Hollysys PLC - PLC should be noted in operation (three, four, five)

3. Main anti-interference measures

(1) Reasonable treatment of power supply to suppress the interference introduced by the power grid

For grid disturbances introduced by the power supply, an isolation transformer with a shielded ratio of 1:1 can be installed to reduce the interference between the device and ground, and the LC filter circuit can also be connected in series at the power input.

Fourth, the correct selection of grounding points, improve the grounding system

Good grounding is an important condition to ensure the reliable operation of PLC, which can avoid the occasional voltage rush hazard. There are usually two purposes for grounding, one is for safety and the other is to suppress interference. The perfect grounding system is one of the important measures to resist electromagnetic interference in the PLC control system.

The ground wire of the PLC control system includes a system ground, a shielding ground, an AC ground and a protective ground. The interference of the grounding system chaos to the PLC system is mainly due to the uneven distribution of the potential of each grounding point, and the ground potential difference between different grounding points, which causes the ground loop current and affects the normal operation of the system. For example, the cable shield must be grounded at one point, if both ends of the cable shield A and B are grounded, there is a ground potential difference, there is a current flowing through the shield, when an abnormal state such as lightning strikes, the ground wire current will be greater.

In addition, the shield, the ground wire, and the ground may form a closed loop, and under the action of the changing magnetic field, an induced current will occur in the shield, which will interfere with the signal loop through the coupling between the shield and the core. If the system is messed up with other grounding processes, the resulting ground circulation may produce unequal potential distribution on the ground wire, affecting 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 logical ground potential can easily affect the logic operation and data storage of PLC, resulting in data confusion, program flight or crash. The distribution of simulated ground potential will lead to the degradation of measurement accuracy, resulting in serious distortion and malfunction of signal measurement and control.

● Safely or mainly grounded

Grounding the power cord and the cabinet connection to the ground are safety grounding. If the power supply leaks or the cabinet is electrified, it can be imported underground from the safety ground, which will not cause harm to people.

● System grounding

The PLC controller is grounded in order to be at the same potential as each device under control, which is called system grounding. The grounding resistance value shall not be greater than 4Ω, and the PLC equipment system and the negative end of the switching power supply in the control cabinet need to be connected together as the ground of the control system.

● Signal and shield grounding

Generally, it is required that the signal line must have a unique reference ground, and the shielded cable must be grounded only in the local or control room when it encounters occasions that may produce conducted interference, so as to prevent the formation of a "ground loop". When the signal source is grounded, the shield 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 multiple measurement point signals is connected with the multi-core twisted pair total shielded cable, each shielding layer should be connected to each other, and the appropriate grounding should be selected at the single-point contact after insulation treatment.

5. Suppression of inverter interference

There are generally the following ways to deal with the interference of the inverter:

The addition of an isolation transformer is mainly aimed at the conducted interference from the power supply, which can block most of the conducted interference before the isolation transformer.

Filters are used, which have strong anti-interference capabilities, and also have the ability to prevent interference from being transmitted to the power supply from the equipment itself, and some also have the function of absorbing spike voltage.

The use of output reactor, the addition of AC reactor between the inverter and the motor is mainly to reduce the output of the inverter, and the electromagnetic radiation generated by the line in the process of energy transmission affects the normal operation of other equipment.

Hollysys PLC - PLC should be noted in operation (three, four, five)

Hollysys PLC - PLC should be noted in operation (three, four, five)

Sections viewed

Contact

working hours