[Abstract] This paper briefly introduces the classification and application of construction hoisting equipment; it also designs and applies a variable frequency speed control system for cranes in construction hoisting equipment by combining advanced variable frequency speed regulation and PLC control technology.
Keywords : Construction hoisting equipment, crane, variable frequency speed control, PLC control
[Abstract] Briefly introduced the building lifting equipment classification and application; In combination with the advanced frequency control and PLC control technology, the architecture of the crane hoisting equipment variable frequency speed regulation control system design and application.
Key: Building lifting equipment crane Frequency control of motor speed PLC control
I. Introduction
In the construction industry, hoisting technology is an extremely important technology. As my country's engineering construction moves towards standardization, factory production, large-scale production, and integration, the weight and height of equipment being hoisted are increasing, as are the difficulties. The hoisting of a large piece of equipment is often a key factor restricting the progress, economy, and safety of a project; it also showcases a company's technical and equipment capabilities. To adapt to current developments, the types of hoisting equipment are also increasing, and the control of hoisting equipment is becoming more advanced and intelligent. Among these, variable frequency speed control and PLC control technologies are widely used in construction hoisting equipment.
II. Construction hoisting equipment
Construction hoisting equipment can be classified according to the nature of the hoisting activity as follows: simple hoisting tools, such as jacks, pulley blocks, hoists, winches, and suspended monorails; and cranes, including mobile cranes, tower cranes, and mast cranes. Construction hoisting equipment can also be classified according to its structural form as follows: bridge cranes (bridge cranes, gantry cranes); cable cranes; and boom cranes (self-propelled, tower, portal, railway, and floating cranes).
Tower cranes are the most commonly used hoisting equipment in the construction industry. They are suitable for hoisting a large number of components or equipment within a given area, each with a relatively small individual weight, and their operation cycle is long. Currently, tower cranes have a hoisting capacity between 3 tons and 100 tons, and their boom lengths can reach 40 meters to 80 meters. Therefore, tower cranes are commonly used in situations where the location is fixed and the service cycle is long, making them relatively economical. Tower cranes have hoisting, slewing, luffing, traveling, jacking, and cable reeling mechanisms, each with significantly different load characteristics. For example, the hoisting mechanism is a potential energy load; it is a constant torque when the load is constant, a resistance load when rising, and a dynamic load when lowering. The slewing and luffing mechanisms mainly transmit horizontal loads, and must overcome significant inertia during starting and braking. Different load characteristics necessitate different requirements in selecting motors, determining transmission schemes and speed control methods, and in operation. Furthermore, due to the harsh outdoor conditions at construction sites, poor power quality, and significant external environmental influences, inspection and maintenance are difficult. Therefore, the reliability of the electrical system of construction hoisting equipment is crucial.
III. Variable Frequency Control System
3.1 Variable Frequency Speed Control
A variable frequency drive (VFD) is a high-tech motor drive device integrating electronic power components, theoretically capable of stepless speed regulation. Furthermore, VFD speed regulation provides comprehensive speed control from constant torque to constant power; a set frequency corresponds to a specific speed. The electrical control system is designed based on the real-time changing requirements of the load, thus requiring comprehensive calculations of the load and motor characteristics. When the frequency f ≤ 50Hz, the motor operates at constant torque; when the frequency f > 50Hz, the motor operates at constant power. Figure 1 clearly shows the mechanical characteristic curves at different frequencies, demonstrating that different loads (torque ratios) can achieve various speeds, fully meeting the requirements of high speed under light loads and low speed under heavy loads.
3.2 Hardware Selection for Crane Variable Frequency Speed Control
(1) Selection of variable frequency motor
Equivalent resistance value; V – DC operating point voltage of the braking unit, generally taken as 700V; Pe – Rated power of the motor; Kc – Braking frequency, taken as 0.2 to 0.4 in bridge cranes; K – Mechanical energy conversion efficiency during feedback, generally taken as 0.7).
(4) Selection of Programmable Controller (PLC)
In bridge cranes, since each mechanism typically has 5 control levels, the total number of PLC points required for each mechanism is generally between 19 and 20. For a bridge crane with three mechanisms, the total number of points required is generally around 60.
(5) Other electrical components shall be selected in accordance with relevant standards, and will not be described in detail here.
3.3 Electrical Diagram Design for Crane Variable Frequency Speed Control
Taking Yaskawa inverters as an example, this section mainly explains the electrical design of the variable frequency speed control section of a crane. The electrical schematic diagram of the crane's hoisting mechanism's variable frequency speed control is shown in Figure 2. The hoisting mechanism's variable frequency motor is equipped with a rotary encoder, which connects to a speed card installed inside the inverter, forming a closed-loop speed control. The control points S1-S7, SC, MA, MB, M1, and M2 on the inverter, as well as the coil control circuits of the motor fan contactor K7 and the brake contactor K8, are connected to the input, output, and power supply points of the selected PLC. Through the execution of the PLC's internal program, closed-loop control of the hoisting mechanism's variable frequency speed control is achieved. A residual current circuit breaker Q1 and an AC reactor are installed between the inverter's power input terminals (R, S, T) and the power supply. The residual current circuit breaker Q1 has a capacity of 1.8 times the rated current of the frequency converter and an induced current of over 30mA. It can detect high-frequency leakage current that is dangerous to the human body and prevent accidents. Its AC reactor and the DC reactor in the frequency converter can effectively improve the power factor on the power supply side and reduce interference to the outside world.
The electrical principle of the variable frequency speed control for the crane's moving mechanism is shown in Figure 3. The variable frequency speed control for the moving mechanism is an open-loop control, eliminating the need for a rotary encoder and speed caliper. Other circuit designs are similar to those for the hoisting mechanism.
3.4 Programming for Variable Frequency Speed Control of Cranes
The programming of a crane's variable frequency speed control system is essentially the programming of the selected PLC. PLC programming generally uses either ladder diagrams or instruction lists. The PLC program for crane variable frequency speed control consists of two parts: the operation control program and the communication interface program. Generally, the main program for crane variable frequency speed control is the operation control program, while the communication interface program is only involved when the PLC is equipped with a touch screen. The operation control program for each mechanism of the crane basically consists of electrical protection, gear control, brake, and motor/fan operation program segments. Gear control involves changing the gear position on the control panel in the crane operator's cab. Through the operation of the program within the PLC, the multi-speed commands of the frequency converter are controlled, causing the frequency converter's output frequency to change according to the preset frequency as the gear position changes.
The crane's PLC control system consists of 96 input points and 64 output points, for a total of 160 input and output points. It uses a Siemens S7-300 PLC (CPU is 315-2DP), which is expanded with 7 digital input modules SM321 and 4 digital output modules SM322.
IV. Installation and Commissioning of Crane Variable Frequency Speed Control
When installing a crane variable frequency speed control cabinet, the vertical distance between the frequency converter and the cabinet or other components should be at least 120mm, and the horizontal distance should be at least 30mm. Furthermore, the frequency converter and PLC should ideally not be in the same cabinet to avoid mutual interference. Shielded cables should be used for the control circuit wiring of the frequency converter and PLC, and their wiring should be kept separate from power lines as much as possible. The grounding of the frequency converter and PLC should use type C grounding, separate from the grounding of welding machines, power equipment, etc., and their wiring should be as short as possible.
The debugging of the variable frequency speed control of the crane generally involves the following steps: (1) Before debugging, check all equipment and wiring, measure the damage of electrical lines during transportation and installation, and check whether there is a phase loss in the power grid; (2) Input the pre-designed program into the PLC. This step can also be done during the cabinet manufacturing process; (3) After powering on the inverter, perform a jog operation on the motor through the control panel on the inverter to see if its rotation direction is correct; then select a suitable self-learning mode for the motor and perform self-learning on the motor. If the rotational self-learning mode is selected, the load must be disconnected and the motor must be ensured to rotate safely; (4) Set the parameters corresponding to the selected control mode and other relevant parameters (see the user manual of the selected inverter for specific settings), and after confirming that all equipment is powered on without abnormalities, perform no-load debugging of the bridge crane. (5) After no-load debugging, perform full-load debugging with the rated load. During full-load debugging, appropriately change the acceleration and deceleration time parameter values of the inverter to achieve the optimal acceleration and deceleration time adjustment. If the hoisting mechanism is running at full load and there is obvious hook slippage, the braking parameters in the frequency converter need to be adjusted.
V. Conclusion
After adopting variable frequency speed control, the crane has demonstrated excellent operational performance, stable and reliable operation, and a significantly reduced failure and maintenance rate, achieving the user's expected goals and generating substantial economic benefits. This has earned widespread praise from users. Furthermore, it exhibits high speed under frequently changing loads and low speed under heavy loads, rationally utilizing power to achieve energy conservation and emission reduction.
References
1. Qiu Weizhang (Chief Editor), Practical Handbook of Crane Electrical Technology, Machinery Industry Press, 2001.
2. Zhang Zhiwen et al., eds., Crane Design Manual, China Railway Publishing House, 1998.
3. Dai Guangping, ed., Electric Motors, Frequency Converters and Electric Drives, China Petrochemical Press, 1999.
4. Wang Zhengmao et al., eds., Electrical Engineering, Xi'an Jiaotong University Press, 2000.
