1. Pay attention to power supply installation
There are two types of power supplies for PLC systems: external power supplies and internal power supplies.
An external power supply is used to drive the PLC's output devices ( loads ) and provide input signals; it is also called the user power supply. The same PLC may have multiple specifications of external power supplies. The capacity and performance of the external power supply are determined by the output devices and the PLC 's input circuitry. Because the PLC 's I / O circuits have filtering and isolation functions, the external power supply has little impact on the PLC's performance. Therefore, the requirements for the external power supply are not high.
The internal power supply is the operating power source for the PLC, that is, the power supply for the PLC 's internal circuitry. Its performance directly affects the reliability of the PLC . Therefore, to ensure the normal operation of the PLC , high requirements are placed on the internal power supply. Generally, PLCs use switching power supplies or power supplies with primary-side low-pass filters for their internal power supply.
In applications with strong interference or high reliability requirements, a shielded isolation transformer should be used to power the PLC system. An LC filter circuit can also be connected in series on the secondary side of the isolation transformer . Additionally, the following points should be noted during installation:
( 1 ) Twisted pair cable should be used to connect the isolation transformer to the PLC and I/O power supply to control common mode interference;
( 2 ) The power line of the system should be thick enough to reduce the voltage drop of the line when large-capacity equipment is started;
( 3 ) When using an external DC power supply for the PLC input circuit, it is best to use a regulated power supply to ensure the correct input signal; otherwise, the PLC may receive an incorrect signal.
2. Keep away from strong interference sources
( 1 ) Power lines, control lines, and PLC power and I/O lines should be wired separately. The isolation transformer should be connected to the PLC and I/O lines using double-insulated wires. The PLC 's I/O lines and high-power lines should be routed separately. If they must be in the same cable tray, AC and DC lines should be bundled separately. If conditions permit, it is best to route them in separate trays. This will not only allow for the largest possible spatial distance but also minimize interference.
( 2 ) The PLC should be kept away from strong interference sources such as welding machines, high-power silicon rectifiers and large power equipment, and should not be installed in the same switch cabinet as high-voltage electrical appliances. Inside the cabinet, the PLC should be kept away from power lines (the distance between the two should be greater than 200mm ). Inductive loads installed in the same cabinet as the PLC , such as the coils of high-power relays and contactors, should be connected in parallel with an RC arc suppression circuit.
( 3 ) AC output lines and DC output lines should not use the same cable. Output lines should be kept as far away as possible from high-voltage lines and power lines, and parallel operation should be avoided.
3. I/O terminal wiring requirements
( 1 ) The input and output of the PLC should be routed separately, and the digital and analog signals should also be routed separately. The transmission of analog signals should use shielded wires, and the shield should be grounded at one or both ends. The grounding resistance should be less than 1/10 of the shield resistance .
( 2 ) Input wiring
● Input wiring should generally not be too long. However, if there is little environmental interference and the voltage drop is not significant, the input wiring can be appropriately longer.
● Input and output lines must not use the same cable; input and output lines must be separate.
● Use normally open contacts to connect to the input terminals whenever possible, so that the ladder diagram is consistent with the relay schematic and easy to read.
( 3 ) Output connection
● Output wiring is divided into independent output and common output. Different types and voltage levels of output voltage can be used in different groups. However, outputs in the same group can only use the same type and voltage level of power supply.
● Since the output components of the PLC are encapsulated on a printed circuit board and connected to a terminal block, short-circuiting the load connected to the output components will burn out the printed circuit board.
● When using relay output, the size of the inductive load will affect the lifespan of the relay. Therefore, when using inductive loads, appropriate selection should be made, or an isolation relay should be added.
● The output load of a PLC may generate interference, so measures must be taken to control it, such as freewheeling protection for DC output, RC snubber circuit for AC output, and bypass resistor protection for transistor and bidirectional thyristor output.
4. Select the correct grounding point
Proper grounding is essential for ensuring the reliable operation of a PLC, preventing damage from accidental voltage surges. Grounding typically serves two purposes: safety and interference suppression. A well-designed grounding system is one of the key measures for PLC control systems to resist electromagnetic interference.
The grounding wires of a PLC control system include system ground, shield ground, AC ground, and protective ground. A chaotic grounding system interferes with the PLC system mainly because of uneven potential distribution at various grounding points. Potential differences exist between different grounding points, causing ground loop currents and affecting normal system operation. For example, a cable shield must be grounded at one point. If both ends A and B of the cable shield are grounded, a potential difference exists, and current flows through the shield. In abnormal conditions such as lightning strikes, the ground current will be even greater.
Furthermore, the shielding layer, grounding wire, and earth may form a closed loop. Under the influence of a changing magnetic field, induced currents will appear within the shielding layer, interfering with the signal loop through the coupling between the shielding layer and the core wire. If the system grounding is inconsistent with other grounding methods, the resulting ground loop current may create unequal potential distributions on the ground wire, affecting the normal operation of the logic and analog circuits within the PLC. PLCs have low logic voltage interference tolerance; interference from logic ground potential distributions can easily affect PLC logic operations and data storage, causing data corruption, program crashes, or system freezes. Analog ground potential distributions will lead to decreased measurement accuracy, causing serious distortion and malfunctions in signal measurement and control.
● Safely grounded or power supply grounded
Connecting the power cord's grounding terminal to the cabinet's grounding point constitutes a safety ground. In the event of a power leak or the cabinet becoming energized, the current can be conducted to the ground through this safety ground, preventing harm to people.
● System grounding
The PLC controller is grounded to ensure it is at the same potential as all the devices it controls; this is called system grounding. The grounding resistance value must not exceed 4Ω . Generally, the PLC device system ground and the negative terminal of the switching power supply in the control cabinet should be connected together as the control system ground.
● Signal and shielding grounding
Generally, signal lines must have a unique reference ground. Shielded cables must also be uniquely grounded locally or in the control room in situations where conducted interference may occur, to prevent the formation of " ground loops . " When the signal source is grounded, the shielding layer should be grounded on the signal side; if not grounded, it should be grounded on the PLC side. When there is a joint in the middle of the signal line, the shielding layer should be firmly connected and insulated, and multiple grounding points must be avoided. When shielded twisted-pair cables for multiple measurement point signals are connected to a multi-core twisted-pair shielded cable, each shielding layer should be interconnected and insulated, and a suitable single-point grounding point should be selected.
5. Inverter interference suppression
Interference handling for frequency converters generally involves the following methods :
Adding an isolation transformer primarily targets conducted interference from the power supply, blocking the vast majority of conducted interference before it reaches the source. Using filters offers strong anti-interference capabilities and prevents interference from the equipment itself from being conducted to the power supply; some filters also have spike voltage absorption functions.
Using an output reactor, adding an AC reactor between the frequency converter and the motor, is mainly to reduce the electromagnetic radiation generated by the frequency converter output during energy transmission, which could affect the normal operation of other equipment.
