PLC installation instructions

1. Introduction
Programmable controller (PLC) is a new type of general automatic control device. It integrates the traditional relay control technology, computer technology and communication technology, and has powerful control functions, high Availability, flexible and convenient use, and easy to expand. However, in use, due to the harsh working environment at the industrial production site, there are many sources of interference, such as low-frequency interference caused by fluctuations in the grid voltage caused by the start or stop of high-power electrical equipment, the normal operation of PLC is affected by the power frequency interference caused by electromagnetic coupling, such as electric welding machine, electric spark machine, and electric brush of motor.
Although PLC is a control device dedicated for field use, many measures have been taken during design and manufacture to make it more adaptable to the industrial environment, but to ensure the stability of the entire system *, we should try to make the PLC have good working environment conditions and take necessary anti-interference measures.
2 PLC installation and maintenance should pay attention to issues
2.1 PLC Installation
PLC is suitable for most industrial sites, but it still has certain requirements for use cases and ambient temperature. Control the working environment of PLC, which can effectively improve its working efficiency and life. When installing PLC, avoid the following places:
(1) The ambient temperature exceeds 0 ~ 50 ℃ range;
(2) relative humidity exceeds 85% or there is dew condensation (caused by sudden changes in temperature or other factors );
(3) direct sunlight;
(4) corrosive and flammable gases, such as hydrogen chloride and hydrogen sulfide;
(5) Check for scrap metal and dust;
(6) frequent or continuous vibration with a vibration frequency of 10 to 10 ~ 55Hz, up to 0.5mm (Peak-peak );
(7) The impact of over 10g (gravity acceleration.
There are mounting holes on the four corners of the small programmable controller housing. There are two installation methods, one is to use screws, different units have different installation sizes, and the other is DIN (German Republican standard) Orbital fixation. One pair of mounting splices for din rail. On the track, first install the Left and Right splint, install PLC, and then tighten the screw. To enable the control system to work, the programmable controller is usually installed in a control cabinet with a protective housing to prevent dust, oil stains, and water splash. To ensure that the temperature of the programmable controller is kept within the specified ambient temperature range during operation, the installation machine should have sufficient ventilation space. There must be an interval of more than 30mm between the basic unit and the expansion unit. If the surrounding environment exceeds 55c, an electric fan must be installed and forced ventilation is required.
To avoid electrical interference from other peripheral devices, the programmable controller should be as far away as possible from the high-voltage power cord and high-voltage equipment. The programmable controller should be at least 200mm away from the high-voltage equipment and power cord.
When the programmable controller is installed vertically, it is necessary to prevent wire head, scrap metal, and so on from the ventilation window into the programmable controller, resulting in a short circuit of the printed circuit board, so that it can not work normally or even permanent damage.
2.2 Power Supply Wiring
PLC power supply is 50Hz, 10% V + AC.
FX series Programmable Controllers have 24 v dc output terminals. The terminal can provide 24 v dc power supply for input sensing (such as photoelectric switch or proximity switch.
If the power supply fails, the interruption time is less than 10 ms, and the PLC operation is not affected. If the power supply is interrupted for more than 10 ms or the power supply drops beyond the allowable value, the PLC stops working and all output points are disconnected at the same time. When the power supply is restored, if the run input is switched on, the operation is performed automatically.
PLC itself has sufficient resistance to interference from power cables. If the power supply interference is particularly serious, you can install an isolation transformer with a ratio of to reduce interference between the equipment and the ground.
2.3 grounding
Good grounding is an important condition to ensure that the PLC can work, which can avoid accidental voltage impact hazards. The ground line is connected to the grounding end of the machine, and the basic unit is grounded. If an extended unit is used, its grounding point should be connected with the grounding point of the basic unit. To prevent interference on the power supply, input, and output ends, a special ground wire should be connected to the programmable controller. The grounding point should be separated from the grounding point of the power equipment (such as a motor. If this requirement is not met, public grounding must also be achieved with other devices, and connection to other devices is prohibited. The grounding point should be * close to PLC as much as possible.
2.4 DC 24 V Terminal
When a passive contact input device is used, the 24 V power supply inside the PLC provides 7mA of the current per point to the input end through the input device.
The 24 V terminal on the PLC can also provide current to external sensors (such as proximity switches or photoelectric switches. When the 24 V terminal is used as the sensor power supply, the com terminal is the 24 v dc Ground end. If an extended crew member is used, the basic unit and the 24 V end of the extended unit should be connected. In addition, any external power supply cannot be connected to this terminal.
In case of overload, the voltage will automatically fall and the input will not work for the programmable controller.
The number of input points for each type of PLC is specified. For each unused input point, it does not consume power, so in this case, the 24 V power supply terminal can increase the ability to provide external current.
The vacant terminals of FX series PLC cannot be used under any circumstances.
2.5 input Wiring
PLC generally accepts switch signals from input parameters such as travel switch and limit switch. The input terminal is the port for converting signals between the PLC and external sensors. The input wiring refers to the wiring between the external sensor and the input port.
The input device can be any passive contact or an open-collector, on an tube. When the input device is connected, the input end is connected, the input line is closed, and the light emitting diode indicated by the input is on.
The primary and secondary circuits at the input end are isolated by photoelectrical coupling. The secondary circuit carries an RC Filter to prevent PLC misoperation due to input contact jitter or electrical noise from input line strings.
If the input contact circuit is connected to a series diode, the voltage on the series diode should be less than 4 V. The number of series diodes cannot exceed two if the flat switch with LEDs is used.
In addition, pay special attention to the following points for the input wiring:
(1) The input wiring generally should not exceed 30 m. However, if the environment interference is small and the voltage drop is not large, the input wiring may be longer.
(2) The same cable cannot be used for the input and output cables. The input and output cables must be separated.
(3) The Pulse Signal width acceptable to the programmable controller should be greater than the scanning cycle time.
2.6 Output Wiring
(1) programmable controllers are provided in three forms: relay output, thyristor output, and transistor output.
(2) wiring at the output end is divided into independent output and public output. When the PLC outputs a relay or Thyristor action, the two outputs of the same number are connected. Different types of output voltage and voltage levels can be used in different groups. However, the output in the same group can only use the same type of power supply with the same voltage level.
(3) Since the PLC output element is encapsulated on the printed circuit board and connected to the terminal board, if the Load connecting the output element is short-circuited, the printed circuit board will be burned out, apply the fuse to protect the output element.
(4) When the relays are output, the inductance load affects the working life of the relays. Therefore, the working life of the relays is required to be long.
(5) the output load of PLC may produce noise interference. Therefore, measures should be taken to control the output load.
In addition to taking into account the control procedures, external emergency parking circuits should also be designed for dangerous loads that can cause damage to users so that when the programmable controller fails, can cause damage to the load power supply cut off.
Do not use the same cable for the AC and DC output lines. The output lines should be kept away from the high-voltage lines and power lines to avoid parallelism.
3 conclusion
PLC is widely used in industrial control with its notable advantages. its practical application involves many problems. This article only puts forward some precautions for its on-site installation and maintenance, for the reference of PLC design and application personnel.