[align=left] 1. Introduction Generally, the starting current of a three-phase asynchronous motor is relatively large, while the starting torque is not. Excessive starting current can cause a drop in grid voltage, affecting the normal operation of other electrical equipment and even preventing the motor itself from starting. Therefore, measures must be taken to reduce the starting current, with reduced-voltage starting being a common method. However, traditional reduced-voltage starting methods, such as star/delta starting, stator series resistance or reactance starting, and autotransformer reduced-voltage starting, are all single-stage reduced-voltage starting methods. During the starting process, the current experiences two surges. Although the amplitude is lower than the direct starting current, the starting process takes a long time, and none of these methods can continuously adjust the motor's starting voltage, resulting in a still significant surge current during motor startup. This paper analyzes in detail the power factor angle characteristics during the starting and running processes of an asynchronous motor. Using the motor power factor angle as a feedback quantity for the system, closed-loop control of the power factor angle is implemented, effectively reducing the motor's starting current while ensuring a relatively large starting torque. The system monitors the motor power factor angle and uses fuzzy control to correct the thyristor firing angle in real time, avoiding electromagnetic torque and current oscillations caused by changes in the motor port input voltage during motor startup, thus ensuring that the motor voltage is regulated according to the expected pattern. Furthermore, obtaining the motor power factor angle information provides a reliable basis for the light-load energy-saving operation of the motor. The system design is simple and reliable, and experiments have proven the effectiveness of the method, demonstrating good dynamic response performance and robustness. 2. Analysis of Asynchronous Motor Power Factor Angle Characteristics 2.1 Power Factor Angle Characteristics During Operation A three-phase asynchronous motor can be equivalently represented by a T-type circuit as shown in Figure 1. The power factor angle φ of the motor is equal to the phase difference between the phase current and phase voltage of any phase, and also equal to the impedance angle of one phase of the motor. The impedance z of one phase of the motor can be expressed as: Under the condition that the power supply frequency is constant, the synchronous speed ns of the motor is constant, and the relationship between the motor speed n and the synchronous speed ns is: n=ns(1-s) (2) According to equations (1) and (2), it can be seen that the power factor angle φ and the motor speed n have the following relationship: φ=f(n) (3) When the motor parameters are known, the relationship curve of φ=f(n) can be obtained, as shown in Figure 2: Figure 2 Relationship curve between power factor angle and speed As can be seen from Figure 2, during the motor starting process, the power factor angle changes significantly with the increase of speed, as shown in section b of the curve in Figure 2. The power factor angle decreases with the increase of speed. When the speed reaches the rated speed, the power factor angle reaches the minimum value. If the motor operates under light load or no load after reaching the rated speed, the speed will increase further, as shown in section a of the curve in Figure 2, at which time the power factor angle increases. 2.2 Power Factor Angle Characteristics During Soft Starting A soft starter for an asynchronous motor is an electronic voltage regulating device, meaning it continuously controls the voltage value for various starting and running operations. This requires the soft starter to continuously and relatively smoothly regulate the voltage according to the actual operating conditions through a certain control strategy. Since a motor is a typical inductive load with a significant freewheeling current phenomenon, voltage control for such an inductive load is much more complex than for a resistive load. Because the input voltage obtained by the motor during soft starting is a chopped, non-sinusoidal voltage, its voltage and current waveforms are relatively complex. The current waveform during soft starting is shown as curve i in Figure 3. In Figure 3, α is the trigger delay angle of the power device, φ is the measurable power factor angle, and θ is the current discontinuity angle. Curve u is the power input phase voltage, and curve i is the stator phase current of the motor. Figure 3: Motor stator current during soft starting. In Figure 3, φ is the measurable power factor angle, which is also the motor freewheeling angle, indicating the motor's freewheeling current behavior. By controlling the trigger delay angle α of the power device to change the value of the current discontinuity angle θ, it is equivalent to controlling the voltage value on the stator side of the motor. From the above figure, the following relationship can be obtained: φ=α- θ (4) 3. Design of closed-loop soft starter for asynchronous motor power factor angle 3.1 Overall design of soft starter Figure 4 Block diagram of closed-loop control principle of power factor angle Figure 4 is the block diagram of the closed-loop control system of asynchronous motor power factor angle. The control of SCR trigger angle mainly includes two parts: (1) Adjusting the SCR trigger angle according to the preset triggering law, that is, the preset trigger angle α'n in the figure; (2) The increment δαn of SCR trigger angle that changes according to the change of power factor angle. The sum of the two parts is the actual SCR trigger angle αn (the subscript represents the nth adjustment amount). In the power factor angle closed-loop control method, calculating the increment δαn of SCR trigger angle according to the change of power factor angle δφn is the key to this control method. Due to the differences in motor parameters, power ratings, operating states, and load types required by the soft starter, the control strategy needs to adapt to different operating conditions and exhibit load type insensitivity. Therefore, this paper adopts a fuzzy control algorithm to calculate δαn, achieving better control characteristics. 3.2 System Control Unit Design The control unit consists of an 80C196KC microcontroller and its peripheral circuits. It is responsible for signal detection, processing, and system control. Its characteristics include not only fast operation speed but also six HSOs specifically for motor control, thus ensuring the simplicity, efficiency, and low cost of the soft starter's hardware and software design. The working principle of the control unit is shown in Figure 5. Figure 5 Block diagram of the working principle of the control unit The voltage signal from the three-phase power supply is processed by the synchronous pulse signal circuit to form a three-phase voltage synchronous pulse sequence of six pulses (300Hz) per voltage cycle, which enters the HSI0 port as the clock reference for SCR triggering; at the same time, the square wave shaping signals of each phase voltage of the power supply enter P1.0-P1.2 for software phase loss and phase failure protection. Voltage and current signals are fed into the MCU's A/D converter via absolute value and filtering circuits, allowing for signal sampling and processing. Simultaneously, the current signal is fed into HSI1 via absolute value and square wave shaping to detect the current discontinuity angle θ, thus determining the measurable power factor angle φ. The protection circuit filters the current and voltage signals and compares them with reference values. When the current or voltage amplitude is too high, it blocks the trigger path and sends a signal to the MCU's external interrupt EXINT for interrupt processing. 3.3 System Software Design Figure 6 Main Program Principle Block Diagram The main program flowchart of the system is shown in Figure 6. The main program first checks for phase loss or phase sequence errors in the three-phase power input. If no fault is found, it enters the system start-up control. During the closed-loop control period, the system status is continuously monitored; if a fault is detected, the system is shut down, and a shutdown signal is sent as an alarm. 4. Experimental Results Figure 7: Thyristor voltage drop signal; Figure 8: Thyristor trigger pulse signal; Figure 9: Line voltage chopper input signal; Figure 10: Phase current envelope waveform during soft start; Figure 11: Motor speed waveform during soft start. Figure 7 shows the thyristor voltage drop signal waveform, Figure 8 shows the thyristor trigger pulse signal waveform, Figure 9 shows the line voltage chopper input signal waveform, and Figure 10 shows the change in motor current during soft start under power factor angle closed-loop control. It can be seen that in the initial stage of motor start-up, the current is relatively large, and the motor speed gradually increases. When the speed reaches the rated value, the current reaches its minimum value. During the start-up process, the current change is stable without oscillation. The starting current during soft start-up is 2 to 3 times its stable operating current. Figure 11 shows the speed waveform during soft start-up; the dynamic process shows no obvious fluctuations, and the operation is stable. 5. Conclusion This paper designs a soft-start control system for a three-phase asynchronous motor using the power factor angle as the feedback control quantity. By monitoring changes in the power factor angle, the thyristor firing angle is corrected in real time, avoiding electromagnetic torque and current oscillations caused by changes in the motor input voltage during motor startup. This ensures that the motor voltage is regulated according to the expected pattern. Furthermore, obtaining the motor power factor angle information provides a basis for energy-saving operation of the motor under light loads.