DETECTING TURN SHORTS IN ROTOR WINDINGS A new test using magnetic flux monitoring

The rotor windings of synchronous motors and generators are usually very reliable. However, the turn insulation in such machines will eventually degrade and puncture due to thermal aging, load cycling, and/or contamination. Although turn shorts do not directly lead to machine failure, they can lead to high bearing vibration, may damage synchronizing systems within brushless motors, and may limit output. Offline tests are available to detect rotor-winding shorted turns, but they may be unreliable because the rotor is not spinning for the test, and, if only a few shorts are present, the shorts may disappear once the rotor is spinning (or vice versa). With funding from the U.S. Electric Power Research Institute (EPRI), a new type of online magnetic flux test has been developed that is suitable for salient pole machines, in particular the salient pole rotors in four- (or more) pole hydrogenerators and synchronous compressor motors that are widely used for air separation. The test requires retrofitting a small magnetic flux probe to the stator core and instrumentation to interpret the small voltage signals that are measured from the probe. This article describes the aging processes of salient pole turn insulation and briefly presents the off line method that has been used to detect turn shorts. © 1975-2012 IEEE.