Common electrical faults of asynchronous motors and troubleshooting methods

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1. Power not connected: Check the switch, fuse, contacts, and motor leads. 2. Winding open circuit: Heat the broken area to the temperature permitted by the insulation rating to soften the paint, then lift the broken wire. Repair the broken section by welding in wire of the same specification, cover it with insulation, and apply paint followed by drying. 3. Winding grounding or inter-phase, inter-turn short circuits: The treatment method is the same as above; simply insulate the grounded or short-circuited area and then apply paint and dry it. 4. Incorrect winding wiring: Verify the wiring diagram, heat the ends and reconnect them in the correct manner (including tying, insulating, and painting). 5. Blown fuse: Identify the cause, resolve the fault, and install a new fuse that matches the motor’s specifications. 6. Incorrect operation during the startup of wound-rotor motors: Check the commutator short-circuiting device and the position of the starting resistor. During startup, the resistor should be connected in series first, and the short-circuiting device should be connected after the startup is complete. 7. The setting value of the overcurrent relay is too low: increase it appropriately. 8. Low oil level in the oil cup of the old-style start switch: Add new oil until the oil level reaches the mark. 9. Incorrect wiring of control equipment: Correct the wiring. (2) The fuse burns out after the motor is connected to the power supply. 1. Single-phase start: Check the power supply cables, the motor leads, the fuse, and the switch contacts; identify any broken wires or incorrect connections and repair them. 2. Grounding or short circuit of the stator and rotor windings: Correct the error. 3. The motor is under excessive load or stuck: Adjust the load to the rated value and resolve any faults in the driven mechanism. 4. Insufficient fuse cross-sectional area: The fuse does not provide protection against motor overload; generally, the fuse should be selected using the formula below – the rated current of the fuse should be equal to the stall current divided by 2 to 3. 5. The starting resistance connected to the wound-rotor motor is too small or is short-circuited: resolve the short-circuit issue or increase the starting resistance. 6. Short circuit in the connection wire between the power supply and the motor: Locate the short circuit point and repair it. (3) After the motor is powered on, it fails to start and only makes a humming sound. 1. Improper selection of slot configuration after pole reversal and rewinding: Selection of an appropriate winding pattern and winding pitch ； Appropriate small rotor diameter of the vehicle ； Recalculate the winding parameters. 2. Short circuit in stator and rotor windings: Locate the break point and carry out repairs ； Check the contact condition between the wound-rotor brush and the slip ring, and check whether the starting resistor is open-circuited or has an excessively high resistance. 3. Incorrect shorting of the start and end of the winding leads, or reversed connections within the winding: Apply direct current to the stator winding and check the winding polarity (using a compass) to determine whether the start and end of the winding are correct. 4. The motor is under excessive load or stuck: Check the equipment and resolve the fault. 5. The power supply is not fully connected: Replace the blown fuse ； Tighten the screws that hold the terminal posts in place ； Use a multimeter to check for broken or incorrectly connected power cables, and then repair them. 6. Low voltage: If a motor connected in delta is mistakenly connected in star, it should be changed back to delta connection ； If the power supply voltage is too low, contact the power supply authority to resolve the issue ； When the voltage drops too low due to excessive voltage drop in the power cable, the cable should be replaced with a thicker one. 7. For small electric motors, hardening of grease or overly tight assembly: Select the appropriate grease and improve the quality of assembly. (4) Live voltage on the motor housing 1. The power cord and ground wire are swapped: correct the mistake. 2. The motor windings are damp and the insulation is severely aged: Motor drying treatment ； The aged insulation needs to be replaced. 3. Grounding of leads and junction boxes: Wrap or replace the insulation of the leads ； Repair the junction box. 4. Grounding of coil ends against the end cover: Remove the end cover, check the grounding point; the coil’s grounding point should be wrapped, insulated, and painted, while insulating paper should be placed on the inner surface of the end cover. (5) Unstable or oscillating pointer of the ammeter when the motor is running under no load or under load. 1. In a wound-rotor motor, one phase of the brushes has poor contact: adjust the brush pressure and improve the contact surface between the brushes and the slip rings. 2. Poor contact of the commutator short-circuiting device in the wound-rotor motor: Repair or replace the short-circuiting device. 3. Welding failure or bar breakage in the cage rotor: Inspect using a transformer or other methods. 4. Open circuit in one phase of the wound-rotor: Use a test light, multimeter, etc. to locate the open circuit and resolve the fault. (6) The motor has difficulty starting, and its speed is 1 lower than the rated speed when a rated load is applied. Low power supply voltage: Check the voltage at the motor’s input terminal using a voltmeter or multimeter, and then take appropriate action. 2. The winding connected in △ is mistakenly connected in Y: Change it back from Y connection to △ connection. 3. Weld separation or fracture in the cage rotor: Inspect and repair after detecting weld separation or fracture. 4. Poor contact between the wound-rotor brushes or the starting resistor: Inspect the contact area between the brushes and the starting resistor. 5. Some of the windings in the stator or rotor are connected incorrectly or in reverse: Refer to item 1 in this table. 6. Too many turns during rewinding: Rewind according to the correct number of turns. 7. Open circuit in one phase of the wound-rotor: Use a test light or multimeter to locate the open circuit and then resolve the fault. 8. Poor contact between the brush and the slip ring: Improve the contact area between the brush and the slip ring, such as by grinding the brush contact surface, adjusting the voltage-regulating brush, or rotating the slip ring surface. (7) Low insulation resistance: 1. The windings are damp or wet from water: Perform heating and drying treatment. 2. The winding insulation is covered with dust and oil residues: Clean the oil residues from the windings, then dry and impregnate them. 3. Damage to the motor terminal block leads to aging and cracking of the insulation: Re-insulate the leads, and replace or repair the outlet box and terminal box. 4. Winding insulation aging: If it is determined to be still usable, it can be cleaned, dried, and repainted. If the insulation is aged and prevents safe operation, it needs to be replaced. (8) The three-phase no-load current is symmetrically balanced, but it generally increases by 1. Insufficient number of turns in the rewound coil: Rewind the coil with a sufficient number of turns. 2. If the motor is connected in △ configuration when it should be connected in Y configuration: change the winding connection to a Y configuration. 3. Excessively high supply voltage: Measure the supply voltage; if it is too high, resolve the issue by consulting the power supply provider. 4. Improper motor assembly (such as incorrect installation, misalignment of the stator and rotor cores, or uneven bolt tightening of the end caps resulting in skewing or loosening of the end caps): Check the assembly quality and eliminate the fault. 5 Uneven or increased air gap: Adjust the air gap; for motors that have already had their rotors machined, it is necessary to replace the rotor or re-wire it in order to address the issue of high no-load current. 6. Overheating and damage to the core during wire removal: Inspect the core or recalculate the windings for compensation. (9) Abnormal noise during motor operation 1. Improper slot fitting during pole rewinding: It is necessary to check the fit between the stator and rotor slots. 2. Rotor rubbing against insulating paper or slot wedges: Trim the insulating paper or inspect the slot wedges. 3. Bearing wear or failure: Overhaul or replace with a new bearing. 4. Loose stator and rotor cores: Investigate the cause of vibration and re-compress the cores to address the issue. 5. Excessively high voltage or unbalanced three-phase voltage: Measure the supply voltage, identify the causes of the high voltage and imbalance, and take action to address them. 6. Incorrect stator winding connection: see item 1 in this table. 7. Fault in the winding (such as short circuit): See item 1 in this table. 8. The number of turns per phase is not equal during rewinding: Rewind to correct the number of turns. 9. Lack of lubricant in the bearing: Clean the bearing and fill it with lubricant, ensuring that it occupies 1/2 to 1/3 of the total volume of the bearing chamber. 10. Fan hitting the air shroud or blocked airflow duct: Repair the fan and air shroud to ensure their correct geometric dimensions, and clean the ventilation ducts. 11. Uneven breath flow, with friction between the stator and rotor. (10) Motor overheating or smoking: 1. The supply voltage is too high, causing the magnetic flux density in the core to become oversaturated and resulting in excessive temperature rise in the motor; if the voltage exceeds the standard by a significant amount, it is necessary to contact the power supply provider to resolve the issue. 2. The supply voltage is too low, resulting in excessive temperature rise of the motor under rated load: If this is caused by a large voltage drop in the power cables, thicker cables can be used ； If the power supply voltage is too low, you can contact the power supply authority to increase it. 3. When the wire is burned, the core is damaged, resulting in increased iron loss: Conduct core inspection tests, repair the core, and eliminate the fault. 4. Rubbing between the stator and rotor cores: Investigate the cause of the fault. If the bearing clearance is excessive, new bearings should be installed; if the shaft is bent, it needs to be inspected and repaired. In cases where the cores are loose or deformed, those cores must be addressed to eliminate the fault. 5. Dust or foreign objects accumulated on the surface of the windings can impair the motor’s heat dissipation: clean or scrub the motor and ensure that its ventilation channels are unobstructed. 6. The motor overheats due to overload or excessive resistance from the machinery it drives: Fix any faults in the driven machinery to reduce resistance. If the current level exceeds the rated value, it is necessary to reduce the load, replace the motor with one of higher capacity, or take measures to increase its capacity. 7. Frequent starting of the motor or excessive number of forward/reverse rotations: Reduce the number of starts and forward/reverse rotations of the motor, or replace it with a suitable motor. 8. If there are broken bars in the squirrel-cage rotor or if the wiring of the wound-rotor windings is loose, the rotor of the motor heats up under rated load, causing an excessive rise in the motor’s temperature: Identify the locations of the breaks and loosenesses, and then resolder them or tighten the fixing screws. 9. Inter-turn short circuits, phase-to-phase short circuits, and winding grounding: Refer to item 1 in this table. 10. Excessively high inlet air temperature: Check for any faults in the cooling system, and verify that the ambient temperature is normal. 11. Fan failure, poor ventilation: Check whether the motor fan is damaged, and whether the fan blades are distorted or not properly secured. Replace the fan if necessary. 12. Two-phase operation of the motor: Check the fusing and switch contacts to eliminate faults. 13. Poor impregnation of the windings after rewinding: A secondary impregnation process should be employed, with vacuum impregnation being the preferred method. 14. Rising ambient temperature or blocked motor ventilation channels: Improve the ambient temperature, take cooling measures, isolate heat sources near the motor, and prevent the motor from being exposed to direct sunlight. 15. Incorrect winding connection: A motor connected in Y configuration is mistakenly connected in Δ configuration, or vice versa; the connection must be corrected. (11) The no-load current is unbalanced when the motor operates under no-load conditions, with a significant difference between the values. 1. During rewinding, the number of turns in the three-phase windings is uneven: correct this by rewinding the windings. 2. Incorrect connection of the winding ends: Identify the start and end points, correct the connection, and then restart the motor for testing. 3. Unbalanced power supply voltage: Measure the power supply voltage, identify the cause, and eliminate it. 4. There is a fault in the windings, such as inter-turn short circuits or incorrect wiring of certain coils: Disassemble the motor to check the winding polarity and identify the fault, then correct or eliminate it. (12) Inter-layer insulation breakdown: 1. The material of the inter-layer spacers is of poor quality or their thickness is insufficient; use spacers made of better materials, such as epoxy glass cloth sheets, or increase the thickness of the spacers. 2. The interlayer shims are misaligned or of inappropriate size: It is necessary to ensure that the cutting dimensions are correct, to work carefully, and to follow the process specifications strictly. 3. Loose coils cause wear of the inter-layer shims: grooved linings or thicker shims can be used ； Or the VPI \"full immersion\" process can be used. (13) Inter-turn insulation breakdown 1. Poor quality of inter-turn insulation material: Reinforce it with resin-coated paint or use \"three-in-one\" mica tape. 2. During winding and embedding, damage to the insulation between turns may occur: follow the process specifications strictly. 3. Insufficient insulation thickness between turns or unreasonable structure: Select the appropriate insulation thickness or structure for the voltage between turns (see Chapter 3). (14) Winding ground fault: 1. Prolonged overload of the motor, along with aging and deterioration of the insulation, can lead to insulation breakdown relative to the ground; adjust the load or replace the motor with one of appropriate capacity to avoid localized overheating. 2. Insulation breakdown due to lightning overvoltage or switching overvoltage on transmission lines: Add or inspect lightning protection devices. 3. Due to the accumulation of conductive dust, the creepage distance decreases, leading to ground breakdown or flashover: regularly clean the insulation and install additional dust-proof sealing insulation devices. 4. After the ventilation duct was installed, the tooth pressure plates came apart at the welds; the core was not pressed tightly enough, causing the teeth to vibrate. Additionally, the bent tooth pressure plates scratched the insulation of the coils, resulting in winding ground faults. : Thoroughly inspect the welding quality and deformation of each part; make corrections or carry out additional welding to ensure that gaskets, toothed compressors, etc. are properly fixed. If the core is not compressed tightly enough, silicon steel sheets should be added or the tooth compression strips raised, and the core should be re-compressed (for cores with internal compression elements, it is not necessary to remove the core from the frame). 5. A ground fault occurred due to the insulation being burned out as a result of coil short circuits: Identify the cause of the short circuit, remove some of the coils, add insulation, and carry out impregnation and baking treatment. (15) Winding open circuit: 1. Mechanical or electromagnetic forces act on the ends of the coil, causing the solder joints to come apart; inspect the solder joints, re-solder them, and strengthen the fixing measures at the ends of the winding. 2. Improper welding techniques, with the weld points overheating and causing separation: Follow the welding procedures strictly. 3. The wire material is of poor quality, with defects such as delamination and peeling: Replace it with a qualified wire and apply insulation treatment. (16) Winding short circuit 1. Line overvoltage: Adjust the overvoltage protection value. 2. Winding insulation aging: Replace the insulation of the windings or related areas. 3. Excessive dust accumulation in the insulation gaps of the windings: Clean or wash the insulation, then dry it – coat it with varnish – and dry it again. 4. Damage to the insulation due to mechanical or electromagnetic forces: Local reinforcement or replacement of the windings and insulation, followed by impregnation and baking. (17) Stator coil insulation wear or electrocorrosion: 1. If the gap between the coil and the slot is too large (for windings formed using the \"molding\" process), 1032 varnish or resin varnish can be used to fill the gaps in the slots. 2. Loose wedge: Replace the wedge (adjust its width or height) or add shims under the gap. 3. Coil dimensional tolerances out of spec: Rewind the coil in accordance with the drawing requirements, see Chapters 2 and 3. 4. Failure of anti-vibration paint: Remove the coil and apply anti-vibration semiconductor paint again. 5. Insulation contaminated with oil and dust: Clean or blow away the dirt on the windings. (18) High leakage current: 1. The motor is damp; dry the windings after cleaning. 2. Oil and dust on the insulated surface: Clean or wash the winding insulation. 3. Insulation aging: Replace the insulation. (19) An increase in the dielectric loss angle: The coil is damaged, resulting in more air gaps within the insulation; vacuum impregnation treatment should be applied. 2. Damaged insulation: Perform local reinforcement after cleaning, then impregnate with varnish and dry it. 3. Improper insulation treatment: Improve the insulation treatment methods. 4. Insulation aging: Replace the insulation. (20) Wear and breakdown between the coil and the end clamp 1. Loose coil: Bind it and then dip it in resin paint, followed by drying. 2. Loose fixing of the end clamp: Same as Article 1 in Chapter 20. 3. Insulation covered in dust: Clean the insulation. If rewiring is done, the material of the end clamp can be changed to a non-metallic one. (21) Mechanical damage to the insulation at the coil ends: Damage caused during installation or removal; if handled in accordance with the prescribed procedures, such local damage can be repaired using epoxy glue. 2. Damage to adjacent coils during local repair or replacement of a coil: Examine the fault condition, and local repair or replacement of some of the coils can be carried out. (22) Loose slot wedges: 1. Aging and shrinkage of the slot wedge material: Replace the slot wedges. Currently, in China, 3240 epoxy glass cloth sheets are used for F-class and B-class insulation; these materials possess stable physical and chemical properties as well as good thermal stability. 2. Aging and loosening of the wedge under-gasket: Replace it with a thicker gasket and reinsert the gasket and wedge. 3. Improper fit between the slot wedge size and the core: Refer to Chapter 3 to select the slot wedge size. 4. The entire magnetic wedge wears out under the action of electromagnetic forces: switch to magnetic slurry ； If a solid mass of magnetic sludge is used, the VPI \"full immersion\" process should be employed. (23) Arching of the cage bars in the protruding core section: 1. When the motor is starting, braking, or operating in forward/reverse directions, large currents flow through the cage bars; the thermal effect caused by these currents leads to local heating and expansion of the cage bars. Once the starting/braking process is over, the cage bars begin to contract. Under the action of centrifugal force, if the strength at the ends of the cage bars is insufficient, arching of the cage bars occurs. 1. Heat the arched portion and use mechanical methods to straighten it out. 2. Remove the cage bars, straighten them, and then insert them back into the slots for welding. 3. Replace the cage bars with those of higher strength. (24) The end bars bend in the direction of the rotor. 1. This fault often occurs on rotors with high circumferential speeds and those with solid end rings; it is caused by poor fixation of the steel end collars, with the bar bending as a result of the inertial forces acting around the circumference of those collars. 2. Replace the end collars with straps that do not have a weft structure, or use end collars made of fiberglass. 3. Improve the fit between the end ring and the rotor bracket by selecting appropriate tolerance values. (25) The welded copper end ring is broken at the weld joint to save copper material. During repairs, circular end rings are sometimes made by welding together several sections of copper; if such assembled end rings are not welded properly, they can expand during operation and damage the stator insulation. 1. An integral end ring is forged from copper. 2. Improve the welding process. 3. Properly cut the welding groove