Health-check: On- and off-line
How are your motors feeling?
Electric motor analysis and testing is an excellent tool to prevent failures. A combination of on-line and off-line testing is necessary for a complete electrical predictive maintenance program. Together, they provide a picture of the motor’s health.
James Li, condition monitoring product manager at SKF Canada Ltd.’s head office in Toronto, contends the issue is not if a motor will fail electrically, but when.
On-line technologies test motors while operating, providing information about power condition and motor load, including specific diagnostic capabilities and overall system function.
Tests, many available in a networked continuously monitored system, include voltage transients, shaft grounding voltage and current, partial discharge, power and torque analysis, current signature analysis, winding temperature, ultrasound and thermography.
Critical motors at one or more sites can be managed from a single location and statistically analyzed trending limits automatically alert maintenance technicians of developing issues.
Off-line technologies test motors during shutdowns, measuring the integrity of the motor’s electromagnetic circuit and insulation system.
Tests include winding resistance, capacitance, insulation resistance, polarization index/dielectric absorption, power factor, breakdown voltage degradation, hipot (high potential) tests, winding inductance and impedance, surge impulse, partial discharge and core loss.
Here is a five-step optimal approach to off-line testing:
- Start with low voltage tests of the winding.
- Test ground insulation with tests of increasing voltage.
- Test turn and phase insulation.
- Test stator core during rewinds.
- Test the rotor.
Information was provided at a MainTrain maintenance conference, developed and produced by the Plant Engineering and Maintenance Association of Canada (PEMAC). Most of the technologies are summarized in IEEE 1415 and discussed in various standards from IEEE, EPRI, NEMA, CSA and EASA.