ELCID test & Core Flux
The Electromagnetic Core Imperfection detector is a test for detection of core faults such as inter-laminar short circuits particularly in large generators, where it can be rather cumbersome to perform a standard loop test. Defects in the inter-laminar insulation cause fault currents to flow locally in the core. These currents can produce dangerous local over heating or hot spots in the damaged areas and the damage to the core may become progressively worse. In extreme cases sufficient heat is generated to melt small parts of the core and even modest rises in core temperature adjacent to the winding can result in the premature failure of the winding insulation.
Around 4% flux will be created in the stator core, with the help of a “loop” wound toroidally around the core. A sensing head is passed over the surface of the core to detect magnetically the presence of fault currents themselves rather than the heating effect they produce. The chattock coils output is fed to the SPU and further to the lap top computer, which gives the graph of fault current v/s slot length. ELCID is considered best technique in assessing the core condition.
Core Flux Test
The EL-CID (electromagnetic core imperfection detection) is the preferred test, but in some cases there is a need for a power flux test. The resistance between laminations is not always linear under different voltage levels and a power flux test with a higher axial potential difference between laminations may therefore reveal core faults that are not detectable by an EL-CID test.
Flux of between 80% and 105% of rated flux is normally used to perform a power flux test or to view hot spots due to shorted lamination if any in the core.The temperatures in the core depend on the flux density. The temperature in the teeth will therefore be lower than the temperature in the area at the back of the core. It is important to compare areas with similar flux density levels when evaluating the core. A difference in “hot spot” versus average core temperature of less than 10ºC is normally acceptable when the test is performed at flux levels between 85% and 100% of the rated level.