Abstract: This paper discusses the design principle of PLC in elevator electrical retrofitting, and determines the scheme for implementing PLC control based on a case study. Actual operation shows that the retrofitted elevator has a good performance-price ratio that justifies the investment.
Keywords: PLC elevator retrofit solution
1. Problem Statement
Currently, a significant proportion of elevators in operation in my country were manufactured in the 1970s and 80s, especially industrial elevators. The vast majority of these elevators rely on relay contact control systems, resulting in high failure rates, heavy maintenance workloads, and low efficiency, severely impacting users' production and management. Most users cannot replace them promptly due to financial constraints, even though the mechanical components are generally in good condition. National policy, considering safety, has mandated the closure of these elevators by labor and construction management departments. Therefore, upgrading the existing electrical systems of older elevator users to improve elevator safety and ensure production and management is an effective approach, especially significant for economically underdeveloped regions.
2. Determination of PLC-based electrical control system upgrade scheme
Elevator electrical control systems are divided into speed control and logic control. The quality of speed control affects the comfort of elevator operation, while logic control is crucial for ensuring safe elevator operation. Therefore, any elevator modification or upgrade must ensure that its control scheme meets both of these requirements.
2.1 Design Scheme of Elevator Electric Drive System
Older elevator models mostly use AC dual-speed motors. These motors employ reactor switching during starting, speed changes, and braking. Regardless of adjustments, a noticeable "step" sensation remains, making it difficult to meet current elevator performance standards in terms of comfort and leveling accuracy. Therefore, replacing the original traction motor with a frequency converter of matching power during the retrofit allows for ideal speed control of the motor's operation and speed regulation. Figure 1 shows the elevator speed curve.
Because elevator operation is a potential energy load, the motor will operate in four quadrants. Therefore, selecting a current source inverter (MICOVERT2000) is more reasonable. This can prevent damage to the inverter from excessively high pump voltage and ensure the safe and stable operation of the drive system. Figure 2 shows the control block diagram of the electric drive system.
The programming parameters in the frequency converter are determined based on the elevator's speed operating curve parameters. The elevator's start-up, speed change, and direction of travel are decided by external signals such as external call signals, internal selection signals, and deceleration signals. After receiving the control signals, the PLC control unit sends instructions to the frequency converter, which then operates according to the speed operating curve. After the modification, the elevator's electric drive system becomes a VVVF drive mode. This greatly improves the elevator's comfort and leveling accuracy, enhancing its overall performance.
2.2 Design Scheme of Elevator Logic Control System
The original elevator used relay control, which resulted in complex wiring and a high failure rate. The replacement uses a new control system, which offers fewer wiring requirements, reliable operation, and convenient maintenance. The number of input and output points of the PLC can be determined based on the number of floors in the elevator to be upgraded, and a suitable PLC model (Fx 2-64 MR), expansion module (Fx-64 EYR), and functional unit (Fx2-40AW) are selected. The integrated electric drive control system and logic control system are shown in Figure 3, which illustrates the schematic diagram of the elevator's electrical control.
Ladder diagrams for the various logic control departments (external call command control, internal selection command control, master command control, floor display control, etc.) are omitted.
3. Conclusion
The upgraded elevator control system has proven to offer smoother and more comfortable operation, significantly improved system reliability and technical performance, increased operational efficiency, and reduced maintenance workload. Upgrading old elevators allows those that are about to be abandoned or have been decommissioned to be put back into service, resulting in substantial economic benefits.
References
1. Chen Jiasheng (ed.). Elevator Structural Principles and Installation Technology. Beijing: Machinery Industry Press, 1990.
2. Yang Changneng et al., eds. Programmable Controllers and Fundamentals. Chongqing: Chongqing University Press, 1992.
3. Xu Yinquan (ed.). AC Speed Control Systems and Applications. Beijing: China Textile Press, 1990.
