The following is an introduction to the common components in the PLC power distribution cabinet:
Circuit breaker
Function: Mainly used to protect the circuit. When a short circuit, overload or leakage occurs in the circuit, causing the current to exceed the rated current, the circuit breaker will automatically cut off the circuit to protect the entire power distribution cabinet and the controlled equipment from damage.
Type: Common ones include air circuit breakers, molded case circuit breakers, etc. Molded case circuit breakers have good insulation performance and arc extinguishing ability, and are suitable for circuits with larger currents; small circuit breakers are often used to control branch circuits.
Fuse
Function: When the current in the circuit is too large, the fuse fuse melts due to overheating, thereby cutting off the circuit and playing a short-circuit protection role. Its action speed is faster than that of the circuit breaker, and it can cut off the fault current in an instant to protect the safety of equipment and lines.
Type: There are plug-in fuses, spiral fuses, enclosed fuses, etc. Different types of fuses differ in rated current, fusing characteristics, etc., and can be selected according to specific circuit requirements.
Contactor
Function: It is an electrical component used to control the start and stop of motors or other load devices. When the contactor coil is energized, a magnetic field is generated, which attracts the moving iron core, closes the contactor contacts, and connects the load circuit to start the device; when the coil is de-energized, the magnetic field disappears, and the contacts are disconnected under the action of the spring, cutting off the load circuit.
Type: It is divided into AC contactor and DC contactor, which are used for AC circuit and DC circuit respectively. AC contactor is widely used, and its rated voltage and rated current have various specifications, which can be selected according to the power size of the controlled load.
Relay
Function: It is generally used for signal conversion, amplification, and control circuit switching. In the PLC power distribution cabinet, the weak current signal output by the PLC can be converted into a strong current signal to control external equipment; or it can play the role of intermediate conversion and isolation in the control circuit to protect the PLC output port from external high voltage and high current.
Type: Common ones include intermediate relays, time relays, thermal relays, etc. Intermediate relays are mainly used for signal transfer and expansion of the number of output contacts; time relays can realize timing control, such as delayed start, delayed stop and other functions; thermal relays are mainly used for overload protection of motors.
Switching power supply
Function: Convert the input power such as AC power into a stable DC voltage to provide the required DC power for PLC, sensors, relays and other equipment. It has the characteristics of small size, high efficiency, stable output voltage, etc., and can ensure the normal operation of various electronic components in the distribution cabinet.
Type: According to the output voltage, it can be divided into switching power supplies of different specifications such as 5V, 12V, and 24V. Among them, 24V switching power supplies are more commonly used in PLC distribution cabinets.
Transformer
Function: Used to change the size of AC voltage to meet the voltage requirements of different equipment. In the PLC distribution cabinet, the AC voltage can be reduced to the working voltage required by PLC, control circuit, etc., or AC voltages of different levels can be converted to ensure the normal power supply of various circuits in the system.
Type: There are step-down transformers, step-up transformers, isolation transformers, etc. Isolation transformers also have the function of isolating interference signals and improving the system’s anti-interference ability. They are often used in places with high requirements for electromagnetic compatibility.
PLC module and I/O expansion module
Function: PLC is the core control element of the entire power distribution cabinet. It receives input signals, performs logical operations and processing according to pre-written programs, and then outputs control signals to achieve automated control of various equipment and process processes. I/O expansion modules are used to expand the number of input and output points of PLC to meet more control needs.
Type: PLC has many brands and models, such as Siemens, Mitsubishi, Omron, etc. PLCs of different brands differ in programming languages, functional features, communication protocols, etc. I/O expansion modules are also divided into digital input and output modules, analog input and output modules, etc., which can be selected and matched according to the signal type and quantity of the actual control object.
Signal isolation module
Function: It is used to isolate and convert input and output signals to prevent field interference signals from entering the PLC system, while protecting the PLC output signal from the influence of external loads, and improving the stability and reliability of the system. It can match and convert signals of different levels and types so that the signals can be accurately transmitted and processed.
Type: Common ones include photoelectric isolation modules, electromagnetic isolation modules, etc. The photoelectric isolation module uses photoelectric coupling devices to realize signal isolation transmission, and has the advantages of fast response speed and good isolation performance; the electromagnetic isolation module uses the principle of electromagnetic induction to isolate signals, which is suitable for some occasions with high requirements for transmission distance and anti-interference ability.
Surge protector
Function: It is mainly used to protect electrical equipment from damage caused by instantaneous high voltage pulses such as lightning strikes and grid operation overvoltage. When a surge voltage appears on the line, the surge protector can quickly turn on and introduce the surge current into the ground, thereby protecting the sensitive electronic components in the distribution cabinet.
Type: According to the protection object, it can be divided into power surge protector, signal surge protector, etc. The power surge protector is usually installed at the incoming line end of the distribution cabinet to protect the power supply system of the entire distribution cabinet; the signal surge protector is used to protect the input and output signal lines of the PLC to prevent the surge voltage on the signal line from damaging the PLC.
Terminal block
Function: used to connect various electrical components and external cables in the distribution cabinet to achieve circuit conduction and signal transmission. It can reliably connect wires and cables of different specifications and types, which is convenient for circuit wiring and maintenance.
Type: There are ordinary terminal blocks, European terminal blocks, plug-in terminal blocks, etc. Ordinary terminal blocks have simple structure and low cost, which are suitable for general electrical connections; European terminal blocks have the advantages of convenient wiring and reliable contact, and are widely used in various distribution cabinets; plug-in terminal blocks are easy to disassemble and replace, and are suitable for occasions where frequent line adjustment and maintenance are required.
Human-machine interface touch screen
Function: As an interactive interface between the operator and the PLC control system, the touch screen can intuitively display the operating status, process parameters, alarm information, etc. of the equipment. At the same time, the operator can input control instructions to the PLC through the touch screen to realize remote monitoring and operation of the equipment, making the operation of the equipment more convenient, fast, intuitive and easy to understand.
Type: Touch screens of different brands and models vary in size, resolution, communication interface, function, etc., and can be selected according to actual needs and operating habits.
Button indicator light
Function: Buttons are used to manually issue control instructions, such as start, stop, reset and other operations; indicator lights are used to display the operating status, fault status and other information of the equipment, so that operators can understand the working status of the equipment in a timely manner. For example, a green indicator light that is always on indicates that the equipment is operating normally, and a red indicator light that indicates that the equipment is faulty.
Type: Buttons include normally open buttons, normally closed buttons, composite buttons, etc.; indicator lights include ordinary indicator lights, LED indicator lights, etc. LED indicator lights have the advantages of high brightness, long life, and low power consumption, and are widely used in distribution cabinets.
Wire trough
Function: Used to organize and fix the wires and cables in the distribution cabinet, making the wiring more neat and beautiful, easy to inspect and maintain, and also prevent mutual interference and wear of wires and cables.
Type: There are plastic wire troughs, metal wire troughs, etc. Plastic wire troughs have the advantages of good insulation performance, light weight, and low cost; metal wire troughs have high strength and good fire resistance, and are suitable for some occasions with high wiring requirements.
Limit switch
Function: Mainly used to detect the position or movement limit of the equipment. When the equipment moves to the set position, the limit switch will act and send a signal to the PLC to control the equipment’s stop, reversal and other actions to prevent the equipment from exceeding the limit position and causing collision or damage.
Type: Common ones include mechanical limit switches, photoelectric limit switches, magnetic induction limit switches, etc. Mechanical limit switches have a simple structure and high reliability, but relatively low accuracy; photoelectric limit switches and magnetic induction limit switches have the advantages of high accuracy, fast response speed, and non-contact detection, and are suitable for some occasions with high position accuracy requirements.
Fan
Function: Used for heat dissipation in the distribution cabinet, reduce the temperature in the distribution cabinet, prevent electrical components from being damaged due to overheating, and ensure the normal operation of the distribution cabinet. The fan is usually installed on the top or side of the distribution cabinet to form air convection and discharge the heat in the distribution cabinet.
Type: There are axial flow fans, centrifugal fans, etc. Axial fans have large air volume and low noise, and are suitable for general power distribution cabinet heat dissipation; centrifugal fans have high pressure and large air volume, and can be used in some occasions with higher requirements for heat dissipation.
