Abstract : This paper introduces the control principle of the bridge crane before modification, analyzes the reasons for its insurmountable asynchrony, and successfully modifies the control loop using a frequency converter and encoder, achieving closed-loop speed control and solving the problem of asynchronous lifting of the two chute sections by the bridge crane. Keywords : Bridge crane, asynchronous operation, PLC, frequency converter, encoder 1. Control Principle Before Modification In hydropower plants, bridge cranes are specially installed to meet the requirements of installing and maintaining generator sets in the main plant, as well as lifting generator rotors, turbines, transformers, and turbine runners for aerial turning. Our company's bridge crane is a product of Taiyuan Heavy Machinery Plant, model 2N100T/20T, with a main hook lifting height of 16m. The speed control system of the lifting mechanism was newly developed by the plant in the 1970s, adopting the self-excited magnetic power braking control principle. The main working principle is shown in Figure 1. When in the upshift gear, ZC and XC are connected, and the three-phase power supply is connected. The speed is adjusted by changing the external resistance of the rotor. In the first gear, the entire resistance is connected; in the second gear, 1JSC is activated; in the third gear, 3JSC is activated; and in the fourth gear, 5JSC is activated and then delayed to activate 7JSC. Different resistances are connected in sequence to achieve the speed adjustment of upshifting to four gears. When in the first, second, or third gear, self-excited magnetic braking is used for control. The three-phase AC power supply to the motor is disconnected, and the DEC (Diverter-Controlled Controller) applies a given initial excitation current (approximately 3-5% of the stator rated current) to the two-phase stator windings of the motor. Under the action of the potential energy load, the rotor of the motor rotates, and its windings cut the magnetic field established by the initial excitation current, generating an induced current. After being rectified by a two-phase bridge rectifier, the induced current is sent to the stator windings of the motor, increasing the excitation current. The braking torque generated by the interaction between the rotor current and the magnetic field also increases accordingly. When the braking torque is equal to the torque exerted on the motor shaft by the potential energy load, the self-excited current sent by the rotor current to the stator windings and the motor speed remain unchanged, and the motor operates stably under self-excited braking. When in different gears, by applying 2JSc, 3JSC, and 4JSC respectively to change the input resistance value of the motor rotor circuit, the rotor current, self-excited current, and braking torque will change, thus causing a change in the motor speed. When the motor reaches another speed, its rotor current, self-excited current, and braking torque will stabilize again, ultimately achieving the purpose of speed regulation during the descent of the load. The fourth gear is the regenerative braking gear; DTX is disconnected, FC and XC are connected, and the three-phase power supply is connected in reverse phase, causing the load to descend at high speed. [b][align=center]For details, please click: Analysis of the Causes of Asynchrony in Bridge Cranes and Introduction to Technical Transformation[/align][/b]