Traditional large and medium-sized motor control devices use contactors and magnetic starters for direct start-stop. The disadvantages are simple and inflexible control methods, significant system impact, easy damage to control components, and high maintenance workload. With the increasing maturity of computer control technology, a new type of intelligent controller, the "soft starter," based on a computer and using bidirectional thyristors as the main control circuit, has been widely applied in this field in recent years. It is gradually replacing traditional control devices due to its flexible and simple control methods, low system impact, less susceptibility to damage to control components, and convenient maintenance. Of course, new products always reveal some shortcomings. This article, based on the soft starter used at Danjiangkou Power Plant, discusses its operation over three years and the problems it has encountered, and proposes several improvement suggestions from a production perspective to further improve its control functions for reference by manufacturers and peers. 1. Functions and Characteristics of the Soft Starter 1.1 Application Situation The 20 m³ air compressor system at Danjiangkou Power Plant is the air source for unit phase adjustment, maintenance, and other air-using equipment. It consists of two air compressors and four air storage tanks, with a working air pressure of 0. The air compressor has a rated current of 272 A and a motor capacity of 132 kW, with a pressure range of 6 to 0.8 MPa. Two independent control systems, each controlling one air compressor, have been in operation for over 20 years, resulting in severe aging and a high failure rate. Due to the limitations of technology at the time, the logic control circuit consisted of relays, the starting method was a traditional frequency-sensitive resistor voltage reduction method, and the motor starter was a contactor. The control system had complex wiring, numerous fault points, incomplete motor protection, high starting current, and significant impact on plant power consumption, which was detrimental to the stable operation of other equipment. In early 1997, a soft starter was applied to upgrade the system. The new control system solved many problems of the original system and facilitated interface with the new computer monitoring system, which was undergoing a comprehensive upgrade. This laid the foundation for improving the overall automation level of the plant and achieving manpower reduction and efficiency improvement. 1.2 Soft Starter The soft starter mainly consists of a main control board with an 8096 CPU, a drive board, a main motor control circuit, and a control panel. The soft starter has control function, motor protection function, display, alarm and parameter setting function. 1.2.1 Control function mode After receiving the external start and stop command, the motor is controlled according to the preset start and stop mode. The selectable start and stop control modes are as follows: (1) Current limiting soft start control mode: As shown in Figure 1, when the motor starts, its output voltage increases rapidly from zero until the output current reaches the set current limit value Im. Then, while ensuring that the output current is not greater than this value, the voltage gradually increases and the motor gradually accelerates to complete the start process. [img=353,200]http://www.cechinamag.com/images/Article/04c57f8c-d7d6-4915-9aab-cb2dbfc06705/231.jpg[/img] (2) Voltage ramp start control mode: As shown in Figure 2, U1 is the initial voltage corresponding to the minimum torque required for motor start. When the motor starts, the output voltage of the soft starter rises rapidly to the set value U1, and then gradually increases at the set rate until it reaches the grid voltage. Then the contactor engages, and the starting process is completed. [img=335,211]http://www.cechinamag.com/images/Article/04c57f8c-d7d6-4915-9aab-cb2dbfc06705/232.jpg[/img] (3) Stop mode: The motor gradually decelerates at the set rate until it stops completely. (4) Free stop: The motor stops freely due to inertia without control. (5) Jog function: Under the control of this mode, the output voltage of the soft starter increases rapidly to the initial voltage U1 and maintains this output voltage value. 1. 2. 2. Motor Protection Functions The motor protection functions of the soft starter include: phase sequence protection, phase loss protection, starting overcurrent protection, running overcurrent protection, running overload protection, and protection against prolonged motor startup failure. When the protection functions of the soft starter are activated, the soft starter will generate a shutdown output and directly display the reason on the control panel. 1.2.3 Display, Alarm, and Parameter Setting Functions The control panel of the soft starter can display the motor current, alarm signals, and set parameter values; set motor protection values ​​and motor operating modes in digital form; and manually start and stop the motor. The working principle of the soft starter is: after receiving the start command, the soft starter performs relevant calculations according to the set start mode to determine the trigger output voltage signal of the thyristor to control the motor startup process. After the motor startup process is completed, the soft starter controls the AC contactor to engage, short-circuiting all thyristors, allowing the motor to directly connect to the grid and run, avoiding unnecessary energy loss. Figure 3 is a schematic diagram of the soft starter. 1.3 Soft Starter Applied to Danjiangkou Power Plant 1.3. 1 System Structure The system structure is shown in Figure 4. It mainly includes a soft starter, a programmable controller, a pressure sensor, and an intake solenoid valve. 1.3.2 Main Functions of the System (1) Control Function Automatic/manual control of the air compressor to start and stop. The control mode is selected by the working mode switch. In automatic working mode, the air compressor is automatically started and stopped according to the change of air tank pressure. In manual mode, the programmable controller is out of operation, and the air compressor is manually started and stopped on the soft starter control panel. According to the actual situation of the 20m3 air compressor system, the motor start and stop mode is selected as current-limited soft start and soft stop. (2) Protection Function The protection function involves the entire air compressor system. When any abnormality occurs, such as excessively high/low air pressure in the first stage cylinder, excessively high/low oil cylinder pressure, excessively high exhaust temperature, phase loss, phase sequence error, motor overcurrent, or overload, the protection action will automatically stop the machine. (3) Monitoring and alarm functions: Real-time monitoring of the system's operating status. The control panel displays the main circuit current and voltage indicators and shows the system's operating status. When a system fault occurs, the control panel displays a detailed fault alarm and sends an alarm signal to the central control room. 1.3.3 System working principle: When the air pressure in the air tank drops to the compressor's starting value, the programmable controller sends a start command to the soft starter. The soft starter controls the motor's starting voltage and current through the thyristor, enabling the air compressor system to start smoothly to no load. When the voltage reaches the rated value, the contactor engages, the thyristor short-circuits, and the three-phase power supply is directly applied to the motor. The soft starter completes the start and sends a start completion signal to the programmable controller. After a 9-second delay, the programmable controller engages the intake solenoid valve, and the air compressor starts running under load, pumping air into the tank. When the tank pressure reaches the shutdown value, the programmable controller disconnects the intake solenoid valve, putting the air compressor system into an unloaded state. Two seconds later, it sends a shutdown command to the soft starter. Under the control of the soft starter, the motor gradually decelerates until it stops completely. [align=center][img=439,453]http://www.cechinamag.com/images/Article/04c57f8c-d7d6-4915-9aab-cb2dbfc06705/233.jpg[/img] [img=449,287]http://www.cechinamag.com/images/Article/04c57f8c-d7d6-4915-9aab-cb2dbfc06705/234.jpg[/img] [/align] 2. Operational Results and Improvement Suggestions The three-year operation of the new control system shows that its reliability, functionality, and performance have been significantly improved compared to the original control system. The main features are as follows: (1) The soft starter is used as the control output execution element in the structure, and the control logic is implemented by the programmable controller, which makes the system structure simple and clear, improves the reliability of the control system, and is also easy to maintain. (2) The current limiting start method of the soft starter controls the starting current within a safe range, which improves the situation where the original control system is affected by the large starting current impacting the power supply of the plant and affecting the normal operation of other equipment. (3) The start-up process adopts bidirectional thyristors. After the start-up process is completed, the contactor short-circuits the thyristor, which avoids the occurrence of faults such as arcing, sticking, and burning of the motor contacts when directly controlling the contactor, and also saves energy. (4) The soft start and soft stop methods reduce mechanical stress, protect the equipment, and extend its service life. (5) The protection function covers various adverse operating conditions of the air compressor system, which is conducive to improving the health level of the equipment. (6) The display function on the control panel makes it easy to fully understand the equipment operation status on site. (7) The digital parameter setting and display functions are intuitive, convenient and time-saving. The anti-interference ability of the soft starter is an important indicator affecting its reliability. During the three years of operation of the soft starter, there was a phenomenon of false start when the device was powered on. Therefore, in view of the current problems of the soft starter, the following suggestions and improvement measures are proposed for the manufacturer's reference: (1) In order to prevent the occurrence of false start after the device is powered on, it is recommended that the manufacturer modify the control software and add a delay function block to the final output signal circuit of the device software. The delay time should be based on avoiding interference signals. This needs to be determined by on-site debugging. Alternatively, a delay relay can be added to the final signal output circuit of the hardware, which can also effectively solve the interference problem. (2) For ease of maintenance, it is desirable that the soft starter can be easily put into and taken out of operation during normal load operation. That is, during normal operation, if the power supply of the corresponding soft starter control device is turned off, the system operation mode remains unchanged. At this time, the faulty soft starter device can be repaired. After the repair is completed, the power supply of the soft starter control device can be turned on directly, and the operation mode (i.e., the automatic following device software) requirements are consistent. This function is very suitable for daily maintenance on site. (3) It is recommended to modularize the soft starter device for easy maintenance. 4 Conclusion In the field of auxiliary machine control such as air compressors, pumps, and fans in hydropower stations and many industries, the traditional control structure has many defects. For large loads, the problem is more prominent. The soft starter not only overcomes the shortcomings of the traditional control structure, but also makes the control function more perfect. With the continuous progress of technology and the continuous improvement of soft starters, the soft starter will become more mature. Therefore, the widespread application of soft starters in large and medium-sized motors is an inevitable trend.