I. Introduction
Motor batch self-starting technology has wide applications in continuous production enterprises such as petrochemical plants. A motor batch self-starting system with a PLC as the core control unit has the following functions and characteristics:
1. It can monitor the motor's operating status in real time;
2. The motor's operating state before the power grid fluctuation is remembered. Only motors that were in operation before the power grid fluctuation and stopped during the fluctuation have the conditions for self-starting.
3. Accurately and promptly capture grid voltage information.
4. The motors that start in batches are pre-set in the PLC according to the process flow requirements. At the same time, in order to avoid the impact of multiple motors on the power grid during self-starting, a batch delay processing method is adopted in the batch self-starting of motors.
5. It has multiple input and multiple output functions, enabling centralized control of multiple motors to start automatically;
6. It has a remote communication interface to enable communication with a host computer or DCS system, allowing for convenient monitoring and maintenance of the system from the host computer or DCS system.
The two sets of PLC-based motor batch self-starting equipment at Luoyang Petrochemical Plant use Siemens S7-300 series PLCs. These PLCs employ a CPU313 as the central processing unit, executing approximately 0.7ms for every 1000 binary instructions . The S7-300 features 128 digital inputs/outputs and 32 analog inputs/outputs, 12KB of RAM, and 20KB of load memory; it fully meets the requirements for real-time monitoring of motor status and system voltage, and timely batch self-starting of motors.
II. System Composition
The two-set PLC-based motor batch self-starting system uses one PLC for each low-voltage busbar section, depending on the substation's power supply method. The system hardware is mainly divided into two parts: peripheral circuitry and the core unit. The peripheral circuitry primarily handles the acquisition, processing, and conversion of signals such as busbar voltage and motor operating status, as well as driving motor start commands. The core unit (i.e., the PLC) mainly processes signals and issues motor drive commands.
2.1 Peripheral Circuit
The peripheral circuit mainly includes the following parts:
1. Bus voltage sampling and monitoring. It converts the 0-380V AC bus voltage into a 4*20mA DC signal through a current-type voltage transmitter.
2. Motor operating status signal monitoring. The motor operating status signal is input to the PLC input module through one dry contact in the motor control circuit. All signal inputs are optocoupled to improve anti-interference capability.
3. Motor Drive Unit. The motor start signal is issued by the PLC. The output unit does not directly drive the motor, but instead drives the motor operation circuit through a 220V, 10AAC intermediate relay. This improves the driving capability and separates the electrical operation circuit from the PLC control circuit, thus enhancing the system's safety and reliability.
2.2 Core Unit
According to the system requirements, its core PLC mainly consists of the following parts:
1. CPU313 and system software. It monitors voltage and motor operating status, performs real-time logical judgments, and issues batch self-start commands for the motor. The CPU313 has four operation options: RUN-P, RUN, STOP, and MRES operating modes.
2. Analog Input Module SM331 (8-channel input). It converts the 4-20mA analog input from the voltage transmitter into a digital signal and sends the digital signal to the PLC control unit for voltage judgment.
3. Digital input module SM321. It has two 16-channel inputs and one 32-channel input, which can monitor the operating status of 62 motors and monitor the operation and debugging status of the PLC motor batch self-starting system. The motor operating status signal is connected to this module through the auxiliary contact of the contactor in the motor operation circuit.
4. Digital output module SM322 (8 outputs). Receives instructions from the PLC control unit, outputs motor drive signals, and drives the motor operation circuit via the output intermediate relay to achieve batch self-starting of the motor.
III. System Software Design
The main tasks of the motor batch self-starting system software are:
1. Complete system initialization;
2. Data monitoring under normal conditions;
3. When the grid voltage fluctuates, i.e., when the grid voltage drops to 70%, all motors will be forced to stop running due to the electrical protection device. Before this, the program has already made a judgment and latched the motor status signal.
4. When the power system returns to normal (bus voltage recovers to 95% within 3 seconds), the program, based on the motor status signals saved before the fault, sends start signals in batches to motors that meet the white-start conditions, in sequence, to restore their operation.
5. The PLC monitors the bus voltage in real time, whether under normal conditions or during motor self-starting.
6. Communication interface program. This includes system monitoring data and fault information. The PLC transmits the collected bus voltage information and motor start status information to the host computer or DCS system, allowing maintenance personnel to understand the equipment's operating status in real time.
