Refrigerator Refrigeration Compressor Controller Based on dsPIC30F4012

Abstract: Based on dsPIC30F4012, a brushless DC motor control system for refrigerator refrigeration compressor is designed. The drive motor is started by a three-stage frequency-up and voltage-up method. When it runs to meet the self-synchronous speed, the microcontroller obtains the rotor commutation point by detecting the terminal voltage of the motor, and improves the control robustness of the system through the PID algorithm. After testing, the system has good stability and controllability, which not only reduces costs, but also has flexible control and high practical value.
Keywords: brushless DC motor (BLDC); digital signal processor (DSP); pulse width modulation (PWM); PID algorithm; dsPIC30F4012

1 Introduction
The driving motor of the household refrigerator compressor is generally a single-phase asynchronous motor. In order to improve the comprehensive performance of the refrigerator, save power and reduce noise, and improve the working efficiency of the refrigerator refrigeration system, the compressor motor needs to be controlled. The brushless DC motor BLDC (brushless DC motor) has the advantages of simple structure, reliable operation, and convenient maintenance of AC motors, as well as the advantages of high operating efficiency and good speed regulation performance of DC motors. Therefore, dsPIC30F4012 is used here to design a brushless DC motor control system for refrigerator refrigeration compressors.

2 System Control Scheme
The system design adopts a three-loop control scheme of speed, current and position, which can achieve good control performance. The control principle is shown in Figure 1.

Since the back EMF is zero or very small when the motor starts, the three-stage frequency-up and voltage-up method is used to start the motor. This process is mainly implemented by software. By providing the controlled motor with a drive signal with increasingly higher frequency and continuously increasing the duty cycle of the drive signal, the motor will switch to the white synchronous operation mode when it reaches a certain speed. When running in self-synchronous mode. The microcontroller determines the motor position by detecting the zero point of the back EMF, and then rewrites the OVDCON (control register) value according to the position signal. The inverter adopts the pwm_on modulation mode, that is, each power device performs PWM modulation in the first 60° of conduction, and maintains a two-to-two conduction mode of constant conduction in the last 60°. This conduction mode has high winding utilization, maximum motor output, and best stability. The PWM modulation mode is simple to control, has no control dead zone, can effectively suppress commutation torque pulsation, and significantly improves the motor's operating performance.

3 Control system hardware design
The system detects the terminal voltage "crossing the midpoint" to obtain the rotor position. The dsPIC30F4012 outputs a PWM pulse square wave, which drives the corresponding power tube to be turned on and off through the drive circuit to achieve the correct power supply to the motor and make the motor run normally.
3.1 Introduction
to dsPIC30F4012 dsPIC30F4012 is a high-performance 16-bit microcontroller with DSP processing capability from Microchip. Its unique module structure and powerful storage performance make it have certain advantages in brushless motor control. dsPIC30F4012 has an on-chip motor-specific PWM controller (MCPWM); through programming, it can generate independent three-phase 6-channel PWM waveforms with the same frequency and working mode. The driving current of each pin is up to 25 mA, and it is directly output to the driver through the RE port, and the three phases are complementary and do not overlap. It can prevent the two power tubes on the same bridge arm from being directly connected and causing a short circuit. This structure greatly simplifies the control software and external hardware that generate the PWM waveform. The system adjusts the BLDC speed through a potentiometer; the resistor pair completes the detection of the terminal voltage VDC, and VDC/2 is the voltage of the terminal voltage "passing the midpoint"; the voltage feedback is achieved through three resistor pairs.
3.2 Drive circuit
The drive circuit is shown in Figure 2. IR2101 is a cost-effective driver produced by IR Company, with good driving effect and simple operation. The IR2101 driver can drive a group of power tubes, and the entire power circuit only needs 3 pieces, which not only saves manufacturing costs, but also improves system stability.

3.3 Current feedback and fault detection circuit The
current feedback and fault detection circuit is shown in Figure 3. The first operational amplifier of the current feedback samples the current on the bus and amplifies the input to the AN2 pin of the dsPIC30F4012 to achieve: the output of the first operational amplifier is connected to the second inverse proportional operational amplifier circuit for motor fault detection. When the bus current is too large, the microprocessor enters the fault processing. The control scheme and algorithm are reasonable and feasible.

4 System debugging and testing
Figure 4 is the main program flow of the system. The main program mainly sets the control words of each device, initializes each variable and flag value, enables the corresponding interrupt and adjusts the speed of the motor. In the three-phase inverter drive, the PWM duty cycle register (PDCX) and the rewrite control register (OVDCON) are used together. The duty cycle register can control the current flowing through the load, and the control register can be rewritten to control the commutation. The dsPIC30F4012 has a built-in motor-specific PWM controller structure, which can greatly simplify the control software and external hardware design for generating PWM waveforms. After successful debugging, the program is downloaded to the dsPIC30F4012 using ICD2, and the entire system is assembled. After testing, the system has good stability and controllability, fast response, smooth operation, and good static and dynamic performance. Figure 5 is the drive waveform of the four power tubes collected by a 4-channel oscilloscope. The waveform shows that the control scheme and algorithm adopted by the system are reasonable and feasible.

5 Conclusion
After testing, the system design has good stability and controllability. Although the motor performance is not as good as the sensored brushless DC motor at low speed startup, the performance after startup is comparable to that of the sensored brushless DC motor. The entire system has low hardware cost and flexible control performance, so it has broad market prospects.