Three-phase asynchronous motors are the most commonly used power sources in industrial equipment. Their performance directly affects production efficiency and operational safety. Evaluating a motor’s condition requires a comprehensive inspection of its appearance, structure, electrical characteristics, and operational performance. This guide provides a detailed step-by-step procedure to help technical personnel systematically assess motor performance.
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Inspect the motor housing for cracks, dents, or rust to prevent mechanical stress concentration and protect the enclosure rating.
Check the terminal box and end covers: bolts should be tight, and covers should be flat and secure.
Ensure cooling vents and fans are unobstructed, and fan blades are intact.
Look for oil stains or burn marks, as a burnt smell may indicate overheated windings or insulation aging.
Rotate the motor shaft manually: it should turn smoothly and freely, without sticking or abnormal noise.
Check axial and radial shaft play: loose or worn bearings can cause excessive vibration.
Verify bearing lubrication: grease should be sufficient, without dryness or leakage.
Confirm nameplate information: rated power, voltage, current, frequency, efficiency class, etc.
Ensure the motor model matches the equipment requirements.
Confirm the manufacturer: motors from reputable manufacturers offer better quality and after-sales support.
Use a megohmmeter to measure insulation resistance between windings and motor frame.
New motors: ≥ 5 MΩ; long-stored motors: ≥ 0.5 MΩ.
If resistance is below the standard, dry the windings or replace the insulation.
Measure the DC resistance of the three-phase windings (R_ab, R_bc, R_ca) with a digital multimeter.
Phase resistances should be nearly equal, with a tolerance ≤ 5%.
Significant differences indicate shorted turns, open circuits, or winding damage.
Verify connection type (Star Y or Delta Δ) matches the rated voltage.
Ensure terminal screws are tight and contacts are secure to prevent overheating.
Motor should start smoothly without vibration or shaking.
Monitor the inrush current, which should be 30%–50% of rated current under no-load conditions.
No-load vibration should be below 4.5 mm/s.
Noise should be uniform, without metallic scraping or humming sounds.
After 20–30 minutes of operation, surface temperature should not exceed 80°C.
Excessive heat or burning smell may indicate winding short circuit, bearing failure, or poor cooling.
Gradually increase load to rated power and observe current, voltage, and speed.
Current should remain within rated range, voltage should be stable, and speed should match the nameplate rating.
Under load, vibration should remain uniform and within acceptable limits.
Abnormal vibration or noise may be caused by bearing issues, rotor eccentricity, or imbalance.
Measure surface and winding temperature after 1 hour under load.
If temperature rises too quickly or exceeds specifications, stop operation and investigate.
| Test Item | Normal Condition | Abnormal Condition | Recommended Action |
| Appearance | Housing intact, fan in good condition | Cracks, rust, oil stains | Repair or replace housing/fan |
| Shaft Rotation | Smooth, no noise | Sticking, abnormal noise | Check bearings, re-lubricate |
| Insulation Resistance | ≥0.5 MΩ (new motor ≥5 MΩ) | <0.5 MΩ | Dry or repair windings |
| Winding Resistance | Three phases balanced | >5% deviation | Inspect windings, repair short/open circuits |
| No-Load Current | 30%–50% of rated | Excessive | Check windings or mechanical obstruction |
| Temperature Rise | ≤80°C | Overheating | Check cooling, bearings, or windings |
| Vibration | <4.5 mm/s | >4.5 mm/s | Dynamic balancing or bearing inspection |
Regular Inspection: Check bearings, winding insulation, and fan ventilation every 3–6 months.
Lubrication: Re-grease bearings every six months to prevent dryness or leakage.
Dust and Moisture Protection: Ensure proper ventilation and avoid moisture ingress.
Electrical Protection: Use circuit breakers, thermal relays, and undervoltage protection to prevent motor damage.
Long-Term Storage: Dry windings before storage, apply anti-rust coating to the surface, and rotate the shaft periodically to prevent bearing rust.
Evaluating the condition of a three-phase asynchronous motor requires a comprehensive assessment of appearance, structure, electrical performance, and operational characteristics. Following this guide step by step can help identify potential faults, extend motor life, and improve production safety and efficiency.
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