IMC-2018 Learning Zone 38:15
by Ed Garcia, EDG Technologies
Electric motors don't last forever, with electrical or mechanical failure always a possibility. This presentation focuses on electrical failures and standards-based testing methods which can reveal problems long before they affect the motors's operation. Early detection, when executed as part of a predictive maintenance program, can avoid unpredicted motor failures which lead to costly unplanned downtime.
There are two main categories of electric motor testing: off-line, where the motor is powered down, and on-line, where the motor is running and driving its load as normal. Both categories have high value in exposing and diagnosing insulation weaknesses, actual electric failures and dynamic issues such as poor power quality or mechanical load problems. Off-line tests include winding resistance, megohm, polarization index, hipot and surge, all of which are recommended and approved internationally by IEEE, IEC, EASA, etc. Online tests reveal issues with poor power quality such as harmonics, ripple and power factor; and loading issues, including transient conditions which adversely affect the motor.
This presentation includes three case studies:
1. Stored motor: A 4000hp AC motor was tested before deployment. The surge test failed, indicating winding insulation problems. Inspection showed significant corrosion in the stator.
2. Resistance testing: A 30hp motor passed all off-line tests except phase resistance, which showed a modest variance. The technician discovered that one of the leads in the junction box had degraded insulation leading to exposed conductors.
3. Torque ripple: In a location with three identical pumps feeding a manifold, the flow of water was less than usual. It was not apparent which motor was causing the problem. Online electrical testing identified that one motor was delivering less torque than designed, and that the torque fluctuated over time. The pump was taken off-line, and inspection revealed that its inlet shroud had become detached, leading to turbulent water flow and cavitation.