At present, the main speed regulation mode of hoisting mechanism in China
The hoisting mechanism is an important transmission mechanism of tower crane. At present, the traditional speed regulation mode requires heavy load and low speed, light load high speed and wide speed range; the advantages and disadvantages of speed regulation mode of lifting mechanism directly affect the performance of the whole machine. There are three principles for the selection of speed regulation mode of hoisting mechanism: first, it should be stable with small impact; secondly, it should be economical and reliable; thirdly, it should be convenient for maintenance.
1. Variable pole speed regulation of multi speed motor
Small and medium-sized tower cranes with 4-rope maximum lifting capacity ≤ 6T are in fierce competition and strict cost control. In China, multi speed motor is mainly used for pole changing speed regulation. The scheme is simple and widely used, and 4 / 8 / 32 pole multi speed motor is often used.
2. Multi speed wound rotor motor with common reducer and eddy current brake
Compared with the disadvantages of multi speed motor driving ordinary single speed ratio reducer, the multi speed wound rotor motor with eddy current brake can obtain soft M-N characteristics by series resistance, stable starting and braking and gear switching, slow positioning speed and higher power consumption than squirrel cage motor. This kind of speed regulation mode has the advantages of simple structure, easy maintenance and high reliability. At present, the problem of heat dissipation of eddy current brake wound rotor motor has been successfully solved, and the reliability of this speed regulation mode has been greatly improved.
At present, domestic 8 ~ 12t hoisting mechanism mostly adopts this speed regulation mode, but this kind of motor still has large peak current and impact when starting, braking and shifting, and the electrical control system is relatively complex. The large tonnage lifting mechanism with more than 16t is AC speed regulation mode. With the gradual decline of international frequency converter price, the application of frequency control technology is more and more widely. The lifting mechanism of tower crane has been used for many years in China, and the effect is good, but it is not widely used. It has the advantages of slow positioning speed, long-time operation, zero speed braking, stable operation without impact, and can prolong the service life of structure and transmission parts, which is beneficial to wire rope arrangement and service life, and improves the safety of tower crane.
Scheme of frequency converter in tower crane lifting mechanism
In the early stage, the frequency converter is mainly used in the large tower crane with the maximum cost of No. 4 tower crane. With the development of power electronic technology, domestic inverter technology is increasingly mature, performance and price advantages are obvious, widely used in small and medium tower crane has become a reality.
The series frequency converter adopts excellent speed vector control strategy to make it have fast dynamic response. Advanced current limiting technology and hardware optimization design can ensure that the frequency converter does not trip under the condition of frequent load fluctuation. It has the following characteristics:
A. Reliability
Perfect reliability design scheme: such as redundancy design, derating design, etc. all components adopt industrial or military level; professional production platform ensures the reliability of the whole machine.
B. Soft start
Compared with the power frequency starting mode, the frequency converter control can reduce the vibration of the mechanical part and the impact of the frame, save the maintenance cost for the user, and the operation is stable.
C. High starting torque
High performance flux vector control mode is adopted to provide stable high output torque in low frequency band.
1. Process introduction
Qtz315 tower crane, a typical specification of small and medium-sized tower crane, is widely used; the main technical indicators of 2-fold ratio are: the maximum lifting weight is 3T, the lifting speed is 52 / 26 / 5.5m/min, the lifting mechanism is 15 / 15 / 5.5kW 4 / 8 / 32 multipole motor, the speed ratio of reducer is I = 12.5, and the drum diameter is 300 mm.
2. Parameter calculation
The frequency conversion and speed regulation system of crane is given by the master controller or potentiometer as the input. Through the use of frequency conversion and frequency regulation electric control equipment, load measurement and control instrument, limit switch, brake, etc., it can control the lifting mechanism of the crane and other AC frequency conversion asynchronous motors for starting, braking, reversible operation and speed regulation.
Equipment selection parameters: moving pulley 2 times, hoisting mechanism 18.5kw 4-pole asynchronous squirrel cage motor, reducer speed ratio i = 31.5; drum diameter of 300 mm, I = 31.5, r = D / 2 = 150 mm = 0.15m, NN = 1450 rpm, PN = 18.5kw.
A. Calculation of maximum lifting weight
Pn=2*π*Nn*Tn/60*1000
The results show that: n * 1 000 * n = 1 45N * 2 * n = 1 45N * 2
The theoretical lifting weight is: F = 2 * TN * I * / R * 10000 = 2 * 121.9 * 31.5 / 0.15 * 10000 = 5.1198t
Considering the transmission efficiency of 0.64, the maximum lifting weight is: Fmax = 0.64 * f = 0.64 * 5.1198 = 3.28t
Compared with the required maximum lifting weight, the safety factor is 1.1 times, which basically meets the requirements.
B. Calculation of maximum lifting speed
Vmax=π*D*Nn/2*I=π*0.3*1450/2*31.5=21.7 m/min
According to the above formula, under the frequency conversion mode, the maximum lifting speed of the motor is greatly reduced, mainly considering the cost saving and competitiveness in the market, and the relevant hardware cost is estimated to be optimized in the design; by shortening the start-up and positioning time, and running the empty hook higher than 50 Hz, the speed is not obviously slowed down in practice.
3. Performance characteristics
Using the advanced modern AC frequency conversion speed regulation technology to carry on the technical transformation to the tower crane electric drive system, enables the crane to realize the smooth operation, enhances the operation efficiency, improves the overload operation, eliminates the brake impact, reduces the electrical maintenance, reduces the electric energy consumption, enhances the power deficiency factor and so on. At the same time, it also has over-current, over-voltage, under voltage and input phase loss protection, as well as inverter over temperature, overload, overspeed, brake unit overheating, I / O fault protection, motor fault protection, etc
A. Wide range of speed regulation can realize the operation with accurate control and positioning requirements;
B. The function of soft start and soft stop can reduce the impact of mechanical transmission, significantly improve the bearing capacity of steel structure and prolong the service life of crane;
C. High integration components and high reliability low-voltage electrical appliances can effectively solve the problem of complex wiring of the original electrical system, which not only reduces the probability of system failure, but also is easy to maintain;
D. When the motor is at zero speed, it can output full torque. Even if the brake is loose or out of order, there will be no weight sliding down to ensure the safety and reliability of the system;
E. It has fast dynamic response, does not appear hook slip and truly realizes "zero speed crossing" function;
G. The inverter used in the system has automatic energy-saving operation mode, which can greatly improve the power factor of the system and the working efficiency of the whole machine. The energy-saving effect is remarkable, and the average power saving rate can reach more than 20%.
