Abstract: PLCs and frequency converters are commonly used automatic control devices in the industrial control field, offering stable and reliable performance. Applying frequency converters and PLCs to bridge crane systems can significantly improve the operational performance and reliability of bridge cranes, representing a currently ideal design approach. Keywords : PLC; frequency converter; bridge crane Bridge cranes, commonly known as overhead cranes, are widely used lifting machinery in industrial and mining enterprises. Traditionally, to improve starting torque, bridge cranes use wound-rotor asynchronous motors, with speed regulation achieved by changing the resistance in series with the rotor through the operation of a drum-shaped cam controller. With the development of new technologies and control equipment, frequency converters are now commonly used as variable frequency speed control power supplies, replacing the original wound-rotor asynchronous motors with squirrel-cage asynchronous motors, and PLCs are used as control devices for contactless control. This improves speed regulation performance and increases system reliability. This paper analyzes the specific application of frequency converters and PLCs in a system through a case study. 1. Bridge Crane Drive System 1.1 Bridge Crane Running Mechanism 1) The trolley drive system moves the entire crane left and right along the direction of the workshop (with the driver's seating position as a reference). 2) The trolley drive system moves the hook and load back and forth along the bridge frame. 3) The hook drive system moves the load up and down. Large cranes (over 10t) have two hoisting mechanisms: the main hoisting mechanism (main hook) and the auxiliary hoisting mechanism (auxiliary hook). Usually, the main hook and auxiliary hook cannot lift loads simultaneously. 1.2 Load Characteristics The load of the bridge crane drive system is constant torque, and its hoisting mechanism is a potential energy load. When the hoisting mechanism lowers the load or decelerates rapidly, the motor is in regenerative braking mode. It is necessary to feed electrical energy back to the grid through a feedback device or consume it in the braking resistor to prevent the DC boost voltage from affecting the braking effect. 1.3 Control Requirements 1) The hoisting mechanism requires a large starting torque and smooth starting and running. 1) The lifting mechanism must be able to operate in both forward and reverse directions and have multiple protections such as overload, limit, and current limiting. 2) The lifting mechanism is prone to "hook slippage" during start-up and shutdown. Because the brake takes time to engage and disengage, and vice versa (approximately 0.65 seconds), while the generation or disappearance of motor torque is instantaneous upon power-on or power-off, problems easily arise in the coordination between the brake and motor. If the motor is energized but the brake has not yet disengaged, it will cause severe motor overload; conversely, if the motor is de-energized but the brake has not yet engaged, the load will inevitably slip, resulting in hook slippage. Therefore, appropriate preventative measures are necessary. 3) The lifting mechanism must have a mechanical brake. While the lifting frequency converter has a zero-speed full-torque function (also known as zero-servo function, meaning the motor can still output 150% of its rated torque at zero speed, keeping the load suspended in the air), if a momentary power outage occurs while the load is suspended in the air, there is a risk of the load slipping. Therefore, a brake must be installed on the motor shaft. Commonly used brakes include electromagnetic brakes and hydraulic electromagnetic brakes. 2. System Application of PLC and Frequency Converter 2.1 System Configuration 1) Frequency Converter. The translation mechanism of the bridge crane does not have high requirements for the performance of the drive system. In order to save costs, a general-purpose frequency converter with V/F control mode can meet the requirements. The hoisting mechanism requires high starting torque and speed regulation performance, so a vector control frequency converter with speed feedback is used. There are many types of such frequency converters. This article takes the Yaskawa VS-616G5 frequency converter as an example for analysis. This frequency converter has zero-speed full torque function, which ensures that when the hook drops from the running state to zero speed, the motor can temporarily stop the load in the air until the electromagnetic brake holds the motor shaft, thereby preventing the hook from slipping. 2) PLC. The Mitsubishi FX-N-08MR can be selected. 3) The translation machine uses a common motor; both the main and auxiliary lifting mechanisms use variable frequency motors and are equipped with photoelectric encoders. The frequency converter is connected to the photoelectric encoder via a PG-based speed feedback control card. To ensure sufficient starting and running torque, the frequency converter of the main lifting mechanism is generally one level larger than the motor capacity (if the motor is 90KW, the frequency converter capacity can be 110KW). The braking unit of the frequency converter should be upgraded to allow for a larger braking current and shorten the braking process. The rated power of the braking resistor should be doubled. 2.2 System Diagram This document only provides the system diagram of the main lifting mechanism, as shown in Figure 1: PLC Terminal and Control Relationship Explanation: In the above system, the PLC is the control center. Its input signals come from the master controller (as shown in Figure 2, used to control the forward and reverse rotation of the main hook motor and multi-speed and zero-position protection) and the frequency converter (fault output, brake control signal), as well as overload, limit, and other detection signals. Its output signals control the on/off of the frequency converter and the main circuit (brake, fan, frequency converter power supply circuit). The terminals of the frequency converter are described as follows: 1: Forward rotation; 2: Reverse rotation; 3: External fault (braking resistor overheat protection); 4: Fault reset; 5: Multi-speed 1; 6: Multi-speed 2; 7: Multi-speed 3; 8: Jog; n: Common terminal of multi-function input terminals (5-8); 25: Zero speed; 26: Speed ​​consistency; 27: Open collector multi-function. 2.3 PLC Program The PLC program of this system is a general sequential control program. The key is to clarify the actual control relationship required during the system operation, and to convert the operation and speed command logic of the master controller into three-bit binary outputs to the multi-speed input terminals of the frequency converter (the program and input/output relationship are shown in Figure 3 and Table 1). 2.4 Working Process After the system is powered on, in the absence of fault feedback, the driver in the cab sends operation command signals to the PLC through the master controller of the linkage console. The PLC outputs the forward, reverse, and multi-speed input terminals of the frequency converter according to the execution result of the internal program, thereby controlling the lifting and lowering operation and speed change of the main hook. 2.5 Prevention of Hook Slippage The Vs-616 GS vector control frequency converter has zero-speed full-torque function. When designing the hoisting mechanism of a bridge crane, only the appropriate coordination between the PLC and the frequency converter needs to be considered to perfectly solve the "hook slippage" problem. For example, in this case, the "zero speed" and "speed consistency" signals at the open collector output terminal can be used to control the mechanical brake's engagement and disengagement respectively, or the "frequency detection signal" can be used as the mechanical brake's engagement and disengagement control command. However, parameter settings need to be coordinated to achieve the best coordination between the brake and the motor operation. 2.6 Functional Expansion 1) If needed, a remote control can be connected to the PLC input terminal to control the bridge crane's drive system. 2) A touch screen or LCD display can be configured to monitor the system's main hook height, load, operating status, fault status, etc. The PLC outputs pulses to the PLC's high-frequency pulse input terminal, which are then calculated by the SPD pulse speed calculation instruction for speed control and display. The PLC and touchscreen are connected via an R32 serial interface. The interface program in the PLC sets up a data reading area and related status flags for the touchscreen, used for touchscreen monitoring. 2.7 System Protection In this system, the frequency converter itself has multiple protection and fault output functions such as short circuit, overload, overvoltage, phase loss, and stall. For the main hoisting mechanism, the frequency converter drives a motor, so the frequency converter's output can be directly connected to the motor without needing a thermal relay for overload protection. The main circuit includes a main contactor and branch contactors, which, in addition to switching the circuit, also provide multiple protections such as short circuit, overload, and undervoltage. The operator can control the main contactor and thus the main power supply via the start/stop button on the control panel. If the contactor cannot be used to switch the circuit, the power circuit can be disconnected by using the emergency stop switch to activate the shunt trip coil of the main circuit breaker. Additionally, a door limit switch and a key switch are connected in series in the main power control circuit as safety protection measures. 3 Conclusion With the powerful support of fully functional, stable and reliable PLCs and frequency converters, bridge cranes have greatly improved in terms of reliability, speed regulation performance, energy saving and operating efficiency compared with traditional bridge cranes. The bridge crane system composed of PLCs and frequency converters has become the typical design mode of bridge cranes and is widely used. References: [1] Zhang Yanbin. Application Practice of Variable Frequency Speed ​​Regulation [M]. Beijing: Machinery Industry Press, 2001. [2] Xie Keming, Xia Luyi. Programmable Controller Principles and Programming [M]. Beijing: Electronic Industry Press, 2002.