Generator Field Thermal Sensitivity test – On Line
The main causes of thermal turbine and generator rotor are :
a)Shaft heat sensitivity
b)Water or oil in the shaft bore
c)Uneven heat transfer between rotor parts
d)Loose wheels or pin bushings
e)Other loose or poorly fitted parts
g)Short-circuited turns in rotor winding.
It is generally believed that most of these causes result in a circumferential temperature gradient in the rotor. This causes the rotor to bow, which results in vibration. A qualified expert can determine the cause of the thermal sensitivity.
All the generator rotor fields exhibit some level of thermally induced vibration. When the shaft vibration exceeds the established limit of 3 mils, the field is considered thermally sensitive. Thermal sensitivity is characterized by a synchronous vibration of the generator rotor, induced by a change in field current.
Generator field should not exceed 3 mils at the bearing journals for any operating condition within the reactive capability curve. If vibrations exceed this limit and vibration levels tend to vary with field current, the field is most likely thermally sensitive and recommended to carry out electrical diagnostic tests like RSO, Flux probe tests to confirm the shorted turns in the rotor winding.