency conversion technology at home and abroad, but most of them are studies on three-phase AC frequency conversion technology for medium and high power levels. There are fewer studies on low-power agricultural appliances, and fewer specific designs for agricultural single-phase frequency conversion systems. With the widespread use of agricultural appliances in agriculture and household appliances, low-power single-phase input and three-phase AC output frequency conversion technology can solve this practical problem. To this end, this paper designs a single-phase AC frequency conversion speed regulation system based on SPWM algorithm, with single-phase AC 220 V as input power supply, single-phase uncontrolled bridge rectification, and then large capacitor filtering to obtain a stable DC 270 V voltage, and then through a three-phase bridge inverter circuit to obtain a three-phase AC power with adjustable voltage amplitude to supply the three-phase AC motor.

　　1 Overall system design

　　The system is based on the basic principle of variable frequency control, adopts constant V/F ratio control, maintains a linear relationship between input voltage amplitude and frequency, and implements SPWM control through a single-chip microcomputer .

　　1.1 System functional structure

　　The block diagram of the system is shown in Figure 1. The system adopts the typical structure of AC-DC-AC power electronic voltage conversion and frequency conversion. The single-phase AC 220 V is input, and after single-phase uncontrolled bridge rectification and large capacitor filtering, a stable DC 270 V voltage is obtained, which is then output through a three-phase full-wave inverter circuit.

　　At the same time, the control circuit calculates the magnitude of frequency modulation and amplitude modulation according to the algorithm principle of SPWM control, and compares and analyzes it with the output current fed back by the inverter, and obtains the ideal control waveform through the drive circuit to drive the switch device IGBT to turn on and off, thereby outputting three-phase AC with adjustable frequency amplitude. The main functions of the system include DC voltage detection, output current acquisition, temperature acquisition and protection, short circuit protection, and overcurrent protection. The control circuit of the control system uses a 16-bit microcontroller dsPIC30F6010A to achieve functions such as adjustment of various parameters and protection of the system.

　　1.2 SPWM waveform algorithm principle

　　Through Fourier analysis, we know that the fundamental frequency amplitude of the SPWM waveform output is:

　　Where: N is the number of pulse widths in half a waveform; Um is the amplitude of the input voltage. When N>1, U1m>Um, so the fundamental wave of the output voltage is the sine wave required for modulation. At the same time, it can be seen that this SPWM generation method can effectively suppress low-order harmonics below k=2N-1, and there are high-order harmonics.

　　The voltages applied to the three-phase windings of the AC induction motor are:

　　Where: Em is the voltage amplitude; ω is the angular frequency of the output voltage; ω = 2πf, f is the frequency of the output voltage, that is, the modulation frequency of the waveform generator. The three-phase voltage with a phase difference of 120° is added to the winding of the induction motor to generate three-phase current, forming a circular rotating magnetic field, driving the motor rotor to rotate. When the constant V/F ratio is controlled, the input voltage amplitude is linearly related to the frequency, and the controlled motor generally has a simple V/F linear relationship, so the calculation method can be used to replace the V/F table [4]. Figure 2 describes the specific calculation process of V/F control.

　　2 System SPWM Control Circuit

　　The SPWM control circuit is mainly composed of dsPIC30F6010A microcontroller, MC54HC244, PNP transistor Q1, power supply, capacitor and resistor, as shown in Figure 3. R1 is the collector resistor of Q1 and the pull-up resistor of MC54HC244, which clamps the signal at a high level, thereby improving the noise tolerance of the chip input signal to enhance the anti-interference ability, and at the same time, it also plays a role in current limiting. R2 is the base resistor of Q1, which plays a role in current limiting. C1 is connected in parallel between the 5 V power supply and the ground, acting as a supporting capacitor and battery. When the inverter is working normally, the current detection value is compared with the SPWM calculated given value to obtain a high level, and the SPWM fault pin FLTA of the microcontroller inputs a high level signal, allowing the SPWM wave to output. At the same time, Q1 is turned on, the enable terminal G of MC54HC244 is low level, and the output terminal Y outputs the SPWM signal. When the inverter fails such as short circuit, Q1 is turned off, the MC54HC244 enable terminal G is high level, SPWM signal output is blocked, and hardware protection is achieved. At the same time, the microcontroller fault pin FLTA is connected to a low level, SPWM wave output is blocked, and software protection is achieved.

　　3 Software Implementation of SPWM System

　　The main task of the system software is to output SPWM waves to the drive circuit to drive the IGBT inverter circuit, and at the same time process the feedback signal of the sampling circuit to achieve voltage and current control. The design of the SPWM wave main program includes five parts, namely initialization, edge parameter identification, frequency and amplitude calculation, phase increment and amplitude modulation coefficient calculation. Initialization also includes the settings of I/O port, SPWM time base register and A/D conversion. Set the control register of SP-WM to make the SPWM timer work in SPWM generation mode, the timer clears the clock using period matching, the period interrupt is set to interrupt once per period, and the counting edge uses the rising edge of the clock. The main program flow chart is shown in Figure 4.

　　4 Experimental testing

　　The system uses the online debugger MPLAB ICD2 in MPLAB software integrated development environment IDE 8.53 to debug the system. MPLAB IDE software is a desktop development environment with a toolkit for programming, developing and debugging microchip microcontroller design applications. MPLAB IDE can be easily applied to Windows XP, Windows 2000 and Win-dows NT operating environments. MPLAB IDE is the user interface shared by various Microchip development system tools such as MPLAB editor, MPLAB ASM30 assembler, MPLAB SIM software simulator, MPLAB LIB30 library, MPLAB LINK30 linker, MPLAB ICE4000 online emulator , PRO MATE II  programmer and online modulator ICD2.

　　The IDE 8.53 operation interface is shown in Figure 5. The MPLAB software is used to debug the obtained single-phase frequency conversion system online to test the performance of the system.

　　5 Conclusion

　　The performance of agricultural low-power single-phase variable frequency speed regulation system was tested through MPLAB software.

　　(1) The system has a reasonable structure, stable input/output process, and has the speed regulation function of a three-phase AC motor with low-power single-phase AC input and controllable system control output.

　　(2) The SPWM waveform output by the microcontroller is stable and reliable, and the sinusoidal AC waveform that the inverter can output can meet the use of small-power three-phase AC motors.