Rotating Machinery and Motor Testing
Rotating Machinery and Motor Testing
This domain tests insulation resistance/PI, winding resistance balance, surge comparison, hipot, vibration, and motor circuit evaluation.
Core rules
- Test IR/PI per IEEE 43: record at 1 and 10 minutes, correct for temperature, and discharge after testing.
- Winding resistance: compare phases; a typical investigation threshold is 5% deviation, with lower limits for large machines.
- Surge comparison: apply a fast-rise pulse to each phase and compare waveforms; a difference indicates a turn-to-turn or phase-to-phase fault.
- Hipot and DC step-voltage tests are proof tests; do not repeat them unnecessarily on aged insulation.
- Check rotation, no-load current, vibration, bearing temperature, and motor circuit conductor/connection resistance.
Worked example
A 460 V motor has phase resistances of 0.812 ohm, 0.815 ohm, and 0.810 ohm. Average = 0.8123 ohm; maximum deviation = (0.815 - 0.810)/0.8123 x 100 = 0.62%, well within a 5% limit. IR is 250 Mohm at 1 minute and 600 Mohm at 10 minutes, PI = 2.4, acceptable. The reasoning: phase balance and PI are both satisfactory, so the motor can proceed to no-load run and vibration checks.
Traps
- Testing a motor immediately after shutdown while it is hot.
- Not discharging the winding.
- Treating a surge-test difference as conclusive without visual inspection.
- Using a hipot as a routine maintenance test on every motor.