Abstract: In the past, several speed control methods for AC asynchronous motors had a small speed range, were stepped speed control, and had low torque at low speeds, which directly affected actual production work. With the progress of society and the development of power electronics technology, frequency conversion technology is widely used in the field of electric drive. Frequency conversion speed control has been implemented in some energy-intensive equipment, achieving significant energy-saving effects. Furthermore, the use of frequency conversion speed control in cranes, replacing the traditional cam controller variable resistance speed control method, fully utilizes its superior performance. Keywords: bridge crane; frequency converter; cam controller; slip [b][align=center]Reconstruction and application of converter for bridge crane Li Zhi-wei (Foshan Mechanical School. Foshan 52800o, China)[/align][/b] Meanwhile, adopting frequency control instead of the resistance control of traditional cam controllers effectively demonstrates its superiority. Key words : bridge crane; conveyor; cam controller; slip ratio 1. Load Characteristics The traditional QD type bridge double girder crane's electrical drive system uses a cam controller for variable resistance speed regulation of the large and small traveling trolleys. This speed regulation method increases or decreases the resistance of the rotor resistor connected to the wound-rotor motor by opening and closing the internal contacts of the cam controller, thus changing the motor speed. In reality, it regulates speed by changing the motor's slip ratio, as shown in Figure 1. [b][align=center]For more details, please click: Application of Frequency Converter Retrofitting in Bridge Cranes[/align][/b]