Tower cranes are developed for construction site. It is used to lifting of building materials and components in industrial and civil construction of reinforced concrete structures.

i The main speed control methods of domestic lifting machinery

The hoisting mechanism is the most important transmission mechanism of the tower crane. Under traditional speed regulation mode, the crane is required to work with heavy load at low speed, light load at high speed, and has wide speed regulating range. The speed regulating performance of the hoist mechanism directly affect the whole crane performance. The speed regulating modes selection has 3 directives. They are stable with small impact, economical and reliable, and easy maintenance.

1) Multi-speed motor switching poles regulation

2) Electromagnetic clutch shifting reducer plus single-speed wound rotor motor with eddy current brake

3) Ordinary reducer plus multi-speed wound rotor motor with eddy current brake

4) Differential planetary reducer plus double motor

5) Variable frequency drives control

Frequency control is the most advanced AC speed control method at present. With the gradual internationally decline in the price of inverters, the application of variable frequency speed control technology is more and more adopted. VFD has been applied to domestic cranes for years with good performance. Its advantage is that it can runs at low speed for a long time, realize zero speed brake, stable no impact running, and extend the structure and driving mechanism service life.

But the cover rate of VFD in crane is not high due to some reasons as below. Firstly, the imported VFD cost very high and there was a time few domestic VFDs can satisfy the technical requirements. Secondly, the VFD normally is not repairable in case of damage in crane application, so new one is needed. Thirdly, previously the lower tonnage hoisting equipment rarely used the imported VFDs due to its high cost. The wide application of VFDs in lower tonnage hoisting cranes waits for the rising of domestic VFDs. 

ii The VFD solution in tower carne hoisting

For a period of time, Yaskawa took the lead positon in applying VFD technology to crane, and its VFD is mainly applied to middle-large tonnage cranes. While the crane whose 4-rope lifting weight lower than 6T competition was too fierce on market and requiring very strict cost control. So VFD cover rate was very low. But with the rapid and great development of China domestic variable frequency drives with extraordinary price advantage, the smaller tonnage cranes using VFDs is becoming a trend and reality.

2.1 Introduction of VFD control techniques for crane

Typical specifications for small and medium-sized tower cranes QTZ315 is widely used; the main technical specification indicators are: maximum lifting weight 3T, lifting speed 52/26/5.5m/min, lifting mechanism 15/15/5.5KW 4/8/32 multi-pole motor, speed reducer speed ratio I=12.5, drum diameter 300mm.

The most critical indicator for small and medium-sized tower cranes is the maximum lifting weight 3T. The diameter of the drum 300mm cannot be changed much, and the speed requirement is relatively high. When using the variable frequency speed control method, it is necessary to make appropriate adjustments to related equipment and technical indicators.

2.2 Parameter calculation

The crane variable frequency speed control system reference is given by the master controller or potentiometer. The frequency and speed regulating device, load measuring instrument, limit switches and brakes work coordinately to control the crane lifting mechanism AC asynchronous motor starting, brake, reversed running and speed regulating.

Equipment selection parameters: movable pulley 2 times, lifting mechanism 18.5KW 4-pole asynchronous squirrel cage motor, speed reducer speed ratio I=31.5; drum diameter 300mm, ie I=31.5, R=D/2=150mm=0.15m, Nn = 1450 rpm, Pn = 18.5 KW.

2.2.1 Calculation of the maximum lifting weight:

Pn=2*π*Nn*Tn/60*1000

So, Tn= Pn*1000*60/2*π* Nn=18.5*1000*60/2π*1450=121.9Nm

The theoretical lifting weight is:     F=2*Tn*I*/R*10000=2*121.9*31.5/0.15*10000=5.1198T

Considering that the transmission efficiency is 0.64, the maximum lifting weight is: Fmax=0.64*F=0.64*5.1198=3.28T

Compared with the required maximum lifting weight, there is a 1.1 times safety factor, which basically meets the requirements.

2.2.2 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, in the frequency regulating mode, the maximum lifting speed of the motor is reduced a lot, mainly considering the cost saving for competitiveness in the market, and so the hardware cost estimation is optimized in design; by coordinate variable methods such as shortening the startup and seating time, make empty hook runs higher than 50HZ, it reach the effect that we do not feel the speed is reduced in actual running.

2.3 Performance characteristics

2.3.1 The wide speed regulating range realizes the precisely controlled positioning task.

2.3.2 The soft start and soft stop functions reduce the mechanical transmission impact, which can significantly improve the bearing performance of the steel structure and prolong the service life of the crane;

2.3.3 Highly integrated components and highly reliable low-voltage electrical appliances can effectively solve the complicated wiring problems of the original electrical system, which not only reduces the probability of system failure, but also easier to maintain;

2.3.4 When the motor is at zero speed, it can output full torque. Even if the brake is loose or malfunctioning, the carriage would not drop, ensuring the system is safe and reliable;

2.3.5 Fast dynamic response avoids the hook slippery

2.3.6 Specific load weight measurement with specialized software enables that the lifting speed can be switched automatically with the load weight change, realizing the lifting requirement of “fast lifting at light load, slow lifting at heavy load”;

2.3.7 The frequency inverter used in the system has an automatic energy-saving operation mode, which can greatly improve the power factor of the system and the working efficiency of the whole machine, and the energy saving efficiency is remarkable which can be more than 20%;

iii Conclusion:

So far, this application of the tower crane with sensorless vector control variable frequency drives has been in test running for months, the whole equipment is running well and more orders are placed by clients.

At present, most of the tower cranes adopt stepped speed regulation. Although step-less speed regulation is the development direction, it is still limited by the higher cost. The researcher has done some calculations. Using the method explained as above, no need to choose higher power rating inverter and make some compromise in lifting speed, the step-less speed regulation by VFD has not much difference in cost from multi-speed winding rotors motors with eddy current braking system. But the performance has been greatly improved.