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The three-phase asynchronous motor maintenance and troubleshooting Abstract: Introduces the three-phase asynchronous motor structural characteristics and damage, according to the three-phase asynchronous motor overhaul experience in recent years, summarizes the three-phase asynchronous motor overhaul method and in the test run test in the common kinds of faults and troubleshooting methods. Keywords: three-phase asynchronous motor, overhaul, stator winding, test Since 1993, our company started to carry out the maintenance of three-phase asynchronous motor, after years of searching and summarizing the practical experience, the overhauling quality of three-phase asynchronous motor and the speed of judging the fault point have been greatly improved, and have been recognized by our customers. Three-phase asynchronous motor, also called induction motor, is a kind of motor with simple structure, sturdy and durable, easy to use and maintain, and reliable operation, which is mainly composed of stator and rotor. At present, it is widely used in most power equipment, such as machine tools, lifting equipment, transportation machinery, blowers, various pumps, as well as fans in daily life, medical equipment and other devices. Three-phase asynchronous motor should be regularly overhauled to ensure reliable operation. There are general maintenance and restoration overhaul. The number of restorative overhauls is increasing year by year as the number of years of use increases and the number of damages continues to increase. I have repaired motors of various sizes, ranging from 0.55kW to 300kW. 1 Structural Characteristics and Damage The three-phase asynchronous motor consists of a fixed part, the stator, and a rotating part, the rotor, and there is an air gap between the stator and the rotor that is necessary for relative motion. The stator is the stationary part of the motor and consists mainly of the stator core, stator windings and seat. Stator core as the magnetic circuit of the motor, generally by 0.35 ~ 0.5mm silicon steel sheet stacked and become, the surface of the steel sheet coated with insulating varnish, the inner surface of the circle punched with an even distribution of grooves, grooves embedded in the stator windings. The function of the stator winding is to pass three-phase alternating current and generate a rotating magnetic field. Usually the windings are made of high-strength enameled wires and embedded in the stator slots. The base is to fix the stator core and stator windings, and support the rotor with two end caps, and at the same time to protect the whole motor and dissipate the heat generated in the operation of the motor. The rotor is the rotating part of the motor, mainly composed of rotor core, rotor winding, rotor shaft, end cover and other parts. The rotor core, as the magnetic circuit of the motor, is made of 0.35-0.5mm silicon steel sheets laminated together and fixed on the rotor shaft. The surface of the rotor is stamped with evenly spaced slots in which the rotor windings are embedded. The rotor windings are used to cut the magnetic field of the stator, generate induced potential and current, and turn the rotor under the action of the rotating magnetic field. The rotor shaft is used to transmit torque and support the weight of the rotor, and is generally made of steel or alloy through machining. The end caps are generally cast iron pieces mounted on both sides of the base to support the rotor. Three-phase asynchronous motors are mainly damaged in the following ways: (1) Incorrect installation of rolling bearings or improper grease lubrication, resulting in friction between the shaft and the bearings, resulting in serious wear and damage to the shaft. (2) stator winding damage. The main reason is that the stator winding is damaged by motor overload, turn-to-turn, phase-to-phase, short-circuit, and ground breakdown. 2 Regular maintenance of three-phase motor In order to avoid and minimize sudden damage to the three-phase asynchronous motor, the three-phase asynchronous motor needs regular maintenance and overhaul. If the motor overheats and the insulation of the stator windings is too low, it must be serviced immediately. Three-phase asynchronous motors should be serviced by disassembling the motor, cleaning the parts, and making a visual inspection for any abnormalities. Then measure the dimensions of the key parts and make electrical inspection of the motor winding. (1) Mechanical inspection. Check whether there are cracks in the shell and end cover of the motor, if there are cracks, weld and replace them. Check the rotor from one side to the other side of the axial clearance, the measurement will be 500 ~ 600mm long ruler, plugged into the stator, rotor, according to 4 or 8 equal position to measure the air gap, and then take its average value. Table 1 lists the reference values for the air gap size of three-phase asynchronous motors, which refers to the sum of the dimensions on both sides. If the average value deviates significantly from the reference value, check that the shaft is not bent and that the assembly process has been carried out correctly. If the rotor does not move, check to see if there are any foreign objects stuck in the rotor and if the bearings are in good condition. Then replace the bearings and bushings according to the situation. Measure and check the upper and lower outer stops of the impeller and the dimensions of the grommets and the inner diameter of the motor to see if the clearance between the two fits is within the range specified in the overhaul standard, and if it is exceeded, it is necessary to replace the parts or take other measures (e.g., surfacing and insertion) to bring the clearance to the specified requirements. Otherwise, the performance and axial balance of the motor will be affected. Observe and check the appearance of the stator and rotor, especially pay attention to the welded joints for any abnormalities. (2) Electrical inspection. DC resistance check: the imbalance of three-phase resistance shall not exceed 2%. Insulation resistance check: three-phase asynchronous motor winding insulation resistance can generally reach more than 100MΩ. If it is lower than 5MΩ, we need to analyze the reason, whether the insulation is damp, or the winding is grounded due to bad insulation, etc. If there is no problem with the balance of three-phase resistance detected by the bridge test, it is purely due to the dampness of the insulation, which needs to be dried out, such as three-phase resistance of the stator is unbalanced, we need to do the three-phase motor coil voltage resistance test to the ground and the inter-turn test, to find out the grounding point. F-class insulation is mostly used. Enameled wires, slot insulation, slot wedges, insulating sleeves, lead wires and impregnating varnish are all required to be insulated with class H. Most of the stator windings under 75kW are replaced with class B insulation. Enameled wires, slot insulation, slot wedges, insulating sleeves, lead wires and impregnating varnish are to be insulated with class B materials. The principle of replacing the winding of the motor is to repair it as it is, especially the number of turns of the coil should not be changed arbitrarily, as the change of the number of turns will obviously affect the main performance of the motor, and the diameter of the wire should only be close to the original total area, and the form of the winding and the span of the coil should not be changed. (2) Assembly and inspection test. After completing the stator and rotor repair, prepare qualified bearings, bushings, seals, etc. for assembly. After the assembly is completed, rotate the rotor by hand, the rotation should be even and flexible, the rotor should have a certain amount of axial runout, and the runout should be within the scope of the maintenance standard: after completing the assembly, check the DC resistance and insulation resistance, etc., and consider the electrical performance normal, then do the three-phase asynchronous motor for the withstand-voltage test, and finally carry out the trial operation to observe whether there is any abnormality in the current, rotational speed and vibration. 4 Common faults and troubleshooting methods of test run test (1) The motor can not rotate after the power is turned on, but there is no abnormal noise, odor and smoke. Check whether there are breaks in the power circuit switch, fuse and junction box, and repair them if there are any. (2) If the motor does not rotate after being energized and then the fuse is blown, it means that there may be a lack of one phase of power supply or a short-circuit between phases of the stator winding, grounding of the stator winding, or incorrect wiring of the stator winding, etc. Then eliminate these faults one by one. Then one by one to eliminate these faults. First of all, check whether there is a phase of the knife gate is not closed, whether there is a phase of the power supply circuit is broken, if there is a power supply circuit repair, if not, then use megohmmeter, multimeter, withstand voltage machine, turn tester, bridge one by one to rule out to find out the trouble point. (3) Motor no-load current imbalance, three-phase phase difference may be rewinding, stator three-phase winding turns are not equal, the winding first and last ends are connected wrongly, the power supply voltage is unbalanced, there are inter-turn short-circuits in the windings, the coil is reversed and other faults. Eliminate these faults one by one by means of winding turn-to-turn withstand voltage tester and bridge test. (4) The motor no-load current is balanced, but the value is large. It is possible that the number of turns of the stator winding is reduced too much during repair, or the Y-connected motor is mistakenly connected as Δ, or the rotor is installed in reverse during motor assembly, so that the stator core is not aligned and the effective length is shortened. Or when dismantling the old windings for overhaul, the core is burnt out by using improper thermal dismantling method. These problems are repaired by eliminating one by one, if the number of turns to reduce the problem, then rewind the stator winding to restore the correct number of turns. If the connection is wrong, then change the connection to Y, if the assembly error and core burned then re 3 three-phase asynchronous motor restoration overhaul Winding damage to the three-phase asynchronous motor, need to carry out restoration overhaul. Damage to the stator winding is generally caused by breakdown to ground or between phases, short-circuiting of the coil turns, and overloading which causes the winding to burn out. All need to replace the stator coil. (1) Stator winding replacement. 75kW or more stator winding replacement of large motor capacity k W 0.5-0.75 1-2 2-7.5 10-15 20-40 50-75 100-180 200-250 Normal air gap m m m 0.25 0.30 0.35 0.40 0.50 0.65 0.80 1.00 Increased air gap m m m 0.40 0.50 0.65 0.65 0.80 1.00 Increased air gap m m 0.40 0.50 0.65 0.65 0.80 1.00 Increased air gap m m 0.40 0.50 0.65 0.65 0.65 0.80 1.00 Increased air gap m m m 0.40 0.50 0.65 0.65 0.80 1.00 1.25 1.50 Normal air gap m m 0.30 0.35 0.50 0.65 0.80 1.00 1.25 1.50 Enlarged air gap m m m 0.50 0.50 0.80 1.00 1.25 1.50 1.75 2.00 500 to 1500r/min Motor speed 3000r/min Table 1 Average air gap values of three-phase asynchronous motors New assembly, overhaul of the core, etc. to solve. 5 Conclusion Through nearly ten years' maintenance of three-phase asynchronous motors, we have constantly summarized our practical experience, which has greatly improved the quality of the motors inspected by our company. We have not only rationalized the management system of motors, but also established a set of standardized maintenance management system, which makes the maintenance work go on the road of standardized management. In the future, we will further strengthen the management, consolidate the results achieved, so that the maintenance work to a new level.