Due to the fact that the PLC of the pure water equipment is controlled by a computer system, it may be difficult to repair problems such as inability to start up, circuit faults, system errors, and so on. So how to deal with PLC circuit faults in pure water equipment?

1、 Common faults in motor control

1. In a narrow indoor control box located between the system software control circuit and the main control circuit, there will be interference between the control switch power supply and the main control circuit switch power supply.

2. The system software PLC, touch screen display, thermal relay alarm point, on-site water pump pressure switch, liquid level instrument high and low control point and other electrical equipment use the same 24V switch power supply. There are too many series and parallel control circuits, and if there is an abnormality at one point, it will cause damage to one or several power consuming machines and equipment on the circuit.

3. The magnetic coil of the DC contactor on the main circuit is immediately connected to the output point of the PLC, without any protective measures in the middle, which can easily cause excessive electrical flow at the PLC output point and damage the PLC.

4. The natural environment in the pure water room is relatively humid and cold, and the electrical equipment exposed outside is raised to a control box. The original control circuit in the control box is transferred to the newly raised control box to separate it from the main control circuit of the system software. This eliminates the impact of switching power supply on the control circuit and the main control circuit, and solves the problem of too many series and parallel control circuits for 24V switching power supply.

5. The communication port number of Siemens PLC series products and the configuration program flow of pure water in the control room are connected to the computer through an RS-485 communication optical cable. There is a floating working voltage between the positive and negative poles of the RS-485 line, which is very easy to cause the PLC communication port number to burn out.

The switch power supply of the EDI control module power supply system is balanced by the supply and demand of a large and medium-sized isolation transformer at the bottom of the control box. Its placement is at the bottom of the control box and the sealing of the control box is not very good, which is very easy to get dust and cause loose wiring terminal blocks, endangering the switch power supply balance of the EDI control module.

2、 Solution

1. Transfer the PLC from the original control box to the newly upgraded control box. Install a wiring terminal block at the location where the PLC was installed in the original control box, and connect the route to the wiring terminal block according to the wire number sequence.

2. Enhance the independent power supply system of the new isolation transformer to the PLC, ensuring the normal operation of all critical PLCs in the automatic control system.

3. Replace the CPU224XPCN Siemens PLCPLC in the original system software with CPU226CN Siemens PLCPLC, and make relative changes to the detailed addresses of the four AI input points.

4. Upgrade 30 OMRON-MY2NJ220/240AC intermediate relays (5 reserved). Connect all 14 points on the magnetic coil of the 30 intermediate relay in series to one end of the output terminal of the newly lifted isolation transformer. Connect the 13 points of the magnetic coil of the intermediate relay to the DO output point of the PLC in accordance with the electrical control system engineering drawing.

5. Upgrade a 24V switching power supply by splitting the original 24V power circuit into two intersections, one dedicated to the touch screen power supply system and the other to the PLC's DI input point and its AI control module power supply system. This includes thermal relay alarm points and on-site water pump pressure switch type high and low liquid level switch key input point switch power supply. The control circuit update and transformation diagram is shown in Figure 1. Causing common malfunctions in the electrical system for producing purified water.

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