According to relevant reports, many overhead cranes in developed countries are controlled by frequency conversion speed regulation systems. Even in developing countries, the proportion is relatively high. Although our country has carried out some frequency conversion speed regulation system transformations to the traditional crane control system in recent years, and many crane manufacturers have begun to adopt the frequency conversion speed regulation system, the vast majority of users still use the traditional "winding motor rotor series resistance adjustment system". Speed system” (hereinafter referred to as the traditional speed control system).
The problem
2.1 Poor transmission performance
In the traditional speed control system, although the rotor series resistance realizes the speed control function (also has a step), because the synchronous speed cannot be changed, the mechanical characteristics of the transmission system are very soft, which makes the speed stability and speed control accuracy very poor. .
In addition, large vehicles are usually driven by two or four or more electric motors. Due to load changes, track flatness, and dispersion of motor parameters, the beam twists and the electronic control equipment on the vehicle shakes greatly, which not only deteriorates. The working environment of the operator is prone to fatigue and operation errors, and the service life, stability, and reliability of the entire equipment are also reduced.
2.2 Poor reliability
◆The contactor frequently opens and closes, especially the operator frequently relies on the reverse connection brake to stabilize the hook (the contactor frequently opens and closes under the condition of high current), which often causes the contactor contacts to sinter and resist burnout.
◆Improper protection measures may cause the motor to burn out.
◆The brake shoe bears relatively large impact and is severely worn, requiring frequent maintenance and replacement.
◆The operating environment of the crane is generally harsh, and the wound motor itself has weak links such as brushes and slip rings, which require frequent maintenance and repair.
2.3 Increase energy consumption
Cranes often run at low speeds, and a large amount of slip power is consumed in the resistance of the rotor in series (generally, it can account for more than 20% of the total energy consumption), which is not desirable in terms of energy saving and economy.
2.4 Increased use and maintenance costs
The shortening of equipment service life and the downtime caused by poor reliability not only increase the maintenance workload, but also increase the cost of equipment use.
2.5 The risk is also relatively high. Cranes are potential loads and have high requirements for safety.
All the above-mentioned problems are caused by the principle of the winding motor rotor series resistance speed regulation system. The final manifestation is that the productivity of the equipment is restricted (frequently fails and is in a downtime or repair state), the maintenance workload is large, and the use and maintenance cost increases.
The following is an example of our company's transformation of a crane's frequency conversion system for a factory to introduce the application of frequency converters in the crane industry.
1. General requirements for equipment transformation
■Modification scope
All the existing electric control cabinets are removed and a new control cabinet is adopted;
Link operation console replacement;
There is a cable replacement;
Replacement of power distribution panel;
On-site lighting, driving warnings, and getting on and off communication loops, etc. remain unchanged;
All on-site detection switches such as the left and right limits of the cart, the front and rear limits of the cart, and the limits of the main and auxiliary lifting mechanisms are all reserved.
■Modification requirements
Try to keep the existing operation method to facilitate the operation of the operator;
After the transformation of each mechanism, the speed is still four gears, and the speed of each gear can be adjusted steplessly according to needs;
Use PLC and industrial control field bus to reduce wiring and failure rate;
Productivity must not be reduced;
The main and auxiliary lifting mechanisms have no hook slipping phenomenon;
The system should have the ability to resist vibration and grid voltage fluctuations;
Perfect protection function;
Detailed working status and fault signal display, which is convenient for operators to monitor, control and maintain by maintenance personnel;
Use the equipment overhaul time to complete the dismantling of the original equipment and the installation and commissioning of the new equipment.
2. Transformation plan
Refer to the following "Main Circuit Scheme Diagram" and "Control Scheme Diagram".
3.1 Main circuit
All use the ACS600 series frequency converter produced by ABB Company (the hoisting mechanism adopts ACC600 special frequency converter for lifting).
ABB frequency converter adopts direct torque control (DTC) method. This control method uses real-time calculated motor torque and speed to form a torque-speed double closed-loop system with high dynamic and static accuracy (control accuracy is better than plus or minus 5%); in terms of torque response, it is more variable-frequency than non-DTC control methods. The device is much faster (torque response time ≤5ms), and overcomes the problem of insufficient output torque in the low-speed section of the traditional speed control system and torque open-loop inverter, and its zero-speed torque can reach 200%. These features fully guarantee the requirements of potential load.
In addition to all the features of the ACS600 series inverter, the ACC600 special frequency converter for lifting has built-in special software for lifting (correspondingly equipped with various interface options, such as local control, bus control, and terminal control. The terminal control is divided into electric potentiometer control. , Radio control, handle control and other control modes), it also has a variety of special functions: pre-excitation, torque verification, power
Special functions such as optimization and gate control logic simplify the peripheral structure of the system, make the entire system simple and clear, and greatly improve safety and reliability.
For cranes with multiple motors driving the lifting mechanism at the same time, multiple frequency converters can be used to implement the built-in master-slave lifting function.
If a general-purpose inverter is used, the above-mentioned special functions must be completed by the control system, which complicates the entire system and reduces stability and reliability.
The specific configuration is as follows:
■The traveling mechanism of the trolley adopts two frequency converters to drive an electric motor respectively. One of them is the master and the other is the slave. The master-slave application software is used to ensure the speed synchronization and load balance of the two motors, and prevent the girder from twisting and rail gnawing.
■The frequency converter of the trolley mechanism adopts one frequency converter.
■The main hoisting mechanism and the auxiliary hoisting mechanism adopt a special hoisting frequency converter (ACC frequency converter) respectively.
Each photoelectric encoder is used for speed feedback and control detection.
■All frequency converters adopt chopper and braking resistor to realize braking function.
■All inverters are equipped with various standard field bus interfaces.
3.2 Control system
Use PLC as the core control device and industrial control field bus to connect PLC, remote I/O, frequency converters of various institutions, and touch screen to reduce cable investment, reduce wiring and maintenance workload, and reduce failure points.
■Remote I/O and touch screen are installed in the driver's cab. Accept instructions from operators and display information about various operating conditions of the system to facilitate operator monitoring, operation and maintenance personnel for fault diagnosis and repair.
■The PLC is installed in the control cabinet. Receive status information of on-site detection devices, inverters and other status information, and control system operation.
■PLC control cabinet, frequency conversion control cabinet, and resistance braking cabinet are placed on the beam.
4.1 Environmental conditions
The working environment of cranes is generally poor: conductive dust, water splash, corrosive gas, high ambient temperature, and high altitude in some cases. Whether it is a PLC or a frequency converter, it is greatly affected by the ambient temperature and altitude. Therefore, it must be used in strict accordance with the relevant conditions of use, and the protection level of the cabinet and the ventilation and heat dissipation methods must be considered.
4.2 Rotary encoder
It is recommended that the lifting mechanism use a rotary encoder. One is for speed feedback; the other is for detection and control.
Many users think that the speed regulation accuracy of the frequency converter is high, because the crane has low requirements on the speed accuracy, in order to save costs and reduce the maintenance workload, they often do not use rotary encoders. This is a misunderstanding in understanding. In fact, the encoder is used to improve the speed and torque calculation accuracy of the motor model at low speeds, to ensure torque verification, opening and closing and other functions, thereby ensuring system safety and preventing hook slipping.
4.3 Anti-interference measures
The use of the frequency converter will inevitably bring some interference. Therefore, some reasonable anti-interference measures must be taken, such as the installation of PLC and inverter, and cable routing, and the EMC compatibility of the system should be fully considered.
According to the operation status of the equipment after the transformation and the feedback information from users, it has the following characteristics:
5.1 The system starts and brakes quickly (compared to the inverter in non-DTC control mode) and the starting process is smooth (which can be clearly felt).
5.2 The failure rate is greatly reduced, and the maintenance workload is greatly reduced. User feedback: For many years, the operation is stable and reliable; the operation is simple and flexible; the productivity is greatly improved than before.
5.3 The safety is obviously improved. Once the maintenance personnel mistakenly shorted the limit switch of the hoisting mechanism, and the operator's operation error caused the hoisting mechanism to overwind, and the system produced brake protection to avoid accidents.
5.4 The system is easy to maintain. Because it provides complete and detailed system operation status information and fault information, fault diagnosis becomes very easy and maintenance work is simple and quick.
5.5 There has been no slipping hook phenomenon.
The frequency converter speed control system used in the crane industry not only meets the crane process requirements, but also overcomes the problems existing in the traditional speed control system. At the same time, many protection functions can be set through the inverter parameters to make the system protection more complete.
The transformation of the frequency conversion speed regulation system requires a relatively large one-time cost investment. But from the perspective of energy saving, productivity improvement, cost reduction, safety improvement, etc., that is to say, it is worth considering factors such as performance, price, and economy.
In order to reduce the one-time investment, according to our years of experience, the following measures can be taken:
1. Non-potential load mechanisms such as large carts and small carts can use one inverter to drive two motors;
2. The selection of the frequency converter should be appropriate;
3. You can consider using the original winding motor or only replacing the motor of the hoisting mechanism with a variable frequency motor;
4. PLC and field bus can be omitted.
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