Three-phase induction motors — windings, bearings and supply
Three-Phase Motor Failure — Diagnosis, and Whether to Repair or Rewind
- Applies to
- Three-phase squirrel-cage induction motors on direct-on-line, star-delta, soft-start and variable-speed drives
- Difficulty
- advanced
- Competence required
- qualified electrician
- Diagnosis complexity
- Moderate. Establishing that a motor has failed is easy; establishing WHY, so the replacement does not fail the same way, is the real work.
- Electrical system
- Three-phase 415 V 50 Hz nominal; motor rating per nameplate
- Safety classification
- live electrical
- Author
- EmersonEIMS Engineering
- Technical review
- Mr. Kararaho
- Last reviewed
- 2026-07-27
Scope — read this before relying on the guide
This guide sets out diagnostic method, ranked causes and safety requirements. It deliberately does not state equipment-specific numbers — test voltages, resistance and component values, torque figures, acceptance windows and pin assignments are referred to the manufacturer's documentation for your exact model rather than given here, because a plausible-looking figure for the wrong machine is more dangerous than no figure at all.
Direct technical answer
Diagnose the supply and the load before you condemn the motor, because most burnt-out motors were killed by something outside them. Single-phasing is the classic: lose one phase and the remaining two carry the load, the motor keeps turning and humming, and the windings cook within minutes, so a motor that failed while apparently running is a strong indicator that the supply, not the motor, is at fault. Measure all three phase voltages under load and compare them, then compare the three winding resistances against each other, since a healthy set is closely balanced and an imbalance points at a winding or connection fault. Insulation resistance to earth tells you whether the winding has failed to earth, but it will not find a turn-to-turn short, which is why a motor can pass an insulation test and still be dead — that requires surge comparison testing. Distinguish electrical from mechanical early: a motor that will not turn by hand, or that is noisy and hot at the drive end, is usually bearings rather than windings, and bearing failure is both cheaper to fix and a common cause of the winding failure that follows it. Take every acceptance value from the standard or the manufacturer, not from memory.
01Symptom description
Controller / display
- Overload relay tripped, or motor protection device operated
- Drive reporting overcurrent, earth fault or phase loss where a VFD is fitted
- Upstream protection tripping on start
- Motor protection trip that resets and trips again on the next start
Indicators
- Overload relay in the tripped position
- Phase failure relay operated
- Drive fault indication with the motor stationary
Sounds
- Loud hum with the motor not turning, or turning slowly — the classic single-phasing signature
- Growling or rumbling from the bearing housings
- Regular clicking or scraping, suggesting bearing damage or rotor rub
- Noise that changes with load rather than staying constant
- Unusually loud magnetic hum, which can indicate a winding or supply problem
Smells
- Burnt varnish smell — the distinctive smell of overheated winding insulation, and a strong indicator the winding is gone
- Hot grease or bearing smell at the drive end
- Any burnt smell means isolate and investigate before another start attempt
Behaviour
- Failed while running rather than on start, which points strongly at supply — commonly a lost phase
- Trips on start under load but starts unloaded, indicating a load or starting-method problem
- Runs hot at normal load, which shortens insulation life long before it fails outright
- Slow to reach speed, or does not reach speed at all
- Repeated failures of successive motors in the same position, which means the cause is the installation and not the motors
- Failed shortly after a VFD was fitted, which raises cable length and reflected-wave stress on the winding
Visible
- Burnt or discoloured windings visible through the terminal box or ventilation
- Overload relay setting compared against the nameplate full-load current
- Terminal box connections — link arrangement for star or delta, and terminal condition
- Cooling fan and cowl condition, and whether the fins are clogged
- Shaft coupling and alignment
- Water ingress, oil contamination or heavy dust in the motor
- Nameplate details, which are needed for every subsequent decision
02What the fault means
In plain language
A three-phase motor turns because three supply phases produce a rotating magnetic field. Most motor failures are not really the motor: losing a phase, an overload set wrongly, a jammed or heavy load, or failed bearings will all destroy a perfectly good motor. Finding which of those did it matters, because if you fit a new motor without fixing the cause, the new one fails the same way.
Technical explanation
A three-phase induction motor develops torque from a rotating field produced by balanced three-phase currents in the stator. Losing one phase while running — single-phasing — leaves the motor able to continue turning on the remaining two, but the current in those phases rises substantially to maintain torque, and because the thermal damage is proportional to the square of the current the winding overheats rapidly while the motor appears to be working. This is the dominant cause of burnt windings in service and it originates outside the motor, in a blown fuse, a failed contactor pole or a broken conductor. Winding faults divide into two classes that require different tests. A fault to earth is found by insulation resistance testing, interpreted against temperature and against the machine history rather than a single number. A turn-to-turn or coil-to-coil short is not detectable by insulation testing at all, because the insulation to earth may be perfectly sound; it requires surge comparison testing, which stresses the inter-turn insulation and compares the response between phases. This is why a motor can pass a megger test and still be electrically failed, and why "it megged fine" is not a diagnosis. Winding resistance measured phase to phase should be closely balanced, and an imbalance indicates a winding fault, a broken conductor or a poor connection. Mechanically, bearing degradation raises friction and temperature and eventually allows the rotor to contact the stator, so bearing failure frequently precedes and causes the winding failure that gets the blame. Where a variable-speed drive is fitted, the fast-rising voltage edges it produces can, over long motor cables, arrive at the terminals with significant overshoot and stress the first turns of the winding — a failure mode absent from direct-on-line installations and one that requires an inverter-rated machine or mitigation.
03Common causes, ranked
These are ordered by likelihood. Presenting every possible cause as equally probable is a failure of diagnosis, not thoroughness.
Most likely
start here- Single-phasing — a lost phase from a blown fuse, failed contactor pole or broken conductor
- Overload: driven load heavier than the motor rating, or an overload relay set incorrectly
- Bearing failure, which raises temperature and can end in rotor rub
- Cooling obstructed — clogged fins, blocked cowl, failed or reversed fan
Possible
check next- Supply voltage imbalance or sustained under-voltage, which raises current for the same load
- Excessive starting frequency, so the motor never cools between starts
- Moisture or contamination ingress degrading the winding insulation
- Misalignment or a badly tensioned belt drive imposing side load on the bearings
- Incorrect star or delta connection in the terminal box
Less common
after the above- Turn-to-turn winding fault, undetectable by insulation testing
- Rotor bar fracture, giving low torque and cyclic current variation
- Reflected-wave stress on long cables from a variable-speed drive
- Shaft current damage to bearings on drive-fed motors without mitigation
- Manufacturing or previous rewind defect
Model specific
verify per unit- Full-load current, insulation class, duty rating and permitted starts per hour are nameplate and manufacturer data — read them rather than assuming
- Star and delta connection arrangements differ; the terminal box link configuration must match the supply and starting method
- Inverter-rated machines have reinforced insulation; a standard motor on a drive with long cables is a different risk
- Bearing types, greasing intervals and grease specification are machine-specific
- Insulation resistance acceptance criteria come from the applicable standard and must be corrected for temperature
Environmental
site conditions- High ambient temperature reducing the motor thermal margin
- Dust and fibre clogging cooling fins, a leading cause in agricultural and industrial settings
- Moisture, condensation and washdown ingress
- Coastal salt air causing corrosion
- Altitude, which reduces cooling air density and requires derating
Installation related
built in- Overload protection set to the wrong value, or omitted
- No phase-failure protection on a critical drive
- Undersized or excessively long supply cabling causing voltage drop
- Motor sized without margin for the real duty and starting frequency
- Standard motor fitted on a variable-speed drive with a long cable run
- Poor alignment at installation, which destroys bearings early
Maintenance related
deferred work- Insulation resistance never tested, so degradation is invisible until failure
- No baseline readings recorded, so later values cannot be compared to anything
- Bearings never greased, or over-greased, which is equally damaging
- Cooling fins never cleaned
- Running current never measured against nameplate, so a developing overload goes unnoticed
- Vibration never monitored on critical machines
Component level
electronics- Stator winding failed to earth
- Turn-to-turn winding short
- Bearings failed
- Rotor bar fractured
- Terminal box connection failed or corroded
- Cooling fan broken
04Safety requirements
Isolation
- Isolate at the starter or drive, lock off, and prove dead at the motor terminals before opening the terminal box
- A motor can be back-driven by its load — a fan windmilling in a duct, or a pump under head, and generate voltage at its terminals
- Where a variable-speed drive is fitted, its DC bus capacitors remain charged after isolation; observe and verify the discharge period
- Confirm the driven machine cannot start or move during the work
Lockout and tagout
- Lock and tag the starter or drive isolator
- Tag any remote or automatic start control, including process control systems
- Where the motor drives a pump or fan that can rotate under process conditions, physically restrain it
- Keep the only key with the person doing the work
PPE
- Arc-rated protection appropriate to the prospective fault energy at the starter
- Insulated tools rated for the system voltage
- Eye protection
- Hearing protection near running machinery
- Gloves suited to hot surfaces — a recently run motor causes contact burns
Stored energy
- Variable-speed drive DC bus capacitors hold a lethal charge after isolation
- Power-factor correction capacitors, where fitted at the motor, retain charge and must be allowed to discharge
- A motor winding is inductive and an insulation tester leaves it charged — always discharge the winding after testing
- Rotating machinery coasts down and may be back-driven by its load
Specific hazards
- ALWAYS discharge the winding after an insulation resistance test. The tester charges the winding capacitance to a high voltage, and it remains charged after the test — an undischarged winding has injured people who assumed the test was over.
- Never restart a motor that smells of burnt varnish to "see if it runs". It will not recover and you risk a further fault or fire.
- A motor humming without turning is drawing heavy current and cooking. Isolate immediately rather than watching it.
- Rotating shafts, couplings and belt drives — never work on them without the drive locked off and the machine restrained
- Hot motor casings cause contact burns long after shutdown
Stop and call a qualified professional if
- The winding smells burnt or shows visible damage
- Insulation resistance is low and you cannot establish whether it is moisture or failure
- A variable-speed drive DC bus cannot be verified as discharged
- The motor is in a hazardous area, where any repair carries certification requirements
- Repeated motor failures in the same position, which need a systems investigation rather than another replacement
05Tools required
| Tool | Why it is needed |
|---|---|
| True-RMS clamp meter | Running current on all three phases against nameplate — the measurement that reveals overload and imbalance |
| True-RMS multimeter | Supply voltage on all three phases under load, and winding continuity |
| Insulation resistance tester (megger) | Winding insulation to earth; interpret against temperature and history, not a single remembered number |
| Low-resistance ohmmeter | Winding resistance balance between phases, which a normal multimeter cannot resolve well enough |
| Surge comparison tester | The only way to find a turn-to-turn fault — a motor can pass insulation testing and still be electrically failed |
| Infrared thermometer or thermal camera | Bearing housing and frame temperature, and locating hot connections at the starter |
| Vibration meter | Distinguishing bearing damage, misalignment and imbalance before they destroy the winding |
| Phase rotation tester | Confirming direction and phase presence after any supply work |
| Motor nameplate details | Every subsequent decision — protection setting, acceptance criteria, repair economics — depends on them |
06Diagnostic decision tree
1. Does the winding smell burnt or show visible damage?
Yes → Do not restart. The winding is gone — move to the repair-or-rewind decision.
No → Continue
2. Did it fail while RUNNING rather than on start?
Yes → Suspect the supply first — single-phasing is the dominant cause of burnt windings
No → Continue
3. Are all three supply phases present and balanced at the motor terminals under load?
Yes → Continue
No → That is the cause. Fix the supply before fitting anything, or the replacement fails identically.
4. Does the shaft turn freely by hand with the drive isolated?
Yes → Continue — the fault is more likely electrical
No → Bearings or a seized load. This is mechanical, and cheaper.
5. Are the three winding resistances closely balanced?
Yes → Continue
No → Winding fault, broken conductor or poor connection
6. Is insulation resistance to earth acceptable for the machine and its temperature?
Yes → Continue, but this does NOT clear the winding
No → Winding failed to earth, or moisture ingress — establish which before condemning it
7. Has a surge comparison test been done?
Yes → Continue to the repair decision
No → A turn-to-turn fault is invisible to insulation testing. Passing the megger is not a clean bill of health.
8. Has the ROOT CAUSE been established?
Yes → Correct it, then repair or replace
No → Do not fit a replacement — successive motors will fail the same way
07Step-by-step diagnosis
Step 1Record the nameplate and the protection settings
- Inspect
- Nameplate rating, full-load current, insulation class, duty; and the overload relay setting actually in use
- Where
- At the motor and at the starter
- Instrument
- Visual record
- Expected result
- Protection set to suit the nameplate and the duty
- If the result is abnormal
- An overload set above the motor rating gives no protection at all and is a frequent finding. Everything downstream — acceptance criteria, repair economics — depends on the nameplate, so capture it first.
- Next
- Step 2
Step 2Inspect before testing
- Inspect
- Burnt smell, winding discolouration, terminal box condition, cooling path, coupling and alignment
- Where
- At the motor, isolated and locked off
- Instrument
- Inspection light
- Expected result
- No burnt smell; cooling clear; terminals sound
- If the result is abnormal
- Burnt varnish smell effectively settles the winding question. Clogged cooling fins are a cause, not a cosmetic issue.
- Next
- Step 3
Safety: Never restart a motor that smells burnt.
Step 3Check all three supply phases under load
- Inspect
- Voltage on each phase at the motor terminals, and balance between them
- Where
- Motor terminal box, with the machine running where it can safely do so
- Instrument
- True-RMS multimeter
- Expected result
- Three phases present and closely balanced
- If the result is abnormal
- A lost phase is the single most common killer of otherwise healthy motors. Imbalance raises current for the same load and shortens insulation life even when the motor survives.
- Next
- Step 4
Safety: Measuring at the terminals with the machine running is live working. Only do it if competent and properly protected.
Step 4Measure running current on all three phases
- Inspect
- Current per phase against nameplate full-load current
- Where
- On each supply conductor
- Instrument
- True-RMS clamp meter
- Expected result
- Balanced currents at or below full-load current for the actual load
- If the result is abnormal
- Current above nameplate means overload — from the driven machine, the supply or the motor itself. Imbalanced current with balanced voltage points at the motor rather than the supply.
- Next
- Step 5
Step 5Turn the shaft by hand
- Inspect
- Whether the rotor turns freely, and whether it feels rough or notchy
- Where
- At the shaft, drive isolated and locked off
- Instrument
- By hand, with the coupling disconnected where practical
- Expected result
- Free rotation with no roughness
- If the result is abnormal
- A stiff, rough or seized shaft is mechanical. This simple check separates a cheap bearing job from an expensive rewind, and it is often skipped.
- Next
- Step 6
Safety: The drive must be locked off and the driven machine restrained before touching the shaft.
Step 6Measure winding resistance and compare the phases
- Inspect
- Resistance of each winding, compared against the other two
- Where
- At the motor terminals, with links removed as appropriate
- Instrument
- Low-resistance ohmmeter
- Expected result
- Three closely balanced readings
- If the result is abnormal
- Balance is the diagnostic, not the absolute value — an imbalance indicates a winding fault, broken conductor or poor connection. A standard multimeter cannot resolve these differences reliably.
- Next
- Step 7
Verify for your unit: Expected winding resistance for the specific machine where available, and the correct link arrangement before measuring.
Step 7Test insulation resistance to earth, and understand its limits
- Inspect
- Insulation resistance from windings to frame, with winding temperature recorded
- Where
- At the motor terminals, fully isolated
- Instrument
- Insulation resistance tester
- Expected result
- A value acceptable for the machine, corrected for temperature and compared against its own history
- If the result is abnormal
- A low reading may be moisture rather than failure, and a dried-out machine can recover. But note clearly: passing this test does NOT clear the winding, because it cannot see a turn-to-turn fault.
- Next
- Step 8
Verify for your unit: Acceptance criteria from the applicable standard and the manufacturer, corrected for temperature — do not apply a remembered figure.
Safety: ALWAYS discharge the winding after testing. The tester leaves it charged, and that charge has injured people who assumed the test had ended.
Step 8Surge comparison test where a turn-to-turn fault is possible
- Inspect
- Inter-turn insulation, compared between phases
- Where
- Workshop, on an isolated machine
- Instrument
- Surge comparison tester
- Expected result
- Comparable response between phases
- If the result is abnormal
- This is the test that finds what the megger cannot. A motor that "megged fine" but will not run properly is the classic case for it.
- Next
- Step 9
Step 9Establish the root cause before deciding anything
- Inspect
- Supply integrity, protection settings, load, cooling, alignment, duty and starting frequency
- Where
- Across the whole installation, not just the motor
- Instrument
- All prior measurements plus the site history
- Expected result
- A specific, identified cause
- If the result is abnormal
- Repeated failures in the same position mean the installation is killing motors. Fitting another one without finding out why is an expensive way to repeat the fault.
- Next
- Proceed to the repair-or-rewind decision
08Repair procedure
Supply and cooling — fix the cause first
cleaning and connections- Repair the lost phase at its source: fuse, contactor pole or conductor
- Correct the overload relay setting to suit the nameplate and duty
- Clean cooling fins and cowl, and repair or replace a damaged fan
- Remake corroded or loose terminal box connections and verify the star or delta links
None of the rest is worth doing until the thing that killed the motor is corrected.
Bearings and alignment
mechanical- Replace bearings with the specified type and grease, neither under- nor over-filled
- Correct shaft alignment and belt tension, which destroy bearings early when wrong
- Check for rotor rub, which indicates the bearing failure went far enough to threaten the winding
A bearing job is a fraction of the cost of a rewind and is frequently the whole fault.
Rewind or replace
component replacement- Rewind where the frame, rotor and shaft are sound and the machine is large enough to justify it
- Replace outright where the motor is small, standard and readily available — a rewind rarely makes economic sense on small frames
- Specify inverter-rated insulation where the motor runs on a variable-speed drive
- Insist on test results with any rewind: winding resistance balance, insulation resistance and a surge test
Protection and drive settings
configuration- Fit phase-failure protection where the drive is critical and none exists
- Set drive parameters to the motor nameplate rather than defaults
- Address cable length and reflected-wave mitigation where a drive feeds a long motor cable
Specialist work
manufacturer level- Refer rotor bar faults, shaft damage and hazardous-area machines
- Provide the measured currents, voltages, resistances and insulation results with the machine
09Post-repair validation
- Confirm all three supply phases present and balanced at the motor terminals under load
- Measure running current on all three phases against nameplate and record it
- Confirm the overload relay is set correctly for the nameplate and duty
- Record insulation resistance and winding resistance balance as a new baseline, with winding temperature
- Thermal-check the frame and bearing housings after a sustained run
- Check vibration after installation, since misalignment shows there before it shows anywhere else
- Confirm direction of rotation before coupling to the driven machine
- Record nameplate, all readings and the root cause identified in the maintenance record
10When not to repair
- Small standard frames, where a new motor usually costs less than a competent rewind
- Damaged frames, shafts or rotors, where the repair cost approaches replacement
- Motors that have been rewound repeatedly, where efficiency and reliability have degraded
- Standard motors repeatedly failing on a variable-speed drive, where an inverter-rated machine is the correct answer
- Hazardous-area motors where certification cannot be maintained through repair
- Any case where the root cause is unresolved — the replacement will fail the same way
11Prevention
- Fit phase-failure protection on drives that matter; single-phasing is the dominant killer and it is cheap to prevent
- Set overload protection to the nameplate and verify it at commissioning
- Take baseline insulation and winding resistance readings when the motor is new, so later values mean something
- Test insulation periodically and trend it rather than waiting for failure
- Measure running current at service visits against nameplate — a developing overload is visible long before a failure
- Keep cooling fins and cowls clean; obstructed cooling silently shortens insulation life
- Grease bearings to the specified interval and quantity — over-greasing damages bearings as surely as under-greasing
- Check alignment after any work on the coupling or driven machine
- Specify inverter-rated motors where a variable-speed drive is used, particularly with long cable runs
12Questions engineers actually ask
The motor megged fine but still will not run properly. How is that possible?
Insulation resistance testing checks the winding against earth. It cannot see a turn-to-turn or coil-to-coil short, where the insulation to earth is perfectly sound but adjacent turns are shorted together. That requires surge comparison testing. "It megged fine" is a useful result but it is not a clean bill of health, and treating it as one is a common and expensive mistake.
What causes a motor to burn out while it is still running?
Most often single-phasing. If one phase is lost the motor keeps turning on the remaining two, but the current in those rises sharply to maintain torque, and heating goes up with the square of the current, so the winding cooks within minutes while the machine appears to be working. That is why a motor that failed while running points at the supply first: a blown fuse, a failed contactor pole or a broken conductor.
Should I rewind it or replace it?
Broadly, small standard frames are cheaper to replace than to rewind competently, while larger machines usually justify a rewind if the frame, rotor and shaft are sound. Two things matter more than the arithmetic. First, find out why it failed — if the cause is unresolved, both options fail again. Second, if the motor runs on a variable-speed drive, specify inverter-rated insulation rather than a like-for-like rewind.
We keep replacing the same motor. What are we missing?
The installation, not the motor. Repeated failures in one position almost always mean an unresolved cause: overload protection set too high or absent, a supply problem, a driven machine that has become stiff, obstructed cooling, excessive starting frequency, or misalignment destroying bearings. Investigate the position rather than buying another motor — measure running current against nameplate, check all three phases under load, and check alignment.
Standards and references
- IEC 60034-1 — rotating electrical machines: rating and performance
- IEEE 43 — recommended practice for testing insulation resistance of electric machinery, including temperature correction
- IEEE 1068 — recommended practice for the repair and rewinding of AC electric motors
- IEC 60204-1 — safety of machinery: electrical equipment of machines
- The motor nameplate and manufacturer data for the specific machine, which is the only valid source for full-load current, insulation class, duty rating, bearing specification and permitted starts per hour referred to throughout
This guidance is written from engineering principle and is not a substitute for the manufacturer's model-specific documentation. Where a figure is model-specific, confirm it against the service data for your unit before acting on it.
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