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July 16,2024
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.

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July 05,2024
Common Motor Failures and Inspection Maintenance

    Common Motor Failures   1.Abnormal startup or abnormal speed after startup 1)Stator circuit (power supply, switch, contactor, leads, windings) missing phase. 2)Rotor cage breakage (ring breakage, bar breakage). 3)Rotor rubbing against stator, or mechanical drag causing jamming. 4)Incorrect stator circuit wiring (winding polarity or star/delta configuration). 5)Low power supply voltage.   2.Overheating or smoking 1)Power aspect High or low voltage, or phase loss. 2)Motor itself Stator winding inter-turn or turn-to-turn short circuit or ground, rotor bar breakage or stator/rotor rubbing. 3)Load aspect Mechanical overload or jamming. 4)Ventilation and heat dissipation aspect High ambient temperature, excessive dirt on casing, blocked air ducts, damaged or improperly installed fan.   3.Bearing operating temperature is too high 1)High bearing running temperature Bearing running temperature should generally not exceed 95°C. 2)Improper, deteriorated, excessive, or inadequate lubricating oil. 3)Bearing wear, rust, spalling, inner or outer race running, or improper assembly of inner and outer covers. 4)Misalignment of couplings or over-tightened belts.   4.Abnormal noise or strong vibration 1)Stator-rotor rubbing or severe wear deformation of driven machinery. 2)Uneven foundation, weak base, or loose anchor bolts. 3)Coupling misalignment or bent shaft. 4)Rotor eccentricity, rotor imbalance, unbalanced driven machinery, or bearing eccentricity. 5)Oil shortage or damage to bearings. 6)Rotor bar breakage. 7)Phase loss or overloaded operation.     Motor Inspection   1.Pre-operation inspection 1)Check if the casing is clean, inspect for dust and dirt inside open motors. 2)Disconnect cables and terminal boards, measure winding resistance and insulation to ground. 3)Verify correct stator winding connection and power supply voltage as per nameplate. 4)Manually rotate motor rotor and drive system, check for obstructions and bearing lubrication. 5)Ensure ventilation system is unobstructed, and all fasteners are secure. 6)Check grounding of motor.   2.Operational inspection 1)During normal operation, current and voltage should not exceed rated values. Phase current imbalance should not exceed 10%, phase voltage imbalance should not exceed 5%, and allowable voltage fluctuation is within -5% to +5% of rated voltage, not to exceed 10%. 2)Ensure temperature measurement devices are working, temperature rise within specified range. 3)Normal sound and vibration, no abnormal odors. 4)Proper bearing lubrication, flexible rotation of oil ring. 5)Cooling system in good condition. 6)Clean surroundings without debris, leaks of water, oil, or air. 7)Protective covers, terminal boxes, grounding wires, control boxes intact.    Motor Maintenance   1)Keep motor surroundings clean and free of debris. 2)Regular inspection, address anomalies, record defects. 3)Prevent water or steam leaks around, avoiding motor dampness affecting insulation. 4)Regularly change lubricating oil, typically every 1000 hours for plain bearings, and 500 hours for roller bearings. 5)Periodically inspect insulation of standby motors, address non-compliance promptly.

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June 20,2024
How to manually control Yaskawa motor?

(1). Manual Control Method The Yaskawa drive can achieve manual control of motor rotation through the control panel. The specific method is as follows: 1. Open the control panel and enter manual mode. 2. Set the frequency to 0Hz first, then press the start button, the motor will stop at this time. 3. Press the forward or reverse button, the motor will rotate in the set direction. 4. The motor speed can be adjusted by setting the frequency. Note: When manually controlling the motor rotation, one should keep a clear mind to ensure their safety.   (2). Precautions 1. Before performing manual control, ensure that the equipment has been correctly electrically connected and mechanically installed. 2. Understand the basic operation methods of the equipment first and then manually control it ensuring safety. 3. When manually adjusting the motor speed, gradually increase or decrease the frequency to avoid frequent changes causing overload and affecting the equipment's lifespan. 4. After manual operation, thoroughly stop the motor rotation, and turn off the control panel to avoid safety hazards.   (3). Common Issues 1. The motor may not rotate steadily during manual control, which could be due to incorrect electrical connections or excessive motor load. 2. Noise and unusual smells during manual control may indicate mechanical faults in the equipment. 3. If the control panel fails to start or adjust the frequency after starting, it could be due to a malfunction in the control panel itself. 4. If the above problems cannot be resolved, promptly contact equipment maintenance technicians for assistance.   In conclusion, Yaskawa drive is a high-precision driving device, and the correct manual control method is crucial for enhancing equipment operation efficiency and ensuring the safety of operators.

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April 15,2024
AB PLC series description

The PLC-5 controller is in the central position of the control system, integrates the existing and future systems through ethernet/ip, ControlNet and DeviceNet, and provides the interconnection between SLC 500, ControlLogix and Micrologix processors. Because the PLC-5 processor has built-in network connection, PLC-5 makes the control structure flexible enough to establish economic connection between a wide range of equipment.       The minimum configuration of a PLC-5/1771 control system includes a programmable controller module and some input and output modules and power supply modules installed on a rack. The controller with communication port can be selected as required. PLC-5 can reach 512 input and output points at most. All PLC-5 processors have remote I / O interfaces. Some PLC-5 processors have local extended I / O interfaces. Some PLC-5 processors have local extended I / O interfaces. Some PLC-5 processors have a ControlNet communication interface. If you want to provide a DeviceNet I / O scanner port for the system, you must add a DeviceNet scanner module (1771-SDN).       PLC-5 is a large, stable and early product of Rockwell Automation   Worldwide, more than 450000 sets of PLC-5 and more than 10000000 PLC-5 1771 i/o modules are operating stably.   PLC-5 has a module MTBF index of more than 400000 hours.   PLC-5 hot standby system can be used for occasions with high control safety requirements.       In recent years, PLC-5 has added ControlNet, DeviceNet, ethernet/ip and other industrial network interface functions.       PLC-5 controllers can be divided into the following categories:       1. Classic PLC-5 controller   There are several CPU models:   Product order number (model) corresponding to processor name   PLC-5/10 1785-LT4   PLC-5/12 1785-LT3   PLC-5/15 1785-LT   PLC-5/25 1785-LT2       2. Enhanced PLC-5 controller   There are several CPU models:   1785-L11B、1785-L20B、1785-L30B、1785-L40B、1785-L60B、1785-L80B   DH+ or (and) remote input / output communication interface (Remote I/O) is generally provided.       3. Ethernet PLC-5 controller   There are several CPU models:   1785-L20E、1785-L40E、1785-L80E   For the above three CPUs, the Ethernet interface is a built-in standard configuration. DH+ or Remote I / O interface is also provided       4. Control network PLC-5 controller   There are several CPU models:   1785-L20C15、1785-L40C15、1785-L46C15、1785-L80C15。   The above four CPUs have built-in ControlNet network communication function, and also provide dh+ and remote input / output communication connection function.       5. Protective PLC-5 controller   There are several CPU models:   1785-L26B、1785-L46B、1785-L46C15、1785-L86B。 The safe controller allows the user to set access to "critical" or "private" program areas, protected memory areas, protected input and output, etc., and can also restrict the operation of the controller. Users can be classified and managed by programming software, so that they have different system permissions.       Except for the classic PLC-5 controller, the above five controllers are all equipped with 25 pin serial communication port.

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April 11,2024

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December 26,2023
ABB Compact 800 Controller

AC 800M   CPU module   Various CPU modules can provide different functions, processing capacity, memory and redundancy support. Each CPU module is equipped with one or more Ethernet ports to exchange data with different controllers or interact with operators, engineers, managers and higher level applications. When availability becomes the most important, these Ethernet ports can be configured as redundant. Each CPU module is also equipped with two RS-232C ports, which can exchange point-to-point data with programming/debugging tools or third-party systems and devices.       Com and I/O module   For each CPU module, multiple communication and I/O modules can be added, such as:   ·Additional RS-232C ports   ·PROFIBUS DPDP - V1 interface   ·ABB INSUM interface   ·MasterBus 300 bus interface   ·S100 interface   ·S800L and S800 modules

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April 04,2024
Yokogawa CPU modules

Yokogawa CPU modules: SCP401, SCP451, SCP461, CP401, CP451, CP461, CP471, CP345, CP701, CP703       It seamlessly integrates distributed control systems (DCS) and safety instrumented system (SIS), simplifies the factory automation design and improves the integration of equipment.       Traditionally, DCS and SIS are two independent systems. Each system requires its own communication platform and hardware structure. Under such circumstances, it is necessary to spend a lot of engineering time and manpower and material resources to realize the optimal operation of the plant.       Advantages: supply Yokogawa DCS card / module / PLC, probe / sensor / cable (some products are available at affordable prices)

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