Diagnosis of rotor inter-turn fault of electrical machine at speed using stray flux test method

The problem of detecting and locating shorted turns in round rotor windings has always been difficult. This is because the winding is completely embedded in the steel and the turn´s insulation is not physically loaded until it is subjected to the centrifugal forces produced at speed. Thus, this problem appears to be without practically known solution. However, efforts have been made by some researchers, especially in the Power Industry to develop systems for detecting and analysing rotor faults which could possibly be caused by shorted turns. Repetitive Surge Oscillograph (RSO) and some other methods developed are not effective enough. A new method called Stray flux test can be used effectively to determine and locate an inter-turn short circuited fault on generator rotor. This paper describes the design and operation of a new shorted turn detector method called Stray flux test. This method provides data at operating speed to locate the coil(s) which have shorted turns and gives indications as to the number of turns shorted in the affected coils while the rotor is at speed.