1 The contact is overheated
Contact heat is mainly caused by the current flowing through the contact, and there is power loss between the contacts (such as contact resistance loss, arc energy loss). It is normal for the contacts to heat up, but severe heat (overheating) is a malfunction.
The main reasons for contact overheating failure should be found from the following aspects.
1. 1 Contact pressure is reduced
During use, the spring of the contact is deformed due to mechanical damage; or under the action of the arc, the spring is annealed at high temperatures; Reduced moving contact pressure; Or the contact itself is deformed, for example, the two flaps of the static contact in the knife switch are opened outward, so that the contact pressure of the contact is reduced.
1. 2 The contact surface is oxidized or there are impurities
Many metal oxides are undesirable conductors that can increase the contact resistance of the contact, causing the contact to overheat. The higher the contact temperature during operation, the more serious the oxidation; The greater the contact resistance, the faster the temperature rises. For contacts with severe oxidation, it can be gently sanded with 00 diamond sandpaper. Many contacts are inlaid with silver nuggets on their contact surfaces, and the oxide of silver is a good conductor and can be left untreated. Once the dust and dirt on the surface of the contact form an insulating film, the contact resistance will be greatly increased, and it should be removed regularly.
1. 3 Excessive contact wear
If the switch is used for a long time, the pressure will be reduced after the contact wear exceeds a certain amount, and the contact will also overheat, so the contact stroke should be adjusted appropriately.
2 Arc extinguishing system failure
The arc extinguishing device is an important part of the switch, especially for the circuit breaker and fuse.
If the switch does not extinguish the arc properly, it will cause the switch to be damaged, which in turn will cause a greater failure of the electrical installation. The main causes of arc extinguishing system failures are described below.
2. 1 The arc extinguishing hood is damp
The arc extinguishing cover is usually made of asbestos cement board or fiberboard, which is susceptible to moisture. After moisture, the insulation is reduced and the arc cannot be elongated. At the same time, when the arc is burned, the water in the arc extinguishing hood vaporizes under the action of high arc temperature, which causes the spatial pressure of the upper part of the arc extinguishing hood to increase, which prevents the arc from entering the arc extinguishing hood and prolongs the arc extinguishing time. This kind of fault is relatively easy to judge. Normally, the arc ejects a small range out of the arc extinguishing hood, and a crisp sound can be heard. If the arc spray range is very large, and you hear a weak "Bubu" sound, as well as the contact is seriously singeed, and there is a phenomenon such as burnt arc extinguishing cover, it means that the arc extinguishing cover has been damp. At this time, just dry the arc extinguishing hood.
2. 2 Arc extinguishing hood carbonization
Under the condition of high temperature, the surface of the arc extinguishing cover is scorched, forming a carbonaceous conductive bridge, which is very unfavorable to arc extinguishing and should be treated in time. The treatment method can be to file it gently with a fine file, but not to increase the roughness, because the rough surface will increase the resistance of the arc.
2. 3 Short circuit of magnetic blowing coil
In order to introduce arc into the arc extinguishing hood, some switches commonly use a magnetic blowing coil. This kind of coil is generally insulated with air, and no additional insulating material is added. If the coil is deformed by collision and conductive dust accumulates too much, there will be a short circuit between the coil lines or turns, which makes the coil unable to work and reduces the arc extinguishing performance of the switch.
2. 4 Arc extinguishing grid is damaged
The metal arc extinguishing grid is damaged, falling off and corroding, so that the arc can not be pulled into the grid smoothly, which affects the arc extinguishing effect, so it should be repaired in time. The arc extinguishing grid looks like copper on the outside, but in fact, most of it is made of steel, and only a layer of copper is plated on the surface. Damaged grids can be replaced with ordinary white iron sheets.
2. 5 Failure of arc extinguishing contacts
The arc extinguishing contact plays the role of attracting arc and protects the main contact.
Its basic working procedure is: close before the working contact, then open before the working contact. If the wear is severe or the assembly is not reasonable, it will lose its function, therefore, it should be checked and adjusted regularly.
2. 6 The quality of the insulating oil decreases
The insulating oil in the switch often has the functions of arc extinguishing, cooling and insulation. The quality of the insulating oil decreases (mainly due to the increase of moisture and impurities), and the insulation performance decreases, which is extremely unfavorable for arc extinguishing, and may even cause serious accidents such as explosion and oil injection. Therefore, it should be inspected regularly, filtered or replaced.
3 Suction solenoid failure
Suction solenoids are one of the main components of many automatic switches (such as contactors, circuit breakers) operating devices, which play the role of making the switch automatically turn on or off. In an AC circuit, the magnetic flux in the core is alternating and the suction force is alternating, which will cause the armature to vibrate and the switch to work unreliably. In order to overcome this shortcoming, a short-circuit ring is installed on the end face of the core, and the phase of the induced magnet generated by the induced current in the short-circuit ring is different from that of the main magnet in the core. In this way, the resultant flux is not equal to zero at any time, and the armature does not vibrate.
The common faults and causes of electromagnets are as follows.
3. 1 Loud noise
The normal operation of the solenoid only emits a uniform, harmonized, slight working sound, if the noise is very large, it means that there is a fault, and the reasons are as follows:
The iron core is in poor contact with the armature end face. Due to the wear and rust of the end face, or the presence of dust, oil scale and other impurities, the air gap between the end faces increases, the excitation current of the electromagnet increases, and the vibration is violent, which increases the noise.
The iron core and armature end face are finished, and generally cannot be smoothed with tools such as files and abrasive cloths, as long as they are cleaned with gasoline and kerosene.
If you use a file or abrasive cloth to repair, you can do it according to the following methods: firstly, line a layer of carbon paper on the end face, after the armature is engaged, the protruding part of the end face is printed with spots on the carbon paper, and then gently file the spots, and after repeating several times, the end face can be flattened.
Damaged short-circuit rings. The short-circuit ring is specially designed to prevent vibration, and the short-circuit ring breaks or falls off. It will make the core noise due to vibration.
Once found, just use copper material to process one and replace it.
The voltage is too low. If the voltage applied to the coil is too low, generally less than 85% of the rated voltage, the suction power will be insufficient, the excitation current will increase, and the noise will also increase.
The movement part is jammed. When the moving part of the armature driving switch is jammed, the reaction force increases, and the armature cannot be engaged normally, resulting in vibration and noise. Therefore, a few drops of light oil, such as engine oil, transformer oil, etc., should be added to the moving friction parts frequently.
3. 2 The coil is overheated or even burned
The reason for the overheating of the coil is due to the current flowing in the coil, which exceeds the rated value for a long time, and the magnitude of the current in the coil is related to the voltage applied to both ends of the coil, related to the load driven by the armature, and mainly related to the coil excitation current required by the magnetic circuit. When the armature is opened, the air distance is large, the magnetic circuit reluctance is large, and the coil excitation current required to generate the same magnetic flux is large. When the armature is closed, the magnetic circuit is less reluctant, and the excitation current is much smaller. It has been calculated that the excitation current of the armature at start-up is dozens of times greater than when it is closed. Prolonged overheating of the coil is the main cause of coil burnout. The main reasons are as follows.
Switching operations are frequent, need to be started frequently, and the coil is frequently subjected to high currents.
The armature is not in close contact with the core end face, and the large air gap makes the current in the coil much larger than the rated value.
If the armature is not installed well, the end face of the core is not aligned with the end face of the armature, which increases the magnetic resistance of the magnetic circuit and increases the current in the coil.
The transmission part is overloaded with a jamming solenoid and cannot be well engaged.
If the coil voltage is too low, the current in the coil will inevitably increase if the same load is driven.
If the voltage at the end of the coil is too high and the magnetic flux of the core is saturated, it will also cause the core to overheat.
The coil insulation is damp and there is an inter-turn short circuit, which will also increase the current in the coil.
4 Failure of the fuse
The fuse is a simple protection appliance in the circuit, which is used in series in the circuit, and can be used to protect the electrical device from the damage of overload current and short-circuit current, and its structure is composed of melt, contact and shell connecting the melt. Some fuses also have a simple arc extinguishing device to improve the arc extinguishing capacity of the fuse. Due to the simple structure and maintenance of the fuse and its small size, it is widely used in low-voltage power distribution devices, and is also widely used to protect voltage transformers and small-capacity electrical equipment in high-voltage power grids from 3 to 35 kV.
The melt is generally made into three shapes, namely filamentous, sheet-like, and cage-like.
Lead-tin melts are generally made into filaments; The zinc-tin melt is generally made into sheets; Silver melt is made into filaments or flakes; Copper melt can be made into filaments, sheets, and cages.
When the current in the circuit, i.e., the current of the two fuse fuses, reaches a certain value, the fuse will blow. The fault of the fuse is mainly manifested in the fact that the fuse is often abnormally burned, the connecting screws of the fuse are burned, and the service life of the fuse is reduced. The following scenarios should be considered to find the fault of a fuse.
4. 1 Whether the selection of fuses is reasonable
The fuse of the fuse should be selected according to the size of the load and the nature of the load.
(1) For electric lighting loads, the rated current of the load is selected, that is, the rated current of the fuse is greater than or equal to the rated current of the load.
(2) When selecting the fuse for the load selection of the asynchronous motor, the rated current should be considered, which is larger than the rated current of the motor.
(3) The rated current of the total fuse of multiple motors should consider the sum of 2-3 times the rated current of one motor with the capacity and the rated current of other motors.
(4) The selection of the rated current of the drop fuse on the high voltage side of the transformer should consider the excitation current when the transformer is closed, which is generally 2-3 times of the rated current. The size of the fuse is too small, which will inevitably make the fuse blow abnormally. For example, a three-phase electric heater with a capacity of 3 kW and a three-phase electric motor may be suitable for the electric heater if the fuse specifications are consistent, but when the motor is started, the fuse will blow abnormally.
(5) The selection of the rated current of the melt on the high and low voltage side of the distribution transformerFor the distribution transformer with a capacity of 100 kVA and below, the rated current of the melt on the high voltage side should be selected according to 2-3 times of the rated current on the high voltage side of the transformer.
For distribution transformers with a capacity of 100 kVA and above, the rated melt current on the high side of the transformer should be 1 of the rated current on the high side of the transformer. 5 - 2 times the selection.
The rated current of the low-voltage fuse can be 1. 5x pick.
(6) The rated current of the fuse should be greater than or equal to the rated current of the fuse. This is often the case, the fuse is suitable, but the fuse box of the fuse is smaller. In this way, the heat dissipation condition of the fuse is poor, and it is easy to make the fuse abnormal blowing.
4. 2 The fuse installation is unreasonable
The fuse end should be wound correctly, if it overlaps or winds in the opposite direction, it will make the fuse and the fuse terminal poor contact or the joint heat, so that the fuse is abnormal fuse.
During installation, the fuse will be blown abnormally if the fuse is pulled too tightly to reduce the cross-sectional area of the fuse, or the fuse will be too bent to increase the heat generation of the fuse.
When one fuse is not enough and multiple fuses are needed, it is generally not possible to twist them into a strand for use, because this will reduce the total capacity of the fuse and may also cause abnormal blowing.
The screws fixing the fuse must be flattened with a gasket, and some also need to add a spring gasket, otherwise it is easy to cause the fuse to blow abnormally.
For fuses equipped with quartz sand, when replacing the fuse, it is necessary to replace all the quartz sand. The new quartz sand must be dry and the purity should not be less than 95%.
Otherwise, the fuse will also be blown abnormally due to the reduction of the arc extinguishing ability of the fuse.
4. 3 Precautions for the use of the fuse
The melt should be selected correctly to ensure the selectivity of its work.
The rated current of the melt installed in the fuse can only be less than or equal to the rated current of the fuse.
After the melt is fused, the melt of the same size and material should be replaced, and it should not be thickened or reduced at will, and other metal wires that are not easy to fuse should be used to replace it, so as not to cause accidents.
When installing the melt, the melt itself should not be damaged. Otherwise, it may be burned out when the normal working current passes, causing unnecessary power outages.
The melt ends of the fuse should be in good contact.
When replacing the melt, the power supply should be cut off, and the fuse should not be pulled out in the case of live electricity.
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