Troubleshooting of Three-Phase Asynchronous Motors

Windings are integral components of an electric motor. Aging, moisture ingress, overheating, corrosion, intrusion by foreign objects, and impact from external forces can all cause damage to the windings. Furthermore, motor overload, undervoltage, overvoltage, and operation under a phase-loss condition can also trigger winding faults. Winding faults are generally categorized into four types: grounding faults, short circuits, open circuits, and wiring errors. The following sections describe, in turn, the symptoms, causes, and inspection methods for each type of fault.

Winding Grounding Faults
This refers to a condition where the insulation between the winding and the iron core or the motor casing breaks down, resulting in an electrical connection to ground.
1. Fault Symptoms
The motor casing becomes live (electrically charged); control circuits may lose functionality; and the winding may overheat due to short-circuit currents, rendering the motor incapable of normal operation.
2. Causes
Moisture ingress into the windings, causing a reduction in insulation resistance; prolonged operation of the motor under overload conditions; corrosion caused by exposure to harmful gases; intrusion of metallic foreign objects into the windings, damaging the insulation; damage to insulation during stator rewinding, causing contact with the iron core; contact between the winding end turns and the end covers or motor frame; burning of insulation caused by friction between the stator and rotor; damage to the insulation of lead wires, causing them to contact the motor casing; or insulation breakdown caused by overvoltage events (such as lightning strikes).
3. Inspection Methods
(1) Visual Inspection Method: Visually inspect the winding end turns and the insulation material within the stator slots for any signs of physical damage or scorching. If such signs are present, the location indicates the grounding fault point.
(2) Multimeter Method: Use a multimeter set to a low-resistance range to perform a check. If the reading is very low (approaching zero), it indicates a grounding fault.
(3) Megohmmeter Method: Select an appropriate megohmmeter based on the motor's voltage class to measure the insulation resistance of each winding phase. If the reading is zero, it indicates that the specific winding phase is grounded. However, in cases where the motor insulation is merely damp or has suffered a partial breakdown due to an incident, judgment may require experience; generally, if the meter's pointer fluctuates unstably around the "0" mark, it suggests that the winding still retains a certain degree of resistance.
(4) Test Lamp Method: If the test lamp lights up, it indicates that the winding is grounded. If sparks or smoke are observed at a specific location, that point identifies the exact location of the winding grounding fault. If the lamp glows dimly, it suggests a partial breakdown of the insulation to ground. If the lamp does not light up, yet sparks occur when the test probe touches the motor casing (ground), it indicates that the winding insulation has not yet suffered a complete breakdown but is merely severely damp. Alternatively, a piece of hardwood can be used to gently tap along the rabbet edge of the motor casing; when tapping a specific spot causes the indicator light to flicker-alternating between off and on-it indicates that the current is intermittently making and breaking contact, thereby pinpointing the exact location of the ground fault.
⑸ Current Burn-out Method: Utilize a variable-voltage transformer connected to a power source. The ground fault point will quickly heat up; the spot where smoke begins to rise from the insulation marks the location of the fault. Particular caution must be exercised with small motors: the current must not exceed twice the rated current, and the duration of the test must not exceed 30 seconds. For large motors, the current should be set to 20%–50% of the rated current-or gradually increased-until smoke just begins to appear at the fault point, at which moment power must be immediately cut off.
⑹ Group Elimination Method: This method is employed when the ground fault is located deep within the iron core and involves severe burning, such that the damaged copper windings have fused together with the core. The procedure involves dividing the grounded phase winding into two halves; this process of subdivision is repeated iteratively until the specific location of the ground fault is isolated.
In addition, other diagnostic techniques exist-such as the high-voltage test method, magnetic needle detection, and power-frequency vibration analysis-but these will not be detailed individually here.
4. Remedial Measures
⑴ If the ground fault is caused by moisture ingress into the windings, the windings must first be thoroughly dried. Once they have cooled to approximately 60–70°C, apply a coat of insulating varnish, and then subject them to a second drying cycle.
⑵ If the insulation at the winding ends is damaged, the affected area at the ground fault point should be re-insulated, coated with varnish, and then dried.
⑶ If the ground fault is located within the winding slots, the entire winding must be rewound, or the specific damaged winding components must be replaced.
Finally, use appropriate megohmmeters to perform measurements to ensure that the windings meet all technical specifications.

Winding Short Circuits
Short circuits in motor windings are typically caused by insulation breakdown resulting from excessive motor current, significant fluctuations in supply voltage, single-phase operation, mechanical damage, or manufacturing defects. These faults are categorized into inter-turn short circuits (within a single coil), inter-coil short circuits (between adjacent coils), inter-pole short circuits, and inter-phase short circuits.
1. Fault Symptoms
The magnetic field distribution becomes uneven, and the three-phase currents become unbalanced, leading to increased vibration and noise during motor operation. In severe cases, the motor may fail to start entirely. Furthermore, a substantial short-circuit current is generated within the affected coil, causing it to heat up rapidly and burn out. 2. Causes
Prolonged motor overload causes insulation aging and a loss of insulating properties; insulation damage occurs during the coil insertion process; moisture ingress into the windings lowers insulation resistance, leading to insulation breakdown; end-turn and inter-layer insulation materials are improperly positioned or damaged during the coil shaping process; insulation on the end-turn connection leads is damaged; overvoltage or lightning strikes cause insulation breakdown; friction between the rotor and stator winding end-turns causes insulation damage; or foreign metal objects fall into the motor interior, or excessive oil/grease accumulation occurs.
3. Inspection Methods
⑴ Visual Inspection: Observe the terminal box and winding end-turns for any signs of scorching; overheating of the windings typically leaves dark brown discoloration and produces a burnt odor.
⑵ Temperature Sensing: Run the motor under no-load conditions for 20 minutes (immediately stopping if any abnormalities are detected), then use the back of your hand to touch various parts of the windings to check if their temperature exceeds normal limits.
⑶ Energized Test: Use an ammeter to measure the current in each phase; if the current in a specific phase is excessively high, it indicates a short circuit within that phase.
⑷ Bridge Measurement: Measure the DC resistance of each winding; generally, the difference between phases should not exceed 5%. If it does, the phase with the lower resistance likely has a short-circuit fault.
⑸ Short-Circuit Detector Method: Place a steel strip near the winding under test; if a short circuit exists within the winding, the steel strip will vibrate.
⑹ Multimeter or Megohmmeter Method: Measure the insulation resistance between any two phase windings; if the reading is extremely low or zero, it indicates a short circuit between those two phases.
⑺ Voltage Drop Method: Connect the three windings in series and apply a safe, low-voltage AC power source; the winding with the lowest measured voltage drop indicates the presence of a short-circuit fault.
⑻ Current Comparison Method: Run the motor under no-load conditions. First, measure the current in all three phases. Then, swap the connections of two phases, measure the currents again, and compare the results. If the current readings do not change in correspondence with the phase swap, the winding with the consistently higher current likely contains a short circuit.
4. Short-Circuit Repair Methods
⑴ Short circuit located at the end-turns: The short-circuit point can be isolated using insulating materials, or the insulation on the affected wire can be re-wrapped, followed by varnishing and re-baking.
⑵ Short circuit located within the stator slots: Soften the insulation to access the fault, locate and repair the short-circuit point, then re-insert the winding into the slot, followed by varnishing and baking. ⑶ For any phase winding with fewer than 1/12 of its turns involved in a short circuit, all shorted turns should be severed; the remaining conductive sections are then connected to form a closed loop, allowing the motor to be used on an emergency basis.
⑷ If the number of turns involved in a winding short circuit exceeds 1/12, the entire winding must be removed and rewound.

Winding Open Circuits
Poor soldering-or the use of corrosive flux that was not thoroughly cleaned away after soldering-can lead to "cold joints" or loose connections. Furthermore, mechanical stress or physical impact can damage coils; similarly, inter-turn short circuits or ground faults can cause conductors to burn through. When one or more conductors within a bundle of parallel-connected wires short-circuit, the remaining conductors experience a rise in temperature due to increased current flow, leading to overheating of the winding and, ultimately, an open circuit. Open-circuit faults are generally categorized into: open connections at the winding ends of a specific phase; open circuits within parallel branches; open circuits in one of several parallel-connected conductors; and broken rotor bars (broken cage).
1. Fault Symptoms
The motor fails to start; the three-phase currents are unbalanced; abnormal noise or excessive vibration is present; the temperature rise exceeds permissible limits; or smoke is emitted.
2. Causes
⑴ Accidental breakage during inspection, maintenance, or repair procedures; alternatively, issues related to manufacturing quality.
⑵ Poor soldering at connection points-such as between individual winding elements, pole (or phase) groups, or winding leads-leading to solder detachment caused by prolonged overheating during operation.
⑶ Physical damage or breakage of the winding caused by mechanical forces or electromagnetic forces.
⑷ Severe charring or melting of the winding resulting from inter-turn short circuits, inter-phase short circuits, or ground faults.
3. Inspection Methods
⑴ Visual Inspection Method: Open circuits most frequently occur at the winding ends; visually inspect for any signs of physical breakage or detached solder joints.
⑵ Multimeter Method: Using the resistance range of a multimeter: for "Y" (star) connected windings, place one probe on the neutral point (center) of the "Y" configuration and touch the other probe sequentially to the starting ends of the three phase windings; the phase that registers infinite resistance indicates the open circuit. For "△" (delta) connected windings, disconnect the delta connections, then measure the resistance of each individual phase winding; the winding that registers infinite resistance is the one with the open circuit.
⑶ Test Lamp Method: The procedure is identical to the multimeter method, but utilizes a test lamp instead; the phase where the lamp fails to light up indicates the open circuit. ⑷ Megohmmeter Method: The phase exhibiting a resistance value trending toward infinity (i.e., not a finite, non-zero value) indicates the location of the open circuit.
⑸ Ammeter Method: While the motor is running, use an ammeter to measure the three-phase currents. If the three-phase currents are unbalanced and no short-circuiting is detected, the winding in the phase with the lowest current likely suffers from a partial open-circuit fault.
⑹ Bridge Method: When the resistance of one motor phase is significantly higher than that of the other two phases, it indicates that the winding in that specific phase has a partial open-circuit fault.
⑺ Current Balance Method: For "Y"-connected windings, connect the three phases in parallel and apply a low-voltage, high-current AC supply. If the current difference between the three windings exceeds 10%, the phase with the lowest current indicates the open circuit. For "△"-connected windings, first disconnect one of the stator winding terminals; then, apply a low-voltage, high-current supply to each phase individually. The phase exhibiting the lowest current indicates the open circuit.
⑻ Broken-Bar Detector Method: During inspection, if the rotor has a broken bar (open cage), the reading on the millivoltmeter should decrease.
4. Open-Circuit Repair Procedures
⑴ If the open circuit is located at the winding ends (end turns), reconnect the broken ends, solder them securely, wrap them with insulating material, cover them with insulating tubing, bind them firmly, and finally, bake the winding to dry it.
⑵ If the winding has been severely scorched-due to inter-turn short circuits, inter-phase short circuits, ground faults, or similar causes-the entire winding should generally be replaced with a new one.
⑶ For open circuits located within the stator slots (slot sections)-provided there are only a few such breaks-emergency repairs may be performed. Use a "group elimination" method to pinpoint the exact location of the break; once found, reconnect the broken ends and ensure the insulation meets safety standards before returning the motor to service.
⑷ For broken rotor bars in squirrel-cage rotors, repair can be effected using welding, cold-joining, or bar-replacement methods.

Incorrect Winding Connections
Incorrect winding connections result in an incomplete or distorted rotating magnetic field, leading to symptoms such as difficulty in starting, unbalanced three-phase currents, and excessive noise. In severe cases, if not addressed promptly, such faults can cause the windings to burn out. The primary scenarios include the following: one or more coils within a specific pole (or phase) are inserted in reverse or have their start and end terminals incorrectly connected; a pole (or phase) group is connected in reverse; an entire phase winding is connected in reverse; branches within a multi-parallel winding circuit are misconnected; or the "Delta" (△) and "Wye" (Y) connection configurations are incorrect.
1. Fault Symptoms
The motor fails to start; the no-load current is excessively high or severely unbalanced; the temperature rises too rapidly; the motor exhibits violent vibration accompanied by loud noise; or fuses blow out.
2. Causes
An intended "Delta" (△) connection was mistakenly wired as a "Wye" (Y) connection; during maintenance or repair, the start and end terminals of one of the three-phase windings were reversed; during reduced-voltage starting, the tap position was selected inappropriately, or internal wiring errors occurred; for new motors, errors were made in winding connections during the winding installation process; or for older motors, the identification of the winding lead terminals (start/end) was incorrect.
3. Inspection Methods
⑴ Ball Bearing Method: If a ball bearing placed inside the stator rotates and rolls along the inner circumference of the stator core, it indicates that the winding connections are correct; otherwise, a winding connection error exists.
⑵ Compass Method: If the windings are connected correctly, a compass moved along a single phase winding should indicate alternating polarities (north and south) as it passes over adjacent pole (or phase) groups. Similarly, in a three-phase winding, adjacent pole (or phase) groups belonging to *different* phases should also exhibit alternating polarities. If the polarity direction remains unchanged, it indicates that a specific pole (or phase) group has been connected in reverse. If the compass needle points erratically or is unstable, it indicates that one or more individual coils within that specific phase group have been connected in reverse.
⑶ Multimeter Voltage Method: By following the wiring diagram, if a voltmeter yields no reading during two separate measurements-or if it yields a reading during one measurement but not the other-it indicates that a winding connection error exists.
⑷ Other Common Methods: These include the dry-cell battery method, the milliammeter residual magnetism method, and the motor rotation direction method.
4. Remedial Actions
⑴ If a single coil or coil group is connected in reverse, resulting in significant unbalance in the no-load current, the motor should be returned to a specialized repair facility for rework.
⑵ If the winding lead terminals were misidentified, the start and end terminals must be correctly identified and the connections re-established accordingly.
⑶ If errors occurred during the wiring for reduced-voltage starting, the connections must be carefully cross-checked against the wiring diagram or schematic and rewired correctly.
⑷ If wiring errors occurred during the initial winding installation of a new motor, or following the complete rewinding of an older motor, the unit should be returned to the manufacturer or a specialized repair facility for rework. ⑸ If one phase of the stator winding is connected in reverse, the current in that specific phase will be exceptionally high; this characteristic can be utilized to diagnose the fault and carry out repairs.
⑹ If the winding is connected in a "Delta" (△) configuration instead of the intended "Wye" (Y) configuration, or if the number of turns is insufficient, the no-load current will be excessive; this condition should be corrected immediately. How to identify the corresponding lead ends among the six output terminals of a three-phase asynchronous motor: use a dry cell battery and a multimeter to make the determination.