The use of shaft voltage to detect air gap eccentricity and shorted turns in salient pole alternators

Electrical machine manufacturers deem that if an induced shaft voltage is measured higher than 250 mV rms on a ball and roller bearing or 400 mV rms on a sleeve type bearing, the nondrive end bearing shall be insulated from the frame to avoid bearing damaging circulating currents. Irregularities in the magnetic circuit may cause a small amount of flux to link the shaft, with the result that an electromotive force is generated between the shaft ends. This electromotive force will produce a current to flow through the shaft, bearings, bearing supports, machine framework and back to the other end of the shaft, unless this circuit is interrupted by insulation. If the rotor pole has shorted turns or the stator to rotor air gap is eccentric, then this will cause a magnetic unbalance as described above. There could also be some electromagnetic effect transmitted to the shaft from the DC cables which run along the shaft to feed the rotor poles. Diesel driven salient pole alternators are the main power generation facility of most offshore drilling rigs. Failure of these units can have drastic effects on the drilling activity as well as safety implications. Prevention of such failures is vital. This paper describes the rationale adopted and noninvasive tests conducted to detect malfunctions within salient pole alternator rotors, while the machine is in service. Results are presented showing that shaft voltage can be used as a predictive maintenance technique and condition monitoring tool on this type of alternator rotor.