Rotating Machinery Testing
Rotating Machinery Testing covers insulation, winding resistance, surge comparison, vibration, and core condition. The exam tests whether you can distinguish an electrical winding fault from a mechanical or thermal problem.
Core rules
- Insulation resistance and PI are performed on stator and rotor windings; discharge after each test.
- Winding resistance should balance between phases; correct to a reference temperature.
- Surge comparison injects a pulse into each phase and compares waveforms; a difference indicates turn-to-turn or phase-to-phase insulation faults.
- Vibration analysis identifies unbalance, misalignment, bearing defects, and looseness.
- Core testing detects hot spots from lamination shorts.
Worked example
A three-phase motor has winding resistances of 1.20, 1.21, and 1.19 Ω. Average = 1.20 Ω. Maximum deviation = 0.01 Ω, or 0.83% of average. This is within a typical 1% balance limit. If one phase reads 1.35 Ω, 12.5% high, check lead connections, splice integrity, and turn count. Do not rely on resistance alone; perform a surge test and insulation resistance test before returning the motor to service.
Traps
Testing a winding at the wrong voltage; not indexing the rotor during repeated tests; ignoring temperature; using a surge test on a wet winding; mistaking bearing vibration for an electrical fault.