PLC use and wiring precautions

1. PLC wiring precautions

1. The PLC must be correctly connected to the system before working. The main connections with the PLC are the PLC power line, input/output device wiring, communication line and grounding line.

2. The PLC AC power line should enter the control cabinet separately from the top of the machine and should not be routed together with the analog signal line and DC signal line to reduce interference with other control circuits.

3. The power supply of the PLC is connected between the L terminal and the N terminal in the input port. To prevent short-circuit faults during wiring, a 250V, 1A fuse should be selected. To prevent output short circuits caused by load short circuits, fuses should be installed in the PLC external output circuit. In the construction of PLC power wiring, the power wiring of the output module must be grouped.

4. PLC has three output modes. If PLC uses transistor output, it cannot be connected to single-phase motors, three-phase motors, or AC loads.

5. If the PLC output terminal is connected to an inductive load, the corresponding protection circuit should be connected according to the different load conditions. An RC series circuit should be connected in parallel at both ends of the AC inductive load.

6. The input interface of FX series PLC generally adopts junction wiring, and the output interface generally adopts group wiring. The input end can be connected to the main control electrical appliance, the contact of the thermal relay, etc., and the output end can be connected to the AC contactor coil, solenoid valve, etc.

7. The I/O connection and control lines of the PLC should be wired separately and at a certain distance. If they have to be wired in the same slot, shielded wires should be used. It is best to separate the AC line from the DC line, the input line from the output line, and the lines that transmit the switch quantity and the analog signal should be laid separately. Shielded wires can be used to transmit analog signals, and the shielding layer should be grounded at one end of the analog module.

8. PLC input/output circuit wiring 1) Determine the type and voltage level of the load power supply according to the nature of the load and the requirements of the output point. Do not choose DC if AC can be used. 2) Correctly determine the capacity of the load power supply. Factors such as the simultaneous operation of loads such as contactors and solenoid valves should be considered. It is not appropriate to simply estimate the total capacity by multiplying the coefficient. Especially in large systems, if the capacity does not meet the requirements, it is easy to cause excessive voltage loss and affect the normal operation of the load. 3) Even if the load power supply is AC 220V, it should not be directly taken from the power grid, and shielding isolation measures should be taken. Moreover, the power supply of the basic unit and expansion unit of the same system should be taken from the same phase as its output power supply. 4) PLC mostly uses positive logic input, requiring a 24V high level output when the proximity switch is actuated.

In addition, all switches and sensors used as PLC input elements are not allowed to be connected to other loads, regardless of whether their capacity allows it. It must be ensured that each input channel is independent to prevent interference or damage to the PLC's internal input circuit.

2. Precautions for using PLC

1. PLC should not be installed in places with high temperature, condensation, rain, dust, smoke, electromagnetic interference, corrosive gas and flammable gas, let alone places prone to vibration or strong impact. If the environment is bad, appropriate ventilation, dust and vibration prevention measures should be taken, and it can be installed in the control room if necessary. The working environment temperature range of PLC is 0~55℃, and the relative humidity should be below 85% (no frost).

2. PLC cannot be installed together with high-voltage electrical appliances, high-power thyristor devices, and high-frequency welding machines. The control cabinet should be away from strong interference sources and power lines.

3. When the PLC backup battery is abnormal, the battery must be replaced within one week. The replacement time should not exceed 5 minutes. In addition, the program must be backed up.

4. Good grounding is an important measure to ensure the safe and reliable operation of PLC. The basic principle of PLC system grounding is single-point grounding.

5. An emergency stop circuit should be set up in the PLC control system to provide the highest safety guarantee, and the emergency stop should not be controlled by the PLC.