Motor Troubleshooting - 70 Interview Q&A

Motor Troubleshooting — real-world interview & field troubleshooting questions with clear, practical answers. Tap any question to open its answer.

Showing the first 70 questions free. The complete question bank is available as a PDF below.

Motor Starting Faults

Q1The motor does not start when the START command is given. What should you check first?

AnswerVerify the incoming supply, starter/contactor operation, control circuit, overload relay, motor terminals, and start command. Check where the voltage path stops instead of replacing the motor immediately.

Q2The contactor pulls in, but the motor does not rotate. How would you troubleshoot it?

AnswerMeasure voltage at the motor terminals while the contactor is energized. If supply is present, check motor winding continuity, phase loss, mechanical locking, and shaft condition.

Q3The motor starts only after pressing the START button several times. What could be wrong?

AnswerCheck the push button contacts, control relay, contactor coil, loose control wiring, and interlocks. Monitor whether the contactor receives a stable command each time the button is pressed.

Q4A motor works in manual mode but does not start from the automatic control system. What would you inspect?

AnswerCheck the automatic start signal, PLC or controller output, interlocks, permissives, remote/local selection, and contactor control circuit. Confirm that the automatic command actually reaches the motor starter.

Q5The motor hums but the shaft does not rotate. What should you check?

AnswerSwitch off and safely isolate the motor before checking for mechanical seizure. Then check for phase loss, winding problems, incorrect connection, excessive load, and bearing or gearbox resistance.

Q6The motor starts when unloaded but fails to start when connected to the machine. What does this indicate?

AnswerThe machine may be demanding excessive starting torque or may have mechanical binding. Check bearings, gearbox, pump, conveyor, brake, alignment, and the driven load before changing electrical protection settings.

Q7A motor does not start after maintenance even though it worked before maintenance. How would you find the fault?

AnswerCompare the current wiring with the previous approved drawing. Check terminals, jumpers, control wiring, overload settings, isolator position, and any connections disturbed during maintenance.

Q8The motor starter shows healthy status, but the motor remains stopped. What would you check next?

AnswerVerify voltage at the motor terminals, contactor main contacts, overload relay contacts, and motor cable. This separates a control-circuit problem from a power-circuit or motor problem.

Q9The motor starts briefly and then stops immediately. What should you investigate?

AnswerCheck the overload relay, protection trip history, control interlock, emergency-stop circuit, phase loss, and motor current. Identify whether the stop is commanded by the control circuit or caused by a protection device.

Q10A motor starts only when the enclosure is moved or tapped. What is a likely troubleshooting direction?

AnswerLook for a loose terminal, damaged control wire, contactor contact, connector, or intermittent cable. Inspect the affected connections safely rather than continuing to operate the motor in this condition.

Overheating & Thermal Issues

Q11The motor becomes very hot during normal operation. What would you check?

AnswerMeasure motor current on all phases and compare it with the nameplate rating. Also check load, cooling airflow, ambient temperature, voltage balance, bearings, and mechanical resistance.

Q12A motor overheats even though its current is close to the rated value. What could cause this?

AnswerCheck cooling, ambient temperature, duty cycle, installation, operating speed, and whether the motor is suitable for the application. Rated current does not guarantee acceptable temperature if cooling is inadequate.

Q13The motor overheats only at low speed. What should you investigate?

AnswerCheck whether the motor's cooling fan provides enough airflow at low speed. A self-cooled motor may require reduced load or an external cooling arrangement for continuous low-speed operation.

Q14A motor overheats after the production load is increased. How would you troubleshoot it?

AnswerCompare motor current and temperature before and after the load increase. Inspect the machine for excessive resistance, material buildup, alignment problems, and process conditions that increase torque demand.

Q15The motor temperature rises after the cooling fan becomes dirty. Why can this happen?

AnswerA dirty fan or blocked cooling path reduces airflow and heat removal. Clean the cooling system safely and inspect the fan, vents, and surrounding clearance.

Q16The motor is hot on one side but relatively normal on the other side. What would you check?

AnswerInspect cooling airflow, blocked vents, bearing condition, mounting, and possible local heating from the driven equipment. Also check winding current balance and motor insulation if required.

Q17The motor overheats after frequent start-stop operation. What should be checked?

AnswerCheck the number of starts per hour, starting current, acceleration time, load inertia, and motor duty rating. Frequent starts can create substantial thermal stress even when running current is normal.

Q18A motor overheats after being enclosed in a smaller cabinet. What is the likely cause?

AnswerRestricted ventilation can raise the ambient temperature around the motor. Check cabinet airflow, heat accumulation, ventilation openings, and the motor's permitted ambient temperature.

Q19The motor is overheating although the mechanical load appears normal. What electrical checks are useful?

AnswerCheck all three phase voltages and currents, terminal tightness, motor connection, supply imbalance, and winding condition. An electrical imbalance can produce additional heating without an obvious mechanical overload.

Q20The motor temperature gradually increases throughout a long shift. How would you diagnose it?

AnswerTrend motor current, ambient temperature, cooling condition, bearing temperature, and load over time. A gradual rise may indicate increasing mechanical resistance, cooling deterioration, or excessive thermal loading.

Overload & Current Faults

Q21The motor overload relay trips during normal production. What should you check?

AnswerMeasure the motor current on all phases and compare it with the overload setting and nameplate current. Then inspect the mechanical load, bearings, gearbox, pump, conveyor, or other driven equipment.

Q22The motor current is higher than normal after a gearbox replacement. What could be wrong?

AnswerCheck gearbox ratio, lubrication, alignment, mechanical drag, and whether the new gearbox changes the load torque. Compare the motor current with the previous healthy condition.

Q23A motor overload occurs only when the machine reaches full production speed. How would you troubleshoot it?

AnswerMonitor current as speed and process load increase. Check for mechanical resistance, process overload, incorrect motor sizing, and ventilation. Identify the exact operating point where current rises abnormally.

Q24The overload relay trips on one phase more than the others. What does this suggest?

AnswerCheck phase voltage balance, motor terminal connections, cable condition, and winding resistance. A significant phase-current imbalance should be corrected before continued operation.

Q25The overload trips after a bearing becomes noisy. What is the likely direction of troubleshooting?

AnswerInspect the bearing and driven shaft for mechanical resistance. A damaged bearing can increase torque demand and cause higher motor current and eventual overload.

Q26The motor current suddenly increases after a conveyor belt is tightened. What should you inspect?

AnswerCheck belt tension, alignment, pulley condition, bearings, and material load. Excessive belt tension or misalignment can increase the torque required from the motor.

Q27The motor is drawing high current but the machine output is low. What could this indicate?

AnswerSuspect mechanical losses, blockage, slipping transmission, incorrect process conditions, or motor problems. Compare motor input current with actual machine output and inspect the mechanical power path.

Q28The motor overload setting has been increased, but the motor still gets hot. What does this tell you?

AnswerIncreasing the protection setting does not remove the underlying overload. Check actual current, mechanical load, cooling, and motor suitability instead of simply raising the protection limit.

Q29A motor overload appears only after a process material change. How would you investigate it?

AnswerCompare torque demand and motor current for the old and new material. Check material buildup, viscosity, weight, friction, cutting resistance, or other process factors that may have increased the load.

Q30The motor trips on overload immediately after replacement with a motor of the same power rating. What should be checked?

AnswerVerify the replacement motor's voltage, current, connection, speed, frequency, and service conditions. Also check whether its thermal protection and starter settings match the new motor.

Voltage & Phase Faults

Q31The motor suddenly loses one phase during operation. What should you check?

AnswerCheck fuses, contactor contacts, terminals, cables, and upstream supply connections. Measure all three phase voltages safely and locate the point where one phase is lost.

Q32A three-phase motor runs but becomes noisy after a fuse fails. What is happening?

AnswerA missing phase can cause severe current imbalance and overheating. Stop the motor, correct the supply fault, and verify phase voltage and current balance before restarting.

Q33The motor voltage is lower than the rated voltage. What problems can this cause?

AnswerLow voltage can increase current under load, reduce available torque, and cause overheating. Check the supply, feeder voltage drop, cable size, loose connections, and upstream loading.

Q34The motor trips when another large machine starts. What should you investigate?

AnswerMeasure the motor supply voltage during the other machine's startup. Check for voltage sag, undersized feeders, transformer capacity limitations, and loose connections.

Q35The motor runs correctly on one supply line but overheats on another. What would you compare?

AnswerCompare phase voltage, frequency, voltage balance, connection quality, and supply stability on both lines. Also confirm that the motor connection matches the available voltage.

Q36A motor starter repeatedly loses one phase at its output. What should you inspect?

AnswerInspect contactor main contacts, terminal tightness, overload relay connections, and cable condition. Look for overheating or arcing at the affected phase.

Q37The motor has correct voltage at no load but voltage drops significantly under load. What does this suggest?

AnswerCheck feeder impedance, cable size, loose connections, transformer capacity, and upstream voltage drop. A healthy no-load measurement can hide a supply problem that appears only under load.

Q38A motor makes abnormal noise after a phase connection was repaired. What should be checked?

AnswerVerify all three phase connections, terminal tightness, voltage balance, and motor current balance. Incorrect or weak connections can cause unbalanced operation.

Q39The motor runs hot even though phase voltages are close to equal. What other voltage-related check is useful?

AnswerCheck phase sequence, actual motor terminal voltage under load, connection configuration, and waveform quality where appropriate. Also inspect current balance because voltage balance alone does not prove healthy operation.

Q40A motor stops whenever a particular contactor operates nearby. How would you troubleshoot it?

AnswerCheck whether the contactor event causes a supply disturbance, control-circuit interruption, or electromagnetic interference. Monitor the motor control voltage and power supply during the event.

Insulation & Winding Faults

Q41A motor trips on insulation failure after being stored in a damp area. What should you check?

AnswerInspect the motor for moisture and contamination and perform an appropriate insulation-resistance test after safe isolation. Follow the motor manufacturer's limits before deciding whether drying or repair is suitable.

Q42The motor winding resistance is different between phases. What could this indicate?

AnswerA significant resistance difference can indicate a winding problem, poor connection, or damaged conductor. Verify the measurement method and terminal connections before concluding that the winding is defective.

Q43A motor repeatedly trips on ground fault after heavy rain. What should you investigate?

AnswerCheck cable insulation, motor terminal box, moisture ingress, glands, and winding insulation. Correct the source of water entry and perform appropriate insulation tests before energizing.

Q44A motor has a burnt smell but still runs. Should it continue operating?

AnswerDo not assume that continued rotation means the motor is healthy. Stop and inspect for overheating, winding damage, loose connections, blocked cooling, or insulation deterioration before returning it to service.

Q45The motor insulation test is poor but winding resistance is normal. Why can this happen?

AnswerWinding resistance checks conductor continuity but may not reveal insulation leakage to ground. Perform the appropriate insulation test and inspect for moisture, contamination, or insulation breakdown.

Q46A motor trips only when it reaches operating temperature. What insulation issue could be involved?

AnswerThermal expansion can worsen an existing insulation weakness or connection problem. Test the motor and cable appropriately and inspect for temperature-dependent leakage or winding deterioration.

Q47The motor terminal box shows carbon marks around one terminal. What should be checked?

AnswerInspect the terminal, lug, cable, connection torque, and insulation for overheating or arcing damage. Repair the connection and replace damaged components before operation.

Q48A motor has repeatedly failed after cable replacement. What should be investigated?

AnswerCheck cable type, termination, insulation condition, grounding, routing, and compatibility with the starter or drive. A recurring cable-related fault may come from installation rather than the motor itself.

Q49A motor trips on earth fault only during startup. What should you check?

AnswerInspect motor cable and winding insulation for weakness that becomes apparent under starting conditions. Also check connections, cable damage, and starter switching behavior.

Q50The motor winding appears discolored after an overload event. What does this suggest?

AnswerDiscoloration can indicate excessive heating. Determine the cause of the overload, inspect winding insulation and connections, and assess the motor before putting it back into service.

Motor Bearing Faults

Q51The motor produces a grinding noise from the bearing area. What should you inspect?

AnswerCheck bearing condition, lubrication, shaft alignment, mounting, and coupling. Excessive bearing noise should be investigated before it causes shaft or winding damage.

Q52A motor bearing becomes hot during operation. What could cause this?

AnswerCheck lubrication quantity and type, bearing condition, alignment, belt tension, shaft load, and mounting. Both insufficient and excessive lubrication can cause overheating.

Q53The motor vibrates strongly near the bearing housing. How would you troubleshoot it?

AnswerCheck bearing wear, shaft condition, coupling alignment, rotor balance, mounting bolts, and foundation. Measure vibration where possible and compare it with the machine's normal baseline.

Q54A bearing fails repeatedly after replacement. What should be investigated?

AnswerCheck bearing fit, shaft and housing condition, alignment, lubrication, belt tension, contamination, and installation method. Replacing the bearing without correcting the underlying cause will not provide a lasting solution.

Q55The motor bearing becomes noisy only at high speed. What would you check?

AnswerInspect bearing condition, lubrication, rotor balance, alignment, and operating speed. Check whether the bearing or motor is being operated beyond its intended speed.

Q56A motor develops vibration after coupling replacement. What is a likely troubleshooting direction?

AnswerCheck coupling alignment, shaft concentricity, coupling condition, mounting, and balance. Compare vibration before and after the coupling change.

Q57The bearing temperature rises after changing lubricant. What should you verify?

AnswerCheck whether the new lubricant has the correct grade and compatibility, and whether the quantity is correct. Excess grease can also generate heat.

Q58A motor bearing fails after belt tension is increased. What could be the connection?

AnswerExcessive belt tension increases radial load on the shaft and bearings. Check belt tension, pulley alignment, bearing load, and the manufacturer's recommended limits.

Q59The motor has bearing noise but electrical current is normal. Can the motor still have a serious fault?

AnswerYes. Bearing and mechanical faults may not significantly change current at first. Inspect vibration, temperature, lubrication, alignment, and bearing condition independently of electrical measurements.

Q60A bearing becomes hot immediately after startup. What should you check first?

AnswerCheck lubrication, bearing installation, shaft alignment, bearing fit, and mechanical preload. Stop the motor if the temperature rises abnormally rather than allowing damage to develop.

Vibration & Coupling Issues

Q61Motor vibration increased after the motor was reinstalled. What should you check?

AnswerCheck motor alignment, mounting bolts, soft foot, coupling condition, shaft alignment, and foundation. Reinstallation can change alignment even when the motor itself is healthy.

Q62A coupled motor runs normally when disconnected but vibrates when connected to the machine. What does this indicate?

AnswerThe problem is likely related to the driven equipment, coupling, alignment, or load. Inspect the complete mechanical train rather than focusing only on the motor.

Q63The coupling heats up during operation. What could cause this?

AnswerCheck alignment, coupling lubrication where applicable, coupling condition, excessive load, and installation. Misalignment can create additional friction and heat.

Q64A motor trips on overload after a pump overhaul. What should be inspected?

AnswerCheck pump alignment, impeller condition, bearings, shaft freedom, valve/process conditions, and mechanical resistance. Compare motor current with the pre-overhaul baseline.

Q65A conveyor motor vibrates after a pulley replacement. How would you troubleshoot it?

AnswerCheck pulley balance, shaft alignment, bearing condition, mounting, belt tracking, and coupling. A rotating component imbalance can produce speed-related vibration.

Q66The motor shaft is difficult to rotate manually when isolated. What should you investigate?

AnswerCheck bearings, brake, gearbox, pump, conveyor, coupling, and other driven components for mechanical resistance. Determine whether the resistance originates in the motor or the machine.

Q67A motor runs correctly without load but makes noise when coupled. What should be checked?

AnswerInspect coupling alignment, driven equipment bearings, gearbox, belt tension, and machine resonance. The coupling may transfer a mechanical fault back into the motor.

Q68A motor's current increases after a shaft alignment adjustment. What could be wrong?

AnswerRecheck alignment, coupling condition, soft foot, bearing loading, and shaft position. An incorrect adjustment can create additional mechanical resistance.

Q69The motor repeatedly loses coupling inserts during operation. What would you investigate?

AnswerCheck alignment, coupling selection, load torque, installation, vibration, and mechanical shock. Repeated insert failure usually indicates an underlying mechanical problem.

Q70The motor operates normally until the machine reaches full load, then vibration increases sharply. What should you check?

AnswerCheck load-dependent alignment, bearing condition, structural resonance, coupling condition, and mechanical loading. Trend vibration and current as the load increases.

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