This paper introduces the structural characteristics and hazards of three-phase asynchronous motor, and based on the maintenance experience of three-phase asynchronous motor in recent years, summarizes the maintenance methods of three-phase asynchronous motor and several common faults and troubleshooting methods in trial operation. Key words: stator winding test of three-phase asynchronous motor. Our company started the maintenance of three-phase asynchronous motor from 1993. After years of exploration and practical experience, so far, the maintenance quality of three-phase asynchronous motor and the speed of judging fault points have been greatly improved, which has been recognized by our customers. Three-phase asynchronous motor, also known as induction motor, is a kind of motor with simple structure, durability, convenient use and maintenance and reliable operation. It is mainly composed of stator and rotor. At present, the vast majority of power equipment, such as machine tools, lifting equipment, transportation machinery, blowers, various pumps, electric fans, medical equipment, etc., are widely used in daily life. Three-phase asynchronous motor should be overhauled regularly to ensure reliable operation. Its maintenance includes general maintenance and recovery overhaul. With the increase of service life and the number of users, the damage situation is also increasing, and the number of repair and overhaul is also increasing year by year. I have repaired motors of various sizes, with power ranging from 0.55 kW to 300 kW. 1 structural characteristics and hazards of three-phase asynchronous motor. Three-phase asynchronous motor consists of a fixed part-stator and a rotating part-rotor, and there is an air gap between stator and rotor which is necessary for relative movement. Stator is the static part of motor, which is mainly composed of stator core, stator winding and frame. Stator core, as the magnetic circuit of motor, is generally made of 0.35 ~ 0.5 mm silicon steel sheet with insulating paint on its surface, and evenly distributed slots are punched on its inner surface, and stator windings are embedded in the slots. The role of stator winding is to introduce three-phase alternating current to generate rotating magnetic field. Usually the winding is made of high-strength enameled wire and embedded in the stator slot. The frame is used for fixing the stator core and stator winding, supporting the rotor with two end covers, and protecting the whole motor and dissipating the heat generated by the motor. The rotor is the rotating part of the motor, which is mainly composed of rotor core, rotor winding, rotating shaft, end cover and other parts. As the magnetic circuit of the motor, the rotor core is made of 0.35 ~ 0.5 mm silicon steel sheets and fixed on the rotating shaft. The rotor surface is punched with evenly distributed slots, and the rotor windings are embedded in the slots. Rotor winding is used to cut the stator magnetic field, generate induced potential and current, and make the rotor rotate under the action of rotating magnetic field. The rotating shaft is used to transmit torque and support the weight of the rotor. It is generally made of steel and alloy by machining. End covers are usually cast iron pieces installed on both sides of the frame to support the rotor. Three-phase asynchronous motors are damaged in the following ways: (1) Incorrect installation of rolling bearings or improper grease, resulting in friction between the shaft and the bearing, leading to serious wear and damage of the shaft. (2) The stator winding is damaged. The main reasons are motor overload, turn-to-turn, phase-to-phase, short circuit and grounding breakdown, which cause stator winding damage. 2 Regular maintenance of three-phase motors In order to avoid and reduce sudden damage accidents of three-phase asynchronous motors, three-phase asynchronous motors need regular maintenance and repair. If the motor is overheated and the stator winding insulation is too low, it must be repaired immediately. The overhaul method of three-phase asynchronous motor is: disassemble the motor, clean all parts first, and then visually check whether there is any abnormality. Then, measure the dimensions of key parts and conduct electrical inspection on motor windings. (1) mechanical inspection. Check the motor housing and end cover for cracks. If there are cracks, weld and replace them. Check the axial clearance of rotor from one side to the other. When measuring, insert a feeler gauge with a length of 500~600mm between the stator and the rotor, measure the air gap at four or eight equal positions, and then take the average value. Table 1 lists the reference value of air gap size of three-phase asynchronous motor, which refers to the sum of the dimensions on both sides. If the average value differs greatly from the reference value, check whether the rotating shaft is bent and whether the assembly process is appropriate. In addition, move the rotor by hand to see if it can rotate. If it can't rotate, check whether there is any foreign body stuck and whether the bearing is in good condition. Then replace the bearing and shaft sleeve as appropriate. Measure and check the dimensions of the upper and lower outer stops of the impeller, as well as the inner diameters of the retaining ring and the motor matched with them, and whether these two matching gaps are within the range specified in the maintenance standard. If it exceeds the difference, it is necessary to replace parts or take other measures (such as surfacing and sleeve insertion) to make the fit clearance meet the specified requirements. Otherwise, the performance and axial balance force of the motor will be affected. Observe and check the appearance of stator and rotor, and pay special attention to whether there is any abnormality at the weld. (2) Electrical inspection. DC resistance inspection: the unbalance of three-phase resistance shall not exceed 2%. Insulation resistance inspection: The insulation resistance of the winding of three-phase asynchronous motor can generally reach more than 100 mω. If it is less than 5mΩ, it is necessary to analyze the reasons, whether the insulation is damp, or whether the winding is grounded due to poor insulation. If there is no problem with the three-phase resistance balance detected by the bridge experiment, it is that the pure insulation is damp and needs to be dried. If the three-phase resistance of the stator is unbalanced, it is necessary to carry out grounding withstand voltage experiment and turn-to-turn experiment on the three-phase motor coil respectively to find out the grounding point. Class F insulation is the most commonly used. Enamelled wire, slot insulation, slot wedge, insulating sleeve, lead wire and impregnating varnish shall adopt H-class insulating material. Most stator windings below 75kW are replaced with class B insulation. Enamelled wire, slot edge, slot wedge, insulating sleeve, lead wire and impregnating varnish shall be made of Class B insulating material. The principle of replacing the motor winding is to repair it as it is, especially the number of turns of the coil cannot be changed at will, which will obviously affect the main performance of the motor. The wire diameter can only be close to the original total area, and the winding form and coil span cannot be changed. (2) Final assembly and inspection test. After the stator and rotor are repaired, qualified bearings, bushings and sealing rings can be prepared for final assembly. After assembly, turn the rotor by hand. The rotation should be uniform and flexible, and the rotor should have a certain axial movement, which should be within the scope stipulated by the maintenance standard: after the final assembly, check the DC resistance and insulation resistance, and after the electrical performance is considered normal, do the withstand voltage test of the three-phase asynchronous motor. Finally, test run, observe whether there is any abnormality in current, speed and vibration. 4 Common faults in commissioning test and troubleshooting methods (1) The motor can't rotate after being electrified, but there is no abnormal sound, odor and smoke. Then check whether there are breakpoints in the switches, fuses and junction boxes of the power supply circuit, and repair them if there are. (2) After electrifying, the motor does not turn, and then the fuse is blown, which indicates that there may be a lack of phase power supply or a short circuit of the stator winding, and the stator winding is grounded and wrongly connected. Then troubleshoot these failures one by one. First, check whether one phase of the circuit breaker is not closed and whether one phase of the power circuit is disconnected. If so, repair the power supply circuit. If not, use megger, multimeter, pressure tester, turn-to-turn tester and bridge to find the fault points one by one. (3) The no-load current of the motor is unbalanced. If the three-phase phase difference is large, it may be that the turns of the stator three-phase winding are unequal, the winding ends are wrongly connected, the power supply voltage is unbalanced, the winding turns are short-circuited, and the coils are reversed. These faults are eliminated one by one by winding turn-to-turn impact withstand voltage tester and bridge test. (4) The no-load current of the motor is balanced, but the value is large. It may be that the number of turns of the stator winding is reduced too much during repair, or that the Y-connected motor is wrongly connected with δ, or that the rotor is installed backwards when the motor is assembled, resulting in the dislocation of the stator core and the shortening of the effective length. Or when the old winding is overhauled and dismantled, the iron core is burnt out due to improper hot dismantling method. These problems are fixed by eliminating them one by one. If the number of turns decreases, rewind the stator winding to restore the correct number of turns. If the connection is wrong, it will be changed to Y, and if the iron core is burnt out by mistake, it will be the overhaul of the three-phase asynchronous motor. The three-phase asynchronous motor with damaged winding needs overhaul. The damage situation is generally stator winding breakdown to ground and phase, turn-to-turn short circuit and overload burning winding. The stator coil needs to be replaced. (1) stator winding replacement. It is normal to replace a large motor with a stator winding above 75kW with a capacity of KW 0.5 ~ 0.751~ 22 ~ 7.510 ~15 20 ~ 40 50 ~ 75100 ~180 200 ~ 250. 80 1.00 Increase air gap MM 0.400 0.500.650 0.650 0.801.001.251.50 Normal air gap MM 0.3000 0.500 0.650 01. Air gap MM MM 0.50 0.50 0.801.001.251.75 The average air gap value of newly assembled three-phase asynchronous motor is 2.00 500 ~ 65438+3000 r/min. Conclusion Through the maintenance of three-phase asynchronous motor for nearly ten years and constantly summing up practical experience, the quality of the motor maintained by our company has been greatly improved. We not only straightened out the management system of the motor, but also established a set of standardized maintenance management system, which made the maintenance work on the road of standardized management. In the future, we will further strengthen management, consolidate the achievements, and bring the conservation work to a new level.