维修电工初级问答(Maintenance electrician primary quiz)

维修电工初级问答（Maintenance electrician primary quiz）1. Why the low voltage winding of the transformer is inside, while the high voltage winding is outside?A: the arrangement of transformer high and low voltage winding is determined by many factors. But for most transformers, the low pressure winding is placed in the high voltage winding. This is mainly about insulation. Theoretically, no matter how the high - voltage winding or low - voltage winding is arranged, it can become a transformer. But because the core of the transformer is grounded, because the low voltage winding is close to the core, it is easy to do from the insulation Angle. If the high voltage winding is close to the core, the voltage of the high voltage winding is high and the insulation requirement is required. This not only increases the volume of the winding, but also wastes the insulation material.Electrical community every day, Moreover, as a result of the transformer voltage regulation is to rely on change of the high voltage winding tap, namely change the number of turns, so the high voltage winding is placed in the low voltage winding outside, lead more easily also.How does the three-phase asynchronous motor rotate?Answer: after the three-phase alternating current flow into the three-phase stator winding, the stator cavity to produce a rotating magnetic rotation before often stationary rotor conductors under the effect of rotating magnetic field, the equivalent of the rotor conductors relative to cut magneticlines, which produced the induced current in the rotor conductors (electromagnetic induction principle). These conductors of the induction current occur in the production field (the effect of the current, the electromagnetic force). Because the inner conductor of the rotor is always arranged symmetrically, the electromagnetic force generated on the conductor is the opposite direction, forming an electromagnetic torque and turning the rotor. Since the current in the rotor conductor is generated by the stator rotating magnetic field induction, it is also known as induction motor. It is also known as the asynchronous motor because the rotor speed is always lower than that of the stator rotating magnetic field.3. Why doesn't the transformer make the direct current transformer?Answer: transformer can change voltage condition is that the original edge to ac electric potential to produce alternating magnetic flux, alternating magnetic flux will generate induced potential, at the edge of the deputy the size of the induction electric potential is proportional to the change rate of flux. When the transformer is in direct current, the current size and direction are the same, and there is no alternating magnetic flux in the iron core, which means that the magnetic flux is constant and the flux rate is zero. Therefore, the induction potential is also zero. At this point, the total dc voltage is added to the winding with a small resistance, causing the current to be very large, causing an approximate short circuit.The alternating current is changed alternately. When theprimary windings are connected to the ac, the magnetic flux generated in the iron core also changes, so that the secondary winding number is greater than the primary one, and the voltage can be raised. Conversely, the subloop number is lower than the initial time. Because of the size of the direct current (dc) and the direction does not change with time, so constant direct current into the primary winding, the core flux was constant, cannot in the secondary winding induction potential, so no pressure.What are the driving modes between motors and machinery?Answer: the back wheel type direct drive; Belt transmission; Gear transmission; The worm drive; The chain drive; 6. The friction wheel drive.5. What kind of inspection inspections should be made in the operation of the transformer?A: the sound is normal; Check the transformer for oil seepage. Oil leakage phenomenon.Oil color and oil level is normal; Whether the current and temperature of the transformer exceed the allowable value; If the transformer sleeve is clean, there is no broken crack and discharge marks. Whether the transformer is grounding or not.6. What are the methods of drying the transformer?Answer:. Induction heating method;Hot air drying method;The oven drying method.7. How to do the motor no-load test?Answer: before the test, the motor is checked, after no problem, connect to three phase power supply, so that the motor does not drag the load. Then check the running sound, the bearing operation and the three-phase current, and the general high capacity high speed motor's no-load current is the rated currentThe no-load current of 20-35% low speed motor is 35 ~ 50% of its rated currentNo-load current is too large and too small and three-phase equilibrium and no-load test time should not be less than 1 hour, should also measure the motor temperature rise at the same time, the temperature rise according to the grades of insulation shall not exceed the allowable limit.How to make short circuit test of motor?Answer: the short circuit test is to use the brake equipment, the rotor of its motor is fixed, the output voltage of the three-phase voltage regulator is gradually increased from zero. When the current reaches the rated current of the motor, the pressure is stopped and the voltage is called the short circuit voltage. The voltage rated at 380 volts is generally between 75 and 90 volts.9. What does transformer overhaul have?Answer: 1. Lift out of the body, the mandrel. Body (core. Coil.(2). Maintenance item cover. Storage tank. Safe airway. Heat pipe throttle and casing; (1). Maintenance of cooling devices and oil filters; For oil or oil change, dry treatment if necessary; 5. Maintenance control and measuring instruments. Signals and protective devices; To clean the shell and paint when necessary; To assemble and conduct prescribed measurements and tests.Daily electrical community! ).10. What are the advantages of the linear asynchronous motor compared with the squirrel cage asynchronous motor?Answer: the advantages of wound asynchronous motor can be through the collector ring and brush, plus resistor in series with the rotor loop, to improve the starting performance and can adjust speed change and resistance within a certain range. However, it is more complicated than the squirrel cage asynchronous motor, and the price is less reliable.11. When the motor is installed and after the test, if it finds that the vibration exceeds the value of the regulation, what should be the reason?A. the rotor balance is not well balanced; The rotor balance is fast and loose; The shaft bends and deforms; The center of the coupling is unnuclear; . The base screws are loose. Thefoundation is uneven or not solid.12. When the motor is running, the bearing temperature is too high.Answer:. The lubricating ester number is not suitable; The quality of lubricated ester is bad or bad; Excessive or too little grease in the bearing room; . There is sundry in the grease; The rotation part is wiped with the stationary part; Bearing inner ring or outer ring; The bearing model is not correct or quality; The coupling is incorrect. Nine. The belt is too tight; The motor vibrates too much.13. Why is the motor rotor more balanced? Which type of motor rotor can only nuclear static balance?Answer: motor rotor in the production process,Due to the influence of various factors (such as uneven material of casting porosity or porosity, and parts of the weight of error and machining error, etc.) will cause the imbalance of rotor in weight, so the rotor school balance after completion of the assembly. The rotor of the motor or motor rated at above 6 poles can only be balanced by the rotor, and the other motor rotor needs to be balanced.14. What should the welder pay attention to before use?A: welding machine new or unused for a long time, often due to be affected with damp be affected with damp between winding. The windings of the insulation resistance between the casingand slash, likely to occur at the beginning of the use of short circuit and grounding, equipment and personal accident. Therefore, the insulation resistance of the meter should be checked before using.Before starting the new welder, check whether the electrical system contactor is in good condition, and the test run can be started when normal. Prove that there is no electric hidden trouble, can only run in the load condition, finally can put into normal operation.The dc welder should rotate according to the prescribed direction, and the rotation direction of the fan with the fan should be paid attention to whether the rotation direction of the fan is correct. To achieve the purpose of cooling the welding machine.15. What are the protections of small and medium sized asynchronous motors?Answer:. Short circuit protection: general fuse is short circuit protection device; Loss of pressure protection: the electromagnetic coil of the magnetic starter has a protective effect on the control circuit of the starting motor. The automatic air switch is equipped with the pressure-free tripping device to protect the motor in the above two situations. Overload protection: the thermal relay is the motor overload protection device.16. What should be noticed in the operation maintenance of asynchronous motor?Answer:. The motor should be kept clean. The voltage fluctuation of the general motors is allowed to be determined by the voltage fluctuation. 5. The difference of the three phase voltage shall not be greater than 5%, and the balance of each phase current shall not exceed 10% and shall pay attention to whether or not the phase operation is missing. To check the temperature rise of the motor regularly, the thermometer should be used to measure the temperature rise, and the maximum allowable value should not exceed the maximum allowable value. The monitoring of the bearing has no abnormal noise, the seal should be good, and the lubricating oil should be changed regularly, the oil change cycle, the general sliding bearing is 1000 hours, the rolling bearing 500 hours; 5. Pay attention to motor acoustics. Odors. Vibration conditions and driving conditions. In normal operation, the motor should be uniform, with no noise and special calls.How to remove the asynchronous motor properly?A; All preparations should be done before removal of the motor, such as tools used, pre-disassembly inspection and record work.Removal of motor steps: (1). Remove the pulley or the shaft coils, and should be marked before removing the pulley and the shaft, and remove the rust before installation. 2. Remove the cover: remove the bearing cap, remove the cover, and mark the front and rear cover, and the installation should be marked reset; (3). Remove the rotor: wear-resisting of thick paper between the stator should be fitted to prevent damage to the stator winding, if the rotor is very heavy, lifting equipmentinstallation available when the rotor to check whether there is sundry, inside the stator and shaft extension end end cover put on first, then the rotor load along with the fan and the back cover.18.How to determine the fault cause from the abnormal vibration and sound of the asynchronous motor?Answer: asynchronous motor produces abnormal vibration and abnormal sound mainly has mechanical and electromagnetism two reasons.Daily electrical communityMechanical reasons: (1) the screw loose of the blade of the motor blades or the fastening blades of the wind blade is loose, causing the wind blade to be touched by the blade of the wind, and the sound that it produces is small when the impact of the noise is heavy; Due to the failure of the spotter or the improper axis, the rotor of the motor will be fixed when the rotor eccentricity is serious. The rotor phase friction will cause the motor to produce violent vibration and uneven touching. The electric motor has caused an abnormal vibration under the action of electromagnetic torque due to the loose screw or foundation of the foot screws. A long-term use of an electric motor is caused by a lack of lubricating oil in the bearing and the steel ball in the bearing is damaged, thereby making the motor bearing room with an abnormal hissing or purr.Electromagnetic reasons: (1). Suddenly appear abnormal sound, normal operation of the motor in the load running speed decreases significantly, while a low along the roar, may be unbalanced three phase current, the heavy load or single phase operation; 2. Normal operating motor, if stator. A short circuit fault in the rotor winding or a high-low weng sound at the time of electric opportunity. The fuselage also vibrates.19. What is the bearing temperature of the asynchronous motor over the shell temperature?Answer: the motor bearing due to chronic lack of oil running, the friction and loss aggravate the bearing overheating. In addition, when the motor runs normally, too much or too much oil can cause the bearing to overheat. (2). When replacing lubrication, the bearing abrasion of the bearing may be damaged due to the impurity of the impurity or bearing in the lubricating oil. Because of the improper assembly, the fixed end cover screws are less elastic, causing the center of the two bearings not to be balanced on a straight line or in the outer ring of the bearing. It makes the bearing turn not flexible, so that the friction increases after the load. If the belt is too tight or the motor is not in the same line as the center of the mechanical shaft, it will increase the bearing load. The bearing is chosen improperly or in poor quality, such as the rust of the inner and outer rings of the bearing, the individual steel beads are not round, etc.; The bearing of the motor is damaged, causing the bearing to overheat.20. Why is the squirrel cage asynchronous motor rotor winding on the ground without insulation and the rotor winding of thecoil induction motor must be insulated?Answer: the squirrel cage rotor can be thought of as a multiphase winding, the phase number is equal to a pair of zhang very conducting bar number, number of turns per phase is equal to 1/2 turn, because each phase rotor induction electric potential is generally small, and silicon steel sheet resistance is greater than the resistance of the copper or aluminum, so most of the current through a conductor, different ground insulation.In the winding rotor winding, the phase number and the stator winding are the same, and the number of turns per phase is also more, according to the formula E 2 = 4.44 K 2 f 2 W 2 & psi; It can be seen that the coil rotor is very large in each phase, and if the ground is not insulated then the ground will be short-circuited or even burned the meter.21. How to repair the general failure of the rotor shaft of the asynchronous motor?Shaft bending: answer: (1). If they are found in the operation of the motor shaft extended terminal have beat phenomenon, is the shaft is bent, shaft bending serious when, the stator is going to happen. The phenomenon of mutual friction between the rotor, shaft bending,The rotor should be taken out and corrected according to the specific situation; The iron core wear of the shaft: since the time operation of the electric captain can sometimes cause the iron loose and iron core of the shaft to loosen, and the shafthas not rolled over the flowers. In this case, consideration should be given to rolling flowers in conjunction. If the lower core has the possibility of displacement on the shaft, a circular groove should be opened on the shaft of both ends, and two curved keys are put in place and welded together with the shaft. . Axis wear: bearing broken down many times, it will wear out the diameter of the shaft, generally can be used in the diameter of roller. If the wear is serious, it can also be deposited at the axial area of the welding pile, and the recar bed can be processed and required to be sized; Shaft crack: if the transverse crack of the axle is not more than 10 ~ 15% of the diameter of the axial crack, it shall not exceed 10% of the shaft. The new axle must be replaced if the shaft cracks are seriously damaged or broken.Why should ac contactors overheat when they operate frequently?A: starting, ac contactor, because of the large space between the core and armature reactance is small, can through the coil excitation current is very big, is often greater than the working current of more than a dozen times, such as frequent starting, the excitation coil through a large starting current, thus causes a coil overheating phenomenon, serious when coil burned.23. What is the regulation of the control cable of the import tray?Answer: (1). The import tray cabinet is arranged in a neat arrangement, which is not crossed, and should be fixed, so that all terminals are not subjected to stress; The cable should notenter the cabinet and the steel belt should be tightened. For the control cables used for transistor protection, control, etc., when the shielded cable is used, the shielded cable shall be connected to the ground when the cable is not shielded. The rubber insulated wire shall be protected by the jacket insulation. 5. The cable core of the tray. Horizontal vertical, non-crossed, the spare conductor has the appropriate leeway.24. What are the rules for cable tube?Answer: when laying the cable, if the cable should be put into the cable pipe, it should comply with the following requirements: the cable and lead cable shall not be inserted into the same pipe; A cable tube is only allowed to penetrate into a power cable; Power cables and control cables shall not be inserted into the same pipe; In the case of bare lead cable, the cable should be protected by the use of hemp or other soft materials.Every electrical community ()25. How to connect the hard bus?Answer: the hard bus usually adopts pressing or welding. Pressing is to use screws to press the bus wire to make it easy to refit and dismantle. Soldering is used for welding or gas welding and is used for non-detachable areas. No tin welding shall be adopted.26. Under what circumstances should a cable be added to the tube for protection? How to choose the pipe diameter?Answer: at the following points: (1). The cable introduces the building, the tunnel, the floor and main walls; (2) the ground is two meters high and 250 mm deep underground; The distance between the cable and the underground pipe is not specified; The intersection of cable and road, tram rail and rail; The factory area may be subject to mechanical damage and accessible location of pedestrians.When selecting pipe diameter, the inner diameter is 50% larger than the outside diameter.27. What are the phase sequence and color of the bus?A: the phase sequence of the busbar (see from the front of the device) is as follows:From left to right, the left-hand side is A phase,B in the middle andC in the right.From top to bottom, the upper side is A phase, the middle is B, and the lower side is C.From the distance to the nearest arrangement, the distance is A phase, the middle is B phase, and the other is C phase.Colour: A - yellow, b-green, c-red, neutral line unearthing purple, positian-chu color, negative - blue, grounding line - black.28. What are the advantages of each pole of the high voltage isolator for two blades?Answer: according to the principle of electromagnetism, two parallel conductor when current flows through the same direction, will produce electromagnetic force close to each other, the size of the force and the distance between the parallel and current, due to the circuit switch control operation, a failure occurs, the blade will flow through the big electric current, make the two blades with a lot of pressure clamp tightly to the fixed contact, which will help the blade due to vibration from the risk of expanding the in situ, on the other hand, due to the effect of electromagnetic force, make the blade (dynamic contact) and close contact between the fixed contact, contact resistance decrease, reason is not caused by fault current flows through contact welding phenomenon.29. What is the length of insulation bandage of the cable head of the indoor cable?Answer: the length should be taken as needed, but not below:Voltage (kv) minimum insulation binding length (mm) 1 below 1603, 2106, 27010, 31530. What kind of electrical equipment should be installedaccording to the explosion danger area?Answer: the junction box used in the electrical wiring box. The pull box should be in accordance with the following. In addition to this circuit, the q-2 level is applied to any type of explosion-proof type; Q - 3. G-2 can be used to prevent dust; The level and group of the pull box are not lower than the level and group of the explosive mixture in the site. The safety of this circuit can be used to prevent dust.31. What are the regulations on the installation of explosion-proof flexible pipes in explosive dangerous places?Answer: there should be no crack in the explosion-proof flexible connection pipe. Hole. Mechanical damage. Deformation and other defects should meet the following requirements in installation. The q-1 level. G-1 level USES the isolation type; The explosion-proof safety type can be used at the q-2 level; Dustproof types used in other explosive places; The class and group of explosion-proof flexible connections used in the q-1, q-2 level should not be lower than the explosive mixing group and level of the site; . Environmental temperature should not exceed & plusmn; 40 ℃. The radius should not be less than five times the diameter of the tube.32. What are the requirements for the grounding of the 10kV transformer (distribution)?Answer: transformer. Metal shell of switch equipment and transformer (PT. CP), distribution cabinet. Control protective disk. Metal frame. Lightning protection equipment. Cable headand metal screen. The docking ground device has the following requirements: (1). The indoor Angle steel foundation and the bracket shall be no less than 25 & times. The flat steel of 4mm2 is connected to the grounding trunk, then the outdoor is connected with the outdoor grounding device. 2. The grounding body should be three meters away from the wall of the transformer. The distance of the grounding body is 2.5 meters and the distance between the two ground bodies shall be 5 meters. In the form of closed loop in the form of the ground network, it can attach the external lead earthing when the grounding resistance fails to meet the requirements. The grounding resistance of the whole grounding network should not be greater than 4 o.33. How to connect different sections,Cable core of different metals?Answer: connect different metals, different sections of cable, should make the resistance of the junction point small and stable. The same metal material shall be used for the same metal material as the cable conductor, and the connecting tube shall be used for the connection of the connected core wire. When the cables of different metals need to be connected, such as copper and aluminum connection, the contact potential difference between the two metal standard electrode positions is large (copper is + 0.345 volts and aluminum is -1.67 volts). When the presence of electrolytes, will form the cathode for aluminium, copper anode of the battery, make aluminum to produce electrochemical corrosion, which lead to an increased contact resistance, so the connection of two different metal cable,besides should meet the requirements of contact resistance, also some anticorrosion measures should be taken. The general method is to apply a layer of tin and then press the copper pressure.34. When the explosion-proof electrical equipment is completed and accepted, what items should be inspected in detail?Answer: when the explosion-proof electrical equipment is completed, the following points must be paid attention to. Explosion-proof certificate no. & rdquo; ; 2. Theexplosion-proof electrical equipment should be in type. The electrical equipment should be free of crack in the shell. The junction box should be fastened, and the fixing bolts and anti-loose devices should be complete. The oil and gas tank should not leak oil and the oil level is highly qualified. The excess incoming line of electrical equipment should be sealed according to the regulations. The installation of the sealing device of the electrical wiring shall comply with the requirements;For the distribution of safety spark electrical equipment, the line direction should meet the design, and the line should have blue color; The grounding or zero of the electrical installation shall be in accordance with the requirements of the design requirements.35. What are the requirements for the installation of the kit cabinet?Answer: 1. The handcart push-pull is flexible and light, no cardresistance collision phenomenon; (2). The center of the movement and the contact center is consistent, the contact is close, the hand is pushing into the work position, meeting the product requirement; The switching contact of secondary circuit auxiliary switch should be accurate and reliable. The mechanical locking device should be accurate and reliable. The cabinet is fully illuminated; 6. The security baffle switch is flexible, and the door of the car is in and out. The position of the control cable in the cabinet should not hinder the carriage of the handcart in and out, and firmly fixed; The grounding contact between the handcart and the cabinet should be close to the cabinet. When the handcart is pushed into the cabinet, the grounding contact should be connected to the head before the main contact, as opposed to pulling out.36. Preparation before installation of the switchboard (cabinet)?A: the installation of the switchboard should be finished in the interior of the room, the foundation shall be the strength of the requirements, and the cleaning will be carried out after cleaning. 2. Before installation, the distributor shall check and check for the model of the switchboard, whether the electrical components in the disk meet the requirements and whether there is any mechanical damage; (3). Under the base steel should cooperate with civil good up, top of the base steel should be 1020 mm above ground level, the level of the basis of the same places the same horizontal plane steel error should not exceed 1/1000 of the length of the maximum error of the level should not be more than 5 mm, car type steel should be the basis of power distribution cabinet and the room on the ground isflat.37. How to repair the general failure of an asynchronous motor shaft?Answer: 1. Bend: take out the rotor and calibrate it according to the specific situation; The iron core of the shaft is worn: it should open a circular slot on the shaft at both ends of the core.The two arc keys are added and welded together with the shaft. Neck wear: usually at the neck of the shaft. If the abrasion is serious, it can be used to build up a layer of electric welding at the neck of the shaft. The shaft cracks: should be replaced seriously.。