To adjust the rotational speed of an asynchronous motor, we can start with changing the following three parameters: the number of pole pairs of the stator winding, the slip ratio of the motor, and the frequency of the power supply. Changing the first parameter affects the synchronous speed. Changing the second parameter only enables speed regulation at the synchronous speed. However, adjusting the motor speed by changing the power supply frequency, that is, variable frequency speed regulation, is a comprehensive speed regulation method covering from constant torque to constant power. As long as a frequency is set, there will be a corresponding rotational speed.
Variable frequency speed regulation can achieve low speed under heavy load and high speed under light load, enabling the motor to work in different constant torque or constant power areas. Its electronic control design is based on the real-time changing requirements of load operation, so it is necessary to comprehensively design and calculate the characteristics of the load and the motor.
(1) Calculation of No-load and Loaded Resistance The hook luffing mechanism of a horizontal jibtower cranegenerally adopts a traction trolley wire rope winding system. When applying frequency converter speed regulation, calculations are made separately according to specific situations. The trolley operation has two cases: no-load and loaded. When it is no-load, the main parameter G (the weight of the spreader, trolley, and rope) is determined during thetower cranedesign, and the resistance it generates is constant. When calculating the no-load resistance, the lifting mass Q is removed, only G and related values are retained, and then the resistance torque generated by the working mechanism itself during operation can be obtained by summation. The load resistance changes with the change of the lifting mass Q. When calculating the load resistance, G is removed, and the lifting mass Q and related values are retained. The resistance at the maximum load is Fjmax. Only by completing the calculations of different loads at different speeds can we determine whether the load is operating in the constant torque or constant power region.
(2) Selection of Operating Speed In traditional design, whether it is no-load or at maximum load, the speed has little relation to the load. When using a frequency converter for speed regulation, the maximum operating speed at maximum load is set as the basic speed. At this time, the motor operates within the constant torque range. According to the performance requirements, the maximum speed that the mechanism can reach is set as the maximum speed. At this time, the motor works within the constant power range. This can expand the speed regulation range, which is conducive to improving work efficiency and makes the variable frequency speed regulation more meaningful.
(3) Selection of Multi-stage Speeds Most of thetower craneluffing mechanisms adopt two-speed, three-speed, or torque motors, with only 2 or 3 speed gears. When applying a frequency converter for speed regulation, choosing an appropriate number of speed gears is a topic worthy of exploration. Currently, the vast majority of designs still have 3 gears, which is clearly not ideal. The choice of the number of speed gears should be determined according to the performance of the selected frequency converter and the requirements of the luffing mechanism. Generally, the frequency converter itself has multi-stage speed settings, ranging from 8 steps to 16 steps. The size of the load dragged by the luffing mechanism determines its requirements for multi-stage speeds: for the luffing mechanism with a lifting mass ≤ 8t, choosing 5 – 8 segments is acceptable; for the luffing mechanism with a lifting mass of 10 – 16t, choosing 8 – 10 segments is acceptable; for the luffing mechanism with a lifting mass ≥ 16t, more speed segments can be selected.
(4) Distribution of Multi-stage Speeds The constant torque output of the motor must meet the requirements of the maximum lifting mass, which is the most basic design concept. When the lifting mass becomes smaller or it is no-load, in order to improve work efficiency, it is necessary to increase the operating speed and achieve constant power output. This is one of the fundamental reasons for applying variable frequency speed regulation. If the multi-stage speed is selected as 8 segments, according to the operation requirements, there must be a starting gear (or in-position gear) and several transition gears in the middle. In the constant torque output range of the motor, setting 5 segments is more appropriate; in the constant power output range, setting 3 speed gears can basically meet the operation requirements.
(5) Determination of the Maximum Speed When applying a frequency converter for speed regulation, the selection of the maximum speed is in the constant power area, which is directly related to the determined values of each speed segment. In the design, the speeds of the following segments can be determined only after determining the maximum speed. The maximum speed mainly depends on the length of the tower crane boom, the maximum lifting mass, and the mechanical precision. In terms of efficiency, when the boom is long, the maximum speed value can be larger, and vice versa. In terms of stable operation, when the lifting mass is large, the maximum speed value should be smaller.
Tower cranes play a crucial role in construction projects, and the frequency conversion speed regulation system significantly impacts their performance. Let’s delve deeper into each aspect to fully understand the design intricacies.
Regarding the basic principle of speed regulation, these three parameters for adjusting the asynchronous motor speed are the foundation. The synchronous speed alteration via the number of pole pairs has its limitations, and the slip ratio adjustment also has constraints. Variable frequency speed regulation, on the other hand, offers more flexibility, enabling seamless transitions between different torque and power requirements. This flexibility is vital because tower cranes often encounter a wide variety of loads, from light building materials to extremely heavy precast components.
The calculation of no-load and load resistance is not a simple task. The traction trolley wire rope winding system in the hook luffing mechanism has multiple components contributing to resistance. In the no-load situation, accurately isolating the fixed resistance factors like G is essential for precisely modeling the base resistance. It gives engineers a starting point to understand how the system behaves even without an external load. When a load is involved, the variable nature of Q completely changes the resistance profile. Different loads will cause different levels of stress on the wire ropes, pulleys, and other moving parts. Precise calculations here ensure that the speed regulation system can adapt to any load condition, preventing issues like overheating of the motor due to excessive resistance or instability caused by insufficient torque.
The selection of operating speed revolutionizes the traditional approach. In the past, a one-size-fits-all speed setting regardless of load led to inefficiencies. With the new method using a frequency converter, setting the basic speed for maximum load within the constant torque range ensures that the crane can handle heavy loads steadily. Then, designating the maximum speed within the constant power range allows for quicker movement when the load is light. This two-tiered approach optimizes the crane’s operation time, reducing the time spent on each lift and maximizing the overall productivity during a work shift.
The concept of multi-stage speeds reflects the growing complexity of modern construction demands. Older two-speed or three-speed motors are no longer sufficient. The availability of frequency converters with multiple speed settings opens up new possibilities. Matching these settings to the lifting mass is a meticulous process. A wrong choice could lead to the crane being either too sluggish when it could be faster or too unstable at speeds it’s not designed for. For smaller lifting masses, fewer speed segments might be enough to achieve the required precision and efficiency. However, as the load gets heavier, more speed options are needed to balance between power and control.
Multi-stage speed distribution further fine-tunes the operation. The starting and transition gears need to be carefully arranged. The starting gear provides a gentle initiation of movement, especially important when picking up a load. Transition gears then smoothly shift the motor from constant torque output for heavy loads to constant power output for lighter loads. This seamless transition not only protects the motor from sudden jolts but also ensures a more comfortable and precise operation for the crane operator.
Finally, determining the maximum speed is a delicate balance. The length of the boom affects the radius of operation. A longer boom can cover more area, but it also requires more power and stability. So, when the boom is long, a higher maximum speed can potentially increase efficiency, but only if the load is within a manageable range. On the other hand, a large lifting mass demands more caution, and a lower maximum speed ensures that the crane can move the load steadily without risking a tip-over or other safety hazards. All these factors interact, and a comprehensive design approach is essential to create an optimal frequency conversion speed regulation system for tower cranes.
