Abstract: By developing and applying the functions of Siemens S7-200 PLC, the automatic control system of a pumping station is realized with self-diagnosis and alarm functions, especially the acquisition and processing of motor operating parameters, which are then used in control to achieve unattended and safe operation of the pumping station with relatively low investment. Keywords: Motor parameters, alarm, analog quantity control, judgment 1. Introduction Due to the special nature of their production processes, metallurgical enterprises widely use water pumps, including various types of submersible pumps. This paper develops and applies the functions of Siemens S7-200 PLC in the submersible pumping station control system, realizing reduced-pressure start-up of submersible pumps, automatic water level control, and mutual switching control between two pumps in case of failure. It also integrates remote control, motor overload protection, leakage monitoring, and abnormal temperature rise detection, combined with rich alarm display functions, to achieve the functions of an intelligent unattended pumping station. 2. Control System Composition and Functional Requirements In general water inlet and outlet applications, since there are no strict requirements for water pressure and flow rate, and the initial investment required for AC frequency converter control and soft starters is relatively large, using an autotransformer for reduced-voltage starting control of the water pump motor is a good solution. The autotransformer is only continuously energized for about 10 seconds during startup (the energizing time may vary depending on the motor power) before being disconnected, so energy consumption is negligible. The main circuit of the motor is equipped with a thermal relay for overload protection and a current transformer for current detection. The autotransformer reduced-voltage control principle of the main circuit of the water pump motor (75kW) is shown in Figure 1. Control loop hardware composition: (1) PLC device includes one Siemens S7-200 series 224 CPU module, expansion unit includes one EM235 analog module, 4 inputs/1 output, and one EM223 digital input/output module; (2) Peripheral devices include one level controller (JY-2A type), two platinum resistance temperature sensors integrated with the motor, two current transformers (150A/20mA), one digital ammeter, and several buttons and indicator lights (DC24V). The hardware configuration of the EM235 analog module is shown in Figure 2. Control function requirements: When the water level is between the set upper and lower limits (the submersible pump motors need to be immersed in water for cooling), and there is no water leakage or abnormal temperature rise (not exceeding 60℃), either pump should be able to start. If the water level drops to the lower limit, the pump should automatically stop to protect the motor. When the water level rises to the lower limit, the water pump should not start. When the water level continues to rise to the upper limit, the water pump should start automatically, with pump #1 starting first. If pump #1 fails to start for any reason, a fault alarm should be issued, and maintenance personnel should be notified to handle the situation. At the same time, pump #2 should be started. If neither pump can be started normally, it is a level one fault alarm. When the water level is between the upper and lower limits, if a pump in use stops due to motor leakage, abnormal temperature rise, motor overload protection activation, or main circuit air switch tripping, another water pump should be started immediately. When starting any water pump, the interval since the last start should be checked to ensure that the interval is greater than 10 minutes before restarting the pump. 3. PLC Programming 3.1 PLC I/O Function Table The digital switch input/output function table is shown in Table 1. The analog signal consists of 4 inputs and 1 output, namely: load current and motor winding temperature detection for pumps #1 and #2; since only one pump operates at any given time, one output is used as a digital display of the motor load current for easy viewing by operators in the main control room. [align=center]Table 1[/align] 3.2 PLC System Control Flow The main control program calls three sets of subroutines to sequentially perform hardware self-test upon PLC system power-on, controller initialization, and analog data acquisition and output processing, before entering the electrical interlock automatic control operation program. The hardware self-test subroutine reads system information to primarily check whether the analog module EM235, the first expansion module, is reliably connected, whether the module itself has any faults, and whether the DC24V power supply provided by the CPU module to the expansion unit is normal. The hardware self-test subroutine LAD is shown in Figure 3. The initialization subroutine initializes and resets the controller to avoid internal register interference and ensure stable program operation. The analog data acquisition and output processing subroutine group includes four subroutines: a subroutine for reading, processing, and displaying the load current of pumps #1 and #2, and a subroutine for detecting the motor winding temperature. Because the input addresses for the four analog quantities (current and winding temperature) of motors #1 and #2 are different, each one must be specified in the control program. However, in actual operation, only one pump is working at any given time; therefore, only the two subroutines for processing the data of one motor are running. The program execution time is also guaranteed. In this control system, the detection of the two analog quantities—motor operating current and winding temperature—is not merely for acquiring and displaying control parameters for operators to observe and judge the system's operating status; it also involves the Siemens S7-200 PLC identifying whether these two parameters are abnormal, issuing warnings to indicate the fault type, and determining whether to stop or start the pump, directly participating in system control. 3.3 Implementation of Motor Operating Parameter Acquisition and Control Functions In the initialization subroutine of the main control program, the sampling counter frequencies for the two analog quantities, motor current and winding temperature, are preset. To increase data stability, multiple sampling and accumulation are performed to calculate the average value; therefore, the relevant operation registers must be cleared beforehand. Then, after the system passes the fault detection of the extended unit, it performs analog quantity acquisition and processing. Two separate subroutines independently perform sampling, calculation, output, and judgment of the motor load current and winding temperature. Both use multiple sampling and summation, and the average value is calculated through shift division. The difference is that the current value (registered in VW20) is output and displayed via the EM235 output channel AQW0 through a transmission instruction; the STL of this part of the control program is shown in Figure 4. On the other hand, it serves as a status detection for whether the water pump is effectively operating, thereby judging motor faults and activating the standby pump. The motor winding temperature, as a parameter with a higher control level, is acquired and processed, and judged by comparison instructions combined with timing instructions. If the temperature rise is abnormal, the motor operation is stopped immediately. Timing instructions are used in multiple places in the system control program to enhance system stability. For example, autotransformers cannot be frequently energized. A timer function is used to set the same water pump to restart only after a ten-minute interval. The motor load current is monitored to determine if the motor is actually running. To avoid the operating time of external contactors and relays and prevent detection errors, a 5-second timer is used, effectively ensuring signal reliability. This provides a reliable basis for the CPU to determine whether to activate another water pump. By setting comparison parameters, if the actual operating current of the motor exceeds 1.1 to 1.2 times the motor's current (Ie) for a prolonged period (generally set to 30 minutes), the pump can be stopped to investigate the cause and prevent motor damage, thus replacing traditional overload protection devices. Simultaneously, a signal can be sent to start the standby pump. Due to the special temperature parameters of the motor windings and the water-immersed cooling system, sudden changes are impossible. Therefore, with a constant sampling frequency, the processed temperature data is checked and compared every five minutes. When the temperature rise exceeds the set value, an alarm is issued, the motor stops, and another pump is started, fully meeting the requirements. Regardless of whether the motor current is overloaded, if the motor temperature rises sharply and exceeds the normal allowable temperature rise, the system can immediately stop the pump. The completion of the above control functions requires the cooperation of comparison instructions and timing instructions to enable the PLC to make correct judgments on the actual situation and initiate the corresponding control execution program. 4. Conclusion Introducing analog parameters into low-voltage control systems compensates for the shortcomings of switch control systems that only reflect the status of input and output devices. When the actuators are operating normally but the transmission parts are not actually in use due to power supply or motor failure, the system remains unaware. Abundant alarm and warning functions, directly pointing to the fault point, greatly facilitate operation and maintenance. Lower investment, simple and reliable main circuit, full utilization of module functions, and near-intelligent control eliminate the need for dedicated personnel to operate the pump station. The author's innovation lies in: by collecting and detecting the pump motor load current and temperature rise, combined with the use of comparison and judgment instructions, realizing the PLC pump station control system's operating status monitoring and numerous equipment protection functions. The processing of analog quantities and their combined use with switch quantities make the system's control functions more scientific, reasonable, and complete. References: [1] Cai Xingjian and Huang Wenyu, eds., In-depth and easy-to-understand Siemens S7-200 PLC, Beijing: Beijing University of Aeronautics and Astronautics Press, 2003. [2] Siemens S7-200 PLC System Manual. [3] Chen Lei and Jiang Zhoushu, Microcomputer Information, Application of PLC in Room Air Conditioner Performance Testing System, Vol. 21, No. 2, 2005.