PLC comprehensive failure reasons

1

Grounding Problems

The grounding requirements for the PLC system are relatively strict. It is best to have an independent dedicated grounding system. Also, attention should be paid to the reliable grounding of other equipment related to the PLC.

When multiple circuit ground points are connected together, unexpected currents can flow, causing logic errors or damaging circuits.

The reason for different ground potentials is usually that the grounding points are separated too far in physical area. When devices that are far apart are connected by communication cables or sensors, the current between the cable and the ground will flow through the entire circuit. Even within a short distance, the load current of large equipment can change between its potential and the ground potential, or directly generate unpredictable currents through electromagnetic effects.

Between power supplies with improper grounding points, destructive currents may flow in the circuit, destroying equipment.

PLC systems generally use a single-point grounding method. In order to improve the ability to resist common-mode interference, shielded floating ground technology can be used for analog signals, that is, the shielding layer of the signal cable is grounded at one point, the signal loop is floating, and the insulation resistance with the ground should be no less than 50MΩ.

2

Interference handling

The industrial field environment is relatively harsh, with many high and low frequency interferences. These interferences are usually introduced into the PLC through the cables connected to the field equipment.

In addition to grounding measures, some anti-interference measures should be taken during the design, selection and installation of cables:

(1) Analog signals are small signals and are easily affected by external interference, so double-shielded cables should be used;

(2) Shielded cables should be used for high-speed pulse signals (such as pulse sensors, counting encoders, etc.) to prevent external interference and high-speed pulse signals from interfering with low-level signals;

(3) The communication cable between PLCs has a high frequency. Generally, the cable provided by the manufacturer should be selected. If the requirements are not high, a shielded twisted pair cable can be selected.

(4) Analog signal lines and DC signal lines cannot be routed in the same wire duct as AC signal lines;

(5) The shielded cables leading into and out of the control cabinet must be grounded and should not be directly connected to the equipment through the wiring terminals;

(6) AC signals, DC signals and analog signals cannot share the same cable, and power cables should be laid separately from signal cables.

(7) During on-site maintenance, the following methods can be used to resolve interference: using shielded cables for the affected lines and re-laying them; adding anti-interference filtering codes to the program.

3

Eliminate inter-wire capacitance to avoid false operation

There is capacitance between each conductor of the cable, and a qualified cable can limit this capacitance within a certain range.

Even if the cable is qualified, when the cable length exceeds a certain length, the capacitance between the lines will exceed the required value. When this cable is used for PLC input, the capacitance between the lines may cause the PLC to malfunction, resulting in many incomprehensible phenomena.

These phenomena are mainly manifested as: the wiring is correct, but there is no input to the PLC; the input that the PLC should have is not there, but the input that it should not have is there, that is, the PLC inputs interfere with each other. To solve this problem, you should do the following:

(1) Use cables with twisted cores;

(2) Try to shorten the length of the cable used;

(3) Use separate cables for inputs that interfere with each other;

(4) Use shielded cable.

4

Output module selection

Output modules are divided into transistor, bidirectional thyristor, and contact type:

(1) The transistor type has the fastest switching speed (generally 0.2ms), but the smallest load capacity, about 0.2~0.3A, 24VDC. It is suitable for equipment with fast switching and signal connection. It is generally connected to signals such as frequency conversion and DC devices. Attention should be paid to the impact of transistor leakage current on the load.

(2) The advantages of the thyristor type are that it has no contacts, has AC load characteristics, and has a small load capacity.

(3) Relay output has AC and DC load characteristics and large load capacity. In conventional control, relay contact type output is generally used first. The disadvantage is that the switching speed is slow, generally around 10ms, and it is not suitable for high-frequency switching applications.

5

Inverter overvoltage and overcurrent processing

(1) When the given speed is reduced to slow down the motor, the motor enters the regenerative braking state, and the energy fed back to the inverter by the motor is also high. This energy is stored in the filter capacitor, causing the voltage on the capacitor to increase and quickly reach the setting value of the DC overvoltage protection, causing the inverter to trip.

The solution is to add a braking resistor outside the inverter and use the resistor to consume the regenerative electric energy fed back to the DC side by the motor.

(2) The inverter is connected to multiple small motors. When an overcurrent fault occurs in one of the small motors, the inverter will issue an overcurrent fault alarm, causing the inverter to trip, thereby causing other normal small motors to stop working.

Solution: Install a 1:1 isolation transformer on the output side of the inverter. When one or more small motors have an overcurrent fault, the fault current will directly impact the transformer instead of the inverter, thus preventing the inverter from tripping. After the experiment, it works well and the previous fault of normal motors stopping has not occurred.

6

Inputs and outputs are labeled for easy maintenance

PLC controls a complex system. All you can see are two rows of staggered input and output relay terminals, corresponding indicator lights and PLC numbers, just like an integrated circuit with dozens of pins. Anyone who does not look at the schematic diagram to repair a faulty device will be helpless and the speed of finding the fault will be very slow. In view of this situation, we draw a table based on the electrical schematic diagram and stick it on the console or control cabinet of the equipment, indicating the electrical symbol and Chinese name corresponding to each PLC input and output terminal number, which is similar to the functional description of each pin of the integrated circuit.

With this input and output table, electricians who understand the operation process or are familiar with the ladder diagram of this equipment can start maintenance.

However, for those electricians who are not familiar with the operation process and cannot read ladder diagrams, they need to draw another table: PLC input and output logic function table. This table actually explains the logical correspondence between the input circuit (trigger element, associated element) and the output circuit (actuator) in most operation processes.

Practice has proved that if you can skillfully use the input-output correspondence table and the input-output logic function table, you can easily repair electrical faults without drawings.

7

Inferring Faults through Program Logic

There are many types of PLCs commonly used in industry today. For low-end PLCs, the ladder diagram instructions are similar. For mid- to high-end machines, such as S7-300, many programs are written using language tables.

Practical ladder diagrams must have Chinese symbol annotations, otherwise it will be difficult to read. If you can have a general understanding of the equipment process or operation process before reading the ladder diagram, it will seem easier.

If an electrical fault analysis is to be performed, the reverse search method or reverse reasoning method is generally used, that is, according to the input-output correspondence table, the corresponding PLC output relay is found from the fault point, and then the logical relationship that satisfies its action is reversed.

Experience shows that if one problem is found, the fault can be basically eliminated, because it is rare for two or more fault points to occur simultaneously in the equipment.

8

PLC self-fault judgment

Generally speaking, PLC is an extremely reliable device with a very low failure rate. The probability of damage to hardware such as PLC and CPU or software errors is almost zero. The PLC input point will hardly be damaged unless it is caused by strong electric intrusion. The normally open point of the PLC output relay will have a long contact life unless the peripheral load is short-circuited or the design is unreasonable, and the load current exceeds the rated range.

Therefore, when we look for electrical fault points, we should focus on the PLC's peripheral electrical components and not always suspect that there is a problem with the PLC hardware or program. This is very important for quickly repairing faulty equipment and resuming production.

Therefore, the electrical fault inspection and repair of the PLC control circuit discussed by the author does not focus on the PLC itself, but on the peripheral electrical components in the circuit controlled by the PLC.

9

Make full and reasonable use of software and hardware resources

(1) Instructions that do not participate in the control cycle or have been entered before the cycle do not need to be connected to the PLC;

(2) When multiple instructions control a task, they can be connected in parallel outside the PLC and then connected to an input point;

(3) Make full use of the PLC internal functional soft components and fully call the intermediate state to make the program complete and coherent and easy to develop. At the same time, it also reduces hardware investment and reduces costs;

(4) If conditions permit, it is best to make each output independent, which is convenient for control and inspection and also protects other output circuits; when an output point fails, it will only cause the corresponding output circuit to lose control;

(5) If the output is a forward/reverse controlled load, not only must the PLC internal program be interlocked, but measures must also be taken outside the PLC to prevent the load from moving in both directions;

(6) PLC emergency stop should be cut off using an external switch to ensure safety.

10

Other considerations

(1) Do not connect the AC power cord to the input terminal to avoid burning the PLC;

(2) The grounding terminal should be grounded independently and not connected in series with the grounding terminal of other equipment. The cross-sectional area of the grounding wire should not be less than 2mm²;

(3) The auxiliary power supply is small and can only drive low-power devices (photoelectric sensors, etc.);

(4) Some PLCs have a certain number of occupied points (i.e. empty address terminals), do not connect the wires;

(5) When there is no protection in the PLC output circuit, a protective device such as a fuse should be connected in series in the external circuit to prevent damage caused by load short circuit.