Abstract : This paper addresses the shortcomings of traditional TPAM starting methods, such as high starting current and long starting time, and studies TPAM soft-start technology based on power electronics. A design scheme for a soft-start device with the P89V51RD2 as its core is proposed. This soft-start device uses thyristor voltage regulation, adjusting the voltage across the TPAM stator by changing the thyristor's firing angle, thus achieving soft starting of the TPAM. Keywords : TPAM; Soft start; Microcontroller Research on the Soft Starter for TPAM Based on P89V51RD2 SCM Yu Bin (Hunan Institute of Technology, Hengyang 421008, China) Abstract : To solve the problems of traditional start defects such as high start current and long start time, a soft starter for TPAM based on power electronics technology has been researched, and a design project for the soft starter based on P89VS1RD2 has been presented in this paper. A voltage adjusting model with SCR is used in this soft starter. By regulating the amplitude of the asynchronous motor's voltage, torque-control start is achieved, and so on. Keywords : TPAM; Soft start; SCM 0 Introduction With the rapid development of today's society and the ever-changing nature of modern technology, intelligent control systems are being used more and more widely, such as in production automation and various remote control and scheduling systems. This also puts forward more requirements, such as increasingly stringent requirements for the fluctuation of the power grid and the intelligent matching of actuators. As a crucial driving actuator, the speed regulation and control technology of electric motors is receiving increasing attention, especially for TPAMs (Three-Phase Asynchronous Motors), which are the most widely used motors, accounting for approximately 70% of all electric motors. Statistics show that general-purpose TPAMs consume about 50% of the world's total electricity annually, and their starting current is as high as 5 to 10 times their rated current. Therefore, large and medium-sized TPAMs generate significant inrush currents during direct starting, causing electrical and mechanical damage to the TPAM and the driven equipment, and also causing a drop in grid voltage, affecting the normal operation of other electrical equipment on the grid. Therefore, soft starting of TPAMs is essential. This paper introduces the research on TPAM soft-start devices. 1 Hardware Design of TPAM Soft-Start Device This system uses a P89V51RD2 microcontroller as the main control chip and thyristors as the main switching devices. The hardware structure of the soft-start device is shown in Figure 1, mainly consisting of a main circuit and a control circuit. Figure 1 Hardware Block Diagram of Soft-Start Device. The main circuit includes a thyristor AC voltage regulating circuit composed of thyristors, a contactor, a power supply, and the TPAM, etc. The voltage applied across the TPAM is changed by controlling the conduction angle of the anti-parallel thyristors, thereby achieving soft start. The capacity of the thyristors is selected according to the capacity of the controlled TPAM. Different TPAMs use different thyristor capacities, but the circuit structure is the same. The main function of the contactor is to bypass the anti-parallel thyristors from the three-phase power supply after the soft start process is completed; when soft stop is required, the soft start device is connected to the TPAM circuit to complete the soft stop. The control circuit includes a microcontroller control circuit, a trigger drive circuit, a detection circuit, a keyboard display circuit, etc. The control circuit is independent of the capacity of the controlled TPAM and can be designed as a general type. The functions and performance indicators that the TPAM soft start device can achieve depend entirely on the control unit circuit structure and the corresponding control algorithm. The physical quantities that need to be detected include voltage, current, speed, power factor, etc. This information needs to be sent to the CPU for processing to form a control signal for the thyristors. The functions of each part in Figure 1 are briefly described as follows: (1) Voltage detection: Two functions are implemented in the voltage detection circuit. Firstly, it has the function of detecting the synchronization signal, sampling the zero-crossing moment of the three-phase voltage, which serves as the synchronization signal for the thyristor pulse trigger signal; secondly, it converts the three-phase power supply voltage signal into a DC signal after being stepped down by a transformer, and then converts it into an A/D signal for fault detection, overvoltage and undervoltage protection, voltage display, etc. (2) Current detection: The three-phase current of the TPAM is detected by the current transformer, and the current information is sent to P89VS1RD2 for overcurrent protection, current display, etc. On the other hand, the phase signal of the current is also detected. (3) Thyristor trigger circuit: The control signal given by P89VS1RD2 is used to send out a pulse signal with a certain pulse width through the trigger circuit composed of the trigger chip TC787 to drive the thyristor to conduct, and the voltage across the TPAM is changed by controlling the conduction angle. (4) Main control chip P89VS1RD2: It is the core unit of the system control, mainly responsible for processing the detection signal, adjusting the phase shift range, giving the drive signal of the thyristor and contactor, output control, etc. (5) Keyboard Display: The keyboard section can preset the start and stop modes and pre-set and modify various parameters. The display section can display various soft start parameters and various fault information. 1.1 Introduction to P89VS1RD2 The P89VS1RD2 is a derivative product of the 8-bit 80C51 microcontroller. It has been greatly enhanced and expanded while fully retaining the 80C51 instruction system and hardware interface framework. It applies the original 16-bit addressing mechanism for external data and program memory, expands the on-chip RAM to 1024 bytes, and expands the on-chip Flash EPROM to 64kB, meeting the needs of this system for large on-chip memory capacity when programming in C language. A typical feature of P89V51 RD2 is its X2 mode option. Using this feature, designers can make the application run at the traditional 80C51 clock frequency (12 clocks per machine cycle) or the X2 mode (6 clocks per machine cycle). Selecting the X2 mode can achieve twice the speed at the same clock frequency. This allows the clock frequency to be halved while maintaining the characteristics. Using Flash program memory, parallel and serial in-system programming (ISP) can be supported. ISP allows the CPU to repeatedly program the finished device under software control without leaving the system, making debugging convenient. This is also an important reason for choosing this chip in this system. P89V51RD2 can also use in-run programming (1AP), which allows the Flash program memory to be reconfigured at any time, even when the application is running. This is very suitable for non-stop modification in the system. 1.2 Thyristor parameter selection How to select a suitable thyristor in the input circuit is closely related to whether the soft starter system can work safely and whether the cost performance is high. The selection of thyristors in this system is mainly considered from the following two aspects: (1) Thyristor current selection In AC circuit, the rated current of the thyristor in the on state refers to the effective value current. For example, for a 3KW TPAM, the starting current can be as high as 5 to 10 times the rated current. When only a safety margin of 5 times is taken, the thyristor on state current is: IM=5×IN=5×4.56=23A. (2) Thyristor Voltage Selection Precisely designing the withstand voltage of a thyristor is difficult because it is related not only to the circuit connection but also to the capacitance and excitation current of the TPAM. Generally, considering overvoltage absorption circuits, the rated voltage of the thyristor should be 2 to 3 times the peak value of the normal operating voltage, i.e., UM is the rated voltage of the thyristor, and UN is the normal operating line voltage of the system, i.e., 380V. Simply increasing the safety margin is not enough when selecting a thyristor. To ensure the thyristor can operate normally without damage, appropriate protection or suppression must be provided for overvoltage, overcurrent, and temperature rise in devices or motors caused by excessive current. Based on market conditions, the thyristor used in this system is the Semikron SKKT 250/18E model, with a rated voltage of 1800V and an average on-state current of 250A, which fully meets the requirements. 2 Control Circuit Design The core of the soft-start device, as an AC voltage regulating device, is the microcomputer control circuit. The main function of this part is to process various signals through mathematical operations and logical judgments, and to control the coordinated operation of various parts of the voltage regulation system. The control circuit of this soft starter mainly includes a microcontroller control circuit, a synchronization circuit, a trigger circuit, a drive circuit, a detection circuit, and a keyboard display circuit. The design of the main circuits is described below. 2.1 Microcontroller Control Circuit Design The block diagram of the P89VS1RD2 microcontroller control circuit is shown in Figure 2. P0.7 is used as the thyristor trigger pulse control command output. When the system is running normally, it outputs a low level and is connected to the Pi pin of the trigger chip TC787 after optocoupler isolation to release the pulse. The microcontroller sends the D/A conversion result to the thyristor trigger control circuit. By adjusting the digital quantity of the D/A conversion at regular intervals, a certain voltage is output to the Vr pin of the trigger chip TC787. The trigger controls the conduction angle of the thyristor according to this value, so that the conduction angle of the thyristor increases at a certain rate from the set initial value, realizing the phase shift control of the thyristor. By comparing and correcting the sampled value and the limit value of the starting current, the thyristor is fully turned on and put into normal operation. The D/A conversion is implemented using the serial D/A conversion chip TLC5615. Figure 2 shows the block diagram of the microcontroller control circuit. P2.0 and P2.1 are used to connect to the serial EEPROM memory AT24C02 for saving set parameters and storing fault information. The keyboard and display section are connected using the microcontroller's external expansion device 8255. The display circuit is implemented by an LCD module, which is used to display parameter settings, operation monitoring, and fault query. The keyboard consists of 8 keys, enabling human-machine interaction, parameter setting, and modification. The buzzer in the figure is used for fault alarm. When a fault occurs, the LCD displays the fault type and corresponding parameter value, and the buzzer sounds an alarm. The A/D conversion circuit is implemented by the TLC2543 chip, used for sampling current and voltage inputs. 2.2 Synchronization Circuit Design The TC787 requires a three-phase synchronous transformer to provide a synchronization signal. A synchronization circuit must be set in the system to ensure that the trigger pulses of each thyristor in the main circuit of the three-phase AC voltage regulator maintain a strict synchronous phase relationship with its anode power supply. In the system, the synchronization signal controlling the thyristor conduction comes from the outputs of three synchronization transformers. The signals output from the synchronization transformers are sent to P89VS1RD2 after opto-isolation and power driving. P89VS1RD2 detects the synchronization signal status to ensure that the phase of the thyristor trigger pulse controlled by P89VS1RD2 is precisely adjustable with the main circuit voltage. Simultaneously, the voltage signals (UA', UB', UC') output from the three-phase power supply after passing through the synchronization transformers are phase-shifted by 30 degrees through a current-limiting phase-shifting network composed of resistors and capacitors, and then sent to pins 1, 2, and 18 of the thyristor integrated trigger chip TC787, i.e., the synchronization voltage input pins. The system uses TLP250 optocouplers for isolation driving. The synchronization circuit is shown in Figure 3. Figure 3 Voltage Synchronization Signal Generation Circuit 2.3 Digital-to-Analog Conversion Circuit Design To enable the microcontroller to specialize in monitoring and improve system reliability, the system uses the high-performance TC787 chip to implement pulse generation. Since the TC787 requires a DC control voltage, a digital-to-analog conversion circuit must be designed. This system uses a serial digital-to-analog converter (D/A converter) TLC5615 to generate a variable reference DC voltage. Its D/A conversion circuit is shown in Figure 4. Figure 4: D/A Conversion Circuit 2.4 Current Detection Circuit Current detection provides the control circuit with a sampled value proportional to the main circuit's operating current. This serves three purposes: first, to provide feedback signals for current-limited start-up; second, to provide feedback signals for overcurrent protection, phase loss protection, and overload protection; and third, to indicate the operating status. The current feedback signal in this system is taken from the stator side of the TPAM. The A-phase current detection circuit is shown in Figure 5. A current transformer is used to detect the magnitude of the three-phase stator current. The outputs IA', IB', and IC' of the current transformer are rectified, filtered, and divided by a full-bridge rectifier to obtain a DC voltage signal. The actual processing is similar to voltage detection. This signal is used as current feedback input to the A/D circuit, and after A/D conversion, it is sent to P89VS1RD2. Figure 5: A-phase Current Detection Circuit 3. Main Program Design of the TPAM Soft-Start Device The main program flowchart is shown in Figure 6. The main program of this soft-start system mainly includes three parts: system initialization, keyboard and display, and operation. The system initialization program primarily completes system initialization and self-test. Initialization includes initializing the microcontroller's input/output ports, internal timers, special registers, interrupt system, and peripheral device 8255. To ensure normal and safe system operation, it is desirable to detect and address potential system faults before system operation; therefore, performing an initial self-test after the main program initialization is crucial. The system initial self-test program performs A/D conversion on the voltage, current, temperature, and other parameters of the system immediately after power-on. The detection process is as follows: First, if the line voltage value obtained from the output voltage detection signal is too high (greater than 420V) or too low (below 340V), the overvoltage or undervoltage fault handling in the protection circuit is initiated. After the voltage detection passes, current detection is performed. If a certain value is obtained from the current detection circuit, the system's transistor is considered to be short-circuited. Because the soft-start system has no trigger signal when powered on, the thyristors are in the off state, so there is no current value. Current only occurs when the thyristor breaks down. The system then enters the breakdown fault handling phase: displaying the breakdown fault. Temperature rise detection is performed after voltage and current detection pass. Only after all three initial detections pass can the initial parameter setting program proceed. The system operation section includes: determining whether startup is required (checking if the start button is valid), startup processing (pressing the start button to enter the soft-start program), determining if startup is complete, stable operation (exiting soft-start and performing only keyboard scanning), determining whether shutdown is required during stable system operation (checking if the stop button is valid), shutdown processing (pressing the stop button to enter the stop subroutine), and proceeding to the next loop. Figure 6 shows the system main program flowchart. 4. System Simulation To verify whether the designed soft-start device meets the design requirements, a simulation model of the TPAM soft-start system is built using Simulink, as shown in Figure 7. It mainly consists of modules encapsulated in three-phase AC voltage source, three-phase AC voltage regulation, TPAM measurement, current feedback, and startup control. The model can be used for simulation of control methods such as ramp start, current limiting start and torque control start. Figure 7 Simulation model of TPAM soft start system This chapter uses MATLAB Simulink to establish simulation models of TPAM full-voltage start and soft start respectively, and conducts simulation experiments. From the simulation results, soft start has the characteristics of small starting current compared with full-voltage start, which verifies the correctness of the soft start control strategy designed in this paper. References : [1] Ren Zhicheng. Application Guide of Electric Motor Electronic Protector and Soft Starter [M]. Beijing: Machinery Industry Press, 2004 [2] Gao Yuenong. Soft start of electric motor [J]. Electrical Drive Automation, 2005.1 [3] Wang Shuhong, Zhu Yuhong. Soft start control system of three-phase asynchronous motor [J]. Machine Tool Electrical, 2000.4 [4] Wang Xiaoguang. Soft starter and its application design and debugging [J]. Machine Tool Electrical, 2004. 1 About the author: Yu Bin (1979-), male, from Yangzhou, Jiangsu, lecturer, major research direction: information processing and DSP. Email: [email protected] Tel: 0734-7134306/13975433788 Address: Department of Electrical and Information Engineering, Building 14, Leigongtang, Hengyang City, Hunan Province, China Postcode: 421008