1. Description and scope of the problem
This guide deals with abnormal thermal overheating of three-phase asynchronous electric motors in critical industrial environments (aerospace, energy). Overheating, defined by constantly exceeding the insulation temperature limits (Classes B, F, H according to NF EN 60034-1), compromises the life of the winding. A rise of 10°C above the permissible service temperature reduces the life of the insulation by 50%. This guide is classified as major criticality.
SAFETY WARNING: Arc Flash and Residual Energy Hazard. Before any intervention, follow the lockout procedure (LOTO). Use appropriate PPE (class 00/0 insulating gloves, face shield, flame-retardant clothing). Never open a live terminal box.
2. Necessary diagnostic tools
| Tool | Recommended specification | Measuring range | Usage |
|---|---|---|---|
| Thermal camera | Resolution 320x240, 8-14 µm | -20°C to +650°C | Location of hot spots |
| RMS multimeter | True RMS, CAT IV 600V | 0-1000V, 0-10A | Measurement of tensions and imbalance |
| Current clamp | True RMS, harmonic compatible | 0-1000A | Measurement of currents per phase |
| Megohmmeter | Test up to 1000V DC | 0-20 GΩ | Insulation resistance measurement |
| Vibration analyzer | FFT, piezo accelerometer | 0-10kHz | Mechanical/bearing diagnosis |
3. Initial Assessment Checklist (Before Diagnosis)
| Action | Observation | Parameter to save |
|---|---|---|
| Measure ambient temperature | Temperature near air inlet | Value in °C |
| Check motor load | Compare current reading to rated current | Amps (A) |
| Observe ventilation | Presence of debris, broken fins | Visual status |
| Alarm history | Consult the frequency converter | Error Code/History |
| Listening to noise | Hissing, growling, rubbing | Type of noise |
4. Decision tree: Systematic diagnostic flow
- Step 1: External thermal analysis
- IF hot spots located on the stator → Check ventilation / obstructions → IF OK → Vibration analysis.
- IF uniform overheating → Electrical analysis (current/voltage).
- Step 2: Electrical analysis
- IF current imbalance > 3% or voltage > 1% → Check connections / drive (harmonics).
- IF rated current OK but overheating → Check insulation resistance (internal degradation).
- Step 3: Mechanical analysis
- IF high vibrations > 2.8 mm/s (RMS) → FFT analysis to identify bearing / misalignment.
5. Fault-Cause Matrix
| Symptom | Probable causes | Diagnostic test | Expected result if cause confirmed |
|---|---|---|---|
| Uniform overheating | Mechanical overload | Amperometric measurement | Current > Rated current (In) |
| Uniform overheating | Obstructed ventilation | Thermal camera | Engine temperature >> Ambient |
| Localized overheating | Voltage imbalance | RMS multimeter | Difference > 1% between phases |
| Noise + Heat | Bearing failure | Vibration analyzer | Harmonic peaks (BPFO/BPFI) |
| Rapid heating at start-up | Short circuit between turns | Megohmmeter | Isolated resistance < 1 MΩ/kV |
6. Detailed root cause analysis
6.1 Mechanical overload
The motor draws too much power to operate the load. Confirmation: measured current higher than nameplate. Damage: overall overheating of the windings.
6.2 Degradation of insulation
Due to humidity, thermal cycles or age. Confirmation: low insulation resistance. Damage: internal arc, total destruction of the motor.
6.3 Electrical unbalance and harmonics
Typical of incorrectly configured frequency converters or loose connections. Confirmation: power analyzer. Damage: heating by eddy currents in the stator.
7. Resolution procedures
- Mechanical overload: Reduce the load or check the mechanical coupling. Check the alignment (tolerance < 0.05 mm).
- Ventilation: Clean the air inlets with dry compressed air. Replace the ventilation cover if damaged.
- Electrical connections: Tighten the terminals to the specified torque (refer to the manufacturer's manual). Use a conductive grease if necessary.
- Insulation failure: If resistance < 1 MΩ/kV, the motor must be removed for re-impregnation or rewinding.
8. Prevention strategies
| Root cause | Strategy | Method | Interval |
|---|---|---|---|
| Overload | Current monitoring | SCADA / Drive | Continuous |
| Bearings | Vibration analysis | FFT analyzer | Quarterly |
| Obstruction | Visual inspection | Checklist | Monthly |
9. Spare parts and components (UNITEC-D)
| Description | Specification | Replacement | UNITEC category |
|---|---|---|---|
| High speed bearing | C3, Long-term lubrication | During FFT maintenance | Dude. Rotary |
| Cooling fan | Reinforced polymer | If fin cracked | Engine Accessories |
| Terminal box | IP66 | If severe oxidation | Electric |
For any replacement, consult our e-catalogue: https://www.unitecd.com/e-catalog/
10. References
- NF EN 60034-1: Rotating electrical machines - Rated characteristics.
- ISO 10816-3: Evaluation of machine vibrations.
- Engine manufacturer's manual (OEM specific).
