PLC control system main circuit design points - Database & Sql Blog Articles

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1. Main Circuit Design Overview In electrical control systems, the main circuit typically refers to high-voltage and high-current circuits. In a standard PLC control system, the main circuit usually includes several key components: - The motor’s main circuit, which involves contactors for starting and stopping the motor, as well as protective circuit breakers. - Power circuits for various drive devices, such as driver input circuits, contact control systems, protection breakers, servo motor armature circuits, and DC motor excitation circuits. - The primary input circuits of control transformers, including contactors for on/off control and protective breakers. - Power and control loops that supply power to different parts of the control system, such as power transformers, rectifiers, voltage regulators, and power supply contactors with protective breakers. The design of the main circuit in a PLC system is similar to other electrical control systems but must adhere to specific standards and consider the unique characteristics of PLCs, ensuring the system's reliability and safety. 2. Main Power Switch Requirements According to EN60204.1 (VDE0113 part 1), the main power switch must be installed to isolate the entire control system from the grid. The switch should have sufficient breaking capacity to handle the total current from the largest motor in a blocked state plus all other consumers and motors. Ideally, it should disconnect all power sources from the equipment. However, exceptions apply for safety devices like maintenance power supplies or emergency lights, which can remain connected directly to the incoming line. These circuits still require separate short-circuit protection devices, such as circuit breakers. 3. Protection Device Installation To ensure reliable protection of the main circuit, each independent component must be equipped with a protection device, such as a circuit breaker, for short-circuit and overcurrent protection. These devices must have enough breaking capacity to reliably disconnect the protected equipment or motor. For different types of main circuits—like motor circuits or drive circuits—additional total protection circuit breakers are recommended. In complex systems with many I/O points, external signals should be grouped and protected by individual circuit breakers to improve maintainability and safety. 4. Grounding and Interference Suppression From a safety perspective, the control system should include a grounding busbar for potential balancing and grounding. All connected electrical devices must use qualified grounding cables to connect to this busbar, reducing interference and enhancing system stability. Devices like PLCs, CNC units, servo drives, and inverters should be powered through isolation transformers or filter reactors to suppress line interference. Surge absorbers should also be installed for high-power loads to manage overvoltage caused by switching operations. 5. Auxiliary Control Power Supply The auxiliary power supply for safety mechanisms, such as emergency stop systems, must remain active even during an emergency stop. Critical components like PLC and CNC power inputs should be supplied by independent regulated power supplies or isolation transformers. They should not share power with other control circuits, such as solenoid valves or 24V/220V control loops. Additionally, sensors, switches, and actuators for PLC I/O should preferably use external power supplies to prevent damage from external faults. Note: In Europe, the standard for industrial control systems has shifted from 380V/220V to 400V/230V for three-phase and single-phase AC. This change should be considered when designing export equipment or repairing imported systems.