PLC installation & wiring 5 points

Pay attention to power supply installation

There are two types of power for PLC systems: external power and internal power.
The external power supply is used to drive the PLC output device (load) and provide input signals, also known as user power. The external power supply of the same PLC may have multiple specifications. The capacity and performance of the external power supply is determined by the input circuitry of the output device and the PLC. Since the I/O circuits of the PLC have filtering and isolation functions, the external power supply has little effect on the performance of the PLC. Therefore, the requirements for external power sources are not high.

The internal power supply is the working power of the PLC, that is, the working power of the PLC internal circuit. Its performance directly affects the reliability of the PLC. Therefore, in order to ensure the normal operation of the PLC, there is a high requirement for the internal power supply. The internal power supply of the general PLC uses a switching regulator power supply or a regulated power supply with a low-pass filter on the primary side.

In the case of strong interference or high reliability requirements, the shield system should be used to supply power to the PLC system. It is also possible to connect the LC filter circuit in series on the secondary side of the isolation transformer. At the same time, you should also pay attention to the following issues during installation:

(1) It is better to use twisted pair connection between the isolation transformer and the PLC and I/O power supply to control the series mode interference;
(2) The power line of the system should be thick enough to reduce the line voltage drop caused by the startup of large-capacity equipment;
(3) When the PLC input circuit uses an external DC power supply, it is better to use a regulated power supply to ensure the correct input signal, otherwise the PLC may receive the wrong signal.
Keep away from strong interference sources

(1) Power line, control line and PLC power line and I/O line should be separately wired. Double-glue line connection should be used between isolation transformer and PLC and I/O. Separate the IO line and the high power line of the PLC separately. If it is necessary to bundle the AC line and the DC line separately in the same line slot, if the conditions permit, the slot line is the best, which not only makes it as large as possible. Space distance and minimize interference.

(2) PLC should be away from strong interference sources such as electric welders, high-power silicon rectifiers and large-scale power equipment, and cannot be installed in the same switch cabinet as high-voltage electrical appliances. The PLC in the cabinet should be away from the power line (the distance between the two should be greater than 200mm). Inductive loads installed in the same cabinet as the PLC, such as relays with larger power and coils of contactors, should be connected in parallel with the RC arc-extinguishing circuit.

(3) Do not use the same cable for the AC output line and DC output line. The output line should be as far away as possible from the high voltage line and the power line to avoid parallelism.

I/O terminal wiring requirements

(1) It is best to separate the input and output of the PLC separately, and the switch quantity and analog quantity should also be laid separately. The transmission of analog signals should be shielded. The shield should be grounded at one or both ends. The grounding resistance should be less than 1/10 of the resistance of the shield.
(2) Input wiring
● The input wiring should not be too long. However, if the environmental interference is small and the voltage drop is not large, the input wiring can be longer.
● The input/output lines cannot use the same cable, and the input/output lines should be separated.
● Connect to the input terminal as often as possible with the normally open contact, so that the programmed ladder diagram is consistent with the relay schematic and is easy to read.

(3) Output connection
● The 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, the output in the same group can only use the same type of power supply of the same voltage level.
● Since the output component of the PLC is packaged on the printed circuit board and connected to the terminal block, if the load connected to the output component is short-circuited, the printed circuit board will be burnt.
● When the relay output is used, the inductive load will affect the service life of the relay. Therefore, when using the inductive load, it should be reasonably selected or an isolation relay.
● The output load of the PLC may cause interference, so measures should be taken to control it, such as the freewheeling protection of the DC output, the RC absorption circuit of the AC output, and the bypass resistor protection of the transistor and the triac output.
Choose the correct grounding point

Good grounding is an important condition to ensure reliable operation of the PLC, which can avoid accidental voltage shock hazards. There are usually two purposes for grounding, one for safety and the other for interference suppression. The perfect grounding system is one of the important measures for the PLC control match system to resist electromagnetic interference.
The ground wire of the PLC control system includes system ground, shield ground, AC ground and protection ground. The disturbance of the grounding system to the PLC system is mainly due to the uneven distribution of potential at each grounding point. There is a ground potential difference between different grounding points, causing ground loop current and affecting the normal operation of the system. For example, the cable shielding layer must be grounded at one point. If both ends A and B of the cable shielding layer are grounded, there is a ground potential difference, and a current flows through the shielding layer. When an abnormal state such as a lightning strike occurs, the ground current will be larger.
In addition, the shielding layer, the grounding wire and the earth may form a closed loop. Under the action of the changing magnetic field, an induced current will appear in the shielding layer, and the signal loop is interfered by the coupling between the shielding layer and the core wire. If the system is confused with other grounding, the generated ground loop may generate unequal potential distribution on the ground, which affects the normal operation of the logic circuit and analog circuit in the PLC. The logic voltage interference tolerance of PLC operation is low, and the distribution interference of logic ground potential easily affects the logic operation and data storage of PLC, resulting in data confusion, program running or crash. The distribution of the analog ground potential will result in a decrease in measurement accuracy, causing severe distortion and malfunction of the signal measurement and control.

● Safely or power grounded
Ground the power cord grounding terminal and the cabinet wiring to a safety ground. If the power supply is leaking or the cabinet is charged, it can be introduced into the ground from the safety ground without causing harm to people.
● System grounding
The PLC controller is grounded in order to be at the same potential as the controlled devices, and the system is grounded. The grounding resistance value shall not be greater than 4Ω. Generally, the PLC equipment system ground and the negative terminal of the switching power supply in the control cabinet shall be connected together as the control system ground.
● Signal and shield grounding
Generally, the signal line must have a unique reference ground. When the shielded cable encounters a possibility of conductive interference, it must be grounded locally or in the control room to prevent the formation of a "ground loop." When the signal source is grounded, the shielding layer should be grounded on the signal side; when it is 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 multi-point grounding must be avoided; When the shielded twisted pair of the point signal is connected with the multi-core twisted pair shielded cable, the shield layers should be connected to each other and insulated, and a single point contact at the appropriate grounding point should be selected.

Inverter interference suppression

The interference handling of the inverter is generally as follows:
The isolation transformer is mainly used for conducting interference from the power supply, and can block most of the conducted interference before the isolation transformer. The filter is used, the filter has strong anti-interference ability, and also prevents the interference of the device itself from being transmitted to the power supply, and some also have the function of peak voltage absorption.
The use of output reactors, adding AC reactors between the inverter and the motor is mainly to reduce the electromagnetic radiation generated by the inverter output during the energy transmission process, affecting the normal operation of other equipment.