Common faults and maintenance methods for low-voltage electrical appliances - Solutions - Huaqiang Electronic Network

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There are many low-voltage electrical appliances used in electrical circuits, and the structure is different. The first steps are to analyze the common faults and maintenance methods of the various components common to all low-voltage electrical appliances, and then analyze the common faults and maintenance methods of some common low-voltage electrical appliances.
1 Common faults and repairs of low-voltage electrical components
1.1 Contact failure and maintenance
(1) The contact is overheated. When the contact is turned on, the current will pass when it passes, and the contact will not overheat under normal conditions. When the dynamic contact contact resistance is too large or the current is too large, the contact will be overheated. When the contact temperature exceeds the allowable value, the contact characteristics will be deteriorated and even fusion welding will occur. The specific reasons for the overheating of the contacts are as follows: 1 The current between the moving and stationary contacts is too large. The contacts of any appliance must be operated at their rated current values ​​or the contacts will overheat. The system current voltage is too high or too low due to excessive contact current; the power equipment is overloaded; the electrical contact capacity is improperly selected and the fault operation is possible. 2 The contact resistance between the moving and moving contacts becomes large. The magnitude of the contact resistance is related to the degree of heat generation of the contact. The reasons for the increase are as follows: First, the pressure is insufficient due to the loss of the spring force of the contact pressure spring or the contact wear is thinned, and the spring or the contact should be replaced according to the situation; Poor contact on the contact surface. For example, in operation, dust and oil cover the surface of the contact, increasing the contact resistance; for example, when the contact is closed, the surface of the contact is burned and burnt due to the arc, resulting in incompleteness and reduced contact area. , resulting in poor contact. Therefore, care should be taken to enhance the maintenance of the running contacts. For the copper contact surface oxide layer and burnt various contacts can be corrected by scraper or fine boring; for large and medium current contact surfaces, not smooth, it is important to flatten; for small capacity contacts, surface quality is required Good; silver and silver-based contacts can be cleaned with cotton-impregnated gasoline or carbon tetrachloride, and the oxide layer does not affect the contact performance. When repairing the contact, the maintenance personnel should remember not to scrape the pin too much, so as not to affect the service life. At the same time, do not use abrasive cloth or grinding wheel to prevent the quartz sand particles from being embedded on the contact surface, which will affect the contact contact performance.
The test for contact pressure can be determined empirically using a strip of paper. A strip of paper (0.01 mm thick) that is slightly wider than the contact is sandwiched between the moving and stationary contacts, and the switch is in the closed position, and then the strip is pulled by hand, and the small-sized electrical appliance is generally used with a little force. It can be pulled out; for larger capacity appliances, the paper strips are torn after being pulled out. The above phenomenon indicates that the contact pressure is appropriate. If the strip is easily pulled out, the pressure is not enough; if the strip is broken, the contact pressure is too high.
Adjusting the pressure of the contacts can be solved by adjusting the contact springs. If the contact spring is damaged, replace it with a new spring or make it according to the original size. The contact pressure spring is usually made of carbon steel spring wire. The newly wound spring is tempered at 250 oC ~ 300 oC for about 20 to 40 minutes. The larger the wire diameter, the longer the time required. . The galvanized spring is deoxidized and held at a temperature of around 200 °C for 2 h to remove brittleness.
(2) Contact wear. There are two kinds of contact wear: one is electric wear, which is caused by gasification of the contact metal due to the high temperature of the spark or arc between the contacts; the other is mechanical wear, due to the impact contact contact when the contact is closed. Surface sliding friction and other reasons.
When the contact is in use, it will become thinner and thinner due to wear. When the original thickness is about 1/2, the new contact should be replaced. If the contact wears too fast, the cause should be identified and the fault should be eliminated.
(3) Contact welding. The phenomenon that the surface of the static and dynamic contacts is melted and welded together and is not opened is called fusion welding of the contacts. When the contacts are closed, due to the impact and vibration, a short current is generated in the small gap between the static and dynamic contacts, and the arc temperature is as high as 3000 oC to 6000 oC, so that the contact surface is burned or melted, and the static and static contact welding is performed. Together. A common cause of contact welding is improper selection, so that the contact capacity is too small, and the load current is too large; the operating frequency is too high; the contact spring damage initial pressure is reduced. After the contacts are welded, only new contacts can be replaced. If the welding is caused by insufficient contact capacity, a larger capacity appliance should be used.
1_2 Electromagnetic system failure and maintenance
(1) The iron core is noisy. The electromagnetic system generates a slight "click" sound during operation, which is normal; if the sound is too large or abnormal, it can be judged that the electromagnetic mechanism has malfunctioned. 1 The contact surface of the armature and the iron core is in poor contact or the armature is skewed. After the iron core and the armature are subjected to multiple magnetic collisions, the end faces will be deformed and worn, or the dirt, oil, rust, etc. accumulated on the contact surface will cause mutual contact and vibration and noise. The vibration of the iron core will cause the coil to overheat. In severe cases, the coil will be burned. An E-shaped core, an air gap of 0.1-0.2 mm between the core center pillar and the armature is left, and the deformation of the core end face will reduce the air gap. It also increases the core noise. If there is grease on the end surface of the iron core, it should be folded and cleaned; if the end surface is deformed or worn, it can be flattened on the flat plate with fine sand cloth to repair the end surface. 2 The short circuit ring is damaged. After many collisions, the iron core is short-circuited in the iron core groove, and may break or fall off. Short-circuit ring breaks often occur in the corners and notches of the groove. When repairing, the break can be welded firmly, and both ends are fixed with epoxy resin. If it can't be welded, the short-circuit ring or core can be replaced. When the short-circuit ring jumps out, it can be first Press the shorting ring into the slot. 3 mechanical reasons. If the contact pressure is too large or the movement of the moving part is blocked, the core can not be fully engaged, and strong vibration and noise will be generated.
(2) Failure and maintenance of the coil. 1 coil failure. When the voltage across the coil is constant, the greater its impedance, the smaller the current passed. When the armature is in the separated position, the coil impedance is the smallest and the current passing through is the largest; during the core pulling process, the gap between the armature and the core is gradually reduced, the impedance of the coil is gradually increased, and the coil current is minimum when the armature is completely attracted. If the armature and the iron core are not completely connected, the coil current will increase, the coil will overheat and even burn out. If the coil insulation is damaged or mechanically damaged to form an inter-turn short circuit, or short-circuit to the ground, a large short-circuit current will be generated locally in the coil, causing the temperature to increase sharply until the entire coil is burned. In addition, if the coil power supply voltage is low or the operating frequency is too high, the coil will be overheated and burned. 2 coil repair. The coil burnout should generally be re-wound. If there are not many turns of the short circuit, the short circuit is close to the end of the coil, and the other parts are still intact, then the damaged several turns can be removed, and the rest can be used continuously. At this time, the impact on the working performance of the electrical appliance will not be affected. Very big.
(3) Failure and maintenance of the arc extinguishing system The failure of the arc extinguishing system refers to the damage, moisture, carbonization, magnetic blown coil short circuit, arc angle and grid falling off of the arc extinguishing cover. These faults can cause an inability to extinguish the arc or extend the arc extinguishing time. If the arc-extinguishing cover is damp, it can be used for drying; when the carbonization is used, the scale can be scraped off; when the magnetic blowing coil is short-circuited, the short-circuited part can be opened by a word screwdriver; when the arc angle falls off, it should be reinstalled; when the grid is peeled off and burned Iron sheets can be used in the original size.
2 Common low-voltage electrical faults and maintenance
2.1 Contactor failure and maintenance Remove the fault analysis and maintenance of the contacts and electromagnetic system already described above. Other common faults are described below.

(1) The contact is out of phase. The phase loss of the motor is caused by a poor contact of a phase contact or a loosening of the coupling screw. At this point, the motor can also rotate, but the speed is low and a strong "click" sound is emitted. If you find this situation, you should stop it for maintenance immediately.
(2) Contact welding. Contactor operating frequency is too high, overload operation, short-circuit on the load side, conductive particles on the contact surface or too small contact spring pressure, etc., will cause the contact welding. If this fault occurs, even if the stop button is pressed, the motor will not stop. Immediately disconnect the previous switch and perform maintenance.
(3) Short circuit between phases. Due to the failure of the contactor's positive reversing interlock, or the two contactors are put into operation at the same time due to malfunction, the phase-to-phase short circuit occurs; or because the contactor moves too fast, the switching time is short, and the arc short circuit occurs during the conversion process. In the case of such a fault, the contactor and the button composite interlock can be used to control the forward and reverse rotation of the motor in the control circuit.
2.2 Thermal relay failure and maintenance Thermal relay failures generally have thermal components burned out, malfunctions and no action.
(1) The thermal element is blown. When the operating frequency of the thermal relay is too high, a short circuit or excessive current is generated on the load side, causing the thermal element to blow. To eliminate this fault, first cut off the power supply, check the circuit to eliminate the short-circuit fault, then re-select the appropriate thermal relay and re-adjust the set value.
(2) The thermal relay malfunctions. The reason for this kind of failure is that the setting value is too small, so that it is not overloaded; the motor starting time is too long, so that the thermal relay may trip during the starting process; the operating frequency is too high, so that the thermal relay is often affected by the starting current. The strong impact and vibration of the place are used to make the thermal relay action mechanism loose and trip; in addition, if the connecting wire is too thin, the thermal relay may malfunction. For the above fault phenomenon, the thermal relay suitable for the above work should be exchanged, and the setting value should be adjusted reasonably or the appropriate connecting wire should be replaced.
(3) The thermal relay does not operate. Since the thermal element is blown or dropped, the current setting value is too large, so that the overload does not operate for a long time; the guide plate is tripped; the connection line is too thick, and the thermal relay does not operate, so the motor is not protected. For the above reasons, targeted repairs can be performed. In addition, after the thermal relay is tripped, it cannot be manually reset immediately. It should be 2 min. After the bimetal is cooled, the contacts are reset.
2.3 Failure of time relay The air bag of the air type time relay is damaged or the seal is not tight and the air leaks, so that the delay action time is shortened or even no delay; the air room is extremely clean, if the dust is disassembled during the disassembly process The airway entering the airway will block and the delay time of the time relay will become very long. In response to the above situation, the air chamber can be disassembled, the rubber film can be replaced or the dust can be removed to solve the problem. The air type time relay will be affected by the change of the ambient temperature and the long-term storage will have a delay time change, which can be adjusted according to the specific situation.
2.4 Speed ​​relay failure and maintenance speed After the relay fails, it is generally manifested that when the motor is stopped, it cannot be braked and stopped. If the fault is not in contact with the contact, it may be caused by improper adjustment of the adjusting screw or breakage of the bakelite pendulum. Just remove the rear cover of the speed relay for inspection.