Failure analysis of 7500 HP induction motors driving reciprocating compressors with three years service

This paper presents the failure analysis and field measurements of five 7500 HP induction motors driving reciprocating compressors for a natural gas compression station. The motors were in service for less than three years when two of them suffered cooling fan failures. Four of the five motors were individually sent one at a time to a motor repair shop to be fitted with a modified cooling fan per the manufacturer´s recommendation. It was during one of the fan replacement processes that the repair shop, accidentally, discovered that many of the magnetic wedges were missing. The end user is not always aware if a motor is provided with magnetic or non-magnetic wedges unless identified in the motor engineering specification. This paper presents the findings on the failure analysis of both the cooling fans and magnetic wedges for these motors. This failure analysis examined fan design, vibration, motor operating temperature, compressor loading profile, number of motor starts, and ambient temperature. The paper also presents a comparison between direct on line motor starting and soft starting using an adjustable speed drive and the impact on motor performance. The settings and historical data gathered from the microprocessor protection relay for the motors will also be covered and discussed.