Engine main bearings fail due to alternator?

[ewenf]

I have an alternator being driven by an engine, output is 1500 kva, 600 volts and rpm is 1200. The alternator has had a supposed history of running the main bearings of the engine that drives it. We were put into a position of having to use the alternator so had the unit completely overhauled. When the alternator was installed all alignment checks were done and checked, all clearances and tolerances were within specs. The alternator was powered up and excitation current and voltages were similar to the other 3 units in the same package. The system is an above earth IT system, there was no ground fault indication. The alternator ran for 3 weeks and the engine main beaings failed. As this alternator is the only common unit for 3 engine failures it is getting the blame but I am having trouble pinpointing the cause, any suggestions.

 

[mplsscooter]

Hi
I have seen this problem b4, check the rotors winding with megger, It only takes a small leakage current (a few amps)to interuput the oil bondary on the bearings and abnormal ware will result assuming brushless exciter a small dc leakage (milliamps) will do this

 

[ewenf]

Thanks, "mplsscooter" and yes the system is a brushless exciter, and unless something moves during rotation resistance checks before the unit was put back into service were all good. To have any current then you would need a circuit and if everything tested above earth then I cannot visulize the circuit, I have reasoned that somehow the rotor shaft becomes magnetized and this causes disruption to the lubrication circuit, hamonics has also been suggested??

 

[MikeHalloran]

A carbon brush between the engine crankcase and the crankshaft would short any currents that happened to be present. You'd probably want multiple redundant brushes.

There's no shortage of magnetic flux in the area, either. I just can't help imagining that long ago, there was an odd looking piece of metal with no apparent function inside the exciter housing, placed there to prevent magnetic leakage flux from getting into the engine structure, and it was lost or omitted during an early maintenance incident. If you look at an Illustrated Parts Breakdown, are all the parts still there?



Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA

 

[mc5w]

Aso, quite a few brushless exciters here is the U.S. use a 300 Hertz permanent magnet generator to drive the voltage regulator and the stator winding of the brushless exciter. The rotor circuit of the brushless exciter runs at 120 Hertz. That is, the rectifier that is mounted on the rotor runs off of a generator that has twice as many poles as the main generator. The 120 Hertz can penetrate insulation deficiencies such as too much capacitance very easily. Therfore, run an alternating current high potential test at 100 or 120 Hertz or connect a capacitance meter to the rotor windings.

Also check the rotor diodes for correct insulation from the heat sink. This could be your leakage source.