With the increasing requirements on speed control performance of tower crane, the traditional speed control methods which have large on-site maintaining requirements, high system fault rates and poor comprehensive technical indicators can not satisfy the special requirements in industrial productions.
This article introduces several main speed control methods in our domestic tower crane mechanism, and points out the advantages and disadvantages of various methods and also the application scopes. For the AC variable frequency speed control method raised for small and medium-sized tower crane with 4 ropes and 6T max hoisting loads, VEICHI AC80 provides complete application solutions.
The tower crane is developed for template technology in the construction, suitable for loads hoisting and components lifting in industrial reinforced concrete structures and civil construction materials.
Ⅱ. Main speed control modes in domestic lifting mechanism
The lifting mechanism is the most important transmission mechanism in a tower crane. At present, new requirements such as slow speed at heavy load, high speed at light load and large speed control scope are added to the traditional speed control modes. The speed control mode of the lifting mechanism will directly affect the overall performance.
There are three principles for selecting speed control mode: the first is smooth operation and small impact; the second is economical efficiency and reliability; the last is easy maintenance.
1. Pole changing speed control for multi speed motor
Due to the fierce competition in small and medium sized tower crane with 4 ropes and max 6T lifting loads, its cost is strictly controlled. The domestic multi speed motors mainly adopt pole changing speed control mode which is with simple scheme and wide application.
2. Ordinary reducer with multi speed winding rotor motor controlled by eddy current brake
Ordinary single speed ratio reducer driven by multi speed motor has overcome the large impact caused by multi speed motor. The eddy current winding rotor motor can get soft M-n characteristic which facilitates smooth brake and gear shift, slow location speed and large power than squirrel cage motor. This speed control mode has advantages of simple structure, easy maintenance and high reliability and has successfully solved motor cooling problem in eddy current braking rotor motor, greatly improving the reliability of this speed control mode.
At present, domestic lifting mechanisms between 8~12T are normally used this speed control mode, while the motor brake and gear shift still has large peak current and impact. As the electrical control system is very complex, lifting mechanisms more than 16T are not suggested to adopt this speed regulation mode.
3. Variable frequency speed control mode is the leading AC speed control mode. With the gradual price decline in international VFD markets, the variable speed control technology is widely used. The lifting mechanism has been used for years with good effects, but it has limited applications. Its advantage is that location speed can facilitate long time operation and achieve zero brake which will ensure smooth operation without impact, extend service life of components and improve the safety of tower crane.
Ⅲ. Solutions of VEICHI AC80 on tower crane
In early days, Japanese Yaskawa inverter is widely used on middle-large size tower crane, due to the fierce competition and strict price control, frequency inverter is rarely used in small and medium-sized type. With the development of power electronic technology, the domestic inverter technology becomes more mature and wins more price advantages, and then it has been gradually used in small and medium-sized tower crane.
1. Brief introduction of VEICHI frequency inverter
AC80 is the newly developed frequency inverter by VEICHI Electric. Compared with the traditional V/F frequency inverter, it can make modular combinations of each series to create a customized platform on basis of the reasonable subdivision of customer requirements.
This series inverter adopts excellent no speed vector control strategy which has fast dynamic response, advanced current limit technology and optimized hardware design, and can ensure no trip even in frequent load fluctuation. It has follow characteristics:
a. Complete reliable designs like redundancy design and derating design;
b. All components meet industrial or military standards;
c. Professional production platform.
B. Soft start
Compared with the power frequency start mode, VFD control can reduce the shock of mechanical part and the impact of framework, which can save maintenance cost and facilitate stable operation.
C. High starting torque
Adopt high-performance flux vector control mode to facilitate stable high output torque at low frequency.
2. Technology introduction
The typical mode of small and medium-sized tower crane is the widely used QTZ315. Two times multiplying power indicators are as listed: Max 3T lifting loads; lifting speed 52/26/5.5m/min; 4/8/32 multipolar motors for 15/15/5.5KW lifting mechanisms; speed ratio of reducer I=31.5; 300mm roller diameter.
3. Parameter calculation
The frequency speed control system of tower crane is given by the main controller or potentiometer. Through the cooperation of electronic speed control device, load controlling instrument, limit switch and brake, the system is used to control the start, brake, reversible operation and speed control of AC asynchronous motor.
Mode selection parameter: two times multiplying power of moving pulley, 18.5KW 4 pole asynchronous squirrel cage motor, speed rate of reducer I=31.5, roller diameter 300mm, that’s I=31.5, R=D/2=150mm=0.15m, Nn=1450 rpm, Pn=18.5kW
A. Calculation of the heaviest lifting load
Tn= Pn*1000*60/2*π* Nn=18.5*1000*60/2π*1450=121.9Nm
theoretical value of lifting load: F=2*Tn*I*/R*10000=2*121.9*31.5/0.15*10000=5.1198T
take 0.64 as the transmission efficiency, the heaviest lifting load is: Fmax=0.64*F=0.64*5.1198=3.28T which has 1.1 times safety factor with the heaviest lifting load, basically meeting the requirement.
B. Calculation of the largest lifting speed.
According to the above formula, the largest lifting speed has greatly reduced under variable frequency mode. Considering to the cost and competition factor, the related cost estimated are optimized. By shortening the starting and location time, and operating at more than 50Hz without hook, it makes it less noticeable about the reduced speed in practice.
4. Performance features
Using the leading AC frequency speed control technology to make transformation on the tower crane electric traction system can achieve stable operation, increase operating efficiency, improve overload performance, eliminate brake impact, reduce electricity consumption and increase power factor. Besides, it has protective functions for over-current, over-voltage, under voltage, input out-phase, overheat, overload, over-speed, I/O fault and motor failure. The variable frequency speed control mode has below outstanding features:
A. Wide speed control scope can achieve accurate control of positioning requirements.
B. The soft start function can reduce the impact on mechanical transmission and significantly improve the bearing capacity of steel structure and extend its service life.
C. High integration components and high reliability low-voltage electrical apparatus can effectively solve the complex wiring problems and reduce system fault rate.
D. The motor can fully output at zero speed, even if the brake is loose or fails, the heavy load won’t slide down, ensuring a safe and reliable system.
B. Fast dynamic response, no hook and real zero speed crossover function.
F. The dedicated load controlling instrument which are set with relevant softwares can realize automatic speed change according to loads change, achieving high speed at light load and slow speed at heavy load.
G. The used frequency inverters are designed with automatic energy-saving mode which can greatly increase the system power factor and the overall working efficiency, and the average power saving rate is up to 20%.
Up to now, that tower crane with no speed variable vector speed control system is under trialed for six months, the whole machine is running well. We have received several new purchasing orders.
At present, most tower cranes adopt step speed regulation, though stepless speed regulation is the future trend, it’s still restricted by factors like cost. The above mentioned methods which need no increase in inverter power and properly sacrifice system speed has no big difference with adopting eddy current winding rotor motor for pole changing speed control, but it can greatly improve the system performance, being of great significance.