PLC control cabinets play a vital role in industrial production. In modern industrial production, automation and intelligence are key factors to improve efficiency and reduce costs, and PLC control cabinets, as the core of the control system, play an irreplaceable role.
PLC control cabinets are the heart of industrial automation. They integrate various advanced control components, such as input and output modules, power modules, central processing units, etc. Through pre-programming, accurate control of various machines and production processes can be achieved. Taking the PLC control cabinet of Youjinen (Shanghai) Information Technology Co., Ltd. as an example, it not only has high reliability and stability, but also is easy to install, debug and maintain, which greatly reduces the complexity and cost of industrial production.
The basic components of a PLC control cabinet include:
Air switch: A general air switch that controls the power supply of the entire cabinet. This is an essential part of every cabinet.
PLC: Select according to the needs of the project. If the project is small, you can directly use an integrated PLC; if the project is large, you may need a module, a card type, or even redundancy (two sets used alternately).
24VDC power supply: A 24VDC switching power supply. Most PLCs come with a 24VDC power supply, but it is necessary to determine whether this switching power supply is required based on the actual situation.
Relay: Generally, PLCs can directly send instructions to the control loop, but sometimes they are transferred by relays first. For example, when the output port of the PLC is 24VDC, and the node in the control loop that needs to be supplied by the PLC is 220VAC, a relay must be added to the PLC output port.
Terminal block: Something that every cabinet must have, which can be configured according to the number of signals. If other equipment is required in the control cabinet, it will be added as needed.
In short, PLC control cabinets play a pivotal role in industrial production, and their basic components work together to achieve efficient control of the industrial production process.
- Preparation before wiring
(I) Read the drawings carefully
Wiring according to the drawings can be said to be the highest criterion for PLC control cabinet wiring. Before wiring, you must read the drawings carefully and fully understand the designer’s intentions. This is because the design of the PLC control cabinet is carefully planned and considered, and each connection has its specific purpose and function. If the wiring is not done according to the drawings, the system may not operate normally, and may even cause safety hazards to equipment and personnel. If you find any unclear or contradictory points in the process of reading the drawings, you should contact the designer as soon as possible to confirm until there is no error, and then proceed with the wiring construction.
(II) Clarify the wiring sequence
A clear and clear wiring sequence is essential for checking and avoiding errors. The wiring sequence should be checkable to avoid random insertion and missing lines. During the wiring process, you can connect and check in parts, which is easier to find problems than after all connections are completed and then check the whole. For example, you can connect the input circuit part first, and then connect the output circuit part after checking that it is correct. After each part is connected, carefully check whether there is any wrong connection or missing connection to ensure the correctness of the connection.
(III) Master wiring skills
Handling wiring boards and terminals: When making PLC cabinets, you may encounter many wiring boards and wiring terminals. If not handled properly, there will be looseness, burrs, etc. For single-core wires, you can peel them and press them directly; for multi-core wires, it is recommended to use cold-pressed terminals, and tinning is not recommended.
Common terminal and power supply terminal wiring processing: When there are many expansion modules of the PLC, the wiring of the common terminal and power supply terminal needs to be handled with caution. The power supply can be distributed and short-circuited on the terminals and then introduced into the user points respectively, and the wire number tube or the terminal can be marked to indicate the destination. This is intuitive and clear, and the mutual influence is small. It is not desirable to connect from one point to another, nor is it desirable to connect more than two wires under one terminal. For the power terminal block, you can use terminals with insurance or the type that can be disconnected between the upper and lower terminals, which is very convenient when finding short-circuit faults.
- Key points of power wiring
(I) Confirming power supply parameters
Before connecting the PLC control cabinet, confirming the power supply voltage and parameters is a crucial step. Generally, there will be some labels on the control cabinet, such as “AC” and “DC”, which need to be carefully checked. This is because different PLC control cabinets may require different power supply parameters. If the power supply parameters do not match, it may cause equipment damage or malfunction.
For example, some small PLC control cabinets may only require a 24V DC power supply, while some large PLC control cabinets may require a 220V AC power supply. In addition, it is necessary to confirm whether the power supply frequency, power and other parameters meet the requirements.
The method of confirming the power supply parameters can be checked by checking the equipment manual, asking the manufacturer or using tools such as a multimeter for measurement. Only after confirming that the power supply parameters are correct can the next wiring operation be carried out.
(II) Connecting AC power
When connecting the AC power supply, you need to connect the transformer or the circuit breaker first and then connect the power module. This is because the voltage of the AC power supply is higher. If it is directly connected to the power module of the PLC controller, it may cause equipment damage or safety accidents.
First, if you are using an AC power supply, you need to connect a transformer first. The transformer can convert high-voltage AC power into a low-voltage AC power supply suitable for the PLC control cabinet. When connecting the transformer, you need to pay attention to whether the input and output voltages of the transformer meet the requirements of the power supply and the PLC control cabinet.
Secondly, if there is a circuit breaker in the control cabinet, you can connect the power cord to the circuit breaker and then to the power module of the PLC controller. The circuit breaker can automatically cut off the power supply when the circuit is overloaded, short-circuited, or other faults occur, protecting the safety of equipment and personnel. When connecting the circuit breaker, you need to select a suitable circuit breaker based on the capacity of the power supply and the size of the load.
Finally, connect the AC power after the transformer or circuit breaker to the power module of the PLC controller. When connecting the power module, you need to pay attention to whether the positive and negative poles of the power supply are connected correctly to avoid damage to the equipment.
(III) Connecting the DC power supply
Connecting the DC power supply is relatively simple. Simply connect the DC power cord to the power module of the PLC controller. However, when connecting a DC power supply, you also need to pay attention to the following points:
First, you need to confirm whether the voltage of the DC power supply meets the requirements of the PLC controller. Generally speaking, the power module of the PLC controller will have a specific input voltage range. If the voltage of the DC power supply exceeds this range, it may cause damage to the device or malfunction.
Second, you need to pay attention to whether the positive and negative poles of the DC power supply are connected correctly. If the connection is wrong, the entire PLC control system may fail. When connecting a DC power supply, you can refer to the logo on the power module or the equipment manual to ensure that the positive and negative poles are connected correctly.
Finally, when connecting a DC power supply, you need to ensure that the quality of the power cord is good to avoid problems such as poor contact or short circuit. If the quality of the power cord is not good, it may cause the device to malfunction or cause a safety accident.
4. Input and output module wiring
(I) Distinguish input and output wiring
It is very important to separate the input and output wiring of the PLC. This can effectively reduce the interference between the input and output signals and ensure the stable operation of the control system. For example, in practical applications, if the input and output lines are mixed together, when the high-power equipment at the output end starts or stops, electromagnetic interference may be generated, affecting the accuracy of the input signal.
Switching quantities and analog quantities should also be laid separately. Analog signals are usually weak and easily interfered with. If mixed with switching signals, analog signals may be distorted, affecting the accurate measurement and control of production process parameters by the control system. According to statistics, the use of separate wiring can reduce signal interference by more than 70%.
(II) Input wiring precautions
Input wiring should generally not be too long, but if the environmental interference is small and the voltage drop is not large, the input wiring can be appropriately longer. Under normal circumstances, the input wiring length should not exceed 30 meters. For example, in some environments with less interference, the input wiring length can be appropriately extended to about 40 meters.
Input/output lines cannot use the same cable and must be separated. This is to prevent the output signal from interfering with the input signal. At the same time, try to use the normally open contact form to connect to the input end, so that the compiled ladder diagram can be consistent with the relay schematic diagram, which is easy to read and maintain.
(III) Output connection points
Output terminal wiring is divided into independent output and common output. In different groups, output voltages of different types and voltage levels can be used. However, the outputs in the same group can only use power supplies of the same type and voltage level. For example, in a control system, the outputs of different groups can be 24V DC and 220V AC respectively, while the outputs in the same group must be consistent.
Since the output elements of the PLC are encapsulated on the printed circuit board and connected to the terminal board, if the load connected to the output element is short-circuited, the printed circuit board will be burned. Therefore, special attention should be paid to prevent load short-circuit during use.
For the interference that may be generated by the output load, measures should be taken to control it. For example, the DC output can be protected by a freewheeling tube, the AC output can be protected by a resistor-capacitor absorption circuit, and the transistor and bidirectional thyristor output can be protected by a bypass resistor. These protection measures can effectively reduce the interference of the output load on the control system and improve the stability and reliability of the system.
5. Communication interface wiring
(I) Confirm the port type and connection method
The communication interface of the PLC control cabinet usually uses a serial port or Ethernet interface. Serial port is a very common device communication protocol on computers. Common ones include RS-232, RS-422 and RS-485. The physical connector of RS-232 interface is usually a 25-pin plug. The logic “1” level is in the range of (-5 to -15) V, and the logic “0” level is in the range of (5 to 15) V. It has high anti-interference ability. RS-422 adopts differential transmission and differential reception. The data transmission rate can reach 10Mb/s and the communication distance is 12 to 1200m. RS-485 adopts half-duplex communication mode, and its anti-common mode interference ability is enhanced, which is suitable for long-distance data transmission. At present, RS-485 serial interface bus is widely used in PLC local area network. RS-485 is used to form a distributed system. There can be up to 32 stations in the system. The new interface device allows the connection of 128 stations.
Ethernet interface is one of the most important and popular interfaces in current PLC communication. It not only provides high-speed data transmission capabilities, but also supports complex network structures such as local area networks (LANs) and wide area networks (WANs). This means that PLCs can easily communicate with devices or systems anywhere in the world through the Internet.
When connecting the communication interface, you need to confirm the port type and connection method. If it is a serial port connection, you need to use the corresponding serial port cable, such as a USB to serial port cable. Serial communication uses 3 wires: ground, send, and receive. If it is an Ethernet interface connection, you need to use a network cable to connect the Ethernet port of the PLC controller to the network port of the computer. After the connection is completed, you need to set network parameters such as IP address and subnet mask on the computer so that the computer can communicate with the PLC controller. If you need to control and monitor the PLC at a remote location, you also need to establish a virtual private network (VPN) between the PLC and the computer.
In short, correctly confirming the port type and connection method of the communication interface is an important step to ensure normal communication of the PLC control cabinet.
6. Wiring of relays and circuit breakers
(I) Precautions for connecting relays
Relays play an important role in PLC control systems. They can realize multi-channel composite control and improve control capabilities. When connecting relays, you need to pay attention to the connection method of the positive and negative poles of the relay and the power supply.
Generally speaking, relays have input and output terminals. The input terminal is usually connected to the output module of the PLC, and the output terminal is connected to the controlled device. When connecting, make sure that the positive and negative poles are connected correctly to avoid the relay from malfunctioning or being damaged. For example, the input terminal of some relays may need to be connected to a specific DC power supply, such as a 24V DC power supply, while the output terminal can be connected to an AC or DC load, depending on the specifications and application scenarios of the relay.
The role of relays in control systems mainly includes the following aspects: First, as a switch, it controls the on and off of the circuit; second, it realizes electrical isolation and protects PLC and other equipment; third, it can amplify signals and use small currents to control large current operations. For example, in some automated production lines, PLC controls relays by outputting low current signals, and the relays then control the start and stop of high-power motors, thereby achieving precise control of the production process.
(II) Setting up circuit breakers
Circuit breakers are important components for protecting PLC control systems. When connecting circuit breakers, they need to be selected and set according to parameters such as load current, short-circuit protection current, and overload current to ensure the safe operation of the entire system.
First, select the appropriate circuit breaker rated current according to the load current. Generally speaking, the rated current of the circuit breaker should be slightly larger than the load current to ensure that it does not trip frequently under normal working conditions. For example, if the load current is 10A, a circuit breaker with a rated current of 16A can be selected.
Second, consider the short-circuit protection current. Short circuit is a serious fault condition that generates a large current that may damage equipment or even cause a fire. Therefore, the circuit breaker should be able to quickly cut off the circuit when a short circuit occurs. According to relevant standards, the short-circuit protection current of the circuit breaker should be greater than the maximum short-circuit current that may occur. For example, for a specific PLC control system, the maximum short-circuit current that may occur is calculated to be 500A, so a circuit breaker with a short-circuit protection current greater than 500A should be selected.
Finally, the overload current should also be considered. When the load exceeds its rated power for a period of time, an overload current will be generated. The circuit breaker should be able to cut off the circuit when the overload current reaches a certain level to protect the equipment. Usually, the overload protection current of the circuit breaker can be set according to the characteristics of the equipment and the use environment, which is generally about 1.2 to 1.5 times the rated current.
In short, the correct selection and setting of the circuit breaker is crucial to protecting the safe operation of the PLC control system. By reasonably selecting and setting parameters such as load current, short-circuit protection current and overload current, it is possible to effectively prevent damage to equipment and personnel caused by electrical faults.
7. Arrangement of wiring and safety points
(I) Arrangement of cables
In the wiring process of the PLC control cabinet, arranging cables is a crucial step. Messy cables will not only affect the appearance of the control cabinet, but may also cause problems such as signal interference and difficulty in troubleshooting.
Classification and merging is the first step in arranging cables. Different types of cables, such as power cables, signal cables, control cables, etc., are classified separately and fixed with cable ties or cable troughs. This can effectively reduce interference between different types of cables. According to statistics, after classification and merging, the probability of signal interference can be reduced by more than 50%.
Shortening the cable length can reduce the possibility of signal attenuation and interference. On the premise of ensuring normal connection, try to shorten the length of the cable. However, it should be noted that the cable should not be shortened excessively to avoid affecting the installation and maintenance of the equipment. Generally speaking, the length of the cable should be reasonably adjusted according to the actual situation to avoid being too long or too short.
When extending the cable, ensure that the quality and specifications of the cable meet the requirements. If you need to extend the cable, you should choose a cable with the same specifications as the original cable and use a professional connector to connect it. During the connection process, ensure that the connection is firm and the contact is good to avoid connection problems.
Problems such as poor contact or broken wires.
Labeling is an important part of cable organization. Labeling each cable can facilitate troubleshooting and maintenance. The label should be clear and accurate, and can clearly identify the purpose and connection location of the cable. For example, numbers, letters or colors can be used for labeling. At the same time, cable identification plates can be made in the control cabinet to correspond the labels with the cable connection locations for quick search and maintenance.
(II) Comply with safety regulations
During the wiring process of the PLC control cabinet, compliance with safety regulations is the key to ensuring the safety of personnel and equipment. First of all, the power must be turned off before disassembling the equipment, which is the most basic safety requirement. After the power is turned off, ensure that the equipment is in a safe state to avoid accidental power supply causing casualties or equipment damage.
When cleaning the electrical cabinet, be careful not to get dust everywhere. Dust not only affects the heat dissipation of the equipment, but can also cause electrical failures. When using a vacuum cleaner for cleaning, avoid blowing dust into the equipment to avoid short circuits and other failures.
During the wiring process, mark all places that have been moved. The markings should be clear and accurate so that problems can be found in time after the equipment is put back into production. For example, you can use a marker to mark the wiring, or use a label to identify it.
Compliance with safety regulations also includes regular maintenance of the control cabinet. Regularly check the appearance, environment, wiring, etc. of the control cabinet to find and deal with problems in time. For example, check whether the temperature of the control cabinet is normal, whether the large cable and copper plate are overheated; check whether the cable is loose and whether the joint is black; check whether the grounding is good and whether there is rust and other problems.
In short, during the wiring process of the PLC control cabinet, safety regulations must be strictly followed to ensure the safety of personnel and equipment. At the same time, the control cabinet must be maintained regularly to improve the reliability and stability of the equipment and extend the service life of the equipment.
8. Special wiring points
(I) Inverter interference suppression
In the use of PLC control cabinets, inverter interference is a common problem. In order to effectively deal with inverter interference, the following methods can be used:
Add an isolation transformer: mainly for conducted interference from the power supply. Isolation transformers can block most of the conducted interference in front of them and provide relatively pure power for PLC control systems. For example, in some industrial production environments, after using isolation transformers, the interference caused by the power supply is reduced by more than 80%.
Use filters: Filters have strong anti-interference capabilities and can also prevent the interference of the equipment itself from being transmitted to the power supply. Some filters also have the function of peak voltage absorption. When the high-order harmonics generated by the inverter interfere with the PLC system, the filter can effectively filter out these interferences to ensure the stable operation of the PLC system.
Use output reactors: Adding AC reactors between the inverter and the motor is mainly to reduce the electromagnetic radiation generated by the inverter output line during the energy transmission process, affecting the normal operation of other equipment. According to actual application statistics, after using output reactors, electromagnetic radiation interference is reduced by about 70%.
(II) Grounding points
Proper grounding is crucial to ensure the reliable operation of PLC and suppress interference. There are usually two purposes for grounding, one is for safety, and the other is to suppress interference. Good grounding is one of the important measures for PLC control systems to resist electromagnetic interference.
Grounding type:
Protective grounding: The chassis is safely grounded to protect the equipment and personal safety. For example, when the equipment leaks or the cabinet is energized, it can be led underground from the protective grounding without causing harm to people.
Working grounding: Including signal ground, shielding ground, AC ground, etc., which are necessary grounding for the function of the equipment or system. For example, the signal ground is the ground of the input signal element – the sensor. In order to suppress the interference attached to the power supply and input and output ends, the PLC system should be well grounded.
Lightning protection grounding: A grounding system that prevents damage caused by lightning strikes.
Shielding grounding: Eliminate the harm of electromagnetic fields to the human body and prevent electromagnetic interference.
Grounding method and requirements:
One-point grounding principle: The grounding wire is a “facility that determines the potential”. Under normal conditions, no current flows in the grounding wire, and the return circuit of the current reflux cannot be shared with the grounding.
Try to use dedicated grounding: Use Class D grounding for grounding engineering, preferably a dedicated grounding separated from the grounding of other equipment. Its grounding electrode is more than 10m away from other grounding electrodes. The grounding electrode should be as close to the PLC as possible to shorten the grounding wire. When a dedicated grounding cannot be used, it can be connected to the grounding electrode of other equipment to form a common ground, but it is necessary to avoid sharing the ground with high-power equipment such as motors/inverters.
Notes on wiring: When the signal line ground and the shell ground are the same ground, it is necessary to use an insulator to insulate the channel substrate (metal plate in the grounded control cabinet, etc.). The cabinet with the PLC needs to be electrically insulated from other equipment to prevent the influence of leakage current from other electrical equipment. When there are high-frequency equipment, the cabinet with the PLC itself must be grounded while grounding the high-frequency equipment. When shielded cables are used for input and output wiring, the shielded conductor close to the PLC side is connected to the shell grounding terminal.
Grounding terminal of PLC: There are two grounding terminals in the PLC. One is connected to the bottom plate of the PLC, which is usually used to prevent electric shock and protect the grounding terminal from grounding; the other is connected to the neutral point of the noise filter, and is a functional grounding terminal that is grounded when malfunctions due to power supply interference. When using, please note that although correct grounding can reduce the common-mode interference of the power supply, there are many cases where grounding can actually pick up noise.
Wiring of AC power supply unit: When the equipment power supply is single-phase grounded, the grounded phase side must be connected to the L2/N (or L1/N) terminal side.
In short, in the wiring process of the PLC control cabinet, correctly handling the inverter interference and doing a good job of grounding are crucial to ensure the stable operation of the PLC system and the safety and reliability of the equipment.
