Advanced Testing Capabilities for Motors

Investigating motor quality issues and the voltage surge environment as factors in the premature failure of motors.

Client

A major North American petrochemical company.

Background

The client asked Kinectrics to review stresses and withstands associated with large induction motors installed on a Floating Production Storage Offloading (FPSO) facility located at an offshore field. The motors, rated up to 13.5 MW, 13.8 kV, provide gas compression and water injection functions.

Client’s problem

The client had experienced a series of failures on motors of similar design, most recently a failure on a 13.5 MW, 4 pole, 13.8 kV induction motor manufactured in 1998. Post-failure analysis indicated failure of the turn-to-turn insulation, which typically relates to one of three issues: (1) manufacturing quality issues (2) the presence of electrical high voltage surges and / or (3) end-winding vibration.

Scope of project work

A visual inspection performed by a consultant of the client found no evidence of end winding vibration and consequently a study was initiated to investigate the voltage surge environment imposed on the winding. The study, carried out by Kinectrics, consisted of two elements: modelling of the voltage surge environment and field measurements of the voltage surge environment due to breaker operations.

Measurements of the voltage surge environment were obtained using preinstalled capacitive voltage dividers connected to a Tektronix TDS 784 oscilloscope. The voltage dividers were installed near the line terminals of the motor to allow for an accurate measure of the voltage surge environment at the motor terminals.

Results and conclusions

The fact that the rise-times, except in one case, were slower than the rise-times stipulated in IEEE Std. 522-2004, and the fact that the measured voltage surge magnitudes were less than stipulated in IEEE Std. 522-2004, indicated that the electrical stress distributed across the turn insulation during normal service would be less than the voltage stress distributed across the turn-insulation were it to be subjected to a maintenance test as described in IEEE Std. 522-2004.

Assuming that the data measured represented the worst-case scenario experienced during a startup, it can be concluded that the data is not indicative of voltage surges having the potential to be the root cause of any turn-to-turn related premature stator winding failure. The voltage surge environment may be a contributing factor. Given that a post failure inspection of the winding did not reveal any evidence of end-winding vibration, it is probable that manufacturing quality issues may be the predominant contributing factor in any premature failures of the motor. The voltage surge environment experienced may, at worst, accelerate the aging process.

Client benefits

Based on the results obtained in this project, Kinectrics recommended that potential manufacturing quality issues be investigated further. This would involve subjecting parts of the failed winding to an array of destructive tests including electrical surge testing. The client would then be in a position to either strengthen purchasing requirements or seek alternate suppliers to eliminate future premature failures.

For more information, contact

John Page, 416.207.6000 x6492, john.page@kinectrics.com