In some circumstances electrical current may be induced into the shafting of rotating machinery.
(a) state the problem that may be caused by this current.
(b) explain with the aid of sketches, how currents may be avoided or reduced in the following instances:
(i) d.c. machines
(ii) main shafting fitted with a bronze propeller.
Reference Answer
### (a) Problems Caused by Induced Shaft Currents Induced electrical currents in the shafting of rotating machinery, if left unmitigated, can lead to severe and premature failure of bearings. The current seeks the path of least resistance to ground, which is often through the machine's bearings and supporting structures. The primary problem occurs at the journal bearings where the rotating shaft is supported by a thin, high-pressure hydrodynamic lubricating oil film. This oil film, while an excellent lubricant, is a poor electrical conductor (a dielectric). For the current to complete its circuit from the shaft to the bearing housing, it must bridge this oil film. This results in the following sequence of damage:
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