Fault analysis method of the hottest industrial in

2022-08-13
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Industrial instrument fault analysis method

investigation method

through the investigation and understanding of the fault phenomenon and its generation and development process, analyze and judge the cause of the fault. Generally, there are the following aspects:

(1) the use situation before the failure and whether there is any omen

(2) whether there is ignition, smoking, abnormal smell and other phenomena when the fault occurs

(3) change of power supply voltage

(4) overheating, lightning, humidity, collision and other external conditions

(5) whether it is disturbed by external strong electric and magnetic fields

(6) whether there is improper use or misoperation

(7) the fault occurred in the normal function or after the repair and replacement of components

(8) what faults and repairs have occurred before

use the investigation method to repair the fault. The investigation and understanding should be in-depth and careful, especially the feedback of the on-site users should be verified, and do not rush to disassemble and repair. Maintenance experience shows that many of the user's comments are incorrect or incomplete, and many problems that do not need maintenance can be found through verification

visual inspection method

it is a method to observe and find faults through human senses (eyes, ears, nose, hands) without any testing instruments

visual inspection is divided into appearance inspection and startup inspection. The appearance inspection mainly includes:

(1) whether the instrument shell and dial glass are in good condition, whether the pointer is deformed or collided with the dial, whether the assembly fasteners are firm, whether the position of each switch knob is correct, whether the movable part rotates flexibly, and whether the adjustment part has obvious changes

(2) whether the wiring is disconnected, whether the connectors are normally connected, whether the spring on the circuit board socket is insufficient in elasticity and poor in contact, and for instruments assembled by unit assembly, pay special attention to whether the connecting screws of each unit board are tightened

(3) whether the contacts of each relay and contactor are misaligned, stuck, oxidized, scorched and stuck

(4) whether the power fuse is blown, whether the electronic tube is cracked, leaked (a layer of white powder is attached to the inner wall of the tube after the leakage), damaged, whether the transistor shell paint changes color, breaks the pole, whether the resistance is charred, whether the coil is broken, and whether the capacitor shell expands, leaks liquid, and bursts

(5) whether the copper coated strip on the printed board is broken, tinned or short circuited, whether the solder joints of various components are in good condition, and whether there is faulty soldering, missing soldering or desoldering

(6) whether the arrangement and wiring of parts and components are skewed, misaligned, fallen off and collided

startup inspection mainly includes:

(1) whether the power indicator light, electronic tubes and other light-emitting components in the machine are powered on and lit

(2) whether there is high-voltage ignition, discharge and smoking in the machine

(3) whether there is vibration and crackling sound, friction sound and impact sound

(4) whether the temperature rise of transformer, motor, power amplifier tube and other easily heated components, resistors and integrated blocks is normal, and whether it is hot

(5) whether there is any special smell in the machine, such as the burnt smell of transformer resistance due to the burning of insulation layer, and the self oxygen smell produced by the ionization of air caused by high-voltage leakage ignition of oscilloscope tube

(6) whether the mechanical transmission part operates normally, and whether there are phenomena such as poor gear meshing, jamming, serious wear, slipping deformation, poor transmission, etc

visual inspection must be very careful and careful, and carelessness and impatience must be avoided. When checking the components and wiring, you can only gently shake it, not too hard, to prevent breaking the components, wiring and copper foil of the printed board. Don't leave the power switch with your hand when you turn on the power supply. If you find any abnormality, turn it off in time. Pay special attention to personal safety and absolutely avoid touching live equipment with both hands at the same time. The high-capacity filter capacitor in the power circuit has charging charge in the circuit, so it is necessary to prevent electric shock

open circuit method

disconnect the suspected part from the whole machine or unit circuit to see whether the fault can disappear, so as to determine the fault

after the instrument fails, first preliminarily judge several possibilities of the failure. In the fault area, disconnect the suspected part of the circuit to determine whether the fault occurred before or after disconnection. If the power on inspection finds that the fault disappears, it indicates that the fault is mostly in the disconnected circuit. If the fault still exists, further open circuit segmentation inspection shall be carried out to gradually eliminate the doubt and narrow the scope of the fault until the real cause of the fault is found

the open circuit method is particularly convenient for the fault inspection of unitized, combined and plug-in instruments and meters, and it is also very effective for some short-circuit faults with excessive current. However, the closed system loop or direct coupling circuit structure whose overall circuit is a large loop should not be used

short circuit method

it is a method to temporarily short circuit a certain level of circuit or component suspected of failure, and observe whether the fault state changes to determine the fault location

when the short-circuit method is used to check the multi-level circuit, the fault disappears or decreases significantly, indicating that the fault is before the short-circuit point, and the fault without change is after the short-circuit point. If the output potential of a stage is abnormal, short circuit the input of the stage. If the output potential is normal, the circuit of the stage is normal. The short-circuit method is also commonly used to check whether the components are normal. For example, use tweezers to short circuit the base and emitter of the crystal triode, observe the change of collector voltage, and judge whether the tube has amplification effect. In TTL digital integrated circuit, the short circuit method is used to judge whether the gate circuit and trigger can work normally. Short circuit the thyristor control electrode and cathode to judge whether the thyristor is invalid, etc. In addition, the input terminal of some instruments (such as electronic potentiometer) can also be short circuited, and the change of instrument indication can be used to judge whether the instrument is disturbed by the bending resistance test of cemented carbide

replacement method

a method to determine the fault at a certain part by replacing some components or circuit boards

replace the suspected components with components with the same specification and good performance, and then conduct power on test. If the fault disappears, it can be determined that the suspected component is the fault. If the fault still exists, the same replacement test can be carried out on another suspected component or circuit board until the fault location is determined

before replacement, take a little time to analyze the cause of the fault, rather than blindly replacing components. If the fault is caused by short circuit or thermal damage, the good component on the replacement may also be damaged. For another example, the basic requirement of a pair of compact models is that the two supporting end faces should be parallel to each other, and only the diode is burnt out, which may be due to the insufficient working current and reverse peak voltage of the tube. If another diode of the same model is replaced at this time, the fault is only temporarily handled, but not eradicated

in addition, the power supply should be cut off for the replacement of components, and it is not allowed to test while welding while being powered on. When the replaced components are installed and welded, they should conform to the original welding installation methods and requirements. For example, insulating fins are usually added between high-power transistors and heat sinks. Do not forget to install them. When replacing, pay attention not to damage other surrounding components, so as to avoid causing human failure

division method

in the process of finding faults, the circuit and electrical parts are divided into several parts to find out the cause of faults

generally, the detection and control instrument circuit can be divided into three parts, namely, external circuit (all circuits from the terminal of the instrument to the detection element and control actuator), power circuit (all circuits from AC power supply to power transformer), internal circuit (all circuits except partial circuit and power circuit). In the internal circuit, it can be divided into several small parts (according to the characteristics of its internal circuit and the structure of electrical components). Divisional inspection refers to the inspection of each part from the outside to the inside, from the big to the small, from the outside to the inside, according to the divided parts, so as to gradually reduce the scope of suspicion. After checking and determining which part of the fault is, conduct a comprehensive inspection on this part to find the fault part

divisional inspection inspects, analyzes and judges each part of the instrument in sequence. Although it is relatively organized, the maintenance time is long, and the key points are often not grasped in the inspection, which wastes a lot of time. This method is applicable to the situation that the maintenance personnel have less maintenance experience, are not familiar with the fault phenomenon of instruments and meters, and the fault is more complex

human body interference method

when people are in a messy electromagnetic field (including the electromagnetic field generated by alternating current), they will induce a weak low-frequency electromotive force (nearly dozens to hundreds of microvolts). When hands touch some circuits of instruments and meters, the circuit will react. Using this principle, we can simply judge some fault parts of the circuit

pay attention to the environment when using human interference method. For example, there are few electrical equipment and lines, basements, some reinforced buildings, etc., and the signal generated by interference will be smaller. At this time, a long wire can be used to replace the hand to obtain a larger interference signal. In addition, when using this method to check the high-voltage part of instruments and meters or the instruments and meters with electrified bottom plate, you must pay great attention to safety to avoid electric shock

voltage method

voltage method is to measure the suspected part with a multimeter (or other voltmeter) with an appropriate range, including AC voltage and DC voltage. Measuring AC voltage mainly refers to AC power supply voltage, such as AC 220V voltage, AC regulator output voltage, transformer coil voltage and oscillation voltage; The measured DC voltage refers to the DC supply voltage, the working voltage of each pole of electronic tubes and semiconductor components, and the voltage of each lead out angle of the integrated block to the ground

voltage method is one of the most basic methods in maintenance work, but the fault scope it can solve is still limited. Some faults, such as slight coil short circuit, capacitor disconnection or slight leakage, are often not reflected in the DC voltage. In case of some faults, such as short circuit, smoking and fire jumping of components, the power supply must be turned off. At this time, the voltage method will not work, and other methods must be used to check

current method

current method is divided into direct measurement and indirect measurement. Direct measurement is to connect the circuit with an ammeter after disconnection, and measure 1 Test standard: air spring performance test standard qc/t 207 ⑴ 996 and jb/t 8064.1 ⑼ 6 output current value shall be compared with the data of the instrument under normal working state, so as to judge the fault. If it is found that which part of the current is not within the normal range, it can be considered that there is a problem with this part of the circuit, at least affected. Indirect measurement does not need to disconnect the circuit, measure the voltage drop on the resistance, and calculate the approximate current value according to the resistance value. It is mostly used for the current measurement of transistor elements

the current method is more troublesome than the voltage method. Generally, the circuit needs to be disconnected and connected into the ammeter until they are broken. However, in some cases, it is more prone to fault than the voltage method. With the cooperation of current method and voltage method, most faults in the circuit can be checked and judged

resistance method

resistance check method is to check whether the input and output resistances of the complete circuit and some circuits of the instrument are normal with the resistance block of the multimeter under the condition of no power supply; Whether each resistance element is open circuit, short circuit, and whether the resistance value changes; Whether the capacitor breaks down or leaks electricity; Whether the inductance coil and transformer are disconnected or short circuited; Forward and reverse resistances of semiconductor devices; The resistance of the lead out pin of each integrated block to the ground; And roughly judge the transistor value; Whether there is a short circuit between the poles of the electron tube and the oscilloscope tube, and whether the filament is

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