Application of PLC in bridge crane

1 Background

The asphalt workshop of the third refining department of Cangzhou Refinery currently has a set of road asphalt equipment with an annual output of 100,000 tons. The workshop is equipped with two bridge cranes for loading. The bridge crane was produced by Zhangjiakou Crane Factory in October 1979 and put into use in our factory in April 1980. After more than 20 years of use, the equipment has become very old. With the increase in production in recent years, the frequency of crane use has increased, and the overhead crane has frequent failures. The electrical maintenance of the asphalt bridge crane (also known as the overhead crane) has always been an important task in our daily maintenance. Often a lot of manpower and material resources are invested, but the normal use of the overhead crane cannot be guaranteed, and a large amount of material costs are consumed every year. In order to solve this problem, we organized technical personnel to carry out QC research in April 2004. After comparison, we decided to use Modicon TSX Neza PLC produced by Schneider to replace the JT3-11/1 time relay in the original circuit. After the transformation, after two years of use, the effect is good.

2 Analysis and modification of control circuit

In the bridge crane circuit, the control circuit of the grab bucket lifting and opening and closing part has the most faults. The main circuit of grab bucket lifting and opening and closing is shown in Figure 1. The control circuit is shown in Figure 2.

Figure 1 Original main circuit

Figure 2 Original control circuit [page]

In Figure 2, KM11, KM33, KM22, and KM44 are the main contactors that control the lifting and opening of the grab bucket, KM1 to KM6 are contactors that cut off the resistor, and KT1 to KT6 are time relays. The function of the time relay is to contact the starting resistor with a graded delay. Due to frequent actions, faults occur frequently. We can see through analysis that:

(1) Since the model of the time relay is JT3-11/1-110V, the working circuit is a half-wave rectifier step-down circuit. In order for JT3-11/1 to work normally, the diode, step-down resistor and contactor auxiliary contacts in the circuit should all work reliably;

(2) The auxiliary contacts of the JT3-11/1 time relay are a major point where electrical faults often occur, such as mechanism failures and poor contact failures, which are very cumbersome to repair;

(3) The coil of JT3-11/1 itself often has short circuit and open circuit faults; in addition, in this part of the control circuit, the contactor that cuts off the resistor and the auxiliary contact of the time relay control each other and are mutually causal, so the circuit is relatively complicated. From the above analysis, we can see that no matter which point has a problem, it will cause the grab motor to start directly, which will greatly reduce the starting torque of the motor. If it is not discovered in time, it is very easy to burn out the motor.

The Modicon TSX Neza PLC produced by Schneider is rich in functions, easy to use and reliable. The CPU unit has a basic structure of 20 I/O points with 12 input points and 8 output points. It can be expanded to 80 I/O points by connecting up to 3 expansion modules as needed. According to the original circuit requirements, the author replaced 6 time relays with two Modicon TSX Neza PLCs and used the output nodes of the PLC to control the KM1~KM6 contactors. The schematic diagram after the transformation is shown in Figures 3 and 4.

Figure 3 Modicon TSX Neza PLC power supply circuit

Figure 4 Output circuit of Modicon TSX Neza PLC

Since the Modicon TSX Neza PLC runs the program as soon as the power is turned on, the grab main contactor is used to control whether the Neza PLC is running. We programmed the Neza PLC so that its output nodes are turned on in sequence according to the set delay time to meet the action requirements of the original circuit. Considering the safe operation of the Neza PLC, the actual working current of the contactor (CJ12-100) coil was measured to be 0.7A. In order to prevent the coil from burning and damaging the output contact of the relay, the rated current of the contact is 2A. A 2A fuse is connected in series in the output circuit to ensure that the contacts of the circuit will not be damaged.

3 Anti-interference measures

Since the PLC is installed on the electrical control box of the bridge crane, it is in a harsh electromagnetic environment formed by strong electric circuits and strong electric equipment. To improve the reliability of the PLC control system, on the one hand, PLC manufacturers are required to improve the anti-interference ability of the equipment; on the other hand, it is required to pay great attention to engineering design, installation and construction, and use and maintenance. Only with the cooperation of multiple parties can the problem be solved perfectly and the anti-interference performance of the system be effectively enhanced.

3.1 Use high-performance power supply to suppress interference introduced by the power grid

In the PLC control system, the power supply plays an extremely important role. The interference of the power grid enters the PLC control system mainly through the power supply of the PLC system (such as CPU power supply, I/O power supply), etc. For the power supply to the PLC system, a power supply with good isolation performance must be used.

3.2 Cable selection and laying

In order to reduce the electromagnetic interference radiated by the power cable, we selected shielded cable. In the project, the use of copper tape armored shielded power cable can greatly reduce the electromagnetic interference generated by the power line, so that the project can achieve satisfactory results.

Different types of signals are transmitted by different cables. Signal cables should be laid in layers according to the type of transmitted signals. It is strictly forbidden to use different wires of the same cable to transmit power and signals at the same time; avoid laying signal lines and power cables close to each other in parallel to reduce electromagnetic interference.

4 Conclusion

The circuit we modified is simple, reliable to use and easy to maintain. Since it was put into use in June 2004, the failure rate of the modified circuits of the two bridge cranes has been zero, which saves a lot of manpower and material resources, reduces labor intensity, and saves tens of thousands of yuan in material consumption each year, solving a major problem of frequent failures in the electrical part of asphalt bridge cranes for many years.