To achieve full energy utilization and meet production needs, motor speed regulation is required. Considering the characteristics of motor starting, running, speed regulation, and braking, an ABB frequency converter is used. The system uses a PLC to acquire data and control the frequency converter, motor, and other equipment. Based on the S7200 PLC programming software, a modular programming approach is adopted, with extensive code reuse to reduce software development and maintenance. The PLC software design enables frequency converter parameter setting, fault diagnosis, and motor start-up and shutdown.
With the development of power electronics and industrial automatic control technologies, AC variable frequency speed control systems have been widely used in the field of industrial motor drives. In addition, due to their powerful functions, ease of use, and high reliability, PLCs are often used for field data acquisition and equipment control. This design utilizes a frequency converter and a PLC to control the water level in a water tank.
Variable frequency drive (VFD) technology is a comprehensive technology built upon control technology, electrical electronics, microelectronics, and computer technology. Compared to traditional AC drive systems, using VFDs for speed control of AC motors offers numerous advantages, such as energy saving, ease of speed control of existing motors, efficient continuous speed control over a wide range, and precise speed control. It facilitates forward and reverse switching of the motor, allows for high-capacity start-stop operation, provides electrical braking, and enables high-speed motor drive. Furthermore, it boasts robust protection functions: VFDs have strong protection capabilities, detecting various faults during operation and displaying the fault type (such as instantaneous voltage drop in the power grid, phase loss in the power grid, DC overvoltage, power module overheating, motor short circuit, etc.), and immediately blocking the output voltage. This "self-protection" function not only protects the VFD but also protects the motor from damage.
PLC Features: First, high reliability and strong anti-interference capability, with a mean time between failures (MTBF) of hundreds of thousands of hours. PLCs also employ numerous hardware and software anti-interference measures. Second, simple programming and easy use. Most PLCs currently use ladder diagram programming with relay control, which is easily accepted by operators. Some PLCs also feature step ladder instructions designed for specific problems, further simplifying programming. Third, easy design and installation, with minimal maintenance. Fourth, suitable for harsh industrial environments. The encapsulated design makes it suitable for applications involving vibration, corrosion, and toxic gases. Fifth, convenient connection to external equipment. The detachable terminal blocks with a unified wiring method provide different terminal functions suitable for various electrical specifications. Sixth, comprehensive functions, strong versatility, small size, low energy consumption, and high performance-price ratio.
The following basic principles should be followed when designing a PLC system to ensure stable system operation.
(1) To meet the control requirements of the controlled object to the greatest extent possible;
(2) The system structure should be as simple as possible;
(3) The system must operate stably and reliably;
(4) The control system can be easily expanded and upgraded;
(5) User-friendly interface.
In this system, to achieve full energy utilization and meet production needs, motor speed regulation is required. Considering the characteristics of motor starting, running, speed regulation, and braking, an ABB ACS800 frequency converter from ABB is used. The system utilizes an S7-200 series PLC to complete data acquisition and control of the frequency converter, motor, and other equipment. The programming software based on the S7-200 PLC employs a modular programming approach, extensively utilizing code reuse to reduce software development and maintenance. The system leverages the PLC software design to achieve frequency converter parameter setting, fault diagnosis, and motor start-up and shutdown.
1. Control requirements for this design:
1) The system requires users to have an intuitive understanding of the working status of the on-site equipment and changes in water level;
2) Users are required to be able to remotely control the start and stop of the frequency converter;
3) Users can set the water level themselves to control the start and stop of the frequency converter;
4) Fault information from frequency converters and other equipment can be promptly reflected on the remote PLC;
5) It has alarm functions for excessively high or low water levels and provides user notifications;
2. Design control structure:
Since there is a motor as the controlled object on site, a single PLC can be used to control a single object. As long as a high-performance PLC is selected appropriately, it can fully perform this function.
The PLC collects information from various objects, including sensors, monitoring motors, and frequency converters. In this system, a frequency converter is used to regulate and control the frequency of the motor. The analog signal output from the PLC is used as the control input signal for the frequency converter, thereby controlling the motor speed and feeding back its operating status signal to the PLC. In case of a fault, it can send an alarm signal to the PLC. Because variable frequency speed control changes the synchronous speed by altering the stator power supply frequency of the motor, it maintains limited slip power from high to low speeds during speed regulation, thus offering high efficiency, wide range, and high precision speed regulation performance.
