Abstract : Variable frequency speed regulation, as a high-performance AC speed regulation device, has been widely used in various cranes. Variable frequency speed regulation has perfect mechanical characteristics, and its good starting and braking performance realizes the rapid and accurate positioning of the crane hook, thereby greatly improving the work efficiency. Keywords: Variable frequency speed regulation; crane; operation mode I. Basic principle of variable frequency speed regulation Generally, the speed regulation methods of three-phase asynchronous motors are: (1) changing the number of magnetic pole pairs P to change the motor speed. The obtained speed can only be 3000, 1500, or 10000, which is stepped speed regulation; (2) changing the slip rate s to regulate speed. The commonly used methods are changing the stator voltage and the slip motor speed regulation. This method has large rotor losses and low efficiency; (3) changing the stator power supply frequency f. Its speed regulation belongs to changing the synchronous speed n. Since there is no artificial change of s, no additional slip power loss is generated in the rotor, so the efficiency is high. It is a relatively ideal speed regulation method. However, changing the stator power supply frequency f to regulate speed, i.e., variable frequency speed regulation, requires a more complex control circuit. The synchronous speed of a three-phase asynchronous motor is: n = 60f/p (1-s) where: P—number of pole pairs; f—stator current frequency, i.e., the frequency of the power supply, f = 50Hz; s—slip rate. Variable frequency speed control can achieve energy saving on the one hand, and better maintain the mechanical characteristics and load capacity on the other. The drive systems of all parts of a crane need speed regulation. The mechanical characteristics of a variable frequency motor are: (i) When a heavy object is lifted, its rotation direction is the same as the direction of the torque generated by the armature current, i.e., the motor is subjected to positive torque, and its mechanical characteristics are in the first quadrant, as shown by curve ① in Figure 1, with the operating point at A and the speed at n[sub]1[/sub]. [align=center] Figure 1: Working state when lifting a heavy object[/align] When decelerating by reducing the frequency. At the instant the frequency drops, the mechanical characteristic curve switches to curve ②. The operating point jumps from A to A', entering the second quadrant. Its torque becomes a reverse braking torque, causing the speed to drop briefly, and it re-enters the first quadrant. At point B, it is in a stable operating state again. Point B is the new operating point after the frequency decreases. At this time, the speed has dropped to n2. (II) When the empty hook (including light load) is lowered, the hook itself cannot lower; it must be achieved by the motor running in reverse. At this time, the torque and speed of the motor are both negative, so the mechanical characteristic curve is in the third quadrant, as shown by curve ③ in Figure 2, the operating point is point C, and the speed is n3. [align=center] Figure 2 Working state when empty hook or light load[/align] When decelerating by reducing the frequency, at the instant the frequency drops, the mechanical characteristics have switched to curve ④, and the working point jumps from point C to point C', entering the fourth quadrant. Its torque becomes positive to prevent the hook from descending, so it is also a braking torque, which slows down the descent speed and re-enters the third quadrant. When it reaches point D, it is in a stable operating state again. Point D is the new working point after the frequency is reduced. At this time, the speed is n4. (III) When descending under heavy load, the heavy object will descend due to its own weight. The rotation direction of the motor is reversed (descending), but the direction of its torque is opposite to the direction of rotation and is positive. Its mechanical characteristics are shown as curve ⑤ in Figure 3. The working point is point E, and the speed is n5. At this time, the function of the motor is to prevent the heavy object from continuously accelerating due to the acceleration of gravity, so as to achieve the purpose of making the heavy object descend at a uniform speed. In this situation, frictional torque will hinder the descent of the load, so the load torque generated during descent is smaller than that during ascent. [align=center]Figure 3 Working state during heavy load descent[/align] II. Frequency Converter and its operation Crane operation is characterized by large inertia and four quadrants, placing more stringent safety and performance requirements on frequency converters compared to other transmission machinery. Generally, the average starting torque of a motor is 1.3-1.6 times its rated torque. Considering factors such as power supply voltage fluctuations and the requirement to pass a dynamic load test at 110% of the rated load, its maximum torque should be 1.8-2.0 times the load torque to ensure safe use. Typically, for ordinary squirrel-cage motors, an equivalent frequency converter can only provide less than 150% of the overload torque value. Therefore, a 200% load torque value can be obtained by increasing the frequency converter capacity or simultaneously increasing the capacity of both the frequency converter and the motor. Since the output waveform of the frequency converter is not a perfect sine wave but contains high-order harmonic components, its current should be increased. There are many types of frequency converters. When configuring them on cranes, it is important to consider their characteristics and the type of crane to ensure a proper match. Currently, there are many types of dedicated frequency converters on the market, such as the Yaskawa VS-616G5 series, the Japanese Fuji G9S series, and the Mitsubishi FRA0241S/A044. Among them, the Yaskawa VS-616G5 frequency converter is a multi-functional fully digital frequency converter with the following characteristics: 1) Full-range flux vector control, providing 150% of the rated torque even at a low frequency of 1Hz without speed feedback; 2) Can be equipped with a braking unit to achieve four-quadrant operation with good dynamic response; 3) Has constant torque characteristics across the entire speed range. The operation methods of mainstream frequency converters can be divided into three types: manual operation, external terminal operation, and communication control operation. (I) Manual operation. As a standard configuration, the inverter is equipped with a dedicated handheld device, which can be used for local reading and writing of inverter parameters and local reading of inverter operating status, as well as simple local stand-alone operation. However, it cannot achieve remote operation and multi-machine linkage, nor can it achieve remote reading and writing of inverter parameters and reading of inverter operating status. (II) External terminal operation. The external terminals of the inverter are connected to external hard-wired logic circuits, which can realize remote operation control of a single inverter or multiple inverters, and can read a limited number of inverter operating statuses. However, it cannot realize remote reading and writing of inverter parameters or reading of all inverter operating statuses. This control method can meet the requirements of crane operation in most cases. (III) Communication control operation. By establishing a network connection between the PLC and the inverter communication interface, it is possible to realize remote operation control of a single inverter or multiple inverters and reading and writing of inverter parameters. At the same time, it is possible to remotely read all operating statuses of a single inverter or multiple inverters. This is a relatively ideal operation method. III. Issues to be aware of when using variable frequency speed control (I) The starting torque is large when lifting heavy objects, usually more than 150% of the rated torque. Considering the possible voltage drop and short-term overload in actual operation, it should generally be selected according to 150%-180% of the rated torque. (II) During the operation of the crane, the load torque changes drastically at the moment the heavy object leaves its original position and rises, and at the moment the heavy object reaches its new position and descends. This should be noted. (III) When adjusting the slack of the crane cable and when performing positioning control, inching operation is required. The working characteristics during inching should be fully considered. (IV) When the heavy object starts to lift or stop, the actions of the brake and the motor must be closely coordinated. The brake takes a certain amount of time (about 6 seconds) to go from tightening to loosening and from loosening to tightening, while the generation or disappearance of the motor torque is reflected immediately at the moment of power-on or power-off. Therefore, problems are very likely to occur in the coordination of their actions. If the motor is energized but the brake is not released, it will cause severe overload of the motor; conversely, if the motor is de-energized but the brake is not engaged, the load will inevitably slide down, resulting in hook slippage. The VS-6165G5 frequency converter has a zero-speed full-torque function, which can effectively prevent hook slippage. Its principle is that when the frequency converter is at zero speed, it maintains sufficient torque in the motor without requiring speed feedback. This ensures that when the hook decelerates from lifting to zero speed, the motor can stop the load in mid-air until the electromagnetic brake engages the shaft, thus preventing hook slippage. Variable frequency speed control completely avoids the shortcomings of wound-rotor asynchronous motors, which cannot accurately control the starting and braking speeds, greatly improving reliability. With the rapid development of electronic components, variable frequency speed control technology will inevitably be more widely used in lifting machinery. At the same time, variable frequency speed control will also experience greater development. References : 1. Wang Jin. Introduction to Construction Machinery. Beijing: People's Communications Press. 2002. 2. Li Fahai, Wang Yan. Fundamentals of Electric Machines and Drives (Second Edition). Beijing: Tsinghua University Press. 2001. 3. Chen Hongli. Modern Speed ​​Control System for AC Motors. Inner Mongolia: Inner Mongolia University Press. 1992. 4. Zheng Di, Tang Kehong. Fundamentals of Mechatronics Design. Beijing: Machinery Industry Press. 2002. 5. Dalian Crane Factory. Crane Design Manual [S]. Liaoning: Liaoning People's Publishing House. 1979. 6. Man Yongkui. General-Purpose Frequency Converters and Their Applications [M]. Beijing: Machinery Industry Press. 1995.