Three-phase inverter system

*Note: This work won the first prize in the Guangzhou Six-School Invitational Competition and the Enterprise Special Award

Figure 1 Hardware block diagram of inverter system

2.  Hardware schematic diagram and principle description

(1) Driving circuit schematic diagram

Figure 2 Driving circuit schematic diagram

The drive circuit uses a high-speed optocoupler isolation module 6N137 to connect the microcontroller and the drive circuit. The optocoupler module uses optical signals as the medium to realize the coupling and transmission of electrical signals. The input and output are completely electrically isolated and have strong anti-interference performance. . For industrial application measurement and control systems that include both weak current control parts and strong current control parts, the use of optocoupler isolation can effectively achieve the isolation of weak current and strong current and achieve the purpose of anti-interference . The specific schematic diagram of the circuit is as follows:

Figure 3 Optocoupler isolation module

The driver circuit uses a half-bridge driver, and the driver chip uses IR2104. The upper and lower arm power tubes of this half-bridge circuit are alternately turned on. Whenever the lower arm is turned on and the upper arm Q1 is turned off , the potential of the Vs pin is the saturated conduction voltage drop of the lower arm Q2 , which is basically close to the ground potential. At this time, Vcc charges the bootstrap capacitor C1 between VB and VS through the bootstrap diode to make it close to the Vcc voltage. When Q2 is turned off, based on the internal CMOS structure (see the internal diagram below for details), HO and Vs are disconnected, and HO and Vb are connected. At the same time, the voltage at the Vs terminal will increase. Since the C1 voltage cannot suddenly change to Vcc , and there is a bootstrap diode between Vcc and Vb , the potential at the Vb point is close to the sum of the Vs point potential and the voltage on C1 . So at this time, the voltage difference between Vb ( HO ) and Vs is the Vcc voltage, and this voltage difference can be used to open the upper arm. According to the chip data sheet, the bootstrap capacitance of IR2104 is selected to be 100nF .

Figure 4 Half-bridge drive schematic diagram

(2) ADC sampling front-end circuit schematic diagram

    The ADC front-end first raises the negative voltage, and then performs impedance transformation through a voltage follower to convert the voltage to be measured to 0-3.3V , and then connects to the built-in ADC of the microcontroller for sampling.

Figure 5   ADC sampling circuit front-end schematic diagram

  The reference source uses TL431 to output a stable 3V reference. The schematic diagram is as follows:

Figure 6   TL431 voltage stabilization schematic diagram

Then the voltage is converted to within 0-3.3V through the resistor network voltage divider and voltage follower .

Figure 7  Resistor network voltage dividing principle diagram

(3) System power supply solution

Figure 8  Dual-channel 5V/3.3V power module

The system power supply scheme is as shown above. The input is 12V and can output two channels of 5V and two channels of 3.3V . The maximum current is 5A . It is used to power the minimum system of the microcontroller, the optocoupler isolation module, the ADC front-end sampling circuit module, the Bluetooth module, etc.

2.      Software flow diagram:

Figure 9   Software flow diagram

3.      System appearance design

Figure 10  System appearance design

(1)  Power module: The power module in the upper left corner is connected to a 12V DC power supply and can generate 3.3V DC voltage and 5V DC voltage to meet different requirements.

(2)  stm32 main control circuit: The stm32 main control circuit on the right connects the Bluetooth module, OLED screen and voltage divider circuit, and simultaneously controls the normal operation of the optocoupler isolation + half-bridge drive module

(3)  Voltage dividing circuit: The left voltage dividing circuit is connected to any end of the three-phase electrical output port, and is connected to the stm32 on-chip ADC after voltage division.

(4)  Optocoupler isolation + half-bridge drive module: The optocoupler isolation + half-bridge drive module in the lower left corner provides spwm wave control to invert DC voltage.

5. Testing and Debugging

After testing, when the input voltage of the inverter system reaches 38.52V , the effective value of the output three-phase electrical phase voltage reaches 12.0214V , and the frequency is 49.83z . The phase difference between the phase voltages is 120.47 °. Matlab performs fft transformation and obtains the result at 50Hz. Get peaks at left and right points.

Table 2 AC voltage (phase voltage) effective value changes with DC voltage

( 4 ) Result analysis

Compare the detected data with the basic requirements of the question. The basic requirements are as follows

a. The DC power supply provides three-phase symmetrical AC power to the resistive load through a three-phase inverter circuit, with a phase difference of 120 °.

b. When the effective value of the line current of the resistive load is 1A , the effective value of the line voltage is 12V ± 1V , and the frequency is 50Hz ± 1Hz.

According to the actual measurement, when the DC voltage is 38.52V , the three-phase electrical phase difference is 120.47 °, the frequency is 49.83Hz , the peak-to-peak value of the phase voltage is 36.00V , and the effective value is 12.014V , which can better meet the requirements of the question.

At the same time, compared with the increased requirements

Select Bluetooth communication transmission data as required. The computer uses matlab to receive the data and analyze the amplitude-frequency characteristics of the voltage waveform. It can analyze harmonics within 10th order and calculate harmonic distortion within 10th order .

( 5 ) Functions realized (indicators achieved)

1 When the input voltage of the inverter system reaches 40.5V , the output three-phase electrical frequency is 50Hz , and the phase difference between the phase voltages is 120 . The effective value of the phase voltage reaches 12V , and the effective value of the current is 1A .

2 stm32 on-chip ADC collects voltage with an accuracy of 12bit .

3 Bluetooth communication transmission data, matlab harmonic analysis, can analyze harmonics within 10th order, draw amplitude-frequency curves and calculate harmonic distortion within 10th order.