Step 1: Visual Inspection
Before any electrical test, inspect physically:
- Shaft seal or bearing cover condition — any oil leakage or damage?
- Terminal box — water ingress, corroded terminals, damaged cable entries?
- Frame — cracked end shields, missing bolts, impact damage?
- Ventilation slots — blocked with dirt, debris, or paint?
- Fan cover — intact? Fan blades complete?
- Shaft — rotate by hand: smooth, no rough spots (bearing roughness), no dragging (shaft touching stator due to bearing wear)
Step 2: Winding Resistance Balance Test
Using a low-resistance ohmmeter (micro-ohmmeter) or Kelvin bridge, measure resistance between each pair of phases (L1-L2, L2-L3, L1-L3) at the motor terminals.
Pass criterion: All three readings within 2% of each other. More than 5% imbalance indicates a fault — shorted turns, open circuit in one phase, or poor connection. Record results and compare to commissioning baseline (if available) — a significant change since new indicates winding degradation.
Step 3: Insulation Resistance (Megger) Test
Using a DC megohmmeter: apply 500V DC for motors rated up to 1 kV; 1,000V DC for 1–6 kV motors. Measure resistance between each winding phase (shorted together) and ground (motor frame/earth terminal). Hold for 1 minute.
| IR reading (MΩ) | Condition | Action |
|---|---|---|
| >100 MΩ | Excellent | Safe to energise |
| 10–100 MΩ | Good | Safe; continue monitoring trend |
| 1–10 MΩ | Fair — moisture or contamination | Dry out; re-test before running |
| <1 MΩ | Poor — do not run | Investigate fault; repair or rewind |
Minimum acceptable: IEEE 43-2013 formula = (kV + 1) × 1,000 MΩ = 1,400 MΩ minimum for a 400V motor. In practice, anything below 10 MΩ warrants investigation.
Step 4: Polarisation Index (PI)
PI = IR at 10 minutes / IR at 1 minute. This ratio indicates whether insulation is contaminated with moisture or carbon tracking. For a clean, dry winding, IR increases over 10 minutes as the applied voltage polarises the insulation dipoles. Contaminated insulation stays at low IR.
| PI value | Interpretation |
|---|---|
| <1.0 | Winding is shorted — do not run |
| 1.0–2.0 | Poor — moisture or contamination; investigate |
| 2.0–4.0 | Good — safe to operate |
| >4.0 | Excellent |
Step 5: No-Load Current Test
Energise the motor uncoupled from its load (or with load disconnected where possible). Measure current in each phase using a clamp meter. Compare to nameplate no-load current (approximately 25–40% of full load current for most 4-pole motors).
Pass criterion: All three phase currents within 5% of each other. Significant imbalance (>5%) indicates a winding problem. If no-load current is much higher than expected, check for shorted turns or partial winding fault. Check that motor runs smoothly without hunting, vibration, or unusual noise.
Step 6: Vibration and Bearing Check
After running for 5–10 minutes uncoupled, measure vibration at bearing housings (horizontal, vertical, axial). Compare to ISO 10816 limits. Listen for: rattling (loose bearing), whining (dry bearing), rumbling (contaminated bearing or fluting).
Measure bearing temperature with an IR thermometer after 30 minutes of no-load running. Bearings should be at ambient + 20–40°C. Above +50°C rise: investigate lubrication or bearing condition immediately.
Record Keeping
Record all test results in a motor test register: date, winding resistance (all 3 phases), IR value, PI, no-load current (all 3 phases), bearing temperatures, and vibration readings. These become the baseline for future condition monitoring and help trend degradation over multiple test cycles.