Abstract: This paper introduces a three-phase asynchronous motor control system based on serial communication between a PC and an AT89S51 microcontroller. The PC, developed using VC6.0, was used to implement the host computer software. The AT89S51 microcontroller, using an AC solid-state relay as the interface device, implemented the drive and control of the three-phase asynchronous motor. The system achieves the goal of software-controlled motor start and stop, and both the hardware and software programs are presented. The entire system has a simple structure, strong practicality, and broad application prospects. Keywords: serial communication; AC solid state relay; VC6.0; three-phase asynchronous motor[b][align=center]Design of Threephase Asynchronous Motor Start Stop System Based on VC and Serial Communication FU Hua-ming,PENG Shuang-ping [/align][/b] Abstract: A control system of the threephase synchronous motor based on the serial communication of PC and AT89S51 is introduced in this paper. The software system on PC is implemented by using VC6.0 as developing environment, while the AT89S51 controls the threephase synchronous motor through the AC SSR (solid state relays). The controlling of the threephase synchronous motor through software on PC is achieved, as well as the program is given out in this paper. The system has a widespread application prospect with its simple structure and high practicability. Keywords: serial communication ; Introduction The starting method of a three-phase asynchronous motor is determined by its power. Generally, it is stipulated that motors with a power of less than 10KW can be started directly (i.e., the motor is started at full voltage, the switch is turned on directly, and the three phases are connected to the power supply). For motors with a power of more than 10KW, the starting method should be determined according to the ratio of the motor capacity to the power transformer capacity. For a given motor power, the maximum power should not exceed 20%-30% of the transformer capacity, so it can be started directly. Otherwise, it is necessary to reduce the voltage to prevent the excessive starting current from generating a large voltage drop on the line, causing large fluctuations in the grid voltage, which will affect the normal operation of other equipment connected in parallel to the grid [1]. For the starting and stopping of small-power three-phase asynchronous motors, relays, contactors and other control electrical appliances are still widely used at home and abroad to achieve automatic control. Since the relay contactor control system switches the main and control circuits through the mechanical movement of the contacts, the contacts are prone to wear due to electrical, mechanical and chemical reasons, and the contacts are prone to malfunction in high-impact and oscillating working environments. If the main circuit has an inductive load, it is prone to arcing and bounce, resulting in significant electromagnetic interference to the outside world. Furthermore, the control system has many intermediate links, requiring each part to operate accurately for the system to function correctly. Therefore, it has several shortcomings and needs improvement. This paper introduces a three-phase AC solid-state relay as the interface device between the microcontroller and the three-phase motor. It uses PC software to directly start or stop the three-phase asynchronous motor, reducing intermediate links and ensuring stable operation of the control system. The visualized motor control system also has broad application prospects. 2 System Design Concept 2.1 System Overview: This system is a three-phase asynchronous motor start-stop system based on serial communication between a PC and an AT89S51 microcontroller. The PC uses VC6.0 as the development environment to implement the host computer software. The host computer software sends control commands to the AT89S51 microcontroller via serial communication. The AT89S51 uses the AC solid-state relay as the interface device, controlling the input level of the AC solid-state relay to control whether the three phases of the three-phase asynchronous motor are connected to the power supply, thereby starting or stopping the motor. 2.2 System Hardware Design 2.2.1 Microcontroller and PC Serial Communication Circuit The AT89S51 is a low-power, high-performance CMOS 8-bit microcontroller with 4k Bytes of Flash read-only program memory that can be repeatedly erased and written 1000 times. The device is manufactured using ATMEL's high-density, non-volatile memory technology and is compatible with the standard MCS-51 instruction system and 80C51 pin structure. The chip integrates a general-purpose 8-bit central processing unit and ISP Flash memory unit. The chip can be programmed in the system, which greatly shortens the development cycle of the microcontroller application system. In addition, the AT89S51 is designed and configured with an oscillation frequency of 0Hz and a power-saving mode that can be set by software. In idle mode, the CPU stops working, while the RAM timer/counter, serial port, and external interrupt system can continue to work. In power-down mode, the oscillator is frozen while the RAM data is saved, and other functions of the chip are stopped until the external interrupt is activated or the hardware is reset. The powerful AT89S51 can provide cost-effective solutions for many embedded control application systems and industrial control systems [2]. Given the above advantages and considering the design goals, this microcontroller was selected. The AT89S51 microcontroller communicates with the PC via the RS-232C serial communication standard interface. Many RS-232C bus interface chips are available; this system uses the MAX232 chip from MAXIM to perform bidirectional level conversion between TTL and EIA, enabling communication between the PC and the AT89S51 microcontroller. The serial communication connection circuit between the AT89S51 microcontroller and the PC is shown in Figure 1. This circuit is sensitive to power supply noise; therefore, it is important to improve the anti-interference capability of the components during circuit design. A 0.1uF or 1.0uF capacitor should be used to decouple the +5V power supply. 1.0uF/16V tantalum capacitors (C1, C2, C3, C4) should be selected as external polarized capacitors and placed as close to the chip as possible. [align=center]Figure 1 Serial Communication between AT89S51 and PC[/align] 2.2.2 Interface Devices for AT89S51 and Three-Phase Asynchronous Motors: The motor control system circuit currently used domestically and internationally is shown in Figure 2. This control system is called a relay contactor control system, which is a type of intermittent control with contacts because the control electrical appliances operate intermittently. [align=center]Figure 2 Traditional Three-Phase Asynchronous Motor Control Circuit Diagram[/align] This system uses a three-phase AC solid-state relay as the interface device between the AT89S51 and the three-phase asynchronous motor. An SSR (Solid State Relay) is a contactless electronic switch with relay characteristics, developed using discrete electronic components, integrated circuits (or chips), and hybrid microcircuit technology. It features long lifespan, high reliability, fast switching speed, low electromagnetic interference, no noise, and no sparks. Solid-state relays are classified into AC solid-state relays and DC solid-state relays according to the output load power supply. AC solid-state relays are classified into zero-crossing conduction type and random conduction type according to the switching method. Zero-crossing conduction type SSR can only conduct when the load power supply voltage crosses the zero zone and turn off when the load current crosses zero. This is to avoid the load being impacted and disturbed by peak voltage. Random conduction type SSR conducts as long as a signal is applied to the input terminal [1]. This system uses zero-crossing conduction type SSR. The schematic diagram and application diagram of zero-crossing conduction type SSR are shown in Figure 3: In the application diagram, 1 and 2 are the control terminals. When a certain DC voltage is applied, the output terminals 3 and 4 are turned on, and the load is connected to the power supply to work. When the control terminal voltage is lower than a certain value, the output terminals 3 and 4 are immediately disconnected, and the load stops working. Its small signal control terminal and main circuit are electrically isolated and insulated, requiring very little control power and switching speed is very fast [3]. [align=center]Figure 3. Block Diagram of Zero-Crossing Conductive SSR[/align] The AC solid-state relay shown in Figure 3 is called a single-phase AC solid-state relay. A three-phase AC solid-state relay integrates three single-phase AC solid-state relays into one unit and directly switches three-phase loads using a single input terminal. It can conveniently control three-phase loads such as three-phase AC motors and heaters. The controlled three-phase asynchronous motor in this system has a power of 3KW and a rated current of 6.4A. Therefore, the three-phase solid-state relay model 3SSR30 produced by Shandong Zibo Qite Electronics Co., Ltd. is selected. This product features zero-crossing triggering, no sparking, a dielectric withstand voltage of 2500V AC, a maximum effective AC current of 30A per phase (the instantaneous current during direct motor start-up is several times the rated current), a maximum effective phase voltage of 450V per phase, and an operating frequency of 50HZ. The control terminal signal must meet the requirements of a voltage of 4-24V DC and a current of 50mA for the output terminal to conduct. The AT89S51 motor control circuit diagram for this system is shown in Figure 4. In Figure 4, the linear optocoupler OPTOISO1 operates in saturation and cutoff states, with a CTR range of 50%-200%. Transistor 9013 operates in the amplification region, providing a sufficiently large input current for the 3SSR30 to conduct. Since the motor is an inductive load, an overcurrent and overvoltage suppression component, an RY820V varistor, must be added to the output circuit to protect the SSR. The selection principle is to use a 500V-600V varistor for 220V and an 800V-900V varistor for 380V. [align=center] Figure 4 Hardware circuit diagram of this system[/align] 2.3 System software design 2.3.1 PC host computer software design At present, most domestic and foreign systems use single-chip microcomputer buttons or switch buttons to input control commands, but this system uses VC6.0 development environment on PC and uses the Windows operating system serial port programming ActiveX control MSComm provided by Microsoft to program the serial interface of PC, compile motor control system visualization software, and input control commands through the software. The PC host computer software interface is shown in Figure 5: [align=center] Figure 5 Host computer software interface of this system[/align] Introduction to important functions: The function OnInitDialog() is responsible for the initialization and opening of the PC serial port, and the functions OnMotoron() and OnMotoroff() are the motor start and motor stop button codes. The function OnComm() is responsible for serial port event capture and processing[4]. 2.3.2 PC and AT89S51 Microcontroller Serial Communication Software Design The serial communication program between the PC and the AT89S51 microcontroller is written according to their communication protocol, as follows: PC: When a software button is clicked, the corresponding button value is sent to the microcontroller, and the PC waits for a response. If the data sent by the microcontroller is the same as the button value, the PC sends 0x00 to the microcontroller, instructing it to complete the task corresponding to the button value; otherwise, the PC sends 0xff to the microcontroller, instructing it not to perform any operation, and the PC resends the button value. Microcontroller: Waits for data from the PC, receives it, and immediately sends it back to the PC, waiting for instructions from the PC. If the PC sends 0x00, the microcontroller completes the task corresponding to the previously received data; if the PC sends 0xff, the microcontroller does not perform any operation and waits for the PC to resend the data. 3 Conclusion Since this system uses modular three-phase AC solid-state relays to directly control the three-phase motor, it makes the long-standing difficult and complex problem of starting and stopping three-phase asynchronous motors simple and practical. Practice has proven that the system is stable and reliable. The whole system has a simple structure, strong practicality, and broad application prospects. The innovations of this paper are: 1. Using a software platform to control the motor start and stop, which is of reference significance for the visualization development of motor control systems. 2. Using three-phase AC solid-state relays to directly control the motor start and stop, simplifying the motor control system. References [1] Wang Honglin. Practical Power Electronics Technology, Beijing: People's Posts and Telecommunications Press, 1986 [2] AT89 Series Microcontroller Technical Manual [S]. Beijing Weili Electronic Technology Co., Ltd. 14-23 [3] Fang Chengyuan. Factory Electrical Control Technology [M]. Beijing Machinery Industry Press, 1992. 39-61 [4] Hu Kai, Zhang Yingchao. Design of Biochemical Analyzer and Communication with PC [J] Microcomputer Information. 2006.22,4-1:208-209