Now, the construction of tower cranes in China has become more and more common. From ordinary multi-storey buildings, real estate projects, high-rise buildings to large-scale railway projects, bridge projects, power engineering, water conservancy projects, there are tower cranes everywhere. Computer-aided design, microelectronic technology, and program-controlled language control technology have all been applied to tower cranes. Among them, the control of hoisting mechanism, swing mechanism and traction mechanism is carried out by the frequency converter.
Ⅱ. Overview of tower crane equipment
A construction site in Shenzhen adopts the QTZ160 (6518) tower-type crane (commonly known as: tower crane). The machine features: the maximum working range of 65m, and the maximum lifting height of 200m.
(1) Hoisting mechanism
It is used to lift heavy objects for lifting or lowering. It consists of a motor, a deceleration device, a reel and a brake. The motor model of the hoisting mechanism is YZRSW280-4/8 double-speed winding motor. The hoisting mechanism has five speeds to achieve high speed for light loads and low speed for heavy loads.
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(2) Swing mechanism
There are two sets of swing mechanisms, which are symmetrically arranged on both sides of the swing bearing. They are driven by 2 motors, and the pinion gear is driven by the planetary gear reducer, thereby driving the upper boom and balance arm of the tower crane to rotate left and right. Disc brakes are attached to the rear of the motor, and are in a normally open state. The swing brakes are used in windy conditions to position the tower crane during work or jacking in the specified position, and it is strictly forbidden to stop with the brakes. Its supporting motor parameters:
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The characteristic of the swing mechanism is that it has a large inertial impact, and the impact is large when it is started too quickly. It is not allowed to stop and reverse the rotation too quickly or too hastily, otherwise it cannot run smoothly, and also damage the mechanism.
(3) Traction mechanism
The traction mechanism is the driving device for the amplitude change of the load trolley. During the hoisting and amplitude change, the motor drives the reel through the planetary reducer (the other end of the motor is equipped with an electro-magnetic disc brake), and through the wire rope, so the load trolley is driven to do the back and forth movement at a speed of 0 to 55 m/min. There are two traction ropes. One end of the two ropes is fixed at the two ends of the traction reel respectively. After winding, they are drawn out to the front and rear of the cargo boom respectively. The other end of the rope is fixed on the load trolley. When luffing, the two ropes are retracted and released to ensure the normal operation of the trolley. When the trolley runs to the minimum and maximum amplitude, the number of turns of the two wire ropes on the reel shall not be less than 3 turns. Its supporting motor parameters:
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Ⅲ. Analysis of disadvantages of traditional control method of tower crane equipment
The traditional control method mostly adopts the method of AC winding motor in series with resistance to start and adjust the speed, which has the following disadvantages:
(1) Due to long-term heavy-load operation, frequent forward and reverse rotation, and large surge current, it is easy to reduce motor insulation and damage electrical components, resulting in high maintenance costs and device aging.
(2) The failure rate is high, the slip ring, carbon brush and contactor of the motor are often damaged, the contacts of the contactor are burned, the carbon brush is on fire, and the motor and resistance are burned out from time to time, resulting in heavy maintenance workload and high maintenance costs.
(3) The circuit is complicated, and the bearing of the swing mechanism often needs to be replaced.
(4) Low efficiency, the power factor is about 0.81 at full load, and the actual operating power factor is much lower than 0.8.
(5) The safety factor is low. When switching from high to low speed, the mechanical impact is large, and the phenomenon of tooth knocking often occurs, resulting in the brake failure of the slow motor, which is very dangerous. Occasionally there will be the phenomenon of broken shaft, and the failure rate is high, causing big impact on production.
Therefore, the use of the control method of driving the three-phase asynchronous motor with the frequency converter to replace the traditional speed regulation method can fundamentally solve the problem of high failure rate of tower cranes, and it is an ideal transmission control device for tower cranes with advanced technology and significant energy saving.
Ⅳ. The problems encountered in the debugging process and solutions
(1) Hoisting mechanism
When the elevator is going down, the motor will do negative work, which will cause the main circuit voltage of the frequency converter to be very high. Therefore, a braking unit and braking resistor must be added to release the energy generated by the elevator in the process of going down. For the KE300A series frequency converters, the braking unit is built in the machine of below 18.5KW, and only the braking resistor should be added.
Parameter P9-04 is the braking threshold, the factory value is 130%; the standard bus voltage of the frequency converter is 540V,
540V*130% = 702V
That is to say, when the bus voltage of the frequency converter reaches 702V, the brake valve will be opened. If the frequency converter still have the over-voltage (E-05/E-06/E-07) alarms, and the parameter P9-04 can be appropriately adjusted to lower the brake opening threshold.
Since the elevator brakes for a long time during the entire downward process, the braking resistance should be appropriately selected to be larger, otherwise the resistance temperature will be very high, causing the abnormality of the system.
(2) Swing mechanism
(2.1) Occasionally, the rotating arm shakes during startup (41Hz, 1 ton of heavy objects are lifted), mainly because the output torque is not enough during startup. By changing the torque boost parameters, the motor has never shaken or encounter other phenomena.
(2.2) Because the inertial impact is relatively large, the power of the braking resistance calculated according to the 10% braking frequency is too small, resulting in the failure to stop. At this time, the tower crane driver is used to reversing, so that the frequency converter often has the over-voltage fault. It can be used normally by amplifying the power of the braking resistor by 2 times.
Ⅴ. Summarize the advantages and characteristics of the KE300A frequency converter
The KE300A frequency converter of MICNO has achieved very good results such as high efficiency and low failure rate in the field use, and has been unanimously recognized by customers. The advantages of the solution are summarized as follows:
(1) It can be used in conjunction with mechanical brakes and safety circuits. In the retrofit system, it can be compatible with the safety circuits of original system, and at the same time perfectly cooperate with the respective types of mechanical brakes, ensuring the safety of the entire system to the greatest extent.
(2) Wide voltage input can meet the requirements of harsh power supply environment.
(3) The modular design is adopted, and the configuration is very flexible.
(4) The main control system adopts 32-bit high-speed DSP, integrating vector control without PG, V/f control, and torque control.
(5) High starting torque: in the vector control mode without PG, 150% starting torque can be provided at 0.5Hz.
(6) Excellent robust performance can ensure that the load can start and stop quickly.
(7) Fast dynamic response: in the vector control mode without PG, the dynamic response time is less than 20ms.
(8) Fast current limiting function: the current can be quickly limited within the protection point, reducing the probability of alarms for frequent overcurrent failures.
(9) High power factor output, reduced reactive power loss, and no large current impact during soft start.
(10) It has the functions of automatic fault reset and power-off reset to ensure continuous production and production efficiency.
(11) It adopts advanced power module drive mode, with high power factor, which can effectively suppress harmonics and eliminate the interference to the peripheral control loop and sensor when the converter is running.
(12) Perfect protection functions: over-voltage/under-voltage protection, overheat protection of frequency converter, ground fault protection, short circuit protection, over-current protection, overload protection, etc.
The application of MICNO KE300A open-loop vector frequency converter in the tower crane actually solves the control problems of the hoisting, swing and traction mechanism on the tower crane very well. And after adopting the frequency converter control, the system control is simpler, and the control system has multiple protections, making the system more secure and reliable.