From diagnosis and rewinding to a full revision and repair, an emergency repair and a replacement motor from stock: Van Bodegraven resolves every failure described below, for industry and marine. For submersible and bow thruster applications we also supply and revise Trident submersible motors.
An electric motor rarely fails without warning. Almost every failure announces itself through measurable signals: rising temperature, a changing vibration spectrum, falling insulation resistance or an abnormal current pattern. This knowledge base brings together the main failure types, their causes and the right diagnosis and repair route, written by the engineers of Van Bodegraven Elektromotoren for maintenance managers, marine superintendents, shipowners, technical managers, OEM engineers, plant managers and service engineers.
Since 1898 we supply, repair and rewind electric motors in our workshop in Dordrecht, with our own shot-blast cabin, paint booth, drying ovens and motor test bench. We work for industry and the marine sector, from standard ABB motors to Trident submersible motors and bow thruster motors up to 5000 kW.
Quick diagnosis: symptom → likely cause → action
Use this table for first triage. The symptom and reading point to the likely cause; the recommended action gives the next step.
| Symptom | Likely cause | First action |
|---|---|---|
| Motor hot, whole frame | Overload, poor cooling, high ambient temperature | Measure current per phase, check cooling and load |
| Motor hot, one phase off | Single-phasing or voltage unbalance | Measure voltage and current per phase, check supply |
| High-pitched whine at the bearing | Bearing damage or lubrication problem | Vibration measurement (FFT), monitor bearing temperature |
| Vibration at 1× running speed | Unbalance | Balance, inspect rotor |
| Vibration at 2× running speed | Misalignment or mechanical looseness | Check alignment (laser) and mounting |
| Earth-fault alarm | Low insulation resistance, moisture or winding fault | Measure insulation (megger), determine PI, do not start |
| Breakdown on start after standstill | Moisture ingress in the winding | Measure insulation, dry in the oven |
| Bearing damage keeps returning | Bearing currents from a VFD (EDM) | Insulated bearing / grounding brush, dV/dt filter |
| Sparking at the DC commutator | Worn brushes, dirty or oval commutator | Inspect brushes and commutator, undercut |
1. Motor runs hot (overheating)
Heat is the winding's worst enemy: every 10 °C above the insulation class halves its life (Montsinger's rule). The main causes are overload, high ambient temperature, blocked or fouled cooling, a failed fan, too-frequent starting, undervoltage and phase unbalance. Diagnosis starts with current per phase, voltage balance (max. 1% unbalance acceptable), ambient temperature and a visual check of the cooling fins and fan cowl. Persistent overload is best solved structurally with the right efficiency class and, where needed, a larger motor.
2. Bearing failures
At 40 to 50%, bearing damage is by far the most common cause of motor failure. Sources are wrong or contaminated lubrication (too much grease is as harmful as too little), misalignment, unbalance, excessive belt tension, moisture ingress and bearing currents from VFDs. The damage pattern reveals the cause: fluting (a washboard pattern) points to electrical erosion, brown discolouration to overheating, and pitting (spalling) to material fatigue. Early detection via vibration analysis (high-frequency peak) and bearing temperature prevents secondary damage to the shaft and core.
3. Excessive vibration
Measure vibration per ISO 10816/20816 on the bearing housings, in axial, horizontal and vertical directions. The frequency is diagnostic: 1× speed = unbalance, 2× speed = misalignment or looseness, high-frequency = bearing damage, and multiples of line frequency point to electrical problems (broken rotor bar, air-gap eccentricity). An FFT spectrum makes unbalance, alignment and bearing damage objectively distinguishable, so you repair the right thing instead of guessing.
4. Low insulation resistance
Measure insulation resistance with a 500 or 1000 V megger. Rule of thumb (IEEE 43): at least 1 MΩ per kV plus 1 MΩ. For a 400 V motor anything above roughly 5 to 10 MΩ is usually healthy; below 1 MΩ it is unsafe to start. More important than the absolute value is the trend and the polarisation index (PI): a PI below 2 indicates contamination or ageing. Low values are caused by moisture, dirt, ageing or an incipient winding fault.
5. Moisture ingress
Moisture is a silent killer, especially on ships and outdoor installations. Symptoms are low insulation resistance and breakdown on the first start after standstill. Never start a wet motor: measure insulation first and, if needed, dry the winding in our oven, followed by reassessment. Prevent recurrence with anti-condensation heaters, the right IP rating, correct cable glands and a drain plug at the lowest point. For marine duty a class-compliant version with IP55/66 and C5-M coating makes the difference.
6. Rotor and stator damage
Stator damage is almost always electrical: a burnt winding from overheating, voltage spikes (often from a VFD), moisture or ageing. The burn pattern tells the cause: symmetrical = overload, one coil/phase = single-phasing or short, localised = winding fault. The fix is rewinding.
Rotor damage usually means broken or cracked rotor bars (squirrel cage) from heavy and frequent starts. Symptoms: pulsating current, vibration at slip-frequency sidebands and difficulty reaching speed. Diagnosis with MCSA (motor current signature analysis) or a growler.
7. Shaft damage and alignment
Shaft damage arises from overload, bearing failure, corrosion or fatigue at the keyway. A bent shaft (runout > 0.02 mm) or worn bearing seat can often still be restored by building up and regrinding. Misalignment (parallel or angular) is a leading cause of repeat bearing and shaft damage: always align with laser, not by eye, and account for thermal growth at operating temperature. A flexible coupling does not hide poor alignment; it only masks it until the bearing fails.
8. VFD-related failures
A variable frequency drive (VFD) solves much but introduces its own failure modes. The three main ones: bearing currents (EDM pitting and fluting from common-mode voltage), winding stress from steep voltage spikes (dV/dt) on long motor cables, and overheating at low speed because self-cooling drops. Solutions: a dV/dt or sine-wave filter, an insulated non-drive-end bearing or grounding brush, screened symmetrical cable, and forced cooling for continuous low-speed duty. When replacing, a correct motor-VFD match is crucial.
9. Marine motor failures
Marine motors operate in a harsh environment: salt, moisture, hull vibration and strict class requirements. Besides the usual failures, accelerated corrosion, condensation on temperature swings and seal wear play a role. A true marine version has C5-M offshore coating per ISO 12944, stainless terminal boxes, IP56/66, anti-condensation heaters and class certification by BV, LR, DNV, ABS, RINA or ClassNK. For non-essential duty the Dormot Core line is class-certified; for essential service we supply new or as-new reconditioned motors with a test report.
10. Bow thruster motor failures
Bow thruster motors combine all marine challenges with intermittent duty and heavy starting torques, often via a VFD. Typical failures: moisture ingress through seals, bearing damage from start-stop cycles, and insulation problems after standstill. Diagnosis and prevention follow the same lines as above, with extra attention to anti-condensation heating and bearing currents. See also our article on replacing a bow thruster motor. We supply bow thruster motors up to 5000 kW from stock, revised-as-new or fully new.
11. DC motor failures
DC motors fail in different places than AC motors. Most problems sit in the commutator and carbon brushes (sparking, wear, an oval or dirty commutator), the commutation (wrong brush position, worn mica undercut), the field or armature winding, and the bearings. Repair covers turning and undercutting the commutator, new brushes of the correct grade, rewinding and alignment. We repair DC motors up to 1500 kW and keep parts and replacement motors from Siemens, Creusen, ABB and Reliance in stock.
12. Root cause analysis
Fixing a failure without knowing the cause leads to recurrence. Root cause analysis (RCA) combines the physical damage pattern with measurement data and operating history. The damage pattern is often half the answer: a symmetrically burnt winding = overload; one burnt phase = single-phasing; fluting on the bearing = bearing currents from the VFD; brown bearings = overheating; broken rotor bar = excessive starts. We record every repair in a test report with measurable values against the factory specification, so the cause is demonstrable and you can prevent it specifically.
13. Repair, rewind or replace
The decision depends on power, availability, remaining life and strategy. The decision tree below gives the practical line; the full cost comparison is in Repair or replace a motor?
Is the frame, core or shaft irreparably damaged? → Yes: replace.
Is it a small standard motor (< ~7.5 kW)? → Yes: replacing is usually cheaper.
Do you want a strategic upgrade to IE4/IE5? → Yes: replace (and cash in the efficiency).
Is it a large, rare or custom motor with healthy frame/shaft? → Yes: repair or rewind.
Is there time pressure or a tight CAPEX budget? → Yes: as-new repair or Dormot Renew from stock.
Rewinding pays off from roughly 7.5 to 11 kW and preserves efficiency when done properly: the burn-out temperature controlled, the original copper cross-section and winding scheme retained, and impregnated in class F or H.
14. Emergency repairs
Downtime costs money, and at sea sometimes a missed tide or charter. For emergencies we work within 24 to 48 hours where possible and offer a loan or replacement motor from stock while your own motor is being repaired. Our technical WhatsApp line is available 24/7 for technical questions (+31 6 41298325); quotations go by e-mail. Good preparation helps: know your critical motors, keep nameplate data ready and consider strategic stock of essential types.
15. Predictive maintenance
Predictive maintenance replaces fixed intervals with condition-based maintenance. The core techniques: vibration analysis (bearing and alignment monitoring), thermography (hotspots, cooling), periodic insulation and PI measurements (winding condition), motor current signature analysis (rotor and load problems) and lubricant analysis. The gain lies in planning maintenance before costly unplanned downtime, and in extending life by catching failures early.
No rights can be derived from the reference values, lead times and specifications mentioned in this article. Contact us for diagnosis, a quotation or current availability.
Frequently asked questions about motor failures
General & diagnosis
What are the most common causes of motor failure? Bearing damage (40-50%), winding faults from overheating or voltage spikes, moisture ingress, misalignment and supply problems (single-phasing, unbalance).
How do I know if a motor fault is electrical or mechanical? Mechanical faults give vibration and noise that track running speed; electrical faults give abnormal current, heat and vibration at line-frequency multiples. A vibration and current measurement separates the two.
Which instruments do I need for motor diagnosis? A clamp meter, multimeter, insulation tester (megger), vibration meter or FFT analyser and an infrared thermometer or thermal camera cover most cases.
Can I diagnose a motor fault myself? First triage (current, voltage, temperature, insulation) is within reach of a maintenance technician. Surge testing, FFT analysis and root cause analysis need specialised equipment and experience; that is where we come in.
How often should I measure insulation resistance? For critical motors annually or six-monthly, and always after a long standstill or suspected moisture. Record the values to track the trend.
Overheating
Why does my electric motor run hot? From overload, poor cooling, high ambient temperature, too-frequent starting, undervoltage or phase unbalance. Measure current per phase and check the cooling.
What is the maximum operating temperature of an electric motor? It depends on the insulation class: class F allows about 155 °C winding temperature, class H about 180 °C. The permissible surface temperature is lower; check the nameplate.
Does overheating shorten a motor's life? Yes, strongly. Every 10 °C structurally above the insulation class halves the winding's life.
Can a dirty motor overheat? Yes. Dirt and dust on cooling fins and fan insulate the motor thermally; regular cleaning is cheap preventive maintenance.
Does a larger motor help against overheating? With structural overload it does, but oversizing lowers the power factor and part-load efficiency. Determine the actual load first.
Bearing failures
How do I recognise incipient bearing damage? By a high-frequency peak in the vibration spectrum, rising bearing temperature and sometimes an audible whine or rumble.
How often should I grease motor bearings? Per the nameplate or supplier; too much grease causes damage just as too little does. Use the correct grease type and do not mix incompatible greases.
What is fluting on a bearing? A washboard pattern on the raceway, caused by electrical erosion (bearing currents), usually from a variable frequency drive.
Which bearings do you use for a revision? Quality bearings from brands such as SKF or NTN, to the original specification; on request with C3 clearance or insulated for VFD applications.
How long does a motor bearing last? The calculated life (L10) is often 20,000 to 40,000 hours, but lubrication, load, alignment and environment determine practice.
Vibration & alignment
What does vibration at 1× speed mean? Almost always unbalance of the rotor or the driven part.
What does vibration at 2× speed mean? Usually a misalignment or mechanical looseness.
Which vibration standard applies to electric motors? ISO 10816/20816 for assessing vibration levels on the housing and bearings.
How do I align a motor correctly? With a laser alignment system, not by eye, and corrected for thermal growth at operating temperature.
Can unbalance be corrected without a new rotor? Often yes, by fine balancing on our balancing machine, provided the rotor itself is undamaged.
Insulation & moisture
What is a safe minimum insulation resistance? Rule of thumb IEEE 43: 1 MΩ per kV + 1 MΩ. Below 1 MΩ do not start.
What is the polarisation index (PI)? The ratio of insulation resistance after 10 minutes to that after 1 minute. A PI below 2 indicates contamination or ageing.
May I start a wet motor? No. Measure insulation first and dry the winding; starting can cause immediate breakdown.
How do I dry a damp motor winding? Under control in a drying oven, or with the motor's own anti-condensation heating; we do this in our workshop with a follow-up insulation measurement.
What is anti-condensation heating and do I need it? A heating element that prevents condensation during standstill. Strongly recommended for marine and outdoor installations.
Rotor, stator & shaft
How do I recognise a broken rotor bar? By pulsating current, sidebands around line frequency in the MCSA spectrum and difficulty reaching speed under load.
What causes a burnt stator? Overheating, voltage spikes, moisture, ageing or a winding fault. The burn pattern points to the cause.
Can a bent motor shaft be repaired? Often yes: straightening, building up and regrinding within tolerance. With cracks or excessive deviation, replacement is needed.
What is air-gap eccentricity? An uneven air gap between rotor and stator due to wear or a bent shaft, which causes vibration and local overheating.
What is a surge test? A test that detects shorts between windings with a voltage pulse; indispensable for diagnosing winding faults.
Variable frequency drives
Can a variable frequency drive cause bearing damage? Yes, via bearing currents (EDM pitting and fluting). An insulated bearing or grounding brush prevents this.
What does dV/dt do to a motor? Steep voltage edges on long cables put extra stress on the first winding coil; a dV/dt or sine-wave filter limits this.
Why does a motor overheat at low speed on a VFD? Because the shaft fan cools less at low speed; forced cooling solves this.
Which cable should I use between VFD and motor? A screened, symmetrical motor cable with a good 360° earth at both ends.
Is every motor suitable for a variable frequency drive? Not automatically; for heavy VFD duty an inverter-duty motor with reinforced insulation and possibly an insulated bearing is advisable.
Marine & bow thruster
What makes a motor suitable for marine use? C5-M offshore coating, stainless terminal box, IP56/66, anti-condensation heating and class certification.
Which class approvals can you supply? Bureau Veritas, Lloyd's Register, DNV, ABS, RINA and ClassNK.
Why does a bow thruster motor fail relatively often? Because of intermittent heavy duty, moisture, salt and VFD stress combined.
Up to what power do you supply bow thruster motors? Up to 5000 kW, from stock, revised-as-new or fully new, with class certification.
What is non-essential duty? Applications that are not critical to propulsion or safety; the class-certified Dormot Core line is suitable for these.
DC motors
Why does my DC motor spark at the commutator? From worn or wrong brushes, a dirty or oval commutator, or an incorrect brush position.
What is commutator undercutting? Milling the mica insulation between the segments slightly lower, so the brushes run smoothly.
Up to what power do you repair DC motors? Up to 1500 kW, including rewinding of armature and field.
Do you keep DC parts in stock? Yes, parts and replacement motors from Siemens, Creusen, ABB and Reliance.
How often should DC brushes be replaced? Depending on load and running hours; check brush length and commutator condition periodically.
Revision, rewinding & replacement
When is rewinding cheaper than replacing? Generally from roughly 7.5 to 11 kW, and for rare or custom motors.
Do I lose efficiency after a revision? Not with a proper revision; we control burn-out temperature, copper cross-section and impregnation.
What is included in a complete revision? Disassembly, diagnosis, rewinding where needed, new bearings and seals, shot-blasting and painting, assembly, alignment, balancing and a test report.
Do I get a warranty on a revised motor? Yes, the same warranty as on a new motor, with a test report.
What is "revised as new"? A revision that returns the motor to as-new electrical and mechanical condition, including test report and warranty.
Is revision more sustainable than buying new? Yes; a revised motor saves a large part of the production CO₂ (less copper, steel and aluminium).
Emergency repair & prevention
Do you offer emergency repairs? Yes, within 24-48 hours where possible, with a loan or replacement motor during the revision.
Do you have a 24/7 line? Yes, a technical WhatsApp line for technical questions; quotations go by e-mail.
What is predictive maintenance? Condition-based maintenance using vibration, thermography, insulation, current signature and lubricant analysis.
How do I prevent recurring failures? With root cause analysis, so the underlying cause (not just the symptom) is removed.
What data do you need for a diagnosis or quotation? The nameplate (power, speed, voltage, frame, IE class), the application, the running hours and a description of the symptom; photos help.
Motor failure? Send us the details.
Describe the symptom and send the nameplate data. Our engineers diagnose, advise on repair, rewinding or replacement, and arrange an emergency repair or replacement motor where needed.