0 Introduction Modern production equipment control involves a large number of switching, digital, pulse, and analog signals. Using traditional relays or discrete electronic circuits as automatic control devices suffers from numerous drawbacks, including complex system structure, high power consumption, and poor reliability. However, using a PLC to implement these controls not only overcomes these shortcomings but also enables logic control, process control, and position control. Furthermore, it is compact and easy to use and maintain. This paper introduces the application of PLC control to achieve logical sequence control and centralized system control in the relay control system for two wound-rotor motors with rotor series resistance starting in our factory. 1 Control Objects and Requirements The main equipment consists of several raw material conveyors. Conveyor No. 1 is driven by a 55kW squirrel-cage asynchronous motor and uses a self-starting control method. Conveyor No. 2 is driven by two 155kW wound-rotor asynchronous motors and uses a rotor series resistance speed regulation starting control method. The material flow is as follows: raw material feeder at the bottom of the raw material silo → No. 2 belt conveyor → No. 1 belt conveyor → homogenization silo. Because the No. 1 belt conveyor has a long distance, poor operating conditions, and frequently starts under heavy load, the mechanical personnel require the belt starting acceleration to be less than 0.3 m/s². Therefore, the system must have a large starting torque and excellent starting characteristics to meet these conditions. The original electrical control used a wound-rotor motor with rotor resistance for starting and speed regulation. Based on the starting characteristic curve requirements, the resistance was calculated and divided into eight different resistance values, which were then connected in parallel to eight contactors. These eight contactors were then used to form a 16-level sequential motor starting and speed regulation control circuit using binary logic encoding to complete the entire starting process of the belt conveyor. Since early sequential control circuits generally used relays, the wiring of the entire control circuit was very complex, with many contacts and a high failure rate. Therefore, replacing the original relay control circuit with a PLC not only solves the above problems but also allows for centralized control of the No. 1 belt conveyor and the feeder, reducing secondary wiring and investment costs, and simplifying the modification process. 2. PLC Programming and Design 2.1 Programming Based on the material flow, the PLC program can automatically control the start/stop of conveyor belt 1 and the feeder, as well as the start/stop control signals for belt misalignment, tearing, and pull rope switches, and the emergency protection stop signals and interlock signals. The working principle is as follows: Start-up status: When the start button is pressed in the interlocked state, conveyor belt 1 starts after a 30-second warning bell. After a 15-second delay, motor 1 of conveyor belt 2 starts, motor 2 starts after a 2-second delay, and the entire start-up process of conveyor belt 2 is completed in 56 seconds. Finally, the feeder starts. Stop-up status: The sequence is reversed from the start-up. When the stop button is pressed, the feeder stops first, then conveyor belt 2 stops freely, and conveyor belt 1 stops after a 120-second delay. 2.2 PLC Selection Through calculation, the control system has 11 external input points (INPUT) and 17 control output points (OUTPUT). Because OMRON series PLCs are widely used in our factory, we selected the OMRON C60P-CDR-AE model, which has 32 INPUT points (24V DC 7mA) and 28 OUTPUT points (2VDC/250VAC 2A). This model meets the system control requirements and has a good cost-performance ratio. 2.3 Programming, Debugging, and Storage: With the PLC in PROGRAM mode, write the designed ladder diagram instruction by instruction into the PLC memory using the OMRON programmer PR015. During instruction input, press the SRCH key to check for errors and make corrections until completion. After verifying the program is error-free, program debugging and system simulation testing can be performed. After disconnecting all main circuit power switches, turn on the control power switch and PLC power switch, press the start signal button, and check the PLC input/output program execution one by one. Once the design requirements are met, the program can be permanently saved. 3. Application Status The system achieved good results after trial operation. Its control circuit is simple, its operation is reliable, and the failure rate is greatly reduced (almost zero failures), overcoming the shortcomings of past relay control, such as frequent failures, unreliable operation, and complicated wiring. This demonstrates that PLCs have significant advantages and high cost-effectiveness in the transformation of switch control and sequential control.