Abstract: To address the problem of excessive starting current in electric motors, an energy-saving soft-start controller based on an embedded system and supplemented by other chips was designed. This controller is a soft-start device for electric motors with energy-saving operation, soft starting, and multiple protection functions. Keywords: Soft start, energy saving, protection, control. One of the important reasons for the damage to many electric motors is the excessive starting current (which can reach 7-8 times the rated current). Therefore, promoting soft-start technology in departments and occasions where electric motors are frequently used is of great significance. The soft-start controller discussed in this paper mainly uses an embedded system, supplemented by other chips, to solve the problem of soft starting in electric motors. It can also realize protection and fault display functions such as overcurrent, phase loss, and incorrect phase sequence. Moreover, it automatically short-circuits the thyristor after starting, connecting the motor to the power grid, achieving energy saving and improving operating efficiency. 1 Soft-start Principle Soft starting is essentially a type of reduced-voltage starting, similar to the principle of "Y-Δ" starting and autotransformer reduced-voltage starting. The difference is that soft starting uses a contactless electronic switch to control the voltage, resulting in a smooth voltage rise rather than a sudden change. The basic principle of soft starting is to utilize the switching characteristics of thyristors by connecting three pairs of anti-parallel thyristors in series to the three-phase circuit of the motor. By adjusting the firing angle of the thyristors, the degree of thyristor activation is changed, thereby controlling the voltage output to the motor and achieving the purpose of controlling the motor's starting characteristics. After the motor starting process is completed, the soft starter controller will control the AC contactor to engage, short-circuiting all the thyristors and allowing the motor to be directly connected to the power grid, thus achieving energy saving. As shown in Figure 1, before determining the firing angle of the thyristors, the synchronization signal of the power grid and the set starting mode must be obtained, and then the change law of the firing angle is determined using this information. 2 Starting Modes of Soft Starter Controller The soft starter controller has a pre-set control program. When the controller receives a starting command, it performs relevant calculations to determine the firing signal of the thyristors, thereby controlling the thyristors to output the corresponding voltage according to the mode set by the soft starter controller to control the motor starting. Starting modes include: voltage ramp soft start, current limiting soft start, voltage control, torque start, and torque plus jump control. 2.1 Voltage Ramp Soft Start Method As shown in Figure 2, this method requires setting the voltage control starting voltage U0 and the acceleration ramp slope. After the motor starts, the controller's output voltage will rise rapidly. When it reaches the controller's set starting voltage U0, the motor's output voltage will increase linearly according to the set acceleration ramp slope until it equals the grid voltage. At this point, the AC contactor will engage, and the starting process ends. The starting voltage U0 and ramp slope can be set according to the user's specific needs. 2.2 Current Limiting Soft Start Method This starting method requires setting a current limit value Im. After the motor starts, the current increases rapidly. When the set current limit value Im is reached, the soft start controller ensures that the output current does not increase further while gradually increasing the thyristor firing angle. The current limit value Im can be set according to the user's grid capacity and the motor's load conditions. The setting range is 40% to 400% of the motor's rated current. 2.3 Voltage Control Method Voltage control starting method is mainly used in light-load starting situations. It maximizes the motor's starting torque while ensuring the starting voltage drop, minimizing starting time and making it the optimal light-load starting method. 2.4 Torque Starting Method This method linearly increases the motor's starting torque from small to large, resulting in smooth starting. It is mainly used in heavy-load situations. 2.5 Torque Plus Sudden Torque Control Method Compared to torque starting method, this method uses a sudden torque jump to overcome the motor's static torque at the moment of starting, followed by a smooth torque increase, shortening the starting time. 3 Soft Starter Controller System Composition and Working Principle 3.1 Hardware Composition This system uses the 89C51 microcontroller as its core, supplemented by AD0809, 74HC164, 74HC273, X25045, and other chips. It mainly includes synchronous signal detection, current detection, parameter setting, overvoltage and undercurrent detection, and trigger pulse output. The hardware structure diagram is roughly shown in Figure 4. Its main working principle is: (1) First, the synchronization signal of the three-phase voltage is obtained by the synchronization detection link, and it can be determined whether the power grid is missing a phase and the phase sequence is incorrect. Once there is a problem, a fault will be displayed and the soft start will not be performed. The obtained synchronization signal is sent to the microcontroller as the synchronization signal of the thyristor gate trigger signal. (2) The working current of the motor is sampled by AD0809 as a reference quantity for the current limiting working mode. (3) The starting mode is selected by the button: if the voltage ramp soft start mode is selected, the starting voltage U0 and the slope of the acceleration ramp must also be input; if the current limiting start mode is selected, the current limiting value Im must also be input. Other modes also require corresponding input of the starting parameters. (4) According to the calculation results of the trigger angle increment of each cycle, 6 trigger pulses are output from the port of the microcontroller. The 6 trigger signals are delayed by 60° in sequence. The trigger pulses must pass through the opto-isolator and the transistor. The opto-isolator is used to improve the anti-interference capability of the system and protect the motor, and the transistor is used to increase the drive current. (5) When the thyristor's conduction reaches its maximum, the motor enters normal operation. At this time, the intermediate relay can be activated, the AC contactor is engaged, the thyristor is short-circuited, and the motor is directly connected to the power grid, achieving energy saving. (6) The device also has a watchdog timer and voltage monitoring function, which is mainly implemented through the X25045 chip. In addition, the X25045 can also save the user-set starting voltage U0, the slope of the acceleration ramp, and the current limit value Im, ensuring that the set parameters are not lost after reset. Furthermore, during the operation of the motor, corresponding protections such as overcurrent, phase loss, and incorrect phase sequence can be performed, and soft stop can be achieved when a fault occurs. 3.2 Software Design The software of this system includes system initialization, keyboard processing, parameter modification and setting, AD conversion, synchronous signal input, pulse signal output, parameter and fault display. Users can select the starting control method according to their load characteristics and set appropriate starting parameters on the operation keyboard. The system, based on the set parameters, continuously sends control signals to the thyristor through sensing and comparison. The entire process, from start to finish, is completed automatically by the system. As shown in Figure 5, the system first checks the mains power supply to see if there is a phase loss or out-of-sequence error. If normal, the keyboard processing program is executed. If parameters need to be modified or reset, they must be set first, and then the start button is pressed to start the motor. During the start-up process, the motor current is continuously sampled to check for overcurrent. When the thyristor's conduction level is at its maximum, the AC contactor is engaged, short-circuiting the thyristor, directly connecting the motor to the grid, and the start-up ends. 4 Conclusion Intelligent soft starter uses an embedded system to control the thyristor's firing angle, thereby controlling the motor's starting voltage. This is unparalleled by other traditional starting methods. It ensures smooth starting under the required load characteristics, reduces the impact on the power grid, guarantees reliable motor starting, and provides protection against overcurrent, phase loss, and phase sequence errors, while also saving energy. This device has been successfully tested and operates well, effectively achieving the soft-start and fault protection functions for the motor. With the development of embedded systems, we hope for even better protection devices, which requires further theoretical and practical exploration by researchers.