PLCs are commonly used in high-voltage squirrel-cage motor water resistance cabinets. They utilize computer simulation software to simulate the motor's starting process, making the entire starting process predictable, adjustable, and controllable. The specific working principle is as follows: The electrolyte rheostat consists of three mutually insulated electrolytic tanks, each containing electrolyte and a corresponding set of conductive plates. The moving plate is controlled by a transmission mechanism and a servo system, which is controlled by the PLC. The PLC system uses internal computer simulation software to control the starting process. At the start of startup, the liquid resistance value (the initial position of the moving plate) is automatically adjusted according to the motor current, ensuring a relatively small starting current, uniform speed increase, and stepless liquid resistance switching, thus achieving a soft start for the motor.
PLC Overview
A PLC is a device used for automated control in industrial production. Generally, it can be used directly in industrial environments without any special precautions. However, despite its high reliability and strong anti-interference capabilities, as mentioned above, harsh production environments with strong electromagnetic interference, or improper installation and use, can lead to program errors or calculation errors, resulting in erroneous inputs and outputs. This can cause equipment malfunctions and misoperations, thus compromising the normal operation of the PLC. To improve the reliability of a PLC control system, PLC manufacturers need to enhance the anti-interference capabilities of their equipment. Furthermore, careful attention must be paid to design, installation, and maintenance; multi-party cooperation is essential to effectively address these issues and enhance the system's anti-interference performance. Therefore, the following issues should be noted during use:
1. Work environment
(1) Temperature PLC requires an ambient temperature of 0~55℃. When installing, it should not be placed under components that generate a lot of heat. The surrounding space for ventilation and heat dissipation should be large enough.
(2) Humidity In order to ensure the insulation performance of PLC, the relative humidity of the air should be less than 85% (no condensation).
(3) Vibration should be prevented by keeping the PLC away from strong vibration sources and preventing frequent or continuous vibration with a frequency of 10~55Hz. When vibration is unavoidable in the operating environment, vibration reduction measures must be taken, such as using vibration damping adhesive.
(4) Avoid air containing corrosive and flammable gases, such as hydrogen chloride and hydrogen sulfide. For environments with a lot of dust or corrosive gases in the air, the PLC can be installed in a well-sealed control room or control cabinet.
(5) The power supply PLC has a certain ability to resist interference from the power line. In environments with high reliability requirements or particularly severe power interference, an isolation transformer with a shield can be installed to reduce interference between the equipment and ground. Generally, PLCs have a 24V DC output to provide input terminals. When using an external DC power supply at the input terminals, a DC regulated power supply should be selected. This is because ordinary rectified and filtered power supplies are prone to causing the PLC to receive incorrect information due to ripple.
2. Interference and its sources in the control system
Electromagnetic interference in the field is one of the most common and easily detrimental factors to the reliability of PLC control systems. As the saying goes, to treat the symptoms, one must first address the root cause; only by identifying the problem can a solution be proposed. Therefore, it is essential to know the source of the field interference.
Interference Sources and General Classifications: Interference sources affecting PLC control systems mostly occur in areas with drastic changes in current or voltage. This is because changes in current generate magnetic fields, which produce electromagnetic radiation on the equipment; changes in magnetic fields generate current, and electromagnetic waves are produced at high speeds. Electromagnetic interference is generally classified into common-mode interference and differential-mode interference based on its interference mode. Common-mode interference is the potential difference between the signal and ground, mainly formed by the superposition of common-mode (same-direction) voltages induced on the signal lines by grid interference, ground potential difference, and electromagnetic radiation from the air. Common-mode voltage can be converted into differential-mode voltage through asymmetrical circuits, directly affecting the measurement and control signals and causing component damage (this is the main reason for the high failure rate of some system I/O modules). This common-mode interference can be DC or AC. Differential-mode interference refers to the interference voltage acting between the two poles of the signal, mainly formed by the coupling induction of spatial electromagnetic fields between signals and the voltage formed by the conversion of common-mode interference by unbalanced circuits. This interference is superimposed on the signal, directly affecting the measurement and control accuracy.
