Using magnetic flux monitoring to detect synchronous machine rotor winding shorts

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. Off-line tests are available to detect rotor winding shorted turns, however they may be unreliable since 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 US Electric Power Research Institute, a new type of on-line magnetic flux test has been developed that is suitable for salient pole machines, and in particular the salient pole rotors in 4 (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.