Application of the hottest PLC and pressure regula

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Application of PLC and pressure regulating cabinet in heavy casting crane

Abstract: Taking the 220t large crown block of steel plant as an example, the application of PLC and pressure regulating cabinet in heavy casting crane is introduced

key words: crown block; PLC; Pressure regulation; Speed regulation

crown block lifting equipment is widely used in the process of steel-making production. It plays an important role in reshipment of materials and connection of various processes. It has many kinds, large load, high temperature, high altitude, three-dimensional, cross and dynamic operations, and completes various complex tasks

at present, the large crown block traditionally designed by the steel plant has complex and scattered control, many lines, and the operation line is easy to be short circuited or grounded. If the fault is found slowly, it is easy to cause shutdown accidents. If it is small, the steel-making production cannot be carried out continuously, if it is large, the whole production line will be paralyzed, and the steel-making losses will be serious. Now the crown block with a new PLC control system overcomes various shortcomings of the traditional crown block. Its high concentration of electrical control, stable operation of PLC and pressure regulating cabinet ensure the normal production of steelmaking and continuous casting

1 electrical control mode of PLC

working principle of PLC: the crown block operator sends out corresponding control instructions by controlling the handle on the linkage platform, and the PLC sends corresponding signals according to the internal programming operation, so as to achieve the purpose of controlling the operation of each component, as shown in Figure 1

in fact, the essence of PLC is to unify some analog devices of traditional relay control system, such as intermediate relay, time relay, normally open and normally closed point, auxiliary point, etc., into PLC, and arrange them organically according to actual requirements. These simulators can be reused indefinitely without increasing the cost, and there is no need to install real devices manually. The more advanced PLC, the more input points and output points, the stronger the logic calculation ability, the faster the program execution speed, and the better the real-time performance

Siemens s is selected as the PLC of this system. There is a main station in the electrical room of the main beam and a sub station in the operation room. All major components of crown block, including main hook, auxiliary hook, main trolley, auxiliary trolley, crane operation, etc., are connected to the input end of PLC through control cables one by one. The output points of PLC all control the intermediate relay, and the normally open contact of the intermediate relay controls the main circuit contactor of each component in the electrical cabinet, so as to control the whole vehicle. This is a centralized and decentralized control system, which meets the operation requirements of crown block

the system has 156 digital input points and 48 digital output points. The communication between PLC and other data interfaces adopts MPI cable connection. MPI cable is the abbreviation of multipoint interface. The physical layer is RS-485, and the maximum transmission rate is 12m bit/s. It has strong anti-interference ability, fast transmission speed and low error rate, and can fully meet the requirements of data transmission. The programming language used in the system is ladder diagram programming language, which is completed by the manufacturer

the crown block with PLC has the following characteristics: he went to Marmara University:

(1) high integration of electrical control, which is convenient for electrical personnel to maintain. According to the information provided by the LED on the PLC or the programming software, the cause of the fault can be quickly found, the fault can be eliminated, and the normal production can be effectively guaranteed

(2) strong function and high cost performance. There are countless programming elements in PLC, including time relay, intermediate relay, etc., which can realize very complex control functions. Compared with the relay system with the same function, they have high cost performance

(3) high reliability and strong anti-interference ability

2 voltage regulation cabinet

2.1 speed regulation

the voltage regulation cabinet adopts meiheng theromat series digital stator voltage regulation device. The voltage regulation and speed regulation system operates in four quadrants, and the device contains three groups of thyristors. The direction conversion of the motor is realized through two external reversing contactors (with mechanical interlocking). The working principle of the voltage regulating cabinet is that the control unit changes the stator voltage of the motor by changing the trigger angle of a group of thyristors connected back-to-back in parallel with each phase of the wound motor

The main hook of the

220t crown block is driven by two 320kw motors working in parallel. It is a large motor with a rated current of about 470A. When starting at full voltage, due to the starting torque of the motor, it is necessary to absorb 7 times the rated current of the motor from the electricity, that is, 3290a. Too much starting current causes serious impact on electricity, affects the work of other crown block, and increases line loss and transformer loss. Due to the large starting current, the excessive starting torque generated by the rotor is also quite destructive to the machinery, causing abnormal wear and impact of mechanical transmission parts, shortening the service life, thereby increasing the maintenance workload and cost, such as damaging the coupling. In view of the above reasons, it is necessary to reduce the starting current ist of the motor

changing the input voltage U1 can change the motor starting current ist. The voltage regulating cabinet is to reduce the motor voltage to adjust the starting current ist. When starting, the output voltage of the thyristor gradually increases, the current also gradually increases, and the motor gradually accelerates until the thyristor is fully connected, and the motor works at the rated voltage. This process ensures the normal starting of the motor

the speed n of the motor plays a very important role in lifting, especially the lifting speed of the main hook. If the lifting speed is too fast, Zetian's lathe worker's reaction will not keep up, and an accident will occur; If the lifting speed is too slow, the waiting time is too long, affecting production. Therefore, the starting speed of the motor should be appropriate in order to control the crown block and serve the production. At the same time, it should also have the same speed under different loads, but once there is damage. Based on the above requirements, the speed of the motor must be adjusted

it can be seen from the motor speed formula that the speed can be adjusted by changing the slip ratio s. 220t uses two methods of winding rotor circuit series resistance and pressure reduction to change the slip rate s and adjust the motor speed n. The higher the series resistance of the wound rotor circuit, the lower the speed; On the contrary, the higher the speed is. If the voltage is low and the torque is small, the speed will be low; On the contrary, the higher the speed is. The purpose of speed regulation can be achieved by adjusting the resistance and motor input voltage U1 connected in series in the wound rotor circuit according to the requirements. Moreover, the series resistance of the wound rotor circuit can make the crown block have better controllability

therefore, the voltage regulating cabinet plays two important roles in this process: making the motor start normally; It can adjust the speed of the motor

2.2 speed negative feedback control system

in the process of speed regulation, 220t crown block has introduced the speed negative feedback voltage regulation and speed regulation control system

The working mode of speed negative feedback is shown in Figure 2. Its working principle is that the decrease of motor speed will reduce the output voltage U of the tachometer generator, while the given voltage u* of the system remains unchanged, and the deviation voltage △ u=u*-u increases. After being amplified by the speed regulator, UK increases, making the trigger circuit α If the angle decreases, the output voltage U of the main circuit composed of thyristors increases, so that the motor operates stably

advantages of speed negative feedback system:

(1) it can overcome large speed changes. When the load torque or power supply voltage of the motor fluctuates, causing a large change in speed, the negative speed feedback will react quickly, adjust the thyristor conduction angle, increase or decrease the input voltage of the motor, and adjust the motor speed

(2) the motor can drive the constant torque load and the speed has a wide adjustment range. If the motor speed is the same under no-load or heavy load, the negative speed feedback will adjust the input voltage to make the motor start with constant torque

speed negative feedback system is to control the motor speed at a preset speed n, so that under the condition of soft start, the motor speed is controlled within an appropriate range of a 6. Full digital closed-loop control and measurement system of steel strand experimental machine, and a good control effect is achieved. After starting, when the measured motor speed reaches the critical value, the system automatically outputs full voltage and enters open-loop control. At this time, the motor works under the rated voltage and rated current. After finishing the general principle,

a relatively precise device encoder is installed on the rotor of the motor, which can measure the motor speed n, rotor position, etc., and provide the necessary data for the speed negative feedback system. The control of the whole system is based on the data measured by the encoder, which is an important part of this system

3 conclusion

from the practice of the Third Steelmaking Plant of Shaogang, it is proved that the control system based on PLC and voltage regulating cabinet is reliable and stable, which reduces the labor intensity of maintenance personnel


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