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

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1. Main Circuit Design Content In an electrical control system, the high-voltage and high-current circuit is typically referred to as the main circuit. In a standard PLC control system, the main circuit usually consists of several key components: - The motor's main circuit, including contactors for starting and stopping the motor, along with protective circuit breakers. - The power circuits for various drive devices, such as driver input circuits, contactor-controlled circuits, protection breakers, servo motor armature circuits, and DC motor excitation circuits. - The primary input circuits for control transformers, which include contactors for on/off control and protection breakers. - The power and control loops that supply power to different parts of the control system, including power transformers, rectifiers, voltage regulators, contactors for power control, and protection breakers. While the basic principles of PLC main circuit design are similar to other electrical systems, it must follow specific standards and consider the unique characteristics of PLC-based systems. Reliability and safety are crucial in this design process. 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 power grid. This switch must have sufficient breaking capacity to handle the sum of the largest motor's locked-rotor current and all other loads. Ideally, it should disconnect all power sources from the equipment. However, exceptions exist for safety-related devices like maintenance power supplies or emergency lighting, which can remain connected directly to the incoming line. These circuits still require separate short-circuit protection devices, such as circuit breakers. 3. Installation of Protective Devices To ensure reliable and effective 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 safely interrupt the protected equipment or motors. For different types of main circuits—like motor circuits or drive circuits—additional total protection circuit breakers should be installed to enhance system reliability and ease of maintenance. In complex systems with many I/O points, shared power supplies should be grouped and each group protected by its own circuit breaker. 4. Grounding and Interference Suppression From a safety perspective, the control system should have a grounding busbar to balance potentials and provide a proper ground. All independent electrical devices connected to the main circuit should use dedicated, qualified grounding cables to connect to the grounding busbar, reducing interference and improving system stability. Devices like PLCs, CNC systems, servo drives, and inverters, which are prone to interference, should be connected through isolation transformers or filter reactors to suppress line noise. Surge voltage absorbers should also be installed for high-power loads to prevent overvoltage spikes caused by switching operations. 5. Auxiliary Control Power Supply The auxiliary power supply for safety functions, such as emergency stop devices (e.g., brakes, safety doors), must remain active even during an emergency stop. Critical components like PLC and CNC power inputs should use independent regulated power supplies or isolation transformers when using AC power. It’s generally not advisable to share power with other system components like solenoid valves or 24V control circuits. Similarly, power supplies for sensors, switches, and actuators in PLC I/O systems should ideally be external to avoid damage from external wiring faults. Note: In Europe, the standard for industrial electrical control systems has shifted to 3-phase AC 400V and single-phase AC 230V, replacing the older 380V and 220V standards. This is important to consider when designing export equipment or repairing imported systems.