(1) Strong electrical interference: the normal power supply of the PLC system are powered by the power grid. Due to the wide coverage of the power grid, it will be subject to all space electromagnetic interference and the induction of voltage on the line. Especially the changes within the power grid, such as the use of knife switch will produce disturbances within the power grid.
Knife switch is what we usually call the gate switch, is a kind of connection and disconnection of AC and DC circuits or for the isolation of the switch with a manual switching device. Operation surge, large power equipment starting and stopping, AC and DC drives caused by harmonics, grid short-circuit transient shock, etc., are transmitted through the transmission line to the original side of the power supply.
(2) cabinet interference: high-voltage appliances in the control cabinet, large inductive loads, chaotic wiring are prone to a certain degree of interference with the PLC.
Interference from the introduction of the signal line: and PLC control system connected to the various types of signal transmission lines, in addition to the transmission of all kinds of effective information, there will always be external interference signal intrusion.
This interference mainly has two ways: one is through the transmitter
Transmitter is a physical measurement signal or ordinary electrical signals into a standard electrical signal output or can be output in the form of communication protocol equipment. Power supply or common signal instrumentation power supply string into the power grid interference, which is often overlooked;
Second, the signal line by the space electromagnetic radiation induction of interference, that is, the signal line on the external induction interference, which is very serious. The introduction of interference from the signal will cause I / O signal work abnormally and greatly reduce the accuracy of the measurement, and when serious, will cause damage to the components.
(3) from the grounding system chaos when the interference: grounding is to improve the electromagnetic compatibility of electronic equipment (EMC) one of the effective means. Correct grounding, both to suppress the impact of electromagnetic interference, but also inhibit the equipment to the outside world to send out interference; while the wrong grounding, but will introduce serious interference signals, so that the PLC system will not work properly.
(4) interference from within the PLC system: mainly by the system components and circuits within the mutual electromagnetic radiation generated, such as logic circuits
Logic circuits are digital circuits containing logical relationships, binary as the principle, the realization of digital discrete signal transmission, logical operations and operation of the circuit. The most basic logic circuits are common gate circuits, while the simplest gate circuits are and circuits, or circuits and non-circuits. Mutual radiation and its impact on analog circuits, analog ground and logic ground and the interactions between components, such as the use of mutual mismatch.
(5) Frequency converter interference: First, the frequency converter starting and running process generates harmonics on the grid to produce conduction interference, causing grid voltage distortion, affecting the quality of power supply to the grid; Second, the output of the frequency converter will produce strong electromagnetic radiation interference, affecting the normal work of the peripheral equipment.