Tower crane is a kind of hoisting machinery that is widely used on construction sites. The use of frequency converters to control the three-phase asynchronous motor instead of the traditional speed regulation method can fundamentally solve the problem of high crown crane failure rate, and With advanced technology and remarkable energy saving, it is an ideal transmission control device for tower cranes.
I. Overview
Tower crane is a kind of hoisting machinery which is widely used in construction site. The traditional control method mostly adopts the method of AC winding motor string resistance to start and adjust the speed. Due to long-term heavy-load operation, frequent forward and reverse rotation, and large impact current, coupled with the poor working environment on some occasions, the motor's slip ring, carbon brush and contactor are often damaged, the contactor's contact burns out, and the carbon brush catches fire. The motor and resistor burnouts occur from time to time, the circuit is complicated, and the bearings of the slewing mechanism often need to be replaced. Since the main winch mechanism is mostly operated by two motors on one shaft, the high-speed motor is in working state, and the low-speed motor is in braking state. It is always in a contradiction between a work and a brake. When switching from fast to slow speed, the mechanical impact is large, and the teething phenomenon often occurs. Once the goods have been positioned, the workers have held the cable, and then switch from fast to slow speed. At this time, the phenomenon of tooth beating occurs, and the slow motor brake fails, which is very dangerous. Occasionally, the phenomenon of shaft folding may occur. The failure rate is high, which has a great impact on production, and the amount of repairs and maintenance costs are also very high. Because asynchronous motors have unparalleled advantages: simple and rugged structure, cheap price and easy maintenance, the use of inverter to control the three-phase asynchronous motor instead of the traditional speed regulation method can fundamentally solve the problem of high crown crane failure rate. Problems, and advanced technology, significant energy saving, is the ideal drive control device for tower cranes.
2. Working principle
1. Cart operating mechanism: used to drag the entire crane to do "horizontal" movement along the construction site, consisting of electric motor, brake, deceleration device and wheels.
2. Trolley running mechanism: It is used to drag hooks and heavy objects to make "longitudinal" movement along the bridge, and is also composed of electric motor, brake, deceleration device and wheels.
3. Lifting mechanism: It is used to drag heavy objects to make up or down lifting movements, and is composed of motor, speed reduction device, drum and brake. Large cranes (over 10 tons) have two lifting mechanisms: "main hook" and "auxiliary hook", usually "main hook" and "auxiliary hook" cannot lift cargo at the same time.
4. Slewing mechanism: Drag the bridge arm to rotate around the axis. Because it prevents deviation, it is usually composed of dual motors, deceleration mechanism and brake.
3. System configuration
In the QTZ crane, the lifting capacity is 8 tons, the lifting mechanism uses Y200L-4 asynchronous motor, power 30KW, speed 1500rpm; the trolley uses Y112M-4 asynchronous motor, power 4KW, speed 1500rpm; the slewing mechanism uses Y112M-4 asynchronous Motor, power 4KW, speed 1500rpm, in the control system, the main function of the PLC is to accept the instructions of the console, and detect the signals of various sensors, and execute the preset program steps; the main function of the inverter is to accept the PLC instructions, according to the program Command to adjust the speed of the motor.
1: Main technical principles
(1) Adopt all digital speed sensor closed-loop vector control to make the system speed regulation range wide and speed regulation precision high. When the inverter is running at low frequency, it also guarantees more than 180% of rated torque output. The maximum torque is twice the rated torque. The basic principle of vector control is to measure and control the stator current vector of the asynchronous motor. According to the field orientation principle, the excitation current and torque current of the asynchronous motor are controlled respectively, so as to achieve asynchronous control. Purpose of motor torque. Specifically, the stator current vector of the asynchronous motor is decomposed into a current component that generates a magnetic field (excitation current) and a current component that generates a torque (torque current), and the amplitude and phase of the two components are controlled at the same time, that is, control Stator current vector, this control method is called vector control method. In this way, a three-phase asynchronous motor can be controlled as a DC motor, so the same static and dynamic performance as the DC speed control system can be obtained.
2: Matters needing attention when using frequency conversion:
2.1 Rotary system:
Main features: mostly adopt separate driving mode (that is, there are two motors to drag), and the speed regulation range is generally within 6:1; transformation scheme: adopt special frequency conversion motor, two motors use two inverters, and adopt 6000s series Alpha advanced master-slave control can be used to make the control accuracy more precise.
The inverter with leading Alpha master/slave control function balances the load of the two traction motors. The 6000s series is a new-generation inverter that uses flux vector control. It can produce 180% Me torque at 0.5Hz. Just input the motor name brand data. The system automatically establishes an accurate motor model and obtains an accurate rotor speed signal. As the feedback signal of the speed loop, it can accurately control the speed and torque of the squirrel cage motor. The 6000s series inverter program has a unique master/slave control. The slave inverter communicates with the master inverter through the input and output ports to track the speed, torque and other fault information of the master inverter in real time, ensuring load balancing and safe operation of the two motors .
2.2 Trolley drag system:
Main features: Only one frequency converter is used. Due to the short stroke, the speed adjustment range is small, and the general range is 4:1. Drag scheme: use a special motor for frequency conversion, and choose the 6000s series as the inverter model.
2.3 Braking method
The braking method uses a combination of DC braking, regenerative braking and mechanical braking:
A. First, through the regenerative braking and DC braking of the frequency conversion speed regulation system, the rotating speed of the slewing or trolley in motion is quickly and accurately reduced to "0", and the operation signal is canceled after the electromagnetic brake holds the shaft;
B. The frequency converter used by the car, because the regenerative braking only occurs during the speed reduction, the regenerative electric energy can be consumed through the braking resistor. However, for the frequency converter used in the slewing mechanism, due to the large braking resistor capacity required, not only the volume is large, but also the heat generated is also very large, it is recommended to always reserve.
C. Anti-slip hook: Before the electromagnetic brake is held and after it is released, it is very easy for heavy objects to fall freely.
The main problems that should be paid attention to to prevent slipping are:
(1) It takes about 0.6 seconds for the electromagnetic clutch from power-on to release (or from power-off to hold) (depending on the model size);
(2) Control logic for lifting and lowering heavy objects.
Four, system advantages
1) Integrated solution: adopting the method of organically combining motor drive and lifting logic control, eliminating the PLC in the original system, reducing potential failure points, simplifying system wiring, simple and flexible debugging, and representing the future development direction of crane control;
2) The wide voltage design can better meet the harsh power environment;
3) Crane professional logic design: Based on years of experience in crane industry design, it has designed expert-level and widely used control and safety logic of hoisting mechanisms, including timing coordination, anti-slip hooks, and other functions, to provide perfect solutions for supporting customers. Program.