Use and maintenance of low-voltage electrical appliances

When using low-voltage electrical appliances, there are many things to pay attention to! When a low-voltage electrical appliance fails, we need to know the repair method of low-voltage electrical appliance failure! Low-voltage electrical appliances are components or equipment that can manually or automatically turn on and off the circuit according to external signals and requirements in order to switch, control, protect, detect, transform and regulate the circuit or non-electric objects. Let's take a look at the precautions for the use of low-voltage electrical appliances!

Precautions for the use of low-voltage electrical appliances

Low-voltage electrical appliances to choose different types of short-circuit breaking capacity of circuit breakers to adapt to different line expected short-circuit current (when I in the same case) the need for the selection of circuit breakers principle is: the short-circuit breaking capacity of circuit breakers ≥ the expected short-circuit current of the line.

1, different loads of low-voltage appliances should be selected with different types of circuit breakers

The most common loads of low-voltage appliances are distribution lines, motors and household and similar household (lighting, household appliances, etc.) three categories. Corresponding to this will have distribution protection type, motor protection type and household and similar household protection type of circuit breakers. The nature of protection and protection characteristics of these three types of circuit breakers are not the same. For the distribution type circuit breakers, it has A class and B class: A class for non-selective type, B class for selective type. The so-called selective type refers to the circuit breaker with overload long delay, short circuit short delay and short circuit instantaneous three protection characteristics. Universal (also known as frame) circuit breakers in the DW15 series, DW17 (ME) series, AH series and DW40, DW45 series most of the B type, while DZ5, DZ15, DZ20, TO, TG, CM1, TM30 and HSM1 series and universal DW15, DW17 some specifications due to only overload long delay, short-circuit transient two-segment protection They are non-selective type A circuit breakers. Selective protection.

2, when the F point short circuit, only the QF2 circuit breaker near the F point action, while the upper direction of the QF1 circuit breaker does not operate, this is selective protection (because QF1 does not operate, so that the failure of QF3, QF4 branch to maintain power supply). If QF2 and QF1 are both Class A circuit breakers, then a short circuit occurs at point F. When the short-circuit current value reaches a certain value, QF1 and QF2 act simultaneously, and the QF1 circuit breaker circuit and all the branches under it are de-energized, which is not selective protection. The reason for selective protection is that QF1 is a class B circuit breaker, it has a short circuit short delay performance, when the F-point short circuit, short-circuit current flowing through the QF2 branch, also flowing through the QF1 circuit, QF2 instantaneous action decoupler action (usually its full breaking time is not greater than 0.02s), because of the short delay of QF1, QF1 will not act within 0.02s (its short delay ≥ 0.1s or 0.2, 0.3, 0.4s). In QF2 action to cut off the fault line, the whole system is restored to normal.

3, it can be seen that if you want to achieve the requirements of selective protection, the upper level of the circuit breaker should be selected with a three-stage protection B-type circuit breaker. Low-voltage appliances for the direct protection of motor motor protection circuit breakers, it is enough to have a long overload delay and short-circuit instantaneous two-segment protection performance, that is, it can choose A circuit breakers (including molded case and universal type), DZ5, DZ15, TO, TG, GM1, TM30, HSM1 and DW15 series in addition to the performance of distribution protection, their 630A and The following specifications all have the function of protecting motors.

Low-voltage electrical fault repair

Various electrical components will be damaged after long term use or due to improper use, then timely maintenance is necessary. There are many electrical appliances used in the electrical circuit, and the structure varies in complexity, so here we will first analyze the common failures and repair methods of each component common to each electrical appliance, and then analyze the common failures and repair methods of some common appliances.

1、Common faults and maintenance of electrical components

1.1 Contact failure and maintenance

(1) contact overheating. When the contact is connected, there will be current through the heat, under normal circumstances, the contact is not overheating. When the static and dynamic contact contact resistance is too large or through the current is too large, it will cause contact overheating, when the contact temperature exceeds the allowable value, it will make the contact characteristics become bad, or even melt welding. The specific causes of contact overheating analysis

As follows: ① The current between the moving and static contacts is too large. The contacts of any electrical appliance must operate at its rated current value, otherwise the contacts will overheat. The contact current is too high or too low because the system voltage is too high or too low; overload operation of electrical equipment; improper selection of electrical contact capacity and fault operation four possible. ②The contact resistance between the static and dynamic contacts becomes large. The size of the contact resistance is related to the degree of contact heating, the reasons for its increase are: one is due to the contact pressure spring loss of elasticity and caused by insufficient pressure or contact wear thinning, for the situation should be replaced spring or contact; two is poor contact surface contact. For example, in operation, dust, oil and dirt covered in the contact surface, increasing the contact resistance; and then, when the contact is closed and broken, because of the arc will make the contact surface burnt hair, burns, resulting in defective uneven and reduced contact area, and cause poor contact. Therefore, attention should be paid to the operation of the contacts to strengthen maintenance. The copper contact surface oxidation layer and burned contacts can be scraped or fine file correction; large, medium current contact surface, not smooth, the important thing is flat; small capacity contacts require good surface quality; silver and silver-based contacts can be cleaned with cotton dipped in gasoline or carbon tetrachloride, the oxide layer does not affect the contact performance. Maintenance personnel in the repair contact, remember not to scrape pin cutting too much, so as not to affect the service life, and do not use abrasive cloth or grinding wheel repair, so as not to quartz grit embedded in the contact surface, but affect the contact contact performance.

For the contact pressure test can be used to determine the paper strip with experience. A strip of paper (0.01 mm thick) slightly wider than the contacts is clamped between the movable and static contacts, and the switch is in the closed position, then pull the paper strip by hand, generally small capacity appliances with a little force, the paper strip can be pulled out; for larger capacity appliances, the paper strip is pulled out after the tearing phenomenon. The above phenomenon indicates that the contact pressure is appropriate. If the paper strip is easily pulled out, it means that the pressure is not enough; if the paper strip is pulled off, it means that the contact pressure is too large.

Adjust the contact pressure can be solved by adjusting the contact spring. If the contact spring is damaged, it can be replaced with a new spring or self-made according to the original size. Contact pressure spring commonly used carbon steel spring wire to manufacture, the new winding of the spring to 250 oC ~ 300 oC conditions for tempering, holding time of about 2O ~ 40 min, the larger the diameter of the wire, the longer the time required. Galvanized springs to be de-oxygenated, held in a temperature of about 200 oC for 2 h in order to go brittle.

(2) contact wear. There are two kinds of contact wear: one is electrical wear, caused by the high temperature of the electrical spark or arc between the contacts to make the contact metal vaporization; the other is mechanical wear, caused by sliding friction of the contact contact contact surface due to impact when the contact is closed, etc. Contact in the process of use, due to wear will become thinner and thinner, when the original thickness of the remaining 1/2 or so, should be replaced with new contacts; if the contact wears too fast, the cause should be identified, troubleshooting.

(3) contact fusion welding. Dynamic and static contact surface is melted and welded together and break off the phenomenon, known as contact fusion welding. When the contacts are closed, due to the impact and vibration, in the small gap between the static and dynamic contacts to produce a short current, arc temperature up to 3000 oC ~ 6000 oC; can make the contact surface is burned or melted, so that the dynamic and static contacts welded together. The common reasons for contact welding are improper selection, so that the contact capacity is too small, while the load current is too large; operating frequency is too high; contact spring damage to reduce the initial pressure. After contact welding, can only replace the new contact, if the contact capacity is not enough to produce melt welding, then you should choose to use a larger capacity appliances.

1.2 Failure and maintenance of electromagnetic system

(1) The iron core is noisy. Electromagnetic system in the work of a slight "buzzing" sound, which is normal; if the sound is too large or abnormal, electromagnetic institutions can be judged to have a failure. ① Armature and core contact surface contact is not good or armature skewed. The core and armature will be deformed and worn after many times of magnetic collision, or due to the accumulation of dirt, oil, rust, etc. on the contact surface, will cause mutual poor contact and vibration and noise. The vibration of the core will make the coil overheat, serious will burn the coil, the E-shaped core, the core column and armature between the air gap of 0.1-0.2 mm, the core end surface deformation will make the air gap reduced, will also increase the core noise. If there is grease on the core end face, it should be folded down and cleaned; if there is deformation or wear on the end face, a fine abrasive cloth can be laid flat on the flat surface to repair the end face. ② short circuit ring damage. After many collisions, the short circuit ring installed in the core slot may fracture or fall off. Short-circuit ring fracture often occurs outside the corner of the slot and the slot part, maintenance can be fracture welded firmly, both ends fixed with epoxy resin; if you can not weld can also be replaced short-circuit ring or core, short-circuit ring jump out, the short-circuit ring can be pressed into the slot first. ③ mechanical reasons. If the contact pressure is too large or due to the moving part of the jammed, so that the core can not be completely absorbed, will produce strong vibration and noise.

(2) Fault of the coil and maintenance. ①Failure of the coil. When the voltage at both ends of the coil is certain, the greater its impedance, the smaller the current through. When the armature in the separation position, the coil impedance is the smallest, the maximum current through; core suction process, the gap between the armature and the core gradually decreases, the impedance of the coil gradually increases, when the armature completely suction, the coil current is the smallest, if the armature and the core, no matter what the reason, not completely suction, will make the coil current increase, the coil overheating, or even burned. If the coil insulation is damaged or mechanically damaged and a short circuit is formed between turns, or a short circuit to ground, a large short-circuit current will be generated locally in the coil, causing the temperature to increase dramatically until the entire coil burns up. In addition, if the coil power supply voltage is low or the operating frequency is too high, it will cause the coil to overheat and burn. ② Repair of the coil. Coil burnout should generally be rewound. If the number of turns shorted is not much, and the short circuit is close to the end of the coil, the other parts are still intact, you can remove the damaged turns, the rest can continue to use, the impact on the working performance of the appliance will not be great.

(3) arc extinguishing system failure and maintenance 0 arc extinguishing system failure refers to arc cover damage, moisture, carbonization, magnetic blowing coil turns ask short circuit, arc angle and grid off, etc.. These failures can cause the arc can not be extinguished or extended arc extinguishing time. If the interrupter cover is damp, drying can be used; charring can be scraped out the accumulated dirt; magnetic blowing coil short-circuit can be used to set aside the short-circuit; arc angle off should be re-installed; grid off and burned when the available iron sheet according to the original size of the match.

2、Common electrical failure and maintenance

2.1 Contactor failure and maintenance

In addition to the above has been introduced contact and electromagnetic system fault analysis and repair. Other common faults are described below.

(1) Contact broken phase. Due to a phase contact contact is not good or coupling screw loose caused by broken phase, so that the motor is out of phase operation. At this time, the motor also; can rotate, but the speed is low and emits a strong "humming" sound. Found in this case, to immediately stop and repair.

(2) Contactor fusion welding. Contactor operation frequency is too high, overload operation, load side short circuit, contact surface conductive particles or contact spring pressure is too small, etc., will cause contact fusion welding. This fault occurs even if you press the stop button, the motor will not stop, should immediately disconnect the previous level switch, and then overhaul.

(3) phase-to-phase short circuit. Due to the contactor forward and reverse interlock failure, or due to misoperation resulting in two contactors into operation at the same time and cause a short circuit between the phases; or because the contactor action is too fast, the conversion time is short, in the conversion process, the arc short circuit. Where such faults, the contactor and button compound interlock can be used in the control line to control the forward and reverse rotation of the motor.

2.2 Fault of thermal relay and repair

The failure of the thermal relay generally has the phenomena of burned out thermal element, misoperation and non-operation.

(1) The thermal element is burnt out. When the thermal relay action frequency is too high, the load side of a short circuit or current is too large, resulting in the thermal element burned out. To eliminate this fault should first cut off the power supply, check the circuit to exclude short-circuit fault, and then re-select the appropriate thermal relay, and re-adjust the fixed value.

(2) thermal relay misoperation. The reason for this failure is: the calibration value is small, so that not overloaded on the action; motor start time is too long, so that the thermal relay in the process of starting may be unbuttoned; operating frequency is too high, so that the thermal relay often by the starting current impact; use the place of strong shock and vibration, so that the thermal relay action mechanism loose and unbuttoned; in addition, if the coupling wire is too thin will also cause the thermal relay false action. For the above failure phenomenon should be replaced with a thermal relay suitable for the above work nature, and reasonable adjustment of the calibration value or replace the appropriate connecting wire.

(3) The thermal relay does not operate. Due to the thermal components burned or off, the current setting value is large, so that a long time overload is still not action; guide plate detachment; coupling line is too thick and other reasons, so the thermal relay does not work, so the motor will not play a protective role. According to the above-mentioned reasons, targeted repair can be carried out. In addition, after the action of the thermal relay, do not immediately reset manually, should be 2 min, wait for the bimetal to cool, and then make contact reset.

2.3 Time relay fault repair

Air time relay air bag damage or seal is not tight and leakage, so that the delayed action time is shortened, or even do not produce delay; air chamber requires extremely clean, if the disassembly process so that dust into the air channel air channel will be blocked, the time relay delay time will become very long. For the above situation can be disassembled air chamber, replace the rubber film or remove the dust, you can solve the fault. Air type time relay by the ambient temperature changes and long-term storage will occur delay time changes, can be adjusted for the specific situation.

2.4 Speed relay failure and repair

When the speed relay fails, it generally shows that the motor can't brake to stop when it stops. This fault is probably caused by improper adjustment of the adjusting screw or broken rubber wood pendulum if the contact is not poor. Just disassemble the back cover of the speed relay for maintenance.