Abstract: This paper studies a control method for high-torque soft starting of a three-phase induction motor. Based on theoretical analysis and simulation, an experimental study of a high-torque soft starter using an Intel 16-bit microcontroller as the microprocessor was completed. Practice shows that the high-torque soft starter enables the motor to start smoothly with high torque and low starting current. The load test results are in good agreement with the simulation results, proving the correctness and effectiveness of this high-torque soft starting theory. English Abstract: This paper studies the control method of a high torque softstarter for a three-phase induction motor. Based on theoretical analysis and simulation techniques, an experiment of a high torque softstarter with a single-chip computer as a microcomputer is completed. The experiment confirms that the softstarter based on high torque theory can start the motor smoothly with high starting torque and low starting current. The results of load test and simulation are identical, which conforms to the theory and is correct and valid. Keywords: High torque soft starter , single-chip computer 1. Introduction Three-phase squirrel-cage induction motors are widely used in industry, agriculture, and transportation due to their advantages such as simple structure, reliable operation, convenient maintenance, low price, and low inertia. With the continuous updating and development of production machinery in various fields, the requirements for the starting performance of motors are becoming increasingly higher. For example: (1) The motor is required to have a large starting torque, be able to start under load, and have a good mechanical characteristic curve; (2) The starting current should be as small as possible; (3) The starting equipment should be as simple, economical, reliable, and easy to maintain as possible; (4) The energy consumption during the starting process should be as low as possible. Three-phase asynchronous squirrel-cage motors generally have two starting methods: direct starting at rated voltage and reduced-voltage starting. Reduced-voltage starting methods include star-delta starting, autotransformer reduced-voltage starting, extended delta starting, and series-connected saturated reactor starting. While these reduced-voltage starting devices partially alleviate the problem of starting large-capacity motors on smaller power grids, they only relatively reduce the impact of high current and do not fundamentally solve the problem. Furthermore, these starting devices have inherent drawbacks, such as poor load adaptability, discontinuous starting current, and high maintenance workload. Additionally, during the entire motor starting process, the electromagnetic torque is greater than the load's reverse torque, causing the motor to accelerate. For the same moment of inertia, the greater the torque difference, the faster the unit accelerates. Machinery with a large moment of inertia starts more slowly. For production machinery that requires repeated starting, the duration of the starting process has a significant impact on labor productivity. Therefore, different production processes require different starting times for the motor. In the past, most starting devices used contactors, which are contact systems and are prone to wear and failure, resulting in poor starting characteristics. In order to achieve contactless control and obtain flexible and good starting characteristics, the electronic soft starter for squirrel-cage asynchronous motors was born in the early 1980s. The soft starter controller is a new type of energy-saving device that has been widely used in Europe and the United States. It uses thyristor AC voltage regulation technology to achieve reduced voltage starting, and later incorporated power factor control technology. In the research of control devices, efforts are being made to improve starting torque, realize computer network remote monitoring, improve detection and self-diagnosis functions, improve product reliability, improve manufacturing process, and reduce costs. Traditional soft starters generally adopt reduced voltage starting or current limiting starting at 50Hz power frequency to suppress the current surge when the motor starts, which has the problem of small starting torque and cannot be used for constant torque loads or loads that require heavy load starting [1]. Based on the simulation in reference [2], this paper develops a high torque soft starter based on graded frequency conversion, which has been proven to be effective by experiments. 2. Principle Analysis When the stator voltage drops, the frequency is lowered, which will reduce the loss of motor torque [3], and can solve the problem of small starting torque of traditional soft starter. The graded frequency conversion proposed in the literature [4] is to make the frequency of the output voltage of traditional soft starter start from a low value, gradually increase, and finally reach 50Hz. Although graded frequency conversion can realize frequency conversion, it cannot make the frequency change continuously. It can only make the frequency change in stages, and each stage frequency is one-n of 50Hz, that is, a 50Hz frequency division. Figure 1 simply shows the process of graded frequency conversion. Figure 1 16.7Hz-25Hz-50Hz three-stage frequency conversion waveform diagram The 50Hz power frequency power supply provided by the power grid can generate each stage of sub-frequency system. The relationship between the power frequency ωnet and the sub-frequency system ωsub is as follows: ωnet＝ωsub×r (1) As can be seen from Table 1, some of the three-phase balanced sub-frequency systems generated from 50Hz are negative sequence, such as 25Hz, and some are positive sequence, such as 12.5Hz. Because positive sequence can generate a large positive torque, for the 25Hz sub-frequency system, it is necessary to break the balance and change the phase angle to make it unbalanced. The relationship between ωsub and phase angle of the sub-frequency system is redefined as follows: ωsub×t -α=0 (2) Assuming the phase angle of phase A is 0°, from equation (1) and equation (2), we can obtain the phase angle of each level of the sub-frequency system from equation (3) and equation (4). This paper uses the Fourier module in the MATLAB/SIMULINK library to calculate the phase angle of the sub-frequency system voltage corresponding to different values ​​of r. Tables 2 to 6 list the phase angles of each phase voltage at 25Hz, 16.7Hz, 12.5Hz, 10Hz, and 6.25Hz. The above are only the voltage phase angles of some sub-frequency systems. The other 50/r algorithms are similar and will not be listed again. Figure 2 shows one of the phase angle combination schemes of the three-phase power supply waveform at 25Hz. All three phases are selected to be fully conducting in the first cycle of two consecutive cycles. Different phase angles correspond to different positive or negative conduction half waves. Figure 2 Three-phase power supply waveform at 25Hz (phase A 90°, phase B -30°, phase C 30°) Figure 3 Simulation model of high torque soft starter To obtain the maximum positive torque during startup, the optimal combination must be selected. The three-phase power supply system of various combinations is analyzed using the symmetrical component method of the three-phase circuit [5]. 3. Modeling and simulation Based on the principle analysis, the startup process model is established as shown in Figure 3. The soft starter has three startup modes. The simulation waveform of the ramp voltage startup mode is shown in Figure 5, which gives the stator current (effective value), speed, and torque waveform. The waveform of direct startup is shown in Figure 4, and the waveform of current-limited startup is shown in Figure 6. It can be seen from the figure that the ramp voltage startup has a smaller startup current impact and a longer startup time than the direct startup. The current-limited startup has a smaller startup current impact and a longer startup time. The graded frequency conversion is shown in Figure 7. The startup torque is larger, the current is also less restricted, and there is a jump at the switching point of each frequency. Figure 4. Direct Start Simulation Waveform; Figure 5. Ramp Voltage Start Simulation Waveform; Figure 6. Current Limit Start Simulation Waveform; Figure 7. Staged Frequency Conversion Start Simulation Waveform; Figure 8. Hardware Design Structure Diagram. 4. Hardware Design The overall hardware design structure is shown in Figure 8. The three-phase power supply is connected to the motor via a bypass contactor, a parallel circuit of thyristors, and a main control MCU (MCU). It receives information from the synchronization detection and phase sequence judgment circuit and the current detection circuit. Based on the input from the keyboard circuit, it determines the triggering method for the thyristor gate pulse. After starting, the contactor is closed, bypassing the anti-parallel thyristors. The entire control circuit includes seven parts: power supply circuit, current detection circuit, voltage synchronization detection and phase sequence judgment circuit, trigger pulse generation and pulse power amplifier circuit, contactor control circuit, display and keyboard circuit, and the microcontroller minimum system circuit. 5. Software Design The main software flow is shown in Figure 9, consisting of three main parts: initialization program, key processing program, and interrupt processing program. Figure 9 shows the main flowchart. The initialization program is responsible for initializing the 8279, enabling synchronous interrupts, and initializing related memory units. The key handling program modifies various parameter values ​​and flag bits in memory according to the key presses, so that the interrupt handling program can process them accordingly, and displays the modified parameter values ​​and status on the digital tube. The synchronous interrupt handling program and the software timer interrupt handling program work together to trigger the six thyristor pulses in the correct sequence, close the contactor after starting, and handle various fault conditions. The interrupt handling program can select current-limiting start, ramp voltage start, or graded frequency conversion start according to the starting mode input by the keyboard. 6. Conclusion Three-phase squirrel-cage induction motors are widely used in industry, agriculture, and transportation due to their advantages such as simple structure, reliable operation, convenient maintenance, low price, and low inertia. With the continuous updating and development of production machinery in various fields, the requirements for the starting performance of motors are becoming increasingly higher. To meet this demand, soft starters are widely used. To improve starting torque, this paper adopts the theory of graded frequency conversion, starting in stages from low to high synchronous speed to reduce energy loss during starting and overcome the shortcomings of traditional electronic soft starters with low electromagnetic torque. This allows the motor to start smoothly under full load and run at low speeds for short periods. Experimental results show that graded frequency conversion starting from 16.7Hz to 25Hz to 50Hz is achieved. The entire device uses a 16-bit microcontroller 80C196KC as the control core, enabling three starting methods: ramp voltage starting, current-limiting starting, and graded frequency conversion. Compared with traditional soft starters, this soft starter using graded frequency conversion effectively improves starting torque, especially at low speeds. It also has advantages such as simple structure, no contacts, light weight, small size, controllable starting current and starting time, and smooth starting process, effectively reducing the current surge during motor starting. Its high-torque starting method makes the soft starter not only suitable for motors with fan and pump loads, but also for motors starting with rated loads, giving it a wider range of applications.